Green Infrastructure and Groundwater Protection

Many types of green infrastructure can be designed to collect stormwater, treat pollutants, and allow stormwater to soak into the ground (i.e., infiltrate) as it would in a natural setting. These types of green infrastructure have the potential to interact with groundwater. For example, some types of green infrastructure, such as constructed wetlands, often intersect with the water table to function properly. For other types of green infrastructure, such as bioretention (pdf), stormwater is filtered through the green infrastructure practice and may infiltrate into the surrounding soil (where additional pollutant removal may occur), promoting groundwater recharge. In these infiltrating practices, the bottom layer of the green infrastructure should be adequately separated from the water table to aid infiltration and maximize pollutant removal before reaching groundwater. Groundwater recharge is often desirable in arid regions but, if not done properly, can also introduce pollutants that could contaminate groundwater.

To manage this potential risk to groundwater quality, stormwater managers and designers should consider a number of factors when planning for, designing, siting, and operating and maintaining green infrastructure that either infiltrates into or intersects with the groundwater. This webpage discusses some key considerations and points to other sources of information to protect groundwater when planning and designing green infrastructure practices.

Green Infrastructure Siting and Design Considerations

Proper siting of green infrastructure can allow stormwater to recharge groundwater where feasible. Improper siting, on the other hand, has the potential to introduce pollutants into the groundwater. If introducing pollutants into the groundwater is a concern (e.g., due to high concentrations of pollutants on or near the site), the green infrastructure design could focus on evapotranspiration or filtration rather than infiltration. Many types of green infrastructure (e.g., green roofs, rainwater harvesting) can be designed to provide water quality benefits without infiltration, which reduces the risk of contaminating groundwater. The following sections provide an overview of siting and design considerations that can help maximize the benefits and effectiveness of green infrastructure while protecting groundwater quality.

Important Siting and Design Questions

To help protect groundwater, stormwater managers should ask the following types of questions when determining site suitability:

• What soil and subsurface conditions are present (e.g., soil type, porosity, presence of confining layers)?
• What are the existing infiltration rates of the underlying soil?
• What is the depth to groundwater or the water table height?
• Is the site located in a karst region?
• What potential pollutants may be in the stormwater?
• What was the prior use of the property?
• Are there drinking water wells or septic drain fields nearby?

There are also design-related questions that design engineers should consider to help protect groundwater:

• What pretreatment can be added to reduce pollutants?
• What soil amendments may be used to reduce pollutants?
• Can adequate separation between the water table and the bottom layer of the stormwater control measure be achieved? If not:
  • Is an impermeable liner needed to prevent the movement of pollutants into groundwater?
  • Is an underdrain needed to reduce infiltration?

By answering questions like these during the planning for green infrastructure, stormwater managers and design engineers can make informed decisions about appropriate siting and design modifications that will help protect groundwater quality. For general information about designing with operation and maintenance (O&M) in mind visit EPA’s O&M considerations for green infrastructure webpage.

Depth to Groundwater

Design engineers must ensure appropriate separation between the bottom layer of green infrastructure practices and the seasonal high water table to protect groundwater. Designers should generally aim to have at least two feet of separation (EPA, 2004). Some types of green infrastructure (e.g., constructed wetlands) may be designed to intersect with groundwater, meaning that separation is not possible. In these cases, the practices should be sited away from hot spots and potentially contaminated soils. Designers can incorporate pretreatment, where appropriate, to further reduce potential harm to groundwater quality.

Groundwater Recharge

In regions where groundwater recharge is the goal, green infrastructure can be designed to increase infiltration. Factors such as land cover and soil conditions influence the amount of feasible recharge. Where site conditions allow, the infiltration achieved by green infrastructure can be increased by using highly porous soil mixtures; not including an underdrain; or increasing the storage capacity of the green infrastructure, which allows the facility to capture more stormwater and infiltrate it over time. In these cases, designers need to ensure adequate design for pollutant removal (which may include a pretreatment component in the green infrastructure practice) and proper siting away from hot spots and potentially contaminated soils.

Stormwater Hot Spots

Hot spots are locations where the designated land use—or related activities—can generate high concentrations of pollutants with the potential to impact stormwater. Green infrastructure that infiltrates into or intersects with the groundwater should not be sited in locations that receive stormwater from hot spots without proper consideration. If green infrastructure is installed downstream from a stormwater hot spot, engineers can make modifications or choose a green infrastructure design that does not infiltrate or intersect with groundwater to mitigate the potential contamination of groundwater. Design choices to consider include installing an impermeable liner; designing green infrastructure that uses evapotranspiration, filtration, and/or biofiltration rather than infiltration; or siting green infrastructure in an area where there is appropriate separation from the water table.

Brownfields

Brownfields are areas of development or land reuse in which there is a presence of hazardous materials or pollutants. Infiltrating stormwater on sites with known or likely contamination may mobilize the pollutants and increase the potential for groundwater contamination. Stormwater managers and designers should take this into account when determining what types of green infrastructure to install on brownfield sites. EPA’s document entitled “Implementing Stormwater Infiltration Practices at Vacant Parcels and Brownfield Sites (pdf)” provides information to communities, developers, and stormwater managers who are considering implementing green infrastructure infiltration practices at brownfield sites. The document notes that “[w]ith careful site analysis and planning, decision-makers can plan for stormwater management practices which promote the infiltration of stormwater while minimizing the potential for mobilizing contaminants.”

