Environ. Sci. Technol. 2010, 44, 2932–2939
Quantitative Source Apportionment of PAHs in Sediments of Little Menomonee River, Wisconsin: Weathered Creosote versus Urban Background S C O T T A . S T O U T * ,† A N D THOMAS P. GRAAN‡ NewFields Environmental Forensics Practice, LLC, 300 Ledgewood Place, Suite 305, Rockland, Massachusetts and Weston Solutions, Inc., 750 E. Bunker Court, Suite 500, Vernon Hills, Illinois
Received November 4, 2009. Revised manuscript received January 14, 2010. Accepted February 19, 2010.
Polycyclic aromatic hydrocarbons (PAHs) in urban environments are often derived from point and nonpoint sources, the latter collectively considered as urban background. Quantifying the contributions of point sources and urban background is important for managing and remediating urban sediments. In this work, the sources of PAHs in 350 sediments from a 1.5-mile portion of the Little Menomonee River (Milwaukee, WI) were determined using principal component analysis (PCA), chemical fingerprinting, and positive matrix factorization (PMF), the combination of which mitigates weaknesses of any one method. At issue was quantifying the contributions of a creosote pointsource formerly located 3.5 to 5.0 miles upstream versus urban background-derived PAHs in the sediments. In total, creosote and urban background contributed 27 and 73% ((14%) of eight carcinogenic PAHs (CPAHs), respectively, in this part of the River. The concentrations of CPAHs derived from urban background were highest in surface sediments (0-6 in.; 20 ( 17 mg/kg), particularly near major roadway crossings, increased in the downstream direction, and (on average) exceeded the 15 mg/kg regulatory cleanup threshold. Weathered creosotederived CPAHs were widespread at low concentrations (4.8 ( 8.1 mg/kg) although some discrete sediments, mostly at depths below 6 in., contained elevated CPAHs derived from creosote. This work demonstrates the value of combining multiple techniques in source apportionment studies in urban sediments. It further demonstrates a means to determine the concentration of PAHs attributable to nonpoint sourced background in urban sediments without the need to identify, collect, and analyze (assumedly) “representative” background samples, which may not even exist in heterogeneous urban watersheds.
Introduction Urban sediments contain polycyclic aromatic hydrocarbons (PAHs) derived from a variety of anthropogenic sources including (1) “point” sources associated with definable acute or long-term industrial activities, such as spills/seeps of * Corresponding author phone: (781) 681-5040; fax: (781) 6815048; e-mail:
[email protected]. † NewFields Environmental Forensics Practice, LLC. ‡ Weston Solutions, Inc. 2932
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petroleum or coal-derived liquids, and (2) “non-point” sources, such as atmospheric fallout of combustion-derived particles, channelized stormwater runoff, or general surface runoff from the surrounding urban or industrial communities. The collective contribution of nonpoint sources to urban sediments is referred to as “urban background” (1). Recognizing and unraveling the relative contributions of PAHs derived from different point sources and from urban background, i.e., source apportionment, is the principal means to control or manage their input and allocate liability for remedial activities. Apportionment is particularly important when managing remediation of sediments containing mixtures of industrial-derived PAHs and urban background wherein a clear understanding of the background may affect the remedial process or help establish an appropriate cleanup standard. Current methods for source apportionment of PAHs generally fall under two categories, viz., chemical fingerprinting or receptor modeling. Chemical fingerprinting relies upon qualitative or quantitative comparisons of PAH concentration profiles, or “fingerprints”, versus those of candidate source materials or reference materials (2). Numerous studies have demonstrated the advantages including data beyond the 16 regulated priority pollutant PAHs, for example, including total hydrocarbon “fingerprints”, alkylated PAHs, sulfur-containing aromatics, or petroleum biomarkers (3). Qualitative chemical fingerprinting can usually recognize a predominant PAH sourcesand in this way roughly provide a source apportionmentsbut can become quickly confounded when sediments contain hybrid fingerprints resulting from mixtures of more than one source. Quantitative chemical fingerprinting is more objective than qualitative fingerprinting and relies upon comparison of diagnostic ratios based on the concentrations of individual PAHs or PAH groups (4–10), the stable isotopic composition of individual PAHs (11–13), or the concentrations of sourcespecific tracers (14). Source apportionment can then be achieved by establishing one or (preferably) more diagnostic metrics based upon the chemical difference