Atmospheric Deposition Rates Research Articles

Factors affecting the occurrence and enantiomeric degradation of hexachlorocyclohexane isomers in northern and temperate aquatic systems

Concentrations of (alpha- and gamma-hexachlorocyclohexane (HCH), alpha/gamma-HCH ratios, and enantiomer ratios (ER) of alpha-HCH were measured in lakes in the arctic, subarctic, Great Lakes, Canada, and temperate regions, and temperate and arctic wetlands and streams. The highest concentrations of alpha-HCH were found in cold, large, and oligotrophic lakes such as those in the arctic, subarctic, and the upper Great Lakes, which is attributed to greater inputs from atmospheric deposition and slower loss rates relative to warmer, temperate lakes. High alpha/gamma-HCH ratios in northern systems indicate aged HCH that has undergone long-range transport to high latitude areas, whereas low ratios in the lower Great Lakes and small temperate systems indicate recent gamma-HCH usage and residual alpha-HCH concentrations. Enantioselective degradation (ERs ranged from 0.31 to 0.7) was greatest in small, high arctic lakes and streams and in large lakes in the subarctic in which alpha-HCH concentrations and contact time between chemical and sediments are highest and nutrient concentrations are lowest. Low ERs were found in wetlands and streams in which contact between chemical and sediments was greatest. Conversely, minimal enantioselective degradation occurred in temperate small lakes and wetlands (ERs ranging from 0.77 to 1.06), despite the warmer temperatures, greater microbial populations, and nutrient availability. The results suggest that enantioselective degradation is optimized by maximal contact between chemical and sediment substrates in nutrient-poor waters in which, it is hypothesized, oligotrophic bacteria may act as biofilms.

Accumulation rate and mixing of shelf sediments in the Monterey Bay National Marine Sanctuary

The distribution of excess 210Pb in 31 sediment cores was used to determine modern (last 100 yr) mass accumulation rates and the depth of sediment mixing on the continental shelf between Pacifica and Monterey, California, USA. Apparent mass accumulation rates average 0.27 g cm −2 yr −1 and range from 0.42 g cm −2 yr −1 to 0.12 g cm −2 yr −1. Accumulation rates were highest at mid-shelf water depths (60–100 m) adjacent to major rivers and near the head of the Ascension submarine canyon. Cores from water depths of less than 65 m had low, uniform 210Pb activity profiles and sandy textures. The uppermost 5–13 cm of 15 cores had uniform 210Pb activity profiles above a region of steadily decreasing 210Pb activity. This phenomenon was attributed to sediment mixing. The thickness of this upper layer of uniform 210Pb activity decreased southward from 13 cm, west of Pacifica, to less than 5 cm, near Monterey Canyon. This southward decrease may be attributed to shallower bioturbation in the southern study area. Integrated excess 210Pb activities were generally higher where sedimentation rates were high. They were also higher with increasing distance from major rivers. Thus, sedimentation rate alone does not explain the distribution of integrated excess 210Pb in this study area. Excess 210Pb in the seafloor is controlled by other factors such as sediment texture, the atmospheric deposition rate of 210Pb, and the residence time of sediment particles in the water column.

Evaluating Critical Soil Acidification Loads and Exceedances for a Deciduous Forest at the Turkey Lakes Watershed

