Well Water in San Luis Valley, Colorado

Alamosa / Rio Grande / Saguache / Costilla County · Population ~46,000 (valley-wide) · Aquifer: San Luis Valley Confined & Unconfined Aquifers

Hardness: Variable

Arsenic Rising

Arsenic in San Luis Valley groundwater is "markedly higher than many other areas of the U.S." according to researcher Kathy James of the Colorado School of Public Health. Her work found that arsenic has been gradually increasing in valley drinking wells over the past 50 years.

The numbers are alarming:

• Arsenic concentrations range from less than 2 to 150 micrograms per liter (the EPA limit is 10)
• 25% of private wells tested by Alamosa's SDC Laboratory show elevated arsenic
• 12 wells in one study exceeded the federal EPA standard; 30 others approached it
• During dry periods, a well hovering around 10 micrograms/L may double or triple in concentration

Zahringer, a chemist and laboratory director in Alamosa, noted that wells she tested just three years ago "now look completely different" because contaminant levels have risen so much. At her own drinking well, arsenic jumped from 13 to 20 micrograms/L.

The Drought-Arsenic Connection

The San Luis Valley is a case study in how drought and overpumping intensify groundwater contamination. Research in California's San Joaquin Valley — a geologically similar confined aquifer — linked rising arsenic to land subsidence from overpumping, which appears to release arsenic from clay layers into the water.

The same mechanism may be at work in the San Luis Valley. Over-pumping of confined aquifers can cause permanent, non-recoverable land subsidence and release arsenic that was previously locked in clay minerals.

After more than 20 years of drought, the valley's water table is declining and arsenic levels are no longer stable. Less water means less dilution of naturally occurring contaminants. The arsenic was always in the geology — but there used to be enough water to keep concentrations below dangerous levels.

Other Contaminants

James's research found that a proportion of valley wells show elevated levels of other heavy metals beyond arsenic:

• Uranium — naturally occurring, about 3% of wells tested showed elevated levels
• Tungsten — an emerging contaminant found in valley groundwater
• Manganese — causes staining and taste issues, health concerns at high levels

Unlike public water supplies, private domestic wells in the valley are unregulated and may go untested for years — or ever.

What Valley Residents Should Do

CDPHE recommends testing private wells when you purchase a property and annually thereafter. In the San Luis Valley, arsenic testing is essential — not optional.

If your arsenic is elevated, a point-of-use reverse osmosis system at the kitchen sink costs about $300 and is the most cost-effective first step. It won't treat your shower water, but it protects your drinking and cooking water.

The valley's six counties are working together on water conservation, including paying farmers to stop irrigating certain lands to replenish the aquifer. But for private well owners, the immediate priority is knowing what's in your water. See our testing guide.