Geological, hydrogeological, and geothermal factors associated to the origin of arsenic, fluoride, and groundwater temperature in a volcanic environment “El Bajío Guanajuatense”, Mexico

Abstract

Arsenic and fluoride groundwater concentrations over national standards for drinking water were measured in the regional aquifer of Juventino Rosas, Guanajuato State, Central Mexico. Also anomalous temperature occurs in groundwater wells of the area. Concentrations of total dissolved solids, silica, and chloride are too low to indicate a geothermal heat source. Additionally, isotopic evidence indicates that groundwater from the studied wells is subject to an evaporation process affected by the humid weather of the zone. The chemical characteristics of the water indicate a deep circulation warm water system in normal geothermal gradient. The warm waters of Juventino Rosas are mainly of three types: Water type I: (Na–HCO3), represented by the highest temperature wells and presence of fluoride and arsenic; water type II (Na–Ca–HCO3) that represents a mixing process between water types I and III. In this group, the sample 13JR contained high concentration of F−; water type III (Ca–HCO3), represented only by one sample (Cen 2) located over the outcrop of shales, limestones, and metamorphic rocks. This sample contains the highest concentrations of sulfate, manganese, and iron. All the geological and geochemical evidences indicate that rhyolite units are the most probable source of As and F−. The area corresponds to a low-temperature and low-enthalpy system and not to a well defined geothermal system.