Critical soil acidification loads (CL) and related exceedances, base cation leaching, N leaching, and forest biomass growth were evaluated for a well-studied deciduous forest site within the Turkey Lake Watershed (TLW). The assessment was done by way of steady-state mass balance considerations of primary inputs for N, Ca, Mg, and K. Critical soil acidification rates were found to be high at TLW. These rates amounted to about 900 or 1400 eq/(ha yr) depending on the forest harvesting regime (selective harvest or maintainence of old-growth condition, respectively). The TLW soil substrate (till derived from basaltic bedrock) appeared to weather well, thereby buffering against natural and anthropogenic soil acidification. As a consequence, soil acidification exceedances were estimated to be relatively low for both the selective harvest and old-growth scenarios. In comparing overall S and N input/output data (atmospheric deposition data vs soil leaching losses), we found that the TLW site was essentially near or at S and N saturation. We also found that atmospheric deposition and soil leaching rates have been declining since about 1980. The figures for CL and exceedance varied to some extent depending on the quality of input data and related uncertainties. Estimated exceedances were increased when dry- as well as wet-deposition rates were considered. They varied depending on the yearly sulfate/nitrate/base-cation mix, and the definition of “acceptable acid leaching.” In addition, they were dependent on whether the forest was considered old growth or not.

Sulfur burial in and loss from the sediments of Little Rock Lake, Wisconsin

Lake sediments often are regarded as accurate records of changes in climatic conditions, rates of atmospheric deposition of substances to the lake, or other processes occurring within lakes. In this study, the sedimentary record of sulfur was examined in Little Rock Lake, Wisconsin. This lake received experimental sulfate additions from 1985 through 1990, and a mass balance showed that 35% (425 kg) of experimental sulfur additions were buried in the sediments. Sediment cores collected before and after sulfur additions confirmed that burial of 180–360 kg of sulfur occurred during the 6 years of experimental sulfur additions. However, cores collected in 1992 and 1996 contained less sulfur than cores taken in 1990 and suggested that sulfur incorporation in sediments is not permanent. To examine seasonal sulfur loss from sediments, sulfur inventories in the top 4 cm of sediments were measured in multiple cores before and after fall overturn. At water depths greater than 5 m, surface sediments retrieved after fall overturn had significantly less sulfur than did those collected before overturn. The small seasonal release from the sediments (25 kg S in 1996) might be caused by organic matter decomposition or sulfide oxidation.

A screening model-based study of transport fluxes and fate of airborne mercury deposited onto catchment areas

Dynamics of airborne mercury deposited onto catchment areas is investigated within the framework of a simulation model. Model results show that, for a particular atmospheric deposition rate, significant interannual variability in mercury transport flux in catchments is caused by climatology and corresponding differences in catchment soil loss rates; in comparison to the normal year, runoff flux increased by a factor of 2–3 for the wet year (rainfall 35% above normal) while for the dry year (rainfall 18% below normal) runoff flux decreased by factors of 5–7. The interaction of parameters describing soil type, topography and vegetation cover causes variability in both transport and emission fluxes among catchments; as soil loss rate increases by a factor of 5 due to variations in these parameters among the examined catchments, annual average transport flux increases by a factor of 3; and annual average emission flux of mercury (as Hg 0) from soil to the atmosphere decreases by a factor of 2 due to the decreased levels of soil mercury associated with catchment soil loss increases. Seasonal variability of transport flux is associated with seasonal changes in precipitation and soil loss rates while seasonal changes of emission flux are primarily due to changes in soil moisture regime and temperature. Although modeled results are consistent with observational data from previous studies, they must be interpreted in a relative sense due to the screening-level character of this study.

Class I areas at risk: event-based nitrogen deposition to a high-elevation, western site.

Between June 1, 2000 and September 30, 2000, 32 precipitation events were sampled near Telluride, CO at an elevation of 3200 m. The wet deposition site was operated following protocols of the Atmospheric Integrated Research Monitoring Network (AIRMoN), a network of the National Atmospheric Deposition Network (NADP). Inorganic nitrogen deposition at the Telluride site of 1.41 kg ha during the study period was 25 to 50% higher than nearby NADP sites. In turn, nitrogen deposition at these NADP sites was similar to high-elevation sites in and near the Colorado Front Range that have been shown to be impacted by atmospheric deposition of inorganic nitrogen in wetfall. Power plant emissions are a likely source of a major portion of this elevated inorganic nitrogen in wetfall to the San Juan Mountains. Principal component analysis (PCA) shows that solutes formed by gases that are emitted from power plants were clustered tightly together, including nitrate, ammonium, sulfate, and chloride. Trajectory analysis, including both backward and forward trajectories, shows that the air masses that contributed to the precipitation events with high amounts of nitrogen deposition at the Telluride site passed directly over or near power plants. Our results suggest that Class I Wilderness Areas in and near the San Juan Mountains are at risk to ecosystem impairment at present rates of atmospheric deposition of inorganic nitrogen in wetfall. Deployment of proposed power plants to this area will likely increase the risk of degradation of resource values in nearby Class I areas. While these data were collected over a short time span, they indicate that establishment of an official AIRMoN site in the southwestern U.S. may be warranted.

