Role of soil freezing events in interannual patterns of stream chemistry at the Hubbard Brook Experimental Forest,New Hampshire

Soil freezing is a disturbance of the below ground environment, potentially resulting in increased losses of NO3- and surface water acidification. Here, we report the effects of soil freezing on interannual variation in stream chemistry at the Hubbard Brook Experimental Forest, New Hampshire. Data from 1970 to 1997 of soil frost depth, snow cover, precipitation, air temperature, and stream discharge and chemistry were used in a stepwise linear regression model to select the variables that best predicted deviations of annual stream concentrations from 4-year running averages. Variables quantifying soil freezing severity were selected as significant predictors of short-term fluctuations in stream K+, NO3-, Ca2+, and Mg2+ concentrations from 1970 to 1989, explaining 59 and 47% of the short-term variability in K+ and NO3-, respectively. Fine-root mortality and disturbance of root-soil-microbe interactions, with subsequent effects on decomposition and nutrient uptake, likely contributed to the mobilization of K+ and NO3- to streamwater following severe soil freezing events. The relationship between soil freezing and stream chemistry, however, weakened during the period 1990-1997. Because soil freezing has had inconsistent effects on stream chemistry during the period 1970-1997, it is unclear whether future changes in the frequency, duration, and depth of soil freezing events as the result of changes in the snow cover regime under a warmer climate will have significant impacts on the losses of NO3- and nutrient-base cations from temperate northern ecosystems.     

Climate-change-driven deterioration of water quality in a mineralized watershed

A unique 30-year streamwater chemistry data set from a mineralized alpine watershed with naturally acidic, metal-rich water displays dissolved concentrations of Zn and other metals of ecological concern increasing by 100-400% (400-2000 μg/L) during low-flow months, when metal concentrations are highest. SO(4) and other major ions show similar increases. A lack of natural or anthropogenic land disturbances in the watershed during the study period suggests that climate change is the underlying cause. Local mean annual and mean summer air temperatures have increased at a rate of 0.2-1.2 °C/decade since the 1980s. Other climatic and hydrologic indices, including stream discharge during low-flow months, do not display statistically significant trends. Consideration of potential specific causal mechanisms driven by rising temperatures suggests that melting of permafrost and falling water tables (from decreased recharge) are probable explanations for the increasing concentrations. The prospect of future widespread increases in dissolved solutes from mineralized watersheds is concerning given likely negative impacts on downstream ecosystems and water resources, and complications created for the establishment of attainable remediation objectives at mine sites.     

Perfluorinated acids in air, rain, snow, surface runoff, and lakes: relative importance of pathways to contamination of urban lakes

Concentrations of perfluorinated acids (PFAs) were measured in various environmental matrices (air, rain, snow, surface runoff water, and lake water) in an urban area, to enable identification of sources and pathways of PFAs to urban water bodies. Total PFA concentrations ranged from 8.28 to 16.0 pg/ m3 (mean 11.3) in bulk air (sum of vapor and particulate phases), 0.91 to 13.2 ng/L (6.19) in rainwater, 0.91 to 23.9 ng/L (7.98) in snow, 1.11-81.8 ng/L (15.1 ng/L) in surface runoff water (SRW), and 9.49 to 35.9 ng/L (21.8) in lake water. Perfluorooctanoic acid (PFOA) was the predominant compound, accounting for > 35% of the total PFA concentrations, in all environmental matrices analyzed. Concentrations and relative compositions of PFAs in SRW were similar to those found for urban lakes. SRW contributes to contamination by PFOA in urban lakes. The measured concentration ratios of FTOH to PFOA in air were 1-2 orders of magnitude lower than the ratios calculated based on an assumption of exclusive atmospheric oxidation of FTOHs. Nevertheless, the mass balance analysis suggested the presence of an unknown input pathway that could contribute to a significant amount of total PFOA loadings to the lake. Flux estimates of PFOA at the air-water interface in the urban lake suggest net volatilization from water.     

