Attenuation of polycyclic aromatic hydrocarbons from urban stormwater runoff by wood filters

https://doi.org/10.1016/j.jconhyd.2006.08.009Get rights and content

Abstract

A significant amount of contamination enters water bodies via stormwater runoff and, to reduce the amount of pollution, retention ponds are installed at many locations. While effective for treating suspended solids, retention ponds do not effectively remove dissolved constituents, such as polycyclic aromatic hydrocarbons (PAH). Previous laboratory studies demonstrates that aspen wood cuttings can be utilized to enhance the removal of dissolved contaminants. The objective of this pilot-scale field test was to determine if wood filters could effectively remove dissolved PAH from the runoff under field conditions. Four wood filter tests were conducted, lasting from 1 to 9 weeks, to determine the degree of PAH attenuation from the aqueous phase as a function of wood mass, residence times, and seasonable changes. The prototype wood filters removed on average between 18.5% and 35.6% (up to 66.5%) of the dissolved PAH contaminants. The PAH removal effectiveness of the wood was not affected by changes in water temperature or pH. The filter effectiveness increased with filter size and was highest in continuously submerged parts of the filter system. Also, heavier molecular weight PAH compounds (e.g. chrysene) were more effectively removed than lighter molecular weight compounds. Disassociation of weakly particle-bound PAH from the filter was identified as the most likely cause for a temporary drop of the wood filter's PAH load during intense storms. Simple filter design changes are likely to double the filter effectiveness and alleviate the disassociation problem.

Introduction

Particularly in urban areas, runoff from impervious surfaces, such as roads and parking lots, contributes significant quantities of pollutants to surrounding surface water bodies (Drapper et al., 2000, Krein and Schorer, 2000, Smith et al., 2000). Stormwater runoff can contain high levels of anthropogenic contaminants, including polycyclic aromatic hydrocarbons (PAH) and heavy metals (US EPA, 1983, Stanley, 1996, Pitt et al., 2004, Mahler et al., 2005). Upon entering the water, organic and inorganic contaminants can remain in the environment for long periods (Sanders et al., 1993), posing a threat to human health and the environment (Tuhackova et al., 2001, Grynkiewicz et al., 2002). Especially PAH originate from numerous, mostly anthropogenic sources including combustion of fossil fuels, vehicle emissions, and industrial effluent. For instance, Mahler et al. (2005) identified parking lot sealcoats as a significant source of PAH in urban runoff waters. PAH concentrations in urban environments have risen over the last two decades due to higher traffic flows and increased consumption of fossil fuel (Van Metre et al., 2000).

Most PAH are genotoxic (Harvey, 1997) and the abatement of PAH in road runoff is typically dealt with by so-called best management practices (BMP). Stormwater BMPs can be either structural or nonstructural to prevent, control, and treat polluted runoff. Retention ponds are an example of a structural stormwater BMP (US EPA, 1999). A retention pond is designed to allow settling of suspended sediment and to control the effluent flow rate into adjacent water bodies (Mallin et al., 2002, Krishnappan and Marsalek, 2002). Contaminants, such as PAH, are generally sorbed to the suspended sediment and co-settle in the retention pond. A fraction of the contaminants, however, persist in the dissolved phase. For instance, Hoffman et al. (1984) determined that between 7% and 21% of PAH in parking lot runoff were not contained in the suspended load, leaving a significant amount of PAH dissolved. Another major source of dissolved contaminants is desorption from polluted sediment. As shown by Ghosh et al. (2001), who performed laboratory desorption studies with dredged silt/clay sediments and simulated the results with four different models and at changing temperature conditions, over 90% of the PAH associated with the sediment eventually desorbed and entered the dissolved phase.

