Environ. Sci. Technol. 2010, 44, 8849–8854
Polycyclic Aromatic Hydrocarbons in Stormwater Runoff from Sealcoated Pavements ALISON W. WATTS,* THOMAS P. BALLESTERO, ROBERT M. ROSEEN, AND JAMES P. HOULE University of New Hampshire, 35 Colovos Road, Durham, New Hampshire 03856, United States
Received June 23, 2010. Revised manuscript received October 18, 2010. Accepted October 20, 2010.
Coal-tar based sealcoat has been identified as a source of polycyclic aromatic hydrocarbons (PAHs) in the environment. This study measured the long-term release of PAHs in parking lot runoff and found that the presence of coal tar sealant increased the mass of PAHs released in runoff by over an order of magnitude. PAH concentrations in stormwater from two coal tar sealed parking lots and one unsealed parking lot (control) were monitored over a two-year period. The measured flow volume and concentrations were used to calculate a mass of 9.8-10.8 kg total Σ16 PAHs per hectare exported in stormwater runoff from the two sealed parking lots and 0.34 kg total Σ16 PAHs per hectare from the unsealed control. The study also measured sediment PAH concentration changes in a receiving drainage and found that even partial coverage of a drainage area by coal tar sealant resulted in measurable increases in PAH sediment concentrations; PAH concentrations in sediment in astormwaterswalereceivingrunofffrombothsealedandunsealed lots increased near the outfall from less than 4 mg/kg prior to sealing to 95.7 mg/kg after sealing. Compound ratio plots and principal components analysis were examined and were able to clearly differentiate between pre- and postsealant samples.
Introduction Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous organic compounds released by both natural and human processes. Some PAHs have been identified as known or suspected human carcinogens (1), and the United States Environmental Protection Agency (US EPA) lists PAHs as one of the compounds most likely to be associated with Tier 1 impacted aquatic sediments, where “adverse effects on aquatic life or human health are probable” (2). The contributors of PAHs in aquatic sediments commonly identified in the literature include manufactured gas plants and other industrial activities, automotive exhaust, stormwater and sewer outfalls, atmospheric deposition, and product spills (3, 4). Recently coal-tar based sealcoat has been identified as an additional source of PAHs in the environment (5-7). Sealcoat, or sealant, is applied to asphalt surfaces to provide a thin black coating that generally lasts several years. The two types of pavement sealants commonly used in the U.S. include asphalt-based and coal-tar based emulsion. Coal * Corresponding author phone: (603)862-0585; fax: (603)862-3957; e-mail:
[email protected]. 10.1021/es102059r
2010 American Chemical Society
Published on Web 11/03/2010
tar is a byproduct of coke used in the production of steel, and coal-tar based sealants are more commonly used in the Midwest and Eastern areas of the United States where coke plants are predominantly located. Coal-tar based sealant commonly contains total PAHs over 50,000 ppm (ppm), while asphalt based sealants generally contain concentrations of less than 100 ppm (8). PAHs are released from sealcoat by dissolution, volatilization, and/or abrasion and are transported from the site by wind and in stormwater runoff in both dissolved and particulate forms. This runoff then flows to and enters receiving waters including stormwater ponds, lakes, estuaries, and streams. From 2001 to 2002, the City of Austin, TX identified unusually high PAH concentrations in creek bed sediment near sealcoat-treated parking lots (8). A subsequent study measured PAH concentrations in particulates washed off of parking lot surfaces by simulated rainfall and identified coaltar based sealant as a previously unidentified source of PAHs in streams. The study found that PAH concentrations in runoff particulates collected from coal-tar sealed surfaces were 65 times higher than in runoff from unsealed surfaces and 10 times higher than an asphalt-sealed lot (9). Because of this concern, the City of Austin, TX banned the use of coal-tar based sealcoat in 2005 (10). Other municipalities followed suit including the District of Columbia and several towns in Minnesota as well as Dane County, MN. Additional studies have identified coal-tar based sealant as a source of PAHs in surface soils (5), house dust (6), and stormwater pond sediments (11). While the overall ecological impacts from the use of coal-tar based sealcoat is not clear, accounts of fish kills attributed to excessive sealant washoff have occasionally been reported in the press (12, 13). Field and laboratory studies have also found that coal-tar based sealcoat adversely affected the growth and development of benthic and amphibian species (14-17) at concentrations within a range consistent with the Probable Effects Concentration for total PAHs in sediments (22.8 mg/kg dry weight) (18). The primary objectives of this study were to 1. determine if coal-tar based parking lot sealcoat is a significant source of PAHs in stormwater runoff and to measure the mass of PAHs exported from coal-tar sealed and unsealed surfaces over a two year period and 2. determine if the use of coal-tar based sealant leads to elevated PAH concentrations in sediments deposited by parking lot runoff. This work extended the results obtained in previous studies to better represent natural environmental conditions with a full-scale instrumented parking lot and represented the first attempt to quantitatively measure multiyear PAH yields from sealed and unsealed parking lots. This study also directly measured changes in PAH concentrations in sediments in a receiving drainage and verified that even partial coverage of a drainage area by coal tar sealant resulted in a measurable increase in PAH concentrations in sediments.