Landscape control of stream water aluminum in a boreal catchment during spring flood

Inorganic aluminum (Al) concentrations are critical for defining the biological effects of acidification. The landscape's role in controlling the spatial variability of Al and its speciation has received only limited attention. We analyzed the speciation of stream Al at 14 sites within a 68 km2 boreal catchment during spring snowmelt, a period of episodic acidity. Three factors that influenced Al at these sites were landscape type (specifically the proportion of wetland areas), stream pH, and dissolved organic carbon (DOC). Forested catchment sites underlain by mineral soils had higher total Al concentrations and greater inorganic Al proportions than catchments with larger wetland areas, despite significantly higher pH. We suggest that this difference results from source limitation of Al in the peat wetlands. The control of Al solubilitywas dominated by organic complexes, with the organic carrying capacity exceeding Al in the majority of samples. When assessing the inorganic phase, only four percent of the samples were oversaturated with regards to commonly forming secondary Al minerals, with no samples showing supersaturation higher than 10 times with respect to any given solid phase. Inorganic Al rarely exceeded biological thresholds, except for short periods during peak flow in forested areas, despite two-thirds of the streams having minimum pH values below 4.9. Streams with a high percentage of wetland area were associated with lower Al:DOC ratios. The Al:DOC ratios were quite stable in each stream before, during, and after snowmelt, with the exception of isolated spikes in the Al:DOC ratio associated with particulate Al at a downstream site during high flow.     

Factors affecting acid neutralizing capacity in the Adirondack region of New York: a solute mass balance approach

High rates of acidic deposition in the Adirondack region of New York have accelerated acidification of soils and surface waters. Annual input-output budgets for major solutes and acid-neutralizing capacity (ANC) were estimated for 43 drainage lake-watersheds in the Adirondacks from 1998 to 2000. Sulfate was the predominant anion on an equivalent basis in both precipitation and drainage export. Calcium ion had the largest cation drainage export, followed by Mg2+. While these watersheds showed net nitrogen (N) retention, the drainage losses of SO4(2-), Cl-, base cations, and ANC exceeded their respective inputs from precipitation. Land cover (forest type and wetlands) affected the export of SO4(2-), N solutes, and dissolved organic carbon (DOC). The relationships of solute export with elevation (negative for base cations and Cl-, positive for NO3- and H+) suggest the importance of the concomitant changes of biotic and abiotic watershed characteristics associated with elevational gradients. The surface water ANC increased with the sum of base cations and was greatest in the lakes with watersheds characterized by thick deposits of glacial till. The surface water ANC was also higher in the lake-watersheds with lower DOC export. Some variation in lake ANC was associated with variability in acidic deposition. Using a classification system previously developed for Adirondack lakes on the basis primarily of surficial geology, lake-watersheds were grouped into five classes. The calculated ANC fluxes based on the major sinks and sources of ANC were comparable with measured ANC for the thick-till (I) and the medium-till lake-watersheds with low DOC (II). The calculated ANC was overestimated for the medium-till with high DOC (III) and the thin-till with high DOC (V) lake-watersheds, suggesting the importance of naturally occurring organic acids as an ANC sink, which was not included in the calculations. The lower calculated estimates than the measured ANC for the thin-till lake-watersheds with low DOC (IV) were probably due to the mobilization of Al as an ANC source in these watersheds that were highly sensitive to strong acid inputs. Our analysis of various drainage lakes across the Adirondacks on the basis of solute mass balances, coupled with the use of a lake classification system and GIS data, demonstrates that the lake-watersheds characterized by shallow deposits of glacial till are highly sensitive to acidic deposition not only in the southwestern Adirondack region where previous field-based studies were intensively conducted but also across the entire Adirondack region. Moreover, the supply of organic acids and Al mobilization substantially modify the acid-base status of surface waters.     

Effects of historical lake level and land use on sediment and phosphorus accumulation rates in Lake Kinneret

Current paradigms of reservoir ontogeny suggest that water-level fluctuations may increase sedimentary nutrient release, causing long-term eutrophication of water bodies formed by dryland flooding. Less is known of the changes in nutrient status following conversion of natural lakes into reservoirs. Here, we use historical hydrological and limnological data and paleolimnological records of sedimentary P accumulation to evaluate changes in nutrient storage in Lake Kinneret, Israel since approximately 1860. Impoundment in 1932 increased water level fluctuations and altered seasonal hydrologic patterns in the lake. Geochemical analysis of sediment deposits indicated that bulk sediment and P accumulation rates in the central lake increased >600% following dam installation (1930s), draining of Lake Hula wetlands (1951-1957), and diversion of surface water outflow (1964 to present). Further, comparison of sedimentary P stratigraphies with long-term chemical records showed that the period of maximum P deposition corresponds to observed increases in whole-lake and in hypolimnetic P content, as well as epilimnetic biological changes indicative of ongoing eutrophication. Together, these patterns suggest that hydrologic management of natural lakes can increase sedimentary nutrient flux under circumstances where lake volume and water levels become more variable.     

