Louis H. Motz

Representing the Coastal Boundary Condition in Regional Groundwater Flow Models

Numerical experiments were performed to investigate how the coastal boundary condition could be approximated in a groundwater flow model to yield accurate values for hydraulic heads and fluxes in the freshwater part of a coastal aquifer. These experiments consisted of obtaining steady-state solutions for hydraulic heads in a vertical cross section using the groundwater flow code MODFLOW and comparing the results to a steady-state solution in a similar cross section for equivalent freshwater heads obtained using the variable-density flow and transport code SEAWAT, which was considered to be the accurate solution to the problem. Six different boundary conditions that have been used to approximate the coastal boundary in groundwater flow models were tested, and simulations were run for both specified-flux and specified-head boundary conditions at the upstream freshwater boundary. For both upstream boundary conditions, the MODFLOW solution that best matched the SEAWAT solution for hydraulic heads and fluxes in the freshwater part of the aquifer was the solution in which equivalent freshwater heads were specified over the full thickness of the aquifer at the coastal boundary.

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

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.

Designing a Groundwater-Level Monitoring Network Using Geostatistics: A Case Study for South and Central Florida, USA

A groundwater-level monitoring network was designed for the Upper Floridan aquifer in southern Florida, U.S.A., within the boundaries of the South Florida Water Management District. The objective of the investigation was to design a groundwater monitoring network for the Upper Floridan aquifer that recommends the number and locations of monitoring wells that will provide equivalent or better quality data compared to the existing monitoring network. This was accomplished by designing a spatial distribution of wells that will improve the accuracy of groundwater-level data measured over time and reduce data estimation errors that occur when spatially interpolating between wells in the Upper Floridan aquifer. Statistical and geostatistical analyses were performed on groundwater-level data, groundwater levels were estimated in unmeasured areas by interpolation, and proposed monitoring well locations were delineated based on acceptable levels of error for groundwater levels between wells and in areas where no wells are located. Semivariograms, which illustrate spatial autocorrelation, were plotted, and a potentiometric surface map representing salinity-adjusted mean heads in the Upper Floridan aquifer based on the current monitoring network was constructed using ordinary kriging to interpolate between wells. Three hexagonal grid network designs with different cell diameters were evaluated to determine the number and locations of proposed monitoring wells along with the resulting errors that would be associated with the different proposed design networks. A hexagonal grid with wells spaced at 29,261 m was considered to be the most practical alternative based on the costs associated with installing new wells. The recommended optimum monitoring network consists of 58 new wells added to 44 existing wells, which would result in a network of 102 wells that has a prediction standard error with a mean of 1.45 m and a range from 0.07 m to 2.62 m.