Geophysical Characterization of Seawater Intrusion In Antigua, West Indies

Abstract

The Inter-American Agency for Cooperation and Development is currently funding a project focused on groundwater management in coastal karst environments in the Caribbean. The islands of Jamaica, Antigua, and Barbados are currently the primary regional participants in this project. Geophysical characterization of pilot sites on each of these islands is being used to provide input into conceptual groundwater flow models, which in turn will be used to support the development of sustainable groundwater management strategies for these coastal aquifers. Geophysical fieldwork at a pilot site located in the northeast portion of the island of Antigua took place in April 2004. Data from the survey support mapping the geology and hydrology of the region and locating the freshwater-seawater interface. 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. In-field analysis of data collected at a regional scale led to a conceptual understanding of seawater intrusion into the study area. This conceptual understanding led to concentrated data collection efforts in specific areas. The first area surveyed in-depth to confirm our conceptual understanding of seawater intrusion at the site was Mercers Creek channel that runs from west of the Collins and Potworks reservoirs to Mercers Creek Bay located along the north coast of the island. TEM soundings, assembled into an eastwest profile across the channel, reveal a conductive region, thought to represent seawater migrating inland in response to the reduced freshwater head caused by pumping at the Collins well field. The second area surveyed in-depth was the Ayres Creek channel that runs by the Collins and Bristol well fields before draining into the extensive tidal flat at Nonsuch Bay located along the northeast coast of the island. This channel contains a secondary dam that captures outflow from the Potworks and Collins dams during periods of high rainfall and flooding, but is otherwise dry. This area was studied in detail using a high-density geophysical data collection strategy. Subsequent data interpretation led to the identification of a conductive zone that appears to be the result of seawater intrusion. The extent of the intrusion in the vicinity of the channel is controlled by the groundwater pumping at the Collins and Bristol well fields and storm surge/tidal fluctuations. This conceptual model has groundwater management implications that are being evaluated using groundwater flow models.