Lake hydrogeophysics; Estimating areas of potential groundwater seepage using reflection seismic, ground penetrating radar and multiple electrode profiling

Lake Hampen, a groundwater seepage lake located on sandy deposits, is one of the cleanest lakes in Denmark. The lake is mainly surrounded by areas with forestry. A small agricultural area has been shown to potentially discharge high concentrations of nitrate to the lake. In order to minimize future nutrient loading to Lake Hampen it is essential that the water balance components and the areas of groundwater in- and outflow are estimated correctly. Groundwater/lake water interaction estimated using seepage meters, hydraulic gradients, and stable isotope measurements give reliable point estimates of groundwater in- and out flow rates. These rates are affected by the heterogeneity in the geological sediments beneath the lake and the lake bed properties. In order to construct a hydrological model covering the entire lake, having an approx. surface area of 760.000 m2, an improved spatial coverage of the geology beneath the lake is necessary. In this study three geophysical techniques; reflection seismic, ground penetrating radar and multiple electrode profiling, are used to map the sediment types and thicknesses at the lake bottom. Areas with thick layers of gyttja or peat are expected to reduce groundwater seepage, while a sandy lake bottom will enable high rates of in- and outflow. The geophysical measurements thereby help identifying areas of potential groundwater seepage that later can be verified using more traditional point measurements.