THE KARSTIC AQUIFER OF THE HUASTECA REGION, SAN LUIS POTOSI, MEXICO

The Huasteca region in eastern Mexico is a karstic macrosystem characterized by highly complex geologic geometry with a tropical climate. High precipitation (1000-1500 mm/year) facilitates sugar cane crops and cattle raising as main economic activities. During the dry season (november-may) base flow is supported by natural groundwater discharge from a karstic aquifer unit. Cienega de Cabezas (CC) is a RAMSAR wetland site supported by groundwater flow in a karstic aquifer composed by Cretacic limestones of the Tamasopo Formation within the carbonate Valles–San Luis Potosí platform. Several localized and diffuse groundwater discharges in this extensive doline structure (11 km long, 2 km wide) support this ecologically fragile area, demanding water management based on scientific information. CC has been investigated at this time using a selection of geochemical indicators (major, minor, and trace elements) and environmental isotopes, to characterize the origin of groundwater, the main geochemical processes, and the mineral/groundwater interactions controlling the baseline geochemistry. Groundwater with carbonate rock interaction and mixing dominates the baseline chemical composition, although nitrate concentrations suggest the impact of diffuse contamination sources derived from agriculture. Springs are HCO3-Ca and SO4-Ca type, in equilibrium with calcite and undersaturated with gypsum and dolomite, long-term sampling indicates seasonal changes for these parameters; in addition, comparison of seasonal variation of temperature and electrical conductivity values in the springs showed the evidence of diffuse and conduit flow in the karstic aquifer. A refinement of the local water meteoric line with the addition of rainwater sampling helped to identify that recharge processes are dominated by fast flow along with the karstic features, no evaporation effect was identified in groundwater. The investigation demonstrated that groundwater flow in the karstic aquifer discharging in CC has a relatively short trajectory and originates in surrounding topographic elevations with dissolution features, making the natural discharges vulnerable to soil use change produced by the economic activities