Changes in Atmospheric Deposition Rates of Heavy Metals in Sweden A Summary of Nationwide Swedish Surveys in 1968/70 – 1995

The Swedish nationwide surveys of atmospheric heavy metal deposition in 1968/70 – 1995 using bryophytes (carpet-forming mosses) as monitors are reviewed. Considered are cadmium (Cd), copper (Cu), iron (Fe), lead (Pb), mercury (Hg), nickel (Ni), vanadium (V), and zinc (Zn). The close agreement between data obtained with deposition (precipitation) collectors and moss carpet analysis is documented briefly, as well as measures of quality control and assessment of reproducibility. Since 1968/70, the deposition rate of the heavy metals considered has declined gradually and evenly, particularly in the southern parts of the country, reflecting an improvement of general air quality due to decreasing dust emission from mainly industry and fossil fuel combustion in northern and western Europe. By far the greatest absolute decrease in deposition rate was measured in Fe, which is a main constituent of most man-generated dust particles. However, the greatest per cent decreases in deposition rates were measured for Pb. The Pb concentration of moss carpets in Sweden as a whole in 1995 was only 11% of the value in 1968/70. Corresponding figures for Fe was 20%, Cd 24%, Ni 28%, Hg 31%, V 43%, Zn 51%, and Cu 52%. For Zn the current deposition rate seems to approach a ‘natural’ baseline, whereas deposition of the other seven elements are still decreasing according to the surveys of 1990 and 1995.

Nitrogen deposition and leaching from two forested catchments in Southwest China--preliminary data and research needs.

Increased nitrogen deposition has resulted in increased nitrogen pools and nitrogen leaching in European and North American forest soils. The development in Asia in general, and China in particular, suggests increased deposition of reduced nitrogen from changes in agricultural practices and of oxidized nitrogen from rapid growth of the transportation sector. Decreased nitrogen retention in forested areas in the future may cause increased NO3– leaching and, thus, acidification and eutrophication in surface waters. The differences in climate, ecosystems, land use, and deposition history make direct application of knowledge from studies in Europe and North America difficult. In Southwest China the potential for nitrogen mobilization from forest soils may be high because of the warm and humid climate, resulting in high decomposition rates of soil organic matter. However, there are very few data available for quantifying the suspected potential for increased nitrogen leaching in forest ecosystems. Here we present data from two forested catchments, dominated by Masson pine (Pinus massoniana), near Guiyang and Chongqing, respectively, in Southwest China. The present nitrogen deposition is moderate, estimated in the range from 10 to 40 kg N ha–1 year–1. The C/N ratios of the soils are generally below 15. Nitrate concentrations in soil water are rather variable in space, with highest values of several hundred microequivalents per liter. The turnover rate of nitrogen in the forest ecosystem is quite high compared to the atmospheric deposition rate. At present, nitrate runoff from the catchments is low and intermediate in Guiyang and Chongqing, respectively. More research is needed to improve our ability to predict future nitrogen leaching from subtropical Asian coniferous forests.

