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[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,

Observed 3-D temperature distributions within a streambed were used to analyze the effects on exchange fluxes between groundwater and the stream during rainfall-runoff events. By combining a dense vertical and lateral monitoring network of streambed temperatures with coupled surface/subsurface 3-D flow and heat transport modeling, we demonstrate how temperature can be used directly as a calibration target. Three model setups with different hydraulic conductivity distributions were evaluated in an optimization approach using temperature and hydraulic head data. The hydraulic conductivity distributions were based on slug test surveys within the streambed and aquifer. A detailed characterization of the hydraulic conductivity of the streambed and aquifer is needed to accurately simulate observed temperatures. Hence, the most sensitive parameters, the vertical hydraulic conductivity and the thermal conductivity, were calibrated within different conductivity zones of the heterogeneous model. Simulated exchange fluxes across the streambed showed variations up to a factor of four within just a meter. Such differences may not have been correctly predicted using 1-D heat transport models due to lateral conduction amongst the different flow paths. During the rainfall-runoff event, fluxes decreased substantially (250%) due to a decrease in the hydraulic gradient with increased stream stage. Although no flow reversals were observed during the studied conditions, it is possible that these can occur during larger rainfall-runoff events. We show that with the current sampling and modeling techniques, 3-D temperature data can be used to estimate dynamic exchange in heterogeneous flow fields encountered in the field.

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Surface water-groundwater interactions were studied in a coastal lagoon performing 180 seepage meter measurements and using heat as a tracer in 30 locations along a lagoon inlet. The direct seepage meter measurements were compared with the results from analytical solutions for the 1D heat transport equation in three different scenarios: (1) Homogeneous bulk thermal conductivity (Ke); (2) horizontal heterogeneity in Ke; and (3) horizontal and vertical heterogeneity in Ke. The proportion of fresh groundwater and saline recirculated lagoon water collected from the seepage experiment was used to infer the location of the saline wedge and its effect on both the seepage meter results and the thermal regime in the lagoon bed, conditioning the use of the thermal methods. The different scenarios provided the basis for a better understanding of the underlying processes in a coastal groundwater-discharging area, a key factor to apply the best-suited method to characterize such processes. The thermal methods were more reliable in areas with high fresh groundwater discharge than in areas with high recirculation of saline lagoon water. The seepage meter experiments highlighted the importance of geochemical water sampling to estimate the origin of the exchanged water through the lagoon bed.

Submarine groundwater discharge (SGD)—including terrestrial freshwater, density-driven flow at the saltwater–freshwater interface, and benthic exchange—can deliver nutrients to coastal areas, generating a negative effect in the quality of marine water bodies. It is recognized that water stable isotopes (18O and 2H) can be helpful tracers to identify different flow paths and origins of water. Here, we show that they can be also applied when assessing sources of nutrients to coastal areas. A field site near a lagoon (Ringkøbing Fjord, Denmark) has been monitored at a metric scale to test if stable isotopes of water can be used to achieve a better understanding of the hydrochemical processes taking place in coastal aquifers, where there is a transition from freshwater to saltwater. Results show that 18O and 2H differentiate the coastal aquifer into three zones: Freshwater, shallow, and deep saline zones, which corresponded well with zones having distinct concentrations of inorganic phosphorous. The explanation is associated with three mechanisms: (1) Differences in sediment composition, (2) chemical reactions triggered by mixing of different type of fluxes, and (3) biochemical and diffusive processes in the lagoon bed. The different behaviors of nutrients in Ringkøbing Fjord need to be considered in water quality management. PO4 underneath the lagoon exceeds the groundwater concentration inland, thus demonstrating an intra-lagoon origin, while NO3, higher inland due to anthropogenic activity, is denitrified in the study area before reaching the lagoon.

An ecohydrological study was carried out at an oligotrophic seepage lake, Lake Hampen, Denmark, to determine the seepage in and out of the lake and determine the mass budgets for nutrients. The lake is primarily surrounded by forest, although there are agricultural fields bordering a small portion of the lake perimeter. The water and mass balances indicate that there is a groundwater input of 1643 × 103 m3 year−1 into the lake (∼70% of total input) and a groundwater output of 1997 × 103 m3 year−1. The groundwater input carries with it 2253 kg N year−1 (∼67% of total input) and 40 kg P year−1 (∼85% of total input). The majority of the nitrogen is leached from the agricultural fields bordering the lake. Concentrations as high as 1750 µM nitrate were measured in the rhizosphere of the littoral zone at this location. It is estimated that the macrophytes are able to take up 1695 kg N year−1 (∼50% of the input). The phosphorus uptake by plants was 310% of the input, indicating that plants are highly dependent on accumulated and remineralized phosphorus in the lake. The nutrients lost through recharge was much lower than input through groundwater discharge entailing a high retention of nutrients in the lake (92–96%). A significant part of the retained nitrogen may be lost through denitrification. Although the plant uptake may have slowed the eutrophication of the lake, the excess nutrients may over time give rise to eutrophication and a shift towards pelagic production. Copyright © 2011 John Wiley & Sons, Ltd.