Freshwater-seawater mixing experiments in sand columns

A study was performed on vertical mixing between freshwater and seawater in controlled column experiments. The aim was to quantify the variation in the thickness of the mixing zone caused by water table fluctuations in aeolian dune sandy sediments. A series of column experiments were setup: first a study on in-well mixing was completed to understand the suitability of down-hole logging in existing piezometers versus multi level sampling; then, an unsaturated-saturated column experiment with sandy aeolian dune sediments of the target aquifer was performed to characterize the mixing zone thickness; finally the third column experiment was conducted by imposing a groundwater fluctuation of 0.1 m to quantify the increase of the freshwater-saltwater mixing zone due to vertical water table oscillations. The freshwater-saltwater mixing zone increase was very limited and did not exceed 0.1 m. All experiments were modeled by means of the SEAWAT numerical code, and calibrated against saltwater concentrations. The model fit was good for all experiments, providing insights into the vertical dispersivity of sandy aeolian dune sediments and also into the reliability and precision of the freshwater-saltwater interface monitoring methods. The down-hole logging technique, in fact, was demonstrated to be inefficient when horizontal flow velocity is negligible; in this case only multi-level sampling techniques provided reliable results.