1 Groundwater impacts on surface water quality and nutrient loads in 2 lowland polder catchments: monitoring the greater Amsterdam area 3 Liang Yu1, 2, Joachim Rozemeijer3, Boris M. van Breukelen4, Maarten Ouboter2, Corné van der Vlugt2, 4 Hans Peter Broers5 5 1Faculty of Science, Vrije University Amsterdam, Amsterdam, 1181HV, The Netherlands 6 2 Waternet Water Authority, Amsterdam, 1096 AC, The Netherlands 7 3Deltares, Utrecht, 3508 TC, The Netherlands 8 4Department of Water Management, Faculty of Civil Engineering and Geosciences, Delft University of Technology, 9 Stevinweg 1, 2628 CN Delft, The Netherlands 10 5TNO Geological Survey of The Netherlands, Utrecht, 3584 CB, The Netherlands 11 Correspondence to: Hans Peter Broers (
[email protected]) 12 Abstract. The Amsterdam area, a highly manipulated delta area formed by polders and reclaimed lakes, struggles with high 13 nutrient levels in its surface water system. The polders receive spatially and temporally variable amounts of water and 14 nutrients via surface runoff, groundwater seepage, sewer leakage, and via water inlet from upstream polders. Diffuse 15 anthropogenic sources, such as manure and fertilizer use and atmospheric deposition, add to the water quality problems in 16 the polders. The major nutrient sources and pathways have not yet been clarified due to the complex hydrological system in 17 such lowland catchments with both urban and agricultural areas. In this study, the spatial variability of the groundwater 18 seepage impact was identified by exploiting the dense groundwater and surface water monitoring networks in Amsterdam 19 and its surrounding polders. Twenty-five variables (concentrations of Total-N, Total-P, NH4, NO3, HCO3, SO4, Ca, and Cl in 20 surface water and groundwater, N and P agricultural inputs, seepage rate, elevation, land-use, and soil type) for 144 polders 21 were analysed statistically and interpreted in relation to sources, transport mechanisms, and pathways.