Increased Total Dissolved Solids, Chloride, and Bromide Concentrations Due to Sea-Level Rise in a Coastal Aquifer

Abstract

Rising sea levels can increase saltwater intrusion in coastal aquifers, impacting well fields by contaminating groundwater with increased total dissolved solids (TDS) and chloride concentrations. A groundwater model was created for Broward County in southeastern Florida, U.S.A., to simulate the increased TDS and chloride concentrations in a coastal well field due to sea-level rise (SLR)-induced saltwater intrusion. The objectives of the modeling were to simulate the increase in TDS and chloride concentrations in a well field for a range of SLR scenarios and quantify the results with respect to Secondary Maximum Contaminant Levels (SMCL’s) for TDS and chloride. Bromide concentrations were also simulated because bromide can form toxic disinfection byproducts (DBP’s) during drinking water treatment. SLR projections for the model were based on projections that follow the Intergovernmental Panel on Climate Change methodology in its Fourth Assessment Report, but they also include the effects of ice sheet melting in Greenland and Antarctica. These projections provide for three scenarios of SLR from 1990 to 2100, corresponding to 5%, 50% and 95% confidence levels. These estimates were extrapolated as part of this investigation to obtain projections of 0.11 m, 0.49 m, and 0.91 m SLR for three 100-year simulations from 2015 to 2115. A three-dimensional numerical groundwater model was constructed using the variable-density groundwater flow and transport code SEAWAT, and simulations were run for three 100-year transient simulations with maximum sea-level rise values at the coastal boundaries corresponding to the 5%, 50% and 95% confidence-level sea-level rise projections. Average TDS concentrations in ten production wells were obtained from the SEAWAT results, and chloride and bromide concentrations were calculated using standard seawater ratios for chloride and bromide relative to TDS. The bromide concentrations were used to model the concentrations of four trihalomethane species (THM4) that represent DPB’s that could be formed following chlorine addition during drinking water treatment. The results from the simulations indicate that the SMCL’s for TDS and chloride, which are based on cosmetic and aesthetic effects, will be exceeded in approximately 65 years from the start of the SLR simulations at the 95% confidence level for SLR. Of even greater significance, the results also indicate that the primary maximum contaminant level for THM4, which is based on health effects, will be exceeded in approximately 30 years from the start of the SLR simulations at the 95% confidence level for SLR.