Rainwater lens dynamics and mixing between infiltrating rainwater and upward saline groundwater seepage beneath a tile-drained agricultural field P.G.B. de Louw a,⇑ , S. Eeman b , G.H.P. Oude Essink a , E. Vermue b , V.E.A. Post c a Deltares, Dept. of Soil and Groundwater, P.O. Box 85467, 3508 AL Utrecht, The Netherlands b Wageningen University, Environmental Sciences Group, Soil Physics, Ecohydrology and Groundwater Management, P.O. Box 47, 6700 AA Wageningen, The Netherlands c School of the Environment, National Centre for Groundwater Research and Training, Flinders University, GPO Box 2100, Adelaide SA 5001, Australia article info Article history: Received 26 March 2013 Received in revised form 12 July 2013 Accepted 17 July 2013 Available online 24 July 2013 This manuscript was handled by Corrado Corradini, Editor-in-Chief, with the assistance of Juan V. Giraldez, Associate Editor Keywords: Rainwater lens Saline seepage Salinization Lens dynamics Mixing Monitoring summary Thin rainwater lenses (RW-lenses) near the land surface are often the only source of freshwater in agri- cultural areas with regionally-extensive brackish to saline groundwater. The seasonal and inter-annual dynamics of these lenses are poorly known. Here this knowledge gap is addressed by investigating the transient flow and mixing processes in RW-lenses beneath two tile-drained agricultural fields in the Netherlands. Evidence of RW-lens dynamics was systematically collected by monthly ground- and soil water sampling, in combination with daily observations of water table elevation, drain tile discharge and drain water salinity. Based on these data, and numerical modeling of the key lens characteristics, a conceptual model of seasonal lens dynamics is presented. It is found that variations in the position of the mixing zone and mixing zone salinities are small and vary on a seasonal timescale, which is attrib- uted to the slow transient oscillatory flow regime in the deepest part of the lens. The flow and mixing processes are faster near the water table, which responds to recharge and evapotranspiration at a time- scale less than a day. Variations of drain tile discharge and drain water salinity are also very dynamic as they respond to individual rain events. Salinities of soil water can become significantly higher than in the groundwater. This is attributed to the combined effect of capillary rise of saline groundwater during dry periods and incomplete flushing by infiltrating freshwater due to preferential flow through cracks in the soil. The results of this study are the key to understanding the potential impact of future climate change and to designing effective mitigating measures such as adapting tile-drainage systems to ensure the future availability of freshwater for agriculture. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction In many coastal areas worldwide, groundwater is brackish to saline because of the combined effects of seawater intrusion and marine transgressions (e.g. Post and Abarca, 2010; Werner et al., 2013). In such areas, freshwater lenses recharged by rainwater are often the only water resource available for agriculture and drinking water. The best-known type of freshwater lens is the Ba- don Ghijben–Herzberg (BGH) lens (Drabbe and Badon Ghijben, 1889; Herzberg, 1901), which develops in areas with saline groundwater where recharge creates an elevated water table in areas like dune belts along the coast (e.g. Stuyfzand, 1993; Vandenbohede et al., 2008), below islands (e.g. Chidley and Lloyd, 1977; Underwood et al., 1992), and even in inland desert areas (e.g. Kwarteng et al., 2000). Another type of a rainwater-fed lens forms in areas where saline groundwater migrates to the surface by upward groundwa- ter flow (referred to here as seepage), such as the coastal area of the Netherlands (De Louw et al., 2011; Velstra et al., 2011) and Belgium (Vandenbohede et al., 2010) and the Po-delta, Italy (Antonellini et al., 2008). They differ from BGH-lenses in that the upward moving saline groundwater limits the penetration depth of rainwater, and thus the volume of the freshwater lens (De Louw et al., 2011; Eeman et al., 2011). Field measurements by De Louw et al. (2011) in the south-western delta of the Netherlands showed that the transition zone between infiltrated rainwater and upward seeping saline groundwater occurs within 2 m below ground level (BGL) and that nearly all mapped lenses lacked truly fresh ground- water (chloride concentration <0.3 g L 1 ). These lenses are the object of the current study, and are referred to as RW-lenses. For the purpose of this study, the vertical extent of the RW-lens is bounded by the water table and the depth below which no rainwa- ter penetrates (B mix , which is the depth at which the salinity equals 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.07.026 ⇑ Corresponding author. Tel.: +31 (0)6 30548000; fax: +31 (0)88 335 7856. E-mail address: Perry.deLouw@deltares.nl (P.G.B. de Louw). Journal of Hydrology 501 (2013) 133–145 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol