High nutrient and chloride loads to surface waters in polder areas due to groundwater seepage

摘要

Both hydrological and hydrochemical processes will affect the contribution of groundwater seepage tot he nutrient and chloride loads to the surface water system in polder areas in the coastal lowlands of the Netherlands. The interaction of these processes is studied in a deep polder called 'De Noordplas'. High loads due to groundwater seepage lead to eutrophication (PO_4 and NH_4) and salinisation of the local and regional surface waters, adversely affecting the aquatic ecosystems and agricultural practices. The seepage water, originating from a confined sandy aquifer, reaches the surface water in ditches and drain tubes via a lowpermeable confining layer of Holocene clay and peat. Buried creek channels and so-called 'hot spots' occur in this confining layer, leading to preferential seepage zones within the polder area. The area of these zones accounts for about 10percent of the total polder area but contributes for about 50-90percent to the solute loads by ground-water seepage. Raising the surface water levels along these preferential seepage zones turns out to be an effective measure to reduce the chloride and nutrient loads by groundwater seepage and to improve local and regional water quality.