Location: Tohono O’odham Indian Reservation, Arizona (Pima, Pinal and Maricopa counties)

Problem: Two drinking water wells had high arsenic and nitrate levels

Outcome: RCAC provided technical assistance to a Tribal water utility to bring the wells back into compliance

The Tohono O’odham Indian Reservation lies across three counties in southern Arizona. It is the third largest Indian Reservation in the United States. According to the 2000 census, nearly 11,000 people live on the Reservation. The Tribe’s enrollment is 25,000.

The Tohono O’odham Utility Authority monitors the Tribe’s drinking water and sewage systems, which are aging. There are 32 public water systems on the Reservation, which is the size of the state of Connecticut. One of those 32 systems, which serves about 340 residents had two wells with chemical contaminants that exceeded the maximum contaminant level (MCL), one exceeded arsenic levels and another exceeded nitrate levels.

A long-term solution would be to join the water system with another neighboring system so that the wells wouldn’t be needed but the Tribe also needed a short-term solution. Tribal water operators needed to know how to blend the well water to balance the chemicals and make it potable, so they sought RCAC’s technical assistance. RCAC regional environmental manager, Pablo Figueroa, provided math training to help the regulators calculate the desired blended water concentrations. Using spreadsheets and blending formulas, he helped the system create a final blending plan and provided procedures to ensure compliance with drinking water standards.

“Now they have a plan and now they are operating according to that plan,” Figueroa said. Recent compliance samples show arsenic values below the MCL of 10 parts per billion (µg/l) at around 5 parts per billion.