Images of a groundwater flow system in Nicaragua

Conceptualization of groundwater flow systems is necessary for water resources planning. Geophysical, hydrochemical and isotopic characterization methods were used to investigate the groundwater flow system of a multi-layer fractured sedimentary aquifer along the coastline in Southwestern Nicaragua. A geologic survey was performed along the 46 km2 catchment. Electrical resistivity tomography (ERT) was applied along a 4.4 km transect parallel to the main river channel to identify fractures and determine aquifer geometry. Additionally, three cross sections in the lower catchment and two in hillslopes of the upper part of the catchment were surveyed using ERT. Stable water isotopes, chloride and silica were analyzed for springs, river, wells and piezometers samples during the dry and wet season of 2012. Indication of moisture recycling was found although the identification of the source areas needs further investigation. The upper-middle catchment area is formed by fractured shale/limestone on top of compact sandstone. The lower catchment area is comprised of an alluvial unit of about 15 m thickness overlaying a fractured shale unit. Two major groundwater flow systems were identified: one deep in the shale unit, recharged in the upper-middle catchment area; and one shallow, flowing in the alluvium unit and recharged locally in the lower catchment area. Recharged precipitation displaces older groundwater along the catchment, in a piston flow mechanism. Geophysical methods in combination with hydrochemical and isotopic tracers provide information over different scales and resolutions, which allow an integrated analysis of groundwater flow systems. This approach provides integrated surface and subsurface information where remoteness, accessibility, and costs prohibit installation of groundwater monitoring networks.