Mercury speciation in open ocean waters

Vital to our understanding of the biogeochemical cycling of Hg and the origin of the enhanced monomethylmercury (MMHg) concentrations in biota is knowledge of the sources, behavior and fate of methylated Hg species in natural waters. Methylated Hg species, dissolved gaseous and reactive mercury were measured in the equatorial Pacific Ocean in early 1990. Both dimethylmercury (DMHg) and MMHg were found in the subthermocline waters of the equatorial Pacific Ocean. Maxima in alkylmercury species in the O₂ minimum region coincided with a decrease in reactive mercury. A significant inverse correlation between DMHg, and MMHg, concentration and O₂ content was observed. A maximum in reactive mercury was observed in the region of the thermocline, with similar concentrations in the surface and deeper waters. Atmospheric deposition is not a significant source of MMHg to this region. The data suggest formation of alkylmercury species in the low oxygen zone, with Hg(II) being the most likely substrate. A model for the cycling of Hg species in the equatorial Pacific Ocean is presented. These results are the first direct evidence of a significant open ocean source of methylated Hg species and suggest a pathway for mercury accumulation in pelagic fish.     

Modeling the elemental mercury cycle in Pallette Lake,Wisconsin, USA

The spatial and temporal distribution of elemental Hg (Hg°) and reactive Hg (Hg_R) has been studied on Pallette Lake, Wisconsin during May – August, 1993 and May, 1994. In general, Hg° concentrations near the lake surface greatly exceeded saturation with respect to atmospheric Hg° indicating a flux out (?) of the lake. Evasional losses were estimated using a thin film model and averaged ?101 pmol m^?2 d^?1 during July and August, 1993. A large portion of atmospherically deposited Hg is re-emitted. Thus, in-lake Hg° production' and evasion to the atmosphere will significantly reduce the amount of Hg which is transported to the sediments, the principal site of methylation. Laboratory experiments were conducted to ascertain the rate of Hg° formation from Hg(II). Reduction was significantly lower in heat sterilized lakewater suggesting Hg° production was biologically mediated. The temporal distribution of epilimnetic Hg°, as measured at the lake center, was influenced by Hg° evasion, Hg° production and advective transport of water parcels of differing Hg content. Spatial gradients in Hg° and Hg_R were identified and a transport model was employed to estimate the advective flux of Hg°. The importance of atmospheric deposition and sediment-water interaction as sources of Hg_R to epilimnetic waters were examined. Porewater concentrations of Hg° and Hg_R were determined on several occasions. During May, 1994, the depletion of lakewater Hg_R following a input pulse due to rain was observed and the estimated removal rate (16–20% d^?1) agrees well with reduction rates obtained in the laboratory (23% d^?1).     

Cadmium and lead contamination of soils near plastic stabilizing materials producing plants in Northern Taiwan

Atmospheric mobilization and exchange at the air-water interface are significant features of biogeochemical cycling of Hg at the Earth's surface. Our marine studies of Hg have been extended to terrestrial aquatic systems, where we are investigating the tropospheric cycling, deposition and air-water exchange of Hg in the mid-continental lacustrine environs of northcentral Wisconsin. This program is part of a multidisciplinary examination into the processes regulating the aquatic biogeochemistry of Hg in temperate regions. Trace-metal-free methodologies are employed to determine Hg and alkylated Hg species at the picomolar level in air, water and precipitation. We have found Hg concentrations and atmospheric fluxes in these fresh water systems to be similar to open ocean regions of the Northern Hemisphere. A well constrained mass balance for Hg has been developed for one of the lakes, Little Rock Lake, which is an extensively studied clear water seepage lake that has been divided with a sea curtain into two basins, one of which is untreated (reference pH: 6.1) while the other is being experimentally acidified (current pH: 4.7). This budget shows that the measured total atmospheric Hg deposition (ca. 10 μg m⁻² yr⁻¹) readily accounts for the total mass of Hg in fish, water and accumulating in the sediments of Little Rock Lake. This analysis demonstrates the importance of atmospheric Hg depositional fluxes to the geochemical cycling and bioaccumulation of Hg in temperate lakes. It further suggests that modest increases in atmospheric Hg loading could lead directly to enhanced levels of Hg in biota. Analogous modeling for monomethylmercury (MMHg) is as yet limited. Nevertheless, preliminary data for the atmospheric deposition of MMHg indicate that this flux is insufficient. to account for the amounts of MMHg observed in biota. An in-lake synthesis of MMHg is implicated. The importance of volatile Hg which is principally in the elemental form, and its evasion to the atmosphere is also illustrated. We suggest that the in-lake production of Hg° can reduce the Hg (II) substrate used in the in-lake microbiological synthesis of MMHg.     

