Denitrification in heterogeneous aquifers: Relevance of spatial variability and upscaling

Abstract: Nitrate contamination of aquifers is threatening the sustainability of water resource production around the globe. Denitrification is an ubiquitous process in groundwater systems and needs to be represented in managerial and predictive efforts. Yet, fully understanding the temporal and spatial dynamics of all biogeochemical and physical properties controlling nitrate reduction at the basin scale remains a challenging task. Conceptual simplifications such as homogenization are used, but their implications on model performance are often not well assessed. This paper (1) analyzes the importance of denitrification spatial variability on the fate of non-point source nitrate contamination, and (2) assesses the performance of upscaled denitrification rate coefficient values. Heterogeneity in the hydraulic conductivity of a typical unconsolidated alluvial aquifer system and its associated uncertainty is systematically represented in our 3D numerical simulations of flow and transport. The spatial variability in the nitrate reduction capacities of the aquifer is accounted for by 3 different conceptual models: a positive and a negative correlation with the aquifer hydraulic properties and a uniform random distribution. Our results indicate that heterogeneity in the denitrification rate coefficient – as well as the model describing spatial variability – has a significant impact on concentration statistics observed at a series of extraction wells. However, we also show that, under the modeling setting used in this study (e.g., stationarity of reactivity), upscaled values of denitrification rate coefficients allow a virtually perfect reproduction of nitrate concentration statistics, a promising outcome for employing parameter estimation (calibration) of effective rate coefficients of denitrification: A detailed description of the spatial variability of the nitrate reduction capacity of the alluvial aquifer is not necessary. Yet, these homogeneous, upscaled denitrification rate coefficients result from a complex interaction between the biochemistry and the physics of the aquifer, which makes their direct estimation a challenging task.