SEDIMENTS OF SAGINAW BAY, LAKE HURON:
ELEMENTAL COMPOSITION AND ACCUMULATION RATES
John A. Robbins
Great Lakes Environmental Research Laboratory
2300 Washtenaw Avenue
Ann Arbor, Michigan 48104
Grant No. R804686
Project Officer
Michael D. Mullin
Large Lakes Research Station
Environmental Research Laboratory-Duluth
Grosse He, Michigan 48138
ENVIRONMENTAL RESEARCH Li^.BORATORY-DULUTH
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
DULUTH, MINNESOTA 55804
Special Report No. 102 of the
Great Lakes Research Division
Great Lakes and Marine Waters Center
The University of Michigan
Ann Arbor, Michigan 48109
1986
ABSTRACT
During the period from 1975 through 1978, sediment cores and grab samples
were obtained from over 100 sites in lower Saginaw Bay. Selected samples were
analyzed for grain size, organic and inorganic carbon, over 30 elements and
both cesium-137 and lead-210. The study has revealed an extensive mud deposit
in the lower bay covering about 400 km^ oriented approximately with
bathymetric contours. The clay content of this deposit exceeds 50% toward the
center with the mean grain size increasing toward deposit margins. Calcium
family elements (Ca, Mg, and inorganic carbon) are preferentially concentrated
at the southwestern end of the deposit either because of the distribution of
source materials or because of prevailing currents in the system. In
contrast, iron and organic carbon exhibit highest concentrations in sediments
with highest content of clay-size particles. Most other elements, including
contaminant metals (Cr, Cu, Ni, Pb, Zn) have surface concentrations which
correlate strongly with concentrations of iron and organic carbon (r>0.9;
N = 30).
Vertical distributions of radionuclides and contaminant metals reveal a
zone of constant activity (or concentration) which extends from the sediment-
water interface to depths ranging from 10 to 25 cm. This zone of uniform
composition varies systematically within the deposit, tending to be greatest
toward the center, and is probably the resiilt of extensive mixing by zooben-
thos (predominantly oligochaetes) . About 90% of the zoobenthos occur within
the zone of mixing determined radiometrically. Benthos densities range from
10,000 to 50,000 per m^ and are sufficient to completely mix sediments
annually.
Ill
Because of extensive mixing, sedimentation rates may not be reliably
determined from profiles of Cs~137 in this system. However, lead-210 dating
appears valid and yields sedimentation rates ranging from about 0.07 to 0.24
g/cm^/yr (0.1-0.6 cm/yr). Highest rates occur toward the southwestern end of
the deposit and decrease with increasing distance from the mouth of the
Saginaw River. Radiometric mixed depths, in combination with sedimentation
rate values, provide estimates of particle residence times in the mixed layer
ranging from 11 to 60 years and averaging 30 years. Contaminant metal
deposition rates as of 1975 are estimated using a model of steady-state mixing
and exponential loading with a 20-year doubling time in combination with
sedimentation rate data.
Annual loadings are estimated (in metric tons/year) as: Cr, 54; Cu, 28;
Ni, 30; P, 420; Pb, 40; and Zn, 86.
Surface concentrations of contaminant metals (and most other elements)
are consistently lower in the bay mud deposits than in open lake deposits.
Intense local sources do not lead to higher concentrations within the deposit
mainly because of extensive downward reworking of surface materials, and to a
lesser degree, because of dilution by inert allochthonous materials. \*Jhen
corrected for "dilution" effects, concentration of chromium are considerably
higher in the bay muds. Relative to underlying sediments, the contaminant
metals are highly enriched in surface materials and mean vertically integrated
amounts (exceeding background) levels (pg/cm^) are: Cr, 280; Cu, 160;
Ni, 160; Pb, 230; and Zn, 490). These values far exceed the excess element
accumulation in deposits of the open lake. Thus, while surface concentrations
of contaminant elements, with the exception of chromium, are not particularly
distinctive, the vertically integrated amounts are strikingly high
IV
illustrate the effectiveness of vertical reworking processes in diluting
contaminants reaching the mud. The total amount of cesium~137 stored in the
deposits is about 64 Ci compared with an estimated 158 Ci deposited over the
same area from cumulative atmospheric fallout. As tributary contributions may
be ignored (but possibly not exchange from the open lake) the muds are no more
than 40% efficient in retention of the radionuclide. Total inventories of
metal contaminants in the lower bay (metric tons) are:: Cr, 1,000; Cu, 590;
Ni, 590; P, 11,000; Pb, 850; and Zn, 1,800.
Fluxes of nutrients from cores collected during the period from April
through November 1978 were determined from changes in concentrations in water
overlying sediments incubated at prevailing in situ temperatures. Mean values
for the period were: P, -530 yg/cm^/yr; N(NH3), +200 yg/cm^/yr; N(N03), -360
yg/cm^/yr; and Si, 3,000 yg/cm^/yr. Releases of Si constitute a major input
of Si into the Bay. The flux of silicon from sediments exhibits an annual
cycle ranging from about 1,500 yg/cm^/yr in the spring to a maximum of about
6,000 yg/cm^/yr in August. The mean flux may be reliably predicted from
thermodynamic expressions and the sediment temperature. During the fall
months the flux (at constant temperature) is strongly correlated with the
numbers of chironomid larvae present. Correlations between other nutrient
fluxes and organism densities are generally insignificant. The mean flux of
silicon based on pore water concentration gradients and estimates of the
effective molecular diffusion coefficient were only about 660 iig Si/cm^/yr.
Significantly higher direct fluxes suggest that Si release from sediments is
not diffusion-limited but dependent on the rate at which materials dissolve at
the sediment-water interface.
CONTENTS
Abstract iii
Introduction 1
Methods 4
Field Methods 4
Laboratory Methods 5
Results and Discussion • • 6
Physical Characteristics of Sediments 6
Composition of Surface Sediments 13
Vertical Distribution of Elements • 39
Sediment Mixing and Sedimentation Rates • 57
Cesium-137 58
Lead-210 • 68
Net Metal Contaminant Fluxes 79
Vertical Distribution of Dissolved Silicon and Phosphorus 83
Nutrient Fluxes 85
Conclusions • 91
Acknowledgments 95
Data File 96
References • 97
Publications and Presentations Receiving EPA Support 100
Vll
INTRODUCTION
This is the second of three reports dealing with the composition of re-
cent sediments of Lake Huron and the rate of accumulation of metal contami-
nants. The aims of the report include: (1) determination of recent
sedimentation rates both by radiometric methods and other means, (2) identi-
fication of elemental contaminants by examination of concentration profiles in
dated sediment cores, (3) development of contour maps for sections of the
lake which show the concentration of metal contaminants and their present and
historical rates of accumulation, (4) estimation of the total amount of
various contaminants stored in sediments, (5) identification of the origins
of metal contaminants in selected cores, and (6) recognition and quantitative
treatment of processes affecting sedimentary records of radioactivity and
metal contaminants and the exchange of substances between sediments and
overlying water. The results of the research reported in this Lake Huron
sediment series represent a natural extension of the work of Thomas et al.
(1973), who provided the first extensive and systematic mapping of the
surficial sediments of Lake Huron, and that of Kemp and Thomas (1976), who
provided the first limited exploratory study of the distribution of metal
contaminants in pollen-dated cores from this Lake.
The reports in the series deal with southern Lake Huron (Robbins 1980),
Saginaw Bay and northern Lake Huron. Areas of the lake treated in each report
are indicated in Figure 1. This report, focusing on Saginaw Bay, emphasizes
the lower part of the bay as can be seen from the distribution of coring
locations shown in Figure 2. The 1973 study of Thomas et al. indicated that
sediments of the outer bay were comprised of sand and therefore relatively
46«00-
45*00
44*00
43*00
85*00'
84*00
83*00'
82*00'
Figure 1. Sediment coring locations in Lake Huron. Sampling was
conducted throughout the entire lake from 1974 through 1978.
stations and Depth Contours (ft)
• 1975ai978Stations
o 1978 Stations
44"'00'
83°50
Figure 2. Sediment sampling sites in Saginaw Bay.
uninteresting from the standpoint of significant accumulations of
contaminants. Although the Thomas et al. study did not include the lower bay,
the earlier work of Wood (1964) showed that the lower part of the bay
possessed an area of fine-grained sediments of considerable extent.
As the fine-grained sediments are the primary carriers of contaminants,
this area is a probable receptor of contamination issuing from the Saginaw
River. In this report, the results of analyses of many samples from the
several hundred cores collected in the lower bay are summarized.
METHODS
FIELD METHODS
Sediment samples were collected at 57 stations in Saginaw Bay during
April and August of 1975. The locations of the 1975 stations, shown in Figure
2, cover in considerable detail most of the corable areas in the lower bay.
In 1978 the entire lower bay was resampled at over 100 stations. Cores were
collected wherever possible and supplemented by Ponar grab samples of
uncorable sediments (sand and gravel). The additional 1978 sampling sites are
shown in Figure 2 as open circles. Sediment cores were collected with a 3-
inch diameter gravity core (Benthos, Inc., Falmouth, Mass.). Cores contained
within plastic liners were hydraulically extruded and sectioned aboard ship.
Details of the sediment collection and processing methods are provided in the
Southern Lake Huron Report (Robbins 1980).
In addition, for this report, a series of cores were taken for nutrient
flux measurements. Cores collected by the above methods were extruded into
short plastic liners approximately 25 cm long. Care was taken to preserve
approximately 10 cm of overlying water and to disturb the core minimally
4
during the transfer process. The short sections of core with overlying water
were placed in a water-bath incubator, the temperature of which was adjusted
to match the in situ sediment temperature. The overlying water in the cores
was continuously aerated and mixed by introduction of filtered air from a 3/8"
section of Tygon tubing extending a few cm into the water overlying each core.
A maximum of 21 cores could be incubated simultaneously. During the period
from April to November 1978, selected sites in the lower bay were revisited
eight times to collect cores for nutrient flux determinations. Fluxes were
inferred from measurement of the concentration of nutrients in overlying water
as a function of time. Approximately every 24 hours for one week, 20 mL of
water was withdrawn from each core (about 5-7% of the total volume of
overlying water), filtered immediately through a phosphate-free pre-rinsed
0.45 micron (Millipore®) filter, and frozen for subsequent analysis. During
the course of the flux study, over 40 cores were examined.
LABORATORY METHODS
The methods for determination of certain sediment characteristics and
composition of sediments are described in considerable detail in the Southern
Lake Huron Report (Robbins 1980). Methods are described for determination of
bulk density, fraction dry weight, zoobenthos composition, activity of cesium-
137, total and inorganic carbon, elements via x\tomic Absorption Spectro-
photometry (AAS) on acid-peroxide extracts, lead-210, amorphous silicon, and
elements in whole sediment as determined via Neutron Activation Analysis
(NAA). Concentrations of nutrients in pore water and, in the flux
experiments, in overlying water are determined by conventional colorimetric
methods as described by Strickland and Parsons (1972).
In addition, for this report, grain-size distributions have been measured
by conventional methods, employing sieving for peroxide-cleaned sediments with
mean grain diameters exceeding 63 ym and pipette analysis of the sieved
materials with mean grain sizes less than 63 ym. Details of these methods are
described by Royse (1970).
RESULTS AND DISCUSSION
PHYSICAL CHARACTERISTICS OF SEDIMENTS
The distribution of grain size in surface sediments of the lower bay is
summarized in Figures 3-5 in terms of the major size categories: clay, silt,
and sand. In analyzing over 150 sediment cores, the 1-2-cm interval was
chosen on the assximption that it provides a better representation of the
distribution of grain sizes of quasi-permanent surface deposits than does the
0-1 cm interval. This uppermost layer (0-1 cm) consists of a fluid, highly
mobile flocculent phase overlying all corable sediments in the bay. The lim-
ited comparisons possible from the vertical dependence of grain size
distributions in cores suggest that there is generally little difference in
grain size distribution between the 0-1 and 1-2 cm intervals of sediment.
