Historical rates of atmospheric Pb deposition using210Pb dated peat cores: Corroboration,computation, and interpretation

Lead-210 dating of peat cores is one approach that has been used to arrive at historical rates of heavy metal deposition. Despite concerns regarding the validity of²¹⁰Pb dating due to Pb mobility,²¹⁰Pb dating can be used if the dates are corroborated with some other independent dating technique. In this study, based on analyses of²¹⁰Pb dated, pollen corroborated peat cores from two sites in the Czech Republic (Jezerní sla and Bo?í Dar Bog), we illustrate a previously unexplored problem concerning the computation of metal deposition, using Pb as an example. When peat cores are collected, sectioned into depth intervals,²¹⁰Pb dated and analyzed for metal contents, the²¹⁰Pb dates most appropriately correspond to the midpoint depth for each interval, whereas the metal contents correspond to the interval between the top and bottom of each section. Thus the²¹⁰Pb dates and metal content values throughout the core are offset by half the distance of each depth interval. In calculating historical rates of heavy metal deposition two approaches are available for correcting for the depth interval offsets, the traditional approach of date interpolation and our newly proposed metal content interpolation. We see noa priori reason for choosing one approach over the other, and suggest simultaneous use of both date and metal content interpolation. Additionally, acid-insoluble ash (AIA), which has been proposed as a dating technique in and of itself, may be more useful as an interpretive tool which may provide insights into the nature or sources of atmospherically deposited Pb. For example, plots of Pb content per core section versus AIA content per core section for Jezerní slat, located in a relatively pristine area, reveal increased Pb content without increased AIA contents in depths shallower than 6 cm, indicating deposition of gasoline-derived Pb after its introduction in 1922. Similar plots for Bo?í Dar Bog, located in a polluted industrialized region, indicate greater inputs of Pb than would be predicted from AIA, based on the Jezerní sla analyses. We interpret the apparent excess Pb deposition at Bo?í Dar Bog as being contributed by soil-derived dust from local metal mining. Elevated rates in Pb deposition at Bo?í Dar Bog are consistent with the history of local mining known to have occurred in the vicinity. Finally, magnetic susceptibility measurements identify combustion of fossil fuels as a source of atmospheric Pb deposition at Bo?í Dar Bog, but not at Jezerní sla      

Study of wet and dry atmospheric deposition at two altitudes in central Switzerland

The analysis of various ions in atmospheric deposition at two altitudes (515 m and 950 m a.s.l.) in Switzerland during 1983/84 revealed that dry deposition depends more on altitude than wet deposition. During autumn and winter, when vertical air exchange is limited, dry deposition of substances from anthropogenic sources (e.g. SO₄-S) was higher at the lower site. In the course of the year, the pattern of wet deposition was independent of altitude and showed highest rates for S, N and Ca in summertime. A comparison between rates of wet, particulate and gaseous S and N deposition suggested that during the growing season wet deposition is the dominant flux. Total (wet + dry) deposition of S, N and Cl was lower than in other parts of Europe, but a substantial increase of NO₃-N deposition during the past 25 years must have occured.     

An exploratory study of potential As and Pb contamination by atmospheric deposition in two urban vegetable gardens in Rome,Italy

Purpose

A preliminary study was carried out in Rome (Italy) to assess the potential role of atmospheric deposition in trace element contamination in urban vegetable gardens relative to human health risk from crop consumption.

Materials and methods

Two sites were selected on the basis of previously known contamination issues. Atmospheric deposition, parent material, soils properties affecting trace element mobility, and various anthropogenic inputs were considered. Soil samples were taken at depth from two points in each garden, within 5 cm of sampled crops. Inputs and crops were sampled and analysed for As and Pb content. A rain and dust gauge was set up in each garden for the duration of 93 days (late spring to late summer) for atmospheric deposition sampling.

