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Investigating Stormwater Hydrocarbon Fate and Biodegradation in Bioretention Areas
April 2012 (volume 7 - issue 3)
Funded by the National Science Foundation (Grant No. DGE-0504195), the University of Minnesota Water Resources Center, NSF Graduate Research Fellowship, and a University of Minnesota Graduate School Fellowship.
Introduction and Background
"Conventional" stormwater mitigation practices, i.e., retention ponds, appear ineffective at controlling some pollutants (Kamalakkannan et al. 2004; Weinstein et al. 2010). For example, polycyclic aromatic hydrocarbons (PAHs) tend to accumulate in retention pond sediments suggesting that ponds are unable to attenuate this important class of pollutants (Kamalakkannan et al. 2004; Weinstein et al. 2010). Low impact development (LID) is an alternative stormwater management approach that reduces negative impacts of stormwater on the watershed by using infiltration to mimic the native hydrology (National Research Council 2008). One common LID best management practice (BMP) in use is bioretention. Bioretention cells, also called raingardens or bioinfiltration practices, are shallow vegetated depressions containing an engineered soil or media (e.g., compost-amended sand) into which stormwater from impervious surfaces is directed for infiltration. Current research indicates that properly designed and installed raingardens are effective at infiltrating the majority of small rainfall events (Davis et al. 2009; LeFevre et al. 2010). Nevertheless, concerns have been expressed about the potential for contaminating groundwater resources due to intentional infiltration of pollutant-containing stormwater (Pitt et al. 1999; Weiss et al. 2008).
Figure 1: Bioretention cell in residential area. Note curb-cut where stormwater enters from street. (Photo: LeFevre)
Relatively little research has been conducted on the fate of stormwater hydrocarbons in bioretention areas, despite the common occurrence of these pollutants in stormwater within urban catchments (Figure 2). Typical sources of petroleum hydrocarbons to the watershed include leaky storage tanks, automotive emissions, elicit dumping, spills, and tire particles (Davis and McCuen 2005). Urban runoff then transports these hydrocarbons to aquatic environments (Menzie et al. 2002). Coal-tar based seal coats, primarily used in the eastern and central portions of the United States, contribute substantial quantities of PAHs to urban stormwater (Mahler et al. 2012; Van Metre et al. 2009). Therefore, determining the removal and fate of stormwater hydrocarbons in bioretention cells will help to to document their role in protecting water resources and ensuring the sustainability of these BMPs. In this research, petroleum hydrocarbon levels in media from bioretention field sites were quantified as well as the fate of a representative hydrocarbon in laboratory-scale bioretention systems.
Figure 2: Petroleum Hydrocarbons are common urban stormwater pollutants. (Photo: LeFevre)
Research Approach and Findings
Field Sampling. Over seventy soil samples from over fifty bioretention areas in the Twin Cities metropolitan area were collected and analyzed for petroleum hydrocarbon residual (LeFevre et al. 2012a). Different catchment land uses were examined, including parking lots, roof runoff, and streets. The mean total petroleum hydrocarbon residual was significantly greater in bioretention soils than in soils from background sites, but all soil concentrations were about one thousand times less than regulatory action levels (LeFevre et al. 2012a). Furthermore, the concentrations observed were all several orders of magnitude less than predicted levels based on conservative estimates of loading rates, suggesting that petroleum hydrocarbons are being degraded in bioretention soils (LeFevre et al. 2012a). To further investigate the potential for petroleum hydrocarbon degradation in bioretention cells, controlled experiments were performed in the laboratory.
Laboratory Experiments. Understanding fate and loss mechanisms and their rates are important for bioretention design. In the laboratory, bioretention cells (Figure 3) were constructed using glass columns wherein all fluxes of a representative hydrocarbon, naphthalene (a PAH), could be determined (LeFevre et al. 2012b). Adsorption to soil, plant uptake, biodegradation, volatilization, and leaching from the bottom of the column were measured. The experiment was run for five months, with repeated pollutant spiking. Overall, pollutant adsorption to soil was the single largest fate (>50%). Biodegradation (i.e., mineralization or conversion to CO2) was substantial (12-18%) and some plant uptake (2 to 23%) occurred. Volatilization was minimal (<0.1%), as was leaching following the first-flush. Biodegradation was further examined in the laboratory using batch testing (Figure 4). Field soils were capable of fully degrading naphthalene (LeFevre et al. 2012a), and biodegradation kinetics were enhanced after extended exposure of the soil bacteria to the pollutant (LeFevre et al. 2012b). Presence of vegetation significantly improved biodegradation kinetics, or the time required to degrade the pollutant (LeFevre et al. 2012b). Substantial enrichment of an important biodegradation gene was also observed in bacteria present in bioretention soils following extended (i.e., 5 months) contact with the hydrocarbon (LeFevre et al. 2012b). This research represents the first comprehensive examination of hydrocarbon fate in bioretention cells, and the first to study hydrocarbon biodegradation in bioretention media (Roy-Poirier et al. 2010).