Inspection of Figures 3-5 shows that the lower bay possesses an extensive
area of mud (clay plus silt) which generally lies within the deeper waters and
covers roughly one quarter of the area of the lower bay. Areas outside the
mud deposit are generally comprised of sand with isolated areas of gravel and
coarser materials. Even within the mud deposit, surface sediments generally
have detectable amounts of fine sand. Only six samples out of 102 contained
no measurable amounts of sand. The mean grain size of surface sediments in
the lower bay is shown in Figure 6. Note that accumulation of finer materials
PERCENT CLAY IN
SURFACE SEDIMENTS
44''00 —
43°50 —
43''40 —
o
N
LOWER SAGINAW BAY
5 10
KILOMETERS
Qyio
Figure 3. Percent clay in surface sediments (1-2 cm depth) of lower
Saginaw Bay.
PERCENT SILT IN
SURFACE SEDIMENTS
30-50
»ll 10-30
in j-10
mm 0-1
44000'
N
LOWER SAGINAW BAY
5 10
KILOMETERS
Figure 4.
Percent silt in surface sediments (1-2 cm depth) of lower
Saginaw Bay.
8
PERCENT SAND IN
SURFACE SEDIMENTS
^m 10-50
EZ3 2-10
CZl <2
44»00'—
43»50 —
43°40
LOWER SAGINAW BAY
5 10
KILOMETERS
83°30
Figure 5. Percent sand in surface sediments (1-2 cm depth) of lower
Saginaw Bay.
MEAN (
3RAIN
SIZE
(Phi
Units)
Wk
>6
^
4-6
^g
2-4
11
0-2
m.
<0
44°00
43°50 —
43M0
Boy City
83° 50
N
LOWER SAGINAW BAY
5 10
KILOMETERS
Figure 6. Mean grain size (phi units) of surface sediments of lower
Saginaw Bay.
10
(phi 2) occurs to a limited extent in the southeastern region of the bay in
locally deep water (cf . Fig. 2 for bathymetric data).
The fractional water content by volume (porosity) of surface sediments
(1-2 cm) shows minor but systematic variations over the mud deposit. Highest
porosities tend to occur toward the center of the deposit and decrease toward
the margins where a greater proportion of sand occurs. No water content
measurements were made outside the mud deposits, as only grab samples could be
obtained for which water content as well as depth determinations are
inaccurate. The vertical distribution of water content in cores shows
systematic variations which are important for interpretation of radioactivity
and metal profiles. An example is provided in Figure 7 (Core: EPA-SB-75~40) .
A qualitative description of sediment textural characteristics is shox^m as a
function of sediment depth along with the porosity and the fraction of dry
sediment by weight which is soluble on treatment with acid and concentrated
hydrogen peroxide. Both the porosity and the soluble fraction decrease with
increasing sediment depth down to about 20 cm, remain constant more or less
between 20 and 35 cm, and then rise slightly below 35 cm to at least 50 cm.
The principal reason for this systematic decrease in porosity over the upper
20 cm is variable dilution of fine grained sediments by sand. Sediments
between 20 and 35 cm contain considerable sand (^50%). Below the layer of
sandy clay-silt is a layer, starting at about 35 cm in the core (Fig. 7), of
virtually sand-free clay. It is important to note that although compaction
can account for some of the decrease in water content near the sediment
surface, its role is comparatively minor below depths of a few cm, as can be
seen by comparison of the water content and acid soluble fraction in Figure 7.
This core typifies the vertical structure of the mud deposits in the lower
11
(uiD) Hld3a lN31Alia3S
12
bay: zone 1 (fluid flocculent sediment) is roughly 1 cm thick, overlaying
zone 2, gray silty clay with a sand content which gradually increases with
sediment depth, followed by zone 3, which is a region of silty clay with a
relatively uniform and high amount of fine sand, underlain by a clay (zone 4)
of higher water content (greasy) of indeterminate thickness. The thickness of
the zones varies systematically with location within the mud deposit.
The depth of the transition between zone 2 (mud with sand content increasing
with depth) and zone 3 (clay-silt with uniformly high amounts of fine sands)
occurs roughly where the porosity, cj), is 0.7 and the rate of change of
d(f) 1 d(j)
porosity, *-*', with depth is greatest (i.e., - r-")* ^^ the case of Core 78-40
dz 9 dz
(Fig. 7), this depth is about 15 cm. Porosity profiles for several other
cores are shown in Figures 8 and 9. Porosity transition depths (mud layer
thickness) vary from 4 to 18 cm in the examples. The porosity of surface
sediments is shown in Figure 10. The systematic variation in the mud
thickness is illustrated in Figure 11. Thickest deposits occur toward the
center of the depositional area and the thickness is correlated with the
fraction of the sediment which is clay.
COMPOSITION OF SURFACE SEDIMENTS
The pH of surface sediments measured in April is shown in Figure 12.
Values tend to be highest (exceeding pH 8.00) toward the margins of the mud
deposit. Lowest pH values tend to be centrally located within the deposit and
are as low as about 7.2 pH units. The platinum electrode potential (Eh) shown
in Figure 13 also tends to be highest toward the deposit perimeter with values
exceeding 400 mv. Within the deposit the electrode potential is less than 100
mv over roughly half the area. These trends toward lower pH and Eh values
13
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POROSITY OF
SURFACE SEDIMENTS
44''00'
43»50 —
43'40 —
Bay City
N
LOWER SAGINAW BAY
5 10
KILOMETERS
83»50
83M0
Figure 10, Porosity (water content) of surface sediments.
16
POROSITY TRANSITION
DEPTH (cm)
^ 15-25
Bi iO-15
HI 0-10
44''00 —
43°50 —
43M0
N
LOWER SAGINAW BAY
5 10
KILOMETERS
Figure 11. Depth of maximum fractional porosity change (see text)
17
Figure 12. pH of surface sediments (April 1975)
18
Figure 13. Eh of surface sediments (April 1975)
19
within depositional basins is typical of the sedimentary environments of the
Great Lakes. Lower pH and Eh values are associated with fine-grained, organic
rich, comparatively reducing sedimentary materials.
The distribution of inorganic carbon (Fig. 14) exhibits an enhanced
concentration in the southwest end of the mud deposit. This trend is
reflected in the distribution of calcium (Fig. 15) and magnesium (Fig. 16).
The association of these constituents as well as their ratios provide
presumptive evidence for preferential deposition of dolomitic materials in
that area of the lower bay. The distribution of organic carbon in surface
sediments (Fig. 17) contrasts markedly with those of the calcium element
group. Organic carbon ranges from less than 1% to greater than 5% over the
lower bay, with maximum concentrations tending to occur in deepest areas of
the bay. Iron (Fig. 18) exhibits a similar pattern. Metals such as lead,
zinc, and copper (which in surface sediments may be partly of anthropogenic
origin) have distributions which are very similar to the distribution of iron.
The distribution of lead is provided as an example in Figure 19.
Concentrations range from less than 20 ppm in regions outside the mud deposit
(sand/gravel) to greater than 80 ppm at several isolated sites within the
depositional zone. In contrast to the above three contaminant metals,
another, cadmium, has a distribution (Fig. 20) which is dissimilar to the iron
group but resembles more closely the distribution of the calcium family
constituents. The weaker association of cadmium with the iron-organic carbon
group also occurs in surface sediments in the southern part of the main lake
(Robbins 1980). The similarity of the distribution of iron in surface
sediments to that of other elements is summarized in Figure 21 in terms of
ordered correlation coefficients. Note that total iron exhibits the best
20
INORGANIC CARBON
( Wt % )
rai >2
^ 1.5-
■2
iii )-i
.5
mm <\
44»00'—
43»50'—
N
LOWER SAGINAW BAY
5 10
KILOMETERS
Figure 14. Inorganic carbon in surface sediments (1-2 cm).
21
44''00 —
43°50
43''40 —
Bay City
o
N
LOWER SAGINAW BAY
83° 50
5 10
KILOMETERS
83°30
Figure 15. Calcium in surface sediments (1-2 cm)
22
MAGNESIUM
Wt %
44''00'
43»50'—
^Z^AO'
Bay City
N
LOWER SAGINAW BAY
83° 50
83M0
5 10
KILOMETERS
83»30
Figure 16. Magnesium in surface sediments (1-2 cm).
23
ORGANIC CARBON
{Wt.%)
■1 5
■ 4-5
Si 3-4
{ZH 1 -3
nz <i
44«00'—
43"50'—
43M0
Bay City
N
LOWER SAGINAW BAY
5 10
KILOMETERS
83° 50
Figure 17. Organic carbon in surface sediments (1-2 cm).
24
IRON
(wt%)
■1 >3
^2.5-3
Hi 2-2.5
EH 1-2
mi <\
44''00 —
Figure 18. Iron in surface sediments (1-2 cm)
25
LEAD
(ppm)
I >80
60-80
^40-60
f 20-40
mm <20
44°00'
43°50 —
N
LOWER SAGINAW BAY
5 10
KILOMETERS
83050
83»40
Figure 19. Lead in surface sediments (1-2 cm)
26
CADMIUM
(ppm)
wa >
4
^ 3-
4
ill 2 -
3
IH t-
2
mm <
1
44°00'—
43»50-
43*'40 —
N
LOWER SAGINAW BAY
5 10
KILOMETERS
Figure 20. Cadmium in surface sediments (1-2 cm).
27
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28
correlation with acid soluble iron (Fe2) (r = 0.96 for N = 33). The elements
Pb, Zn, Cu, Mn, and Ni are all well correlated with iron while organic carbon
is comparatively less well-correlated in this environment. In contrast, in
surface sediments of the main lake, both iron and organic carbon are
significantly better correlated (Robbins 1980). Reasons for this difference
are unclear; however, a possible explanation is that the hydrodynamic
properties of iron and organic carbon-containing particulate matter may tend
to be more similar in deep water depositional environments remote from
allochthonous sources. It should also be noted that the inorganic carbon
content of surface sediments is more poorly correlated with calcium and
magnesium in sediments of Saginaw Bay than in sediments of the main lake
(Robbins 1980).
The surface sediment composition data are summarized in Table 1. Values
are in micrograms/gram (ppm) unless noted. The index 2 following an element
symbol (e.g., Fe2) indicates a concentration based on MS analysis of acid
extracts. Elements without this index xvrere determined by neutron activation
analysis of whole sediment. The extent of variability of the concentrations
is indicated by the ratio of the standard deviation of the values to the
average (coefficient of variation). This ratio is shown versus element in
Figure 22. Two major groups of elements may be distinguished: those with
coefficients greater than 0.4 and generally (with the exception of As) in the
range of 0.4-0.6, and those with values below 0.4, generally occurring within
a somewhat narrower range from about 0.25 to 0.35. Elements falling within
the high variability group include the calcium family constituents and the
contaminant metals group as determined from acid extracts. The low
variability group includes the rare earth elements, elements which are not
29
TABLE 1. Summary of surface concentration data.
