Environmental archives of atmospheric Hg deposition - A review.

Environmental archives offer an opportunity to reconstruct temporal trends in atmospheric Hg deposition at various timescales. Lake sediment and peat have been the most widely used archives; however, new records from ice, tree rings, and the measurement of Hg stable isotopes, are offering new insights into past Hg cycling. Preindustrial Hg deposition has been studied over decadal to millennial timescales extending as far back as the late Pleistocene. Exploitation of mercury deposits (mainly cinnabar) first began during the mid to late Holocene in South America, Europe, and Asia, but increased dramatically during the Colonial era (1532-1900) for silver production. However, evidence for preindustrial Hg pollution is restricted to regions directly downwind or downstream of cinnabar or precious metal mining centers. Excluding these areas, there has been an approximately four-fold increase in atmospheric deposition globally over the industrial era (i.e., since 1800-1850), though regional differences exist, especially during the early 20th Century. Lake sediments, peat, ice, and tree rings are all influenced by (and integrate) a range of processes. For example, lake sediments are influenced by atmospheric deposition, sediment focusing, and the input of allochthonous material from the watershed, peat records reflect atmospheric deposition and biotic uptake, ice cores are a record of Hg scrubbed during precipitation, and tree rings record atmospheric concentrations. No archive represents an absolute record of past Hg deposition or concentrations, and post-depositional transformation of Hg profiles remains an important topic of research. However, natural archives continue to provide important insight into atmospheric Hg cycling over various timescales.

Evidence for Different Reactive Hg Sources and Chemical Compounds at Adjacent Valley and High Elevation Locations.

The spatial distribution of chemical compounds and concentration of reactive mercury (RM), defined as the sum of gaseous oxidized mercury (GOM) and <3 µm particulate bound mercury (PBM), are poorly characterized. The objective of this study was to understand the chemistry, concentration, and spatial and temporal distribution of GOM at adjacent locations (12 km apart) with a difference in elevation of ∼1000 m. Atmospheric GOM measurements were made with passive and active samplers using membranes, and at one location, a Tekran mercury measurement system was used. The chemistry of GOM varied across time and location. On the basis of data collected, chemistry at the low elevation site adjacent to a highway was primarily influenced by pollutants generated by mobile sources (GOM = nitrogen and sulfur-based compounds), and the high elevation site (GOM = halogen-based compounds) was affected by long-range transport in the free troposphere over the marine boundary layer into Nevada. Data collected at these two locations demonstrate that different GOM compounds exist depending on the oxidants present in the air. Measurements of GOM made by the KCl denuder in the Tekran instrument located at the low elevation site were lower than that measured using membranes by 1.7-13 times. Accurate measurements of atmospheric concentrations and chemistry of RM are necessary for proper assessment of environmental impacts, and field measurements are essential for atmospheric models, which in turn influence policy decisions.

Mercury distribution in two Sierran forest and one desert sagebrush steppe ecosystems and the effects of fire.

Mercury (Hg) concentration, reservoir mass, and Hg reservoir size were determined for vegetation components, litter, and mineral soil for two Sierran forest sites and one desert sagebrush steppe site. Mercury was found to be held primarily in the mineral soil (maximum depth of 60 to 100 cm), which contained more than 90% of the total ecosystem reservoir. However, Hg in foliage, bark, and litter plays a more dominant role in Hg cycling than the mineral soil. Mercury partitioning into ecosystem components at the Sierran forest sites was similar to that observed for other US forest sites. Vegetation and litter Hg reservoirs were significantly smaller in the sagebrush steppe system because of lower biomass. Data collected from these ecosystems after wildfire and prescribed burns showed a significant decrease in the Hg pool from certain reservoirs. No loss from mineral soil was observed for the study areas but data from fire severity points suggested that Hg in the upper few millimeters of surface soil may be volatilized due to exposure to elevated temperatures. Comparison of data from burned and unburned plots suggested that the only significant source of atmospheric Hg from the prescribed burn was combustion of litter. Differences in unburned versus burned Hg reservoirs at the forest wildfire site demonstrated that drastic reduction in the litter and above ground live biomass Hg reservoirs after burning had occurred. Sagebrush and litter were absent in the burned plots after a wildfire suggesting that both reservoirs were released during the fire. Mercury emissions due to fire from the forest prescribed burn, forest wildfire, and sagebrush steppe wildfire sites were roughly estimated at 2.0 to 5.1, 2.2 to 4.9, and 0.36+/-0.13 g ha(-1), respectively, with litter and vegetation being the most important sources.

Anthropogenic impacts on mercury concentrations and nitrogen and carbon isotope ratios in fish muscle tissue of the Truckee River watershed, Nevada, USA.

The lower Truckee River originates at Lake Tahoe, California/Nevada (NV), USA and ends in the terminal water body, Pyramid Lake, NV. The river has minimal anthropogenic inputs of contaminants until it encounters the cities of Reno and Sparks, NV, and receives inflows from Steamboat Creek (SBC). SBC originates at Washoe Lake, NV, where there were approximately six mills that used mercury for gold and silver amalgamation in the late 1800s. Since then, mercury has been distributed down the creek to the Truckee River. In addition, SBC receives agricultural and urban nonpoint source pollution, and treated effluent from the Reno-Sparks water reclamation facility. Fish muscle tissue was collected from different species in SBC and the Truckee River and analyzed for mercury and stable isotopes. Nitrogen (delta(15)N) and carbon (delta(13)C) isotopic values in these tissues provide insight as to fish food resources and help to explain their relative Hg concentrations. Mercury concentrations, and delta(15)N and delta(13)C values in fish muscle from the Truckee River, collected below the SBC confluence, were significantly different than that found in fish collected upstream. Mercury concentrations in fish tissue collected below the confluence for all but three fish sampled were significantly greater (0.1 to 0.65 microg/g wet wt.) than that measured in the tissue collected above the confluence (0.02 to 0.1 microg/g). Delta(15)N and delta(13)C isotopic values of fish muscle collected from the river below the confluence were higher and lower, respectively, than that measured in fish collected up river, most likely reflecting wastewater inputs. The impact of SBC inputs on muscle tissue isotope values declined down river whereas the impact due to Hg inputs showed the opposite trend.