A collaboration between Stanford scientists and the Orange County Water District (OCWD) recently diagnosed the cause of transient spikes in trace arsenic levels seen in a Southern Californian groundwater basin — the introduction of highly purified water. The study was published in the journal Environmental Science and Technology, and OCWD has modified post-treatment operations in light of its findings.

Arsenic levels in OCWD’s aquifers occasionally exceeded the drinking limit of 10 micrograms per liter. OCWD Director of Health and Regulatory Affairs Jason Dadakis reported that public water supplies were at no time affected by the spikes, which were transient in nature.

The fact that highly purified water can instigate the release of naturally occurring subterranean contaminants was previously unknown. The study’s findings are particularly relevant for drought-stricken California, where groundwater basins have emerged as attractive alternatives to evaporation-vulnerable surface reservoirs.

OCWD built and operationalized its groundwater replenishment system in 2008. The world’s largest treatment system for potable reuse, it recharges a subterranean basin with local surface water, imported water and thoroughly treated recycled water. Water is piped into surface ponds, from which it percolates and infiltrates the basin below.

A network of wells provides over 1,400 sampling points and enables OCWD to conduct over 350,000 analyses per year, and it was with this monitoring infrastructure that OCWD observed arsenic traces. The spikes, which were first seen in 2009, were difficult to reason about — blending of different water types and their aggregate flow through the basin, for example, complicated analyses. And so, three years later, OCWD enlisted the help of Stanford’s earth system science professor Scott Fendorf.

“It was very difficult to understand exactly what was going on all the time in this uncontrolled field [setting],” Dadakis said. “That led us to contact Stanford… to do work in [Fendorf’s] laboratory where we could control some of these factors.”

Fendorf’s laboratory received samples of sediment from underneath Anaheim’s Miraloma Basin. The scientists packed the sediment — which itself had naturally occurring arsenic — into small vials, flowing differing water samples through them and measuring the extent to which arsenic bled into the water. They found that OCWD’s recycled water was particularly effective in coaxing arsenic to unbind from sediment, precisely because it was poor in calcium and magnesium cations.

“What we discovered is that, essentially, their water is too pure,” said Sarah Fakhreddine, co-author of the study and a third-year graduate student advised by Fendorf. “Some of the arsenic is bound on the clay particles in the aquifer, and it’s bound by calcium and magnesium…. As you introduce high purity water that has [low concentrations of] ions, you start to lose the calcium and magnesium in the system and you release the arsenic.”

Dadakis said that OCWD has responded to these findings by increasing the amount of calcium added to the recycled water post-treatment. Preliminary results do not show any negative consequences of doing so, though it is too early to tell whether the attempted solution will be effective.

This work was not the first collaboration between Stanford and OCWD (Stanford began consulting for OCWD as early as the late 1970s), nor will it be the last. Fendorf and company have already taken up a second project: OCWD is evaluating the feasibility of injecting water directly into the groundwater basin, and the Stanford scientists have come on board to study the geochemistry of that particular system.

 

Contact Akshay Agrawal at akshayka ‘at’ stanford.edu.