The role of microorganisms in elemental mercury formation in natural waters

Gas evasion of elemental Hg (Hg°) from the open ocean plays a prominent role in the global mercury cycle. Elemental Hg is formed primarily by reduction of ionic Hg in the mixed layer of aquatic systems. By culturing phytoplankton in defined media, and by incubating natural seawater and freshwater samples, we have demonstrated that Hg° is produced by microorganisms, with formation rates (0.5 to 10% d^?1) similar to those estimated from mass balance studies. Our results also suggest that <3μm microorganisms are the primary Hg reducers in natural waters. Eucaryotic phytoplankton are capable of reducing ionic Hg to Hg° but the rate of reduction is insufficient to account for the observed reduction rates found in incubated field samples. Bacteria are thus the more likely Hg reducers. In seawater, cyanobacteria such asSynecococcus may account for much of the mercury reduction, while in the eutrophic, polluted Upper Mystic Lake north of Boston other procaryotic microorganisms are contributing to the overall Hg reductive capacity of the medium. By reducing ionic Hg, microorganisms play a pivotal role in the aquatic biogeochemistry of Hg, not only by enabling evasion to the atmosphere, but by directly decreasing the amount of ionic Hg available for methylation.     

Mercury from Combustion Sources: A Review of the Chemical Species Emitted and Their Transport in the Atmosphere

Different species of mercury have different physical/chemical properties and thus behave quite differently in air pollution control equipment and in the atmosphere. In general, emissions of mercury from coal combustion sources are approximately 20–50% elemental mercury (Hg°) and 50–80% divalent mercury (Hg(II)), which may be predominantly HgCl₂. Emissions of mercury from waste incinerators are approximately 10–20% Hg° and 75–85% Hg(II). The partitioning of mercury in flue gas between the elemental and divalent forms may be dependent on the concentration of particulate carbon, HCl and other pollutants in the stack emissions. The emission of mercury from combustion facilities depends on the species in the exhaust stream and the type of air pollution control equipment used at the source. Air pollution control equipment for mercury removal at combustion facilities includes activated carbon injection, sodium sulfide infection and wet lime/limestone flue gas desulfurization. While Hg(II) is water-soluble and may be removed from the atmosphere by wet and dry deposition close to combustion sources, the combination of a high vapor pressure and low water-solubility facilitate the long-range transport of Hg° in the atmosphere. Background mercury in the atmosphere is predominantly Hg°. Elemental mercury is eventually removed from the atmosphere by dry deposition onto surfaces and by wet deposition after oxidation to water-soluble, divalent mercury.     