Materials and Methods Study Design. The University of New Hampshire Stormwater Center (UNHSC) collected runoff from two coal-tar sealed parking lot surfaces and one unsealed (control) parking lot. The site provided a semicontrolled environment with a known construction and maintenance history. The parking lot drainage area was divided into three flat subwatersheds comprising the two sealed test sites (lots A and B) and the unsealed control test site (lot C), which were all sampled separately (Figure 1). Lot C covers 3.65 ha (nine acres) and drains through a series of catch basins and underdrains to a sampling gallery. Lot A is located west of lot C and includes VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Stormwater runoff samples were collected from two coal-tar sealed lots (A and B) and from an unsealed control (C). The sample locations for A and C are located in drainage piping 10 and 20 m from the lots. Sediment samples were collected from a stormwater swale and a small wetland downstream of the swale. a 0.13 ha (0.3 acre) section that drains to a separate catch basin, while lot B, to the southeast, is a 0.05 ha (0.15 acre) area that drains to an outlet structure. After passing through the sampling locations, runoff from lots A and C flow into a vegetated swale, while runoff from lot B exits the site through a small weir and is directed into a tree filter. Lots A and C were installed in 1996, and lot B was installed in 2006. All of the lots are constructed of asphalt over a gravel base, and neither of the lots were previously sealcoated. Lots A and B have similar traffic use and are heavily used when the university is in session, while lot C receives approximately 50% less traffic. All lots are used lightly in the summer (JuneAugust). All traffic enters and exits through the main entrance on lot C. Snow is plowed as needed and is piled along the edges of the lots where it melts in the spring. Snow piles from each lot were not specifically separated, and some mixing probably occurred near the intersection of each lot. Sealant was applied to both lots on October 5, 2007 by a local sealant contractor. The contractor was requested to apply a coal-tar based sealant to lot A and an asphalt based product to lot B using standard industry practices (mixed with sand and water as recommended by the manufacturer). However, after the sealant was applied it was found that coal-tar based sealant had been applied to both lots and the study design was adjusted to accommodate two coal-tar sealed lots, rather than one. Immediately prior to sealing, the lots were swept and blown clean with leaf blowers, and the lot edges masked with tape, including the catch basins. No primer or surface treatments were applied, and no vegetation was present in the pavement. Two coats of sealant were used, with the second coat applied after the first coat had been allowed to dry for approximately three hours. Afterward, both sites were closed to traffic for 48 h. The temperature on October 5, 2007, as recorded at the University of New Hampshire (UNH) weather station (19), ranged from 50 °F at night to 83 °F in the afternoon and was approximately 70-80 °F while the sealant was being applied. The day was sunny, with no previous rain in six days. Starting at 7 p.m. on the following day 1.5 cm of rain fell at the site. The site was dry for 28 h after the sealant application was completed. The site conditions and sealant application methodology complied with the manufactures’ specifications, which require a minimum temperature of 50 °F during application and at least 8 h of drying time before rainfall (20). Stormwater Sampling. Twenty-four storms were sampled between October 6, 2007 and December 12, 2009. Stormwater samples were collected with automated ISCO samplers, which are refrigerated programmable devices designed for water 8850
