Unravelling the Aquifer-Scale Competition for Organic Substrate in a Polluted Aquifer by Interpreting Multivariate Statistics Through Scenario-Based Hydrogeochemical Modeling

Chlorinated solvents in groundwater degrade mainly via Reductive Dechlorination–RD, which is a microbially-mediated redox reaction . Degrading bacteria, called organohalide-respiring bacteria– OHRB, oxidize the available organic substrate (electron donors–EDs) to gain energy and reduce chlorinated solvents for respiration, instead of other inorganic Terminal Electron Acceptors–TEAs (e.g., NO3−, Mn(IV) and Fe(III) oxi-hydroxides). However, OHRB are usually less efficient in oxidizing EDs than the bacteria that use inorganic TEAs; thus, substrate-limiting conditions cause competition for organic substrate. As a consequence, chlorinated solvent degradation is inhibited when high concentrations of inorganic TEAs occur naturally or anthropogenically in groundwater. Identifying the occurrence of the competition for organic substrate is critical to evaluate whether natural attenuation is a viable option for groundwater decontamination yet a challenging task when it comes to investigate this process at a field-scale in polluted aquifers, especially using sparse and incomplete datasets. For this reason, we propose a scenario-based modeling approach consisting of a multi-scenario batch model, which takes advantage of mechanistic knowledge to reproduce multivariate statistical patterns (the real data Pearson correlations) and assess whether the competition for organic substrate is occurring in groundwater. The selected site case study is a coastal silty sandy aquifer, where different cases of contamination are present: a MTBE-BTEX plume, several foundry waste dumps, and a residual plume of chlorinated ethenes. 108 scenarios with different levels of availability of inorganic TEAs, MTBE-BTEX and chlorinated ethenes were modeled both in competitive and non-competitive conditions, and Pearson coefficients were calculated on numerical results and compared with the real data correlations. The satisfactory match between the model correlations in competitive conditions and the field correlations confirms the occurrence of competition of organic substrate in groundwater. Substrate-limitating conditions as well as the presence of mostly nitrate and intermediate-to-high soluble Mn(IV) oxi-hydroxides seem to inhibit biodegradation of chlorinated ethenes.