Foliar exchange of mercury vapor: Evidence for a compensation point

Historical studies for crop and weed species documented elemental Hg vapor (Hg°) deposition to foliage, but they used Hg° concentrations that were orders of magnitude higher than levels now known to occur under background conditions, possibly creating artificially high gradients between the atmosphere and landscape surfaces. Measurements of Hg° exchange with white oak (Quercus alba L.), red maple (Acer rubrum L.), Norway spruce (Picea abies L.), and yellow-poplar (Liriodendron tulipifera L.) foliage were conducted in an open gas exchange system that allows for simultaneous measurements of CO₂, H₂O and Hg° exchange under controlled environmental conditions. When Hg° concentrations were held at 0.5 to 1.5 ng m^?3, red maple (Acer rubrum L.), Norway spruce (Picea abies L.), yellow-poplar (Liriodendron tulipifera L.), and white oak (Quercus alba L.) foliage exhibited mean Hg° emissions of 5.5, 1.7, 2.7, and 5.3 ng m^?2 h^?1, respectively. At Hg° concentrations between 9 and 20 ng m^?3 little net exchange of Hg° was observed. However at concentrations between 50 and 70 ng m^?3 the Hg° was deposited to foliage at rates between 22 and 38 ng m^?2 h^?1. These data suggest that dry foliar surfaces in terrestrial forest landscapes may be a dynamic exchange surface that can function as a source or sink dependent on the magnitude of current Hg° concentrations. These data provide evidence of species-specific compensation concentrations (or compensation points) for Hg° deposition to seedling foliage in the 10–25 ng m^?3 range.     

FIELD OBSERVATIONS OF TOTAL GASEOUS MERCURY BEHAVIOUR: INTERACTIONS WITH OZONE CONCENTRATION AND WATER VAPOUR MIXING RATIO IN AIR AT A RURAL SITE

Total Gaseous Mercury (TGM) data (n = 5125) measured from September 9 to November 30, 1994 in Southern Québec with an automatic TGM analyser (Tekran®) are used to study the behaviour of TGM with ozone and the water vapour mixing ratio. Results show, as a whole, no straight-forward correlation between TGM and ozone concentrations. A significant correlation between TGM and water vapour mixing ratio was measured. Some back reduction of Hg(II) to Hg° in the aqueous phase and subsequent evaporation of Hg° according to Henry‘s Law is expected. The latter correlation seems to be partly ozone dependent since under ’clean conditions‘ (ozone < 30 ppbv) the slope of TGM vs the water vapour mixing ratio is twice that during ‘polluted conditions’ (ozone ≥ 30 ppbv) which suggests some compensation effects under polluted conditions (counteracting the back-reduction of Hg(II) to Hg° in the aqueous phase). Oxidation of TGM by ozone during high moisture conditions (water vapour mixing ratio ～14g/kg) seems to appear at the threshold ozone concentration ≥ 30 ppbv.     

Biogeochemical Cycle of Mercury and Methylmercury in Two Highly Contaminated Areas of Tagus Estuary (Portugal)

Mercury (Hg) dynamics was evaluated in contaminated sediments and overlying waters from Tagus estuary, in two sites with different Hg anthropogenic sources: Cala Norte (CNOR) and Barreiro (BRR). Environmental factors affecting methylmercury (MMHg) production and Hg and MMHg fluxes across sediment/water interface were reported. [THg] and [MMHg] in solids (0.31–125 μg g^?1 and 0.76–201 ng g^?1, respectively) showed high variability with higher values in BRR. Porewater [MMHg] (0.1–63 ng L^?1, 0.5–86% of THg) varied local and seasonally; higher contents were observed in the summer campaign, thus increasing sediment toxicity affecting the sediment/water Hg (and MMHg) fluxes. In CNOR and BRR sediments, Hg availability and organic carbon were the main factors controlling MMHg production. Noteworthy, an upward MMHg diffusive flux was observed in winter that was inverted in summer. Although MMHg production increases in warmer month, the MMHg concentrations in overlying water increase in a higher proportion compared to the levels in porewaters. This opposite trend could be explained by different extension of MMHg demethylation in the water column. The high concentrations of Hg and MMHg and their dynamics in sediments are of major concern since they can cause an exportation of Hg from the contaminated areas up to ca. 14,600 mg year^?1 and an MMHg deposition of up to ca. 6000 mg year^?1. The results suggest that sediments from contaminated areas of Tagus estuary should be considered as a primary source of Hg for the water column and a sink of MMHg to the sedimentary column.     

