Locating an optimal lake bank filtration site
Session
Resumé
The project focuses on finding an ideal site for the establishment of a lake bank filtration borehole around Lake Tissø and assessing the feasibility of water extraction by lake bank filtration. To locate potential aquifer outcrops in the lake, the shallow subsurface sediments of the lake and the surrounding areas were mapped by offshore and on-land Multi Electrode Profiling (MEP) and Ground Penetrating Radar (GPR) measurements. At a prospective site, the hydraulic connection between the aquifer and the lake was further investigated. It was expected that in winter the natural discharge of warm groundwater to the cold lake would be indicative of a sufficient hydraulic connection that could be traced by thermal methods. Both the temperature distribution of the lakebed measured by Distributed Temperature Sensing and the thermal drone surveys indicated groundwater upwelling in the area.
Målgruppe
Abstract
Lake bank filtration can be a successful method for extracting water from surface waters provided that there are shallow aquifers near the lake with good hydraulic connection to the surface water body. At these sites, surface water bodies provide a continuous supply of water, while the seepage of water through the sediment-water interface provides a natural physical, chemical and biological filtering of the extracted water.
Due to the increasing demand for water resources, Kalundborg Forsyning aims to raise the present surface water extraction of 5 million m³/year from Lake Tissø by an additional 5-10 million m³/year. As the lake is continuously replenished by Halleby stream, bank filtration could potentially be an additional water extraction method to meet the water supply demand. The aim of this study is therefore to establish if lake bank filtration is a feasible method for water extraction at Lake Tissø by (i) mapping the sediments under the lakebed to find shallow aquifers and their outcrops in the lake and (ii) investigating if groundwater discharges from these aquifers to the lake indicating a good hydraulic connection.
To locate shallow underwater sediments and potential aquifer outcrops, a 5-km long section along the northern shoreline of the lake was mapped with offshore geophysical methods. Multi Electrode Profiling (MEP) with electrodes floating on the water surface was used to study the shallow sediments along the shore, while Ground Penetrating Radar (GPR) measurements were carried out in the lake along 200 m long transects perpendicular to the shoreline. At a prospective site, the natural hydraulic connection between the shallow aquifer and the lake was evaluated by thermal tracer methods assessing groundwater upwelling in the area. Assuming that the discharge of warm groundwater to the colder shallow near-shore lake during the winter will result in warmer temperatures at the sediment-water interface, Distributed Temperature Sensing (DTS) was used to map the temperature distribution of the lakebed. The lake and land surface temperature was further surveyed by airborne thermal imagery.
The MEP survey along the lake shoreline located several disconnected lenses of coarser material extending up to 12 m depth. At the selected field site, both the MEP and GPR surveys detected a potential aquifer outcrop approximately 100-140 m offshore. DTS results also showed consistent warm temperature anomalies in the area 100-180 m offshore. Groundwater fluxes estimated using vertical sediment temperature profiles showed groundwater discharge to the lake with fluxes up to 0.06 m/d.
