Relationship between the concentrations of dissolved organic matter and polycyclic aromatic hydrocarbons in a typical U.K. upland stream

Concentrations of total and freely dissolved polycyclic aromatic hydrocarbons (PAHs) and dissolved organic carbon (DOC) were measured in water collected during 4 sampling events at five sites from the River Wyre. The sites are typical of streams draining upland organically rich soils in northwest U.K. Freely dissolved PAHs were separated from those associated with DOC using a flocculation method. The sum of concentrations of the total and freely dissolved PAHs analysed ranged from 2.71 to 18.9 ng/L and 2.61 to 16.8 ng/L respectively. PAH concentrations, and PAH fluxes derived from concentrations and water flow rates, generally increased downstream, the trend in the latter being more pronounced. The concentration of individual PAHs containing five or more aromatic rings was found to be strongly correlated to the DOC concentration (p < 0.0001), suggesting common terrestrial sources and hydrological pathways. In contrast, no significant relationships were observed between concentrations of PAHs with four or less rings and DOC. Concentrations of PAHs with 4+ rings showed similar seasonal variation to DOC concentration (peaking in the late Summer), while variation in 2-3 ring PAHs was out of phase with DOC (peaking in the Winter). As the PAH-DOC relationship appeared partly dependent on the molecular weight of the PAHs, a linear regression function that included an interaction between this variable and DOC concentration was used to model PAH concentrations over a two year period to estimate annual fluxes. The relationship identified between PAH concentrations and DOC should help to enhance interpretation of PAH monitoring data that are currently sparse both spatially and temporally, and thus enable more robust assessments of the potential risks of these environmental pollutants to sensitive aquatic organisms and human water supplies.