Anna Störiko

and 4 more

The interface between rivers and groundwater is a key driver for the turnover of reactive nitrogen compounds, that cause eutrophication of rivers and endanger drinking-water production from groundwater. Molecular-biological data and omics tools have been used to characterize microorganisms responsible for the turnover of nitrogen compounds. While transcripts of functional genes and enzymes are used as measures of microbial activity it is not yet clear how they quantitatively relate to actual turnover rates under variable environmental conditions. We developed a reactive-transport model for denitrification that simultaneously predicts the distributions of functional-gene transcripts, enzymes and reaction rates. Applying the model, we evaluate the response of transcripts and enzymes at the river–groundwater interface to stable and dynamic hydrogeochemical regimes. While functional-gene transcripts respond to short-term (diurnal) fluctuations of substrate availability and oxygen concentrations, enzyme concentrations are stable over such time scales. The presence of functional-gene transcripts and enzymes globally coincides with the zones of active denitrification. However, transcript and enzyme concentrations do not directly translate into denitrification rates in a quantitative way because of non-linear effects and hysteresis caused by variable substrate availability and oxygen inhibition. Based on our simulations, we suggest that molecular-biological data should be combined with aqueous chemical data, which can typically be obtained at higher spatial and temporal resolution, to parameterize and calibrate reactive-transport models.

Olaf Cirpka

and 3 more

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.