Hydrochemical assessment of Hydrological Environmental Services in the recharge area in the Southwest of Mexico City

The aquifer recharge area that supplies water to Mexico City is being protected with the implementation of land use restrictions and with the use of payment schemes for maintaining environmental services (PES). In order to assess the effects of PES in the conservation soil area, the chemical and isotopic compositions of water from springs were characterized and the water baseline chemistry was established to provide a useful standard to monitor the water resources and to support the decision-making under current water management policies. A total of 32 water samples taken in four field surveys, during the dry and rainy season, and background chemical data from different information sources, were studied. The water salinity increases from southwest to northeast towards the urbanized area. The chemical composition of water is due to the occurrence of natural chemical interactions between water and rocks in aquifers and to the presence of effluents generated by human activities. In the forest area, water chemistry has not changed significantly over the past three decades and there are no major discernible trends in the compiled water chemistry data, suggesting that PES has contributed in part to maintain the initial condition in the aquifer recharge area. The natural reference levels proposed have allowed the current chemical conditions in the system to be understood and can be implemented to monitor water resources in the area of conservation soil in the Sierra de Las Cruces with an acceptable degree of reliability.