Estimating the depth of fresh and brackish groundwater in a predominantly saline region using geophysical and hydrological methods, Zeeland, the Netherlands

ABSTRACT The province of Zeeland is situated in the coastal zone of the Netherlands. The ground surface level is around or below mean sea level. Therefore seepage of brackish to saline groundwater is very common. Sea level rise as a result of climate change will very likely increase the pressure on the coastal groundwater system, leading to an increased salinization of the groundwater and surface water system. Still, freshwater agriculture is being practiced in large parts of the province. The vegetation extracts its fresh water from the unsaturated zone and thin rainwater lenses that ‘float’ on top of brackish and saline groundwater. Geophysical and hydrogeological data have been combined on two spatial scales to obtain a better insight into this fresh‐brackish‐saline groundwater distribution. This information is being used to: assess groundwater abstractions, plan landuse and improve the input of variable‐density groundwater flow and coupled solute transport models. For over 6000 locations various types of data have been used to estimate the depth of the brackish‐saline groundwater interface of 1000 mg [Cl‐]/L. These types of data are both of geophysical (vertical electrical soundings, EM34, geoelectrical well logs and electrical cone penetration tests) and of hydrological origin (water samples and abstraction wells). These data have been interpreted and combined with knowledge on the distribution of geological units to make an estimation of the depth of the brackish‐saline water transition for the whole province (~66 km × 63 km). In addition to the regional brackish‐saline interface map, continuous vertical electrical soundings (CVES) have been executed to map the fresh‐brackish‐saline distribution on a local scale. The CVES profiles were made at eight different plots where brackish‐saline water is occurring at shallow depths (<5 m below the surface) according to the regional map and where freshwater agriculture is still being practised. Six of the eight sites have thin (0–3 m) brackish to slightly saline water lenses. At two sites up to 15 m thick brackish water lenses have been observed with CVES. The thickness of the brackish water lenses varies laterally over short distances. Sandy sediment and a higher topography are favourable factors for the development of such lenses.