Groundwater Age and Recharge in Five Aquifers Supplying Monterrey Metropolitan Area Using Environmental Tracers

Monterrey Metropolitan Area (MMA) with approximately 4.6 million habitants is the third most populated urban area and second most important economical center of Mexico. MMA’s water demand is supplied by surface and groundwater sources in an approximate ratio of 50:50. This study estimates groundwater age and recharge rates from five aquifers (Campo Buenos Aires, Zona Metropolitana de Monterrey, Citricola Norte, Cañón del Huajuco and Campo Topo Chico) that provide groundwater sources for MMA water demand, by using environmental tracers (CFCs, ¹⁸O, ²H, ³H, ¹⁴C and Cl^-). Mean annual precipitation in the study zone (~7,800 km²) ranges from 342mm in the northwestern to 1,040mm in the central portion. Groundwater stable isotopes (²H and ¹⁸O) indicated that the groundwater recharge source was local meteoric precipitation. Radiocarbon results showed that groundwater age within the study area ranged from 3,800 years BP at the northwestern deeper wells to recent water at the central valley shallower wells. CFCs revealed apparent ages ranging from 1967 to recent water. CFCs, ³H and radiocarbon results demonstrated that several samples were the product of a mixture of waters from different ages. A map showing estimated average recharge rates from the study zone was developed by using chloride mass balance method and considering different soil, climate and vegetation characteristics.