The San Luis Valley is composed of sediment up to 30,000 feet in thickness. Several layers of lava flows are embedded within these sediments. In addition, an impermeable layer of clay, 10 to 80 feet thick, is present throughout the central and northern valley. The clay layer blocks the downward movement of water.
The clay layer, at depths of 50 to 130 feet underground, creates two separate aquifers. Both aquifers yield large quantities of water. The lower aquifer is confined. The uppermost aquifer is unconfined and lies above the clay lens. This results in a relatively high water table (less than 12 feet to water) throughout half of the valley.
Both the confined and the unconfined aquifers are composed of unconsolidated clay, silt, sand, and gravel. The confined aquifer is imbedded with the layers of lava flows.
The San Luis Valley is estimated to contain over 2 billion acre-feet of ground water. Over 140 million acre-feet of ground water is estimated to be recoverable.
The ground water quality is generally better in the confined aquifer than in the unconfined aquifer. Concentrations of dissolved solids in the confined aquifer range from 70 to 437 milligrams per liter. Concentrations of dissolved solids in the unconfined aquifer range from 52 to 13, 800 milligrams per liter.
Water with the greatest concentrations of dissolved solids is found east of Great Sand Dunes National Monument.
Five thermal springs are found in the San Luis Valley. The combined discharge of these springs is 695 gallons per minute. The springs contribute over 4,000 tons of dissolved solids to the Rio Grande Basin each year.
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