between known or conceived “end-member” sources, and then mathematically determining their mass contributions in mixed samplesspreferably using absolute concentrations rather than ratios, which do not mix linearly (15). This approach becomes increasingly difficult when more than two sources are present. Receptor modeling involves the application of multivariate statistical methods to identify and quantify pollutant sources (16). Receptor models rely upon either chemical mass balance (CMB) approaches or factor analyses and are not confounded by any number of candidate sources. CMB models rely upon input of PAH concentration profiles from a predetermined set of candidate PAH sources within a given study area (17). The linear combination of the candidate source profiles is then used to best explain the profiles in each receptor (18–20). In contrast to CMB models, factor analyses do not require knowledge of the candidate PAH sources or their profiles a priori. As such, factor analysis can sometimes recognize PAH sources not recognized by (included in) CMB models of the same data set (21). Factor analyses for source apportionment most often rely upon (1) principal component analysis (PCA) in combination with multiple linear regression (MLR) or (2) positive matrix factorization (PMF). PCA-MLR uses PCA to determine a set of principal components that increasingly explain the variance within the data. PCAs factor scores for each sample are then used to calculate the percent contributions of the major 10.1021/es903353z
2010 American Chemical Society
Published on Web 03/29/2010
dibenz[a,h]anthracene, and benzo[g,h,i]perylene, collectively known as CPAHs - was of practical importance to remediation of the sediments in the study area (Figure 1). Specifically, a 15 mg/kg (dry wt) CPAHs cleanup standard had been established for sediments over the entire 5-mile stretch of the LMR in 1998. This cleanup standard was determined from the concentrations of CPAHs measured in “background” sediment samples collected upstream of the facility and from multiple tributaries. It was hypothesized that background CPAHs may be higher than 15 mg/kg due to the increased urbanization within the study area (Figure 1). Such variability is not atypical in urban waterways and demonstrates the difficulty of how to adequately represent “background” in a spatially heterogeneous urban watershed. The objective of this study was to use PCA, chemical fingerprinting, and PMF to quantify the contribution of background-derived CPAHs in the study area and, in doing so, demonstrate a means to quantify background-derived (nonpoint source) PAHs without collecting and analyzing “representative” background samples, which may not even exist in heterogeneous urban watersheds.
Experimental Section
FIGURE 1. Map showing the 5-mile stretch of the Little Menomonee River (LMR) between the former Moss-American wood treating facility and the confluence with the Menomonee River. The study area described herein spans from miles 3.5 to 5.0 downstream of the former wood treating facility. Inset shows the location of LMR in the greater Milwaukee area. sources using MLR (22). PCA-MLR has been used to apportion the sources of PAHs in urban soils (23) and sediments (24). PMF was developed in the early 1990s (25) and has been most often applied to atmospheric, temporally distributed data sets (22, 26, 27). PMF offers an advantage over PCA in that it can handle missing values much better than PCA and can account for precision of the data. In recent years, PMF has been successfully applied to spatially distributed data sets to apportion the sources of PAHs in soils (28, 29) and sediments (21, 30, 31). In this work, a combination of chemical fingerprinting, PCA, and PMF are used to recognize and apportion the contributions of high molecular weight PAHs derived from weathered creosote versus urban background in sediments from a 1.5 mile section of the Little Menomonee River (LMR) in northwest Milwaukee, Wisconsin (Figure 1). The study area is 3.5 to 5.0 miles downstream from a former wood treatment facility, the Moss-American Superfund Site, which had operated between 1921 and 1976. Sediments proximal to and downstream from the facility had been impacted by the historic discharge of creosote wastes. Remediation of the facility’s soil and groundwater and sediments up to 3.5 miles downstream of the facility was completed between 2000 and 2006. Assessment work conducted in advance of remediation of sediments in the study area suggested an increasing contribution of PAHs derived from urban background, which is reasonable given the River traverses an increasingly urbanized watershed that includes two major road crossings and numerous drainage culverts and tributaries (Figure 1). Determining the sources of eight high molecular PAHss benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[j/ k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-c,d]pyrene,