Seasonal and long-term variation of 210Pb concentration in air, atmospheric deposition rate and total deposition velocity in south Germany

The activity concentration in air and atmospheric deposition rate of the long-lived radon progeny 210Pb has been investigated at Munich–Neuherberg, south Germany, from 1972 (activity concentration) and from 1981 (atmospheric flux) to 1999. For these periods, the continuous measurements yielded an average 210Pb activity concentration at ground level of 0.57 mBq m −3, and an average total 210Pb deposition rate to ground of 180 Bq m −2 year −1. The average total deposition velocity, which relates the total 210Pb deposition rate to the 210Pb activity concentration was calculated to be 1.0 cm s −1. The variation of the data with time was studied by time-series analysis and distinct seasonal patterns were identified. Maximum 210Pb activity concentrations in air are observed in the autumn and winter months (October through February) of each year. By contrast, the maximum 210Pb deposition rate is observed during summer (June–August), i.e. in the months with the highest amount of rainfall at this site. Like the 210Pb deposition rate, the total deposition velocity exhibits a seasonal pattern with maximum values in summer. Due to the long observation period of 18 years, it was possible to observe for the first time a strong positive relationship between 210Pb deposition and precipitation, especially for the months May and June and to a smaller extent for several other months. In the long-term, variations of approximately a factor of 2 were observed in the annual average 210Pb activity concentrations, the annual deposition sums and the annual average deposition velocities. Since around 1981 210Pb concentrations in air steadily decreased while 210Pb depositions increased. As a consequence of these significant trends, the time series of the total deposition velocity exhibits a trend of the data from approximately 0.7 cm s −1 in 1981 to 1.7 cm s −1 in 1999.

Decreasing Mercury in Northern Wisconsin: Temporal Patterns in Bulk Precipitation and a Precipitation-Dominated Lake

Long-term monitoring of a precipitation-dominated lake in northern Wisconsin indicates decreasing mercury concentrations over the past decade. In the surface waters of Little Rock Lake, aqueous HgT has decreased at an average rate of 0.04 ng/L/yor roughly 40% from 1988 to 1999. The concentration of aqueous meHgT in Little Rock Lake has decreased tooat an average rate of 0.004 ng/L/y (roughly 50% since 1988). Atmospheric monitoring also indicates decreased mercury in bulk precipitation. For those years when we have a continuous data record (1994 through 1999), the annual Hg cycle in bulk precipitation has been damped and the average rate of bulk atmospheric Hg deposition decreased by roughly 50% (from 11.2 ug/m2/y to 5.5 ug/m2/y). These trends suggest that the lake waters have responded to decreased atmospheric Hg depositionperhaps driven by reduced anthropogenic emissions in the region. Ancillary data and mass balance modeling are used to more fully investigate recent changes in the aquatic mercury cycle.

Radionuclides in the terrestrial ecosystem near a Canadian uranium mill--Part III: Atmospheric deposition rates (pilot test).

Atmospheric deposition rates of uranium series radionuclides were directly measured at three sites near the operating Key Lake uranium mill in northern Saskatchewan. Sites impacted by windblown tailings and mill dusts had elevated rates of uranium deposition near the mill and elevated 226Ra deposition near the tailings compared to a control site. Rainwater collectors, dust jars, and passive vinyl collectors previously used at the Ranger Mine in Australia were pilot-tested. Adhesive vinyl surfaces (1 m2) were oriented horizontally, vertically, and facing the ground as a means of measuring gravitational settling, wind impaction, and soil resuspension, respectively. Although the adhesive glue on the vinyls proved difficult to digest, relative differences in deposition mode were found among radionuclides and among sites. Dry deposition was a more important transport mechanism for uranium, 226Ra, and 210Pb than rainfall, while more 210Po was deposited with rainfall.