Numbe r
Range
Standard
of
Element
Minimum
Maximum
Average
Deviation
Ratio*
Samples
FSOL(%)
1.40
54.6
27.7
10.3
0.372
34
IOC(%)
0.110
2.49
1.42
0.619
0.437
29
0C(%)
0.320
5.21
3.47
1.14
0.330
28
As
1.14
60.2
16.6
13.5
0.814
35
Ba
183
551
422
82.9
0.196
35
Be
1.42
46.7
15.1
7.77
0.513
35
Ca2(%)
0.012
4.87
2.35
1.48
0.628
46
Cd2
0.170
4.32
2.40
1.22
0.507
33
Ce
7
55.6
41.1
11.1
0.270
35
Co
1.25
11.1
8.22
2.29
0.278
35
Cr
5.67
106
71.6
21.8
0.304
35
Cr2
8.93
173
63.5
36.8
0.508
46
Cs
0.216
5.15
3.25
1.05
0.323
35
CsR
0.110
7.04
2.97
1.42
0.479
35
Cu2
0.720
49.7
25.5
15.9
0.624
45
Eu
0.206
1.12
0.895
0.222
0.248
35
Fe(%)
0.310
3.43
2.38
0.686
0.288
35
Fe2(%)
0.026
3.21
1.78
0.948
0.532
46
Hf
0.880
9.19
5.60
1.51
0.269
35
K2(%)
0.104
1.23
0.652
0.247
0.379
33
La
5.13
31.9
24.2
6.30
0.269
35
Lu
0.044
0.423
0.283
0.075
0.264
35
Mg2(%)
0.020
5.56
1.54
1.05
0.682
46
Mn2
20
964
496
298
0.602
46
Na
4,020
9,090
6,550
1,130
0.172
35
Ni2
3.10
66.5
31.9
17.5
0.548
44
P2
187
2,100
1,330
486
0.366
26
Pb2
3.20
87.5
45.3
26.9
0.593
44
Sb
0.089
0.957
0.522
0.167
0.320
35
Sc
0.640
12.3
8.80
2.67
0.304
35
Se
0.574
3.00
1.76
0.474
0.269
35
Sm
0.770
5.28
3.76
1.08
0.287
35
Th
0.802
8.57
6.28
1.79
0.286
35
U
0.249
5.00
1.41
0.918
0.650
35
Zn2
4.90
188
96.3
56.2
0.584
45
Values in yg/g unless indicated. *Ratio = SD/mean = coefficient of variation.
FSOL = fraction soluble in acid; IOC = inorganic carbon; OC = organic carbon;
CsR = radiocesium (Cs-137).
30
COEFFICIENT OF VARIATION (SD/AV)
0.2
0.4
0.6
0.8
1 ( 1 1
1 1 i
Asl
Mq2|
lOCI
Ul
Ca2|
Cu2l
Mn2l
Pb2l
Zn2l
Ni2l
Fe2
Br
Cr2
Cd2
CsRI
K2I
h-
FSOLI
2
P2I
OCI
UJ
Sb
_J
III
Cs
S
c
C
r
Fe
Th
Co
Sm
Ce
Hf
La
Se
Lul
Eul
Bal
Na
Figure 22. Relative variability in concentrations of elements in
surface sediments (see text).
31
likely to be contaminants and (where comparison is possible) the whole sedi-
ment concentrations of elements whose acid extract concentrations fall in the
high variability group (e.g., Fe/Fe2, Cr/Cr2, and Cs/CsR).
A comparison of the concentration of elements in surface sediments of the
lower bay with those in surface sediments in the southern part of Lake Huron
is provided in Table 2. It can be seen that, generally, concentrations are
lower in the bay mud deposits. A regression of In Cgg vs In Cslh yields the
following relationship:
CsB = 0.71 CsLH^-03 (1)
with r = 0.99 (N = 33). The mean ratio of Csb/CslH ^s 0.77. Thus, on the
average, concentrations in the mud deposit of the bay are 30% less than in mud
deposits in southern Lake Huron, presumably because of the greater dilution of
bay sediments by inert (quartz/clays) materials. If the effects of this
dilution are taken into account, only a few elements have significantly
different average concentrations in the bay relative to the open lake. Most
notable is whole-sediment chromium (Cr) which has an average concentration of
72 ppm. As the mean concentration in southern Lake Huron is 47 ppm, the above
regression equation yields a predicted value of 0.71*47"'-*03 s 37.5 ppm.
Hence, if the effects of dilution are removed, concentrations of total
chromium are 72/37.5 = 1.9 times higher in the bay than the lake. This
relative enhancement is also seen in acid-soluble chromium, although to a
lesser degree (63/0.71*66-*'*03 =s 1.2). Also occurring in relatively higher
concentration (on an inert-material corrected basis) is organic carbon
(3.5/0.71*3.2^*03 = 1,5). The relative enhancement (or reduction) in the mean
concentration of elements in the bay muds is shown in the last column of
32
TABLE 2. Comparison of average concentrations of elements in surface sedi-
ments of Saginaw Bay and southern Lake Huron.
Lower
Southern
Ratio of
Relative
Saginaw Bay
Lake Huron**
means
Enhancement
Element*
(Csb)
(Cslh)
(Csb/^slh)
(Csb/^sb)
FSOL(%)
28
31
.90
1.15
IOC(%)
1.4
2.4
.58
0.80
0C(%)
3.5
3.2
1.08
1.48
As
16
27
.61
.78
Ba
422
432
.98
1.14
Br
15
51
.29
.37
Ca2(%)
2.4
2.7
.89
1.22
Cd2
2.4
2.9
.83
1.13
Ce
41
54
.76
.95
Co
8.2
10.6
.77
1.02
Cr
72
47
1.53
1.92
Cr2
63
66
.95
1.18
Cs
3.3
3.6
.91
1.24
Csl37
3.3
7.7
.42
.56
Cu2
25
37
.67
.85
Eu
.90
1.1
.81
1.15
Fe(%)
1.8
2.4
.75
1.03
Fe2(%)
2.4
3.1
.77
1.05
K2(%)
.65
.68
.96
1.36
La
24
31
.77
.98
Lu
.28
.33
.85
1.24
Mg2(%)
1.5
2.0
.75
1.03
Mn2
.050
.13
.38
.58
Na
6,550
7,300
.90
.97
Ni2
31.9
50.6
.63
.79
P2
1,330
1,500
.88
1.00
Pb2
45.3
73.6
.62
.76
Sb
.52
.87
.60
.85
Sc
8.80
11.2
.79
1.03
Sm
3.76
5.23
.72
.96
Th
6.28
8.24
.76
1.00
U
1.41
2.79
.50
.69
Zn2
96.3
116.3
.83
1.01
*Values in pg/g unless indicated. **From Robbins (1980).
33
Table 2. Some elements, notably Br, Cs-137, Mn2, and U are significantly less
in the bay muds even after correction for dilution. Reasons for this
reduction are not clear. In the case of the radionuclide, Cs-137, which
enters the lake primarily from atmospheric fallout, the reduction may be
attributable to focusing effects. Cesium-137 deposited over a wide area (lake
surface) accumulates in comparatively limited areas of the lake bottom. In
the bay, the area of accumulation is comparable to the area of initial
deposition. Also, more Cs-137 may be exported to the open lake from shallow
bay waters through resuspension.
Coefficients of correlation between contaminant metal concentrations
(namely Cd, Cr, Cu, Ni, Pb, and Zn) , provided in Table 3, illustrate their
very high degree of similarity in areal distribution within surface mud
deposits. In all cases but the pair Zn-Cr, for which r = 0.89, values of the
correlation coefficient exceed 0.9 for N >^ 32 in the case of Cd and N >^ 43 for
the other element pairs, not involving Cd. Analysis of one contaminant
element in surface sediments serves to establish levels of the others with a
high degree of confidence.
TABLE 3. Correlation coefficients for pairs of contaminant metal concentra-
tions (acid soluble) in surface sediments.
Cd
Cr
Cu
Ni
Pb
Zn
Cd
1
.90
.94
.91
.91
.92
Cr
.90
1
.93
.91
.94
.89
Cu
.94
.93
1
.94
.97
.98
Ni
.91
.91
.94
1
.97
.95
Pb
.91
.94
.97
.97
.1
.99
Zn
.92
.89
.98
.95
.99
1
For Cd N >_ 32; for other elements N >_ 43.
34
x\s both grain size distribution and concentration data are available for
surface sediments, some inferences may be made concerning the association of
elements with sediment grain size. Ideally, the elemental composition of
sediment size fractions should be measured, but such an approach is beyond the
scope of this study. However, to the extent that the concentration of an
element in sediments of a well-defined size range (e.g., 3 to 4 phi units) is
independent of location over the lower bay, the concentration may be inferred
by statistical analysis of a sufficiently large concentration-grain size data
set. If fj_ is the fraction of the total dry weight of sediment which is in
the ith size class, and the concentration of a certain element in that size
class is CjL, then the measured concentration in the sample is
N
C = I Cifi. (2)
i = 1
If Cj is the measured concentration in the jth sample, then the assumed
independence of C^ on location means that
N
Cj = I Cifij (3)
i = 1
where fj_j is the fraction of the total sediment dry weight in the ith size
class at the jth location. It is further assumed that the dependence of C*^
on mean grain size in each size class ((^±) is essentially Gaussian so that,
-(6.-6) 2 / lo'l
1 m 9
Ci = C^ax^ (^)
35
A nonlinear least squares routine was used (Dams and Robbins 1970) to deter-
mine values of the three parameters (C^ax ^m ^^^ ^^ ) ^^^ each element using
the surface concentration data and grain size data. The results are presented
in Table 4 and Figure 23. It can be seen that the elements (Fe2, Fe, Pb2,
Zn2, and Br) are apparently associated with the smallest mean grain sizes
(greater than 10 phi units). These elements not only are associated with
particularly fine grained sediments but concentrations are distributed over a
wider range of sizes (especially Fe2, for which the value of a. is 6.3 phi
units). The model concentrations for these elements shown in Figure 23
indicate that Fe2 is, in fact, not strictly Gaussian, in the sense that the
apparent position of the maximum is beyond the range of the data. Also shown
in Figure 23 (upper panel) is the correlation of the measured concentration
with each size class. Generally, for all elements (including those shown),
correlations are high (>0,7, N = 30) for grain sizes greater than 7 phi.
Below 7 phi units, the correlation decreases rapidly, reaching zero at 4 phi
units and remains zero or negative for even lower phi values (larger grain
sizes). Thus, the measured concentrations of all elements of this study are
derived from partial concentrations of the element on grain sizes smaller than
about 60 microns (4 phi). The presence of materials with grain sizes greater
than 4 phi (coarse silt) merely serves to dilute sediments with inert
constituents (inert with respect to elements determined in this study). In
the case of Fe2 and Pb2 (Fig. 23), the correlations vs. grain size in the
interval beyond 7 phi units are comparable, although the correlations with
lead remain slightly higher. The behavior of calcium, however, is
significantly different. For this element, the maximum correlation occurs
around 7-8 phi units and drops significantly for smaller particle sizes
36
TABLE 4. Grain size/concentration model parameters.
Correlation
Number
Am
(phi units)
(phi units)
Coeffi-
of
Element
Cmax*
cient**
Samples
FSOL
209.
7.1
0.66
.51
34
IOC
9.65
6.8
0.68
.52
29
OC
205.
8.0
0.25
.65
28
Br
76.6
13.0
2.2
.49
35
Ca2
19.7
6.7
0.64
.81
46
Cd2
7.8
7.8
1.8
.86
33
Ce
93.9
8.1
2.2
.67
35
Co
23.1
8.0
1.8
.70
35
Cr
266.
7.6
1.4
.70
35
Cr2
142.
10.8
3.8
.80
46
Cs
7.82
11.0
3.0
.68
35
CsR
159.
8.8
0.21
.81
35
Cu2
318.
7.0
0.50
.89
45
Eu
1.63
8.4
2.8
.62
35
Fe
4.93
10.5
3.2
.72
35
Fe2
9.25
18.4
6.3
.87
46
Hf
9.72
6.2
2.2
.41
35
La
52.6
8.4
2.4
.69
35
Lu
0.65
7.6
2.0
.56
35
Mg2
24.6
8.2
0.52
.82
46
Mn2
1,200.0
8.4
2.5
.83
46
Na
10 , 000 .
5.4
2.7
.44
35
N12
168
7.1
1.1
.85
44
P2
3,500.
8.4
2.1
.56
26
Pb2
128.
