San Pedro Groundwater (Hydrological Investigations in the Upper San Pedro Basin, Coshise County, AZ)

1. Purpose and Objectives: The purpose of this study is to obtain a better definition of stream-aquifer interactions and water budget components in the Sierra Vista sub-basin.

2. Background: Demands in the water supply in the basin are increasing and may threaten perennial flow in the San Pedro River and associated riparian area, San Pedro Riparian National Conservation Area (SPENCA) managed by the Bureau of Land Management. Poorly quantified stream-aquifer interactions and groundwater budget prevent a reasonable estimate of the timing, magnitude, and location of the effects of groundwater withdrawals on river flows and the riparian area.

3. Funding: No estimate was provided.

4. Partners: USGS

5. Description and Approach: Stream-aquifer interactions will quantified through geochemical, continuous water-level, and geophysical exploration studies. Mass-balance analysis of common ions and stable isotopes will be used to quantify sources of in-house flow of the San Pedro River. The hydraulic response of the aquifer to natural and artificial stress will be used to estimate aquifer properties and hydraulic connection between the river and aquifer. Temporal gravity measurements will be used to estimate aquifer storage change and specific yield.

6. Status: Study is ongoing.

7. U.S. Contact: Donald R. Pool, USGS Tucson, (520) 670-6671