THE EFFECT OF LANDSCAPE FEATURES ON DEPOSITION TO HUNTER MOUNTAIN, CATSKILL MOUNTAINS, NEW YORK

Atmospheric deposition to montane ecosystems is higher than to adjacent lowlands. However, because of the heterogeneity of mountainous landscapes, rates of deposition are likely to vary considerably across major landscape features. Estimates of total atmospheric deposition for montane areas in the United States are wide ranging and usually based on models that do not take into account landscape heterogeneity. Thus, little is known about the spatial variability of atmospheric deposition to these high-elevation ecosystems. We identified four landscape features that are likely to control patterns and rates of atmospheric deposition in mountainous terrain, including (1) forest edges or gaps, (2) elevation, (3) aspect, and (4) vegetation type, and we measured patterns of atmospheric deposition across them in the Catskill Mountains of New York State. We measured lead amount in the forest floor as an index of atmospheric deposition, used these data to estimate relative deposition to the Hunter Mountain massif, a...

The Effect of Landscape Features on Deposition to Hunter Mountain, Catskill Mountains, New York

Atmospheric deposition to montane ecosystems is higher than to adjacent lowlands. However, because of the heterogeneity of mountainous landscapes, rates of deposition are likely to vary considerably across major landscape features. Estimates of total atmospheric deposition for montane areas in the United States are wide ranging and usually based on models that do not take into account landscape heterogeneity. Thus, little is known about the spatial variability of atmospheric deposition to these high-elevation ecosystems. We identified four landscape features that are likely to control patterns and rates of atmospheric deposition in mountainous terrain, including (1) forest edges or gaps, (2) elevation, (3) aspect, and (4) vegetation type, and we measured patterns of atmospheric deposition across them in the Catskill Mountains of New York State. We measured lead amount in the forest floor as an index of atmospheric deposition, used these data to estimate relative deposition to the Hunter Mountain massif, and with a geographic information system (GIS) database, scaled up atmospheric deposition estimates to the Hunter Mountain landscape. Area-weighted mean deposition to the region above 1000 m (high-elevation) was 13–43% greater than to a nearby, low-elevation site, depending upon how edges were defined. In addition, we identified “hotspots” of deposition, such as high-elevation, coniferous forest edge zones, where atmospheric deposition of pollutants and nutrients was 300% greater (44 kg N·ha−1·yr−1 and 46 kg S·ha−1·yr−1) than to low-elevation forests (11.1 kg N·ha−1·yr−1 and 11.6 kg S·ha−1·yr−1). Our data suggest that estimates of atmospheric deposition to mountainous terrain that do not include landscape heterogeneity may be inadequate. Furthermore, current models of atmospheric deposition cannot predict “hotspot” regions of extremely high deposition. Landscape features such as forest edges, elevation, aspect, and vegetation type should be considered in future modeling efforts.

Atmospheric nitrogen in the Mississippi River Basin — emissions, deposition and transport

Atmospheric deposition of nitrogen has been cited as a major factor in the nitrogen saturation of forests in the north-eastern United States and as a contributor to the eutrophication of coastal waters, including the Gulf of Mexico near the mouth of the Mississippi River. Sources of nitrogen emissions and the resulting spatial patterns of nitrogen deposition within the Mississippi River Basin, however, have not been fully documented. An assessment of atmospheric nitrogen in the Mississippi River Basin was therefore conducted in 1998–1999 to: (1) evaluate the forms in which nitrogen is deposited from the atmosphere; (2) quantify the spatial distribution of atmospheric nitrogen deposition throughout the basin; and (3) relate locations of emission sources to spatial deposition patterns to evaluate atmospheric transport. Deposition data collected through the NADP/NTN (National Atmospheric Deposition Program/National Trends Network) and CASTNet (Clean Air Status and Trends Network) were used for this analysis. NO x Tier 1 emission data by county was obtained for 1992 from the US Environmental Protection Agency (Emissions Trends Viewer CD, 1985–1995, version 1.0, September 1996) and NH 3 emissions data was derived from the 1992 Census of Agriculture (US Department of Commerce. Census of Agriculture, US Summary and County Level Data, US Department of Commerce, Bureau of the Census. Geographic Area series, 1995:1b) or the National Agricultural Statistics Service (US Department of Agriculture. National Agricultural Statistics Service Historical Data. Accessed 7/98 at URL, 1998. http://www.usda.gov/nass/pubs/hisdata.htm). The highest rates of wet deposition of NO 3 − were in the north-eastern part of the basin, downwind of electric utility plants and urban areas, whereas the highest rates of wet deposition of NH 4 + were in Iowa, near the center of intensive agricultural activities in the Midwest. The lowest rates of atmospheric nitrogen deposition were on the western (windward) side of the basin, which suggests that most of the nitrogen deposited within the basin is derived from internal sources. Atmospheric transport eastward across the basin boundary is greater for NO 3 − than NH 4 + , but a significant amount of NH 4 + is likely to be transported out of the basin through the formation of (NH 4) 2SO 4 and NH 4NO 3 particles — a process that greatly increases the atmospheric residence time of NH 4 + . This process is also a likely factor in the atmospheric transport of nitrogen from the Midwest to upland forest regions in the North-East, such as the western Adirondack region of New York, where NH 4 + constitutes 38% of the total wet deposition of N.

