Abstract
Abstract. The Amsterdam area, a highly manipulated delta area formed by polders and reclaimed lakes, struggles with high nutrient levels in its surface water system. The polders receive spatially and temporally variable amounts of water and nutrients via surface runoff, groundwater seepage, sewer leakage, and via water inlets from upstream polders. Diffuse anthropogenic sources, such as manure and fertiliser use and atmospheric deposition, add to the water quality problems in the polders. The major nutrient sources and pathways have not yet been clarified due to the complex hydrological system in lowland catchments with both urban and agricultural areas. In this study, the spatial variability of the groundwater seepage impact was identified by exploiting the dense groundwater and surface water monitoring networks in Amsterdam and its surrounding polders. A total of 25 variables (concentrations of total nitrogen (TN), total phosphorus (TP), NH4, NO3, HCO3, SO4, Ca, and Cl in surface water and groundwater, N and P agricultural inputs, seepage rate, elevation, land-use, and soil type) for 144 polders were analysed statistically and interpreted in relation to sources, transport mechanisms, and pathways. The results imply that groundwater is a large source of nutrients in the greater Amsterdam mixed urban–agricultural catchments. The groundwater nutrient concentrations exceeded the surface water environmental quality standards (EQSs) in 93 % of the polders for TP and in 91 % for TN. Groundwater outflow into the polders thus adds to nutrient levels in the surface water. High correlations (R2 up to 0.88) between solutes in groundwater and surface water, together with the close similarities in their spatial patterns, confirmed the large impact of groundwater on surface water chemistry, especially in the polders that have high seepage rates. Our analysis indicates that the elevated nutrient and bicarbonate concentrations in the groundwater seepage originate from the decomposition of organic matter in subsurface sediments coupled to sulfate reduction and possibly methanogenesis. The large loads of nutrient-rich groundwater seepage into the deepest polders indirectly affect surface water quality in the surrounding area, because excess water from the deep polders is pumped out and used to supply water to the surrounding infiltrating polders in dry periods. The study shows the importance of the connection between groundwater and surface water nutrient chemistry in the greater Amsterdam area. We expect that taking account of groundwater–surface water interaction is also important in other subsiding and urbanising deltas around the world, where water is managed intensively in order to enable agricultural productivity and achieve water-sustainable cities.
Highlights
The hydrology of many lowland delta areas is highly manipulated by human activities such as ditching, draining, and embanking, to enable agriculture and habitation
This study aimed at identifying the impact of groundwater on surface water quality in the polder catchments of the greater Amsterdam city area, which is the management area of Waternet, the organisation which manages dikes, regulates water levels and pumping regimes, and is responsible for the clean surface water, drinking water supply, and waste water treatment
This study focuses on the polder catchment landscape around the city of Amsterdam in the Netherlands
Summary
The hydrology of many lowland delta areas is highly manipulated by human activities such as ditching, draining, and embanking, to enable agriculture and habitation. L. Yu et al.: Groundwater impacts on surface water quality and nutrient loads deltas account for 2 % of the world’s land, but accommodated around 600 million people in 2000, and will accommodate about 1400 million by 2060, as was estimated by Neumann et al (2015). The reclamation of swamps and lakes and the drainage of peat areas to enable urbanisation and agriculture severely changed the hydrological, chemical, and ecological environment of these areas (Ellis et al, 2005; Yan et al, 2017). Lowland delta areas are vulnerable for water quality deterioration by processes like salinisation and eutrophication, which can be amplified by climate change (Wu et al, 2015) and land subsidence (Minderhoud et al, 2017)
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