Linking biogeochemical potential to depositional processes

Biogeochemical reactions are microbially mediated chemical reactions that occur naturally in the subsurface, involving naturally occurring and anthropogenically enriched reactants as well as microbes. These reactions play a crucial role in determining the fate of reactive solutes in groundwater. In Quaternary aquifers, the depositional (sedimentological) processes modulate the composition of the sedimentary matrix, leading to strong spatial variability in both hydraulic and reactive properties. For example, the presence or absence of reduced minerals or organic matter in the sediment matrix determines its “reactivity” with respect to oxidized reactants, auch as dissolved oxygen and nitrate. Additionally, the depositional processes determine properties relevant to groundwater flow, notably the hydraulic conductivity. In this work, we attempt to link depositional processes and the physico-chemical makeup of sediment matrices with the ability of an aquifer to naturally attenuate electron acceptors. We focus on nitrate and denitrification as dissolved electron acceptor and associated biodegradation pathway, respectively. Traditional numerical modeling approaches that account for physical heterogeneity in reactive transport rely on geostatistical methods using, e.g., multi-Gaussian random fields, and often fall short in capturing the link to the geological generating processes. We propose the use of object-based modeling to realistically represent the subsurface's physical characteristics and bridge the gap between geology and biogeochemical potential. Object-based models depict various sedimentary features as 3-D geometries (geo-bodies) within a hierarchical framework. On the largest spatial scale, the model represents strata, corresponding to the sedimentary deposition setting. Within each stratum, facies elements (with internal structure such as crossbedding or layering) representing architectural elements, such as channels or scour-pool fills, are assigned. We illustrate the construction of an object-based aquifer scale groundwater flow model informed via sedimentological descriptions (core logs), integrating field and lab-derived information. Furthermore, we apply a travel-time based reactive transport modelling approach to quantify the effect of the realistic distribution of reactive sedimentary faces on the extent of denitrification. We expect our ongoing analysis to shed light on the quantitative link between the sedimentological architecture of an aquifer and its denitrification potential.