COMBINING PHYSICAL AND CHEMICAL METHODS TO ESTIMATE GROUNDWATER RECHARGE IN AN ALLUVIAL AQUIFER IN SAN LUIS POTOSÍ, MÉXICO

It is worldwide recognized that, of all the factors included in the groundwater evaluation, recharge (natural or induced) is the most difficult parameter to estimate. Because uncertainties associated with each approach for estimating recharge, the use of many approaches is recommended to constrain the recharge estimates. The objective of this investigation is to integrate geophysical, chemical, and isotopic methods to estimate natural groundwater recharge along ephemeral creeks in the Rio Verde fluvial aquifer, characterized by highly permeable sediments. The geophysical characterization of 2 sections (transversally to ephemeral creeks) with field measurements in separate seasons, resulted in temporal changes for the vertical and lateral resistivity values. These variations allowed the indentification of the associated resistivities of the saturated zone (5-10 ohm-m), no-saturated zone (13-30 ohm-m) and the thickness of this one (10-30 m). Rain, surface, and groundwater samples were collected in different seasons for major, minor, trace, and environmental stable isotopes (δ¹⁸O, δ²H).Field parameters (electrical conductivity) and ions (sulfates) showed an increase during the rainy season and a decrease after this season, as a result of recharge and mixing processes of groundwater. The weighted average chloride concentrations (0.6 mg/L) in rainwater were calculated considering the rain amount. With this value, the chloride mass balance was used for the estimation natural groundwater recharge estimation. Chloride concentrations in groundwater indicated that natural recharge is less than 5% of total rain, it represents values of recharge between 16 to 25 mm/year. The characteristics of natural recharge indicate that groundwater has low electrical conductivity (180-784 umhos/cm), dissolved oxygen (2.63-11.07 mg/l), pH in the range of 5.25-9.3 and is HCO₃-Ca type. The previous local isotopic water line (δ²H= 8.00 δ¹⁸O + 14.87) was adjusted (δ²H= 8.09 δ¹⁸O + 14.87) with the information of additional rain water simples , enviromental isotopic values showed considerable evaporation effects, indicating that the recharge process is associated with low diffuse flow throughout the vadose zone, resulting in a low recharge percentage of the total rainfall value.