Continuous analysis of dissolved gaseous mercury (DGM) and mercury flux in two freshwater lakes in Kejimkujik Park,Nova Scotia: evaluating mercury flux models with quantitative data

Diurnal patterns for dissolved gaseous mercury (DGM) concentration, mercury flux, several water variables (pH, oxidation reduction potential (ORP), water temperature), and meteorological variables (wind speed, air temperature, % relative humidity, solar radiation) were measured in two lakes with contrasting dissolved organic carbon (DOC) concentrations in Kejimkujik Park, Nova Scotia. A continuous analysis system made it possible to measure quick changes in DGM over time. Consistently higher DGM concentrations were found in the high DOC lake as compared to the low DOC lake. An examination of current mercury flux models using this quantitative data indicated some good correlations between the date and predicted flux (r ranging from 0.27 to 0.83) but generally poor fit (standard deviation of residuals ranging from 0.97 to 3.39). Cross-correlation analysis indicated that DGM dynamics changed in response to solar radiation with lag-times of 65 and 90 min. This relationship with solar radiation was used to develop new predictive models of DGM and mercury flux dynamics for each lake. We suggest that a generalized approach using time-shifted solar radiation date to predict DGM can be incorporated into existing mercury flux models. It is clear from the work presented that DOC and wind speed may also play important roles in DGM and mercury flux dynamics, and these roles have not been adequately accounted for in current predictive models.     

Fluctuations of dissolved organic matter in river used for drinking water and impacts on conventional treatment plant performance

Natural organic matter (NOM) in drinking water supplies can provide precursors for disinfectant byproducts, molecules that impact taste and odors, compounds that influence the efficacy of treatment, and other compounds that are a source of energy and carbon for the regrowth of microorganisms during distribution. NOM, measured as dissolved organic carbon (DOC), was monitored daily in the White River and the Indiana-American water treatment plant over 22 months. Other parameters were either measured daily (UV-absorbance, alkalinity, color, temperature) or continuously (turbidity, pH, and discharge) and used with stepwise linear regressions to predict DOC concentrations. The predictive models were validated with monthly samples of the river water and treatment plant effluent taken over a 2-year period after the daily monitoring had ended. Biodegradable DOC (BDOC) concentrations were measured in the river water and plant effluent twice monthly for 18 months. The BDOC measurements, along with measurements of humic and carbohydrate constituents within the DOC and BDOC pools, revealed that carbohydrates were the organic fraction with the highest percent removal during treatment, followed by BDOC, humic substances, and refractory DOC.     

Effect of dissolved organic carbon on the photoproduction of dissolved gaseous mercury in lakes: potential impacts of forestry

