Physico-Chemical Model Of Toxic Substances In The Great Lakes
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Date
1983-08
Authors
Thomann, Robert V.
Di Toro, Dominic M.
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Abstract
A physico-chemical model of the fate of toxic substances in the
Great Lakes is constructed from ash balance principles and incorporates
principal mechanisms of particulate sorption-desorption, sediment-water
and atmosphere-water interactions, and chemical and biochemical decay.
The steady state mass balance model of the suspended solids in the open
lake water yields net solids settling velocities from 0.02 m/d for
Saginaw Bay to 1.33 m/d for Lake Ontario. Calibration of the toxic
model is through comparison to plutonium-239 data collected in the
1970's using a 23 year time variable calculation.
239Pu partition coefficient of 400,000 l/kg, a particulate settling
velocity of 2.5 m/d, associated lake specific net sedimentation and
resuspension velocities provided a good calibration to the observed time
variable 239Pu behavior. An assumption of zero resuspension of the
sediment did not provide a good calibration. The results indicate that,
in general, the sediments are interactive with the water column in the
Great Lakes through resuspension and horizontal transport. Fifty
percent response times of 239Pu tollowing a cessation of load extend
beyond 10 years with sediment resuspension. The calibrated model was applied to polychlorinated biphenyl (PCB)
using a high and low estimate of contemporary external load and with and
without volatilization. The results of the application indicate that
the upper load level (lake range of 400-9500 kg/yr) without volatilization
is not representative of the surface sediment data and very limited
water column data. The lower load level (lake range 640-1390 kg/yr)
with volatilization (at an exchange rate of 0.1 m/d) appears to be more
representative of observed surface sediment data for the open lake
waters. Calculated water column concentrations for the lower load level
with and without volatilization ranged from 0.25 to 0.90 ng/l for open
lake waters. Fifty percent response times for PCB following cessation
of load and including volatilization varied from less than 5 years to
10-20 years for the other lakes without volatilization. Comparison of
these response times to decline of concentrations of PCB in Lake Michigan
indicates that at least for that lake volatilization is occurring at an
exchange rate of about 0.1 m/d.
Calculations using a solids dependent partition coefficient for PCB
indicate that the total and dissolved PCB concentration in the water
column and sediment PCB concentration are affected to less than an order
of magnitude. Interstitial PCB concentration however increases by about
two orders of magnitude over the case with a solids independent partition
coefficient. Higher exposure concentrations to benthic organism may
then result with a potential route of PCBs to the top predators in the
food chain.
Calibration of the model to limited data on benzo(a) pyrene is obtained with a partition coefficient about one order of magnitude higher than published empirical relationships. The model confirms that on a lake-wide scale the principal external source is the atmosphere and for the larger lakes such as Michigan the response time of the lake to external loads is about 6-10 years while for Lake Erie response time is about 2 years. Application of the model to cadmium in the lakes, using a solids dependent partition coefficient indicates that the lakes do not reach equilibrium over a 100 year period. For constant partitioning, cadmium concentrations reach steady state in about 10-25 yeats. An estimate of the preceding 50 year average cadmium input ranges from 200-600 gCd/km2-yr for the upper lakes to 2000-10,000 gCd/km2-yr for Lake Erie. Calculated high concentrations of cadmium in interstitial water (e.g. 10 ug/l,) indicate the importance of measuring interstitial cadmium concentrations.
Calibration of the model to limited data on benzo(a) pyrene is obtained with a partition coefficient about one order of magnitude higher than published empirical relationships. The model confirms that on a lake-wide scale the principal external source is the atmosphere and for the larger lakes such as Michigan the response time of the lake to external loads is about 6-10 years while for Lake Erie response time is about 2 years. Application of the model to cadmium in the lakes, using a solids dependent partition coefficient indicates that the lakes do not reach equilibrium over a 100 year period. For constant partitioning, cadmium concentrations reach steady state in about 10-25 yeats. An estimate of the preceding 50 year average cadmium input ranges from 200-600 gCd/km2-yr for the upper lakes to 2000-10,000 gCd/km2-yr for Lake Erie. Calculated high concentrations of cadmium in interstitial water (e.g. 10 ug/l,) indicate the importance of measuring interstitial cadmium concentrations.
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Keywords
Physico-Chemical Model , Toxic Substances , Great Lakes