Distribution of Mercury and Monomethylmercury in Sediments of Vigo Ria, NW Iberian Peninsula

Fifty four samples of surface sediments (1-cm of thickness) and a gravity core of 154-cm long were collected in the Vigo Ria, NW of Iberian Peninsula. In the laboratory the sediments were oven dried at 40°C, desegregated, homogenised and analyzed for total Hg, monomethylmercury (MMHg), Al, and particulate organic carbon (POC). Mercury in sediments varied within a broad range: 0.09 to 8.9 nmol g^?1 the higher concentrations being found in the shipyards and docks areas of Vigo port (southern–middle Ria zone). No correlation was found between Hg and POC. The good exponential correlation between total Hg and Al levels suggests that Hg is closely associated with the fine fraction of sediments. Monomethylmercury concentrations in surface sediments ranged between <1.4 (DL) and 8.5 pmol g^?1 being the spatial distribution similar to that observed for total Hg. The lack of correlations MMHg–Al and MMHg–POC suggests that in situ processes of methylation/demethylation are responsible for MMHg distribution rather than the sediment characteristics. Additionally, these processes appear mercury-dependent only for Hg concentrations above 2.5 nmol g^?1. On the basis of these results it was estimated that 155 kg of Hg (0.1% MMHg) are present in the surface sediments of Vigo Ria, being 61% of anthropogenic origin.     

Linkages between atmospheric mercury deposition and the methylmercury content of marine fish

Enhanced Hg deposition to productive marine systems may result in concurrent increases in monomethyl Hg (MMHg) concentrations of marine fish. Consequently, it is important to understand what effects an increasing Hg supply may have on the marine food chain. A simple ocean model is employed to estimate the fraction of total Hg inputs which is required to sustain “average” marine fish MMHg concentrations annually. Calculations show that upwelling zones require 20% of total annual Hg inputs, coastal zones 5%, and open-ocean regions only 0.02%. The value for coastal areas is similar to that calculated for the acidified basin of Little Rock Lake, Wisconsin, a small fresh water seepage lake. These calculations point to Hg source strength and rates of particle scavenging as being key factors in controlling the rate of transport to sites of methylation (and subsequent entry into the marine food chain). If biological variables (scavenging rates, primary productivity) remain constant while anthropogenically-derived Hg deposition increases, it is likely that concentrations in marine biota (including fish) will rise in accord.     

Volatilization of demethylmercury and elemental mercury from river Elbe floodplain soils

It has been shown that an untreated mercury-polluted floodplain soil (containing 10 μg/g per dry weight (d.w.) total Hg and 12 ng/g (d.w.) monomethylmercury compounds (MMM)) of the river Elbe in Northern Germany contains both dimethylmercury (DMM) and elemental mercury (Hg°). This is the first time ever that DMM has been detected in unmodified soils. A novel purge- and-trap-technique involving a sequential thermodesorption-separation of the two species after trapping on a carbon molecular sieve (CMS) has been developed that allows the determination of the two species DMM and Hg° from aqueous solutions or soil samples by GC-CVAFS. The compounds' identities as Hg-species were confirmed by GC-ICP/MS. A DMM-concentration of 740 pg/g (d.w.) was determined in the soil; the Hg°-concentration was found to be at least four times larger, but could not yet be quantified. Since no precautions against losses via evapoartion were taken during sampling and storage, the original concentrations were probably much higher. Both DMM and Hg° are easily purged with N₂ from soils as well as from soil suspensions, indicating that the two species may readily evaporate from those soils under natural conditions. The amount of DMM determined in the soil suspension was significantly lower (80 pg/g (d.w.)) compared to that in the original soil sample, suggesting that DMM might not be stable under these conditions. Also, it was shown that in natural samples, MMM can be converted into DMM in the presence of sulfide, at S^2?-levels as low as 100 μg/g.     

