Solving the mystery of arsenic-contaminated water

The purified water is then piped thirteen miles from the treatment plant to recharge stations, where it seeps into underground aquifers and is stored for at least six months before it is released for use by the county’s 2.4 million residents. The OWCD carefully tracks the water through each step of the purification and storage process.

Beginning in 2009, results from groundwater monitoring wells near the recharge basins first detected increased arsenic levels, and in some cases the levels were just above the acceptable U.S. drinking water standard of ten micrograms per liter. The arsenic spikes were transient, and returned to acceptable background levels by the time the water was extracted for use farther away.

“At no point was the groundwater delivered for public consumption in the area unsafe, but the OCWD was considering expanding its recharge of purified recycled water, so we thought it was prudent to get a better understanding of what was going on,” Dadakis said.

An OCWD investigation revealed that when the recycled water first arrived at the recharge basins, it was free of arsenic, so the contamination must have happened as the water seeped underground.

However, none of the normal trigger mechanisms for arsenic contamination seemed to apply. For example, in Southeast Asia, arsenic contamination is largely due to bacteria removing oxygen from the soil and creating anaerobic conditions that cause arsenic atoms to migrate from sediments into the water. But the OCWD aerated their water, so low oxygen levels were not to blame.

Triggering an arsenic spike
OCWD investigators also noticed another curious thing: Only the purified recycled water triggered the arsenic spike. Local runoff and imported water from the Colorado River did not pick up arsenic as it percolated into the recharge stations. Puzzled, Dadakis enlisted the help of Fendorf, a soil scientist at Stanford’s School of Earth, Energy & Environmental Sciences.

Fendorf’s team analyzed sediment samples from the recharge stations and discovered that arsenic was present in very low concentrations in a thin band of clay above the aquifers. That explained where the arsenic was coming from, but not how the arsenic was getting into the water.

Woods Institute says that further experiments eventually revealed the culprit: The water was too pure. In particular, the distilled water from the treatment plant was lacking in calcium and magnesium; this deficiency caused calcium and magnesium atoms in the sediments to migrate into the water and off of charged clay particles that harbored the arsenic. With the calcium and magnesium ions leaving the clay surface, the arsenic ions were repelled from the clay surface and entered the water. The other water sources used to replenish the groundwater basin didn’t draw in arsenic because they already contained abundant calcium and magnesium ions.

“This is a new trigger for arsenic contamination that wasn’t appreciated before,” Fendorf said.

Now that the cause of the arsenic spike is known, OCWD is experimenting with ways to fix the problem. One possible solution is to add more calcium to the water during the treatment process.

“We’ve altered some of our post-treatment operations here,” Dadakis said. “We keep a closer eye on the calcium level and have actually boosted it recently, in part due to the recommendations coming out of Scott’s work.”

Fendorf noted that as more communities consider manipulating groundwater resources and increasing subsurface water storage, the risk of large-scale contamination increases. “It only takes a little bit of arsenic or other elements to contaminate a big aquifer,” Fendorf said. “In Orange County, the contaminant was arsenic, but in other areas, it might be uranium, chromium, selenium or boron, as examples.”

— Read more in Sarah Fakhreddine et al., “Geochemical Triggers of Arsenic Mobilization during Managed Aquifer Recharge,” Environmental Science & Technology 49, no. 13 (25 June 2015): 7802–09 (DOI: 10.1021/acs.est.5b01140)