Christopher L. Babiarz

Influences of Watershed Characteristics on Mercury Levels in Wisconsin Rivers

Total and monomethyl mercury were measured at 39 river sites in Wisconsin during fall 1992 and spring 1993. Using a Geographic Information System (GIS), we delineated watersheds with unique and homogeneous physical characteristics. Mean unfiltered total Hg (Hg T ) was higher in spring (7.94 ng L -1 ) than in fall (3.45 ng L -1 ). Major differences in Hg T yields were observed among various land-use groupings. In wetland/forest watersheds, elevated Hg T fluxes were associated with the filtered phase, while in agricultural watersheds, increased Hg T fluxes were due to particle loading. Monomethylmercury (MeHg) yields from wetland/forest sites were higher than agricultural/forest sites and agricultural only sites. Percent wetland surface area was positively correlated with MeHg yield. These results identify the importance of land use and land cover in influencing Hg concentrations, speciaton, and transport in rivers.

Whole-ecosystem study shows rapid fish-mercury response to changes in mercury deposition

Methylmercury contamination of fisheries from centuries of industrial atmospheric emissions negatively impacts humans and wildlife worldwide. The response of fish methylmercury concentrations to changes in mercury deposition has been difficult to establish because sediments/soils contain large pools of historical contamination, and many factors in addition to deposition affect fish mercury. To test directly the response of fish contamination to changing mercury deposition, we conducted a whole-ecosystem experiment, increasing the mercury load to a lake and its watershed by the addition of enriched stable mercury isotopes. The isotopes allowed us to distinguish between experimentally applied mercury and mercury already present in the ecosystem and to examine bioaccumulation of mercury deposited to different parts of the watershed. Fish methylmercury concentrations responded rapidly to changes in mercury deposition over the first 3 years of study. Essentially all of the increase in fish methylmercury concentrations came from mercury deposited directly to the lake surface. In contrast, <1% of the mercury isotope deposited to the watershed was exported to the lake. Steady state was not reached within 3 years. Lake mercury isotope concentrations were still rising in lake biota, and watershed mercury isotope exports to the lake were increasing slowly. Therefore, we predict that mercury emissions reductions will yield rapid (years) reductions in fish methylmercury concentrations and will yield concomitant reductions in risk. However, a full response will be delayed by the gradual export of mercury stored in watersheds. The rate of response will vary among lakes depending on the relative surface areas of water and watershed.

Seasonal influences on partitioning and transport of total and methylmercury in rivers from contrasting watersheds

Seven Wisconsin rivers with contrasting, relativelyhomogeneous watershed composition were selected toassess the factors controlling mercury transport.Together, these watersheds allow comparisons ofwetland, forest, urban and agricultural land-uses.Each site was sampled nine times between September1993 and September 1994 to establish seasonalsignatures and transport processes of total mercury(HgT) and methylmercury (MeHg). Our resultsclearly show that land use and land cover stronglyinfluence mercury transport processes. Under base-flowconditions, unfiltered MeHg yield varies by a factorof sixteen (12–195 mg km-2 d-1), andincreases with the fraction of wetland area in thewatershed. Elevated mercury yields during high floware particle-phase associated in agricultural sites,but filtered-phase associated in wetland sites.Methylmercury represented less than 5% of totalmercury mobilized during the spring thaw across allwatersheds. Autumn MeHg yield was generally 11–15%of HgT in wetland influenced watersheds, thougha maximum of 51% was observed. In some cases, singlehigh-flow events may dominate the annual export ofmercury from a watershed. For example, one high-flowevent on the agricultural Rattlesnake Creek had thelargest HgT and MeHg yield in the study (107 and2.32 mg km-2 d-1, respectively). The mass ofmercury transported downstream by this single eventwas an order of magnitude larger than the eight other(non-event) sampling dates combined. These resultsunderscore the importance of watershed characteristicsand seasonal events on the fate of mercury in freshwater rivers.