The production of dissolved gaseous mercury (DGM) in freshwater lakes is induced by solar radiation and is also thought to be linked to processes mediated by dissolved organic carbon (DOC). Studies investigating these processes using comparisons between lakes are often confounded by differences in DOC content and structure. In this study, we investigated the link between DOC concentrations and DGM production by using tangential ultrafiltration to manipulate DOC concentrations in water samples taken from a given lake. In this way, a range of samples with different DOC concentrations was produced without substantial changes to DOC structure or dissolved ions. This was repeated for four lakes in central Quebec: two with highly logged drainage basins and two with minimally logged drainage basins. On two separate days for each lake, water samples (filtered to remove >99% of microorganisms) with varying DOC concentrations were incubated in clear and dark Teflon bottles on the lake surface. DGM concentrations were measured at 3.5-h intervals over the course of 10.5 h. Levels of DGM concentrations increased with increasing cumulative irradiation for all lakes until approximately 4000 kJ m(-2) (400-750 nm, photosynthetically active radiation (PAR)), when DGM concentrations reached a plateau (between 20 and 200 pg L(-1)). When we assumed that DGM production was limited by the amount of photoreducible mercury, reversible first-order reaction kinetics fitted the observed data well (r2 ranging between 0.59 and 0.98, p < 0.05 with the exception of N70 100% DOC, 0% DOC, and K2 0% DOC with p = 0.06, 0.10, and 0.11, respectively). The DGM plateaus were independent of DOC concentrations but differed between lakes. In contrast, photoproduction efficiency (DGMprod) (i.e., the amount of DGM produced per unit radiation (fg L(-1) (kJ/m2)(-1)) below 4000 kJ m(-2) PAR) was linearly proportional to DOC concentration for both logged lakes (r2 = 0.97, p < 0.01) and nonlogged lakes (r2 = 0.52, p = 0.018) studied. Furthermore, logged lakes had a lower DGMprod per unit DOC (p < 0.01) than the nonlogged lakes. In these four lakes, the rate of DGM production per unit PAR was dependent on the concentration of DOC. The DGM plateau was independent of DOC concentration; however, there was a significant difference in DGM plateaus between lakes presumably due to different DOC structures and dissolved ions. This research demonstrates an important mechanism by which logging may exacerbate mercury levels in biota.     

Influence of climate change, tidal mixing, and watershed urbanization on historical water quality in Newport Bay, a saltwater wetland and tidal embayment in southern California

Historical coliform measurements (n = 67,269; 32 years) in Newport Bay, a regionally important saltwater wetland and tidal embayment in southern California, have been compiled and analyzed. Coliform concentrations in Newport Bay decrease along an inland-to-ocean gradient, consistent with the hypothesis that this tidal embayment attenuates fecal pollution from inland sources. Nearly 70% of the variability in the coliform record can be attributed to seasonal and interannual variability in local rainfall, implying that stormwater runoff from the surrounding watershed is a primary source of coliform in Newport Bay. The storm loading rate of coliform from the San Diego Creek watershed--the largest watershed draining into Newport Bay--appears to be unaffected by the dramatic shift away from agricultural land-use that occurred in the watershed over the study period. Further, the peak loading of coliform during storms is larger than can be reasonably attributed to sources of human sewage, suggesting that nonhuman fecal pollution and/or bacterial regrowth contribute to the coliform load. Summer time measurements of coliform exhibit interannual trends, but these trends are site specific, apparently due to within-Bay variability in land-use, inputs of dry-weather runoff, and tidal mixing rates. Overall, these results suggest that efforts to improve water quality in Newport Bay will likely have greater efficacy during dry weather summer periods. Water quality during winter storms, on the other hand, appears to be dominated by factors outside of local management control; namely, virtually unlimited nonhuman sources of coliform in the watershed and global climate patterns, such as the El Nino Southern Oscillation, that modulate rainfall and stormwater runoff in southern California.     

Arsenic redistribution between sediments and water near a highly contaminated source

Mechanisms controlling arsenic partitioning between sediment, groundwater, porewaters, and surface waters were investigated at the Vineland Chemical Company Superfund site in southern New Jersey. Extensive inorganic and organic arsenic contamination at this site (historical total arsenic > 10 000 microg L(-1) or > 130 microM in groundwater) has spread downstream to the Blackwater Branch, Maurice River, and Union Lake. Stream discharge was measured in the Blackwater Branch, and water samples and sediment cores were obtained from both the stream and the lake. Porewaters and sediments were analyzed for arsenic speciation as well as total arsenic, iron, manganese, and sulfur, and they indicate that geochemical processes controlling mobility of arsenic were different in these two locations. Arsenic partitioning in the Blackwater Branch was consistent with arsenic primarily being controlled by sulfur, whereas in Union Lake, the data were consistent with arsenic being controlled largely by iron. Stream discharge and arsenic concentrations indicate that despite large-scale groundwater extraction and treatment, > 99% of arsenic transport away from the site results from continued discharge of high arsenic groundwater to the stream, rather than remobilization of arsenic in stream sediments. Changing redox conditions would be expected to change arsenic retention on sediments. In sulfur-controlled stream sediments, more oxic conditions could oxidize arsenic-bearing sulfide minerals, thereby releasing arsenic to porewaters and streamwaters; in iron-controlled lake sediments, more reducing conditions could release arsenic from sediments via reductive dissolution of arsenic-bearing iron oxides.     

