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Scientists have engineered mineral-coated sand to remove contaminants from storm water. In places where water resources are strained, engineered sand could transform storm water into a valuable asset.
"The way we treat storm water, especially in California, is broken. We think of it as a pollutant, but we should be thinking about it as a solution," Joseph Charbonnet, a graduate student in civil and environmental engineering at the University of California, Berkeley, said in a news release. "We have developed a technology that can remove contamination before we put it in our drinking water in a passive, low-cost, non-invasive way using naturally-occurring minerals."
Afraid of the toxic chemicals and pollutants picked up as storm water runs across herbicide-soaked lawns, down oil-soaked streets and past overflowing sewers, most cities do their best to divert runoff away from water reservoirs and into rivers and streams.
The latest research -- published Thursday in the journal Environmental Science and Technology -- suggests this unwanted storm water, if treated properly, could provide a lifeline to water-strapped cities like Los Angeles.
Charbonnet and his researcher adviser, David Sedlak, are currently working with a local community in the Sun Valley neighborhood of Los Angeles to turn a 46-acre gravel pit into a wetland outfitted with a water filtration system.
The project, a scaled-up version of a rain garden, will see storm water diverted to the wetland where it will pass through layers of the mineral-coated sand before seeping back into underground aquifers.
"Before we built the buildings, roads and parking lots that comprise our cities, rainwater would percolate into the ground and recharge groundwater aquifers," said Sedlak, a professor of civil and environmental engineering at UC Berkeley. "As utilities in water stressed regions try to figure out how to get urban storm water back into the ground, the issue of water quality has become a major concern. Our coated sands represent an inexpensive, new approach that can remove many of the contaminants that pose risks to groundwater systems where storm water is being infiltrated."
In the lab, Charbonnet mixed sand with two types of manganese, which react to form manganese oxide. The mineral coating binds and reacts with organic toxins, including herbicides, pesticides and harmful chemicals like bisphenol-A, breaking them down into smaller, less toxic molecules that are more easily biodegraded.
While engineered sand can't totally purify contaminated water, it can be combined with other water treatments to turn storm water into a safe source of drinking water.
In tests, Charbonnet found the sand effectively removed almost all of the BPA from a contaminated water. However, the sand's decontamination abilities diminished over time. To recharge the sand's ability to clean water, he exposed the sand to a solution containing a low concentration of chlorine. The solution boosted the mineral coating's reactivity.
"If you have to come in every year or two and dig up this sand and replace it, that is incredibly labor intensive, so in order to make this useful for community stakeholders it's really important that this stuff can be regenerated in place," Charbonnet said.
Even in places where water isn't in short supply, improved stormwater filtration systems could improve the ecological health of local water ways. In the Pacific Northwest, studies have shown contaminated storm water is harming local salmon populations. Elsewhere, storm water runoff has been blamed for fueling toxic algae blooms that threaten fish stocks and endangered species.
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