Alluvial aquifer thickness and bedrock structure delineation by electromagnetic methods in the highlands of Bolivia

Abstract

The porous aquifers in the area called Challapampa are the most important groundwater reservoirs that supply drinking water to Oruro city in the highlands of Bolivia. They consist of unconsolidated fluvial–lacustrine deposits, resting on a complex sedimentary bedrock and covered by a thin surficial clay layer. The settings of these geological units and the structures governing the flow patterns have barely been investigated, despite this reservoir having been utilized during the last 50 years. This study applied transient electromagnetic (TEM) soundings and electrical resistivity tomography (ERT) in the middle part of the alluvial fan of River Paria to investigate the thickness of the porous aquifer and detect the relief of the bedrock. Likewise, some results expressed as resistivity models indicate the possible existence of geological structures below the unconsolidated sediments. The average depth of investigation reached in this study is between 200 and 250 m below the surface, for both the applied methods. The geological structures inferred have similar directions as the major faults in the vicinity, from southeast to northwest, which in turn are assumed as part of fractured aquifers underlying the porous aquifers. The geo-electrical techniques were successfully tested in the study area and the resistivity models from TEM complement very well those obtained from ERT. Therefore, extended investigations using the same techniques would help to develop a more complete description of the hydrogeological settings of the aquifer system.