Regional changes in groundwater flow patterns and effects on groundwater composition

Abstract

Wetlands on the Vecht river plain in the Netherlands are threatened by pollution of ground water on the adjacent ridge ‘Het Gooi’. To assess the impact of this pollution, information is needed on the present groundwater flow pattern and hydrochemical processes occurring during flow. In the determination of hydrochemical processes past changes in flow patterns must be taken into consideration. Over the past 600 years impoldering and groundwater extraction have induced important hydrological changes in the study area. Exercises with a two-dimensional finite difference groundwater model were used to study the effects of these changes on regional groundwater flow patterns. Steady-state simulations along a vertical section were carried out for four different points in time, namely, the 14th century, 1885, 1941 and 1985. Changes in flow patterns are inferred from a comparison of the steady-state simulations. The results indicate that groundwater flow changed from a simple pattern under natural conditions to a complex flow pattern dominated by artificially man-controlled hydraulic heads at present. The computer simulations are used to estimate the effect of changes in flow patterns on regional groundwater composition. Data on the distribution of chloride and oxygen-18 in ground water provide a verification of the estimated effects and information on the present position of the fresh-brackish groundwater interface in the study area. Isochrones calculated by the model are used to estimate the position of this front where data on water composition are absent. The future displacement of the fresh-brackish groundwater front is inferred from the position of successive isochrones, assuming that the present flow pattern will remain in steady state. The computer simulations provide a general framework for the determination of hydrochemical processes in future studies addressing the impact of groundwater pollution on wetlands in the river plain.