Upscaling groundwater quality models to the global domain scale

Groundwater acts as a long-term water quality buffer due to its wide range of residence times, spanning days to thousands of years (Maxwell et al., 2016). This makes groundwater a critical freshwater resource, especially where surface water quality or quantity is limited. However, contamination from emerging pollutants, influences of climate change, and changing water management, land use, and agricultural practices, likely increasingly lead to undesirable groundwater quality worldwide (Lapworth et al., 2023). Despite identification of this trend, the extent of these changes remains poorly understood, except for specific local and regional groundwater systems.

Mechanistic, physically-based models for predicting groundwater quality at global scales are not yet available, and field data are sparse. In this study, we review groundwater quality models for catchment scales and above and assess whether these approaches are scalable for global applications. Current models are constrained by computational demands, and insufficient subsurface data and knowledge of kinetic processes. Using numerical experiments for nitrogen cycling, we highlight the associated geochemical uncertainties to these subsurface conditions.

With the upscaling of 2D transport models to the kilometer grid scale we demonstrate what limitations follow from global hydrological models when used for groundwater quality modelling. At coarse kilometer-scale grids, sinks and sources tied to landscape features are poorly represented, causing inaccuracies in flow paths and therefore groundwater composition and fluxes. Additionally, when using kilometer scale grids, low flow velocities compared to grid dimensions and subgrid source heterogeneity prevent meaningful groundwater quality representation over the century timescales used in climate change and socio-economic development scenarios.

Future hyperresolution hydrological models may enable direct numerical simulations of groundwater quality, though subsurface transport property and reactivity data could remain limiting to a model’s coverage. For short-term global groundwater quality assessments, we recommend using data-driven approaches combined with conceptual groundwater cycling models as alternatives to mechanistic methods. This work highlights the need for improved modeling frameworks to enhance global understanding of groundwater quality dynamics, critical for informed water management and sustainable use of the groundwater resource under climate and land-use change.

 

Lapworth, D., Boving, T., Brauns, B., Dottridge, J., Hynds, P., Kebede, S., Kreamer, D., Misstear, B., Mukherjee, A., Re, V., Sorensen, J., & Vargas, C. R. (2023). Groundwater quality: global challenges, emerging threats and novel approaches. Hydrogeology Journal, 31(1). https://doi.org/10.1007/s10040-022-02542-0

Maxwell, R. M., Condon, L. E., Kollet, S. J., Maher, K., Haggerty, R., & Forrester, M. M. (2016). The imprint of climate and geology on the residence times of groundwater. Geophysical Research Letters, 43(2). https://doi.org/10.1002/2015GL066916