Ami L. Riscassi

Evaluation of automated streamwater sampling during storm events for total mercury analysis

Understanding the processes by which mercury is mobilized from soil to stream is currently limited by a lack of observations during high-flow events, when the majority of this transport occurs. An automated technique to collect stream water for unfiltered total mercury (HgT) analysis was systematically evaluated in a series of laboratory experiments. Potential sources of error investigated were 1) carry-over effects associated with sequential sampling, 2) deposition of HgT into empty bottles prior to sampling, and 3) deposition to or evasion from samples prior to retrieval. Contamination from carry-over effects was minimal (<2%) and HgT deposition to open bottles was negligible. Potentially greater errors are associated with evasive losses of HgT from uncapped samples, with higher temperatures leading to greater evasion. These evasive losses were found to take place primarily within the first eight hours. HgT associated with particulate material is much less prone to evasion than HgT in dissolved form. A field test conducted during a high-flow event confirmed unfiltered HgT concentrations sampled with an automated system were comparable to those taken manually, as the mean absolute difference between automated and manual samples (10%) was similar to the mean difference between duplicate grab samples (9%). Results from this study have demonstrated that a standard automated sampler, retrofitted with appropriately cleaned fluoropolymer tubing and glass bottles, can effectively be used for collection of streamwater during high-flow events for low-level mercury analysis.

Seasonal and flow-driven dynamics of particulate and dissolved mercury and methylmercury in a stream impacted by an industrial mercury source

Sediments and floodplain soils in the East Fork Poplar Creek watershed (Oak Ridge, TN, USA) are contaminated with high levels of mercury (Hg) from an industrial source at the headwaters. Although baseflow conditions have been monitored, concentrations of Hg and methylmercury (MeHg) during high-flow storm events, when the stream is more hydrologically connected to the floodplain, have yet to be assessed. The present study evaluated baseflow and event-driven Hg and MeHg dynamics in East Fork Poplar Creek, 5 km upstream of the confluence with Poplar Creek, to determine the importance of hydrology to in-stream concentrations and downstream loads and to ascertain whether the dynamics are comparable to those of systems without an industrial Hg source. Particulate Hg and MeHg were positively correlated with discharge (r(2)  = 0.64 and 0.58, respectively) and total suspended sediment (r(2)  = 0.97 and 0.89, respectively), and dissolved Hg also increased with increasing flow (r(2)  = 0.18) and was associated with increases in dissolved organic carbon (r(2)  = 0.65), similar to the dynamics observed in uncontaminated systems. Dissolved MeHg decreased with increases in discharge (r(2)  = 0.23) and was not related to dissolved organic carbon concentrations (p = 0.56), dynamics comparable to relatively uncontaminated watersheds with a small percentage of wetlands (<10%). Although stormflows exert a dominant control on particulate Hg, particulate MeHg, and dissolved Hg concentrations and loads, baseflows were associated with the highest dissolved MeHg concentration (0.38 ng/L) and represented the majority of the annual dissolved MeHg load. Environ Toxicol Chem 2016;35:1386-1400. Published 2015 Wiley Periodicals, Inc. on behalf of SETAC. This article is a US Government work, and as such, is in the public domain in the United States of America.

The use of novel biomarkers to determine dietary mercury accumulation in nestling waterbirds

Mercury (Hg) depuration into growing feathers is a well-studied phenomenon in waterbirds. Although the kinetics of Hg excretion in relation to molt and diet has been studied extensively, the relationship between the individual nutritional condition of nestlings and dietary Hg accumulation has not been investigated. In the present study, a body-condition index (BCI) and nutritional condition index (NCI) for nestlings of two waterbird species occupying different trophic positions on the aquatic food web were determined and used to predict Hg accumulation through diet. Candidate models consisting of these indices and nestling age were compared using Akaikes information criterion corrected for small sample sizes. For both species, the top-performing model contained the sole parameter of nutritional condition index (NCI). The relationship between Hg and NCI was stronger in the species foraging higher on the trophic web, which experienced higher rates of Hg depuration into feathers. Models containing BCI could not be discounted (AICc < 2) for one of the species and the utility of this index is discussed.

Association of dissolved mercury with dissolved organic carbon in U.S. rivers and streams: The role of watershed soil organic carbon

Streams and rivers are important pathways for the export of atmospherically deposited mercury (Hg) from watersheds. Dissolved Hg (HgD) is strongly associated with dissolved organic carbon (DOC) in stream water, but the ratio of HgD to DOC is highly variable between watersheds. In this study, the HgD:DOC ratios from 19 watersheds were evaluated with respect to Hg wet deposition and watershed soil organic carbon (SOC) content. On a subset of sites where data were available, DOC quality measured by specific ultra violet absorbance at 254 nm, was considered as an additional factor that may influence HgD:DOC . No significant relationship was found between Hg wet deposition and HgD:DOC, but SOC content (g m−2) was able to explain 81% of the variance in the HgD:DOC ratio (ng mg−1) following the form: HgD:DOC=17.8*SOC−0.41. The inclusion of DOC quality as a secondary predictor variable explained only an additional 1% of the variance. A mathematical framework to interpret the observed power-law relationship between HgD:DOC and SOC suggests Hg supply limitation for adsorption to soils with relatively large carbon pools. With SOC as a primary factor controlling the association of HgD with DOC, SOC data sets may be utilized to predict stream HgD:DOC ratios on a more geographically widespread basis. In watersheds where DOC data are available, estimates of HgD may be readily obtained. Future Hg emissions policies must consider soil-mediated processes that affect the transport of Hg and DOC from terrestrial watersheds to streams for accurate predictions of water quality impacts.

Using ptilochronology to determine daily mercury deposition in feathers of nestling waterbirds

Feathers are commonly used biomarkers of mercury (Hg) contamination in waterbird species. Most studies that analyze waterbird feathers for Hg content report concentrations on a per-unit mass basis. While this is appropriate for intraspecific comparisons, we suggest a more effective method for studies comparing multiple species of similar size and with similar foraging habits. Ptilochronology is a technique for determining the rate of feather growth in individuals based on their nutritional condition. When paired with Hg analysis, feather growth rates can be used to calculate the average daily Hg deposition into a feather. In this study we used this technique in comparison with the commonly used metric of Hg per-unit feather mass in two waterbird species. Average daily Hg deposition into feathers was 26.7% more sensitive to differences in Hg between the two species, suggesting that this may be a more biologically meaningful metric to use in interspecific comparisons.