Gaseous Elemental Mercury Fluxes in New York City

As with many urban environments, a number of sources of airborne elemental mercury (Hg°) exist in New York City, yet little research has been conducted to examine the flux and sources of mercury in New York. In this study, we conducted ambient monitoring of Hg° at six locations in New York City. Airborne Hg° averaged 3.84±0.10 ng m^-3 and 3.70±0.08 ng m^-3 in the boroughs of Manhattan and Brooklyn respectively, yet only 2.69±0.03 ng m^-3 in a more residential neighborhood in Queens. Both precipitation and ambient temperature were significantly correlated with ambient Hg° levels in New York City, suggesting that the surface emission of mercury from urban surfaces plays a role in urban Hg° concentrations. Local sources were also seen to contribute to urban Hg° levels by leading to `spikes' of Hg° in which elevated concentrations were recorded for short periods of time.     

Mercury in the surface water of Swedish forest lakes —concentrations,speciation and controlling factors

Concentrations of total Hg and five operationally defined Hg species were determined in the surface water of 25 Swedish forest lakes of different type. Regional and seasonal variations were studied during the ice-free season of 1986. The concentration of total Hg was usually in the range of 2 to 10 μg m-3. Hg concentrations were highly correlated to the concentration of humic matter measured as water color. Hg concentrations were about twice as high in acidic lakes (pH 5) than in circumneutral lakes, which is attributed basically to the acidity of humic compounds acting as Hg carriers in boreal waters. Significant seasonal variations were caused by hydrological processes. During periods of high water flow, Hg concentrations increased dramatically, especially in humic lakes. Between spring and autumn, chemically reactive Hg compounds were gradually replaced by more inert species. Hg/C ratios were higher than in surface runoff from forest watersheds, indicating a significant impact of direct deposition of Hg on lake surfaces during summer. Regional differences were small despite differences in Hg contamination.     

Development of a fugacity/aquivalence model of mercury dynamics in lakes

A simple, mechanistic model of mercury (Hg) dynamics in a lake has been developed, based on the fugacity/aquivalence approach of Mackay (1991) and Mackay and Diamond (1989) and its extension to treat several interconverting chemical species (Diamond et al., 1992). The model considers the distribution of inorganic (HgII), elemental (Hg°) and methyl (MeHg) mercury species between dissolved and particle-sorbed phases, and fate and transport in a system consisting of a well-mixed water column and an active sediment layer. Hg can enter the lake from watershed runoff and by atmospheric deposition directly to the lake surface. Once in the lake, Hg exchanges between water and air, and water and sediments, and exits by sediment burial, advective flow and volatilization. The model was applied to a hypothetical drainage lake on the Canadian Shield. Model estimates of water and sediment concentrations compare well with measured values. The results suggest that the three Hg species experience significantly different fates and persistence, with overall Hg dynamics dominated by the fate of HgII (the predominant species). A sensitivity analysis illustrates the importance of physical/chemical properties and lake characteristics on the total amount and behavior of Hg in the lake.     

Mass balance and systemic uptake of mercury released from dental amalgam fillings

The release of mercury (Hg) from dental amalgam fillings has been verified by several authors. In this study, the emission rate of Hg°-vapor from the oral cavity (O-Hg) and the urinary Hg-excretion rate (U-Hg) have been studied with 34 healthy individuals. In ten cases, the urinary excretions of silver (U-Ag) and the fecal excretions of Hg and Ag (F-Hg, F-Ag) were also monitored. All variables, except U-Ag, were significantly related to the load of amalgam. According to this study, an individual with a moderate load of amalgam, i.e. 30 restored surfaces, is predicted to exhibit the following emission rates: O-Hg=22, U-Hg=3, F-Hg=60 and F-Ag=27 μg/d (d=24 hours), consistent with a gross mass balance for Hg of approximately 60 μg/d. The corresponding systemic uptake of Hg was estimated to 12 μg/d based on external data relating air Hg°-exposures to urinary Hg-excretions. The worst case individual showed a gross mass balance of 200 μg Hg/d connected to a systemic uptake of 70 μg Hg/d. These values were compared to the average intake of total-Hg by a Swedish diet (2 μg/d) and to the WHO's tolerable value for intake of total-Hg by food (45 μg/d). Upscaled to the entire Swedish population (8 mill.), the data suggests a fecal/urinary emission to the environment of 100 kg Hg yearly originating from a population load of amalgam fillings containing 90,000 kg of Hg.     

