Understanding groundwater chemistry and residence times of two thermal springs in east-central Mexico.

Thermal springs are natural discharge points that can offer valuable information on groundwater circulation. The use of tracers to determine residence times can help us understand complex hydrogeochemical processes despite limited data availability. The present work aims to determine groundwater chemistry composition of two thermal springs located in east-central Mexico as well as understand some of the processes that may impact residence time estimation. Tritium and carbon-14 tracers indicated a significant component of pre-modern water. Major ions data collected showed both springs have concentrations of HCO3- greater than 1,000 mg/l and temperatures around 41 °C. Saturation indices showed water-rock interaction with geological formations present in the area, such as limestone sequence ‘El Doctor’, that could influence groundwater residence time. Isotope data (δ18O) was used to determine a recharge elevation ranging from 2900 to 3000 meters above sea level. Additionally, SiO2 geothermometers were also applied to quantify circulation depth and reservoir temperature. Analysis of hydrochemical composition, residence times, and any other information obtained from tracers, such as tritium and C-14, allows us to gain a better understanding of how groundwater systems work, along with a more accurate interpretation of results.