Modeling the effect of snow and ice on the global environmental fate and long-range transport potential of semivolatile organic compounds

Snow and ice have been implemented in a global multimedia box model to investigate the influence of these media on the environmental fate and long-range transport (LRT) of semivolatile organic compounds (SOCs). Investigated compounds include HCB, PCB28, PCB180, PBDE47, PBDE209, alpha-HCH, and dacthal. In low latitudes, snow acts as a transfer medium taking up chemicals from air and releasing them to water or soil during snowmelt. In high latitudes, snow and ice shield water, soil, and vegetation from chemical deposition. In the model version including snow and ice (scenario 2), the mass of chemicals in soil in high latitudes is between 27% (HCB) and 97% (alpha-HCH) of the mass calculated with the model version without snow and ice (scenario 1). Amounts in Arctic seawater in scenario 2 are 8% (alpha-HCH) to 21% (dacthal) of the amounts obtained in scenario 1. For all investigated chemicals except alpha-HCH, presence of snow and ice in the model increases the concentration in air by a factor of 2 (HCB)to 10 (PBDE209). Because of reduced net deposition to snow-covered surfaces in high latitudes, LRT to the Arctic is reduced for most chemicals whereas transport to the south is more pronounced than in scenario 1 ("southward shift"). The presence of snow and ice thus considerably changes the environmental fate of SOCs.     

Influence of dissolved organic carbon on methylmercury bioavailability across Minnesota stream ecosystems

Stream ecosystems are widely contaminated by mercury (Hg) via atmospheric transport and deposition in watersheds. Dissolved organic carbon (DOC) is well-known to be the dominant ligand for aqueous methylmercury (MeHg), the bioaccumulative form of Hg in aquatic food webs. However, it is less clear if and how the concentration and character (e.g., aromaticity) of DOC influences the availability of dissolved MeHg to stream food webs. In this work, we analyzed total-Hg and/or MeHg concentrations in water, seston, and macroinvertebrates (filter-feeding hydropsychid caddisflies), and other physiochemical properties in 30 streams along a south-north geographic gradient in eastern Minnesota that corresponds to substantial changes in dominant land cover (i.e., agriculture, urban, wetland, and forest). In general, MeHg concentrations in seston and hydropsychids were higher in watersheds with more forest and wetland coverage, and increased with dissolved MeHg concentration. However, we found that the efficiency of MeHg incorporation into the stream food webs (i.e., bioconcentration factors of MeHg in both seston and hydropsychids, BCF(MeHg) = solid MeHg ÷ dissolved MeHg) decreased significantly with DOC concentration and aromaticity, suggesting that MeHg bioavailability to the base of food webs was attenuated at higher levels of terrestrial DOC. Therefore, our findings suggest that there is a dual role of DOC on MeHg cycling in streams: terrestrial DOC acts as the primary carrier ligand of dissolved MeHg for transport into surface waters, yet this aromatic DOC also attenuates dissolved MeHg uptake by aquatic food webs. Thus, consideration of MeHg bioavailability and its environmental regulation could help improve predictive models of MeHg bioaccumulation in stream ecosystems.     

