The influence of a confining layer on saltwater intrusion under surface recharge and groundwater extraction conditions

In many coastal areas intensive irrigation and groundwater withdrawal can exert counterbalancing effects on the dynamics of seawater intrusion in the aquifer, particularly when non-aquifer irrigation water (artificial recharge) is also used. When, as is often encountered, a shallow phreatic aquifer is separated from a deeper aquifer by a possibly discontinuous and less permeable confining layer, the effect of this semi-permeable layer on the dynamic balance between freshwater and saltwater can be very important. It can act as a barrier to seawater encroachment but also hinder attempts to replenish an over-pumped confined aquifer or to flush salts out of the phreatic aquifer. The presence of localized areas of severe upconing or salt buildup will depend not only on the distribution of pumping wells but also on the heterogeneity of the confining layer. In this study we want to quantify the impact of the confining layer by running simulations based on different realizations of its hydraulic conductivity distribution. One of the questions we seek to address is how important it is to accurately characterize (and therefore how extensively one needs to measure) the properties of such a confining layer. The study site we will use is a 273 km(2) region that forms part of the plain of Oristano on the western coast of central Sardinia (Italy) characterized by Quaternary alluvial deposits, the presence of several lagoons, and about 25000 active and inactive pumping wells. The simulations will be performed with the CODESA-3D model, a three-dimensional finite element model of coupled density-dependent and variably saturated flow and transport.