Saltwater Intrusion in Coastal Aquifers II Posters
Presiding: T Van Biersel, Louisiana Geological Survey; D Carlson, Louisiana Geological Survey
H21A-01 0830h
Application of Geophysical Surveying Techniques to Hydrologic Analyses on Antigua, West Indies
Groundwater with high total dissolved solids (TDS) is commonly observed in wells drilled into limestone aquifers located along the northern portion of the island of Antigua. High TDS generally limits the direct use of this ground water for human consumption. Less stringent limits are placed on the use of the water for agricultural purposes. One source of the high TDS is seawater intrusion which is enhanced by groundwater pumping. To improve the quality of pumped groundwater, Antigua is currently updating its groundwater management program. As part of this update, authorities on the island are revising the conceptual models that describe groundwater flow and seawater intrusion in the limestone aquifers on the island and their respective relationships to the high TDS observed in groundwaters. Geophysical surveys were proposed, along with improved groundwater sampling and monitoring, to support revisions of the conceptual model of groundwater flow and subsequent groundwater modeling. The geophysical surveys were considered important for mapping the geology and hydrology of the region and locating the freshwater-seawater interface. Geophysical fieldwork in the northeast portion of the island of Antigua was performed during two separate field campaigns; one conducted in 2003 and a second conducted in 2004. Geophysical methods used in the fieldwork included time-domain electromagnetic (TEM) central-loop soundings, Schlumberger-array resistivity soundings, EM-31 terrain conductivity profiling, and multi-electrode resistivity profiling using a dipole-dipole configuration. Regional surveys using the TEM method were designed to map large-scale geology and hydrology (including variations in water quality). Focused surveys in localized coastal areas using a variety of techniques (e.g., multi-electrode resistivity profiling, TEM, and EM-31 terrain conductivity mapping) were used to map areas where large-scale mapping indicated a strong likelihood of seawater intrusion. Analyses to date indicate that focused areas of seawater intrusion occur along coastal inlets where low-lying stream valleys coupled with tidal fluctuations and storm surges provide suitable pathways for overland flow and subsequent infiltration of seawater. Adjacent to these stream valleys, but further inland, extensive pumping occurs. This pumping is currently believed to exacerbate the inland migration of seawater along the stream valleys thereby increasing the TDS of pumped water. The data also show the subsurface to be quite electrically conductive (electrical resistivity generally less than 25 ohm-m). This low subsurface conductivity is consistent with water samples across the aquifer that show high TDS and alkalinity. This suggests in-part that dissolution of limestone may be widespread. Estimated electrical conductivities for the marine clay layers that underlay the limestone units are also found to quite low indicating that these units are saturated with depositional formation waters (i.e., seawater).
H21A-02 0830h
Sustainability Analysis of the Water Resources and Supply of the Vieux Fort Region of Saint Lucia
In the Vieux Fort region of the Caribbean island of St. Lucia, water needs are becoming acute. The water supply shortfalls during the dry season will continue to grow as population and development increase, unless action is taken. Actions to address the problem should include measures to optimize the present water delivery system and the development of a new supply, through new intakes, groundwater, or reservoir construction. An investigation into the potential for groundwater resources using electrical resistivity soundings indicated a likely pervasive, shallow aquitard of clay materials below the water table; the shallowness of this aquitard virtually precludes the existence of productive perched aquifers. Consequently, a model of Grande Riviere du Vieux Fort (Big Vieux Fort River) seasonal surface-water flow was developed, based on a digital elevation model and rainfall data, allowing us to analyze the possible productivity of any new intakes placed along the river. A specific site downstream of the present intake was recommended for potential development. Recommendations were given for short, medium and long-term development of the resources and supply of the Vieux Fort region of southern St. Lucia.
H21A-03 0830h
2-D and 3-D Visualization of the Freshwater/Saltwater Mixing Front, and Zones of Preferential Groundwater Flow in the Karst Biscayne Coastal Aquifer using Electromagnetic Induction Techniques, Miami, Southeastern Florida.
The Biscayne aquifer is unconfined, composed primarily of Karst limestone, and underlies all of Miami-Dade County and much of Biscayne Bay in southeastern Florida. It is the sole source of drinking water for the 3 million inhabitants of the city of Miami and Miami-Dade County, as well as portions of Broward and Monroe Counties. Saltwater intrusion is a prominent problem for all coastal aquifers including the Biscayne aquifer. Simple and quick detection of the three-dimensional saltwater/freshwater interface has been problematic without the use of extensive sounding surveys or multiple well sampling. We are developing a technique combining rapid EM-31 surface surveys with EM-31 vertical soundings to model the depth to the saltwater/freshwater front at two sites located within a half mile of Biscayne Bay. The EM-31 has a maximum signal penetration of about 25ft allowing for accurate near shore surveys. Depths to the saltwater have ranged from over 25 ft inland to less than 2-3 ft near the Bay and saltwater mangroves. Changes in conductivity along survey lines of equal elevation that are equidistant from the Bay may indicate zones of preferential flow due to conduit networks or the presence of backfill, both of which exacerbate saltwater intrusion. All surveys show a rapid change from fresh to brackish water as you move toward the Bay indicating a shallow and abrupt mixing zone. Using a simple depth-modeling program, a wire frame contour map of the mixing zone can be constructed. This technique has proven to be a quick, inexpensive method for first-order hydrogeological and spatial analysis of the saltwater/freshwater interface. In an allied study we are using down-hole electromagnetic induction techniques with an EM-39 tool on existing wells, analyzing fluctuations in conductivity within the saltwater zone to look for zones of high permeability in the aquifer. Conductivity fluctuates within the mixing zone from brackish values to values equivalent to Biscayne Bay surface water. These conductivity changes may indicate areas of preferential flow into and out of the aquifer.