Evaluating sediment cap performance with PDMS profilers : field study of McCormick and Baxter Superfund Site
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During the Fall of 2009, a pilot study was conducted at the McCormick and Baxter (M&B) Superfund Site to evaluate polydimethylsiloxane (PDMS) profilers as a method for sediment cap performance monitoring. The profilers are shielded solid phase microextraction fibers, silica rods coated with polydimethylsiloxane (PDMS) as the sorbent. The deployment explored whether profiling PDMS could be used as a low impact, highly sensitive, long term monitoring strategy at M&B since the sediment operable unit will be transitioning to the “Operable & Maintenance” phase of Superfund cleanup. To evaluate sediment cap performance, a good understanding of the flux of contaminants from the sediment is required. While surface waters can illustrate this flux, they can also contain contaminants originating from upstream sources. The existing sampling plan at M&B uses a conventional porewater pumping sampling technique that measures both dissolved and particulate fractions. PDMS profilers measure only the freely dissolved fraction which has been shown to be a good indicator of bioavailability. These profilers also have lower detection limits (ng/L) and the ability to measure vertical concentration gradients which can help identify sources and mechanisms of the contamination. Each sampler was analyzed at three depths for the USEPA Priority Pollutant List of 16 polycyclic aromatic hydrocarbons (PAHs). PDMS measurements showed clear vertical profiling with large reductions in PAH concentrations through the sediment cap offshore. Nearshore, uniform concentration profiles were observed indicating increased vertical mixing as a result of tidal smearing. Further, ANOVA and Tukey-Kramer HSD analysis of sample variability revealed PAH concentrations were statistically different at two locations compared to the remainder of the sampling locations- indicating areas requiring closer attention. PDMS profilers were co-located at 13 of the 22 conventional porewater extraction sampling locations. The correlation of PDMS and conventional porewater extraction techniques was limited due to the low detection frequency by conventional porewater extraction methods as a result of the higher detection limits by that method. The correlation was good for light molecular weight PAHs with most measurements of the same order of magnitude and improved with increasing depth (due to the greater number of detections). 72% of the direct comparisons between PDMS-derived and conventionally-derived porewater concentrations were of the same order of magnitude. Any comparison between the two datasets is necessarily limited especially for the higher molecular weight PAHs, however, due to the large number of non-detects in the conventionally collected data.