Ground Penetrating Radar and Electrical Resistivity to Delineate Volcaniclastic Aquifers, La Laguneta, El Salvador

The village of La Laguneta in south-central El Salvador relies solely on groundwater for its freshwater needs. La Laguneta is located in a ca. 0.5 km2 basin on the flank of the inactive volcano El Vicente. The basin's principal aquifer consists of Quaternary siliceous pyroclastics and subordinate epiclastic volcanics underlain by a bedrock unit of Tertiary epiclastic volcanics and pyroclastics with intercalations of basic to intermediate lavas. Water depths in wells in the basin range from 10 to 25 m, with poor yields (<4 L/min) during the dry season when groundwater levels decline by as much as 10 m. Ground penetrating radar (GPR) and electrical resistivity (ER) were collected to determine the thickness of the principal aquifer unit across the basin. The 50 MHz GPR survey consisted of 11 2-D transects with 2 m trace spacing. The ER dipole-dipole survey consisted of 5 2-D transects with 5 m electrode spacing. Four ER transects coincided with GPR transect locations. Two wells, located adjacent to 3 GPR transects and 2 ER transects, provided ground truth. The GPR data provided good images to depths of 10 m at ca. 2 m resolution, revealing sedimentary onlap and fluvial cut and fill structures. This is underlain by a less-reflective and/or noisy interval, with some stronger horizontal reflectors visible to >30 m, that is interpreted as a volcanic/volcaniclastic aquitard beneath the fluvial sequence. A bedrock surface contour map based on the GPR data indicates that the present-day basin is longitudinally divided into two subsurface, partially-closed sub-basins with depths of ca. 13 m. The ER results are less conclusive but generally confirm the GPR interpretations. Many of the wells in the basin were drilled into the basement high between the two sub-basins explaining their low productivity. A well located in the northeast sub-basin documents the presence of a shallow water table at 4 m below ground surface. Assuming a horizontal water table and low seepage rate into the underlying volcanics, the center of each sub-basin should have the greatest saturated interval and best potential for a productive well.