Large-scale assessment of onshore and offshore fresh-saline groundwater resources in deltas and coasts

Abstract:

Coastal areas, including deltas, are hotspots for population growth and economic development. The rising demand for fresh water that results from these developments has resulted in increased rates of groundwater pumping and an associated enhanced risk of groundwater salinization. Future sea-level rise, climate change and surface sealing due to urbanisation are likely to further increase salinization risk in the near future. In order to correctly project the future fate of fresh groundwater resources in coastal areas under climate and socioeconomic change, and to design effective management strategies, a correct estimate of the current fresh-brackish-salt groundwater distribution is imperative. This not only includes onshore fresh groundwater volumes, but also offshore stocks as a strategic resource. Here, we make a case that estimating the current fresh-brackish-salt groundwater distribution by itself is a major challenge. The presence of conductivity contrasts in coastal areas, the past occurrence of sea-level transgressions and the aforementioned system inertia makes that traditional estimation methods such as interpolations between in-situ salinity observations or equilibrium (steady-state) modelling approaches are incapable of producing sufficiently realistic fresh-brackish-salt groundwater distributions. We show how advancements high-resolution paleo-groundwater modelling may be key to providing distributions that are more realistic and accurate. We illustrate this by 2D modelling examples of 15 of the largest global deltas and a large set of 3D models estimating of onshore and offshore groundwater volumes of the global coast.