Cation exchange in a temporally fluctuating thin freshwater lens on top of saline groundwater

Abstract

In coastal-zone fields with a high groundwater level and sufficient rainfall, freshwater lenses are formed on top of saline or brackish groundwater. The fresh and the saline water meet at shallow depth, where a transition zone is found. This study investigates the mixing zone that is characterized by this salinity change, as well as by cation exchange processes, and which is forced by seepage and by rainfall which varies as a function of time. The processes are first investigated for a onedimensional (1D) stream tube perpendicular to the interface concerning salt and major cation composition changes. The complex sequence of changes is explained with basic cation exchange theory. It is also possible to show that the sequence of changes is maintained when a two-dimensional field is considered where the upward saline seepage flows to drains. This illustrates that for cation exchange, the horizontal component (dominant for flow of water) has a small impact on the chemical changes in the vertical direction. The flow's horizontal orientation, parallel to the interface, leads to changes in concentration that are insignificant compared with those that are found perpendicular to the interface, and are accounted for in the 1D flow tube. Near the drains, differences with the 1D considerations are visible, especially in the longer term, exceeding 100 years. The simulations are compared with field data from the Netherlands which reveal similar patterns.

Cation Exchange in Groundwater‐Chemical Evolution and Prediction of Paleo‐Groundwater Flow: A Natural‐System Study

“…In the study of cation exchange in aquifers, the exchange of calcium (Ca) in recharging‐meteoric water, for sodium (Na) on clays in clastic aquifers, is well established (Appelo & Postma, 2005; Back, 1966; Chapelle, 1983; Drever, 1997; Foster, 1942; Kreitler et al, 1977; Stumm & Morgan, 1996; Whittemore, 1995). There are numerous numerical simulations of the progressive changes in groundwater chemistry as a consequence of cation exchange (e.g., Appelo, 1994a; Bjerg et al, 1993; Bjerg & Christensen, 1993; Cederberg et al, 1985; Charbeneau, 1988; Dance & Reardon, 1983; Eeman et al, 2017; Engesgaard & Traberg, 1996; Grove & Wood, 1979; Martinez & Bocanegra, 2002; Miller & Benson, 1983; Schulz & Reardon, 1983; Valocchi, Roberts, et al, 1981; Valocchi, Street, & Roberts, 1981; van Breukelen et al, 1998 and Wilkinson et al, 1992). However, these studies often include a limited number of reactions, with most studies limited to Na‐Ca exchange, and only two employing a full chemical‐speciation model, Appelo (1994a) and Eeman et al (2017).…”

“…There are numerous numerical simulations of the progressive changes in groundwater chemistry as a consequence of cation exchange (e.g., Appelo, 1994a;Cederberg et al, 1985;Charbeneau, 1988;Dance & Reardon, 1983;Eeman et al, 2017;Engesgaard & Traberg, 1996;Grove & Wood, 1979;Martinez & Bocanegra, 2002;Miller & Benson, 1983;Schulz & Reardon, 1983;van Breukelen et al, 1998 andWilkinson et al, 1992). However, these studies often include a limited number of reactions, with most studies limited to Na-Ca exchange, and only two employing a full chemical-speciation model, Appelo (1994a) and Eeman et al (2017). In addition, none of these studies include calculation of the cation-exchange front that develops within the clay-exchange complex, and its contribution to downgradient changes in the groundwater composition.…”

Water‐rock interactions drive chemostasis

“…Quantifying the role that cation exchange plays in modulating stream chemistry is important as cation exchange can buffer stream‐cation concentrations and influence downgradient geochemical reactions and stream chemistry (Capuano & Jones, 2020; Eeman et al, 2017; Kim et al, 2017). Despite the geologic pervasiveness of clays, the role that cation exchange plays in influencing stream C – Q patterns and subsequent impacts on downstream water quality in headwater streams is not commonly included in studies investigating drivers of chemostasis.…”

Simulations of freshwater lens recharge and salt/freshwater interfaces using the HYDRUS and SWI2 packages for MODFLOW

“…In particular, in view of possible climate change, increased water demands and sea level rise, the development of freshwater lenses supplied with infiltrated rainwater on top of saline aquifers is a subject of intense research (e.g., Werner et al, 2013). A large number of contributions have focused on freshwater lenses in deltaic regions, as well as on islands, peninsulas, or sandbars located in tropical, arid, and temperate climatic zones (e.g., Comte et al, 2014;Contractor and Jenson, 2000;Chang et al, 2016;De Louw et al, 2011;Eeman et al, 2017;Holding and Allen, 2015;Houben and Post, 2016;Illangasekare et al, 2006;Jocson et al, 2002;Mahmoodzadeh et al, 2014;Sadurski et al, 1987;Sinclair et al, 2016;Stuyfzand, 2016;Sulzbacher et al, 2012).…”