INVESTIGATION OF THE EXTENT OF FRESH AND BRACKISH GROUNDWATER TO INFORM BETTER MANAGEMENT OF DECREASING GROUNDWATER LEVELS IN THE WILLCOX BASIN, SE ARIZONA

Declining surface water supplies and adoption of the recent Lower-Colorado River Basin Drought Contingency Plan will make Arizona more dependent on groundwater to meet growing water resource demands (Colorado River Drought Contingency Plan Authorization Act 2019). Knowing the extent and distribution of fresh and brackish groundwater in relation to existing wells and water table elevations would enable water managers and users to better quantify how much water is available. This study focuses on the Willcox Basin in southeastern Arizona, where groundwater levels have experienced and continue to experience significant declines, yet there is increasingly high demand for groundwater for irrigated agriculture. The current water table is approaching the terminal depths of numerous wells in some locations. Based on the lithology and data obtained from local wells, the saturated thickness of fresh groundwater averages 85.3 meters across the basin. Near the Willcox Playa, the saturated thickness of brackish water averages 30.5 meters; no other brackish groundwater was found with depth from wells in other parts of the basin. It is still unknown how much deeper the fresh water extends, or if water becomes brackish with depth, because the data are limited by a lack of deep wells in the basin, and a similar lack of geophysical surveys.