Shallow rainwater lenses in deltaic areas with saline seepage
- 1Deltares, Dept. of Soil and Groundwater, P.O. Box 85467, 3508 AL Utrecht, The Netherlands
- 2Wageningen University, Environmental Sciences Group, Soil Physics, Ecohydrology and Groundwater Management, P.O. Box 47, 6700 AA Wageningen, The Netherlands
- 3Federal Institute for Geosciences and Natural Resources, Dept. of Groundwater and Soil Science, Stilleweg 2, 30655 Hanover, Germany
- 4KWR Watercycle Research Institute, P.O. Box 1072, 3430 BB, Nieuwegein, The Netherlands
- 5TNO Geological Survey of the Netherlands, P.O. Box 80015, 3508 TA Utrecht, The Netherlands
Abstract. In deltaic areas with saline seepage, freshwater 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 measurements at point scale (groundwater sampling, temperature and electrical soil conductivity (TEC)-probe measurements, electrical cone penetration tests (ECPT)) to field scale (continuous vertical electrical soundings (CVES), electromagnetic survey with EM31), and even to regional scale using helicopter-borne electromagnetic measurements (HEM). The measurements show a gradual 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 is found 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. The model results show that the constantly alternating upward and downward flow at low velocities in the confining layer is the main mechanism of mixing between rainwater and saline seepage and determines the position and extent of the mixing zone (Dmix and Bmix). Recharge, seepage flux, and drainage depth are the controlling factors.