Abstract
Polycyclic aromatic hydrocarbons (PAHs) are environmental contaminants associated with adverse human health outcomes. Environmental justice neighborhoods experience disproportionate environmental health risks. Hurricane Harvey made landfall on August 25, 2017, bringing record rainfall and catastrophic flooding to Houston, Texas, redistributing PAHs in residential soil. We aimed to describe PAH distributions in soil in the Manchester neighborhood of Houston, TX, and identify their potential sources. Soil samples were collected from 24 residential addresses and analyzed for 16 priority PAH concentrations using an accelerated solvent extractor. PAH distribution and source determination were conducted using spatial analysis and isomer ratios. All sample sites detected PAHs in soil, with the total mass ranging from 0.75 to 69.9 ng/g, which were predominantly four-ring structured PAHs. Total PAH concentrations were highest on the northeastern border of the neighborhood, whereas lower overall concentrations of PAHs were found on the southwestern border, at the highest elevation in the watershed. The ratio indeno[1,2,3-cd]pyrene (IP) to indeno[1,2,3-cd]pyrene plus benzo[ghi]perylene indicated vehicular combustion as the primary source in 19 of 23 samples. After heavy rainfall from Hurricane Harvey in the Manchester neighborhood, PAHs in soil were unevenly distributed throughout the neighborhood, with an accumulation of PAHs in the northeastern edges. Using isomer ratios and spatial analysis, the likely source of PAHs is from use of transportation infrastructure.
Keywords: GIS, PAH, environmental exposure
Introduction
Polycyclic Aromatic Hydrocarbons (PAHs) are a group of chemicals constructed of carbon and hydrogen atoms that are created through the burning of organic substances such as oil and gas, as well as organic matter such as tobacco and meats.1 Of the >100 PAHs, 16 have been designated by the Environmental Protection Agency (EPA) as priority pollutants.2 Of the 16 priority PAHs, 15 are classified as Group B2 by the EPA, signaling that these PAHs are reasonably anticipated to be human carcinogens (NTP 2016). The priority PAHs have been linked to adverse health outcomes in humans primarily through occupational exposures, with the majority of adverse health effects from PAHs associated with chronic rather than acute exposures.3 For example, in one study of the Australian workforce, researchers estimated that 6.7% of the adult workforce is chronically exposed to PAHs on the job.4 However, there are multiple routes of exposure to PAHs in nonoccupational settings; these can include ingestion, inhalation, and dermal contact.5 Since these routes of exposure to PAHs are possible among the general population (i.e., not limited to the workplace), residents of areas located in proximity to sources of PAHs may also be vulnerable to exposure and associated health effects.6
The Houston Ship Channel (HSC) is a 80-kilometer long manmade port linking Houston, TX, to the Gulf of Mexico. Along the length of the HSC, there are more than 200 petrochemical facilities, which account for ∼25% of the petroleum refining capacity in the United States.7 Residential neighborhoods located near the HSC have documented increased risks for negative health impacts due to their proximity to the high concentration of industrial sites on the channel.8 The Manchester neighborhood of Houston is located east of downtown Houston and along the HSC near the confluence of the Sims and Buffalo Bayous and on the boundary line between two of Houston's watersheds, Sims Bayou and the San Jacinto Watershed (Fig. 1). Manchester is considered an environmental justice neighborhood and is surrounded by the HSC, a major refinery complex, a 24-line railyard, and the elevated Interstate 610 Ship Channel Bridge. Twelve facilities in Manchester report to EPA's Toxic Release Inventory.9
FIG. 1.
Study area of Manchester neighborhood in Houston, TX. Color image is available online.
