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The Influence of Green Infrastructure Practices on Groundwater Quality: The State of the Science
Brumley, J., C. Marks, A. Chau, R. Lowrance, J. Huang, C. Richardson, S. Acree, R. Ross, AND D. Beak. The Influence of Green Infrastructure Practices on Groundwater Quality: The State of the Science. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-18/227, 2018.
Green infrastructure (GI) is increasingly being used to manage urban stormwater runoff. How the soils and subsurface geology/sediments interact with the stormwater runoff constituents has received little attention and the possible risks of groundwater quality impairment is poorly understood. The goal of literature review is to provide the current understanding of potential impacts or impacts to groundwater quality that could result from the use of GI to manage stormwater runoff. The results of the literature review were mixed; in some cases, there were impacts or potential impacts, and in other cases there were no impacts found. Many of the studies’ results were problematic. In most cases, the results—reflected only what occurs in the vadose zone or the infrastructure—were extrapolated to predict what may occur to the groundwater. This extrapolation ignores other processes that could facilitate the transport of contaminants to the groundwater, such as preferential flow. In other cases there was no attempt made to measure concentrations of contaminants in aquifers or deeper in the vadose zone, and therefore, no definitive evidence for changes in groundwater quality. These results indicate that more research is needed to address potential risk of groundwater contamination that could result from the use of GI. These results will inform decision makers in EPA Office of Water, EPA Regional Staff, States and Local Governments of the potential for groundwater impacts resulting from the use of GI for stormwater management.
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.