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Groundwater • Groundwater movement in the basin 6 miles south of the Town of Saint David at and within St David Springs is from the higher elevations in the mountains toward the valley and then northwest along the riverbed. Groundwater moves readily between the younger and older basin-fill units and between the streambed alluvium and the younger basin-fill unit. In the confined areas, water from the artesian aquifers may leak upwards into the water-table aquifer. In 1990, the Arizona Department of Water Resources estimated the total groundwater in storage in the Upper San Pedro basin to be 59 million acre-feet. Groundwater is water that is lifted to the surface by a well. • Water enters the aquifers by mountain-front recharge and by streambed infiltration. A smaller amount enters the basin as groundwater underflow from Mexico. Mountain-front recharge consists of surface runoff that flows off the bedrock in the mountains. It infiltrates into the permeable basin-fill sediments on the alluvial fans surrounding the mountains, and eventually reaches the water table. Streambed infiltration occurs when surface-water flows in the San Pedro River channel and its tributary washes infiltrate the coarse streambed sands down to the water table.
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