An Electron‐Balance Based Approach to Predict the Decreasing Denitrification Potential of an Aquifer

Abstract

Numerical models for reactive transport can be used to estimate the breakthrough of a contaminant in a pumping well or at other receptors. However, as natural aquifers are highly heterogeneous with unknown spatial details, reactive transport predictions on the aquifer scale require a stochastic framework for uncertainty analysis. The high computational demand of spatially explicit reactive‐transport models hampers such analysis, thus motivating the search for simplified estimation tools. We suggest performing an electron balance between the reactants in the infiltrating solution and in the aquifer matrix to obtain the hypothetical time of dissolved‐reactant breakthrough at a receptor if the reaction with the matrix was instantaneous. This time we denote as the advective breakthrough time for instantaneous reaction (τinst). It depends on the amount of the reaction partner present in the matrix, the mass flux of the dissolved reactant, and the stoichiometry. While the shape of the reactive‐species breakthrough curve depends on various kinetic parameters, the overall timing scales with τinst. We calculate the latter by particle tracking. The effort of computing τinst is so low that stochastic calculations become feasible. We apply the concept to a two‐dimensional test case of aerobic respiration and denitrification. A detailed spatially explicit reactive‐transport model includes microbial dynamics. Scaling the time of local breakthrough curves observed at individual points by τinst decreased the variability of electron‐donor breakthrough curves significantly. We conclude that the advective breakthrough time for instantaneous reaction is efficient in estimating the time over which an aquifer retains its degradation potential.

Strategies for Simplifying Reactive Transport Models: A Bayesian Model Comparison

“…To model the release of DOC from natural organic matter (NOM) of the aquifer matrix, we choose a linear driving‐force‐expression. We assume an infinite supply of NOM; that is, the long‐term depletion of NOM is neglected because this process usually takes decades (Loschko et al, 2018; 2019). …”

Nitrate reduction potential of a fractured Middle Triassic carbonate aquifer in Southwest Germany

“…However, only a few studies have provided sound estimates of the amount of electron donors available in the aquifer matrix for the observed nitrate reduction processes (e.g., Kludt et al 2016). Particularly for fractured aquifers, detailed information on the extent of nitrate turnover and the inventory of available electron donors is usually sparse, which often limits the estimation of the nitrate reduction potential to geostatistical or stochastic modeling approaches (Rivas et al 2017;Loschko et al 2019).…”

Managed aquifer recharge with marginal water for irrigation and contaminant attenuation

“…The complexity of multicomponent biodegradation in steady-state saturated zone transport is already quite large, as current complexity reducing approaches are mostly accurate for specific conditions that assume reactants are well-mixed to some extent. For example, quasi-steady-state chemical conditions (Schäfer et al, 2020), reaction rates dependent on averaged concentrations (Massoudieh & Dentz, 2020), predominantly rate-limited conditions (Wright et al, 2021), (near-)instantaneous kinetics (Loschko et al, 2019), or large time asymptotic behavior (Wright et al, 2017). Unsaturated zone solute leaching occurs on a much shorter length and time scale than saturated zone transport.…”

“…Thus, for general analytical characterizations to be possible, it is necessary to first develop methods to simultaneously upscale continuously changing physical and chemical heterogeneity, and take into account coupled non-Markovian behavior in both the physical and chemical aspects of the problem. Accordingly, a possible starting approach is to assume that physical and chemical heterogeneity are correlated, as has been studied recently for saturated zone transport (Zhou et al, 2019;Mohamed et al, 2010;Jang et al, 2017;Chaudhuri & Sekhar., 2007;Loschko et al, 2018;Loschko et al, 2019), though it is unclear to what extent this assumption is valid.…”