Dust samples collected in the homes of Manchester residents during December 2016 were analyzed for PAHs; all of the EPA's 16 priority PAHs, including the subgroup of 7 probable human carcinogens, were found in the sampled homes.10 Results from this study were compared with an analysis of PAHs in soil samples collected in the yards of the same homes in September 2017, 1 week after Hurricane Harvey made landfall along the Texas coast on August 25, 2017, resulting in flooding that was called catastrophic, historical, devastating, and life threatening by the National Weather Service.11 Comparison of outdoor soil and indoor dust samples demonstrated probable redistribution of PAHs by Hurricane Harvey.12 Soils exposed to heavy rainfall and or flooding have been found to have higher levels of PAHs.13
The objective of this study was twofold: first, to describe the spatial distribution of PAHs found in soils in the Manchester neighborhood after a major rainfall event, and second, to describe potential sources of PAHs in the neighborhood using spatial analysis. Several potential sources of PAHs are located in or adjacent to Manchester, including a petrochemical complex that has documented leaks and emissions of hazard to the northeast,14 a heavily trafficked elevated interstate bridge to the west, and an active 24-line rail yard to the south. In addition, on the northern edge of Manchester, there are 89 chemical storage tanks and a chemical manufacturing facility.15 Finally, the HSC, which is also a potential source of pollution for Manchester due to heavy cargo ship and tanker traffic, borders Manchester to the northeast. The Port of Houston, which includes the terminals along the HSC, is one of the world's largest ports, and the second busiest in the United States in overall tonnage.
Materials and Methods
Soil
Samples of soil from the top 2 to 3 cm were collected in prepared 8-ounce glass jars on September 1, 2017, at 24 randomly selected residential addresses in Manchester linked to previous studies where a more detailed account of the site selection and initial analysis can be found.16 In short, the samples were collected primarily where the soil was still saturated from precipitation association with Hurricane Harvey: typically along the banks of drainage ditches located in front of the homes and adjacent to neighborhood streets. Before each collection, trained two-person teams donned powder-free nitrile gloves. Metal hand trowels used for soil collection were rinsed after each collection with distilled water. Jars were filled to a maximum of three-quarters capacity to prevent breakage during freezing. The latitude and longitude of each collection site were recorded and jars were stored in a cooler for transport back to Texas A&M University (∼90 miles), where they were immediately stored in a −20°C freezer.
Instrument analysis
An accelerated solvent extractor with dichloromethane was used for the extraction of organic compounds in the soil samples. The samples were extracted at 100°centigrade (C) and 1500 pounds per square inch (psi). The extracts of each sample were filtered using a sodium sulfate filtration funnel and transferred into a 250 mL volumetric flask. Activated copper was added to the samples to remove any background sulfur present and the samples were run on alumina/silica columns to further filter the extract. The extracts were then transferred to a water bath preheated to 60°C where they were concentrated to a volume of 1–2 mL through evaporation technique with a hexane exchange occurring as the final concentration approached 1–2 mL. The final sample was then run on gas chromatography/mass spectrometry. All instrument analyses were conducted by the Texas A&M Geochemical and Environmental Research Group.17 Quality control and quality assurance were performed as stated in a previous study, Horney et al.17
Data analysis
Data analysis was conducted with Microsoft Excel 2013 (Redmond, WA) and Stata Version 14 (College Station, TX), whereas spatial analysis was performed using ArcGIS Version 10.4 (ESRI, Redlands, California). For factor analysis, factor loading was set to 0.8 due to small sample size. Data were transformed and normalized for spatial analysis; this method was used in previous studies to describe the distribution of environmental pollutants.18 In our study, the spatial distributions of PAHs were interpolated using simple kriging without detrending after diagnostic semivariograms were fitted as described in prior studies.19 All maps were created using the North American 1983 Geographic Coordinate System with the North America Albers Equal Conic projection. Base map shapefiles were downloaded from the City of Houston GIS Open Data and the Texas Natural Resources Information System websites.20 The study area was defined as residential parcels east of Interstate 610 and north of the Manchester rail lines, (West: −95.265927; East: −95.252709; North: 29.722526; South: 29.714624).
Results
Twenty-three of the 24 soil samples collected were analyzed for PAH abundance as previously described. One sample was not included in the spatial analysis because it was determined to be an outlier by at least 4 standard deviations in all PAH measurements. Leaving this outlier in the analysis did not change the overall PAH distribution or ratio diagnostics but did invalidate the geostatistical assumptions for interpolation predictions, which is why it was left out.
Soil PAH levels
All 16 priority PAHs were detected at every sampling location (Table 1). However, each site varied with respect to concentrations of individual PAHs. Overall, the most abundant PAHs were benzo(b)fluoranthene, fluoranthene, chrysene, indeno(1,2,3-cd)pyrene, benzo(a)pyrene, and benzo(g,h,i)perylene. The total concentration for these compounds was 11.5535, 8.1451, 7.4829, 6.3456, 6.2186, and 6.1164 ng/g, respectively.
