Oehler, Till, Ramasamy, Murugan, Babu, Suresh, Santos, Isaac and Moosdorf, Nils (2019) Nitrogen behavior in a tropical subterranean estuary and a groundwater seepage zone. [Poster] In: EGU. , 07.04.2019, Wien .

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Abstract

Submarine groundwater discharge (SGD) can transport large amounts of nutrients into the coastal ocean. During
transport from land into the ocean, nutrients are often modified in the subterranean estuary (STE). In some cases,
denitrification can remove nitrogen, while in some STEs mineralization can produce nitrate. As a consequence, nutrient
concentrations of discharging groundwater are often different than terrestrial groundwater, making it difficult
to estimate nutrient concentrations of SGD.We investigated nutrient concentrations in a STE and in a groundwater
seepage face at Varkala Beach in southwestern India, in order to clarify the groundwater nitrogen transformations
along the land-ocean continuum. The coastal area is densely populated with an intensive agricultural land use.
Therefore groundwater flowing into the sea may have high nutrient concentrations, typical for a coast in India.
During post monsoon period of 2018, groundwater samples were obtained from the STE across the beach on a
meter scale, and from the seepage face on a centimeter-decimeter scale. Groundwater nitrogen concentrations varied
across the salinity gradient in the STE, indicating non-conservative removal of nitrogen. The seepage face had
fresh groundwater (salinity < 1 PSU) at 10 centimeters sediment depth. In these samples, nitrogen concentrations
followed a conservative mixing between fresh groundwater and seawater with high nitrogen concentrations of 400
�mol/L in fresh groundwater. Sediment core incubations also resulted in high nitrogen fluxes from the groundwater
seepage zone, when compared to sediment cores sampled outside of the seepage zone. STE samples over the
meter scale suggest non-conservative removal of nitrogen, probably due to long residence times of groundwater
or mixtures of different types of groundwater. However, nitrogen concentrations in the groundwater seepage face
are high and similar to groundwater nitrogen concentrations in the hinterland, probably promoted by rapid groundwater
flow from the hinterland in permeable sediments. SGD studies may use terrestrial nutrient endmembers in
groundwater to calculate a nutrient flux from a water flux in such settings.

Document Type: Conference or Workshop Item (Poster)
Research affiliation: Affiliations > Not ZMT
Biogeochemistry and Geology > Submarine Groundwater Discharge
Date Deposited: 01 Sep 2020 12:48
Last Modified: 01 Oct 2020 13:01
URI: http://cris.leibniz-zmt.de/id/eprint/4133

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