Abnormal rainfall this year has eased our concerns over drought, but climate change is increasing the likelihood of prolonged dry spells in our region. Around the globe, an increasing number of water-stressed cities are seeking ways to conserve and find new sources of water. One possibility is the capture and reuse of stormwater.
Growing demands for water, bigger populations, and longer dry spells are leading to more depleted aquifers. Researchers are exploring ways to replenish these underground reservoirs with urban runoff. Aquifer recharge is not new: Injection wells and infiltration ponds have long been used to direct water underground. For example, there are large basins around Los Angeles that direct hillside runoff into aquifers. Urban runoff from roads, parks, and rooftops is different, however, in that it is dirty, containing harmful contaminants from pesticides, blacktop, vehicle exhaust, grease, and oil, plus human and animal waste. The challenge is how to scrub out these contaminants before directing runoff water into underground reservoirs.
Three promising approaches are being investigated. One technology is using a sand filter where the grains of sand are coated with a thin layer of manganese, one of the most common elements. The coating allows the grains to bind to and break down the chemicals found in herbicides and many consumer products as well as the endocrine-disrupting chemical bisphenol A. Over time, the sand loses its scrubbing effectiveness, but researchers have found a way to renew the potency of the sand filter by adding a tiny amount of chlorine — an amount similar to that used in treating most drinking water.
Another de-contaminating technology uses biochar, basically a charcoal made from wood or plants cooked at high temperature. Since it removes different pollutants from those scrubbed out by the coated-sand filter, the two technologies can be combined to create a more comprehensive treatment. Biochar is getting a lot of positive attention as a fertility booster in soils and as a carbon sink to pull climate-changing carbon from the air. Using it to clean polluted water could have multiple benefits.
A third approach uses carefully selected plants, especially those with long, fine root systems, to filter out nitrogen, phosphorus, and even heavy metals carried by urban runoff. Although appearing simple, assembling a balance mix of plants is anything but simple, because to function effectively, it must work like a living machine tailored to local conditions.
Since urban stormwater varies in kinds of pollutants and dirtiness, the desired goal is to create a menu of approaches that can be used together, or separately, to match specific local needs. This important and ongoing research has direct applicability for capturing our local stormwater runoff and using it to recharge our aquifers.