Saltwater Intrusion in Coastal Aquifers I
Presiding: T Van Biersel, Louisiana Geological Survey; D Carlson, Louisiana Geological Survey
H13C-01 13:30h
Application of a Density-Dependent Numerical Model (MODHMS) to Assess Salinity Intrusion in the Biscayne Aquifer, North Miami-Dade County, Florida
Miami-Dade County is located at the Southeastern part of the State of Florida adjoining the Atlantic coast. The sole drinking water source is the Biscayne Aquifer, which is an unconfined freshwater aquifer, composed of marine limestone with intermediate sand lenses. The aquifer is highly conductive with hydraulic conductivity values ranging from 1,000 ft/day to over 100,000 ft/day in some areas. Saltwater intrusion from the coast is an immediate threat to the freshwater resources of the County. Therefore, a multilayer density-dependent transient groundwater model was developed to evaluate the saltwater intrusion characteristics of the system. The model was developed using MODHMS, a finite difference, fully coupled groundwater and surface water flow and transport model. The buoyancy term is included in the equation for unconfined flow and the flow and transport equations are coupled using an iterative scheme. The transport equation was solved using an adaptive implicit total variation diminishing (TVD) scheme and anisotropy of dispersivity was included for longitudinal, transverse, vertical transverse, and vertical longitudinal directions. The model eastern boundaries extended approximately 3.5 miles into the Atlantic Ocean while the western boundary extended approximately 27 miles inland from the coast. The northern and southern boundaries extend 6 miles into Broward County and up to the C-100 canal in Miami-Dade County respectively. Close to 2 million active nodes were simulated, with horizontal discretization of 500 feet. A total of nine different statistical analyses were conducted with observed and simulated hydraulic heads. The analysis indicates that the model simulated hydraulic heads matched closely with the observed heads across the model domain. In general, the model reasonably simulated the inland extent of saltwater intrusion within the aquifer, and matched relatively well with limited observed chloride data from monitoring wells along the coast. Saltwater intrusion in the Biscayne aquifer is the result of a combination of natural variations in recharge, evapotranspiration, ground-water withdrawals from the aquifer, and relative canal stages in comparison to tidal stages in the Biscayne Bay. Pre- and post wellfield pumpage scenarios were conducted to determine the effect of wellfield pumpage on saltwater intrusion. The model is being used for future water supply management scenarios and for evaluating the best placement of saltwater intrusion monitoring wells along the coast.
H13C-02 13:45h
Saltwater Sources for Saltwater Intrusion in Shallow Aquifers in South Louisiana
Shallow southward-dipping Quaternary and Upper Neogene aquifers across South Louisiana are used as sources of freshwater. Local accumulations of brackish waters occur within these aquifers on the north shore of Lake Pontchartrain, in the Baton Rouge area, in Southwest and South-central Louisiana, and in the New Orleans area. The low Br/Cl ratios, high Na/Cl ratios, low K/Cl ratios and low 87Sr/86Sr ratios in these brackish waters form linear mixing trends which are inconsistent with the saline-water sources being in situ marine formation fluids of similar age. Extrapolation of these ratios to fluids of high salinity suggests the saline-water sources are Lower Neogene or Paleogene marine formation fluids that dissolved halite. South Louisiana overlies the deep Jurassic Louan salt, which is the source of salt diapirs. The region is cut by a series of east-west trending gravity faults with southward-dipping fault planes. Deeper migrating formation fluids are hypothesized to be dissolving halite through contact with salt diapirs and moving up fault planes to enter shallow aquifers and mix with the in situ groundwaters. High freshwater withdrawal rates are accelerating this process, causing updip movement of the shallow saltwater fronts, e.g., within the Pliocene and Upper Miocene Baton Rouge aquifers and in the Pleistocene Lake Charles "500-foot" sand. Conversely, the absence of significant freshwater withdrawal has apparently caused a static saltwater front, e.g., in the Pliocene Big Branch Aquifer on the north shore of Lake Pontchartrain.
H13C-03 14:00h
Resistivity Surveys of Seasonal Groundwater Salinity Variations Along Forest-Marsh Transects at North Inlet, South Carolina.
Salinity boundary changes each season in a saltwater marsh will help determine if rising and falling tides over a one-year period are threatening groundwater. Monthly resistivity measurements of Crabhaul Creek, a finger marsh basin in North Inlet, South Carolina, were made using Super MiniRes with 530 Vpp at 10 mA. Previous studies show a salinity gradient that disappears and returns to the same area in a one-year period. The goal of this project was to measure the seasonal changes of the salinity in this basin. Three transects across Crabhaul creek are used to form a 2-D interpretation of the salinity variations along the creek and their changes over two seasons, winter and spring. All transects cross the same zones of saline tolerant vegetation. One transect crosses the basin and Crabhaul creek, a second transect upstream extends through the basin where there exists a gradient but there is no definition of the tidal creek, and a third transect 100 meters between the two. A gradient array with a target depth of 3 meters and M-N spacing of 1 meter was used to profile each transect at the end of the month. A dipole-dipole array with A-B spacing of one meter and a target depth of three meters was done every 2-3 months to image the subsurface for any inconsistencies occurring seasonally aside from the salinity gradient. A finite difference inversion of each dipole-dipole survey was made with DCIP2D Version 3.2 developed by UBC - Geophysical Inversion Facility. The first meter of salinity variations is dominated by evapotranspiration and surface water however below this boundary there is a distinct decrease of resistivity from 10 ohm-meters to 2 ohm-meters on the eastern side of each transect corresponding to an increase in salinity. The resistivity remains constant from the upland forest to the tidal creek consistent with fresh water upwelling from the forest. The resistivity drops at the creek and remains low eastward through the basin and into the forested barrier island. Seasonal movement of this gradient will help determine seasonal groundwater flux for this forest-marsh boundary.
