IS MOUNTAIN BLOCK RECHARGE A SIGNIFICANT SOURCE OF GROUNDWATER? A STUDY IN THE SANTO TOMAS VALLEY, BAJA CALIFORNIA, MEXICO

Mountain block recharge (MBR) to adjacent basin aquifers can be a significant source of groundwater in arid and semi-arid regions. Unfortunately, geologic complexities within the mountain block often limit our understanding of this indirect form of recharge. Secondary permeability, resulting from faults and fractures, allows rainwater to infiltrate crystalline mountain rock, ultimately recharging the basin aquifer. Therefore, it is essential to consider mountain block geological features, especially faults, in recharge studies. We attempt to better understand MBR by creating a detailed fracture-trace map and by sampling springs and groundwater throughout the Santo Tomas valley, located in Northern Baja California, Mexico. Bounded by active faults, the valley is heavily fractured. These fractures enable fluid flow within the mountain block. Stable isotope (δ¹⁸O and δ²H) data show two distinct types of spring water within the watershed, possibly representing local and regional flow paths. Thermal springs, believed to be regional flow, display a -1.9‰ δ¹⁸O depletion when compared to all other spring water, indicative of recharge from higher elevations or older waters; both of the latter would be less affected under local drought conditions. This distinct isotopic signal was found 15 km downstream in the alluvial aquifer, indicating a significant amount of water is recharging the basin aquifer via the mountain block along this flow regime. A quantitative permeability value for the faults and fracture zones is difficult to attain due to their heterogeneous nature. However, the thermal system and most cold-water springs surface along active faults, which appear to transmit more water than undifferentiated fractures.