10.3
2.8
.91
44
Sb
0.99
9.4
3.0
.54
35
Sc
36.8
7.3
1.2
.69
35
Sni
16.4
6.9
1.0
.69
35
Th
21.9
7.3
1.4
.71
35
U
3.3
7.0
1.8
.25
35
Zn2
284.
12.0
3.6
.90
45
* Units are identical with those in Table 1.
**The coefficient of correlation between measured and predicted values of
surface concentrations .
37
UJ
o
UJ
o
o
or
o
o
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>
(D
<
UJ
O
Z
o
o
too-
2 4 6
GRAIN SIZE (phi units)
8
Figure 23. Model concentrations of calcium, iron, and lead as a
function of grain size (see text).
38
((j) = 9). This result, which does not depend on any complex model analysis,
suggests an association of Ca with a narrower range of grain sizes than Pb2
and Fe2. This is borne out by the concentration model analysis as xvell. The
distribution of Ca as well as the other calcium family elements (FSOL, Mg,
IOC) peaks at a distinctly lower value of phi ((|)= 6.7) and has a very narrow
grain size range (0.64 phi units). The model concentration of calcium vs.
grain size (Fig. 23) indicates that this element is associated with fine silt
(8-16 micron dia) whereas both lead and iron are associated with fine clays
(<2 micron dia). Other elements are relatively undistinguished in terms of
grain size, but several have relatively narrow grain size ranges (like the Ca
group) for reasons which are not clear. These include: organic carbon, Cs-137
(CsR), and Cu2.
VERTICAL DISTRIBUTION OF ELEMENTS
The vertical distribution of FSOL (fraction soluble on acid treatment)
and selected acid-soluble major and minor elements are illustrated in Figures
24 through 34. The behavior of FSOL as exemplified in Figure 7 for a core at
station 40 is characteristic of its behavior in other cores. As seen in
Figure 24, FSOL at first decreases with increasing sediment depth (through
zones 1-2) and increases once again in zone 4. Note that the porosity remains
more or less constant in zone 4 for the cores used for illustration (Fig. 9).
Both calcium and magnesium (Figs. 25 and 26, respectively) follow a similar
trend, decreasing within zones 1 to 3 but rising dramatically (up to a factor
of 4) within zone 4. The concurrent rise in both magnesium and calcium
suggests an increase in the amount of dolomitic materials within zone 4.
The behavior of iron contrasts strongly with that of FSOL, Ca, and Mg.
39
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CALCIUM (wt%)
Figure 25. Vertical distribution of calcium in selected sediment cores.
41
10
— •
IT
4
X
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20-
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CL
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^ 30h
40-
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MAGNESIUM (wt.7o)
4
Figure 26. Vertical distribution of magnesium in selected
sediment cores.
42
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MANGANESE (wt.%)
Figure 28. Vertical distribution of manganese in selected sediment cores.
44
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LEAD(/xg/g)
40 80 20 40 80
Figure 31. Vertical distribution of lead in selected sediment cores.
47
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Figure 33. Vertical distribution of acid soluble phosphorus in
selected sediment cores.
49
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50
As seen in Figure 27, the concentration of Fe tends to be constant within
zones 1-2, decreases abruptly within zone 3, and remains constant within
zone 4, For manganese (Fig» 28), the behavior is similar to that of iron
except for a significant increase within zone 1 (the flocculent material in
the upper 1 cm). The minor elements (Figs. 29-34), for the most part, exhibit
the same behavior as iron: constant or slightly decreasing concentrations in
zones 1-2, an abrupt decrease in zone 3, and again constant concentrations in
zone 4. These profiles suggest that zones 1 and 2 are comprised of sediments
containing recent anthropogenic constituents overlying older, uncontaminated
materials. As the minor constituents are of relatively uniform concentration
within these upper sediments, zones 1 and 2 appear to be either recent
overlays of translocated materials or materials thoroughly reworked to a depth
of well-defined extent which varies systematically with location (Fig. 38).
The notion of well-mixed recent materials comprising zones 1-2 is further
supported by radiometric evidence discussed below.
The extent of enrichment of elements within the upper sediment zones
(1 and 2) relative to underlying sediments (zone 4) is given in Table 5 in
terms of the ratio of element concentrations in zone 2 to concentrations in
zone 4. On the average, concentrations of Cu, Pb, and Zn are four times
higher in zone 2 than in zone 4. Elements Cr, Ni, and Mn are twice as high,
and the remaining elements, while possessing elevated concentrations relative
to underlying sediments, do not exhibit a major degree of enrichment. At
least part of the enrichment is due to the increase in inert (quartz?)
constituents in underlying sediments (zone 3 and possibly 4). To eliminate
effects of dilution by inert materials, the degree of enrichment may be
recalculated in terms of the concentrations of elements on a weight soluble
51
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52
basis. If Cg is the concentration of a given element in surface sediments in
g/g, then the concentration on a weight soluble basis is given by
Cg- = Cg / Fg (5)
where Fg = Fraction soluble (g soluble / g total)
= FSOL
The sediment enrichment factor is then calculated as
Cs' - ^d'
SEF =
^d'
Cs Fd
-1
Cd Fs
(6)
where C^ is the characteristic (mean) concentration of an element in deep
sediments (zone 4) and C^' = ^d^-^d ^^ above. When the concentration of an
element on a weight soluble basis is the same in surface and underlying
sediments, SEF = 0.0. Kemp et al. (1974), who introduced the concept of the
SEF, used aluminum rather than FSOL as a basis for normalizing concentration
values. SEF values are given in Table 6 for selected cores in terms of
percent enrichment. Elements divide into four discrete categories of
enrichment: (1) Cu, Pb, and Zn have average SEFs close to 200%, (2) Cr and Ni
have values around 100%, (3) Mn, P, Fe, K have values of around 50%, and
(4) Mg and Ca are negatively enriched by about -15%. Because major enrichment
of Mn occurs within zone 1 the calculated SEF is sensitive to details of the
sampling and averaging. Use of only zone 1 (0-1 cm or less) in the estimate
of the average would greatly (x2 - x3) increase the SEF for Mn.
53
Included in Table 6 are mean SEFs for sediments of southern Lake Huron.
A comparison of values indicates that, while enrichments of Pb and Zn are
comparable in the bay and open lake sediments, the elements Cr, Cu, and possibly
P are significantly more enriched in sediments of the bay. Thus, while dilution
of anthropogenic constituents by inert materials in bay sediments suppresses
concentration values, and while removal of such effects by regression techniques
indicates concentrations in surface sediments undistinguished from the open
lake, it is evident that, relative to sediments originally present (underlying
materials), surface materials are significantly more enriched in copper and
chromium. This contrast with the open lake suggests the importance of local
sources (tributary?) of these elements.
If the enrichment of elements in zones 1 and 2 (Mn excepted) is due
solely to anthropogenic loadings, then the excess accumulation at each
location may be calculated as follows:
Excess accumulation (pg/cm^^)
^ (C^total - CBg)wi (7)
i = 1
where C^^Q^-g^^. ~ the concentration of a given element in the ith sediment
interval (yg/g), Cgg is the background concentration of the element and w^ is
the mass per unit area of sediment in the ith interval (g/cm^). It must be
emphasized that the excess accumulation is truly the anthropogenic element
storage only if Cgg is the same in zone 2 as it is deeper in the core (i.e.,
in zones 3 and 4 below the enriched layer). Values of the excess accumulation
are given in Table 7 for selected cores. A comparison of these values with
those for southern Lake Huron provides a striking contrast. Far more excess
54
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55
TABLE 7. Excess element accumulation in selected cores from lower Saginaw Bay
(yg/cm^).
Site
'.r
Element
Numbe
Cr
Cu
Ni
P
Pb
Zn
2
97
54
60
1,620
61
230
11
71
40
52
1,100
81
160
16
130
68
59
3,700
140
230
25
460
230
190
4,500
340
690
28
-
150
130
-
270
600
29
-
100
60
-
200
380
30
340
140
130
2,936
200
460
36
120
150
150
-
150
320
38
180
130
130
-
220
420
40
460
220
200
-
290
570
43
320
230
160
-
340
720
49
630
350
580
-
470
1,064
Mean
(SB)
280
160
160
2,800
230
490
Mean
(SLH)*
19
25
75
94
*Data from Robbins (1980),
(anthropogenic) Cr, Cu, Ni, P, Pb, and Zn is stored per unit area in the lower
bay than in open lake sediments • How can this be reconciled with the
generally lower element concentrations in surface sediments of the bay?
Simply because the overlay of enriched sediments is generally of far greater
depth in lower bay deposits. Evidence provided below indicates that the
zoobenthos present in these deposits redistribute contaminants throughout a
considerable depth in the sediments. Thus, in the lower bay, contaminant
concentrations in surface sediments are apparently reduced as a result of two
important processes: (1) dilution by inert materials and (2) redistribution
of materials throughout a portion (up to '\.20 cm) of the sedimentary column.
An approximate estimate of the total excess storage of selected "anthro-
pogenic" elements in the lower bay is provided in Table 8. Insufficient data
are available to compute very accurate averages. Hox^ever, an order of magni-
56
Mean Excess
Total
Total
Storage in
Accumulation
Storage
Southei
rn Lake Huron
(yg/cm^)
(metric tons)
(metric tons)
280
1,500
-0
160
830
710
160
830
950
2,800
15,000
=^0
230
1,200
2,400
490
2,500
2,900
TABLE 8. Approximate total storage of anthropogenic metals in lower Saginaw
Bay.*
Element
Cr
Cu
Ni
P
Pb
Zn
*Based on acid soluble concentrations.
tude comparison indicates that amounts of Cu, Ni, Pb, and Zn stored in the
lower bay are comparable to amounts stored in deposits of southern Lake Huron,
which are roughly five times as extensive as those in the lower bay.
SEDIMENT MIXING AND SEDIMENTATION RATES
In the preceding discussion, it has been possible to investigate patterns
of contaminant metal deposition, inter-element associations, and even total
anthropogenic element loadings without reference to sedimentation rates.
To gain information about the rates of contaminant deposition, however, it is
necessary to associate a time scale with concentration profiles in individual
cores. Two independent methods have been used for sediment geochronology in
this report. The first relies on measurement of the vertical distribution of
cesium-137 and the occurrence of a horizon corresponding to the onset of
nuclear testing about 25 years ago. This method provides a measure of the
average sedimentation rate over the past 25 years. The second method is based
on the radioactive decay of lead--210 (ti/2 = 22.26 years) following burial in
sediments. In principle this method is capable not only of yielding average
sedimentation rates over a period of roughly 100 years, but is capable of
revealing changes in the rate of sedimentation over this period of time.
57
The computation of contaminant fluxes is sensitive to the interpretation
given to individual profiles. This fact has been emphasized in a number of
recent studies (Robbins et al. 1977, Robbins and Edging ton 1975, Edgington and
Robbins 1976, Robbins et al. 1978) which show that the mixing of surface
sediments, probably by benthic organisms, has a significant influence on
radioactivity and metal contaminant profiles and in the estimate of
contaminant fluxes (Edgington and Robbins 1976). In the section below on
cesium-137, the effects of sediment mixing on radioactivity and contaminant
profiles are discussed.
Cesium-137
Cesium-137 is a uniquely anthropogenic radionuclide first introduced into
the environment as a result of atomic weapons testing which began roughly 25
years ago. Many studies have now demonstrated the utility of cesium-137 for
investigation of sedimentation processes in aquatic systems such as lakes and
reservoirs. Cesium-137 is an especially useful tracer because its input to
the lakes may be accurately inferred from atmospheric and precipitation
radioactivity measurements (Sr-90) made for about a 20-year period within the
watershed of the Great Lakes and elsewhere.