Figure 3: Column experiments determined fate of naphthalene in bioretention cells. (Photo: LeFevre)
Figure 4: Batch tests were used to determine biodegradation kinetics. (Photo: LeFevre)
Implications of Research
Overall, this research suggests that bioretention can be an effective means of mitigating hydrocarbon pollutants in stormwater and demonstrates that biodegradation of stormwater hydrocarbons is occurring in these systems. In the case of hydrocarbon pollutants like naphthalene, biodegradation typically destroys the contaminant, rather than simply retaining or transforming the contaminant, and is thus a desirable outcome. It must be stated, however, that this research did not examine every possible hydrocarbon that could be present in stormwater, and thus biodegradation rates and the relative importance of the various fate mechanisms may vary with compound.
One can envision hydrocarbon removal largely as a two-stage process. First, hydrocarbons are removed from the infiltrate via adsorption to organic matter in the bioretention media (a very fast process); subsequent biodegradation between storm events keeps hydrocarbons in the bioretention cell from accumulating (slower, occurring between storm events). The findings in this research (LeFevre et al. 2012a; LeFevre et al. 2012b) contrast findings from research concerning stormwater retention ponds, where PAHs have been observed to accumulate to potentially harmful levels (Kamalakkannan et al. 2004; Weinstein et al. 2010). One probable reason that bioretention cells perform differently than ponds regarding hydrocarbons is because bioretention soils are well-drained and maintain aerobic conditions favorable for biodegradation, whereas stormwater pond sediments are typically devoid of oxygen (Kamalakkannan et al. 2004). Although anaerobic biodegradation of hydrocarbons is possible, the rates are much slower.
The research also suggests three important design parameters that can improve bioretention performance for stormwater hydrocarbon removal. First, a bioretention area must not become clogged and waterlogged, or anoxic conditions (as found in retention pond sediments) can prevail and substantially reduce biodegradation rates. Second, planting bioretention with deep-rooted vegetation creates an enhanced environment for hydrocarbon-degrading bacteria and increases biodegradation kinetics. Finally, bioretention media should contain some organic matter (not pure sand) to allow adsorption of hydrocarbon pollutants (and future biodegradation) while protecting the underlying groundwater.
- Davis, A. P., Hunt, W. F., Traver, R. G., and Clar, M. (2009). Bioretention Technology: Overview of Current Practice and Future Needs. Journal of Environmental Engineering-ASCE, 135(3), 109-117.
- Davis, A. P., and McCuen, R.,H. (2005). Stormwater Management for Smart Growth. Springer, New York.
- Kamalakkannan, R., Zettel, V., Goubatchev, A., Stead-Dexter, K., and Ward, N. I. (2004). Chemical (polycyclic aromatic hydrocarbon and heavy metal) levels in contaminated stormwater and sediments from a motorway dry detention pond drainage system. Journal of Environmental Monitoring, 6(3), 175-181.
- LeFevre, G. H., Hozalski, R. M., and Novak, P. J. (2012a) The role of biodegradation in limiting the accumulation of petroleum hydrocarbons in raingarden soils." Water Res., DOI: 10.1016/j.watres.2011.12.040.
- LeFevre, G. H., Novak, P. J., and Hozalski, R. M. (2012b). Fate of Naphthalene in Laboratory-Scale Bioretention Cells: Implications for Sustainable Stormwater Management. Environ.Sci.Technol., 46(2), 995-1002.
- LeFevre, N. B., Watkins, D. W., Jr., Gierke, J. S., and Brophy-Price, J. (2010). Hydrologic Performance Monitoring of an Underdrained Low-Impact Development Storm-Water Management System. Journal of Irrigation and Drainage Engineering-ASCE, 136(5), 333-339.
- Mahler, B. J., Metre, P. C. V., Crane, J. L., Watts, A. W., Scoggins, M., and Williams, E. S. (2012). "Coal-Tar-Based Pavement Sealcoat and PAHs: Implications for the Environment, Human Health, and Stormwater Management." Environ.Sci.Technol., 46(6), 3039-3045.
- Menzie, C. A., Hoeppner, S. S., Cura, J. J., Freshman, J. S., and LaFrey, E. N. (2002). Urban and suburban storm water runoff as a source of polycyclic aromatic hydrocarbons (PAHs) to Massachusetts estuarine and coastal environments. Estuaries and Coasts, 25(2), 165-176.
- Pitt, R., Clark, S., and Field, R. (1999). Groundwater contamination potential from stormwater infiltration practices. Urban Water, 1(3), 217-236.
- Roy-Poirier, A., Champagne, P., and Filion, Y. (2010). Review of Bioretention System Research and Design: Past, Present, and Future. J.Environ.Eng., 136(9), 878-889.
- Van Metre, P. C., Mahler, B. J., and Wilson, J. T. (2009). PAHs Underfoot: Contaminated Dust from Coal-Tar Sealcoated Pavement is Widespread in the United States. Environ.Sci.Technol., 43(1), 20-25.
- Weiss, P., LeFevre, G., and Gulliver, J. (2008). Contamination of Soil and Groundwater Due to Stormwater Infiltration Practices: A Literature Review. University of Minnesota, St. Anthony Falls Laboratory Project Report no.515. Prepared for Minnesota Pollution Control Agency. Available June 23, 2008. http://www.pca.state.mn.us/index.php/view-document.html?gid=7732
We want to hear from you!!!
Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions:
- Are there any siting concerns regarding hydrocarbon pollution?
- What could be the fate of higher molecular weight PAHs in bioretention areas?
- What are the best ways to prevent clogging of bioretention soils, better design, better maintenance, upstream treatment for suspended solids, or a combination of these?