Details of the chemical and data analysis methods employed are provided in the Supporting Information. Briefly, a set of 350 sediment samples were collected from shallow cores representing surface (0-6 in.), intermediate (6-15 in.) and deep (15-24 in.) intervals between mile 3.5 and 5.0 of the LMR study area in June 2008. The concentrations (dry wt.) of 16 priority pollutant PAHs (Table S1) were determined using a modified EPA Method 8270C (32) that yielded a robust data set of 5600 data points (350 samples × 16 PAHs) that included zero nondetect values. These data were analyzed using principal component analysis (PCA) in order to recognize any apparent mixing trend among samples containing PAHs derived from creosote versus noncreosote, or background sources. Twenty-one samples from this trend were analyzed for (1) total petroleum hydrocarbons (TPH; C9-C44 dry wt.) using EPA Method 8015B, which provided a high-resolution “fingerprint” useful for qualitative chemical fingerprinting (33) and (2) a larger suite of 52 PAHs, alkylatedPAHs, and sulfur-containing aromatic compounds (Table S1 of the Supporting Information) using the same modified EPA Method 8270C (32). Positive matrix factorization (PMF) was performed using the U.S. EPA PMF 1.1 program, which is based upon Paatero (34), and was recently summarized by Sofowote et al. (31). Input data for PMF included the concentrations of 10 high molecular weight 4- to 6-ring priority pollutant PAHs (Table S1 of the Supporting Information) for all 350 samples analyzed. Lower molecular weight 2- and 3-ring PAHs were excluded since these PAHs are prone to the effects of weathering and therefore, can confound source discrimination (35).
Results and Discussion Priority Pollutant PAH Concentrations and Isomer Ratios. The concentrations of priority pollutant PAHs in sediments studied ranged from 78 to 790,000 µg/kg (Figure S1 of the Supporting Information). Isomer ratios sometimes used to reflect PAH source differences were evaluated. When compared to reference materials from the literature (7, 36) and creosote from upstream in the LMR, the ratios of benz[a]anthracene/Σ(benz[a]anthracene + chrysene/triphenylene) and indeno[c,d]pyrene/ Σ(indeno[c,d]pyrene + benzo[g,h,i]perylene) demonstrated that most sediments contained PAH consistent with urban background with variable contributions of creosote possible (Figure S2 of the Supporting Information). Although suggestive of mixing of creosote- and background-derived PAHs, these isomer ratios are unable to VOL. 44, NO. 8, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Principal component factor score plot (PC1 vs PC2) showing overall trends. Most samples appear as mixtures of weathered creosote and urban background. PAH profiles for representative samples (A-C) are shown. See Table S1 of the Supporting Information for compound abbreviations. Factor loadings are found in Table S2 of the Supporting Information. render any quantitative conclusions concerning the sources of PAHs in these sediments. Principal Component Analysis. PCA conducted on the priority pollutant PAH data set (16 variables × 350 samples) also revealed the presence of an apparent mixing trend. Figure 2 shows the resulting factor score plot of the first and second principal components (PC1 and PC2), which were responsible for 60% and 25% of the total variance respectively (Table S2 of the Supporting Information). Samples are spread across the PC1 and PC2 factor score plot in an arc and three representative samples’ PAH histograms are shown (Figure 2A-C). Sample A contains naphthalene, prominent phenanthrene with lesser amounts of acenaphthene, fluorene, fluoranthene, and pyrene, and minimal amounts of higher molecular weight PAHs. This distribution is generally consistent with moderately weathered creosote from Brenner et al. (37) and in upstream locations in the LMR. PAHs in sample B resemble sample A except for a reduced abundances of naphthalene, phenanthrene, acenaphthene, and fluorene (Figure 2B), which is consistent with severely weathered creosote (37). Thus, samples spanning between A and B (Figure 2) reflect a creosote weathering trend in which the more weathering-susceptible, lower molecular weight PAHs are progressively lost. Sample C is also dominated by fluoranthene and pyrene but, unlike Sample B, it also includes a marked abundance of phenanthrene and all higher molecular weight PAHs (Figure 2C). Sample C’s PAH distribution is generally consistent with urban background (1) and other combustion-derived sources of PAH typical in urban atmospheres and runoff (7, 38). Most of the 350 sediments studied tend to plot between samples B and C (Figure 2) suggesting most contain mixtures of PAHs derived from severely weathered creosote (Figure 2B) and urban background (Figure 2C). Chemical Fingerprinting. Interpretation of the PCA results was based upon the priority pollutant PAH data onlysand some knowledge of the LMR’s history. However, because reliance on priority pollutant PAHs alone can limit our ability to distinguish specific PAH sources, more detailed chemical fingerprinting data were acquired for 21 samples along the apparent mixing trend. The TPH and PAH concentrations resulting from these analyses are given in Table S3 of the Supporting Information. 2934
