Using hydrogeophysical methods to assess the feasibility of lake bank filtration

Abstract

Lake Tissø is the fourth largest lake in Denmark with sufficient, continuous surface water input making lake bank filtration an optimal surface water extraction method provided that there are suitable shallow aquifers with a good hydraulic connection to the lake. The shallow subsurface under the lake was mapped by five waterborne Electrical Resistivity Tomography (ERT) and eight Ground Penetrating Radar (GPR) surveys to locate potentially suitable aquifers. Distributed Temperature Sensing (DTS) was used for the general characterisation of the hydraulic connection between the lake and aquifer. The waterborne ERT surveys showed several zones of coarser material along the shore extending up to five meters depth below the lake surface, while on-land ERT surveys confirmed that these zones extend to the lakeshore with similar thickness. Both waterborne ERT and GPR profiles agreed that at a selected field site the highest possibility of coarse sediments is 60–140 m from the lakeshore. Lakebed temperatures measured in December 2016, May and June 2017 all indicated potential groundwater discharge to the lake at approximately 135 m from the shoreline supporting the results of geophysical surveys showing coarser sediments in that area. A 2D numerical flow model of the area with a geological setup based on geophysical information and slugtest data, also showed upward groundwater discharge peaks at 60 and 140 m from the shoreline confirming the findings of the field surveys. Thus, this study shows that the combination of waterborne ERT and GPR surveys with DTS measurements is a fast and efficient way to assess the feasibility of lake bank filtration in these settings. This approach of combined hydrogeophysical methods is robust and has a potential to site lake bank filtration areas not only next to large rivers where coarse sediments are found in abundance, but even in different sedimentary environments where suitable layers for lake bank filtration are more difficult to identify.