Resources

Name	Summary	Key Words
The Influence of Green Infrastructure Practices on Groundwater Quality: The State of the Science	This 2018 EPA literature review provides an overview of the current knowledge on potential risks for groundwater contamination from green infrastructure infiltration practices. Main issues addressed include: 1) pollutant risks that need further research, 2) new infrastructure that has not been researched in depth, and 3) determining local considerations when planning for green infrastructure.	Groundwater Contamination, Literature Review, Planning Considerations
Green Infrastructure Opportunities and Barriers in the Greater Los Angeles Region: An Evaluation of State and Regional Regulatory Drivers that Influence the Costs and Benefits of Green Infrastructure (pdf)	This EPA report outlines regulatory opportunities and barriers for implementing green infrastructure with a focus on practices that contribute to groundwater recharge.	Regulations, Implementation, Groundwater Recharge
Cost Effective Tools for Assessment of Infiltration at Green Infrastructure Stormwater Management Sites	EPA’s 2019 Science Brief provides an overview of cost-effective technologies that can be used to monitor the performance of infiltration-based green infrastructure.	Resistivity, Monitoring, Technology, Groundwater Recharge
Implementing Stormwater Infiltration Practices at Vacant Parcels and Brownfield Sites (pdf)	This 2013 EPA publication outlines the decision-making process for determining whether infiltration practices are appropriate for brownfield sites and vacant parcels. Successful implementation of infiltration practices on these types of sites requires careful site analysis and planning.	Groundwater Contamination, Implementation, Brownfield Sites
Stormwater Best Management Design Guide: Volume 2, Vegetative Biofilters	This 2004 EPA design guide provides information on the selection and design of grass swales, filter and buffer strips, and bioretention cells.	Best Management Practices, Design Considerations, Groundwater Recharge
National Menu of Best Management Practices (BMPs) for Stormwater-Post-Construction	EPA’s fact sheet series on post-construction best management practices provides information about the applicability, implementation, and effectiveness of various BMPs for stormwater managers who are developing their programs.	Best Management Practices, Implementation
Enhanced Aquifer Recharge Research	This EPA webpage introduces Enhanced Aquifer Recharge (EAR) and its potential application in multiple groundwater-related processes. The webpage also provides links to related research, modeling tools, and publications.	Enhanced Aquifer Recharge, Technology, Groundwater Contamination
Memorandum: Clarification on Which Stormwater Infiltration Practices/Technologies Have the Potential to Be Regulated as "Class V" Wells by the Underground Injection Control Program (pdf)	EPA’s 2008 memorandum supports the use of infiltration practices to manage stormwater and clarifies the Underground Injection Control Program requirements for stormwater infiltration practices that can be classified as Class V wells.	Underground Injection Control, Injection Wells, Implementation, Regulations
The Influence of Stormwater Management Practices and Wastewater Infiltration on Groundwater Quality: Case Studies	EPA’s 2020 publication presents the results from a 2015 study conducted under the Safe and Sustainable Water Resources program regarding potential impacts to groundwater quality from green infrastructure. Three sites were selected and studied, all differing in climate, geology, type of infrastructure used, and geographic location.	Groundwater Contamination, Case Study
Know Before You Grow: Understanding Site Risks for Garden and Growing Activities (pdf)	This EPA infographic from 2021 provides guidance for those who want to convert a brownfield site into a successful community gardening project. The infographic covers details such as important questions to consider before beginning these projects, how to assess risks from potential site contamination, understanding differences in intended use, and where to turn for technical assistance.	Brownfield Sites, Community Garden
It Is Not Easy Being Green: Recognizing Unintended Consequences of Green Stormwater Infrastructure	This article from Water journal discusses the danger in implementing green infrastructure without fully understanding the potential consequences. The article examines six practices and proposes a decision-making framework for properly implementing green infrastructure.	Sustainability, Environmental Justice, Implementation, Groundwater Contamination
Green Stormwater Infrastructure Redirects Deicing Salt from Surface Water to Groundwater	This 2020 study, published in Science of The Total Environment, takes the practice of winter deicing salt application and analyzes lesser-known impacts on water quality. The study evaluates soil chemistry and long-term storage of salt ions to better understand how infiltration-based green infrastructure manages salt.	Road Salt, Soil Chemistry, Groundwater Contamination
Modeling Effects of Low Impact Development on Road Salt Transport at Watershed Scale	This 2019 study from the Journal of Hydrology focuses on low-impact development practices and their limited consideration of road salt pollution. A modeling approach was implemented to assess the effectiveness of these practices against salt contamination in an urban headwater watershed by monitoring stream chloride concentrations.	Low Impact Development, Road Salt, Groundwater Recharge
Effect of Spatial Allocation of Green Infrastructure on Surface-Subsurface Hydrology in Shallow Groundwater Environment	This paper from the 2019 World Environmental and Water Resources Congress looks at the challenge of balancing surface runoff control and groundwater quality protection in shallow groundwater locations. A hydrological model was used to analyze the impact of spatial allocation of green infrastructure on surface hydrologic performance and groundwater dynamics.	Groundwater Height, Modeling, Technology, Performance
Performance and Water Table Responses of Retrofit Rain Gardens	Published in the Journal of Hydrologic Engineering in 2013, this case study focuses on the lack of knowledge about the application of rain gardens in urban retrofitting. The study focuses on evaluating the performance of terraced, street-side rain gardens, including monitoring water tables during simulated runoff events.	Case Study, Best Management Practices, Performance
A Review on Implementing Infiltration-Based Green Infrastructure in Shallow Groundwater Environments: Challenges, Approaches, and Progress	This 2019 review from the Journal of Hydrology identifies both the challenges and progress of using infiltration-based green infrastructure in shallow groundwater areas.	Groundwater Height, Design Considerations, Literature Review