Mercury speciation in the Scheldt Estuary

Surface waters of the Scheldt Estuary were sampled on various occasions between 1991 and 1994. Longitudinal particulate Hg (PM) concentrations ranged from 0.4 – 1.7 μgHg/g and are essentially controlled by physical mixing of polluted fluvial particulates with relatively unpolluted marine particulates. Total dissolved mercury (TDM)concentrations ranged from 0.5 to 5.2 ng/L and are strongly influenced by removal and mobilization processes in the upper estuary, while in the lower estuary mixing processes cause a progressive decrease in TDM towards the mouth. Speciation studies showed that dissolved Hg is predominantly bound to strong complexing ligands (organic substances) in the upper estuary, but this fraction decreases with increasing salinity. In June 1993, however, the reactive mercury fraction was also high in the upper estuary. Model calculations showed that a conditional stability constant for Hg- humic acid interactions of 10¹⁹ was a good estimate for the Scheldt estuary. Dissolved methylmercury was analyzed on three occasions. Significant seasonal variations were observed with concentrations ranging from 11 to 120 pg/L in the winter and 80 to 400 pg/L in summer. Supersaturation of Hg° is observed throughout the whole estuary resulting in an estimated evasion flux of 140–1400 ng/m² day.     

The distribution of dissolved and particulate mercury in three Siberian estuaries and adjacent Arctic coastal waters

Dissolved and particulate mercury distributions were determined in the three largest Siberian rivers and in adjacent Arctic coastal waters during two cruises. Water samples were collected in the Lena River and its mixing zone in the Laptev Sea in September 1991, and in the Ob and Yenisei Rivers and the adjacent Kara Sea in September 1993. Average total dissolved Hg concentration was 5.0 pM in the Lena River, 2.8 pM in the Ob River and 1.5 pM in the Yenisei River. Mercury content of suspended particulate matter was low, averaging 0.17 mg kg^?1 in the Lena and 0.05 mg kg^?1 in the Ob and Yenisei Rivers. These concentrations are lower than those observed in other world rivers affected by local input of man-made origin. In the estuarine mixing zones, higher concentrations of dissolved and particulate Hg which may originate from the spring flood were found. The carbon cycle is apparently a driving mechanism for Hg distribution in Arctic coastal waters. Particulate Hg content was positively correlated with the content of organic matter of the particles. In the Kara Sea, uptake by phytoplankton is suspected to be responsible for the increase in particulate Hg levels. Mercury fluxes from the three rivers to the Arctic Shelf are estimated and compared to direct atmospheric inputs.     

Seasonal variability in the Mercury speciation of Onondaga Lake (New York)