Sources of methylmercury to a wetland-dominated lake in northern Wisconsin

Several lines of evidence suggest that wetlands may be a major source of methylmercury (MeHg) to receiving waters, perhaps explaining the strong correlation between concentrations of waterborne MeHg and dissolved organic carbon (DOC) in regions such as northern Wisconsin. We evaluated the relative importance of wetland export in the MeHg budget of a wetland-dominated lake in northern Wisconsin using mass balance. Channelized runoff from a large headwater wetland was the major source of water and total mercury (HgT) to the lake during the study period. The wetland also exported MeHg in high concentrations (0.2-0.8 ng L(-1)), resulting in an export rate similar to those reported for other northern wetlands (ca. 0.3 microg MeHg m(-2) y(-1)). Yet, based on intensive sampling during 2002, the mass of MeHg that accumulated in the lake during summer was an order of magnitude greater than the export of MeHg from the wetland to the lake. Hence, a large in-lake source of MeHg is inferred from the mass balance. Most of the accumulated MeHg built-up in anoxic hypolimnetic waters; and the build-up was roughly balanced by losses of inorganic Hg (Hg(II)) implying a chemical transformation within the anoxic water column. An abundance of sulfate-reducing bacteria (SRB) in hypolimnetic waters, established by DNA analysis of the pelagic microbial community, along with a previous report documenting high methylation rates in the hypolimnion of this lake (ca. 10% d(-1)), suggest that this transformation was microbially mediated. These findings indicate that the direct effect of wetland runoff may be outweighed by indirect effects on the lacustrine MeHg cycle, enhancing the load of Hg(II), the activity of SRB, and the retention of MeHg, especially in northern lakes with flushing times longer than six months.     

Mercury emission to the atmosphere from experimental manipulation of DOC and UVR in mesoscale field chambers in a freshwater lake

Mesocosm experiments in an optically transparent lake allow the manipulation of both dissolved organic carbon (DOC) and incident ultraviolet radiation (UVR) in order to study mercury reduction and emission processes. In the absence of UVR and the presence of visible light, mercury emission is very low (approximately0.3 ng/m2/h). When UVR is permitted in the mesocosm chambers, mercury emission increases, with emission rates ranging from 0.3 to 2.5 ng/m2/h. At concentrations between 1.5 and 2.5 mg/L DOC, mercury emission does not appear to depend on either the concentration or the optical properties of the DOC. In particular, the addition of 1.0 mg/L DOC from a nearby wetland to a photobleached mesocosm did not increase the emission of mercury. The similarities between mercury emission from highly photobleached 1.5 mg/L DOC and from terrestrially enriched 2.5 mg/L DOC suggest that the moieties responsible for mercury reduction are far in excess of that needed for mercury reduction. Using the measured flux rate of mercury from the water surface, we calculated a dissolved gaseous mercury (DGM) concentration thatwould need to be present to drive the emissive flux. The buildup of DGM was used to approximate a kinetic rate constant for the net mercury reduction in this system of approximately 0.17 h(-1), which is consistent with existing published values.     

Interaction between urbanization and climate variability amplifies watershed nitrate export in Maryland

We investigated regional effects of urbanization and land use change on nitrate concentrations in approximately 1,000 small streams in Maryland during record drought and wet years in 2001-2003. We also investigated changes in nitrate-N export during the same time period in 8 intensively monitored small watersheds across an urbanization gradient in Baltimore, Maryland. Nitrate-N concentrations in Maryland were greatest in agricultural streams, urban streams, and forest streams respectively. During the period of record drought and wet years, nitrate-N exports in Baltimore showed substantial variation in 6 suburban/urban streams (2.9-15.3 kg/ha/y), 1 agricultural stream (3.4-38.9 kg/ha/y), and 1 forest stream (0.03-0.2 kg/ha/y). Interannual variability was similar for small Baltimore streams and nearby well-monitored tributaries and coincided with record hypoxia in Chesapeake Bay. Discharge-weighted mean annual nitrate concentrations showed a variable tendency to decrease/increase with changes in annual runoff, although total N export generally increased with annual runoff. N retention in small Baltimore watersheds during the 2002 drought was 85%, 99%, and 94% for suburban, forest, and agricultural watersheds, respectively, and declined to 35%, 91%, and 41% during the wet year of 2003. Our results suggest that urban land use change can increase the vulnerability of ecosystem nitrogen retention functions to climatic variability. Further work is necessary to characterize patterns of nitrate-N export and retention in small urbanizing watersheds under varying climatic conditions to improve future forecasting and watershed scale restoration efforts aimed at improving nitrate-N retention.     

