Physico-Chemical Model Of Toxic Substances In The Great Lakes

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.