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\begin{document}

\title{Primary Controls on the Natural Denitrification Potential of Aquifers}



\author[1]{Olaf Cirpka}%
\author[2]{Matthias Loschko}%
\author[3]{Thomas Wöhling}%
\author[4]{David Rudolph}%
\affil[1]{University of Tübingen}%
\affil[2]{University of Stuttgart}%
\affil[3]{University of Tuebingen}%
\affil[4]{University of Waterloo}%


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  \date{June 15, 2024}


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\begin{abstract}
Elevated nitrate concentrations in groundwater are observed in regions
of intensive agriculture worldwide, threatening the safety of
drinking-water production. Aquifers may contain geogenic reduced
constituents, such as natural organic matter (NOM), pyrite, or biotite,
facilitating aerobic respiration and denitrification. Because these
electron donors are not replenished, the breakthrough of nitrate (and
eventually dissolved oxygen) in production wells is only delayed.
Frameworks of modeling nitrate fate and transport that assume constant
rate coefficients of nitrate elimination cannot address the reduction of
the aquifer's denitrification potential by the reaction itself. We have
tested several approaches of modeling the fate of dissolved oxygen and
nitrate in aquifers, including multi-dimensional bioreactive transport
models with dynamic abundances of aerobic and denitrifying bacteria,
approaches neglecting the dynamics of biomass and dispersive mixing, and
simple models based on an electron balance. We found that the primary
control on the timing of nitrate breakthrough is the ratio of the
bioavailable electron-donor content in the aquifer material to the
electron-acceptor load in the infiltrating water. Combined spatial
variability of groundwater velocities and electron-donor content can
explain most of the spread in nitrate breakthrough, whereas kinetics of
the reaction plays a minor role under most conditions. Our modeling
study highlights the need for field surveys on joined physical and
chemical heterogeneity of aquifers under the stress of pollutants that
can react with the aquifer material.%
\end{abstract}%



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