Large shift in source of fine sediment in the upper Mississippi river

Although sediment is a natural constituent of rivers, excess loading to rivers and streams is a leading cause of impairment and biodiversity loss. Remedial actions require identification of the sources and mechanisms of sediment supply. This task is complicated by the scale and complexity of large watersheds as well as changes in climate and land use that alter the drivers of sediment supply. Previous studies in Lake Pepin, a natural lake on the Mississippi River, indicate that sediment supply to the lake has increased 10-fold over the past 150 years. Herein we combine geochemical fingerprinting and a suite of geomorphic change detection techniques with a sediment mass balance for a tributary watershed to demonstrate that, although the sediment loading remains very large, the dominant source of sediment has shifted from agricultural soil erosion to accelerated erosion of stream banks and bluffs, driven by increased river discharge. Such hydrologic amplification of natural erosion processes calls for a new approach to watershed sediment modeling that explicitly accounts for channel and floodplain dynamics that amplify or dampen landscape processes. Further, this finding illustrates a new challenge in remediating nonpoint sediment pollution and indicates that management efforts must expand from soil erosion to factors contributing to increased water runoff.     

Anthropogenic sources of arsenic and copper to sediments in a suburban lake,Northern Virginia

Mass balances of total arsenic and copper for a suburban lake in densely populated northern Virginia were calculated using date collected during 1998. Mass-balance terms were precipitation; stream inflow, including road runoff; stream outflow; and contributions from leaching of pressure-treated lumber. More mass of arsenic and copper was input to the lake than was output the 1998 lake-retention rates were 70% for arsenic and 20% for copper. The arsenic mass balance compared well with a calculated annual mass accumulation in the top 1 cm of the lake sediments; however, the calculated contribution of copper to the lake was insufficient to account for the amount of copper in this zone. Leaching experiments were conducted on lumber treated with chromated copper arsenate (CCA) to quantify approximate amounts of arsenic and copper contributed by this source. Sources to lake sediments included leaching of CCA-treated lumber (arsenic, 50%; copper, 4%), streamwater (arsenic, 50%; copper, 90%), and atmospheric deposition (arsenic, 1%; copper, 3%). Results of this study suggest that CCA-treated lumber and road runoff could be significant nonpoint sources of arsenic and copper, respectively, in suburban catchments.     

Trace metal and major ion inputs into the Olentangy River from an urban storm sewer

Trace metal clean techniques were used to sample and analyze the input of dissolved trace metals, major ions, and dissolved organic carbon (DOC) from a storm sewer along an urban highway in Columbus, OH. The outfall, draining a 3.6 ha sewershed with 100% impermeable surface area, discharges into the Olentangy River. Dissolved Pb (average concentration of 3 nM) and dissolved Zn (average concentration of 127 nM) were found to be much lower in concentration than reported in previous investigations of dissolved metals in urban stormwater runoff. Average concentrations of dissolved Cr (1 microM), Ni (0.087 microM), and Cu (0.33 microM) were similar to those reported in previous studies. The storm sewer is shown to be a significant source of V, Ni, and Zn to the river. The outfall is also a significant source of Na, NH4, Cl, and DOC. The storm sewer input is depleted in NO2 and NO3 as compared to the river, reflecting the highly agricultural land use of the watershed upstream of the sewershed. Input from the storm sewer is also depleted, as compared to the river, with respect to dissolved Mg, Sr, and U with probable sources in the limestone/shale bedrock and glacial till-derived soils in the watershed.     

A dynamic model of the fate of organic chemicals in a multilayered air/soil system: development and illustrative application

A new site-specific, dynamic model (SoilPlus) was developed to simulate the fate of nonionized organic chemicals in the air/litter/soil system; key features of the model are the double-layered air compartment interacting dynamically with multilayered litter and soil compartments, with seasonal dissolved organic carbon (DOC) fluxes. The model describes the soil environment calculating separate mass balances for water, chemical, and organic matter. SoilPlus underwent a process of benchmarking and evaluation in order to reach a satisfying confirmation of its predictive capability. Several simulations were performed to estimate the role of litter and DOC in affecting the fate of a model contaminant for POPs (hexachlorobenzene). The model shows that litter can behave as a buffer in the process of transferring hexachlorobenzene from air to the mineral soil and as a trap when hexachlorobenzene tends to move from a contaminated field toward clean air. DOC seems to behave as a leaching-enhancer in certain climatic conditions (heavy rainfall, high DOC concentrations), but it does not appear to move significant amounts of HCB in a year calculation.