Evaluating the salinity distribution of a shallow coastal aquifer by vertical multielectrode profiling (Denmark)Evaluation de la distribution de la salinité dans un aquifère côtier peu profond au moyen de profils verticaux multi-électrode (Danemark)Evaluación de la distribución de la salinidad de un acuífero costero somero por perfilajes verticales multielectrodos (Dinamarca)垂向多电极剖面估算滨海浅部含水层盐度分布 (丹麦)Análise da distribuição de salinidade num aquífero costeiro pouco profundo através de perfis multi-eléctricos verticais (Dinamarca)

A monitoring system, including five groups of piezometers and five vertical multielectrode profiling probes (VMEP), has been installed in an aquifer beneath a coastal dune in Denmark. In order to assess the salinity distribution within the aquifer, geoelectrical data were gathered in March, June and September 2008, by measuring a dipole-dipole and gradient array using multielectrode profiling. Interpretation of the processed resistivity data was performed by regularized inversion using a one-dimensional, horizontally layered model of formation resistivity. The standard deviation on estimated layer log-resistivity was 0.01–0.03. By estimating two parameters of a power function, observed fluid conductivities derived from samples of porewater were related to corresponding estimated formation resistivities. The conductivity profiles correlate with a winter situation in March with high sea level, active recharge and significant wave activity, causing increased hydraulic heads, a thicker freshwater lens and salt water overlying freshwater close to the sea. In June, the thickness of the freshwater lens is reduced due to less recharge and prevailing offshore winds, imposing density-stable conditions and a sharper transition between fresh and brackish water. During the autumn, aquifer recharge is enhanced and hydraulic heads increase, resulting in a thicker freshwater lens.