Are springs and tinajas in Saguaro National Park threatened by groundwater withdrawal outside the park?

Swann, Don; Pearl, Jessie; King, Courtney; Gaun, Janelle; Filippone, Colleen

Surface water in the desert is necessary for both people and wildlife. Saguaro National Park and science partners have been studying the park’s intermittent streams, springs, and spring-fed stream pools called tinajas for more than 10 years. Analysis of groundwater chemistry suggest that the source of water in some springs and tinajas could be mountain-block scale water-bearing zones along which water flows downward from upper elevations until it is ponded at the base of the Rincon Mountains behind the Catalina detachment fault. The water trapped below the fault extends beyond the park boundaries into areas that are subject to active groundwater pumping. The purpose of this study was to better understand the chemistry, origins, and hydrologic relationships of the park’s surface waters. An important tool that can be used to determine these links is analysis of the water itself. The number of dissolved ions such as calcium and sodium reflect the travel time and distance as well as types of rocks or soil the water has flowed through before reaching the surface, and can help identify different “types” of water in an area Perennial tinajas and springs in the Rincon Mountain District were selected for water chemistry and isotope analysis based on their geological and hydrological properties. Water from these sources was compared to water in local aquifers. Strong evidence was found showing that base flow into some tinajas during the driest months of the year is from a combination of local alluvial and near-surface bedrock sources as well as older, regional water from deeper within the Rincon Mountain block. A fault associated with the Catalina fault, the Javelina sub detachment fault, appears to be critical for maintaining water in many low-mid elevation tinajas through the driest seasons. Chemistry data suggests that there is some potential, but not conclusive evidence, that base flows could be impacted by groundwater pumping outside the park. Recommendations include future hydrogeological investigations and participation in long-term efforts to protect regional water resources in the face of Tucson’s growing population, rising temperatures, and changing precipitation regimes. A Visitor Center interpretive video about water in the park was developed as a part of this project.