Table 1.
Polycyclic Aromatic Hydrocarbon (PAH) Concentration in Residential Soil in Manchester, Houston (N = 23).
| PAH compound | Minimum | Maximum | Mean | Median | Standard deviation |
|---|---|---|---|---|---|
| Acenaphthene | 0.099 | 1.327 | 0.548 | 0.512 | 0.260 |
| Acenaphthylene | 0.014 | 0.326 | 0.090 | 0.058 | 0.085 |
| Anthracene | 0.009 | 0.702 | 0.152 | 0.040 | 0.211 |
| Benz(a)anthracene | 0.023 | 4.779 | 0.578 | 0.128 | 1.150 |
| Benzo(a)pyrene | 0.026 | 6.219 | 0.616 | 0.126 | 1.341 |
| Benzo(b)fluoranthene | 0.054 | 11.554 | 1.118 | 0.219 | 2.435 |
| Benzo(g,h,i)perylene | 0.026 | 6.116 | 0.524 | 0.114 | 1.254 |
| Benzo(k,j)fluoranthene | 0.006 | 3.552 | 0.257 | 0.036 | 0.729 |
| Chrysene/triphenylene | 0.043 | 7.483 | 0.828 | 0.195 | 1.643 |
| Dibenzo(a,h)anthracene | 0.004 | 1.249 | 0.109 | 0.020 | 0.261 |
| Fluoranthene | 0.044 | 8.145 | 1.021 | 0.242 | 2.007 |
| Fluorene | 0.018 | 0.550 | 0.230 | 0.180 | 0.148 |
| Indeno(1,2, 3-c, d)pyrene | 0.014 | 6.346 | 0.513 | 0.087 | 1.315 |
| Naphthalene | 0.316 | 2.020 | 0.869 | 0.787 | 0.448 |
| Phenanthrene | 0.022 | 2.675 | 0.453 | 0.121 | 0.760 |
| Pyrene | 0.037 | 6.909 | 0.853 | 0.194 | 1.677 |
| ∑PAHs | 0.753 | 69.952 | 8.759 | 3.061 | 15.722 |
PAH, polycyclic aromatic hydrocarbon.
PAHs with four-ring structures made up nearly 42% of the total PAH concentrations in the Manchester soil samples, whereas two- and three-ring structures comprise ∼20% each. Fourteen percent of the total PAH concentrations were five-ring structures, whereas 6% were six-ring structures (Fig. 2). PAHs with four-ring structures have been found in previous studies to be indicative of anthropogenic sources (e.g., coal, vehicular emissions, etc.).21
FIG. 2.
Average PAH ring concentrations by ring number in residential soil samples in Manchester, Houston (N = 23). PAH, polycyclic aromatic hydrocarbon. Color image is available online.
Spatial distribution of PAHs
To help residents and community stakeholders in Manchester understand potential exposure to PAHs, soil sample concentrations were used to create spatial distribution maps (Fig. 3A–D). The highest total PAH (Fig. 3A) concentrations were generally on the northeastern border of the neighborhood, whereas lower overall concentrations of PAHs were found on the southwestern border, at the highest elevation in the watershed. The same spatial pattern was seen for concentrations of benzo[a]pyrene (Fig. 3B) and pyrene (Fig. 3D). Naphthalene concentrations were distributed differently, with higher concentrations along the neighborhood's southern and northwestern border. However, concentrations of naphthalene were relatively low overall.
FIG. 3(A–D).
Spatial distribution of PAH concentrations in Manchester residential study area. Color images are available online.
