Shallow rainwater lenses in deltaic areas with saline seepage

In deltaic areas with saline seepage, fresh water availability is often limited to shallow rainwater lenses lying on top of saline groundwater. Here we describe the characteristics and spatial variability of such lenses in areas with saline seepage and the mechanisms that control their occurrence and size. Our findings are based on different types of field measurements and detailed numerical groundwater models applied in the south-western delta of The Netherlands. By combining the applied techniques we could extrapolate in situ measurements at point scale (groundwater sampling, TEC (temperature and electrical soil conductivity)-probe measurements, electrical cone penetration tests (ECPT)) to a field scale (continuous vertical electrical soundings (CVES), electromagnetic survey with EM31), and even to a regional scale using helicopter-borne electromagnetic measurements (HEM). The measurements show a gradual S-shaped mixing zone between infiltrating fresh rainwater and upward flowing saline groundwater. The mixing zone is best characterized by the depth of the centre of the mixing zone Dmix, where the salinity is half that of seepage water, and the bottom of the mixing zone Bmix, with a salinity equal to that of the seepage water (Cl-conc. 10 to 16 g l-1). Dmix manifests at very shallow depth in the confining top layer, on average at 1.7 m below ground level (b.g.l.), while Bmix lies about 2.5 m b.g.l. Head-driven forced convection is the main mechanism of rainwater lens formation in the saline seepage areas rather than free convection due to density differences. Our model results show that the sequence of alternating vertical flow directions in the confining layer caused by head gradients determines the position of the mixing zone (Dmix and Bmix and that these flow directions are controlled by seepage flux, recharge and drainage depth.