The time dependence of the input of this isotope is discussed in greater
detail in the previous report (Robbins 1980). Most of the cesium-137 was
introduced into the lakes during the period from 1958 to 1964. Thus, to a
good approximation, the input is in the form of a pulse whose subsequent
behavior in the water column and sediments serves to trace long-term aquatic
transport processes. Studies by others have shown the residence time of
cesium-137 in the water column to be very short (for details, see Robbins
58
1980), about one year. Thus the radionuclide is rapidly removed from the
water, and changes in its aqueous concentration mimic the history of
atmospheric deposition as a result. As the hydraulic retention time of the
lake is long (about 30 years) in comparison with the overall residence time of
the radionuclide, the dominant process for removal of cesium-137 is particle
scavenging and sedimentation. This inference is consistent with the knox^
high affinity of radiocesium for certain clay minerals present in the water
column. It is therefore expected that the flux of cesium~137 to the sediments
would follow the time-dependence of aqueous concentrations as well as that of
atmospheric inputs. While the flux may do so, profiles of cesium-137 do not,
but are generally smoothed out both in open lake sediments (see Robbins 1980)
and in those of Saginaw Bay as well.
As part of this and the previous report , a model has been developed to
account for the discrepancy between observed and expected cesium-137 profiles.
The details are presented in Robbins et al . (1977). The smearing is assumed
to occur only in the sedimentary column and, as a result of the rapid steady-
state mixing of sediments over a zone of fixed depths, at the sediment x>7ater
interface. This depth is referred to as the mixing depth. It can be shown
(Robbins et al. 1977) that if the activity of radiocesium added to the
sediments at time t (with t = corresponding to a very deep sediment layer)
is Ag(t), the expected cesiura-137 distribution is given by:
A = Aijj(T) z<_s
A = A^(T + (s - z) /d) ) e""^(z-s)/a3 z>s
and t (8)
A^(t) = Y e-a + Mt Je+(y + X) ^^(^ ) ^^
59
where y = r/s, t = T corresponds to the sediment water interface, and ^ is the
sedimentation rate (cra/yr) neglecting the effects of compaction. A is the ra-
dioactive decay constant for cesium-137 (A = 0«69315/ti/2 ; tx/2 ^ 30.0 years)
and z is the depth below the sediment-water interface. The effects of
compaction are automatically taken into account by expressing z in terms of
the cumulative mass per unit area and o) in terms of the mass sedimentation
rate (g/cm^/yr). The mixing depth, in consistent units, is expressed in terms
of mass /unit area.
A computer program was developed to find the value of the mixing depth,
sedimentation rate, and surface activity Ag(T) giving the best least squares
fit of the above equation to the observed profiles. The results of the
calculation are illustrated in Figure 35 for the distribution of cesium-137 in
a core from station 10. The solid line represents the model distribution with
values of 2.2 g/cm^ for the depth of mixing and 0.11 g/cm^/yr for the
sedimentation rate. In linear terms this mixed depth corresponds to about
10.5 cm and a sedimentation rate of about 0.53 cm/yr (a mean value for the
upper 10 cm of sediment). The shaded area in Figure 35 is the distribution of
cesium-137 expected if there were no mixing at this location but if the
sedimentation rate remained the same. The effect of mixing is thus to
drastically smear out the distribution. Distributions of cesium-137 in
Saginaw Bay are in fact so dominated by mixing that often the peak (such as
that seen at about 15 cm in the core at station 10) is completely absent.
Such distributions provide little reliable sedimentation rate information but
yield data on mixing depths. Profiles of cesium-137 in other cores are
illustrated in Figure 36. It can be seen that the activity is smeared over an
interval ranging from about 10 to 25 cm.
60
Cesium-137 (pCi/g)
2 4 6
T
Figure 35. Vertical distribution of cesium-137 in a sediment core at
station 10. Also shown is a theoretical distribution based on the
rapid steady-state mixing model (see text). The shaded region
shows the distribution of cesium-137 expected in the absence of mixing.
61
Cesium- l37(pCi/g)
^0 24602460246024602460246
e
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Figure 36. Distribution of cesium-137 in selected sediment cores
(1975 data).
62
As part of this report, the vertical distribution of zoobenthos was
determined in a number of sediment cores (Batac-Catalan et al. 1980).
Previously, Robbins et al. (1977) observed that the zoobenthos occurred
primarily in the zone of mixing as defined by radiocesium and radiolead
profiles and that they were present in sufficient numbers to account for
estimated mixing rates. A summary plot for cores examined to date from this
study area, from Lake Huron and from Lake Erie, is given in Figure 37.
The 90% cutoff depth is very well correlated with the depth of the mixed zone
(S = -0.2 -f 1.16 Z; r = 0.91, N = 10). Thus, there is strong circumstantial
evidence that benthic organisms are primarily responsible for the mixing of
near-surface sediments and the resultant alteration of radioactivity and
contaminant metal profiles. Previously it had been suggested (Lerman and
Lietzkie 1975) that profiles of cesium-137 as well as those of strontium-90
might be affected or even largely determined by diffusional migration.
However, Robbins et al . (1977) showed that cesium-137 cannot migrate
significantly in sediments by molecular diffusion as it is strongly bound to
sediment solids. This result cannot be generalized to include other
contaminants, tracers, or sedimentary environments however. Cesium-137 can
experience significant diffusional migration in sediments which do not contain
minerals with a specific affinity for this radionuclide (cf . Alberts et al.
1979). Also, a nearly conservative tracer such as Sr-90 undergoes consider-
able diffusion in sediments of the Great Lakes (Lerman and Lietzkie 1975).
Shown in Figure 38 is the depth of sediment mixing as determined from
inspection of Cs-137 profiles. In all cases the depth of penetration of this
isotope is comparable to or slightly less than the depth of penetration of
contaminant metals. This result suggests that the organisms mixing sediments
63
16
12
E
(J
CO
o Saginaw Bay
• Lake Huron
■ Lake Erie
8
Z90 (cnn)
12
16
Figure 37. Relation between the depth of sediment mixing based on cesium-137
profiles and the range of zoobenthos which actively redistribute sediments
(primarily ollgochaetes) .
64
MIXED DEPTH (cm)
5-10
44«00 —
43°50 —
43M0 —
o
N
LOWER SAGINAW BAY
5 10
KILOMETERS
Figure 38. Contour plot of the depth of sediment mixing as determined
radiometric ally.
65
were able to completely homogenize the overlay (zone 2) in the time period
from about 1964 to 1975 (or about 10 years). Such inferences are based on the
assumption that local contributions of Cs-137 (such as from the Saginaw River)
are unimportant. Generally, loading of the isotope from watershed erosion is
small. The mixed depth contours shown in Figure 38 possess features
comparable to those in open lake sediments: greatest depths occur toward the
centers of depositional basins. However, while mixed depths range from about
2-5 cm in the open lake basins, those in Saginaw Bay range from 5-20 cm.
The total cesium-137 stored in bay sediments is shown in Figure 39.
Highest accumulations occur toward the southwestern end of the deposit and in
areas of deeper water. While the pattern may be interpreted as implying a
riverine source of the isotope, it must be kept in mind that the gyre-like
circulation in the lower bay could easily redistribute incoming atmospheric
and open-lake loadings so as to produce the observed pattern of deposition.
The mean deposition within the muddy deposits was 13.9 + 1.4 pCi/cm^ in 1975
and 12.8 +2.3 pCi/cm^ as remeasured in 1978. As very little new Cs-137 was
introduced during the period from direct fallout, and if river , loadings are
negligible, then the expected amount stored in 1978 should be less as a result
of radioactive decay. Assuming losses of Cs-137 through resuspension, the
expected 1978 activity would be
13.9 X e-0-69315 x 3/30 =: 12. 96 pCi/cm^
which is close to the observed value of 12.8 pCi/cm^.
If the loading to the lower bay were directly from the atmosphere and
there were no losses via outflow or gains either from tributary inputs or from
importation of the isotope from the main lake, the expected amount within the
lower bay would be
66
TOTAL CE
:siuM-
137
(pCi/9)
H
>20
■
15-20
^
10-15
■
2-10
r-
<2
44000'—
43»50-
43M0
L
N
LOWER SAGINAW BAY
5 to
KILOMETERS
83°30
Figure 39. Total amount of cesium-137 deposited in sediments of the
lower bay.
67
10 pCi/cm^ (fallout value decay corrected to 1975) x 15.8 x 10^^ cm^
(area of the lower bay) = 158 Ci.
The observed inventory is
13.9 pCi/cm2 x 3.74 x 10^^ (area of the mud deposit) = 52 Ci.
Allowing for a maximum of 1 pCi/cm^ deposited over non-muddy areas of the
lower bay, this amount would contribute
(15.8 - 3.74) X 10^2 ^ 1.0 pCi/cm^ = 12 Ci.
Thus, the total amount of Cs-137 in the lower bay is estimated as
52 + 12 = 64 Ci. Thus, in the absence of watershed contributions,
there is a net storage of 64/158 = 40% of the cesium-137 entering via at-
mospheric fallout. The effect of watershed inputs remains to be evaluated.
Lead-210
The lead-210 method of dating coastal marine and lacustrine sediments has
been used with increasing frequency since its first application by Krish-
naswami et al. (1971). The extensive literature concerning radioactive lead
isotopes and lead-210 in particular has been recently reviewed by Robbins
(1978). The method has been shown to be of value in dating fine-grained
sediments from all of the Great Lakes (Superior: Bruland et al. 1975, Evans
et al. 1981; Michigan: Robbins and Edgington 1975, Edgington and Robbins 1976;
Huron: Robbins et al. 1977; Erie and Ontario: Robbins et al. 1978, Farmer
1978). Generally, the method has been applied to a very limited set of cores.
Only a brief account of the lead-210 method of dating sediments is given in
this report. For a detailed discussion of the principles underlying the
method, see Robbins (1978).
68
Lead-210 is produced as the indirect result of the decay of uranium
present in crustal and sedimentary materials. The principal components in the
decay scheme are :
U238 -> R^226(t3^/2 = 1*620 yr) -> Rn222(t-^^2 = 3.8 d)
-> Pb210(ti/2 = 22.26 yr) -> Po210(t;^/2 = ^38 d)
-> Pb206
Short-lived products in the decay scheme are not shown.
Some lead-210, present in sediments, is produced by in situ decay of
radiura-226. Generally this is a small and nearly constant activity referred
to as supported lead-210. In addition to supported lead-210, there is an
excess which is supplied to sediments from atmospheric deposition.
Atmospheric lead-210 originates from a unique property of the uranium series
decay scheme shown above. Radium-226 decays to form the radioactive noble
gas, radon-222, which diffuses out of crustal materials into the atmosphere.
Radon-222 decays through a series of very short-lived nuclides to lead-210
which has a high affinity for atmospheric particulate matter and is rapidly
scavenged from the air. The flux of lead-210 over the Great Lakes has not yet
been measured but is expected to be around 0.5 pCi/cm^/yr and quite constant
from year to year. Once in the water, lead-210 is rapidly transferred to
sediments and this excess, not being supported by radium activity, decays
toward supported levels during burial. In sediment cores from the Great
Lakes, excess lead-210 may be roughly twenty times higher than supported
levels (cf. Robbins and Edgington 1975).
69
In undisturbed cores where the sedimentation rate is constant, the
activity of excess lead-210 is given by
A(m) = A^.e-^t = a^ e""^^/^ (9)
where Aq is the excess activity at the surface (m = o) , t is the time before
collection (years), m is the cumulative dry weight of sediment (g/cm^), and r
is the mass sedimentation rate (g/cm^/yr). The radioactive decay constant is
given by 0.69315/22.26 = 0.311 yr^^.
The total (observed) activity is given by
^tot = A(^) + Af
where Af is the activity of supported lead-210.
The above equations suffice to describe profiles in sediments where there
is no mixing. Incorporation of the effects of rapid steady-state mixing is
comparatively straight-forward for lead-210 (Robbins et al. 1977) as Ag(t) in
Eq. 8 is essentially constant over time. Using mass units rather than linear
(cm) units and substituting Aq for Ag(t) in that equation, the distribution of
excess lead-210 is given by
A^ = Aq / (1 4- Xs/r)
A(m) = Anj m £ s > (10)
= A^e-^ (m-s)/r ^>s.
