Using Reactive Transport Model to simulate a hydrothermal system in the Pocuro Fault Zone (Central Chile)

Abstract

Groundwater from detritic aquifers is used intensively for agriculture in the Aconcagua basin, Central Chile. The area is well known for its water scarcity and since 2010 precipitation decreased generating serious problems concerning the water uses. The intensive pumping from boreholes located in the upper basin generated a drop in piezometric levels greater than 4 m. These strong changes occur in the lower part of the upper basin where the recharge could occur. In this area, the rocks are fractured around to a regional scale master fault (Pocuro fault). In addition, 10 cold springs and 8 thermal springs outflow at different elevation spatially related to the fault. This hydrogeological assessment point to the relevance of asses and understand the recharge processes and the role of the faults in the groundwater flow of the Aconcagua basin.During winter 2016 41 water samples from springs, boreholes, river and rain were collected. To assess the hydrogeochemical processes major, minor and trace elements were analyzed at CEGA labs. Stable isotopic data where obtained at ZAIDIN lab. Data have been interpreted using classical hydrogeochemical and statistical tools. The electrical conductivity (EC) range between 31 µ/cm to 714 µ/cm. The measured pH is between 3,86 to 9,62. The waters types are mainly HCO3-Ca and SO4-Ca. The HCO3-Ca waters have neutral to slightly alkaline pH while the SO4-Ca waters have the lowest pH values (<6). The data indicates that the main hydrogeochemical process is the water-rock interaction that gives most of the ionic concentration observed.Isotopic data show that all samples from springs fall on the Local Meteoric Water Line, suggesting a short residence time and/or shallow circulation depth. The study of structural and geometrical faults assessment is a key tool to determine the most probable pathways of groundwater and to define the presence of fractured aquifer.