Development of a Fully Coupled Biogeochemical Reactive Transport Model to Simulate Microbial Oxidation of Organic Carbon and Pyrite Under Nitrate‐Reducing Conditions

Abstract

In regions with intensive agriculture nitrate is one of the most relevant contaminants in groundwater. Denitrification reduces elevated nitrate concentrations in many aquifers, yet the denitrification potential is limited by the concentration of available electron donors. The aim of this work was to study the denitrification potential and its limitation in natural sediments. A column experiment was conducted using sediments with elevated concentrations of organic carbon (total organic carbon 3,247 mg C/kg) and pyrite (chromium reducible sulfur 150 mg/kg). Groundwater with high nitrate concentration (100 mg/L) was injected. Measurements were taken over 160 days at five different depths including N‐ and S‐isotope analysis for selected samples. A reactive transport model was developed, which couples nitrate reduction with the oxidation of organic carbon (heterotrophic denitrification) and pyrite (autolithotrophic denitrification), and considers also transport and growth of denitrifying microbes. The denitrification pathway showed a temporal sequence from initially heterotrophic to autolithotrophic. However, maximum rates were lower for heterotrophic (11 mmol N/(L*a)) than for autolithotrophic denitrification (48 mmol N/(L*a)). The modeling showed that denitrifying microbes initially preferred highly reactive organic carbon as the electron donor for denitrification but were also able to utilize pyrite. The results show that after 160 days nitrate increased again to 50 mg/L. At this time only 0.5% of the total organic carbon and 46% of the available pyrite was oxidized. This indicates that denitrification rates strongly decrease before the electron donors are depleted either by a low reactivity (total organic carbon) or a diminishing reactive surface possibly due to the presence of coatings (pyrite).

Developments and applications of the HYDRUS computer software packages since 2016

“…The latter used the DVLO theory to calculate the attachment efficiency during variable geochemical conditions. Applications of pyrite oxidation in acid sulfate soils are given by Knabe et al (2018), who implemented pyrite oxidation under nitratereducing conditions in aquifers, and by Salo et al (2023) and Mosley et al (unpublished data, 2024), who simulated geochemical changes in acid sulfate soils following prolonged drought events due to climate change. Tremosa et al (2020) assessed the weathering of shale tailings by combining transient water flow, mobile-immobile regions, and gas diffusion with geochemical reactions.…”

“…Applications of pyrite oxidation in acid sulfate soils are given by Knabe et al. (2018), who implemented pyrite oxidation under nitrate‐reducing conditions in aquifers, and by Salo et al. (2023) and Mosley et al.…”

Biocolmation and the finite element modeling of its influence on changes in the head drop in a geobarrier

“…The important problem of denitrification in agricultural production was studied in paper [9]. During the experiments, as a part of general dynamics, the development of microbes as one of the factors of denitrification (neutralization and decomposition of nitrates) was studied.…”

Constructing a mathematical model and studying numerically the effect of bio-clogging on soil filtration consolidation

“…The authors of [6] investigated, among other factors, the impact of the development of microbes as one of the factors of denitrification in agricultural production. The cited work shows that bio-processes in porous environments, including the development of microorganisms, are important; studying them is an urgent task.…”