Anders W. Andren

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.

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.

Generator column determination of octanol/water partition coefficients for selected polychlorinated biphenyl congeners.

A generator column technique has been used to directly determine log K/sub ow/ values for 15 PCB congeners and biphenyl. The method circumvents many of the experimental difficulties encountered with the traditional shake-flask system. Log K/sub ow/ values generated by this procedure are compared with data obtained from the shake-flask method, two chromatographic estimation techniques, and the computational method of Hansch and Leo. Log K/sub ow/ values reported for the higher chlorinated congeners, including octa- and decachlorobiphenyl, are some of the highest ever measured directly.

The effect of sorption on the oxidation of polychlorinated biphenyls (PCBs) by hydroxyl radical

Abstract Sorption of hydrophobic organic compounds, such as polychlorinated biphenyls (PCBs), to particulate matter (diatomaceous earth) affects transformation rates by aqueous-phase hydroxyl radical. The reaction of three PCB congeners with hydroxyl radical (OH.), generated in a photo-Fenton system (i.e. iron, hydrogen peroxide and light at pH 3), was followed to evaluate predictive models of transformation kinetics in the presence of particulates. The OH. generated by this system only reacted with dissolved PCBs, and the overall rate of removal for each congener could be modeled using congener-specific data on desorption kinetics and solution phase reaction rates. Observed kinetics for the least hydrophobic congener evaluated, 2-monochlorobiphenyl, agreed with predictions based upon measured steady-state hydroxyl radical concentrations and second-order rate constants. 2,2′,5-Trichlorobiphenyl was also oxidized by OH., but sorption to particulate matter resulted in slower transformation rates. A predictive model which utilized measured adsorption/desorption rate constants, steady-state hydroxyl radical concentrations and second-order rate constants adequately predicted 2,2′,5-trichlorobiphenyl transformation kinetics. Concentrations of 2,2′,4,5,5′-pentachlorobiphenyl, the most hydrophobic compound analyzed, did not change over the time scale of the experiments because dissolved concentrations were extremely low and desorption kinetics were very slow.

Estimation of octanol/water partition coefficients: Evaluation of six methods for highly hydrophobic aromatic hydrocarbons

Abstract Six methods for estimating log K ow were evaluated using a set of experimental values for 64 aromatic compounds. All log K ow values used in this evaluation were experimentally measured using a generator-column technique. This provided an accurate, self-consistent set of compounds having log K ow values ranging from 2.13 to 8.58. The estimation methods examined included two group contribution methods and four correlative methods utilizing molecular weight, HPLC retention time, molecular connectivity index, and total molecular surface area.

Estimation of vapor pressures for polychlorinated biphenyls: a comparison of eleven predictive methods.

Eleven methods were used to predict vapor pressures at 25.0/sup 0/C for 15 polychlorinated biphenyls with experimental values. These results permitted an assessment of the predictive ability of these methods for compounds with low vapor pressures (<1.0 Pa) and one or less experimental determinations. The error for theoretically based methods was high and increased with decreasing vapor pressure. The correlative methods, based on a set of compounds with known vapor pressures, had much better predictive power. The best correlative method was based on a relationship between ..delta..G/sub v/ and gas-liquid chromatographic retention indexes. The predictive error for this method was estimated to be a factor of 1.75. Vapor pressures obtained by using the three best correlative methods for all polychlorinated biphenyls are reported.

Seasonal variations of 210Pb and 210Po concentrations in an oligotrophic lake

Abstract The vertical distribution of 210Pb and 210Po in the dissolved ( 0.4 μm) phases was measured in Crystal Lake, Wisconsin, to examine the spatial and temporal variability during the seasonal cycle of this oligotrophic lake. The concentration of unsupported 210Pb in the water column is maintained principally by atmospheric input. However, most of the 210Po in Crystal Lake is produced in situ from radioactive decay of 210Pb. Mass balance considerations indicated that the removal rates of 210Pb and 210Po from the water column to the sediment varied temporally by nearly an order of magnitude. During transient periods of high biological productivity, a large net flux of these nuclides into the sediment occurred. In addition, 210Pb was rapidly stripped from the water column during fall turnover. It was during these short-lived events that most of the annual net removal of 210Pb and 210Po occurred. The mean removal residence time was estimated to be 0.095 yr for 210Pb and 0.26 yr for 210Po. These residence times suggest that there is a difference between 210Pb and 210Po in the extent of their recycling in the water column. Calculations indicated that there was a cyclic response of the water column 210Po inventory corresponding to successive time periods where there was a net loss or net gain. This cycling is attributed to rapid biological removal and subsequent release from the sediment of freshly deposited 210Po. For 210Pb, replenishment of the water column appeared to occur mainly from atmospheric input.

Aqueous solubilities of six polychlorinated biphenyl congeners at four temperatures.

Aqueous solubilities of six polychlorinated biphenyls (PCBs) were measured at 25/sup 0/C and three other temperatures by using a slightly modified generator column method. Solubility increased exponentially with temperature in the range 0.4-80/sup 0/C for all six PCBs investigated, as well as for biphenyl and 4-chlorobiphenyl. Enthalpies of solution for the PCBs and biphenyl were determined, ranging from 28.5 (4-chlorobiphenyl) to 66.6 kJ/mol (decachlorobiphenyl). These values can be used to interpolate solubilities in the experimental temperature ranges, with average errors of less than or equal to 10%, for all congeners except decachlorobiphenyl. The average absolute error for decachlorobiphenyl was 16%. 17 references, 2 tables.

Precipitation scavenging of polychlorinated biphenyl congeners in the great lakes region

Abstract Ten precipitation events were sampled in the fall of 1986 in Madison, WI and analyzed for individual congener and total polychlorinated biphenyl (PCB) levels in both the dissolved and particulate phases. Total PCB concentrations were generally at the lower end of ranges recently reported for precipitation. Operationally defined dissolved and particulate phase congener distribution patterns for the two events of highest concentration were qualitatively similar to gas-phase and particle-bound patterns for northern Wisconsin air samples. Higher than predicted dissolved-phase concentrations may indicate non-equilibrium processes during scavenging and/or sample processing, the presence of colloids and micro-particulates, and/or more efficient gas-phase transfer to hydrometeors with organic coatings. Observed organic carbon-normalized distribution coefficients increased slightly with increasing octanol-water partition coefficient, giving the relationship log Koc = 0.22 log Kow + 4.64. The data indicate that a third organic-rich colloidal phase could be influencing partitioning, and could explain the higher than expected apparent gas scavenging efficiency for PCBs from the atmosphere. Precipitation-weighted mean fluxes of PCBs in the dissolved and particulate phases were 1.2 and 1.4 μg m−2 year−1, respectively, indicating that precipitation remains a significant source of PCBs to the upper Great Lakes.