PAHs in the Fraser River basin: a critical appraisal of PAH ratios as indicators of PAH source and composition

Abstract

Parent and alkyl PAHs (51 compounds and alkyl homologues) have been quantified in suspended particulates and sediments (345 samples) from the Fraser River system, British Columbia, Canada. The best potential to distinguish natural and anthropogenic sources is exhibited by ratios of the principal mass 178, 202, 228 and 276 parent PAHs, 1,7/2,6+1,7-DMP (dimethylphenanthrene), the phenanthrene/anthracene and fluoranthene/pyrene alkyl PAH series and several less commonly applied PAHs (e.g. acephenanthrylene and pentaphene). Using these ratios we infer sources of PAH to the Fraser basin and evaluate the consistency of these source assignments and the suitability of various commonly applied PAH ratios as indicators. PAH ratios and total concentration data reveal a basin lightly impacted by a variety of sources in its remote regions, especially near roads, but heavily impacted in urban areas, particularly near Vancouver. Contamination sources shift from biomass (e.g. wood and grass) burning to vehicle emissions between remote and urban locations. Stormwater and wastewater discharges appear to collect PAH from urban areas and release them as point sources. In contaminated areas ratios are specific for combustion vs. petroleum sources, and some ratios (202 and 276) distinguish biomass or coal from liquid fossil fuel combustion. At lower concentrations multiple sources at times make interpretations based on a single ratio misleading and the higher mass ratios (228 and 276) may be most applicable to urban areas. In all cases the examination of a variety of PAH indicator ratios that encompass a range of masses is necessary for a robust interpretation.

Introduction

The Fraser River discharges 140 km³/year of fresh water and about 20 Mt/year of sediments to the Strait of Georgia from a 234,000 km² drainage basin (Fig. 1). The river runs through diverse landscapes including mountains, coniferous and deciduous forests, semi-deserts, small urban centres, transport corridors and, at its estuary, the large metropolis of Vancouver (Dorcey, 1991). Valued for its natural beauty and, especially, for the world's largest salmon runs (Northcote and Larkin, 1989), the Fraser River is threatened by human encroachment. Industry, agriculture, transport and population, all of which are expanding, discharge contaminants to the river via municipal and industrial wastewater, and these loadings are further augmented through runoff and the atmosphere (Krahn and Shrimpton, 1988, Macdonald and Crecelius, 1994, West et al., 1994, Bieger et al., 1996, Larkin and Hall, 1998, Macdonald, 1998, Bertold and Stock, 1999, Gray and Tuominen, 1999).

PAHs (polycyclic aromatic hydrocarbons) have emerged as a concern in the Fraser basin because their concentrations often exceed Canadian Federal and Provincial guidelines for the protection of aquatic life (Brewer et al., 1998). Even more alarming are observations of organisms exhibiting impairments typically caused by PAH exposure (Goyette and Boyd, 1989, Wilson et al., 2000). However, to assess correctly the significance of these findings and formulate adequate pollutant control strategies it is necessary first to determine the source(s) and composition of the PAHs responsible.

Parent and alkyl-substituted PAHs have both natural sources (oil seeps, bitumens, coal, plant debris, forest and prairie fires) and anthropogenic sources (fossil fuels and combustion). Since the PAH compositions of the two sources overlap, especially for parent PAHs, the significance of anthropogenic PAH in the environment must be evaluated against a dynamic background of natural PAH. Parent and alkyl-substituted PAHs provide some of the most definitive and ubiquitous tracers of organic matter in aquatic systems (e.g. Laflamme and Hites, 1978, Wakeham et al., 1980a, Wakeham et al., 1980b, Barrick and Prahl, 1987, Readman et al., 1987, Bouloubassi and Saliot, 1993, Yunker et al., 1993, Yunker et al., 1999, Budzinski et al., 1997, Fernandes and Sicre, 1999). Despite this widespread applicability of PAHs, most studies have been limited to a specific location or type of sample, with the result that few studies have comprehensively addressed the relative suitability of various commonly applied PAH ratios as indicators.

Here, we take advantage of an unprecedented opportunity to investigate PAH patterns throughout North America's 10th largest watershed (the Fraser River and Strait of Georgia) by assembling sediment and suspended particulate data collected between 1987 and 1997 for varied objectives by several groups (Yunker et al., 1989, Yunker et al., 1999, Sekela et al., 1995, Brewer et al., 1998, Sylvestre et al., 1998a, Sylvestre et al., 1998b, Bertold and Stock, 1999). These data include a large suite of parent and alkyl PAHs and have all been analysed by a single laboratory. From a large number of compounds, we have selected a limited set of PAH ratios that exhibit the best potential to distinguish natural and anthropogenic sources. Perhaps of equal importance, this comprehensive PAH data set provides an opportunity to evaluate the consistency and completeness of PAH source assignments using ratios, homologue distribution patterns, and less-commonly used biomarkers.