Geophysical Methods Applied to Saltwater Intrusion in Antigua

The island of Antigua has been suffering from an acute shortage of potable water due largely to drought conditions on the island. Because of the persistent drought conditions one of the island's primary sources of potable water, the Potworks Dam, is dry. In addition to the Potworks Dam, the nearby smaller Collins Dam is also dry. In addition to these dry surface impoundments, the problem is further complicated by the fact that the reduced recharge has resulted in a lowering of the watertable in the nearby Collins abstraction region. As a consequence, salinity levels in abstraction wells at Collins have increased. In extreme cases, the watertable has declined below the depth of the abstraction pump. A geophysical survey was conceived to generate information related to the hydrogeologic framework of the area, while taking advantage of the simplified logistics resulting from the dry conditions of Potworks reservoir. During the period from 14 April to 21 April, 2003, geophysical data were acquired in the dry bed of the reservoir, and at two locations east toward the coastline. Geophysical methods included EM-31, Schlumberger array resistivity soundings, and TEM central-loop soundings. Geophysical results show lateral variation in subsurface resistivity related to both lithologic variation and groundwater salinity. Shallow resistivity variation, mapped with the EM-31 system, is indicative of a channel system within the bed of the Potworks reservoir. Variation in resistivity mapped using the TEM and resistivity soundings likely indicates structure of the Karst aquifer, which has variable resistivity due to porosity/salinity variation.