Physio-chemical and ecological modeling the fate of toxic substances in natural water systems

Abstract

A basic framework for the modeling of the fate of chemicals in natural water systems is presented. Both the physical/chemical and ecological fate of the substance are considered. The fully time variable equations do not present any significant insight into the general behavior of chemical fate. However, under a steady state assumption and including bottom sediment interactions, relatively simple formulations result that permit rapid calculation of the maximum concentrations that might be expected. It is shown that for a completely mixed lake the ratio of the areal loading rate of the toxicant to the water column concentration is determined by the hydraulic overflow rate and the net loss rate of the toxicant from the water column. Thus, c T = W Ta/q + w T for c T as the total toxicant concentration, W Ta as the areal loading rate of the chemical, q as the hydraulic overflow rate, and w T the net loss of the chemical. The net loss rate w T is a function of all water column and sediment decay rates and interactions. The steady state food chain model of the accumulation of a toxic chemical from both water and food sources is shown to be a means for estimating the parameters of loss and sediment interaction for a lake. The steady state case of the discharge of a chemical into a stream provides a simple means for estimating net loss rates under some reasonable assumptions. Also coupling the food chain model to the physio-chemical model permits the use of chemical concentrations in the fish to estimate the net loss of toxicant in a stream. Application is made to metals concentrations in Lakes Ontario and Michigan and to 1,4 dichlorobenzene in Lake Zurich. The theory indicates that under zero decay of the chemical in the sediment and equal partition coefficients in the sediment and the water column that the chemical in particulates form in the sediment should equal that in the water column. Metals data for Lake Ontario appears to confirm this result at least to order of magnitude. For streams, application is made to a) cadmium in the Sajo River in Hungary where a net loss rate of about 0.12 m/d is estimated, and b) PCBs in the fish and sediment of the South Branch of the Shiawassee River in Michigan where net loss rates of about 3/day are estimated.