WELL CATCHMENTS ESTIMATION IN HETEROGENEOUS AQUIFERS WITH A STRATIFIED MODEL

Abstract

A new method is proposed to delimitate wellhead protection areas in a confined aquifer where hydraulic conductivity is assumed as spatially random function. In a natural formation the heterogeneity of porous media influences the capture zone delimitation and considerable differences can result from the homogenous case. In the last decade several approaches were proposed to solve the problem but they need of a priori description of statistical properties of aquifer that are usually unknown or that requires a very expensive in situ investigation. To give a probabilistic estimation of well catchments, we suggest here that a 3-D aquifer can be described with a perfectly stratified model. Only three parameters are needed to completely define the latter: the log-conductivity expected value and variance, and the number of identical layers on the known depth of aquifers. While the log- conductivity expected value can be derived from a standard pumping test, the variance and the number of layers need of a multilevel tracer test to be defined. To validate the proposed method, pumping and tracer tests were developed in a fully 3-D synthetic aquifer and their results were adopted to define the parameters of the stratified model. Several situations were investigated with different hydraulic log- conductivity spatial structure. In all cases the well catchment and isochrones deduced from the latter show a good agreement with corresponding areas that can be recognized in the synthetic aquifer.