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