PAH potential source analysis
Pyrogenic source PAHs are typically found at elevated concentration in urban areas.22 Isomer ratios, including anthracene to phenanthrene (A/P) and fluoranthene to fluoranthene and pyrene (Fl/Fl+P), have been used to identify potential sources of PAHs in urban environments.23 An A/P ratio of >0.1 indicates combustion sources, whereas an A/P ratio of <0.1 indicates a petroleum source.24 The ratio of Fl/Fl+P can also be used to distinguish between petroleum combustion sources and other forms of combustion that result in PAHs Yunker et al.24 In urban environments, a Fl/Fl+P ratio >0.5 (Fl/Fl+P > 0.05) indicates pyrogenic sources of PAHs, whereas a ratio <0.4 (Fl/Fl+P < 0.4) indicates petrogenic sources. A Fl/Fl+P ratio between 0.4 and 0.5 (0.4 < Fl/Fl+P < 0.5) indicates liquid fossil fuel as a source of PAHs, such as cars and light trucks.25 Isomer ratios from the Manchester soil samples indicate that the main sources of PAHs in the area are pyrogenic (Fig. 4). The A/P ratios for each sample were all >0.1, indicating a pyrogenic source rather than a petrogenic source. Most of the samples had an Fl/Fl+P ratio >0.5, also indicative of pyrogenic sources of PAHs. Three samples have Fl/Fl+P ratios between 0.4 and 0.5, indicating that the potential source of PAHs in these samples could be automobile exhaust.
FIG. 4.
Isomer ratios of PAHs from residential soil in Manchester, Houston. Color image is available online.
The isomer ratio of indeno[1,2,3-cd]pyrene (IP) to indeno[1,2,3-cd]pyrene plus benzo[ghi]perylene (Bghi) (IP/[IP+Bghi]), although more limited for source specification, was also used to create a map for potential sources of PAHs to the Manchester neighborhood because of its utility in identifying vehicular emissions (Fig. 5). Ratio values between 0.2 and 0.5 are indicative of vehicle emissions and crude oil combustion, values <0.2 are indicative of petroleum products, and values >0.5 indicate grass, wood, and coal combustion.26 These values effectively divide the ratio into three categories for potential source of PAHs. Of the 23 samples analyzed with the IP/(IP+Bghi) ratio, 4 samples in the north eastern corner were with ratio >0.5, indicating grass, wood, or coal combustion, and 19 samples comprising the rest of the neighborhood were with ratio in between 0.2 and 0.5, indicating vehicle emissions as the main source for PAHs in this area.
FIG. 5.
Spatial distribution of IP/(IP+Bghi) isomer ratio in residential soil of Manchester, Houston. Color image is available online.
Discussion
PAH concentrations in soil samples from the Manchester neighborhood of Houston, TX, were unevenly distributed across the study area. When soil samples were collected 1 week after Hurricane Harvey made landfall, residents of Manchester reported no flooding of homes, only flooding in yards from open ditches that could not drain the amount of precipitation that was associated with the Hurricane over the short period of time. Our ability to collect soil samples 1 week after Hurricane Harvey provided a unique opportunity to improve our understanding of PAH sources and distribution subsequent to a flooding event. Higher total PAH concentrations were consistently found in the lowest lying areas of the neighborhood, where floodwaters accumulated. Since Manchester is located on the border between two major watersheds, when heavy rains occur, the neighborhood does not flood; however, there is standing water in yards and open ditches as water flows toward the HSC. As PAHs settle from air onto soil, heavy rainfall can redistribute PAHs.27
In urban settings where PAHs from anthropogenic sources are more abundant in the environment, it is important to know where water flows to understand where soil PAHs may be accumulating. Although PAH exposure can occur through ingestion, inhalation, and dermal routes,28 for this study the main concern was dermal exposure of residents due to contact with soils in their yards and nearby ditches. Although it is rare for PAHs to present acute toxicity, symptoms from chronic exposure such as those of residents in Manchester may include chronic bronchitis, dermatitis, cutaneous photosensitization, and pilosebaceous reactions.29 Direct biological measurement of PAHs is not typically possible because of the limitations in our knowledge of human background levels as well as the high cost.30 Indirect biological measurements through metabolites can be used, but there is similarly little clinical use for these measurements due to the lack of understanding of background levels of PAHs in humans.31 Understanding where PAHs accumulate in the soil can prevent some types of chronic exposure by educating residents about the importance of not having contact with soil in these areas.
Wind direction can play an important role in determining potential sources of PAHs in ambient air and soil as well as understanding the potential redistribution of PAHs from their sources.32 The soils analyzed in this study were collected 7 days after Hurricane Harvey made landfall. Air-monitoring towers located in Manchester gauge wind speed and direction and collect other data including ambient air chemical concentrations. Typically, Manchester experiences winds from all directions during a given month.33 For the 3 months before Hurricane Harvey, Manchester's average wind direction was south-southeastern. Based on wind direction alone, the PAH density of the neighborhood would be expected to be higher in the southern part of the neighborhood, but in our case, the accumulation was in the north-northeastern borders. This is potentially due to the redistribution of soils contaminated with PAHs through the draining of precipitation associated with Hurricane Harvey transporting soils in that direction. Knowing that wind direction is predominantly south and that water flows to the northeast, it is difficult to rule out any potential PAH sources based only on water flow and wind.
