Groundwater impacts on surface water quality and nutrient loads in lowland polder catchments: monitoring the greater Amsterdam area
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,...
Main Authors: | , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2018-01-01
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Series: | Hydrology and Earth System Sciences |
Online Access: | https://www.hydrol-earth-syst-sci.net/22/487/2018/hess-22-487-2018.pdf |
Summary: | 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), NH<sub>4</sub>, NO<sub>3</sub>, HCO<sub>3</sub>, SO<sub>4</sub>,
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 (<i>R</i><sup>2</sup> 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. |
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ISSN: | 1027-5606 1607-7938 |