One of the most promising abatement strategies for dissolved contaminants in roadway runoff relies on low-cost biofilters. For instance, Clark et al. (2000) investigated municipal leaf compost and peat moss for treating stormwater runoff and compared the effectiveness of these biofilters to that of conventional adsorbents, including activated carbon and cation-exchange resin. Although about 3 to 4 times less effective than the conventional filters, the cost of the biofilter systems was significantly lower. Other organic materials that demonstrated heavy metal treatment capabilities for stormwater runoff include sawdust, banana peels, peanut and hazelnut shells (Brown et al., 2000, Cimino et al., 2000, Schneegurt et al., 2001, Annadurai et al., 2003). Besides for heavy metals, biofilters have been successfully used to treat dissolved pesticides, chemical dyes, orthophosphate, monoaromatics (benzene, toluene, and o-xylene) and PAH (Bras et al., 1999, Morais et al., 1999, Cimino et al., 2000, MacKay and Gschwend, 2000, Trapp et al., 2001, Palma et al., 2003, Boving and Zhang, 2004, Karthikeyan et al., 2004).

Wood in particular appears to be a promising biofilter matrix for the several reasons. First, wood is an inexpensive and renewable resource and current forest management practices produce large quantities of low quality/low value wood (US DA, 2003) that could be used for filter construction. Second, Boving and Zhang (2004) have already shown that under laboratory conditions over 90% of dissolved pyrene – one of the most prominent PAH in stormwater runoff– can be removed. Similarly high removal effectiveness was determined for three other PAH (naphthalene, anthracene, fluorene). Importantly, Boving and Zhang (2004) also showed that once the PAH were removed from solution, the wood greatly resisted contaminant remobilization when flushed with clean water. The capability of wood to remove and contain contaminants from the aqueous phase is related to the composition of the wood. Aspen wood (Populus tremula), which is the wood species used in the Boving and Zhang (2004) study, typically consists of 51% cellulose, 26% hemicellulose, 21% lignin, 1% ash, and less than 1% inorganic matter (Fengel and Wegner, 1989). Lignin is a hydrophobic polymer and is generally recognized as the principal wood sorbent (Garbarini and Lion, 1986). PAH are hydrophobic compounds and therefore expected to sorb to hydrophobic organic matter or sediment rather than remaining in aqueous solution. Wood is organic matter and because the wood lignin is also very hydrophobic, the wood is an attractive sorbent for dissolved PAH and other hydrophobic compounds (Chefetz et al., 2000, Salloum et al., 2002). Based on these considerations, placing wood in stormwater runoff is expected to enhance the contaminant removal efficiency of existing BMPs.

The goal of this study was to field test wood filters under pilot-scale conditions. The main objective was to evaluate the effectiveness of the wood filter for removing dissolved PAH from roadway runoff. Multiple filters of different sizes were investigated and the potential effects of heavy precipitation, pH, and temperature changes due to seasonal fluctuations were monitored. Potential spatial and temporal variations of PAH removal effectiveness in the filter matrix were also studied. Because of the large number of factors considered and their inherent variability no modeling of the results was attempted in this study.

Section snippets

Site selection

The field site was located off Interstate 195, exit 3 (Gano Street), in Providence, RI. The catchment area was approximately 5.26 ha and the traffic count averages 150,000 vehicles per day (RI DOT, 1999). The retention system was built in 1999 and consisted of three ponds (from inflow to outflow): settling pond, constructed wetland, and micro-pool. The wood filters were installed in the concrete-lined micro-pool because it was the deepest of the three ponds (up to 0.8 m in the center, depending

Field parameters

Table 2 summarizes the average pH, EC, DO, temperature, and total amount of precipitation measured during each of the four test. The average annual pH for storm water entering the detention pond system was 7.3, but fluctuated over the seasons. During the height of summer the pH was more acidic (6.3); while during the rest of the year the pH was close to or slightly higher than neutral (7.0 to 8.0). Similar seasonable changes were observed in electric conductivity. For instance, average EC was

Conclusion

The assessment of the wood filter removal efficiency for dissolved PAH under field conditions was the principle research objective of the pilot-scale tests. The results demonstrate that the wood filter effectiveness did not change with seasons. Particularly, there was no evidence of increasing temperatures causing decreasing PAH uptake or vice versa. Other factors, such as pH or EC, did not significantly influence the filter effectiveness either. Further, the PAH removal effectiveness was

Acknowledgements

This study was made possible by a grant from the University of Rhode Island Transportation Center. Additional support was provided by the Cooperative Institute for Coastal and Estuarine Environmental Technology (CICEET). Mr. Tony Johnson, American Excelsior Inc., generously supported wood cuttings and technical advice.

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