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sampling and water quality monitoring. The samplers were triggered by a threshold flow condition and collected up to 24 preset aliquots at evenly spaced volume intervals throughout each storm to provide a volume weighted sample. Flow was measured based on pressure readings at a calibrated weir. Sample events were rejected if they did not represent the full storm, contained inadequate volume, or were influenced by equipment errors such as partially blocked sample tubes. Most samples were analyzed as whole water samples, but a subset of samples collected in 2009 was also filtered to develop an initial understanding of the partitioning of PAHs between the dissolved and particulate phase. Samples to be filtered were split using a Teflon churn splitter and then analyzed after passing through a 1.2 µm glass fiber filter. Six filtered samples were collected and analyzed from the sealed lots (A and B), and one sample was analyzed from the unsealed lot (Lot C). Sediment Sampling. Surface sediment samples were collected from three locations in a stormwater swale that drains mixed runoff from lots A and C and from two locations in a small wetland downstream of the swale (Figure 1). The swale surface is highly inhomogeneous, with sediments dispersed among pebbles, plant material, and a rip rap base layer. Sample locations were selected to represent depositional sediments (as indicated by lack of vegetation or erosion). Initial samples were collected at all five locations in early October 2007, prior to sealant application, with subsequent samples collected in June and October 2008, November 2009, and April 2010. Samples were collected by hand from approximately the top 2 cm of the sediment using a clean disposable glove. Mechanical sampling methods were not able to reliably extract sediment from the selected locations. In November 2009, a dust sample was collected from the surface of lots A and C. Samples were collected by sweeping the surface with a clean disposable hand broom and dustpan to collect approximately 30 g of particles. The selected locations were situated in drive lanes and were free from visible oil stains, cracks, or other irregularities. The sample from lot C was collected 20 m from the border of lot A. Sample Handling, Analysis, and Quality Control/Quality Assurance. Samples collected in this study were stored in polytetrafluoroethylene (PTFE) bags which were refrigerated and protected from light until analyzed. Samples were processed within 48 h of each storm. Samples which were not analyzed within 14 days of collection were frozen and held at -4 °C. PTFE bags can be frozen without breakage, allowing batch analyses or archiving of samples. Most of the water samples were analyzed at UNH for the 16 EPA Priority PAHs. Using separatory funnels and following US EPA method 3510C, water samples were extracted in methylene chloride, spiked with internal standards, and subsequently analyzed by GC/MS following US EPA Method 8270d. A subset of the water samples and all of the sediment and dust samples were sent to a contract commercial laboratory (CL) for extended PAH analyses that included alkylated homologues by GC/MS Selective Ion Monitoring (SIMs) to provide lower detection limits and additional compounds for use in PAH fingerprinting. The sediment and dust samples were prepared by dichloromethane solvent extraction (EPA Method 3570) and analyzed by GC/MS/SIMs (EPA Method 8270M) with internal standards. Any samples which exceeded the calibration range for the instrument were diluted and reanalyzed, and all solids were reported as dry weight concentrations. Laboratory quality control methods, including surrogate spike compounds, matrix spike samples, blank spikes, and lab duplicates were included with each batch analyses. All results were reviewed to verify that surrogate recoveries and relative percent deviations (RPD) were within acceptable
FIGURE 2. Total (Σ16) PAH concentrations in stormwater runoff from two coal-tar sealed parking lots and an unsealed control lot. Mean concentrations were calculated for the first three months after the sealant was applied (Oct-Dec, 2007) and for six month intervals thereafter. The means, number of samples (n) and standard deviation (σ) for each time period is shown in the table. limits (40-120% and 50%, respectively). The median surrogate recoveries for water and sediment samples were 84% (UNH water), 86% (CL water), and 65% (CL sediment). Seven field duplicate water samples (median RPD 38%) and one field replicate sediment sample (RPD 46%) were also collected. Laboratory duplicate samples were analyzed (median RPD water 39%, sediment 43%). Two water samples were split between UNH and the CL to verify comparability (mean RPD 28%). Low concentrations of PAHs were detected in rinsate samples at levels up to 50% of the lowest environmental sample as well as in trip blanks at concentrations that exceeded the estimated detection limit but were less than the reporting limit. After each storm the sample process was reviewed to determine if the sampling collection criteria