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The Influence of Green Infrastructure Practices on Groundwater Quality: the State of the Science

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The Influence of Green Infrastructure Practices on Groundwater Quality: the State of the Science EPA/600/R-18/227 | August 2018 | www.epa.gov/research The Influence of Green Infrastructure Practices on Groundwater Quality: The State of the Science photo Office of Research and Development National Risk Management Research Laboratory – Groundwater, Watershed, and Ecosystem Restoration Division EPA/600/R-18/227 August 2018 The Influence of Green Infrastructure Practices on Groundwater Quality: The State of the Science by Jessica Brumley† , Christopher Marks‡ , Alexis Chau* , Richard Lowrance‡ , Junqi Huang‡ , Cassie Richardson# , Steven Acree‡ , Randall Ross‡ , and Douglas Beak‡ † NRC Post Doctoral Research Associate Ada, OK 74820 ‡ U.S. Environmental Protection Agency Office of Research and Development National Risk Management Research Laboratory Ada, OK 74820 * ORAU Student Contractor Ada, OK 74820 # Environmental Research Apprenticeship Program Ada, OK 74820 Notice/Disclaimer Statement This document has been reviewed in accordance with U.S. Environmental Protection Agency policy and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. ii Abstract Green infrastructure (GI) technologies applied to stormwater are developed to mimic natural infiltration and hydrologic processes. GI is a design strategy that enhances runoff storage volume, infiltrates runoff, and contributes to groundwater recharge. Urban development often leads to the removal of vegetation and soil, and replacing them with large stretches of impervious surfaces. This disturbance of the natural hydrologic cycle due to urbanization is closely connected to deteriorating urban water quality and enhanced flood risks. When GI is used for urban runoff, there are concerns as to how the soils and subsurface geology/sediments interact with the stormwater runoff constituents, thus providing possible risks of groundwater quality impairment. Groundwater can be contaminated by many constituents: nutrients, metals, dissolved minerals, pesticides, other organics, and pathogens. This review provides insight into the current state of knowledge of the influence of GI on the subsurface environment and groundwater. All types of GI were assessed, both surface and subsurface infiltration infrastructures from peer- reviewed literature, published reports, and conference proceedings. 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. When managing water resources, the tendency for contaminants to move between the ground and surface water needs to be considered. This requires an understanding of the native soil characteristics in the unsaturated zone and saturated zone as well as the hydrology. The primary geochemical processes that need to be considered as stormwater infiltrates are dissolution and precipitation, redox, ion exchange, adsorption/desorption, complexation/chelation, kinetics, mixing relationships, and colloid- facilitated transport. Simulation models are a potentially affordable way to predict risk as well as provide a decision-making tool for implementing GI. While many models are used to assess surface water and groundwater transport, few integrate GI; those that do integrate GI do not address groundwater contaminant transport. The biology of the system can have various impacts. Microorganisms such as bacteria, viruses, and parasites can be a contamination risk depending on the unsaturated and saturated zone conditions, incubation time, and native microbial populations. Macrobiological organisms can enhance or cause complications for green infrastructure, but research on these is limited. Riparian zones do not have any studies specific to urban GI, but previous studies on riparian zone restoration show they could restore denitrification to urban streams, induce recharge, and serve as a less manipulative approach for enhancing infiltration into alluvial groundwater. Overall, a better understanding of the risks associated with GI is needed to recognize the implications of GI on a longer temporal scale and wider spatial scale. When implementing GI, the local geology, climate, hydrology, biology, geochemistry, type of infrastructure, and contaminant loads need to be carefully considered to reduce risks to groundwater. iii Foreword The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting the Nation's land, air, and water resources. Under a mandate of national environmental laws, the Agency strives to formulate and implement actions leading to a compatible balance between human activities and the ability of natural systems to support and nurture life. To meet this mandate, EPA's research program is providing data and technical support for solving environmental problems today and building a science knowledge base necessary to manage our ecological resources wisely, understand how contaminants affect our health, and prevent or reduce environmental risks in the future. The National Risk Management Research Laboratory (NRMRL) within the Office of Research and Development (ORD) is the Agency's center for investigation of technological and management approaches for preventing and reducing risks from pollution that threaten human health and the environment. The focus of the Laboratory's research program is on methods and their cost-effectiveness for prevention and control of pollution to air, land, water, and subsurface resources; protection of water quality in public water systems; remediation of contaminated sites, sediments and ground water; prevention and control of indoor air pollution; and restoration of ecosystems. NRMRL collaborates with both public and private sector partners to foster technologies that reduce the cost of compliance and to anticipate emerging problems. NRMRL's research provides solutions to environmental problems by: developing and promoting technologies that protect and improve the environment; advancing scientific and engineering information to support regulatory and policy decisions; and providing the technical support and information transfer to ensure implementation of environmental regulations and strategies at the national, state, and community levels. As part of the Safe and Sustainable Water Resources Research Program (SSWR) a report on the current understanding of the potential impacts to groundwater quality from the use of Green Infrastructure (GI) for stormwater management was initiated as part of the SSWR 5.02.B research effort. The goal of this report is to enhance scientific information on groundwater impacts and interactions from green infrastructure. In addition, the effort will provide the basis for long-term research on the effectiveness of GI as a suite of Best Management Practices for water resources. Potential impacts to groundwater quality are of at least three general types: 1) direct contamination of groundwater by infiltrated dissolved and suspended surface contaminants (e.g. microbials, oil and gas, pesticides); 2) indirect contamination through changing aquifer conditions that allow a potential contaminant to be mobilized (e.g., arsenic mobilization due to redox changes); 3) interaction of infiltrated water with existing subsurface contaminants (in either soil, subsoil, or groundwater) that could alter the spatial extent of existing contamination. The results of this report will be used to inform current and future GI research efforts on the potential pathways for changes in groundwater quality, as well as knowledge gaps requiring further investigation. Ann Keeley, Acting Director ORD/NRMRL/GWERD Ada, Oklahoma iv Table of Contents Notice/Disclaimer Statement ..................................................................................................... ii Abstract .................................................................................................................................... iii Foreword .................................................................................................................................. iv Acronyms and Abbreviations .................................................................................................... x Acknowledgments .................................................................................................................. xiii 1.0 Introduction ........................................................................................................................ 1 1.1 Stormwater Definitions ................................................................................................ 1 1.2 Urban Stormwater ...................................................................................................... 1 1.3 Green Infrastructure ................................................................................................... 3 1.3.1 Green Roofs .................................................................................................. 8 1.3.2 Bioretention or Swales ................................................................................... 9 1.3.3 Permeable Pavement .................................................................................. 11 1.3.4 Dry Wells ..................................................................................................... 12 1.4 General Information on Groundwater.......................................................................
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