Dissolved and particulate Hg speciation was determined on four occasions in the Spring to Fall interval of 1989, at three depths of the water column of Onondaga Lake, New York; an urban system in which the sediments and fish flesh are contaminated with H_g. Species determined included total Hg (Hg_t), reactive (‘ionic’) Hg (Hg_i), monomethylmercury (CH₃HgX), elemental Hg (Hg°) and dimethylmercury (CH₃)₂Hg). Onondaga Lake was found to contain very high levels of Hg_t (2 to 25 ng L^?1 Hg), Hg_j (0.5 to 10 ng L^?1 Hg), and CH₃HgX (0.3 to 7 ng L^?1 Hg), which generally increased with depth in the lake. These concentrations represent a significant level of contamination, based upon comparisons with other polluted and pristine sites. Elemental Hg levels were typically about 0.05 ng L^?1 and (CH₃)₂Hg was near the limits of detection (?0.001 ng) L^?1 in most samples. The greatest CH₃HgX concentrations in the hypolimnion, as well as the largest gradients of both CH₃HgX and (Hg_t), were observed upon the first onset of stratification, in early summer. These concentrations did not become more pronounced, however, as stratification and H₂S levels in the hypolimnion increased throughout the summer. The very low concentrations of (CH₃)₂Hg in these MeHg and sulfide-rich waters calls into question the belief that CH₃HgX and H₂S will react to yield volatile dimethyl-mercury, which can then escape to the atmosphere by diffusion. Mercury speciation was highly dynamic, indicating active cycling within the lake, and an apparent sensitivity to changes in attendant Iimnological conditions that track the stratification cycle.     

Modelization of the mercury fluxes at the air-sea interface

Aqueous and atmospheric Hg° concentrations for remote marine areas such as the equatorial Pacific Ocean and for coastal seas such as the North Sea and the Scheldt Estuary are discussed. Biological processes seem to be at the origin of the supersaturated Hg° concentrations in the water. On the other hand, transfer velocities across the air-sea interface were calculated with a classical shear turbulence model and with a wave breaking model. With these data, Hg° fluxes from the sea to the atmosphere were calculated: in the Pacific Ocean they range from 0.43 to 6.5 μg g Hg.m^?2. yr^?1 at a wind speed of 2.8 m.s^?1 and from 10.3 to 156 μg Hg.m^?2 yr^?1 at a wind speed of 54 m.s^?1, but they are still higher when wave breaking is considered (from 11 to 168 μg Hg.m^?2.yr^?1). These transfer fluxes are an order of magnitude higher in the Scheldt Estuary.     

Results of the international aqueous mercury speciation intercomparison exercise

Twenty-seven laboratories from around the world agreed to participate in an intercomparison exercise for total Hg (Hg_t) and methyl Hg (MMHg) in pristine lake water. Unfiltered samples from a remote brown water lake in northern Wisconsin (USA) were collected into acid cleaned Teflon^® bottles using ultra-clean sample handling techniques. The samples were acidified in the field with 0.4% by volume of pre-analyzed HCl (12N; <5 pg Hg/mL), and sent to the primary reference laboratory (PRL) by overnight mail. Within one week of receipt, the samples were randomized, and 10% analyzed for Hgt and MMHg at the PRL to verify the homogeneity of the set. Each participating laboratory was then sent 3 randomly selected 1 L bottles, while the PRL retained 30, and the secondary reference laboratory (SRL) retained 12 samples. The participating laboratories were asked to analyze each bottle in triplicate for both Hgt and MMHg, reporting all QA data including blanks, spike recoveries, and detection limits. The PRL analyzed samples in triplicate at both the beginning and the end of the analytical window, to provide a controlled estimate of any changes in concentration or speciation over that time. Of the 23 laboratories that returned results, 18 utilized BrCl oxidation, gold trapping, and cold vapor atomic fluorescence (CVAFS) detection for Hg_t. Four laboratories reported similar techniques, varying either in detector (cold vapor atomic absorption), or wet chemistry. Only 16 laboratories reported MMHg results, with 15 using a variation of the aqueous phase ethylation, GC separation, and CVAFS detection technique. The results show good convergence between the participating labs for both Hg_t and MMHg. For Hgt 18 of 23 labs reported means within 20% of the consensus value and PRL results (1.27±0.18 ng/L and 1.27±0.14 ng/L respectively). For MMHg, 13 of the 16 labs reported results within 20% of either the consensus value (0.420±0.055 ng/L) or the PRL value (0.446±0.041 ng/L).