by Robert Marcos, photojournalist
The half-million residents of Southern California’s Coachella Valley have a lot to worry about. Toxic dust storms sweeping up from the Salton Sea. New concerns about the Coachella Canal, after the USGS reported that the canal crosses directly over the San Andreas Fault which is 150 years overdue for an major earthquake.1
Residents also worry about the future viability of the Colorado River – which provides 430,000 acre feet of water that’s used to irrigate 76,000 acres of farmland, 120 golf courses, and to replenish the aquifer in order to prevent subsidence issues like those that first appeared in 1948. And it was this replenishing that caused toxicity problems to develop in the aquifer. 2
Colorado River water that’s diverted to the Coachella Valley is both highly-oxygenated and has a high pH, (meaning it’s alkaline). When raw Colorado River water was injected into the Coachella Valley’s aquifer, its oxygen chemically reacted with (naturally occurring) chromium-3 that’s bound up in the sediment. The new oxygen oxidized the chromium-3 and converted it into soluble, toxic, chromium-6. The chromium-6 was subsequently measured at levels that exceed California’s new limit of 10 parts per billion – at 33 of the Coachella Valley Water District’s 92 wells. 3
The worst contamination is found on the aquifer’s southeastern side, which not only contains high levels of chromium-6 but also arsenic, agricultural nitrates, and perchlorate runoff. When the aquifer on the Valley’s eastern side is replenished the river water’s alkalinity triggers a chemical reaction that releases (naturally occurring) arsenic from the sediment which then disperses into the groundwater. 4
Tough new standards: The State of California’s tough new standards for chromium-6 is causing major headaches for the Valley’s five water districts. California had formerly limited chromium-6 contamination to 50 parts per billion in drinking water. Conforming to the state’s new limit of 10 parts per billion is going to be costly. The Coachella Valley Water District estimates that it will cost them $350 million dollars, with no funding provided by the State. Meanwhile the standards that were established by the EPA for arsenic, which are also 10 ppb, have remained unchanged since 2001. 5
Mitigation Efforts
The four water agencies that manage the Coachella Valley’s groundwater – the Coachella Valley Water District, Desert Water Agency, Indio Water Authority, and Coachella Water Authority, have submitted structural compliance plans to the State Water Resources Control Board. They plan to mitigate naturally occurring chromium-6 to meet California’s strict limit through a mix of infrastructure overhauls, advanced filtration, and operational adjustments. Because California’s mandate depends on the size of the water system, these agencies are working against a rolling compliance deadline spanning from October 2026 through October 2027. 6
Advanced Water Treatment Infrastructure
- Ion-Exchange Technology: Agencies like the Indio Water Authority (IWA) are expanding the use of strong base anion exchange systems. These systems route groundwater through tanks filled with resin beads that magnetically trap and remove chromium-6 molecules.7
- Reduction Coagulation Filtration (RCF): The IWA has also piloted RCF treatment, which chemically alters the chromium-6 into a harmless, filterable solid form before water enters the distribution grid. 8
Well Offline Management and Decommissioning
• Taking Impacted Wells Offline: The largest provider, Coachella Valley Water District (CVWD), has identified 34 out of its 96 drinking water wells that exceed the 10 ppb threshold. Impacted wells across the valley are being selectively pulled from service or restricted during off-peak seasons to prevent contaminated groundwater from mixing into municipal supplies. 9
Drilling New Deep-Water Wells
- Bypassing the Contaminated Strata: Chromium-6 in the Indio Subbasin is entirely naturally occurring, leaching out of specific geological rock layers and sediment over centuries. Water districts plan to construct new, deeper wells that tap into lower layers of the aquifer where chromium-6 levels are safely below the state limit. 10
Well Clustering and Pipeline Blending
- Dilution and Interconnection: Agencies are designing “well-clustering” projects. This technique involves building miles of new connecting pipelines to merge water from higher-chromium wells with water from pristine wells. Blending the water dilutes the total chromium-6 footprint down to legally compliant levels before it reaches household taps. 11