Groundwater resources and quality variations caused by gravel mining in coastal streams

Abstract

We study the effects of gravel mining on the aquifer–river system in the Baix Fluvià area (NE Spain). Field data show that instream mining has caused a decline of the water-table head of the unconfined aquifer along the Fluvià river. Further, dredging in its lowermost reaches reduces its stage and decreases its slope to zero, which facilitates mixing with sea-water, and thus salty-water intrusion from the river into the aquifer. A dimensionless solution of the Boussinesq’s equation is derived to estimate the water-table shape and the amount of groundwater lost as runoff for any given decline of the river stage. A flow and solute transport finite-element model is also used to calculate groundwater loses for the Fluvià case. We find that results of the general analytical solution are consistent with those of the numerical model, which reproduces the actual layered aquifer and a more appropriate domain geometry. Finally, the observed chloride distribution and time evolution are broadly reproduced using the numerical model. It shows that significant chloride plumes develop after the cone of depression of the nearby wells reaches the river. However, a natural clean-up takes place in the absence of pumping when the natural water gradient turns completely towards the river.