PCBs and PAHs in Southern Lake Michigan in 1994 and 1995: Urban Atmospheric Influences and Long-Term Declines

Surface water samples were collected in southern Lake Michigan off Chicago, IL to determine if elevated levels of urban atmospheric pollutants influence concentrations of organic contaminants in nearshore surface waters. Water was sampled every 12 hours for 2 to 12 days during May 1994, July 1994, and January 1995 sampling intensives. Total (dissolved + particulate; sum of 85 congeners) polychlorinated biphenyl (∑-PCB) concentrations ranged from 80 pg/L (26 July 1994) to 350 pg/L (18 January 1995), while total polycyclic aromatic hydrocarbon (∑ 24-PAH) concentrations during the same period ranged from 7 ng/L (18 May 1994) to 19 ng/L (17 January 1995). PCB concentrations in surface waters were higher when winds were from the S/SW, suggesting that the rate of direct atmospheric deposition of urban PCBs to the surface water may be fast relative to aquatic horizontal mixing and removal mechanisms. Additionally, PCB concentrations were higher during the winter than in the spring or summer, but PAH concentrations were not significantly different throughout the year. Lipid normalized PCB concentrations in zooplankton ( > 202 μm) appear to be higher during the winter (780 vs. 14,200 ng ∑-PCBs/glipid; July vs. January), but PAH levels in zooplankton were invariant with season (21,000 vs. 21,200 ng ∑-PAH/g lipid; July vs. January). The seasonal trend in surface water concentrations suggests relatively constant, elevated deposition of urban PAHs throughout the year, but net PCB exchange that varies seasonally. In spite of these elevated atmospheric deposition fluxes, the concentration of ∑-PCB in Lake Michigan surface waters has declined ten fold over the past 14 years. The resulting pseudo-first order loss rate constant for Lake Michigan is 0.17 ± 0.3/yr (t 1/2 = 4.0 years), which closely matches those found for the atmosphere and biota of Lake Michigan.

The effect of long-term nitrogen additions on the bryophyte cover of upland acidic grasslands