were met. This included reviewing the storm hydrograph and sample volume to ensure that the selected interval represented the full storm and that adequate sample was collected in each interval. Blockage of the ISCO tubing by either sediment or ice was a common cause of sample collection failure. Samples that met the sampling criteria were processes for analyses. Chemical data were reviewed following analyses completion to ensure that all method quality control criteria were met. Samples which failed to meet the quality control criteria were rejected. Calculations. The rate of stormwater runoff was recorded every five minutes at the outlet of lot C, while flow volume from lots A and B was calculated as a percentage of flow based on the difference in area between those lots. The mass of PAHs exported from each lot during the course of the study was calculated from the total volume of runoff and from mean concentration data for five time intervals. Concentration data were divided into intervals which included an October-December 2007 time frame, followed by four, subsequent six-month intervals through December 2009. Six-month intervals provided sufficient data to develop a mean while capturing possible trends over time. The first three-month interval was separated in order to isolate differences in wear and release rates early in the sealant history. Duplicate field samples were averaged to yield a single value for the mass calculations. Statistical analyses were performed using SAS JMP8. A principal components analysis (PCA) was performed on the sediment data. The PCA data were normalized by multiplying each compound by a constant such that Σ16 PAH )1 for each sample, and one-half of the method detection limit was substituted for nondetect values.
TABLE 1. Calculated Mass of Total PAHs Exported in Stormwater Runoff from Coal-Tar Sealed and Unsealed Surfaces during the Two Year Study Period total mass of PAHs (Σ16) in kg time period Oct-Dec 07 Dec-June 08 July-Dec 08 Jan-June 09 July-Dec 09 total per lot total per hectare
C-unsealed 3.65 A-sealed 0.13 B-sealed 0.055 hectare hectare hectare 0.05 0.18 0.1 0.4 0.5 1.23 0.34
0.59 0.27 0.18 0.16 0.21 1.41 10.8
0.09 0.06 0.15 0.12 0.12 0.54 9.8
Results Water Concentrations and Mass. Total PAH concentrations ranged from 0.13 to 7.3 µg/L in runoff from the unsealed control (14 samples), 4.6 to 5890 µg/L from lot A (17 samples), and 8.12 to 642 µg/L from lot B (17 samples) (Figure 2). Concentrations from the two sealed lots appear to show a decreasing trend with the highest concentrations measured in the first three months. ANOVA analysis (R)0.05) showed a significant decreasing trend line for lot B. However, there was no statistically significant trend in concentrations from lot A, most likely due to high variance. High variability in stormwater samples is common: for example, total suspended solids measured in runoff from 2004-2007 at the UNHSC site had a mean concentration of 60 mg/L with σ ) 58. No significant trend in concentrations was found in runoff from the control lot. PAH concentrations in the filtered samples were reduced by 41-98% relative to the unfiltered splits (data included in the Supporting Information). The calculated mass of total PAHs exported from each lot is shown in Table 1. In order to facilitate comparison to other studies, the results were normalized to mass per-hectare for each time period. Sediments. Prior to sealant application, total PAH concentrations in the swale and wetland were less than 4 mg/kg dry weight. In June 2008, nine months after the sealant was applied, concentrations in the swale near the outfall had increased by over an order of magnitude (Figure 3). In April 2010, 30 months after the sealant was applied, concentrations near the outfall were approaching presealant levels, while concentrations in downstream wetland had increased slightly. VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Total PAH concentrations in sediments collected from a stormwater swale and receiving wetland prior to and after sealant was applied to a portion of the watershed. Sealant was applied on October 5, 2007. Figure is not to scale. NS ) not sampled.
FIGURE 4. a. Double ratio plots of sediment and dust samples. The baseline samples collected prior to sealant application are shown as blue triangles and are generally separate from samples with high PAH concentrations impacted by sealant. The colors indicate total PAH concentration for each sample, grading from blue (
