Chemical and stable isotope composition of surface and groundwater in the surroundings of the Los Humeros Caldera, Puebla, Mexico

It is important to develop a regional hydrogeological model to identify possible recharge and discharge areas for a sustainable use of a geothermal reservoir. The Los Humeros geothermal area is situated within five surficial watersheds and coveres an area of more than 15.000 km(2). A total of 208 well and spring samples were collected between June 2017 and November 2018. The stable isotope data for this region define a regression line of delta D-H2O = 8.032 delta O-18 + 12 and indicate that groundwater is recharged by regional precipitation. At least 39 groundwater wells, with a maximum temperature of 35 degrees C, show temperatures above the reported mean average surface temperature of 15 degrees C. Characteristic elements for geothermal reservoir fluids (B, Li, As) are also present in these groundwaters, indicating a possible connection between the reservoir fluid and the local groundwater through local fracture systems. Concentration of B in these hot wells is between 150 and 35000 ppb.