Geothermal evidence for groundwater flow through Quaternary sediments overlying bedrock aquifers below Lake Vättern, Sweden

Abstract

Groundwater 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.

Modelling borehole flows from Distributed Temperature Sensing data to monitor groundwater dynamics in fractured media

“…They also used Ramey's equation to see if the vertical flow of warm fluid in the upper part can explain this discrepancy between measured and modeled geothermal temperature. They estimated the vertical flow velocity and concluded that this discrepancy can be explained by the infiltration of warm fluid transported from depth by fractures (Preto et al, 2019). In this work, the use of Hasan & Kabir's solution for converting temperature profile into flow profile is validated numerically (section numerical validation) and also experimentally by cross-checking the estimated flow rates with measured flow rates by a heat pulse flow meter.…”

“…Using a laboratory prototype, they obtain very good agreement between flow rate measurements derived from temperature profiles using Ramey's model with imposed flow rates (Silva et al, 2019). Preto et al (2019) investigated the reason for nonlinearity in the recorded geothermal gradient in the upper part of a deep wellbore which could not be explained by the variability of measured thermal properties of sediments and rocks. They also used Ramey's equation to see if the vertical flow of warm fluid in the upper part can explain this discrepancy between measured and modeled geothermal temperature.…”