[A-HW24] Human- and Climate-induced variability in water cycle and (sub)surface water resources

[AHW24-07] Natural and anthropogenic drivers of unconventional distribution of salinity between land-ocean interface

*Xuan Yu¹ (1.Chinese Academy of Sciences, Institute of Soil Science)

Keywords:Submarine groundwater discharge , Coastal hydrology, aquaculture, soil salinization

Groundwater flow and solute transport models that simulate how groundwater moves across the land-ocean interface and seabed are essential to science and engineering because these processes affect our coastal water resources as well as the chemical composition of the ocean. Models are particularly useful to estimate the saline component of SGD, which can occur far offshore and is difficult to measure directly. These models have been applied over different domain sizes to simulate natural and anthropogenic driver of salinity distribution and groundwater fluxes. On the one hand, depending on the system and application, the level of geologic detail represented can range from homogeneous or layered to fully heterogeneous hydraulic conductivity fields. These features strongly affect model results, limiting understanding of subsurface salinity distributions and associated density-driven saltwater circulation along coasts worldwide. The impact of the scale of representation of heterogeneity on salinity distributions and SGD was investigated using numerical simulations. Upscaling hydraulic conductivity can significantly modify salinity distributions and flow paths, resulting in unpredictable variations in simulated SGD, though the values for homogeneous fields with equivalent hydraulic conductivity show consistent trends. Simulated density distributions control both the rate and direction of subsurface saltwater circulation. The length of the mixing zone perimeter, a measure of salinity distribution complexity, is shown to correlate with both the rate of subsurface saltwater circulation and the amount of groundwater circulating in the reverse direction from homogeneous cases. The results demonstrate a strong dependence of salinity distributions and saltwater circulation on the scale and distribution of geologic heterogeneity represented in numerical models. On the other hand, the recent expending of onshore aquaculture farms introduces considerable SGD variability spatially and temporally. To understand the impacts of shrimp farm irrigation on groundwater salinization and SGD, we numerically simulated a series of aquaculture management scenarios in a two-dimensional conceptual coastal aquifer using a coupled surface-subsurface approach. We characterized sensitivities to pond water salinity, pond water depth, and farm width. Salinization was assessed by three indicators (salinized area, infiltrated salt mass, and recovery rate), and three SGD indicators were evaluated (fresh SGD, saline SGD, and saltwater circulation rate). Our results show that pond water depth is the primary control on the mass of saltwater infiltration while farm width is the primary control for recovery rate. Pond water salinity and depth affect both fresh and saline SGD. We show that aquaculture is a previously unrecognized mechanism of salinization affecting coastal aquifer vulnerability and SGD. A regional graphical information system analysis shows transformation into aquacultural ponds could introduce considerable SGD variability spatially and temporally. These findings will enable coastal managers to better evaluate the salinization of groundwater, as well as the chemical composition and solute loads delivered by SGD.