Harold F. Hemond

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.

Determination of gas exchange rate constants for a small stream on Walker Branch Watershed, Tennessee

The steady state tracer gas method was used to determine gas exchange rate constants (k) for a small stream (annual average flow ≤1 m3/min) draining the West Fork of Walker Branch watershed in eastern Tennessee. Chloride was used as a conservative tracer to account for dilution by lateral inflow, and propane and ethane were used as volatile tracers. Gas exchange rate constants for propane (kp) were about 100 d−1 over a wide range of flow conditions, while those for ethane (ke) were about 117 d1; an equivalent rate constant for O2 (KO2) would be about 118–139 d−1, depending on the method used for its calculation. These rate constants are much larger than those typically found in rivers and large streams. Much lower kp values (about 50d−1) were found during one experiment conducted at low flow with much of the stream surface covered with floating leaves. Nineteen previously published empirical equations were used to predict kO2 values for one 72m stream reach; agreement between the predicted and measured values was generally very poor, underscoring the importance of field-measured gas exchange rates in studies of the transport and fate of volatile compounds. Because ethane and propane have similar gas exchange rates and similar aqueous diffusion coefficients (ke/kp and De/Dp are both close to 1, where D is the compounds diffusion coefficient), accurate determination of the exponent n in the relationship ke/Kp− (De/Dp)n was not possible. The ratio ke/kp (1.17) is much closer to De/Dp (1.24) than to He/Hp (0.82, where H is the compounds Henrys law constant), suggesting that stripping of dissolved volatiles by air bubbles was not a dominant mode of gas exchange for the study stream.

POTENTIAL EFFECTS OF CLIMATE CHANGE ON FRESHWATER ECOSYSTEMS OF THE NEW ENGLAND/MID‐ATLANTIC REGION

Numerous freshwater ecosystems, dense concentrations of humans along the eastern seaboard, extensive forests and a history of intensive land use distinguish the New England/Mid-Atlantic Region. Human population densities are forecast to increase in portions of the region at the same time that climate is expected to be changing. Consequently, the effects of humans and climatic change are likely to affect freshwater ecosystems within the region interactively. The general climate, at present, is humid continental, and the region receives abundant precipitation. Climatic projections for a 2 x CO1 atmosphere, however, suggest warmer and drier conditions for much of this region. Annual temperature increases ranging from 3-5°C are projected, with the greatest increases occurring in autumn or winter. According to a water balance mode!, the projected increase in temperature will result in greater rates of evaporation and evapotranspiration. This could cause a 21 and 31% reduction in annual stream flow in the southern and northern sections of the region, respectively, with greatest reductions occurring in autumn and winter. The amount and duration of snow cover is also projected to decrease across the region, and summer convective thunderstorms are likely to decrease in frequency but increase in intensity. The dual effects of climate change and direct anthropogenic stress will most likely alter hydrological and biogeochemica! processes, and, hence, the flora! and fauna! communities of the region’s freshwater ecosystems. For example, the projected increase in evapotranspiration and evaporation could eliminate most bog ecosystems, and increases in water temperature may increase bioaccumulation, and possibly biomagnification, of organic and inorganic contaminants. Not a!! change may be adverse. For example, a decrease in runoff may reduce the intensity of ongoing estuarine eutrophication, and acidification of aquatic habitats during the spring snowmelt period may be ameliorated. Recommendations for future monitoring efforts include: (1) extending and improving data on the distribution, abundance and effect of anthropogenic stressors (non-point pollution) within the region; and (2) improving scientific knowledge regarding the contemporary distribution and abundance of aquatic species. Research recommendations include: (1) establishing a research centre(s) where field studies designed to understand interactions between freshwater ecosystems and climate change can be conducted; (2) projecting the future distribution, activities and direct effects of humans within the region; (3) developing mathematical analyses, experimental designs and aquatic indicators that distinguish between climatic and anthropogenic effects on aquatic systems; (4) developing and refining projections of climate variability such that the magnitude, frequency and seasonal timing of extreme events can be forecast; and (5) describing quantitatively the flux of materials (sediments, nutrients, metals) from watersheds characterized by a mosaic of land uses. 0 1997 by John Wiley & Sons, Ltd.

Speciation and fate of arsenic in three lakes of the aberjona watershed.