H13C-04 14:15h
Movement of Saltwater in the "2,000-Foot" Sand of the Baton Rouge Area, Louisiana
Ground-water withdrawals in southeastern Louisiana have caused saltwater to encroach into some freshwater aquifers. The most heavily pumped area, East Baton Rouge Parish, includes the city of Baton Rouge and surrounding areas. The Baton Rouge aquifer system includes 10 extensive aquifers that were pumped about 150 Mgal/d (million gallons per day) in 2003. Ground-water investigations in the 1960's delineated a freshwater-saltwater interface located at the Baton Rouge fault. Test drilling and water well data collected near the fault indicate that the fault is associated with major discontinuities in water levels and water quality. Generally, aquifers south of the fault contain saltwater and aquifers north of the fault contain freshwater with chloride concentrations less than 10 mg/L (milligrams per liter). Saltwater encroachment into freshwater areas north of the fault has been monitored and delineated with well networks in several aquifers. Saltwater was initially detected as early as the 1960's; by the 1990's saltwater had been detected in six of the aquifers north of the fault, including the "2,000-ft" sand. Northward encroachment of saltwater across the fault in the "2,000-ft" sand in East Baton Rouge Parish is in response to large water withdrawals from the aquifer totaling about 22.7 Mgal/d in the Parish in 2002. Encroachment of saltwater has been monitored by a system of wells. The approximate location of the freshwater-saltwater interface north of the fault was delineated for the years 1966, 1977, and 1992. At present, wells in the "2,000-ft" sand at two public-supply well fields located north of the fault are producing water with chloride concentrations approaching 200 mg/L. Wells at these fields may help protect the aquifer from northward advancement of saltwater by intercepting and discharging saltwater. In 2002, about 0.8 Mgal/d from the aquifer was withdrawn by the impacted wells. If withdrawals from the impacted wells were discontinued, saltwater could advance more rapidly toward the industrial district, unless remediation steps were taken to reduce northward encroachment. The resulting saltwater contamination in wells underlying the industrial district, located about 3 miles north of the fault, could affect approximately 70 percent of the withdrawals from the "2,000-ft" sand in East Baton Rouge Parish.
H13C-05 14:30h
Using Electromagnetic Soundings to Determine Fresh and Saline Water Relationships on Barbados, Antigua, and Socorro Islands
Seawater intrusion can significantly limit the amount of potable groundwater available to coastal communities. Management of the water resources available to such communities is increasingly being studied using an interdisciplinary scientific approach that includes geophysical methods, existing and new well information, and conceptual hydrogeological models as input for complex groundwater modeling. Calibration and validation of groundwater flow models depend on an accurate knowledge of water table elevations, the nature of the fresh - saline water interface at depth, and the spatial variation of hydrologic parameters and variables (including permeability, porosity, storativity, and concentration). Determining the spatial variability of hydrologic variables and parameters sufficient to support detailed hydrologic modeling (including conceptual model development, calibration, and validation) can be achieved though non-invasive geophysical methods at a fraction of the cost associated with more traditional methods, such as drilling. In this study, electromagnetic soundings are used to provide basic information to support an analysis of the density-driven interaction between fresh and saline porewaters on the Caribbean islands of Barbados and Antigua, and Socorro Island in the Pacific. Studies in Barbados and Antigua represent coastal aquifers affected by pumping, and hence modification of the location and widths of the fresh/seawater interface. Data from Socorro Island represents a natural interface, unaffected by anthropogenic modification. Electromagnetic sounding-derived data are shown to provide significant basic information that supports some or all of the following (i) choice of conceptual models of flow, (ii) characterization of the geologic and hydrolgeologic environments, and (iii) model calibration and validation. For example, on Barbados, data from the soundings demonstrated the existance of a diffused freshwater/seawater interface and the presence of karst conduits. On Antigua, the soundings demonstrated that seawater intrusion was focused along stream valleys and was more than likely exacerbated by pumping inland and adjacent to the valleys.
H13C-06 14:45h
Ground Water Salinization by Tsunami Event of Coastal Aquifer, India
Consequences of the December 26th, 2004 tsunami event on ground water quality are investigated. Two coastal aquifers, part of HELP (UNESCO) program are monitoring. They are modeling to: i) better understand processes which increased salinity up to 20,000 æS/cm in some locations in the shallow dune aquifer; ii) quantify the remediation time to flush out salinity under natural flow condition; iii) evaluate the risk of contamination to the underlying one, Cuddalore.