Investigating groundwater vulnerability variation under future abstraction scenarios to estimate optimal pumping reduction rates

Abstract

Groundwater vulnerability variation over time due to groundwater depth variation caused by pumping reduction is an issue of great interest that should be properly investigated; this is because pumping reduction results in groundwater level increase on the one hand and groundwater vulnerability increase, as a result of groundwater depth decrease, on the other hand. In this context, the present study introduces a novel approach, which considers the spatio-temporal variation of groundwater vulnerability and at the same time focuses on the relationship between vulnerability change and pumping reduction to estimate the optimal pumping reduction rate (i.e. the highest rate of pumping reduction under which the highest increase in groundwater levels occurs without an increase in groundwater vulnerability taking place) in over-exploited and contaminated aquifers. In a first stage, a dynamic and time-dependent approach involving the linkage between a GIS-based DRASTIC model and a groundwater flow model was developed in order to determine groundwater vulnerability changes under different pumping reduction rates (0%, 10%, 20%, 30% and 40%), and thus to produce the necessary data sets for investigating their relationship. Subsequently, regression analysis was conducted to define this relationship and estimate the optimal pumping reduction rate. Our implementation of the methodology showed that groundwater vulnerability displays small changes during the projection period for all reduction rates, while decreasing for 0% and 10% reduction of withdrawals and increasing for 20%, 30% and 40% reduction. After finding that a quadratic equation accurately describes the relationship between vulnerability change and pumping reduction (R2 = 0.999), an optimal reduction rate of 16.4% was estimated. By confirming that no change in vulnerability occurs under this value during the projection period, the validity of the equation defined was affirmed.