This page provides information on surface water and groundwater impacts associated with infiltration of stormwater runoff.
Although data are still limited, there has been increasing work done on assessing groundwater impacts associated with stormwater infiltration. The following discussion summarizes the current state of knowledge on the topic.
The risk of groundwater contamination from different chemicals is summarized below. Specific information for each chemcial can be found at the links below.
Metals are typically present at low levels in urban stormwater and are generally retained in the upper soil layers via adsorption to solid particles (Violante et al., 2010). They therefore represent a low risk to groundwater. Exceptions may occur under the following conditions:
Management strategies to reduce the risk of metals leaching to groundwater include periodic replacement of the upper soil layer within infiltration systems, preventing runoff water containing high concentrations of sodium from entering infiltration BMPs, and maintaining soil conditions favorable to metal attenuation (e.g. near neutral pH).
Because of the diversity of organic compounds, it is difficult to generalize, but typically these are at low concentration in stormwater runoff. Many compounds are attenuated within the infiltration media, where they may be degraded or immobilized. Risk to groundwater from organic chemicals is typically low. BMPs with little or no organic material, particularly underground practices, may present some risk if concentrations of organic chemicals are elevated in stormwater runoff.
Management strategies include limiting or avoiding infiltration in areas where organic chemicals may be present in runoff (e.g. vehicle fueling areas, chemical storage areas) and developing response plans in areas where spills may occur (e.g. transportation corridors). In areas where organic chemicals may be present in runoff, bioretention practices, including tree-based BMPs, should be encouraged since the media will facilitate attenuation of organics.
Although nitrate is poorly attenuated in most infiltration BMPs, concentrations in stormwater are generally low, resulting in low risk to groundwater. Conditions that may result in elevated nitrate concentrations include development in areas with historical application of nitrogen fertilizer, areas where turf is being established (if turf is being fertilized), and use of media with high concentrations of organic nitrogen that can be converted to nitrate.
Phosphorus is strongly attracted to solid particles and the resulting risk to groundwater is low. Elevated concentrations in infiltration water may occur in areas with large inputs of soluble phosphorus (e.g. areas with large inputs from leaves) or if the infiltration media has a high concentration of organic matter. Phosphorus does not represent a risk to drinking water supplies but may negatively impact surface waters if groundwater discharges locally to lakes or streams. In areas where surface waters are impaired for phosphorus, infiltration practices should be designed to minimize the risk to the impaired water, including use of media with lower organic matter content and locating the infiltration practice away from the impaired water.
Chloride is not attenuated within infiltration systems. Chloride presents a concern for groundwater where concentrations are high in stormwater runoff. This occurs in areas where application of choride-based deicers is high, such as major transportation and high density commercial areas.
Concentrations of pathogens in stormwater runoff are often high. There is limited information on fate and transport of pathogens through infiltration systems. Infiltration practices that utilize media with organic matter are likely to be most effective in attenuating pathogens, while systems with little or no organic matter, particularly underground systems, are less effective. More information is needed on the fate of pathogens in stormwater infiltration systems. Once in groundwater, survival of microorganisms depends on factors such as temperature, pH, and presence of organic matter. Die-off of most microorganisms is fairly rapid, with a three order magnitude decrease in population within 100 days or less. Some organisms, viruses in particular, may survive for one year or more, however (Krauss and Griebler, 2011; Bitton et al., 1983: Toze, 2003).
Nieber et al. (2014) monitored leachate through three infiltration BMPs using lysimeters. The study was conducted over an 18 month period. The three BMPs included an infiltration basin, a large infiltrating rain garden ed within Como Park, and an infiltration gallery constructed in a former industrial area. Below is a summary of results. Note, the authors do not provide summary statistics in the report.
Nitrate and nitrogen: Hsieh and Davis (2005) and Dietz and Clausen (2005) observed less than 35 percent nitrogen removal in rain gardens. Hunt et al (2006) observed less than 40 percent reduction in nitrogen for rain gardens. These and other studies typically show reductions in reduced nitrogen (organic nitrogen and ammonia) with occasional increases in nitrate concentrations in leachate. This is expected as reduced forms of nitrogen can be converted to nitrate in the presence of oxygen. Although nitrate was not attenuated in these studies, nitrate concentrations in the runoff entering these BMPs was well below the drinking water standard of 10 milligrams per liter.