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Figure 3 shows the GC/FID chromatograms and extended PAH histograms for three samples spanning the apparent mixing trend. The chromatogram for sample “U” (Table S3 of the Supporting Information) in which the TPH is predominantly composed of resolved peaks that include numerous PAHs (Figure 3A), which also are evident in the corresponding PAH histogram (Figure 3B). This sample contains virtually no unresolved complex mixture (UCM), the presence of which is a characteristic feature of petroleum (39). The PAH histograms show that the nonalkylated (parent) PAHs predominate over the alkylated equivalents, which is a characteristic feature of pyrogenic PAHs (40). Thus, the chromatogram and PAH histogram for sample “U” (Figure 3A-B) appears consistent with weathered creosote (36, 37). The remaining 20 samples’ GC/FID chromatograms exhibited prominent UCM “humps” indicating the presence of a petroleum component in each (Figure 3C, E). The shapes of the UCMs were largely symmetrical, ranged from ∼C20 to C40, and reached a maximum around C28. There were no homologous series of n-alkanes observed in any of the samplessa feature characteristic of most nonbiodegraded petroleumssalthough resolved petroleum (triterpane) biomarkers were evident (Figure 3). These chromatographic features are typical of lubricating or hydraulic oils (41), which are prominent petroleum contributors to urban runoff/ background (1, 42). All of the sediments studied from along the apparent mixing trend also contained prominent resolved PAHs, dominated by fluoranthene and pyrene, which is typical of urban background (1). Higher molecular weight PAHs were present in varying amounts. For example, the PAH histograms shown in Figure 3, parts D and F, show that the relative abundance of eight CPAHs (black bars) progressively increase relative to fluoranthene and pyrene. It is difficult to explain this increase in HPAH by weathering of the creosote (e.g., Figure 3B) given that weathering-susceptible, lower molecular weight PAHs, such as phenanthrene, are retained. Thus, the relative increase in the proportion of HPAH observed must reflect variation in the sources of the PAHs, viz., an increased contribution of urban background-derived PAHs. Supporting this conclusion is concurrent variability in the relative abundances of alkylated PAHs compared to the corresponding parent PAHs. For example, the relative abundance of
FIGURE 3. GC/FID chromatograms of extractable hydrocarbons (left) and corresponding PAH and alkyl-PAH profiles (right) for sediments containing (A,B) weathered creosote, (C,D) weathered creosote mixed with urban background, and (E,F) prominent urban background. Black bars represent the eight CPAHs. See Table S1 of the Supporting Information for compound abbreviations. Data from Table S3 of the Supporting Information represent samples U, O, and A, respectively. * - internal standards. alkylated fluoranthenes/pyrenes and alkylated benz[a]anthracenes is lowest in the creosote (Figure 3B) and progressively increases, being highest in Figure 3F. The increase in the relative abundance of alkylated PAHs to nonalkylated PAHs is a feature typical of petrogenic PAH sources (40) and, in these samples, is consistent with the presence of a lubelike petroleum also indicated by the UCMs in the GC/FID chromatograms (Figure 3). The chemical fingerprinting results for selected sediments confirm that the apparent mixing trend evident from the PCA of priority pollutant PAHs (Figure 2) represents mixtures of hydrocarbons, including PAHs, derived from weathered creosote (Figure 3A,B) and urban background in the LMR (Figure 3E,F). The increased influence of urban background in the study area is due to the increased urbanization compared to upstream (Figure 1), whereby runoff from streets and parking lots and atmospheric deposition increase the load of noncreosote derived PAHs in this part of the River, where they become mixed with any allochthonous creosote historically transported downriver from the former wood treating facility (Figure 1). It is suspected that urban background in this area not only includes uncombusted petroleum and combusted petroleum particulates (soot), but that in this region, combustion of coal and wood could also
