The objective of this project was to evaluate the performance of passive samplers, called diffusive gradients in thin film devices (DGTs), as biomonitoring tools for total mercury (THg) and methylmercury (MeHg) in benthic organisms. Mercury-specific DGTs were developed to measure labile THg and MeHg in sediments. This research was based on the premise that the porewater and solid phase labile pool of THg and MeHg available to the DGTs is correlated to the bioavailable pool of THg and MeHg in that same matrix.
The project team conducted a series of bench-scale laboratory experiments where benthic organisms were co-deployed with DGTs in a series of exposure vessels containing sediment. The team then analyzed the uptake patterns of THg and MeHg in both the tissue samples and DGT samples and evaluated whether the tissue and DGT data were correlated. Various iterations of these bench-scale experiments were conducted to investigate the performance of the DGTs as biomonitoring tools for a variety of organisms and under a variety of sediment conditions (i.e., salinity, sediment organic carbon content, and sediment THg concentrations). In addition, DGTs were deployed at a marine field site in Washington.
In some of the experiments, a very close correlation was noted between the DGT data and the tissue data. However, several experiments indicated much weaker correlative relationships; DGTs appeared to be capable of taking up THg and MeHg in cases where no statistically significant uptake was observed by the organisms, though the reverse was noted in one of the experiments. Overall, it appears that relationships between DGT and tissue data are highly variable and may depend on the sediment characteristics at individual locations. It also appears that neither DGT nor tissue samples are consistently more sensitive than the other with respect to THg or MeHg concentrations in contaminated sediments. This diminishes the utility of DGTs, as currently configured, as potential off-the-shelf direct biomonitoring tools for THg and MeHg.
The project’s results indicate that mercury uptake by benthic macroinvertebrates is more complex than what can be measured by a simple passive sampler such as a DGT, as currently configured. Future research should focus on developing a deeper understanding of the relationship between DGT and tissue data, including a closer look at the effects of mercury speciation and complexation, as well as continued work employing DGTs as mercury porewater samplers.