CHARACTERIZATION OF ARIDLAND SPRINGS AND TRANSIENCE OF THE SOCIO-HYDROLOGIC SYSTEM IN THE SIERRA DE LA GIGANTA MOUNTAINS OF BAJA CALIFORNIA SUR, MEXICO

In the arid remote Sierra de la Giganta mountains of Baja California Sur, Mexico, there is a ranching community self-identified as Choyeros, who subsist off of springs for personal use, ranching and garden oases. Springs and the associated ecosystems are culturally important to the Choyero community. Community structures are controlled in part by canyon morphology- where social connections are strongest within a single canyon. Spring location greatly influences the internal community structure, where power-holding members often have the cheapest access to water, and legal water rights. Geomorphic relicts such as mountain lake beds and river channels demonstrate wetter climates in the past. These are possibly related to differences in the North American Monsoon system, and records of the timing of this transition are under investigation. The geologic context and the chemical and hydrological evolution of the springs are characterized and evaluated in terms of the social systems they support. Spring locations are controlled by the geologic evolution of the tectonically active landscape. Helocrene springs commonly occur along NE-SW trending fault/joint systems from a rhyolitic sandstone aquifer overlain by confining Miocene volcaniclastics. Social and economic drivers have increased the population and use of spring water, leading to challenges with water quality and availability. Due to the complex morphology of the aquifer and lack of groundwater monitoring wells, spring chemistry is leveraged to understand the recharge regime and aquifer structure. Analyses of major ions, trace elements, noble gasses, Sr isotopes, ẟ²H and ẟ¹⁸O isotopes show evidence of isotopic enrichment with decreasing elevation, suggesting that spring waters are recharged from spatially local and distinct areas. ¹⁴C analyses of spring water in a canyon transect indicate mean transit times of 500-750 years, with no component of modern recharge as seen by the absence of modern ³H (0.3 ± 0.084 TU). The mean transit times of the spring waters suggest that recharge is reliant on paleoclimates and not actively being recharged. In combination with the geochemical evidence that the springs are recharged locally, long term extraction will lead to depletion, exacerbating social water issues.