WaterSolving the mystery of arsenic-contaminated water

Published 3 September 2015

Can water ever be too clean? If the intent is to store it underground, the answer, surprisingly, is yes. In a new study, scientists have shown that recycled water percolating into underground storage aquifers in Southern California picked up trace amounts of arsenic because the water was too pure. The research sheds light on a poorly understood aspect of groundwater recharge with purified recycled water, namely the potential mobilization of arsenic. Arsenic is a naturally occurring element that can cause organ failure and cancer in humans with prolonged exposure above established health thresholds. The findings pose a problem for Orange County, California, which differs from most communities in that it purifies treated wastewater instead of discharging it directly into rivers and oceans – but the problem goes beyond Orange County.

Can water ever be too clean? If the intent is to store it underground, the answer, surprisingly, is yes. In a new study, Stanford scientists have shown that recycled water percolating into underground storage aquifers in Southern California picked up trace amounts of arsenic because the water was too pure.

The research, published online in the journal Environmental Science & Technology, sheds light on a poorly understood aspect of groundwater recharge with purified recycled water, namely the potential mobilization of arsenic. Arsenic is a naturally occurring element that can cause organ failure and cancer in humans with prolonged exposure above established health thresholds.

Woods Institute reports that the finding has implications beyond California, as more communities are increasingly tapping into and actively managing their groundwater resources to combat drought and dwindling water supplies.

Globally, as we’re pushing our water resources, the use of groundwater, the replenishment of groundwater and subsurface water storage are all on the rise,” said study co-author Scott Fendorf, the Huffington Family Professor in Earth Sciences and a senior fellow by courtesy at the Stanford Woods Institute for the Environment.

The problem first came to light when the Orange County Water District (OCWD) in Fountain Valley, California, noticed that recycled wastewater that had undergone a rigorous purification process showed temporary, low-level increases in arsenic after it percolated into soils and sediments from recharge basins (essentially large, man-made surface ponds) into underground storage aquifers.

Advanced water purification
Orange County differs from most communities in that it purifies treated wastewater instead of discharging it directly into rivers and oceans. The water purification process at OCWD, known as the Groundwater Replenishment System, is one of the most advanced in the world and involves three major steps: microfiltration, reverse osmosis, and a final cleansing with ultraviolet light and hydrogen peroxide.

Reverse osmosis is really the heart of our process, and it involves forcing water through a semipermeable membrane that is essentially designed as a molecular sieve, allowing water molecules to pass through but rejecting other dissolved molecules and ions,” said study co-author Jason Dadakis, OCWD’s director of health and regulatory affairs.