MODELING THE TRANSPORT OF SOLUTES TO GROUNDWATER USING TRANSFER-FUNCTIONS

Transport of chemicals through the unsaturated zone into groundwater i s in general a highly nonlinear process with a pronounced spatial stru cture of which only a very limited number of measurements is economica lly and technically feasible. In most applications, it is thus not pra cticable to model these processes with high spatial and temporal resol utions. However, in an agricultural environment chemicals are usually applied to large areas, and we are generally interested in their long- term effects on groundwater quality. With these restrictions, the tran sport processes may be averaged in space and time over sufficiently la rge regions such that it may be permissible to use a stationary, linea r approximation. An efficient way to study such systems exploits that a stationary, linear system is completely determined by its transfer f unction, that is, by its response to a narrow pulse input. We use a ge neral formalism to represent the transport processes implicitly by the relation between the flux and the resident concentration of a conserv ative chemical. To model the transport of nonconservative chemicals, t his description is coupled, as it was done by (Villermaux, 1981), with a model of the local interactions. We obtain an expression for the tr ansfer function of a linearly adsorbing chemical with linear adsorptio n kinetics and first-order decay in the water and in the adsorbed phas e which is based on the measured transfer function of a conservative t racer. This procedure has the major advantage that parameter estimatio n and model validation tests can be applied to the chemical reaction p rocesses alone.