Chloride transport in a recently reclaimed Dutch polder

Knowledge about the origin of solutes is required to define reasonable limits for surface water quality. The origin of the high chloride (Cl) concentrations in the drainage water of a recently reclaimed Dutch polder is analysed. Historical data on soil formation are used to understand the layering of the soil profile and its hydraulic properties. The salinisation process of the lake bottom in the period 1600–1932 explains the Cl profiles in the soil. After reclamation in 1942, the average application rate of Cl from fertiliser was 118 kg ha−1 yr−1. Natural Cl deposition was 37 kg ha−1 yr−1 and average Cl crop uptake was 10 kg ha−1 yr−1. Upward diffusion from the subsoil (>120 cm depth) of about 220 kg ha−1 yr−1 is the major term in the Cl mass balance. Annual Cl drain discharge is estimated at 365 kg ha−1 yr−1. A field monitoring study in the winter leaching period 1991–1992 showed that the Cl drain discharge was only 180 kg ha−1 yr−1, probably, due to the relatively small precipitation excess in that period. The dynamics of water flow and solute transport in a two-dimensional flow domain are well described by the HYDRUS-2D model based on the Richards and Laplace equations for movement of water and the convection–dispersion equation for solute transport. The large lateral flow component towards the drain in the topsoil (0–120 cm depth) in the zone just above and below the phreatic surface explains the peaks in Cl and nitrate concentrations in the drainage water. The combination of historical data, field experiments and a simulation model was fruitful in explaining the origin and dynamics of Cl concentrations in the drainage water. The high Cl concentrations in the drainage and surface water are natural background concentrations originating mainly from the subsoil of the reclaimed polder.