Groundwater occurs in all four rock units in the Agua Fria basin. The volcanics and crystalline rocks yield only small amounts of water. Because the basin-fill unit is thin it does not contain large quantities of groundwater in storage. The volcanic rocks provide small amounts of water to low-yield stock wells in the northeastern sections of the basin. Well yields are best from cinder beds and fractured sections of the volcanics.
Conglomerates occur widely throughout the basin and contain the largest volume of groundwater.
Faulting formed the present-day drainage basins and separated the unit into several smaller, discrete groundwater basins that are separated by impermeable crystalline rocks. The water-bearing ability of the igneous and metamorphic rocks depends on their degree of fracturing. A number of perennial springs flow from the crystalline rocks. Normal discharges are 1 to 5 gallons per minute. Development of groundwater resources is increasing in the Agua Fria basin.