Metals: Note these reviews are summarized in Weiss et al. (2008). Hunt et al. (2006) observed an 81 percent reduction in mass loading for lead in a rain garden field study (Hunt et al., 2006) and Davis (2007) observed a lead removal rate of 83 percent in a field study. Davis et al. (2003) observed a 64 percent reduction in zinc. In another field study Dietz and Clausen (2006) observed zinc below reporting levels in effluent. Backstrom (2003) observed a 66 percent reduction in zinc in grassed swale field sites. Hunt et al. (2006) observed a 98 percent reduction in mass loading for zinc in a rain garden field study. Davis et al. (2003) observed a 43 percent reduction in copper in one field study and a 50 to 60 percent removal in another study (Davis, 2007). Dietz and Clausen, 2006 found 98% of influent copper was retained in a mulch layer. Backstrom (2003) studied field sites with grassed swales and found the swale provided a total reduction of 34 percent for copper. Hunt et al. (2006) observed a 99 percent reduction in mass loading for copper in a rain garden field study. Limited data exist for other metals, although laboratory column studies show removal rates of 95 percent or more for cadmium (Sun and Davis, 2007). Datry et al. (2004) did not observe increases in metal concentrations in groundwater beneath an infiltration basin.
Organics: Hsieh and Davis (2005) found that removal efficiency of oils and greases from a synthetic stormwater was nearly 100 percent and 99 percent removal was observed during a natural rain event. Datry et al. (2004) did not observe hydrocarbons in groundwater beneath an infiltration basin.
Pathogens: Weiss et al. (2008) state "Documented pathogen contamination of groundwater due to infiltration practices has occurred (Clark et al. 2006) and E. coli have been shown to pass through stormwater sand filters (Clark and Pitt 2007). Dietz and Clausen (2005) found fecal coliforms concentrations to be less than 10 colony forming units (CFU) per 100 mL in both the influent (from roof runoff) and the effluent from a rain garden. Rusciano and Obropta (2007) created a bioretention column in which horse manure was fed to the column to simulate a [bacterial] pollutant source. Results indicated the median reduction in fecal coliform was 98.6%".
|Concentrations (ug/L) of select chemicals in shallow groundwater (Source: MPCA, 2001) and comparison with Class 2B surface water standards.|
|Surface water standard (ug/L)1||230000||In development. Likely to be in 3000 to 5000 ug/L range. See .||59 to 343, depending on hardness||30 to 90 for lakes and 50 to 150 for rivers and streams, depending on ecoregion2|
|1 Source: Minnesota Administrative Rules, 7050.0222|
2 Many lakes, rivers, and streams have site-specific standards. See the above source for further information.
Surface waters (lakes, streams, rivers, wetlands) may be impacted by infiltration practices if infiltrating water moves laterally to the surface water body. Understanding surface water impacts from infiltration is complicated and will depend on local hydrogeologic conditions. We were unable to find studies directly linking effects of stormwater infiltration on surface water quality.
Two sources of data can be used to estimate potential effects of infiltration on surface water. First is a comparison of chemical concentrations in shallow groundwater under urban areas with surface water standards. For this we used data from MPCA, 2001. Concentrations of chemicals in shallow groundwater under established urban areas should reflect long-term inputs from infiltration, though not necessarily from infiltration through infiltration practices. The table at the right provides a summary for chemicals that occur at elevated concentrations in shallow groundwater under urban areas compared to undeveloped areas. The table includes surface water standards for Class 2B waters. The results show that concentrations in shallow groundwater are below surface water standards.
A second source of information would be to compare concentrations in infiltrating stormwater with surface water standards. There is limited data to complete this analysis. An additional complication is that this comparison represents a worst-case scenario since infiltrating water will mix with groundwater. The extent to which mixing occurs varies with each hydrogeologic setting. Nieber et al. (2014) observed an exceedance rate of the 230 mg/L standard for chloride in about 15 percent of samples collected beneath three infiltration BMPs. About 80 percent of samples exceeded 100 mg/L for phosphorus. Median zinc concentrations were about 20 ug/L. The report did not specifically compare concentrations in leachate with surface water standards. The results suggest that caution should be exercised when locating infiltration practices near surface water impaired by phosphorus.