ELECTRICAL AND ELECTRICMAGNETIC METHODS (FREQUENCY DOMAIN) APPLIED TO THE IDENTIFICATION OF SALINE DISPERSION ZONE IN PLAYA PANAMA COSTAL AQUIFER, COSTA RICA

Abstract

In Costa Rica, only a few coastal aquifers have been studied with the employment of geophysical methods.No scientific investigation which uses simultaneously two different methods to analyze the hydrogeologic properties ofcoastal aquifers, particularly to define salt intrusion has been done before in the area of study.The aquifer of Playa Panama,localized on the Pacific Coast, in the northwest part of the country (picture 1), has a moderate hydric potential, to be usedfor tourist purposes. The Average monthly rainfall in the region is 1944 mm/year, the potencial evapotranpiration is of 1930mm/year (table 1). The aquifer is composed of basaltic angular conglomerates, mainly eroded from the Nicoya Complex.The aquifer geometry was analyzed through the application of Vertical Electrical Soundings (VESs) using a Schlumbergerarray, and conductivity measurement in the frequency domain. The results were complemented with stratigraphic,hydrogeological and hydrochemical data and suggest a decreasing of the aquifer’s thickness towards the sea, and a nonpolluted condition. The saline dispersion zone was also studied with both techniques, together with an electric sectionobtained with Wenner array. Both methods showed a high sensitivity in the area corresponding to the salt intrusion (pictures8,9), which occurs at a depth of 15 m in the sediments and at a distance not exceeding 65 m landward from the high tideline.