b'25A novel method to evaluate the hydrochemical interactionbetween groundwater and surface waters in DenmarkVant Veen SGW1, Nilsson B2, Troldborg L2, Thorling L2, Boutrup S1, Larsen MM3, Rasmussen J1, Hinsby K2, Kazmierczak J2, and Kronvang B11Aarhus University, Department of Bioscience, Silkeborg, DK-8600, Denmark2Geological Survey of Denmark and Greenland, Department of Hydrology, Copenhagen, DK-1350,Denmark3Aarhus University, Department of Bioscience, Roskilde, DK-4000, DenmarkThis study investigates the hydrochemical interaction between groundwater and surface water in order to develop a method to evaluate the chemical impact from aquifers on surface water chemical quality in Denmark. The proposed method involves a catchment analysis for evaluating, whether groundwater aquifers is a significant source of nutrients and hazardous substances.The novel method proposed involves a five tier approach: Tier 1: Assesses whether there are exceedances of the environmental quality requirements and / or significant upward trends for specific nutrients or hazardous substances. Tier 2: Source apportionment of the specificsubstances identified as problematic in tier 1. Tier 3: Analysis of the potential contact (PC)between the groundwater aquifers and surface water bodies in the catchment based on the National Danish hydrologic model (Troldborg et al, 2014). Tier 4: Quantification of the discharge of relevant substances from sub-surface point sources to surface water bodies Tier 5: Analysis of the potential hydraulic contact (PHC) where the PC is supplemented by the potential hydraulic contact (PHC) calculated by the DK-model. A screening is performed for the occurrence of the substances within a buffer of 250 m and 500 m. A combination of modelled groundwaterdischarge to the water body, chemical quality of the groundwater aquifer and expert assessment of the substance transport and attenuation, the potential influence of each groundwater aquifer for the chemical status of surface water is quantified and evaluated.The method was tested in three different pilot catchments in Denmark. The suggested screening method was found to be useful and that missing data made it impossible to complete some of the proposed tiers. Moreover, we concluded that more monitoring data for hazardous substanc-es is needed and that specific knowledge about the fate of hazardous substances between the groundwater aquifer and surface waters is generally poorly established.References:Troldborg L, Srensen BL, Kristensen M, Mielby S. 2014: Afgrnsning af grundvandsforekomster. Tredje revision af grundvandsforekomster i Danmark. GEUS rapport 2014/5869'