Determining the Ages, Sources, and Connections Between Groundwaters and Surface Waters in the Upper and Middle Babocomari Watershed

Abstract

Groundwater and surface water are two intimately connected water storage regimes whose interactions impact the economic and ecological health of the regions they sustain. However, many growing communities in the southwestern United States with evolving land use lack comprehensive hydrologic studies characterizing their water resources. One such region is the Babocomari Watershed in southeastern Arizona, home to the winery communities of Elgin and Sonoita, as well as the Babocomari Cienega and River that are crucial habitats for many rare and endangered species of flora and fauna. Determining the age, sources, and chemistry of waters in this area could inform water and ecosystem management policies through knowledge of recharge seasonality and vulnerability to modern contamination. This research builds upon previous studies in the area through the collection and analysis of groundwater and surface water samples within the Babocomari Watershed, which were analyzed for solute chemistry, stable water isotopes, and age tracers. Elgin groundwater appears to come from local winter and summer precipitation that has experienced evaporation, while surface waters in the Cienega reflect evaporated summer precipitation. This seasonality can be affected by climate change, which shifts precipitation patterns and thus both the timing and magnitude of recharge. Principal Component Analysis indicates that Babocomari Cienega waters are more chemically similar to waters along O’Donnell Creek than waters from Elgin, implying the Cienega receives more water from the former than the latter. Groundwater in the upper and middle watershed is relatively young, with modern corrected radiocarbon signatures less than 500 years old. Some groundwaters also had detectable tritium, implying they contained a component of modern (post-1950s) recharge. Groundwater with modern recharge is susceptible to modern contamination, like nitrate from leaky septic tanks—a common problem in rural areas lacking centralized sewage infrastructure. Elevated nitrate levels were found in two wells in the Elgin area, with their nitrate isotope values consistent with septic contamination.