Given our limited sample, a better way to estimate the potential sources of PAHs in Manchester may be using spatial analysis and isomer ratios. Diagnostic isomer ratios indicated that the majority of PAHs are from combustion sources. However, these sources are nonspecific and could include PAHs from oil refining, vehicular emissions, and the combustion of coal, wood, and other materials. In an effort to narrow down potential sources of combustion, additional isomer ratios were used to assess PAH composition and distribution inside the study area. The isomer ratio of IP/(IP+Bghi) strongly suggests that the PAHs found in Manchester, save one small area in the northern part of the neighborhood, are primarily from vehicular combustion.34 Although PAH concentrations are highest in areas closest to the refinery complex and the HSC, isomer ratios indicate Interstate 610, which borders the neighborhood to the west, may play a larger role in PAH pollution than other potential sources. Interstate 610 is a 38-mile loop around the City of Houston that is heavily trafficked, carrying a daily average of 157,814 vehicles.35 The portion of Interstate 610 that borders Manchester includes the elevated Houston Ship Channel Bridge, which carries 10 lanes of freeway traffic over the HSC at a height of 135 feet over Manchester on the west side.36 The interstate is not the only potential source for vehicle emissions in Manchester. The industrial sites bordering the neighborhood have heavy truck traffic that travels through the neighborhood with great frequency. In 2017, nearby industries purchased several Manchester streets for parking and storage, which further increased truck traffic.37
Studies have linked air pollution associated with vehicle exhaust to a number of deleterious health outcomes.38 For example, exposure to increased levels of air pollution in early life stages has been shown to diminish lung function and increase susceptibility to acute respiratory illnesses and asthma.39 In cohort studies, chronic exposure to traffic exhaust has been linked to the development of respiratory symptoms in healthy children.40 There is also mounting evidence linking exposure to PAHs to inducing oxidative stress through biotransformation in the liver using cytochrome P450 and dihydrodiol dehydrogenase.41 Human and animal laboratory studies have also indicated that diesel exhaust particles can lead to enhancement of allergic inflammation and developments of allergic immune response.42
Limitations
This study has several limitations. Soil samples were collected outside of selected homes where residents had previously agreed to take part in a small pilot cohort study of environmental exposures among residents of the Manchester neighborhood being conducted between December 2016 and August 2017. This use of this sample limited the distribution and interpolation analysis to the study area, instead of allowing for the use of a gridded matrix over a larger area.43 In the collection of the soil samples, only 1 sample per location was taken, yielding a total of 23 samples after 1 sample was excluded due to being an outlier. Without having soil samples from all sides of each potential PAH source, it is difficult to use spatial statistics to predict the source.44 Spatial analysis results did not vary greatly with the outlier in the calculations; however, leaving the outlier in the calculations invalidated the kriging assumptions, thus it was not included. The sample size for this study was very small, increasing standard errors. Additional samples, which may provide additional information about possible point sources or better interpolation and source identification geospatially, were collected at the same time along the border of Manchester but have not yet been analyzed. Finally, the data used in this study were cross-sectional; all samples were collected in a single day 1 week after Hurricane Harvey made landfall. Therefore, we cannot provide any information related to the temporality of exposures.
Conclusion
After heavy rainfall from Hurricane Harvey in the Manchester neighborhood of Houston, Texas, PAH distributions in soil were found to be unevenly dispersed throughout the neighborhood, with an accumulation of PAHs in the northeastern edges. Using isomer ratios and spatial analysis, the likely source of PAHs in Manchester is vehicle combustion. Further research is warranted to verify these findings using larger sample sizes collected over a larger geographical area that includes all sides of the potential point sources.
Acknowledgments
Research reported in this publication was supported by the National Institute of Environmental Health Sciences of the National Institutes of Health under Award Number P42 ES027704 and the Texas A&M Institute for Sustainable Communities. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the article.
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