Abstract
ABSTRACTGroundwater discharge into lakes is an important component of the fluid and nutrient budgets, and a possible route for contaminant transport. However, groundwater flow beneath lakes is difficult to investigate due to the need for drilling deep boreholes. In 2012, a 2,000 m deep borehole was drilled in Lake Vättern, the second largest lake in Sweden. A continuous temperature profile was collected from the borehole. The geothermal gradient in the upper 180 m is highly non-linear, and not controlled by variability in the measured thermal properties of the sediments and rocks. The anomalous temperature profile is best explained by fluid flow into the borehole and subsequent vertical flow of warm waters towards the lake floor. Combining the temperature profile with stratigraphic information from drilling logs and seismic data, we find that fluid flow into the borehole occurs in glacial and glaciofluvial sediments deposited on top of a large sandstone aquifer (the Visingsö Group). The warm waters flowing through the glacial and glaciofluvial sediments are likely sourced from the underlying Visingsö Group sandstones. There is no evidence for substantial vertical migration of these waters through the overlying glacial and postglacial sediments. We speculate that they escape either along lake margins where overlying sediments become thinner, or along faults that are known to exist in the deeper basin.These results highlight an important hydraulic transport pathway between recognised regional aquifers and Lake Vättern. Further work is needed to evaluate the significance of groundwater discharge on the water and nutrient budget of the lake.
Highlights
Lake Vättern is the second largest lake in Sweden and the sixth largest in Europe in terms of area (1,856 km2) and volume (74 km3; Kvärnas 2001) (Fig. 1)
Sandstones belonging to the Visingsö Group were recovered between 341 and 155 mblf (Fig. 3)
The glacial and glaciofluvial sediments through which the groundwater is flowing are confined by a thick pro- and postglacial sedimentary sequence
Summary
Lake Vättern is the second largest lake in Sweden and the sixth largest in Europe in terms of area (1,856 km2) and volume (74 km3; Kvärnas 2001) (Fig. 1). It is located in south-central Sweden and stretches over 140 km in an NNE-SSW direction (Fig. 1A). The catchment area is 6,359 km, of which the lake comprises approximately 30% (Fig. 1B). Kvärnas (2001) summarized the water balance for the lake, with 612 mm/yr (or 1.14 km3/yr) of freshwater input, of which 510 mm/yr (0.95 km3/ yr) is in the form of precipitation. There is no reason given for the apparent imbalance in yearly inflow and outflow, which may reflect uncertainties in the underlying measurements
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