INFLUENCE OF BATHYMETRIC CHANGES ON HYDRODYNAMICS AND SALT INTRUSION IN ESTUARINE SYSTEM1

ABSTRACT: A vertical (laterally integrated) two‐dimensional numerical model has been applied to study the hydrodynamic characteristics and salt water intrusion in the Tanshui River estuarine system. The cross‐sectional profiles measured in 1978 and 1994 are schematized for model simulations. Detailed model calibration and verification have been conducted with water surface elevations, tidal current, salinity distributions, and residual velocities measured. The overall performance of the model is in qualitative agreement with the available field data. The model was then used to study how hydrodynamics and salt water intrusion change in response to changes in bathymetry. The model simulations indicate that more tidal energy propagates into the estuarine system in 1994 because of the substantial increase in river cross‐sections. The limits of salt intrusion in 1994 extended farther inland than those in 1978. On the other hand, the extent of mangrove wetland in the lower estuary has increased over the past 20 years and is likely a result of the increased salinity in the estuary.     

Water quality impacts on sorbent efficacy for per- and polyfluoroalkyl substances treatment of groundwater

Per- and polyfluoroalkyl substances (PFAS) are a large group of synthetic compounds that have emerged as chemicals of concern in drinking water and groundwater. Typically, such waters are treated to remove PFAS by passing the water through a bed of sorbent material (e.g., activated carbon and anion exchange resins [AIX]). However, the efficacy of these sorbents varies depending on the types and concentrations of PFAS, in addition to water quality conditions such as organic matter content and conductivity (ionic strength). The choice of sorbent material to effectively treat PFAS in complex natural waters will, therefore, depend upon site water quality and PFAS conditions. To help inform these decisions, a series of evaluations using a rapid small-scale column test approach was conducted with two sorbent materials (a granulated activated carbon [GAC] and an AIX), individually and combined, under conditions where conductivity, pH, and organic carbon concentrations were varied in a semifactorial approach. Artificial groundwater batches were prepared to meet these test conditions and spiked with six PFAS compounds (perfluorobutane sulfonic acid [PFBS], perfluorobutanoic acid [PFBA], perfluorohexane sulfonic acid [PFHxS], perfluorohexanoic acid [PFHxA], perfluorooctane sulfonic acid [PFOS], and perfluorooctanoic acid [PFOA]), passed through small columns packed with ground sorbent material for ∼30,000 bed volumes of water for single sorbent treatments and ∼20,000 bed volumes for combined sorbent treatments, during which samples of effluent were captured and analyzed to quantify breakthrough of PFAS from the sorbent materials over time. AIX was found to be more effective than GAC at removing the tested perfluoroalkyl sulfonic acids (PFBS, PFHxS, and PFOS), but GAC was similarly or more effective than AIX at removing perfluorocarboxylic acids (PFBA, PFHxA, and PFOA) under high conductivity conditions. Overall, the efficacy of AIX at removing PFAS was more strongly impacted by organic carbon and conductivity than GAC, while pH had less of an effect on either sorbent's efficacy compared to the other test conditions.