High-resolution distributed temperature sensing to detect seasonal groundwater discharge into Lake Vaeng, Denmark

Abstract

[1] Distributed temperature sensing (DTS) was used to map spatial and temporal changes in temperature on a 25 m by 6 m lakebed area in the winter (February), spring (May), and summer (August) of 2012. A constant and high discharge of groundwater with the average temperature of around 8 C to the lake will result in either lower (summer) or higher (winter) daily temperatures and reduce temperature variability at the sediment-water interface (SWI). DTS data were used as a proxy for groundwater discharge using three metrics; daily minimum temperature, diel amplitude, and daily standard deviation of temperature. During the seasons, the daily minimum temperatures at the SWI indicate a discharge zone 4-6 m offshore. From winter to summer, the extent of this zone changes and the SWI temperatures also show a shift of discharge locations toward the shore. Fluxes estimated on the basis of vertical temperature profiles from the top 50 cm of the lakebed and seepage meters in August compare well with the locations of the high-discharge zones detected by the DTS in the same period, giving confidence in the ability of the method to map both the areas and spatial variability of groundwater discharge to lakes. Compared to February, the DTS was able to detect new relatively cold temperature zones at the SWI in May and August indicating that groundwater discharge to the lake changes over time and that DTS can be used to monitor temporal variability in areas of discharge.Citation: Sebok, E., C. Duque, J. Kazmierczak, P. Engesgaard, B. Nilsson, S. Karan, and M. Frandsen (2013), High-resolution distributed temperature sensing to detect seasonal groundwater discharge into Lake Vaeng, Denmark, Water Resour. Res., 49, 5355-5368,

Identifying, characterizing and predicting spatial patterns of lacustrine groundwater discharge

“…However, isotropic and homogenous conditions rarely exist and spatial distribution of groundwater inflow differs strongly from lake to lake . Experimental studies highlighted a large variety of observed exchange patterns including decreasing seepage with distance from the shoreline (McBride and Pfannkuch, 1975;Brock et al, 1982;Cherkauer and Nader, 1989;Kishel and Gerla, 2002), increasing seepage with distance from the shoreline (Cherkauer and Nader, 1989;Schneider et al, 2005;Vainu et al, 2015), local hotspots of offshore seepage (Fleckenstein et al, 2009;Ono et al, 2012) and a high smallscale variability in near-shore zones (Kishel and Gerla, 2002;Blume et al, 2013;Neumann et al, 2013;Sebok et al, 2013). Most often, complex hydrogeological settings are the reason for deviations from the theoretical pattern of LGD .…”

“…Kidmose et al, 2013;Kishel and Gerla, 2002;Schneider et al, 2005;Vainu et al, 2015) and piezometers (Kishel and Gerla, 2002). Two other methods have also been used successfully to investigate exchange patterns between lakes and groundwater: fibre-optic distributed temperature sensing (FO-DTS) (Blume et al, 2013;Sebok et al, 2013) and vertical temperature profiles (VTPs) (Blume et al, 2013;Neumann et al, 2013;Sebok et al, 2013). Both methods use heat as a tracer.…”

“…The measurement of radon activities can also help to identify groundwater inflows (Kluge et al, 2012;Ono et al, 2012;Shaw et al, 2013). Existing lake studies have investigated LGD patterns with either a high spatial resolution (1-2 m 2 ) but a local focus (10 m × 17 m-25 m × 6 m) (Kishel and Gerla, 2002;Blume et al;Sebok et al, 2013) or focused on the entire lake but used a relatively low spatial resolution (measurements along the shoreline every 200-3000 m) (Schneider et al, 2005;Shaw et al, 2013). However, the experimental effort required rarely allows their extension to cover the lateral, alongshore dimension in sufficient extent and detail to identify the spatial variability and patterns of LGD along the shoreline.…”

“…Analysis of 24 h amplitude or daily minimum temperature did not provide useful information of groundwater inflow patterns as the impact of solar radiation was too strong and spatially variable. Sebok et al (2013) recommended using nighttime data to avoid the uncertainties caused by solar radiation. However, at night, shallow near-shore water cooled down and it was not easy to distinguish if temperature shifts resulted from groundwater inflow or from a decrease of water temperature due to decreasing air temperature.…”

Geophysical Methods for Monitoring Temperature Changes in Shallow Low Enthalpy Geothermal Systems

“…Sebok et al [121] used DTS to map spatial and temporal changes in temperature on a lakebed area to confirm the presence of a relatively high discharge of groundwater. The cable was spread out an area of 25 m × 6 m. The measurement through the seasons showed that the extent of the discharge zone was changing as well as its position relative to the shore.…”