Nitrate transport through groundwater into the sea: a ticking time bomb?

Christoph Merz1, Nils Moosdorf2, Jörg Steidl1
1 AG Tieflandhydrologie und Wassermanagement, Leibniz Zentrum für Agrarlandschaftsforschung (ZALF), Müncheberg
2 Leibniz-Zentrum für Marine Tropenforschung (ZMT), Bremen

2.5 in Wasserquantitäts- und qualitätsmodellierung auf regionaler Skale - Herausforderungen und neue Ansätze

27.03.2020, 15:30-15:45, Weißer Saal

Groundwater resources play a crucial role in the whole water cycle. Therefore, its quality must be of high standard and the directives defining water quality must be complied. However, there are still many water bodies in regions worldwide which do not show a good chemical status or are endangered to lose their good chemical status. In many cases, nitrate from agricultural land use is the reasons for this issue. Tackling this problem, standard approaches of hydrological analysis often fail reproducing clear spatio-temporal correlations between nutrient source and contaminated aquatic environments.

Although groundwater primary discharges into rivers and lakes a certain amount of groundwater flows into the sea along the shorelines as submarine groundwater discharge (SGD). The transported nutrients and pollutants are a serious risk for sensitive marine coastal ecosystems, whose tolerance for nutrients can be far smaller than that of terrestrial ecosystems. Due to high groundwater residence times, groundwater can transport increased nutrient levels resulting from present changes until even after 2025, when the United Nations Sustainable Development Goals commit to reduce nutrient input to the oceans.

The principle item of our approach is an innovative GIS based tool for the spatial and temporal analysis and evaluation of nitrate dynamics in large scale aquifer systems to assess the nitrate flux from agricultural sources via groundwater to the ocean by SGD. The model-based study presented here uses globally available data to identify sensitive global areas where nutrient input from submarine groundwater discharge is potentially expected. The data used represent 1) nutrient input into groundwater based on land use classes, 2) regional groundwater transit times, 3) nitrate degradation by biogeochemical processes, 4) global submarine groundwater runoff distribution and 5) connected sensitive coastal ecosystems. Global geodata sets (land use, lithology, groundwater elevation, hydraulic conductivity, global distribution of groundwater runoff, spread of sensitive coastal ecosystems) are coupled with literature data of regional, redox based denitrification rates in the related groundwater systems. The model calculations include results for more than 20,000 coastal catchments worldwide.

Within the uncertainties associated with global scale modeling approaches, coastal regions impacted by SGD with increasing nutrient loads can be identified based on the comprehensive model analysis. Especially in regions connected to sensitive ecosystems, e.g. coral reefs, future changes in nutrient load will pose a high risk to the ecosystem health. The presented approach of a catchment-scale nitrate modeling focuses on a realistic spatio-temporal assessment of the nitrate contamination path from the land use into marine ecosystems in order to enable land use changes in time.



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