Diel mercury concentration variations in a mercury-impacted stream.

Filtered and particulate mercury (Hg) and methylmercury (MMHg), and associated water chemistry parameters, were evaluated bi-hourly for several 30 h periods during the summer and winter seasons at several distinct locations (downstream forested, midstream urban/suburban, upstream industrial) along a creek contaminated with high levels of inorganic Hg to determine if biogeochemical Hg and MMHg cycles respond to the daily photocycle. In summer particulate Hg and MMHg concentrations doubled overnight (excluding the upstream industrial site) concurrent with increases in turbidity and total suspended sediment; no such pattern was evident in winter. Seasonal and diel changes in the activity of macrobiota affecting the suspension of contaminated sediments are likely responsible for these patterns as other potential explanatory variables (e.g., instrument drift, pH, discharge) could not account for the range and timing of our observations. Diel patterns in filtered Hg (HgD) were significant only at locations and times of the year when channel shading was not present and daytime concentrations increased 22-89% above nighttime minima likely caused by direct and indirect photochemical reactions. Relationships between HgD and dissolved organic carbon (DOC) concentration or character were inconsistent between sites. Unlike HgD, there were significant diel patterns in filtered MMHg (MMHgD) at all sites and times of year, with summer concentrations peaking in mid to late afternoon while the timing differed in winter, with concentrations peaking after sunset. Daily variability in MMHgD concentration ranged between 25 and 75%. The results imply key controls on net methylation occur within the stream or on the stream bed and include factors such as small-scale temperature changes in the water column and photosynthetic activity of stream biofilm. With respect to stream monitoring, results from this study indicate (1) consistent timing in stream Hg and MMHg sampling is required for accurate assessment of long-term trends, (2) in situ measurements of turbidity can be used to quantify diel dynamics of both particulate Hg and MMHg concentrations, and (3) in situ fluorescing dissolved organic matter (FDOM), a potential proxy for DOC, was not capable of resolving diel dynamics of filtered Hg or MMHg.

Hg isotopes reveal in-stream processing and legacy inputs in East Fork Poplar Creek, Oak Ridge, Tennessee, USA.

Natural abundance stable Hg isotope measurements were used to place new constraints on sources, transport, and transformations of Hg along the flow path of East Fork Poplar Creek (EFPC), a point-source contaminated headwater stream in Oak Ridge, Tennessee. Particulate-bound Hg in the water column of EFPC within the Y-12 National Security Complex, was isotopically similar to average metallic Hg(0) used in industry, having a mean δ202Hg value of -0.42 ± 0.09‰ (1SD) and near-zero Δ199Hg. On average, particulate fraction δ202Hg values increased downstream by 0.53‰, while Δ199Hg decreased by -0.10‰, converging with the Hg isotopic composition of the fine fraction of streambed sediment along the 26 km flow path. The dissolved fraction behaved differently. Although initial Δ199Hg values of the dissolved fraction were also near-zero, these values increased transiently along the flow path. Initial δ202Hg values of the dissolved fraction were more variable than in the particulate fraction, ranging from -0.44 to 0.18‰ among three seasonal sampling campaigns, but converged to an average δ202Hg value of 0.01 ± 0.10‰ (1SD) downstream. Dissolved Hg in the hyporheic and riparian pore water had higher and lower δ202Hg values, respectively, compared to dissolved Hg in stream water. Variations in Hg isotopic composition of the dissolved and suspended fractions along the flow path suggest that: (1) physical processes such as dilution and sedimentation do not fully explain decreases in total mercury concentrations along the flow path; (2) in-stream processes include photochemical reduction, but microbial reduction is likely more dominant; and (3) additional sources of dissolved mercury inputs to EFPC at baseflow during this study predominantly arise from the hyporheic zone.

Emerging investigator series: the effect of wildfire on streamwater mercury and organic carbon in a forested watershed in the southeastern United States.

Wildfires alter forested ecosystems, which include large stores of mercury (Hg) and organic carbon, two compounds that are closely linked in vegetation, soils, and streamwater. Studies have shown that wildfires release elevated levels of mercury to the atmosphere which can be locally redeposited and leave charred organic material (vegetation and litter) on the soil surface. Both can contribute to the elevated mobilization of Hg into lakes and streams. However, no studies have conducted a detailed examination of hydrological transport of Hg following a wildfire. This study investigates the coupled transport of mercury and carbon at Twomile Run, a headwater stream located in the forested mountains of Shenandoah National Park, in the year following a low-severity wildfire. Weekly baseflow samples and bi-hourly high-flow storm samples were analyzed for dissolved and particulate mercury (HgD and HgP, respectively), dissolved organic carbon (DOC), UV absorbance at 254 nm (UV254, surrogate for DOC quantity and character), and total suspended solids (TSS), and were compared with identical measurements taken from a nearby unburned watershed. For all flow conditions sampled at the burned site (which did not include the 2 months following the fire), streamwater HgD and DOC concentrations, and corresponding UV254, were similar to the unburned system. TSS concentrations varied between sites but overall differences were relatively small in magnitude and likely attributable to site differences rather than fire effects. Notably, the HgP per unit of TSS at the burned site was an order of magnitude higher than the unburned site (2.66 and 0.13 ng HgP per mg TSS, respectively) for 8 months following the fire, resulting in elevated HgP concentrations for the range of flow conditions, after which there was a rapid return to non-disturbed conditions. Streamwater total Hg fluxes roughly doubled (0.55 to 1.04 µg m-2 yr-1) as a consequence of the fire, indicating that in addition to changing atmospheric and terrestrial Hg cycling, fires can rapidly and significantly alter the streamwater Hg which has implication for downstream ecosystems. These findings are particularly relevant as the occurrence and severity of wildfires are expected to increase in the mid-latitudes in response to climate change.

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.