HR: 0830h
AN: H21A-01    [Abstracts]
TI: Application of Geophysical Surveying Techniques to Hydrologic Analyses on Antigua, West Indies
AU: * Farrell, D A
EM: dfarrell@swri.org
AF: CNWRA, Southwest Research Institute, 6220 Culebra Rd, San Antonio, TX 78238 United States
AU: Sandberg, S K
AF: Geophysical Solutions Inc., P.O. Box 50248, Albuquerque, NM 87181-0248 United States
AU: Barnes, H
AF: University of the West Indies, Mona, Kingston, Jamaica
AU: Sutherland, A
AF: University of the West Indies, Mona, Kingston, Jamaica
AU: Browne, J
AF: Antigua Public Utilities Authority, Casada Gardens, St. John's, Antigua and Barbuda
AU: Humphrey, J
AF: Caribbean Institute for Meteorology & Hydrology, Husbands, St. James, Barbados
AU: Mayers, B
AF: University of the West Indies, Cave Hill, St. Michael, Barbados
AB: 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).
DE: 0684 Transient and time domain
DE: 1803 Anthropogenic effects
DE: 1884 Water supply
SC: Hydrology [H]
MN: 2005 Joint Assembly