Fresh groundwater resources in coastal deltas

Coastal deltas house more than 335 million people worldwide in some of the largest population centers in the world, including growing megacities such as Shanghai, Dhaka, and Bangkok. These populations often rely heavily upon groundwater resources to meet domestic, agricultural, and industrial water demands—making the sustainability of fresh groundwater resources critical to ensuring the longevity of coastal communities. This research uses delta morphology as a tool for understanding the distribution of fresh and saline groundwater within coastal deltas. Morphodynamic modeling simulating the formation of coastal deltas is used to explore how delta morphology impacts hydrogeologic characteristics within deltas. Data from deltas around the world are used to create 207 unique models that span the full range and combination of fluvial and marine influences. Simulated landforms depict the characteristics expected in fluvial, wave, and tidal influenced deltas; these landforms are used to generate spatially varying permeability profiles for each simulated delta. High permeability areas within coastal deltas are often associated with the river network and are highly connective. A distance-based sensitivity analysis shows that deltaic permeability, hydraulic gradient, and groundwater flow rates are sensitive to changes in delta morphology and geomorphic characteristics. Two-dimensional density-dependent groundwater flow and solute transport modeling is used to simulate the horizontal fresh-saline water distribution within the shallow subsurface in representative fluvial, wave, and tidal deltas. The volume of saline water in the shallow subsurface within deltas is estimated to vary between 36% and 89% of the total groundwater volume, depending on the morphodynamic influences and the amount of recharge the delta receives. Results show that deltas located in dry climates are most susceptible to salinity and that wave or fluvial delta are especially susceptible. The generic groundwater models of the three delta types are used to understand the vulnerability of 55 real deltas to groundwater salinization. The result of this research provides an initial estimate of the amount of freshwater within deltas, identifies where salinity is most likely to occur within a delta, and suggests which delta types are most vulnerable to groundwater degradation.