[1] Measurements of submarine groundwater discharge (SGD) in coastal areas often show that the saltwater discharge component is substantially greater than the freshwater discharge. Several mechanisms have been proposed to explain these high saltwater discharge values, including saltwater circulation driven by wave and tidal pumping, wave and tidal setup in intertidal areas, currents over bedforms, and density gradients resulting from mixing along the freshwater-saltwater interface. In this study, a new mechanism for saltwater circulation and discharge is proposed and evaluated. The process results from interaction between bedform topography and buoyancy forces, even without flow or current over the bedform. In this mechanism, an inverted salinity (and density) profile in the presence of both a bedform on the seafloor and an upward flow of fresher groundwater from depth induces a downward flow of saline pore water under the troughs and upward flow under the adjacent crest of the bedform. The magnitude and occurrence of the mechanism were tested using numerical methods. The results indicate that this mechanism could drive seawater circulation under a limited range of conditions and contribute 20%-30% of local SGD when and where the process is operative. Bedform shape, hydraulic conductivity, hydraulic head, and salinity at depth in the porous media, aquifer thickness, effective porosity, and hydrodynamic dispersion are among the factors that control the occurrence and magnitude of the circulation of seawater by this mechanism.
Rapid infiltration basin systems (RIBS) are used for the application of treated wastewater to soil for wastewater disposal. To ensure sufficient additional wastewater treatment, U.S. regulations require a minimum separation distance between the infiltration basin and groundwater. Analytical and numerical models that predict groundwater mounding beneath basins have assumed a uniform specified flux boundary condition across the basin. In many systems, however, the basins are only partially flooded, with overland flow and soil infiltration controlling the extent of basin inundation. The iTOUGH2 computer code was modified to describe the coupled surface–subsurface flow in RIBS. After testing the model with published laboratory and field data, simulations were used to estimate groundwater mounding beneath RIBS for four hydraulic loading rates and two flooding periods in two representative soils. Because of interest in nitrate (NO3−) removal beneath RIBS, a simplified approach using a domain‐average denitrification reduction factor, Fs, was used to assess the impact of pore water saturation on denitrification. Simulations using the conventional specified flux boundary condition underpredicted groundwater mounding by as much as a factor of 25 in loamy sand and a factor of 6 in sand. The impact of the basin boundary condition on Fs was less significant, with Fs reduced by up to 50% if the specified flux boundary condition was used. Thus, ignoring overland flow underpredicts denitrification and groundwater mounding for the cases studied here. For a fixed amount of wastewater discharged during a weekly flooding–drying cycle, simulations indicate that longer flooding periods result in less groundwater mounding but a reduction in denitrification.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.