The speciation of arsenic in the environment is controlled by reduction, methylation, and oxidation processes and is therefore influenced by redox conditions. However, in the lakes studied, speciation was found to be far from thermodynamic equilibrium. A Superfund site is a major source of As to the watershed, with the Aberjona River being the main conduit for As found in the Mystic Lakes. Total As concentrations in the water column decrease downstream, from >100 nM in the Halls Brook Storage Area (HBSA), where As(III) enters via reducing ground-water, to <20 nM in the Lower Mystic Lake (LML); the sediments of all the lakes are sinks for As.

Use of radon-222 and calcium as tracers in a three-end-member mixing model for streamflow generation on the West Fork of Walker Branch Watershed

Genereux, D.P., Hemond, H.F. and Mulholland, P,J., 1993. Use of radon-222 and calcium as tracers in a three-end-member mixing model for streamflow generation on the West Fork of Walker Branch Watershed. J. Hydrol., 142:167-211. Measurements of naturally occurring radon-222 (222 Rn) and calcium (Ca) in surface and subsurface waters on the West Fork ef Walker Branch suggest that a simple three-end-member mixing model provides a realistic and useful framework for streamflow generation over a wide range of flow conditions. The three end members are vadose zone water, soil groundwater, and bedrock groundwater. Bedrock groundwater was distinguished from the soil end members on the basis of its high Ca content; 222 Rn concentration was the basis for the distinction between vadose zone water (low 222 Rn) and soil groundwater (high 222 Rn). The behavior of the end members with changing flow was consistent with a wide variety of environmental observations, including temperature and flow variations at springs, water table responses, the general lack of saturated zones on hillslopes and even near the stream in some places, and the importance of water movemen t through bedrock. Variability in the chemistry of the end members precluded using other solutes (Na, K, and SO4) to test the mixing fractions derived from 222 Rn and Ca data; during those times of year when the soil temperature is most different from that of the underlying bedrock (late summer and late winter), temperature may be a useful tracer for distinguishing between water from the soil end members and that from bedrock. The mixing model provides a simple framework for analyzing the essential features of streamflow generation in this highly heterogeneous terrain.

Relationship between DOC concentration and vadose zone thickness and depth below water table in groundwater of Cape Cod, U.S.A.

Changes in concentration ofdissolved organic carbon (DOC) reflectbiogeochemical processes that determinechemical composition of groundwater and othernatural waters. We found that the deeper thevadose zone, the lower the concentration of DOCin groundwater near the water table, indicatingthat considerable attenuation ofsurface-derived DOC occurred in the vadosezone. Under vadose zones <1.25 m, DOCconcentrations at the surface of the watertable ranged to >20 mg C l−1, while forvadose zones >5.0 m, DOC never exceeded2.0 mg C l−1. DOC concentrations alsodecreased exponentially with increasing depthbelow the water table, most notably in theupper two meters, implying continuedattenuation in the upper layer of the saturatedzone. Ninety-nine percent of the DOC wasattenuated by the time the water reached adepth of 19 m below the water table. A stronginverse relationship between DOC and nitrateconcentrations suggests that nitrate isdepleted where DOC supplies are high, providingevidence that some portion of the DOC losses ingroundwater are due to microbialtransformations, including denitrification. DOC concentrations in shallow groundwater showconsiderable spatial variability, butconcentration of DOC at any one site issurprisingly stable over time. The largestsource of variation in DOC concentration ingroundwater therefore is spatial rather thantemporal, suggesting that local heterogeneitiesplay an important role in DOC delivery toshallow groundwater. Our results highlightboth the importance of shallow vadose areas inDOC delivery to groundwater and the need todistinguish where samples are collected inrelation to flow paths before conclusions aremade about mean groundwater DOC concentrations. The substantial losses of DOC in the vadosezone and in shallow depths within the aquifersuggest quite active biogeochemical processesin these boundary environments.

