Determining the ages, sources, and connections between groundwater and surface waters in the upper Babocomari watershed
Jonathan Hasenstab1, Jennifer McIntosh1, and Andrew Salywon2
1Department of Hydrology and Atmospheric Sciences
The University of Arizona, Tucson AZ
2Desert Botanical Garden, Phoenix, AZ
Groundwater and surface water are two intimately connected water storage regimes whose interactions impact the health of the regions they sustain. However, many growing communities with evolving land use lack comprehensive hydrologic studies characterizing their water resources. One such region is the Babocomari Watershed, home to the winery communities of Elgin and Sonoita, as well as the Babocomari Cienega—a crucial habitat for endangered 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 paper builds upon previous studies in the watershed through collecting groundwater and surface water samples within Elgin and the Babocomari Cienega, and analyzing them for solute chemistry, stable water isotopes, and age tracers. Elgin groundwater appears to come from local winter and summer precipitation, 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. Groundwater from 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 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.