This is the equation used to obtain sedimentation rates and mixed depths from
excess lead-210 profiles. Eq. 10 describes a theoretical profile in which the
activity of lead-210 is constant from the sediment-water interface down to
depths. Below that depth (mixed depth) the activity falls off exponentially.
An example of the lead-210 distribution is given in Figure 40, together
with the distribution of cesium-137 and ragweed (ambrosia) pollen in a core at
70
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71
station 25 (1975). Note that over the zone of about 12 cm depth, where
cesiuTn-137 activity is essentially constant, the activity of lead-210 is also
essentially constant as expected if both isotopes are uniformly mixed. Below
12 cm, the activity of excess lead-210 falls off more or less exponentially,
as expected, due to a combination of radioactive decay and burial. A least
squares fit of model parameters to the distribution yields a mixed depth of
2.5 g/ car- (12 cm) and a sedimentation rate of 0.078 g/cm^/yr (0.31 cm/yr
average over upper 20 cm). The ragweed distribution shown in the figure was
provided by Dr. A. Kemp, formerly with Canada Centre for Inland Waters. Use
of ragweed pollen data to obtain sedimentation rates in the Great Lakes is
discussed in detail elsewhere (cf. Robbins et al. 1978). Following forest
clearance in the Great Lakes region roughly 130 years ago, a dramatic increase
in production of ragweed pollen occurred. As the grains are well preserved in
recent sediments of the Great Lakes, the depth at which an increase occurs
(horizon) may be used to obtain an average sedimentation rate for the past 130
years or so. In the core at station 25 the horizon is at 30-35 cm, cor-
responding to a cumulative sediment weight of 10.1 g/cm^. Thus the average
sedimentation rate is 10.1/130 = 0.078 g/cm^/yr. Such good agreement between
dating methods is completely accidental in view of the many approximations,
assumptions, and uncertainties.
Distributions of excess lead-210 (on a weight soluble basis) are shown
for additional cores in Figures 41-43. As can be seen, profiles exhibit the
characteristic zone of mixing below which the activity decreases more or less
exponentially. Summary data and model parameters derived from least squares
fits are given in Table 9. These rates must be treated with some reservation
because exponential portions of the profile occur at depths where there may be
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gross compositional changes as evidenced by variations in major element com-
position and because straggling of zoobenthos into sediments below the mixed
zone may produce profiles with exponential features. Trends in the apparent
sedimentation rates over the mud deposit are shown in Figure 44. Highest
rates occur toward the southwestern end of the deposit and decrease almost
monotonically with increasing distance along the main axis of the deposit
toward the far end of the lower bay. This trend could be interpreted as the
effect of river loading. However, a similar trend was found earlier in
sediments of the Goderich basin. Higher mass sedimentation rates occurred
toward the eastern margin of the basin where enhanced deposition of dolomite
occurs. Similarly, in the bay, highest apparent sediment accumulation rates
tend to occur in areas of greatest concentrations of calcium and magnesium.
Preferential accumulation of these silt size constituents in the southwestern
end of the mud deposit may reflect prevailing current structure as much as the
distribution and loading of source materials.
Also shown in Table 9 is the time resolution associated with each core.
This concept has been discussed in detail elsewhere (Robbins 1980). Briefly,
the effect of steady-state mixing is to smear out signals in the sedimentary
record. A measure of the fidelity of the record is the time resolution with
which two events may be distinguished. This time is given approximately as
the residence time of a particle within the mixed layer, which in turn is
given as the ratio of the mixed depth (g/cm^) to the sedimentation rate
(g/cm^/yr). Within the mud deposits of the lower bay this time varies from 11
to 60 years, with a mean value for cores examined of 30 years. This latter
value may be taken as the approximate mean particle integration time for these
deposits.
77
APPARENT SEDIMENTATION
RATE (g/cm2/yr)
44^00
43*50'- -
43M0'
Bay City
LOWER SAGINAW BAY
Figure 44. Apparent sedimentation rates at selected locations in the
lower bay based on lead-210 profiles.
78
If resuspension of particles out of the mixed layer is an important means
of exchange of contaminants between sediments and water, the effect of this
integration process is twofold. At the onset of contamination loadings, rapid
mixing of sediments over an interval corresponding roughly to 30 years serves
to remove contaminants from the water coluinn more efficiently. However, with
the cessation of contaminant loadings, the mixed layer serves to maintain
levels in the bay through resuspension with a time constant of about 30 years.
NET METAL CONTAMINANT FLUXES
In the absence of sediment mixing, the estimate of the current and
historical rates of deposition of a particle bound (non-mobile) metal
contaminant would be straightforward. If the concentration of a given element
in the sediments corresponding to a time t in the past was C(t), then the rate
of deposition would be F(t) = r*C(t) where r is the sedimentation rate
(g/cm^/yr). The net (anthropogenic) flux would be r*(C(t) - C^) where C^ is
the estimated "background" concentration.
Because of mixing, however, present and historical net fluxes cannot be
estimated unless some assumption is made concerning the time dependence of the
flux. Previous work (Robbins 1980, Edgington and Robbins 1976) has shown that
the vertical distribution of many contaminant elements (Pb, Cu, Cd, Zn) is
consistent with a roughly exponential increase in loading since 1800 and that
the doubling time is approximately 20 years. Assuming that form of the
loading, the rate of deposition may be inferred for the time of collection of
the cores as follows:
79
For stable elements (with no radioactive decay), the net concentration of
an element in the mixed zone is given by Eq. 8 with A = or
^t Te + At
C^(t) = Ae-^t / e -^ At c^Ct) dT (11)
where Cg is the concentration of an element in materials deposited at the
sediment surface.
In accordance with the above discussion, Cg has the form
Cs(t) = Cs(o) e^'t (12)
where A' = 0,6932/t^ and t^ is the doubling time -20 years.
Substituting Eq. 11 into Eq. 12 and integrating
Y
Cm(t) = eA't Cg(o) (13)
Y+A '
The flux to the surface in 1975 is then
Fo(o) = r*Cs(o) eA't (^=0) (14)
y +A '
y
^m ^
Note then that the product C^*r is simply the net or excess metal concentra-
tion in mixed layer times the sedimentation rate. If the time dependence of
the input is essentially exponentially increasing, the correction for mixing
is given as
y + A
Fcor = (15)
y
80
where Y is the ratio s/r and Y"-^ is simply the time resolution discussed
above. Values of this correction factor and estimated fluxes corrected by
time factors are given in Table 10. Net fluxes are consistently much higher
in Saginaw Bay than in Goderich basin of Lake Huron (main depositional area)
because of the much higher sedimentation rates in the bay. The total rate of
accumulation of selected contaminant metals generally exceeds that expected
from atmospheric loadings. Approximate atmospheric loadings are summarized by
Robbins (1980). For copper, the atmospheric flux is approximately 0.41
IJg/cm^/yr which implies a loading to the lower bay of
0.41 X 15.8 X 10^2 ^g/yr = 6.5 metric tons/yr
whereas the estimated deposition is 28 metric tons/yr. Similarly for nickel,
the atmospheric loading of about .23 3Jg/cm'-/yr corresponds to 3.6 metric
tons/yr as compared with an estimated deposition of 30 metric tons/yr.
These elements would appear to have major non-atmospheric sources.
In contrast, a lead atmospheric deposition of about 1.7 yg/cm^/yr implies a
total loading of 27 metric tons per year which is very comparable to the
calculated sediment deposition rate of 40 metric tons. As most anthropogenic
lead in the environment results from combustion of fuel additives and coal,
the atmospheric pathway is the primary route to loading of aquatic systems.
Thus the agreement between these numbers supports the validity of this
approach. Atmospheric loadings of zinc (2.6 yg/cm^/yr) imply a total
deposition of 40 metric tons per year as compared with 86 metric tons per year
into sediments. Thus zinc is comparable to lead in terms of the predominance
of atmospheric inputs into sediments of the lower bay.
81
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VERTICAL DISTRIBUTION OF DISSOLVED SILICON AND PHOSPHORUS
The vertical distribution data are available for dissolved Ba, Ca, Fe, Mg,
Mn, P(SRP), K, Na, Si(SRS), and Sr (Table 31 of the Data File, see page 96) for
two cores from Saginaw Bay (station 30) and from Lake Huron sites as well.
Results for the Saginaw Bay cores are generally comparable to those from the
open lake (in terms of pore water concentrations, gradients, and fluxes). For a
detailed discussion see Robbins (1980). Of particular interest are the dis-
tribution of SRP and SRS. Soluble reactive phosphorus concentrations in over-
lying water are essentially undetectable (<0.1 ppm) . In pore water, concen-
trations rise to a maximum of 5.5 ppm at 4-5 cm depth, decreasing with increas-
ing sediment depth to values of around 3 ppm. Values in a replicate core are
somewhat sporadic because of minor oxygen contamination (see Robbins 1980).
The gradient at the sediment water interface is about 1.5 pg P04/cm^.
Gradients of PO4 in the open water sediments were 0.5-0.8 yg P04/cm^.
Assuming an effective diffusion coefficient of about 5 x 10""^ cm^/sec or 160
cm^/yr, the apparent flux across the sediment surface is about 240 yg/cm^/yr.
The net downward flux of acid soluble P (as PO4) is 1,680 yg/g x 0.2 g/cm^/yr
= 330 yg/cm^/yr. Within the uncertainties of the calculation these fluxes are
comparable, suggesting steady-state conditions with respect to incoming and
outgoing fluxes of SRP. For silicon, the concentration in overlying water is
about 2 yg/ml. The distribution in core 30A and 30A2 is shown in Figure 45.
The break in concentration values around 5 cm is unexplained but reproducible
in two cores. Equilibrium concentrations are around 25 yg Si/ml. The
gradient at the sediment water interface is about 5.5 yg/cm^ which implies a
flux of 660 yg/cm2/y-c, assuming a diffusion coefficient of 120 qxqZ fy^ (See
Robbins 1980). This estimated flux is at the low end of the range of fluxes
83
SAGINAW BAY CORES
(EPA-SLH-75)
5 10 15 20 25
DISSOLVED SILICON ()jig/ml)
Figure 45. Vertical distribution of soluble reactive
silicon (SRS) in two cores from station 30.
84
calculated from pore water gradients in southern Lake Huron cores (Robbins
1980), 750-1,700 pg Si/cm^/yr, and as measured directly in northern Lake Huron
cores by Remmert et al. (1977), 1,020-2,050 yg Si/cm^/yr.
NUTRIENT FLUXES
The results of direct flux measurements for silicon are shown as a func-
tion of season in Figure 46 for cores taken at station 31. Fluxes measured
directly in cores are consistently higher than values based on pore water
gradients and show a marked seasonal variation with maximum silicon release
occurring during August. The seasonal variations in this flux may be
attributed primarily to sediment temperature variations as illustrated by
Figure 47. The solid line represents the conventional thermodynamic
description (Arrhenius equation) of the temperature dependence of dissolution
with an activation energy of 16.3 k/Cal/mole as determined by least squares
methods .
The role of organisms (especially chironomids) in silica regeneration is
well documented for other lakes (Tessenow 1964) and may be expected to occur
in Saginaw Bay where such organisms occur. The silica fluxes measured in a
series of cores during fall cruises when chironomids were present are provided
in Table 11. It can be seen that the dominant benthic taxon in terms of
numbers are the immature tubificid worms. However, densities of these
organisms correlate poorly with the silicon flux, as can be seen from
Table 12. Because of the limited number of observations, most correlations
are not significant. However, the correlation between the Si flux and the
density of chironomids is high and significant for both observation periods
(Fig. 48). In this experiment, other nutrients were measured as well and
85
o
to
o
lU
(jA/2UJ0/6Ty)!S iO xnid
B
•H
CD
B
o
d
o
o
•f-f
CO
>
a
u
3
o
o
3
0)
en
C
o
03
>
C
O
CO
CT3
0)
0)
85
8000
6000
CM
(f)
4000
5.