Methane transport and oxidation in the unsaturated zone of a Sphagnum peatland

Efflux rates and oxidation rates of methane (CH4) were measured in a northern Sphagnum bog, Thoreaus Bog in Concord, Massachusetts, by using a gradient methodology which does not change in situ conditions. A remote-sampling technique was devised to obtain undisturbed CH4 profiles as a function of depth in the peat; effective diffusion coefficients in peat were estimated both physically and by using propane as a tracer. By combining these techniques we estimated the average late summer CH44 flux from deep, anaerobic methanogenic sediments to the unsaturated zone to be 3.5 × 10−11 mol cm−2 s−1 (± 1.0 × 10−10 mol cm−2 s−1), while the CH4 flux from this unsaturated zone to the atmosphere was 3.7 × 10−12 mol cm−2 s−1 (± 5.0 × 10−12 mol cm−2 s−1). Therefore a large fraction of the CH4 flux was consumed before it reached the atmosphere. Most CH4 consumption, presumably by oxidation, occurred between the water table, located 12 to 15 cm below the bog surface, and about 6 cm below the bog surface. In this region, CH4 concentrations and oxidation rates were unevenly distributed, probably following patterns of upward transport of CH4 by bubbles via fissures and tubes in the saturated zone. Between the surface of the bog and 6-cm depth, CH4 concentrations were more uniformly distributed, most likely because of greater horizontal mixing in this depth range. Analysis of CH4 distributions in unsaturated peat is a straightforward and practical technique to measure both net CH4 efflux and CH4 oxidation with minimal disturbance to the peat structure and gas exchange conditions.

Cumulative impacts on water-quality functions of wetlands

The total effect of cumulative impacts on the water quality functions of wetlands cannot be predicted from the sum of the effects each individual impact would have by itself. The wetland is not a simple filter; it embodies chemical, physical, and biotic processes that can detain, transform, release, or produce a wide variety of substances. Because wetland water quality functions result from the operation of many individual, distinct, and quite dissimilar mechanisms, it is necessary to consider the nature of each individual process.Sound knowledge of the various wetland processes is needed to make guided judgements about the probable effects of a given suite of impacts. Consideration of these processes suggests that many common wetland alterations probably do entail cumulative impact. In addition to traditional assessment methods, the wetland manager may need to obtain appropriate field measurements of water quality-related parameters at specific sites; such data can aid in predicting the effects of cumulative impact or assessing the results of past wetland management.

Mercury Methylation by Desulfovibrio desulfuricans ND132 in the Presence of Polysulfides

ABSTRACT The extracellular speciation of mercury may control bacterial uptake and methylation. Mercury-polysulfide complexes have recently been shown to be prevalent in sulfidic waters containing zero-valent sulfur. Despite substantial increases in total dissolved mercury concentration, methylation rates in cultures of Desulfovibrio desulfuricans ND132 equilibrated with cinnabar did not increase in the presence of polysulfides, as expected due to the large size and charged nature of most of the complexes. In natural waters not at saturation with cinnabar, mercury-polysulfide complexes would be expected to shift the speciation of mercury from HgS0(aq) toward charged complexes, thereby decreasing methylation rates.

Spatial and temporal variability in streamflow generation on the West Fork of Walker Branch Watershed

Abstract Spatially intensive measurements of streamflow were used to document the spatial and temporal variability in streamflow generation on the West Fork of Walker Branch Watershed, a 38.4 ha forested catchment in Oak Ridge, Tennessee. The study focused on a 300 m section of a small stream, and covered a wide range of flow conditions ( Q weir , streamflow at the basin outlet, varied from about 350 to 3500 l min −1 ). There was enormous spatial variability in the stream inflow, down to the finest scale investigated (reaches 20 m in length). Lateral inflow to longer reaches (60–130 m) was linearly correlated with Q weir over the full range of flows studied, making it possible to estimate the spatial pattern of stream inflow from measurement of Q weir alone. The heterogeneous nature of the karstic dolomite bedrock was the dominant control on the observed spatial variability in streamflow generation. This thesis is consistent with the results of field investigations using natural tracers, reported in a companion paper. Bedrock structure and lithology may affect streamflow generation directly (via water movement through fractured rock), and indirectly (by influencing the slope and thickness of the overlying soil). While the West Fork contains all the topographic and surface hydrologic features of larger basins (ridge tops, valleys, hollows, spurs, ephemeral and perennial stream channels), it covers an area which is relatively small with respect to the bedrock heterogeneity. Therefore, while the hydrologic processes observed on the West Fork are no doubt typical of those occurring elsewhere in karst terrain, the particular patterns of spatial and temporal variability observed are somewhat specific to the study site.