Mercury cycling in the Allequash Creek watershed, northern Wisconsin

Although there have been recent significant gains in our understanding of mercury (Hg) cycling in aquatic environments, few studies have addressed Hg cycling on a watershed scale. In particular, attention to Hg species transfer between watershed components (upland soils, groundwater, wetlands, streams, and lakes) has been lacking. This study describes spatial and temporal distributions of total Hg and MeHg among watershed components of the Allequash Creek watershed (northern Wisconsin, USA). Substantial increases in total Hg and MeHg were observed as groundwater discharged through peat to form springs that flow into the stream, or rivulets that drain across the surface of the wetland. This increase was concomitant with increases in DOC. During fall, when the Allequash Creek wetland released a substantial amount of DOC to the stream, a 2–3 fold increase in total Hg concentrations was observed along the entire length of the stream. Methylmercury, however, did not show a similar response. Substantial variability was observed in total Hg (0.9 to 6.3) and MeHg (<0.02 to 0.33) concentrations during synoptic surveys of the entire creek. For the Allequash Creek watershed, the contributing groundwater basin is about 50% larger than the topographic drainage basin. Total Hg concentrations in groundwater, the area of the groundwater basin, and annual stream flow data give a watershed-yield rate of 1.2 mg/km2/d, which equates to a retention rate of 96%. The calculated MeHg yield rate for the wetland area is 0.6 to 1.5 mg/km2/d, a value that is 3–6 fold greater than the atmospheric deposition rate.

The role of groundwater transport in aquatic mercury cycling

Mercury, which is transported globally by atmospheric pathways to remote aquatic environments, is a ubiquitous contaminant at very low (nanograms Hg per liter) aqueous concentrations. Until recently, however, analytical and sampling techniques were not available for freshwater systems to quantify the actual levels of mercury concentrations without introducing significant contamination artifacts. Four different sampling strategies were used to evaluate ground water flow as a mercury source and transport mechanism within aquatic systems. The sampling strategies employ ultraclean techniques to determine mercury concentrations in groundwater and pore water near Pallette Lake, Wisconsin. Ambient groundwater concentrations are about 2–4 ng Hg L−1, whereas pore waters near the sediment/water interface average about 12 ng Hg L−1, emphasizing the importance of biogeochemical processes near the interface. Overall, the groundwater system removes about twice as much mercury (1.5 g yr−1) as it contributes (0.7 g yr−1) to Pallette Lake. About three fourths of the groundwater mercury load is recycled, thought to be derived from the water column.

Total concentrations of mercury in Wisconsin (USA) lakes and rivers

We analyzed surface waters from 30 Wisconsin lakes and rivers for total mercury ([Hg]T) and total suspended particulates (TSP) on a state-wide basis with trace-metal ‘ultraclean’ techniques. Mercury concentrations ranged from 0.3 to 2.9 ng L−1 in lakes and from 0.7 to 8.9 ng L−1 in rivers. TSP concentrations ranged from 0.9 to 6.6 mg L−1 in lakes and from 3.1 to 31.4 mg L−1 in rivers. Spatial trends were weak; however, [Hg]T was generally higher in the spring than in the autumn of 1991. Total mercury concentration was weakly dependent on TSP with the coefficient of determination (r2) ranging 0.06 to 0.49 across seasonal and geophysical differences.

Watershed and discharge influences on the phase distribution and tributary loading of total mercury and methylmercury into Lake Superior.

Knowledge of the partitioning and sources of mercury are important to understanding the human impact on mercury levels in Lake Superior wildlife. Fluvial fluxes of total mercury (Hg(T)) and methylmercury (MeHg) were compared to discharge and partitioning trends in 20 sub-basins having contrasting land uses and geological substrates. The annual tributary yield was correlated with watershed characteristics and scaled up to estimate the basin-wide loading. Tributaries with clay sediments and agricultural land use had the largest daily yields with maxima observed near the peak in water discharge. Roughly 42% of Hg(T) and 57% of MeHg was delivered in the colloidal phase. Tributary inputs, which are confined to near-shore zones of the lake, may be more important to the food-web than atmospheric sources. The annual basin-wide loading from tributaries was estimated to be 277 kg yr(-1) Hg(T) and 3.4 kg yr(-1) MeHg (5.5 and 0.07 mg km(-2) d(-1), respectively).

Biogeochemical controls on mercury methylation in the Allequash Creek wetland

We measured mercury methylation potentials and a suite of related biogeochemical parameters in sediment cores and porewater from two geochemically distinct sites in the Allequash Creek wetland, northern Wisconsin, USA. We found a high degree of spatial variability in the methylation rate potentials but no significant differences between the two sites. We identified the primary geochemical factors controlling net methylmercury production at this site to be acid-volatile sulfide, dissolved organic carbon, total dissolved iron, and porewater iron(II). Season and demethylation rates also appear to regulate net methylmercury production. Our equilibrium speciation modeling demonstrated that sulfide likely regulated methylation rates by controlling the speciation of inorganic mercury and therefore its bioavailability to methylating bacteria. We found that no individual geochemical parameter could explain a significant amount of the observed variability in mercury methylation rates, but we found significant multivariate relationships, supporting the widely held understanding that net methylmercury production is balance of several simultaneously occurring processes.