contribute PAH-laden particulates to the River. Evidence for the contribution of wood-derived PAHs lies in the presence of retene in the sediments studied (Table S3 of the Supporting Information), which has been attributed to wood burning in the Great Lakes region (43). Positive Matrix Factorization. PCA and chemical fingerprinting of the sediments from the LMR have indicated that they contain mixtures of PAHs derived from weathered creosote and urban background. Understanding the absolute concentrations of PAHs derived from both of these sources was important to developing a prudent remedial strategy for sediments in the study area (Figure 1). Of particular concern was the degree to which sediments might become recontaminated by nonpoint, background sources following remediation. This was of practical concern too since, according to the 1996 Consent Decree governing the cleanup of the Moss-American Superfund Site, the River’s sediments up to 5 miles from the facility were required to be remediated if the concentration of eight carcinogenic 5- and 6-ring PAHs, or CPAH-benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[j/k]fluoranthene, benzo[a]pyrene, indeno[1,2,3c,d]pyrene, dibenz[a,h]anthracene, and benzo[g,h,i]perylene (Table S1 of the Supporting Information) exceeded 15 mg/ kg (dry wt.). This cleanup criteria was based upon the VOL. 44, NO. 8, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. High molecular weight PAH profiles determined by PMF analysis. Black bars represent the eight CPAHs. See Table S1 of the Supporting Information for compound abbreviations. “background” concentration of CPAHs for the LMR developed prior to any remediation and based upon a limited number of “representative” sediment samples collected upstream of the former facility and in tributaries upstream of mile 3.5. The representativeness of these “background” samples and the 15 mg/kg cleanup criteria was arguable within the more urbanized study area (Figure 1). The large robust PAH data set provided an opportunity to quantitatively determine what the actual contribution of background CPAHs in the study area using PMF. Figure 4 shows the two source profiles (factors) determined by the PMF model, which were able to well predict the CPAH concentrations of sediments in the LMR as judged from a high correlation between the predicted and measured CPAH concentrations (R2 ) 0.989; Figure S4 of the Supporting Information). This two-source model also produced robust Q values that were extremely close to the theoretical Q value (Table S5 of the Supporting Information). These metrics validate that the PMF two-source solution well explains the concentrations and profiles of PAHs in the sediments studied. The source 1 profile is dominated by 5- and 6-ring PAHs whereas the source 2 profile is dominated by 4-ring PAHs, particularly fluoranthene and pyrene (Figure 4). As a result, source 1 contributes a higher percentage of CPAHs per total HPAH (92%) than source 2 (32%). These two source profiles were generally exhibited by “end-member” sediments along the apparent mixing trend recognized by PCA (Figure 2). For example, Figure 2, parts B and C, shows profiles from either end of the apparent mixing trend thatswhen interpreted in 2936
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light of the additional chemical fingerprinting data (Figure 3)sare representative of weathered creosote and urban background in the River, respectively. These two endmembers’ profiles closely match the two PMF source profiles such that it can be confidently concluded that PMF profiles 1 and 2 are attributable to urban background in the LMR and weathered creosote, respectively. One difference is that the PMF-based urban background profile (Figure 4A) contains relatively less fluoranthene and pyrene than observed in any authentic samples enriched in background HPAHs (e.g., Figure 2C or Figure 3F). This difference likely indicates that even the most background-enriched sediments studied contained at least some creosote (that contributes excess fluoranthene and pyrene relative to CPAHs). Because the concentrations of the eight CPAHs were of greatest concern to sediment remediation, the PMF results are discussed in terms of the contributions of CPAHs by creosote and background. The relative contributions of CPAHs by weathered creosote and urban background for the sediments shows little spatial variation between the three depth intervals or position (upstream to downstream) within the River (Figure S5 of the Supporting Information). Background makes a higher percentage contribution of CPAH in most samples studied regardless of their depth or position. Weathered creosote makes a higher percent contribution in only a few discrete samples, which were slightly more common in upstream locations (near Appleton Ave.) and in intermediate and deep sediments (Figure S5 of the Supporting Information). The discrete distribution of creosote-derived