3 2000
AE= 16.3 kcal /mole
10
T(«C)
20
Figure 47. Temperature dependence of the soluble reactive silicon flux
at station 31.
87
TABLE 11 • Zoobenthos density and soluble reactive silicon fluxes from cores
collected in Saginaw Bay, Lake Huron.
Core
Density
( number /m'
■2)
Cruise
Tublfj
Lcidae
Naididae
Ch
ironomidae
Silicon Flux
Number
Mature
Immature
(Ug/cm2/yr)
7*
1
850
40,000
5,900
1,100
3
850
65,000
7,900
770
1'
280
8,200
850
560
1,800
2'
1,700
29,000
8,200
850
2,700
3'
2,300
18,000
560
560
1,680
8**
1
1,400
23,000
1,130
3,600
2
280
5,600
1,300
8
280
5,600
280
1,600
2'
280
29,000
13,000
560
2,400
* October, 1978. **November, 1978.
TABLE 12. Correlations* between nutrient fluxes and benthic organism densi-
ties in cores from station 31 in Saginaw Bay, Lake Huron.
(CRUISE 7)
Organism
Phosphate
Ammonia Nitrate
Sulfate
Silicon
Group
(PO4)
(NH3) (NO3)
(SO4)
(Si)
Tub. Mature
0.93
-0.09 -0.17
-0.75
-0.07
Immature
0.07
-0.82 0.69
-0.54
-0.36
Naididae
-0.19
-0.49 0.13
-0.41
-0.29
Chironomidae
0.11
0.74 0.04
0.04
0.97
Total
0.06
-0.69 0.55
-0.56
-0.26
(CRUISE 8)
Organism
Phosphate
Ammonia Nitrate
Sulfate
Silicon
Group
(PO4)
(NH3) (NO3)
(SO4)
(Si)
Tub. Mature
0.41
0.92 0.23
0,97
0.88
Immature
0.23
0.14 0.93
0.22
0.49
Naididae
-0.30
0.63 0.78
-0.50
0.13
Chironomidae
0.09
0.63 0.25
0.76
0.99
Total
-0.9
-0.05 0.94
0.01
0.62
*Correlations which are significant on both sampling periods are underlined.
88
4000-
CM
E
-^
(J)
o»
J.
X
3
O
O
2000
^ 4000-
2000
STATION 31/31'
SAGINAW BAY
CRUISE 7: OCT 1978
F=873+ 1.86 N,
1
CRUISE 8 : NOV. 1978
F= 1130+ 2.15 N,
400
800
1200
CHIRONOMID DENSITY (m^)
1600
Figure 48. Relation between the SRS flux at station 31 and density
of Chironomid larvae during the fall months (1978).
89
correlations which are persistently higher over both cruises are underlined in
Table 12 •
The results for silicon suggest the relationship:
Flux = 1,000 + 2 X chironomid larvae density,
where the flux is in micrograms Si/cm^/yr and the density is in numbers m~^.
As the mean density of chironomid larvae at this location is about 500 m"^,
roughly half the flux of silicon from the sediments is attributable to the
presence of these organisms. This circumstantial evidence for the effect of
chironomids is strengthened by considering Tessenow's experiments with
sediments from Lake Heiden, Germany (Tessenow 1964), in which he demonstrated
a causal relationship. Addition of chironomids (Plumosus group) to his
sediments resulted in enhanced silicon release. Converting Tessenow' s results
to the above form, his experiments yield:
Flux = 1,000 + 4 X chironomid larvae density.
Graneli (1977) has also observed that Chironomus plumosus larvae increase
the release of silica as well as phosphorus from sediments of several lakes in
Sweden. It would therefore seem likely that, at least in shallow waters pf
the Great Lakes where fine-grained sediments can be found, such as lower
Saginaw Bay and most of Lake Erie, chironomid larvae may play a major role in
the regeneration of silicon from sediments. In Lake Erie, average chironomid
densities may be as high as 1,000 m"^ (P. McCall, Dept. of Geological
Sciences, Case Western Reserve University, Cleveland, Ohio, pers. comm.).
That these organisms may enhance silicon fluxes does not necessarily mean that
their removal or exposure to aquatic pollutants will result in a long-term
reduction in the capacity of the sediments to return silicon to overlying
waters. It is always possible that the ecological niche represented by
90
processing of silica-rich diatom detritus can be filled by another biotic or
abiotic component. It should be noted that Robbins and Edgington (1979) found
that the flux of Si from sediments in Lake Erie is proportional to the
concentration of amorphous silicon in surface sediments. This result suggests
that the flux is dominated by dissolution of particulate silica recently
deposited on the sediment surface. Processing of such materials by
chironomids, which can selectively attract and ingest diatom fragments by
controlling water movements in the vicinity of their burrows, (D. White,
School of Natural Resources, University of Michigan, Ann Arbor, Michigan,
pers. comm.), would enhance silica fluxes but reduce pore water silica
gradients.
CONCLUSIONS
During the period from 1975 through 1978, sediment cores and grab sam-
ples were collected from over 100 sites evenly arrayed in lower Saginaw Bay.
This intensive coverage of the lower bay, with subsequent comprehensive
analysis of sediments for over 30 elements,, as well as other properties, has
provided a detailed picture of the type of sedimentary materials present and
patterns of major element and metal contaminant concentrations. This study
has revealed an extensive mud deposit in the lower bay covering about 400 km^,
oriented predominantly with the bathymetric contours but skewed toward the
western side of the bay in shallower regions, presumably as a result of
prevailing gyre-like current patterns. The clay content of this deposit
exceeds 50% toward the center and in parts lying in deeper water. The mean
grain size in the high clay areas exceeds 6 phi units. Grain sizes increase
toward the margins of this basin, exhibiting the classic pattern associated
91
with hydrodynamic particle sorting. Calcium family elements (Ca, Mg, and
inorganic carbon) are preferentially concentrated at the southwestern end of
the deposit, either because of the distribution of source materials or current
structure in the bay.
Iron and organic carbon exhibit highest concentrations in the clay-rich
sediments. Most contaminant metals such as lead, zinc, copper, nickel, and
chromium are strongly correlated with iron concentrations (r>0.9 in most cases
for N>30) and thus have very similar surface concentration patterns. Concen-
trations of contaminant metals in surface sediments are consistently lower
than concentrations in deposit ional basins of the open lake. Because the
Saginaw River is considered to be a strong tributary source of contaminants,
low metal concentrations in lower bay sediments require explanation. At least
two processes reduce concentrations: (1) dilution of contaminants by a
greater amount of allochthonous material in the bay, and (2) intensive re-
working of sediments by zoobenthos. Corrections made for dilution effects
show that chromium is significantly higher in sediments of Saginaw Bay than in
open lake deposits. Vertical distributions of minor elements, cesiura-137 and
lead-210, show that the upper 10-25 cm of sediment are extensively reworked.
The depth of sediment mixing varies systematically over the deposit, with
highest values occurring toward the centers in fine-grained materials.
Vertical distributions of benthic organisms (primarily worms) indicate
that 90% of the zoobenthos occur within the zone of mixing as determined
radiometrically. Thus, organisms are probably responsible for sediment mix-
ing. Numbers of benthos range from 10,000 to 50,000 m~^ and are sufficiently
abundant to recycle particle bound contaminants in the mixed zone annually.
92
The total amount of cesium-137 stored in sediments of the lower bay is
estimated to be 64 Ci , as compared with an estimated 158 Ci deposited over the
same area from cumulative atmospheric fallout (decay corrected to 1975).
In the absence of tributary contributions, the muddy deposit appears to be
about 40% efficient in retaining this radionuclide. Integrated amounts of
contaminant metals such as Cr, Cu, Ni, Pb, and Zn (yg/cm^) far exceed amounts
stored in offshore deposits. A comparison with atmospheric loadings shows
that Cr, Cu, and Ni especially have strong non-atmospheric sources in the bay.
Thus, relatively low surface concentrations of these elements conceal the fact
that much contamination has been reworked dox^nward to appreciable depths.
Total inventories of metal contaminants in metric tons are: Cr, 1,100;
Cu, 590; Ni, 590; P, 11,000; Pb, 850; and Zn, 1,800.
Sediment mixing is, in fact, so deep that cesium-137 provides little or
no information on sedimentation rates. However, lead~210 appears to give
valid rates. Sedimentation rates based on ragweed pollen data and lead-210 in
one core are very consistent. Values determined from lead-210 in 12 cores
range from 0.07 to 0.24 g/cm^/yr and imply a total mass accumulation in the
deposit of 6.3 x 10^ metric tons per year. The relatively few sedimentation
rate values indicate a systematic variation in rate over the deposit, with
highest rates occurring at the southwestern end in proximity to the Saginaw
River. Contaminant metal current deposition rates are estimated using a model
of steady state mixing and exponential increase in loading with a 20 year
doubling time. Annual loadings are estimated in metric tons/year as: Cr, 54;
Cu, 28; Ni, 30; P, 420; Pb, 40; and Zn, 86.
In combination with sediment mixing depths, sedimentation rates provide a
measure of the particle residence time in the mixed layer. This time is also
93
equivalent to the time resolution in cores and, to the extent that resuspen-
sion of particle-bound contaminants from the mixed layer controls concen-
trations in the water column, is also the response time for recovery of the
bay as a closed system to cessation of external contaminant loads. In the
cores examined, this time varies from 11 to 60 years with a mean of 30 years.
This latter value is presently the best estimate of mixed layer residence time
to be used in bay-wide eutrophication models involving resuspension processes.
Direct flux measurements provided the following average values over the
period from April 1978 to November 1978: P, -530 yg P/cm^/yr; N(NH3), +200 pg
N/cm^/yr; N(N03), -360 yg N/cm^/yr; and Si, 3,000 yg Si/cm^/yr. Releases of
Si constitute a major input of Si to the bay. The flux of silicon from
sediments exhibits an annual cycle ranging from about 1,500 yg/cm^/yr in the
spring to a maximum of about 6,000 yg/cm^/yr in August. The mean flux may be
reliably predicted from thermodynamic expressions and the sediment tempera-
ture. During the fall months the flux in individual cores is strongly corre-
lated with the numbers of chironomid larvae present. Correlations between
other nutrient fluxes and organism densities are generally insignificant.
Further studies attempting to account for the behavior of contaminants
and materials in the lower bay system must take account of zoobenthos-mediated
integration processes occurring in sediments. A further deception occurring
as a result of dilution of contaminants by inert materials and extensive
reworking should be considered: low concentrations of contaminants in bulk
sediment do not necessarily imply reduced transfer up the food chain by
benthos. Their particle selective feeding behavior may allow them to take
full advantage of contaminants on food particles distributed throughout the
reworked zone. Experience with intensive coring in the lower bay as well as
94
the open lake demonstrates the high degree of spatial heterogeneity in
sediment composition and accumulation/mixing rates within depositional basins.
Sediment budget calculations, if they are to be realistic, must take account
of sediment variability. In modelling the behavior of contaminants and
nutrients in the water column, depositional and non-depositional zones should
be treated separately with realistic model elements characterizing integration
and resuspension processes. Direct flux measurements are to be preferred to
pore water gradient flux estimates because of a multiplicity of effects,
including surface dissolution processes and zoobenthos mediated exchange of
solutes .
