Modelling Concepts for Hydrophobic Organic Pollutants in Lakes

ABSTRACT Schwarzenbach, R.P. and Imboden, D.M., 1984. Modelling concepts for hydrophobic pollutants in lakes. Ecol. Modelling, 22: 171–212. A simple model concept is developed to evaluate the behavior of hydrophobic organic compounds in lakes. Hydrophobic compounds, defined as being readily soluble in nonpolar organic solvents but only sparingly soluble in water, include important environmental pollutants such as halogenated hydrocarbons, fuel and mineral oil compounds, polycyclic aromatic, hydrocarbons plasticizers, non-ionic detergents, etc. Simple physico-chemical concepts are employed to portray the relevant transfer and reaction processes: gas exchange is described by the two-film model, sorption/desorption on particulate organic carbon (POC) by a reversible, instantaneous linear sorption isotherm and hydrolysis and photochemical transformations by linear reaction kinetics. Based on the estimation of typical time and length scales for mixing in lakes, the aquatic environments are described either by a completely mixed water body (1-box model), or by a annual cycle between stratification (two-box model, consisting of epilimnion and hypolimnion) and overturn (1-box model). Two test lakes (schematic representations of Lakes Zurich and Biel, Switzerland) and several model compounds, which represent a wide range of typical reaction characteristics, serve to demonstrate how the spatial distribution pattern depends on the relative importance of mixing versus reaction rates. The model is applied to the distribution of tetrachloroethylene (PER) and hexachlorobenzene (HCB) and compared to measured data of PER in lake Zurich. For compounds dominated by adsorption and removal on particles, a more sophisticated model is needed which includes the exchange between sediments and water (Imboden and Schwarzenbach, to be published).

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