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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 ...

Abstract. We have investigated the potential of 2D electrical imaging for the characterization of seawater intrusion using field data from a site in Almeria, SE Spain. Numerical simulations have been run for several scenarios, with a hydrogeological model reflecting the local site conditions. The simulations showed that only the lower salt concentrations of the seawater-freshwater transition zone could be recovered, due to the loss of resolution with depth. We quantified this capability in terms of the cumulative sensitivity associated with the measurement setup and showed that the mismatch between the targeted and imaged parameter values occurs from a certain sensitivity threshold. Similarly, heterogeneity may only be determined accurately if located in an adequately sensitive area. At the field site, we identified seawater intrusion at the scale of a few kilometres down to a hundred metres. Borehole logs show a remarkable correlation with the image obtained from surface data but indicate that the electrically derived mass fraction of pure seawater could not be recovered due to the discrepancy between the in-situ and laboratory-derived petrophysical relationships.Surface-to-hole inversion results suggest that the laterally varying resolution pattern associated with such a setup dominates the image characteristics compared to the laterally more homogeneous resolution pattern of surface only inversion results, and hence surface-to-hole images are not easily interpretable in terms of larger-scale features. Our results indicate that electrical imaging can be used to constrain seawater intrusion models if image appraisal tools are appropriately used to quantify the spatial variation of sensitivity and resolution. The most crucial limitation is probably the apparent non stationarity of the petrophysical relationship during the imaging process.