FIGURE 5. Stacked histograms showing the concentrations of CPAHs contributed by weathered creosote and background for sediments (upstream to downstream from mile 3.5 to 5.0) in (A) surface sediments (0-6 in.; n ) 154), (B) intermediate sediments (6-15 in.; n ) 135), and (C) deep sediments (15-35 in.; n ) 61) as determined by PMF. Locations of the two major road crossings (per Figure 1) and the 15 mg/kg CPAH cleanup threshold are indicated. CPAHs suggests that creosote had been deposited and (mostly) buried at only a few locations (“pockets”) in the River. What is notable is that urban background-derived CPAHs are pervasive and dominant in nearly all locations and depths (Figure S5 of the Supporting Information) suggesting that runoff and direct deposition of PAH-laden particles have significantly impacted sediments in the study area. The PMF-based concentrations of CPAHs attributable to urban background and creosote are shown in Figure 5. Several trends are evident. First, in surface sediments the concentrations of CPAHs derived from urban background (1) predominate over those derived from creosote, (2) are highest near or downstream from the major road crossings, (3) generally increase downstream, and (4) regularly exceed the 15 mg/kg CPAH cleanup threshold (Figure 5A). Second, in intermediate depth sediments, the concentrations of CPAHs derived from urban background are as follows: (1) lower than are observed in surface sediments and (2) rarely exceed the 15 mg/kg CPAH threshold, except in the vicinity of the major road crossings (Figure 5B). Finally, in the deep sediments the concentrations of CPAHs derived from urban background are lower than in surface or intermediate sediments with the
only threshold exceedence occurring in one sample beneath Silver Spring Rd. (Figure 5C). If it is assumed that deeper sediments represent older sediments, then these trends indicate that the contribution of urban background (1) has increased over time, (2) has its greatest impact in sediments near the major road crossings, and (3) overall increases in a downstream direction. Each of these results would not be surprising for the study area wherein increased urbanization over time and runoff from major roadways would be expected. Although high concentrations of CPAHs derived from creosote are not observed in the surface sediments (Figure 5A) they are present at discrete sample locations in several intermediate and deep sediments, being most common near Appleton Ave. (Figure 5B,C). The presence of low levels of creosote-derived CPAHs in the surface sediments could be attributed to remobilization of creosote-impacted sediments from upstream locations (before they were remediated in 2006), e.g., during typical flooding events over the previous 80 years. The higher concentrations of creosote-derived CPAHs in discrete intermediate and deep sediments indicates the presence of creosote historically deposited and buried in isolated “pockets”. The fact that some of these occur in proximity to the major road crossings could be due to the VOL. 44, NO. 8, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Percent Contributions and Concentrations of CPAHs Derived from Background and Weathered Creosote As Determined by PMF
depth samples
shallow
intermediate
deep
all depths
0-6 in. 154
6-15 in. 135
15-35 in. 61
0-35 in. 350
73
77
70 ( 15
73 ( 14
%CPAH due to background median 81 73 mean and 78 ( 10 69 ( 15 st. dev. %CPAH due to Creosote median 19 27 mean and 22 ( 10 31 ( 15 st. dev.
27
23
30 ( 15
27 ( 14
conc. CPAH due to creosote (mg/kg) median 4.3 2.6 1.5 mean and 5.2 ( 4.3 4.9 ( 8.7 3.9 ( 13 st. dev.
8.1 13 ( 14
2.9 4.8 ( 8.1
hydrodynamics of the River, in which particle settling near bridges may be favored. The data from Figure 5 were used to calculate the statistics shown in Table 1, which bear out the trends described above. Of practical relevance is the fact that the mean concentration of CPAHs derived from urban background in shallow sediments (20 ( 17 mg/kg) exceeds the 15 mg/kg CPAH threshold that was established in 1996 for remediation of the entire 5 miles of the LMR by the Consent Decree. The 15 mg/kg threshold was based upon a limited number of background samples collected upstream of the facility (i.e., a relatively rural area) and in tributaries to the River. In retrospect, these “background” samples were probably not representative of the sediments present in the study area, i.e., between 3.5 and 5.0 miles downstream from the former wood treating facility and in an area with a greater degree of urbanization (Figure 1). This situation is not uncommon in urban waterways wherein it is difficult to represent “background” conditions by collecting and analyzing discrete and “representative” background samples. This work demonstrates a means by which background concentrations can be quantitatively determined without collecting and analyzing (assumedly) “representative” background samples. Of practical importance for the LMR is the likelihood that unless the input of background-derived CPAHs is reduced in the future it should be anticipated that surface sediments in the study area will become recontaminated to this approximate concentration (20 ( 17 mg/kg CPAH) if they were remediated to