ACKNOWLEDGMENTS
I should like to thank J. Murphy, captain of the R/V Simons, and the crew
as well as E. McCue of the R/V Bluewater for their good-natured assistance in
carrying out the sampling operations. Thanks are due R. Rossmann and
E. Seibel and their staff for help in analysis of samples for grain-size
distributions. I appreciate the assistance of K. Johansen and J. Krezoski in
making radionuclide measurements, and K. Remmert , G. Burin, L. Hess, and
M. Willoughby for their help in the laboratory work. Thanks are due J. Jones
for his assistance in conducting the neutron activation analysis of samples at
the Phoenix Memorial Laboratory, University of Michigan. I appreciate the
help of Z. Batac-Catalan and D. White in the determination of the composition
of benthos in sediment samples. I wish to express my appreciation to
S. Schneider, Lisa Tabak, and to the technical illustration group for their
help in production of this report. Finally, a special note of appreciation to
several individuals at the USEPA Grosse lie Laboratory and the Great Lakes
95
Environmental Research Laboratory (NOAA) for their support of this work:
M. Mullin, N. Thomas, B. Eadie, and £• Aubert .
DATA FILE
The complete set of data on which this report is based includes grain
size distributions, elemental concentrations in sediments and in pore water,
nutrient fluxes, abundances of zoobenthos and element regression tables.
The 278-page data file is available from the author upon request (J. A.
Robbins, Great Lakes Environmental Research Laboratory, 2300 Washtenaw Ave.,
Ann Arbor, Michigan 48104).
96
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of Cesium-137 in a reservoir. Science 203: 649-651.
Batac-Catalan, Z., J. R. Krezoski, J. A. Robbins, and D. S. White. 1980.
Distribution and abundance of zoobenthos in the muddy deposits of
Saginaw Bay, Lake Huron. 23rd Conf . on Great Lakes Research of the
International Association for Great Lakes Research, Kingston, Ontario.
May 19-22. Abstracts p. 63.
Bruland, K. W., M. Koide, C. Bowser, and E,. D. Goldberg. 1975. Lead-210
and pollen geochronologies on Lake Superior sediments. Quater. Res.
5: 89-98.
Dams, R., and J. A. Robbins. 1970. Nondestructive neutron activation
analysis of environmental samples. Great Lakes Research Division,
University of Michigan, Special Report No. 48. 101 pp.
Edgington, D. N., and J. A. Robbins. 1976.. Records of lead deposition in
Lake Michigan sediments since 1800. Environ. Sci. Technol. 10: 266-274.
Evans, J. E., T. C. Johnson, E. C. Alexander, Jr., R. S. Lively, and
S. J. Eisenreich. 1981. Sedimentation rates and depositional processes
in Lake Superior from lead-210 geochronology . J. Great Lakes Res.
7: 299-310.
Farmer, J. G. 1978. Lead concentration profiles in lead-210 dated Lake
Ontario sediment cores. Sci. Total Environ. 10: 117-127.
Graneli, W. 1977. Sediment respiration and mineralization in temperate
lakes. Ph.D. dissertation. Institute of Limnology, University of Lund,
Sweden. Summary 9 pp.
97
Kemp, A. L. W., and R. L. Thomas. 1976 • Cultural impact on the geochemistry
of the sediments of Lakes Ontario, Erie and Huron. Geosci. Can. 3: 191-
207.
Kemp, A. L. W. , T. W. Anderson, R. L. Thomas, and A. Mudrochova. 1974.
Sedimentation rates and recent sediment history of Lakes Ontario,
Erie and Huron. J. Sediment. Petrol. 44: 207-218.
Krishnaswami, S., D. Lai, J. M. Martin, and M. Meybeck. 1971. Geochronology
of lake sediments. Earth Plant. Sci. Lett. 11: 407-414.
Lerman, A., and T. A. Lietzke. 1975. Uptake and migration of tracers in
lake sediments. Limnol. Oceanogr. 20: 497-510.
Remmert, K. M. , J. A. Robbins, and D. N. Edgington. 1977. Release of dis-
solved silica from sediments of the Great Lakes. 20th Conference on
Great Lakes Research, International Association for Great Lakes Research.
Robbins, J. A. 1978. Geochemical and geophysical applications of radio-
active lead. In J. 0. Nriagu (ed). The Biogeochemistry of Lead in the
Environment. Elsevier/North-Holland Biomedical Press, New York.
Vol. lA, 285-393.
Robbins, J. A. 1980. Sediments of southern Lake Huron: Elemental compo-
sition and accumulation rates. U.S. Environmental Protection Agency,
Research Reporting Series EPA-600/3-80-080 August 1980. 310 pp. +
Appendix. 198 pp.
Robbins, J. A., and D. N. Edgington. 1975. Determination of recent sedimen-
tation rates in Lake Michigan using Pb-210 and Cs-137. Geochim.
Cosmochim. Acta. 39: 285-304.
98
Robbins , J. A., and D. N. Edgington. 1979. Release of dissolved silica
sediment of Lake Erie. 22nd Conference on Great Lakes Research,
International Association for Great Lakes Research. Abstracts, p. 19.
Robbins, J. A., J. R. Krezoski, and S. C. Mozley. 1977. Radioactivity in
sediments of the Great Lakes: Post-depositional redistribution by
deposit-feeding organisms. Earth Planet. Sci. Lett. 36: 325-333.
Robbins, J. A., D. N. Edgington, and A. Lo W. Kemp. 1978. Comparative
lead-210, cesium-137 and pollen geochronologies of sediments from Lakes
Ontario and Erie. Quater. Res. 10: 256-278.
Royse, C. F. 1970. Introduction to sedimemt analysis. Arizona State
College, Tempe, Arizona.
Strickland, J. D. H., and T. R. Parsons. 1972. A practical handbook of sea
water analysis. Fisheries Research Board, Canada, Bull. No. 125.
Ottawa. 185 pp.
Tessenow, U. 1964. Experimental investigations concerning the recovery of
silica from lake mud by Chironomid larvae (Plumosus group). Archiv. f.
Hydrobiol. 60: 497-504.
Thomas, R. L. , A. L. W. Kemp, and C. F. M. Lewis. 1973. The surficial
sediments of Lake Huron. Can. J. Earth Sci. 10: 226-271.
Wood, L. E. 1964. Bottom sediments of Saginaw Bay, Michigan. J. Sed .
Petrol. 34 173-184.
99
PUBLICATIONS AND PRESENTATIONS
RECEIVING EPA SUPPORT
UNDER GRANT R804686 TO DATE:
Burin, G« , and J. A. Robbins. Polychlorinated biphenyls (PCBs) in dated
sediment cores from southern Lake Huron and Saginaw Bay. 20th Annual
Conference on Great Lakes Research of the International Association for
Great Lakes Research, Ann Arbor, Michigan. May 10-12, 1977.
Johansen, J. A., and J. A. Robbins. Fallout cesium-137 in sediments of
southern Lake Huron and Saginaw Bay. 20th Annual Conference on Great
Lakes Research of the International Association for Great Lakes Research,
Ann Arbor, Michigan. May 10-12, 1977.
Robbins, J. A. Recent sedimentation rates in southern Lake Huron and
Saginaw Bay. 40th annual meeting of the American Society of Limnology
and Oceanography. Lansing, Michigan, June 20-23, 1977.
Ullman, W. , and J. A. Robbins. Major and minor elements in sediments of
southern Lake Huron and Saginaw Bay: patterns and rates of deposition,
historical records and interelement associations. 40th annual meeting of
the American Society of Limnology and Oceanography. Lansing, Michigan,
June 20-23, 1977.
Robbins, J. A. Limnological applications of natural and fallout radio-
activity in the Great Lakes, S3miposium on limnology of the Great Lakes,
40th annual meeting of the American Society of Limnology and Ocean-
ography, East Lansing, Michigan, June 20-23, 1977.
100
Robbins , J. A., and L. W. Hess. Concentration profiles of heavy metals in
recent sediments of Saginaw Bay, Lake Huron. Annual Meeting of the
Ecological Society of America, Michigan State University, East Lansing,
Michigan, August 21-26, 1977.
Robbins, J. A., P. McCall, J. B. Fisher, and J. R. Krezoski. Effect of
deposit feeders on migration of radiotracers in lake sediments. American
Society of Limnology and Oceanography ,» 41st Annual Meeting, Victoria,
British Columbia, June 19-22, 1978.
Robbins, J. A. Aspects of the interaction between benthos and sediments
in the North American Great Lakes and effects of toxicant exposures.
Invited paper for the Symposium on the Theoretical Aspects of Aquatic
Toxicology, Borok, USSR, July 3-5, 1979.
Robbins, J. A., and J. R. Krezoski. Effect of benthos on transport of
particles and solutes in freshwater sediments. 9th Northeast Regional
Meeting of the American Chemical Society, Syracuse, New York, October 2-
5, 1979. Abstract.
Krezoski, J. R. , and J. A. Robbins. Radiotracer studies of sediment rework-
ing by Lumbriculid Ollgochaetes , 28th Annual Meeting of the North
American Benthological Society, Georgia State University, Savannah,
Georgia, March 26-28, 1980.
Robbins, J. A. Seasonal variations in the flux of silica from sediments in
Saginaw Bay. 23rd Conference of the International Association for Great
Lakes Research, Queen's University, Kingston, Ontario, May 19-22, 1980.
Abstracts, p. 41.
101
Batac-Catalan, Z., J. R. Krezoski, J. A. Robbins, and D. S. White. Distri-
bution and abundance of zoobenthos in the muddy deposits of lower Saginaw
Bay, Lake Huron. 23rd Conference of the International Association for
Great Lakes Research, Queen's University, Kingston, Ontario, May 19-22,
1980 • Abstracts, p. 63.
Krezoski, J. R. , and J. A. Robbins. Radiotracer studies of interactions
between sediments and freshwater macrobenthos. The 21st Congress of the
International Association of Theoretical and Applied Limnology, Kgoto,
Japan. Aug. 24-31, 1980. Verb. Internat. Verein. Limnol. 21, 382, 1981.
Robbins, J. A. Stratigraphic and kinetic effects of sediment reworking by
Great Lakes zoobenthos. 2nd International Symposium on the interactions
between sediments and freshwater. Queen's University, Kingston, Ontario,
Canada. June 14-18, 1981.
Robbins, J. A. Geochemical and geophysical applications of radioactive
lead isotopes. In: Biogeochemistry of lead in the environment. Part A.,
J. 0. Nriagu (Ed.), Elsevier Scientific Publishers, Amsterdam,
Netherlands. Vol. lA. (1978) 285-393.
Robbins, J. A., J. R. Krezoski, and S. C. Mozley. Radioactivity in sedi-
ments of the Great Lakes: postdepositional redistribution by deposit-
feeding organisms . Earth Planet. Sci. Lett. 36, (1977) 325-333.
Krezoski, J. R., S. C. Mozley, and J. A. Robbins. Influence of benthic
macroinvertebrates on mixing of profundal sediments in Lake Huron.
Limnol. Oceanogr. 23 (1978) 1011-1016.
Robbins, J. A., D. N. Edgington, and A. L. W. Kemp. Comparative lead-210,
cesium-137 and pollen geochronologies of sediments from Lakes Ontario and
Erie. Quatern. Res. 10 (1978) 256-278.
102
Robbins, J. A., P. L. McCall, J. B. Fisher, and J. R. Krezoski. Effect of
deposit feeders on migration of cesium~137 in lake sediments •
Earth Planet. Sci. Lett. 42 (1979) 277-287.
Meyers, P. A., N. Takeuchi, and J. A. Robbins. Petroleum hydrocarbons in
sediments of Saginaw Bay, Lake Huron. J. Great Lakes Res. 6 (1980)
315-320.
Robbins, J. A. Aspects of the interaction between benthos and sediments of
the North American Great Lakes and effects of toxicant exposures.
In : Proc. of the US-USSR Symposium on Theoretical Aspects of Aquatic
Toxicology, Borok, Jaroslavl, USSR, July 3-5, 1979.
Robbins, J. A. Stratigraphic and dynamic effects of sediment reworking by
Great Lakes zoobenthos. Hydrobiologia 92 (1982): 611-622.
103
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