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The spa al distribu on of seepage at a fl ow-through lake in western Denmark was invesgated at mul ple scales with integrated use of a seepage meter, lake-groundwater gradients, stable isotope frac ona on (δ 18 O), chlorofl uorocarbon (CFC) apparent ages, land-based and off -shore geophysical surveys, and lake bed coring. Results were compared with a three-dimensional catchment-scale groundwater fl ow model using the MODFLOW and LAK3 codes for simula ng lake-groundwater interac on. Seepage meter and model results of discharging groundwater to the lake compared well, if direct seepage measurements from near shore were combined with measurements from deeper parts of the lake. Discharge rates up to 9.1 × 10 −7 m s −1 were found. Ground-penetra ng radar used to map the lake bed sediments proved very eff ec ve in recognizing low-and high-permeability areas but also in understanding the complex recharge pa ern of the lake and rela ng these to the geologic history of the lake. Recharge of the surrounding aquifer by lake water occurs off shore in a narrow zone, as measured from lake-groundwater gradients. A 33-m-deep δ 18 O profi le at the recharge side shows a lake δ 18 O plume at depths that corroborates the interpreta on of lake water recharging off shore and moving down gradient. Inclusion of lake bed heterogeneity in the model improved the comparison of simulated and observed discharge to the lake. The apparent age of the discharging groundwater to the lake was determined by CFCs, resul ng in ages between 3 and 36 yr with an average of 16 yr. The simulated average groundwater age was 13.2 yr.Abbrevia ons: CFC, chlorofl uorocarbon; GPR, ground-penetra ng radar; MEP, mul electrode profi le; MLW, mul level well.Groundwater-dominated lakes are especially vulnerable to deterioration in lake water quality due to inputs from polluted groundwater. An understanding of the distribution and rate of seepage to and from lakes is therefore needed for environmental management or restoration of lake ecosystems (Hayashi and Rosenberry, 2002;Sophocleous, 2002;Gleeson et al., 2009). Several studies have demonstrated the infl uence of seepage on (i) lake water quality, e.g., the discharge of nutrient-rich groundwater (Loeb and Goldman, 1979;Brock et al., 1982;Belanger et al., 1985;Ito et al., 2007), groundwater rich in cations (Dean et al., 2003;Cullmann et al., 2006), dissolved inorganic and organic C (Striegl and Michmerhuizen, 1998;Staehr et al., 2010), or, in general, changes in lake alkalinity as a result of weathering processes in the watershed (Schafran and Driscoll, 1993); or (ii) biological communities, e.g., biodiversity and species distribution in seepage zones (Lodge et al., 1989;Hagerthey and Kerfoot, 1998;Rosenberry et al., 2000;Hayashi and Rosenberry, 2002;Sebestyen and Schneider, 2004).Th e exchange of water and solutes between groundwater and lakes is complex and there is still a challenge in understanding the temporal and spatial variability across diff erent scales (Käser et al., 2009). To address this challenge, a ...

[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,

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