Thoughts From Engineers: Enhanced Aquifer Recharge Goes Mainstream

Thoughts From Engineers: Enhanced Aquifer Recharge Goes Mainstream

Sometimes it takes converging events to suddenly ramp up interest in a particular water-management technology. Articles with headlines such as “America is using up its groundwater like there’s no tomorrow” (bit.ly/IsNoTomorrow) do little to quell fear, but data show that the United States has hit all types of records in recent years—from record heat and drought to precipitation and flooding. Notwithstanding the many climate and aquifer model projections in circulation, the common theme is uncertainty—in terms of climate and precipitation trends and future availability of freshwater resources.

National initiatives such as the Water Reuse Action Plan, which advocates for the reclaiming and reuse of water, have helped shift the discussion to nontraditional avenues for securing water. These efforts, combined with research confirming the depletion of aquifers in specific regions of the United States (bit.ly/GroundwaterFuture), have generated greater interest in use of alternative water sources to bolster groundwater supplies.

Managed aquifer recharge, also known as enhanced aquifer recharge (EAR), has been in use for decades in some parts of the country such as California, which has many active projects and more on the way. Other parts of the country may have different objectives, and EAR can address several at once. The most-crucial element moving forward is to ensure that with growing interest in EAR and use of various sources of water—such as stormwater—to bolster groundwater supply, our knowledge base of potential risks and safeguards keeps pace accordingly.

Mechanics of EAR

At its most basic level, EAR has been called a form of “water banking.” It represents a tool to capture water—everything from reclaimed and treated wastewater to stormwater—to be stored in an aquifer, via some form of passive surface infiltration or through mechanical means such as underground injection. Aside from the obvious benefit of bolstering local groundwater supply, the state of California has been using some form of EAR since the early 1900s to combat land subsidence from high rates of groundwater withdrawal. Other benefits include curbing seawater intrusion, improving water quality through an aquifer’s natural filtration capabilities, restoring stream flow and ecosystem viability, and intercepting flood waters.

According to the International Groundwater Resources Assessment Center, there are more than 1,200 EAR projects worldwide (bit.ly/ManagedAquifers), of which a few hundred projects are in the United States. Of these projects, stormwater use as the primary water source is uncommon (bit.ly/EnhancedAquiferRecharge), but positive recharge results have been reported. In Nassau County, N.Y., for example, a series of infiltration basins led to increases in the water table by 5 feet above predevelopment levels. A project draining a basin of roughly 220 acres resulted in 41 acre-feet per year of additional groundwater recharge in Los Angeles (bit.ly/LA-StormwaterRecharge). Another study in Flanders, Belgium, quantified the potential for enhanced groundwater recharge and determined it to be on average 17 percent higher due to stormwater infiltration, considering total impervious area, average precipitation and runoff coefficients (bit.ly/FlandersRecharge).

What’s notable with respect to the last two studies mentioned is that the project regions had some form of zoning protection in place to protect EAR source water and groundwater from contamination—and this is key. Stormwater as a source water may be best employed in areas where land uses are restricted to those unlikely to generate highly contaminated runoff.

The Contaminants Stormwater Carries

Notwithstanding the many successes of EAR, the U.S. Environmental Protection Agency acknowledges there are still significant unknowns with respect to best practices for design, location, long-term management and monitoring of EAR projects (bit.ly/RechargeRisk). Moreover, the agency acknowledged that one of the most-concerning aspects relating to stormwater use in EAR projects is the risk of inadvertently introducing contamination to the receiving groundwater source.

Without venturing into an excessive amount of detail, at a minimum, EAR projects necessitate analysis that includes an understanding of the hydrogeology of the receiving aquifer, the geochemical character of the native groundwater and reactions likely to result given the character of incoming source water. The range of potential contaminants is broad and could include everything from basic nutrients to inorganic compounds such as metals and ions, organic compounds, polycyclic aromatic hydrocarbons as well as perfluoroalkyl and polyfluoroalkyl substances (PFAS) and pathogens.

The infinite number of possible contingencies given the complex and unique character of each subsurface basin underlines the full range of what we don’t know. Although studies have investigated the behavior of PFAS and other toxins in different hydrogeological settings, it’s unlikely we could anticipate the behavior of these chemicals in all hydrogeochemical conditions.

The Value of Safeguards

Some may say the increased use of EAR is an indication that we’ve come full circle in terms of how we manage water. After all, we’re building fewer dams and reservoirs, and we’re incorporating green infrastructure into many more water-planning projects in a nod to where water naturally wants to go. With EAR technology, the interconnected character of groundwater and surface water is at last recognized as one vital and critical resource. But although EAR is a technology that mimics a core hydrologic process, it’s still a tool of engineering—and with it comes its share of risks. Above all, we shouldn’t throw caution to the wind as more projects move forward. Our groundwater supplies are generally still free of anthropogenic contaminants, but it’s in our interests to ensure this remains the case for years to come.

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About Chris Maeder

Chris Maeder, P.E., M.S., CFM, is engineering director at CivilGEO Inc.; email: chris.maeder@civilgeo.com.

The post Thoughts From Engineers: Enhanced Aquifer Recharge Goes Mainstream first appeared on Informed Infrastructure.

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