Regular applications of NH4NO3 (3.5–14 g N m−2 yr−l) and (NH4)2SO4 (14 g N m−2 yr−l) to areas of acidic grassland in the Derbyshire Peak District over a period of six years have resulted in marked changes in the abundance of the bryophyte species present on the site. A dose-related reduction in bryophyte cover, significant at all levels of nitrogen addition, was obtained after only one year of applications and this effect has been maintained over a further five years of treatment.(NH4)2SO4 additions at the same rate as the highest NH4NO3 input (14 g N m−2 yr−1) produced greater reductions in cover with losses of 80–95% compared with 45–55% for the 14 g NH4NO3 treatment. Soil pH measurements taken from cores collected in December 1994 showed a small reduction in the pH of the (NH4)2SO4 treated plots (4.03 compared with 4.41 for the controls), whereas the NH4NO3 treatments were unaffected.The mean stem density of the dominant bryophyte species Rhytidiadelphus squarrosus was significantly reduced at the lowest level of nitrogen addition and this was associated with marked increases in the total stem nitrogen content of this species. Pleurozium schreberi, present at much lower cover values, however showed no significant change except at the highest level of application, suggesting differential effects of the treatments on these two species.These results are indicative of significant losses in the bryophyte cover of acidic grasslands at atmospheric input rates of 5 g N m−2 yr−1 or less, and suggest that these species may be particularly at risk from the high atmospheric nitrogen deposition rates that are becoming a feature of many upland areas.

Controls on nitrogen flux in alpine/subalpine watersheds of Colorado

High‐altitude watersheds in the Front Range of Colorado show symptoms of advanced stages of nitrogen excess, despite having less nitrogen in atmospheric deposition than other regions where watersheds retain nitrogen. In two alpine/subalpine subbasins of the Loch Vale watershed, atmospheric deposition of NO3− plus NH4+ was 3.2–5.5 kg N ha−1, and watershed export was 1.8–3.9 kg N ha−1 for water years 1992–1997. Annual N export increased in years with greater input of N, but most of the additional N was retained in the watershed, indicating that parts of the ecosystem are nitrogen‐limited. Dissolved inorganic nitrogen (DIN) concentrations were greatest in subsurface water of talus landscapes, where mineralization and nitrification augment high rates of atmospheric deposition of N. Tundra landscapes had moderately high DIN concentrations, whereas forest and wetland landscapes had low concentrations, indicating little export of nitrogen from these landscapes. Between the two subbasins the catchment of Icy Brook had greater retention of nitrogen than that of Andrews Creek because of landscape and hydrologic characteristics that favor greater N assimilation in both the terrestrial and aquatic ecosystems. These results suggest that export of N from alpine/subalpine watersheds is caused by a combination of direct flushing of N from atmospheric deposition and release of N from ecosystem biogeochemical processes (N cycling). Sensitivity of alpine ecosystems in the western United States to atmospheric deposition of N is a function of landscape heterogeneity, hydrologic flow paths, and climatic extremes that limit primary productivity and microbial activity, which, in turn, control retention and release of nitrogen. Conceptual and mechanistic models of N excess that have been developed for forested ecosystems need to be modified in order to predict the response of alpine ecosystems to future changes in climate and atmospheric deposition of N.

History of the atmospheric deposition of major and trace elements in the industrialized St. Lawrence Valley, Quebec, Canada

The history of the atmospheric deposition of major and trace elements over southwestern Quebec, Canada, was reconstructed using multielemental analysis of lacustrine sediments sampled in a small and undisturbed lake located on top of a mountain in the heart of the industrialized St. Lawrence Valley. Acid leachable and residual elements were extracted from a 37-cm long core (1-cm resolution) using clean techniques and analyzed by inductively coupled plasma mass spectrometry. Organic matter and sulfur concentrations were high and played a major role in the low postdepositional diagenetic remobilization of many trace elements. Sulfur, manganese, iron, arsenic, molybdenum and barium displayed a high mobility making it exceedingly difficult to infer unambiguously time-dependent changes in atmospheric deposition for these elements. Atmospheric deposition rates for the less mobile elements (e.g., potassium, vanadium, chromium, cobalt, nickel, copper, zinc, rubidium, cadmium, tin, antimony, mercury, thallium, lead, and bismuth) increased regularly between 1942 and 1960–1975 in the Lake Hertel area and then stabilized for most of these elements, with the exception of nickel, copper, zinc and tin. Lead deposition rate was reduced by about 25% between 1982 and 1995, and a slight decreasing trend was also found for cobalt, mercury, and thallium during the same period. Present-day atmospheric deposition of metals directly on the lake surface represents a small percentage of the sedimentary deposition rates at this location. Deposition followed by surface runoff and outwash of terrestrial organic and inorganic matter most likely is the driving mechanism leading to the non-diagenetic enrichment of metals in Lake Hertel sediments.

