When The Water's Too Clean: An Arsenic Problem In Orange County And The Future Of Reuse

The Orange County Water District (OCWD) has been a leader in the recycled water movement. Its Groundwater Replenishment System (GWRS) operates as a possible model for the potable reuse systems of the future, taking treated wastewater that would normally be discharged into the ocean, purifying it, and then piping it into “recharge stations” where it is stored in underground aquifers for at least six months before traveling to consumers (that buffer puts the “indirect” in indirect potable reuse).

Since operations began in 2008, the GWRS gauges its output at 165.8 billion gallons of locally-controlled, drought-proof water treated to exceed all state and federal standards. Skeptics note that as a relatively new method of treatment, reuse has to find its footing before it can achieve public acceptance and establish broad operations. Shortly after the GWRS was founded, a new hurdle presented itself.

A study conducted by researchers from Stanford University and OCWD, “Geochemical Triggers of Arsenic Mobilization during Managed Aquifer Recharge,” found that recycled water picked up trace amounts of arsenic on its way to storage aquifers in Orange County, CA. The researchers posit that because the recycled water had all of its calcium and magnesium eliminated during treatment, it attracted atoms of the very same from the sediment leading to the aquifer, which pulled arsenic ions in along with it.

Although the study was released in September, arsenic levels have been a concern for OCWD for some time.

“OCWD began a proactive voluntary groundwater monitoring program focused on potential metals mobilization in the lead up to and then after the onset of GWRS operations in 2008,” said Jason Dadakis, the director of health and regulatory affairs for OCWD. “The first meaningful arsenic increases were observed in 2009.”

While the arsenic levels reverted within drinking water standards before reaching any consumers, the study was an effort to investigate the cause of this mobilization. Conducting laboratory batch and soil column studies, the researchers found arsenic in the clay above the aquifers. They noticed that local runoff and imported water did not pick up arsenic from the recharge stations. Further testing revealed that the purity resulting from OCWD’s three-tier treatment process left the recycled water vulnerable to arsenic mobilization.

Dadakis and his team got to the bottom of the arsenic mobilization and shed light on a little-understood aspect of the still-burgeoning method of groundwater recharge.

“[The study] has made OCWD and others in the groundwater management and water recycling community more aware of this potential consideration for indirect potable reuse projects using highly purified recycled water,” Dadakis said. “Our findings support the idea that geochemical characterization of the receiving aquifer should be considered during project planning and design, especially with regard to post-treatment stabilization.”

Dadakis added that OCWD will keep this issue in mind as it plans future recharge of recycled water in other locations within the basin. Now that they’ve gotten to the root of the arsenic problem, OCWD will be adding lime to its existing post-treatment process to ensure the presence of divalent cations and keep the sediment from unleashing its calcium and magnesium into the water.

“We are also currently conducting additional, similar laboratory tests with Stanford on soil core samples obtained from a deeper and geochemically distinctive portion of the groundwater basin that we would like to target for recycled water replenishment via new injection wells,” he said.

The study can be seen as a major contribution to the reuse cause as it clarified an issue plaguing one of the nation’s leading water recycling systems. Or it can be an argument that potable reuse projects have a long way to go in proving viable for wide adoption. Either way, it goes to show that water can be too clean for its own good.