Background
The Torres Martinez Desert Cahuilla Indians (TMDCI) reservation spans roughly 24,000 acres in Riverside and Imperial Counties, overlying the Coachella Valley groundwater basin. Naturally occurring arsenic in this aquifer has long exceeded the federal and California maximum contaminant level of 10 parts per billion (ppb). The tribe’s public water system serving administrative buildings, the health clinic, and approximately 260 members has been out of compliance since at least 2009, with recorded arsenic levels reaching 16 ppb. Many individual allotment wells serving another 180 tribal members have shown similar exceedances.
For years, the tribe relied on bottled-water deliveries and temporary in-home filters funded by the Indian Health Service. Nearby private mobile home parks located on reservation land — such as Oasis Mobile Home Park — have faced even higher arsenic levels (up to 97 ppb), but these systems are privately owned and outside direct tribal control.
The Partnership
In 2022, Cadiz Inc. entered a landmark 50-year supplemental water-supply agreement with the Torres Martinez Desert Cahuilla Indians, the Salton Sea Authority, and the Coachella Valley Water District. As part of that agreement, Cadiz committed up to $5 million to fund pipeline connections and wellhead treatment systems that would permanently remove arsenic and nitrates from tribal drinking-water sources — at no capital cost to the tribe.
Cadiz’s wholly owned subsidiary, ATEC Water Systems, was selected to design, install, and support the treatment systems. ATEC specializes in modular, chemical-free adsorptive-media filtration proven effective on arsenic-bearing groundwater across the western United States.
The Challenge
Scattered wells, remote locations, and limited tribal funding made a traditional centralized treatment plant impractical. The tribe needed a solution that could:
- Treat arsenic reliably to below 10 ppb (and ideally to non-detect levels) despite high silica, phosphate, and other competing ions common in the basin
- Be installed quickly at multiple small sites
- Require minimal ongoing maintenance and waste handling
- Be operated and maintained by tribal staff to strengthen sovereignty and create local jobs
The Solution
ATEC deployed its regenerative iron-based hybrid anion-exchange filtration systems at several tribal wells in Thermal, California, near the Salton Sea. The skid-mounted units treat water directly at the wellhead, eliminating the need for new buildings or extensive piping.
Working with the Farmworkers Institute of Education & Leadership Development (FIELD), ATEC provided hands-on training so tribal water technicians could operate and maintain the systems themselves. The entire capital cost was covered by Cadiz under the 2022 agreement, and the systems were designed for low operating cost and long media life.
Results
By early 2024, ATEC had completed installation of permanent arsenic-removal systems at multiple wells serving the tribe’s administration buildings, clinic, and community areas. The treated water now meets federal and California drinking-water standards, significantly reducing the tribe’s long-term dependence on bottled water for these facilities.
The project has been publicly recognized as a successful public-private-tribal partnership that delivers safe water while building local capacity. It has also positioned the Torres Martinez tribe as a leader in sustainable groundwater management in the region and opened the door for potential expansion of similar wellhead treatment to other disadvantaged communities near the Salton Sea.
Project Update – EPA Lifts Alternative Water Requirement Following Sustained Arsenic Compliance
In May 2024, the U.S. Environmental Protection Agency Region 9 issued an Emergency Administrative Order (EAO) to ATEC Water Systems related to elevated arsenic levels in the drinking water system serving Angelique Meli Properties at the Avenue 66 Mobile Home Park in the eastern Coachella Valley (PWS ID No. 090600043).
The EAO was issued under Section 1431 of the Safe Drinking Water Act to address arsenic concentrations that had exceeded the federal Maximum Contaminant Level (MCL) of 10 parts per billion (ppb). As part of the Order, ATEC implemented treatment improvements and monitoring requirements to stabilize water quality and protect public health.
Based on monthly sampling data submitted to EPA, the system has demonstrated sustained compliance. According to EPA’s January 21, 2026 determination, the system’s Running Annual Average (RAA) arsenic concentration is 2.8 ppb, well below the federal MCL. As a result, EPA formally lifted the requirement to implement an Alternative Water Source Plan under Paragraph 30 of the Emergency Administrative Order. Other elements of the Order remain in effect pending further notice.
This outcome underscores the importance of timely intervention, proven treatment technology, and ongoing regulatory coordination in addressing naturally occurring arsenic challenges across the Coachella Valley. It also reflects how targeted groundwater treatment upgrades can restore compliance and long-term system stability without requiring permanent alternative water supplies.