By U.S. EPA
Typical stormwater management design – pipes, sewers, and collection systems – is intended to transport rainwater runoff to sewage treatment plants or surface water bodies, since the impervious surfaces of streets and cityscapes do not allow rainwater to soak into the ground. While this design is intended to reduce flooding of streets and buildings, it can actually increase the risk of flooding and erosion when large volumes of stormwater overwhelms the treatment plants and directly enter surface waters. When the rainwater does not soak into the ground where it falls, the underground storage volume of water decreases, and the underground water supply is not replenished. Green Infrastructure (GI) is designed to mimic natural systems by allowing more rainwater to soak into the ground rather than be transported away. GI is designed to reduce stress on wastewater systems, decrease sewer overflows, and improve watershed health.
To support the adoption of beneficial GI practices, potential users like city planners need data and science-based guidance. To date, research has typically focused on water movement, GI system performance, and surface water contamination, but effects of GI on groundwater quality and quantity have been less studied. Studies have not investigated whether GI systems could potentially create a risk to groundwater by creating new pathways for contaminants to infiltrate, mobilize, and be transported to the subsurface and into groundwater resources. These contaminants can include nutrients from lawn fertilizers, pesticides, heavy metals, chloride from road de-icing salts, pathogens from human waste, and other organic compounds from sources like auto fluids.
EPA recently completed a comprehensive multi-year study on the potential impacts on groundwater from the use of GI, the first known study of its kind. Researchers intensively monitored three locations with established green infrastructure systems that are also diverse in climate, geology, and geography:
- The Portland neighborhood in Louisville, Kentucky, where raingardens were planted between the street and sidewalk to collect rainwater in underground storage galleries. All household trash, leaves, and other solids are filtered out and the infiltrating water is dispersed throughout the subsurface.
- Yakima, Washington, where a channel collects the discharge from the City’s wastewater treatment plant and transports it to a river floodplain. The slow transport rate also allows water to infiltrate from the bottom of the channel. This indirect discharge allows the treated effluent to be dispersed across the floodplain, rather than directly to the Yakima River.
- The Fort Riley, Kansas, Army base, where a permeable pavement system was installed at an elementary school parking lot on the base. The permeable pavement allows rainwater to soak through the surface to the underlying soil, unlike traditional paving surfaces, which cause water to run off.
“These three sites demonstrate the shared interests of EPA and municipalities,” says Dr. Doug Beak, EPA research geologist and principal investigator of the project. “The Fort Riley and Louisville study sites were selected to provide data on groundwater hydrology and chemistry (water quality); and Yakima was selected at the request of Region 10 to evaluate ecosystem services due to a river floodplain expansion project.”
Each location’s GI system was outfitted with various instruments to monitor and collect data like temperature, pH, and movement of the infiltrating water. Scientists also collected groundwater samples to measure materials like total dissolved solids, metals, and nutrients. At Fort Riley, researchers also monitored organic contaminants like chlorinated solvents, petroleum hydrocarbons, and pesticides. The full dataset is captured in the case study report.
The data indicates changes in groundwater quality at all three study sites. As the rainwater fell, infiltrated into the subsurface, and mixed with the groundwater supply deep underground, it altered the chemistry of the groundwater. Concentrations of chemicals, salts, dissolved metals, and other compounds in the groundwater changed as rainwater entered the supply. The compounds and their concentrations varied at each study site, due to the different GI systems and conditions, but the traditional stormwater contaminants detected did not pose a health concern. Water quality index analysis indicated that groundwater quality at these sites was suitable for human consumption, and the concentrations of organic compounds were not a regulatory concern.
While the levels of these compounds in the groundwater may not post a direct human health risk, the bigger concern is the changing composition of groundwater. Especially in areas where groundwater is used as a drinking water source, changes to the groundwater composition from infiltrating rainwater must be monitored. Understanding source water chemistry is an important factor in drinking water treatment, and a change in source water chemistry can make the current water treatment approach ineffective. Changes in water type can also affect the water supply distribution system, as the new water chemistry can react with the scale that builds up within the pipes.
While regulatory guidelines and treatment plants typically monitor contaminants with direct health impacts, like pathogens and metals, the impact of cations and anions like chloride (often from road salt application) may be underestimated or overlooked. Additionally, excess sodium from road salts can cause clogging within the subsurface, reducing infiltration and water movement. Chloride is also potentially increasing the mobility of some metals of concern.
“Changes in any source water chemistry affect drinking water processing, and treatment plants may not necessarily be looking for the kinds of geochemical changes we observed in this study,” Dr. Beak explains, “We need to have a full understanding of how green infrastructure can affect or impact our water sources to provide the best information to local decision makers and water treatment plant operators.”
This study resulted in a large, comprehensive collection of groundwater monitoring data from these sites, providing information on the potential effects these GI systems have on the quality of area groundwater. The data collected can be used by state and local agencies to understand under what conditions groundwater quality may or may not be adversely impacted by GI. Observed geochemical processes, movement of water within the system, and potential for changes to groundwater chemistry provide new information for communities and city planners to consider and use in their GI planning, and potentially their water treatment processes.
These case studies will also help inform future research directions, such as aquifer recharge studies in areas with water scarcity. Insights from this effort confirm the importance of monitoring for dissolved water quality parameters like chloride, in addition to pathogens and general water quality parameters like temperature, pH, and alkalinity, to optimize water treatment for drinking or other uses.
View the full EPA Report
Related Science Matters story: Using Cost-Effective Tools for Assessment of Infiltration at Green Infrastructure Stormwater Management Sites