Atmospheric heavy metal deposition plumes adjacent to a primary lead–zinc smelter

A method for the determination of atmospheric heavy metal deposition rates has been developed using 0.5-m 2 deposition trays at 0.1 m from the ground. Trays were spaced at 150-m intervals along a 1500-m line 500 m east of a Pb–Zn smelter. Ten sampling events of 1–3-h duration were conducted under westerly wind conditions so as to determine the sources of heavy metals deposited near the smelter. Deposited materials were sampled from the trays using wipes. There was good agreement between deposition trays placed side-by-side and exposed in pairs. Under certain conditions, however, the method is not appropriate owing to the potential for local contamination. Geometric mean deposition rates for Pb, Zn, Fe, Cu, As and Cd averaged over a nominal plume width of 600 m amounted to 18.8, 22.2, 12.2, 0.614, 0.403 and 0.052 mg m −2 day −1, respectively. Gaussian deposition profiles were seen for Pb, Zn, Fe, Cu, As and Cd downwind from the blast furnace, sinter plant, and refinery area. Zinc deposition could also be attributed to a northern Zn production area. This northern site was not generally associated with elevated Pb deposition. On the basis of this work, the deposition of heavy metals in residential areas adjoining the smelter is likely to occur downwind from the smelter site, with deposition rates increasing with wind speed. The strategic measurement of heavy metal dry-deposition rates over short periods of time using large collection surfaces provides source-specific information not obtainable by conventional long-term ‘passive’ deposition sampling. Lower detection limits than those achieved here are likely to be achieved in non-smelter settings. Previous suggestions implicating a sink of city surface dusts as the probable source of Pb recontamination of residential settings in the absence of ongoing smelter emissions are not supported by this work.

A new sampler for collecting separate dry and wet atmospheric depositions of trace organic chemicals

Studies conducted in Saskatchewan and elsewhere have demonstrated the atmospheric transport of agricultural pesticides and other organic contaminants and their deposition into aquatic ecosystems. To date these studies have focused on ambient concentrations in the atmosphere and in wet precipitation. To measure the dry deposition of organic chemicals, a new sampler was designed which uses a moving sheet of water to passively trap dry particles and gasses. The moving sheet of water drains into a reservoir and, during recirculation through the sampler, is passed through an XAD-2 resin column which adsorbs the trapped organic contaminants. All surfaces which contact the process water are stainless steel or Teflon. Chemicals collected can be related to airborne materials depositing into aquatic ecosystems. The sampler has received a United States patent (number 5,413,003 – 9 May 1996) with the Canadian patent pending. XAD-2 resin adsorption efficiencies for 10 or 50 μg fortifications of ten pesticides ranged from 76% for atrazine (2-chloro-4-ethylamino-6-isopropylamino- S-triazine) to 110% for triallate [ S-(2,3,3-trichloro-2-phenyl)bis(1-methylethyl)carbamothioate], dicamba (2-methoxy-3,6-dichlorobenzoic acid) and toxaphene (chlorinated camphene mixture). Field testing using duplicate samplers showed good reproducibility and amounts trapped were consistent with those from high volume and bulk pan samplers located on the same site. Average atmospheric dry deposition rates of three chemicals, collected for 5 weeks in May and June, were: dicamba, 69 ng m -2 da -1; 2,4-D (2,4-dichlorophenoxyacetic acid), 276 ng m -2 da -1: and, γ-HCH ( γ-1, 2, 3, 4, 5, 6-hexachlorocyclohexane), 327 ng m -2 da -1.