Because of its highly permeable nature, the San Joaquin River's valley is underlain by one of the largest aquifers in the Western United States. The aquifer underlying the San Joaquin River and Tulare Basin is estimated to hold nearly of water, of which about half can be pumped economically or is clean enough for human use. The aquifer receives in excess of of inflow per year, mostly from precipitation and irrigation water seepage. Concentration of chloride and other minerals generally increases from east to west across the basin.