Results and discussion

Atmospheric deposition influx was high at both sites (2.22 and 2.32 As and 2.67 and 3.42 Pb μg m^?3 day^?1). Soil pH was between 6.70 and 7.57 and texture varied from loamy sand to clay loam (3.4 to 31.9 % clay content). CEC ranged between 21.6 and 54.2 meq/100 g within rooting depth, rising almost commensurately with soil organic carbon (SOC) content (1.87–8.37 %). Somewhat high total soil Pb content (80.8–522.7 ppm) contrasted with negligible exchangeability and crop content (<0.01 ppm). Total soil As (17.0–32.0 ppm) corresponded with exchangeable and crop As for one site in one of the gardens. Leaves evinced high As accumulation levels (16.0–41.2 ppm) in all crops. High amounts of atmospheric Pb combined with negligible amounts of extractable Pb and Pb plant tissue content point to particulate inhalation and ingestion as a main health threat in the case of Pb. In contrast, food intake seems to be of greater concern relative to high As contamination. Greater soil As solubility may be explanatory, but the mostly low amounts of extractable As and the high atmospheric As suggest an airborne route being playing an important role.

Conclusions

Preliminary results suggest that research on trace element contamination in urban gardens should consider atmospheric deposition as a major contributing source.

     

Bulk and wet atmospheric deposition chemistry at pallanza (N. Italy)

The chemistry of wet only and bulk depositions collected weekly at Pallanza from May, 1984 to December, 1986 is compared and discussed in relation to the composition of dust fall. The comparison shows a very good agreement between pH values (volume weighted mean values of 4.35 and 4.36, respectively, for wet only and bulk samples) and differences between 6 and 12% for ammonium, sulphate, nitrate, and Na. For Ca, Mg, and K the difference range is 30 to 50%. The amount of precipitation over Pallanza (mean value 1951–1985,1709 mm), together with the solute concentrations in atmospheric deposition, mean that there is high bulk deposition of acidifing substances. The values found during the study period were 76, 93, 143, and 81 meq m² yr^?1 for H⁺, ammonium, sulphate, and nitrate, respectively.     

Forest canopy transformation of atmospheric deposition

Solute fluxes to the ground in open plots and under the forest canopy of different species were investigated in a number of long-term ecosystem studies in West Germany. From the canopy flux balance, rates of interception deposition and canopy/deposition interactions were assessed. Chemically, both open precipitation and throughfall are dilute solutions of H₂SO₄ and HNO₃ and their salts. For the sites investigated, mean pH in bulk precipitation ranged from 4.1 to 4.6, and in throughfall from 3.4 to 4.7. The increase in acidity after canopy passage at most sites indicates considerable interception deposition of strong acids to the forest stands, exceeding the rate of H+ buffering in the canopy. Evidence for buffering processes can be directly deduced from the fact that on sites with high soil alkalinity and high foliage base status, throughfall pH is usually higher than precipitation pH. Furthermore, the same idea can be concluded from changes in solution composition after canopy passage: the H⁺/SO _inf4 ^sup2? ratio is decreasing at most sites, while alkali earth cations from exchange processes occur in throughfall (Ca²⁺/SO _inf4 ^sup2? ratio increases). Solution composition and element flux data are presented for each of the sites, and the regional, orographical and site specific (species composition, ecosystem state) differentiations are discussed. A method for the assessment of total deposition and of canopy interactions such as H⁺-buffering and cation leaching is described, and results of calculations are shown. From these calculations it is concluded that forest ecosystems in Germany receive mean H⁺ loads of ca. 1 to 4 keq H⁺ · ha^?1 · a^?1 from atmospheric deposition. Acidity deposition rates seem to be related to a few key factors such as regional characteristics and ecosystem characteristics.     

The acidifying potential of atmospheric deposition in Canada

It is proposed that the value of ([SO₄ ^??] - [Ca⁺⁺ + Mg⁺⁺]) in precipitation is a suitable way to describe the acidifying potential (AP) of the wet desposition. In eastern North America, the AP of precipitation varies from 20 to 85% of the total sulphate, the remainder of the sulphate being neutralized H₂SO₄, sulphate from salts in dust or from sea salt. The AP ranges from 20 to 80% of the H⁺ in the wet deposition. The rest of the H⁺ is contributed by the net effect of N compounds. Ammonium and nitrate ions from ammonia and NO_Xemissions do not represent a net acidic loading to the terrestrial ecosystem if they are taken up by vegetation. However, when N leaches from watersheds in the form nitrate, it constitutes an acidifying demand on the ecosystem. Therefore, the overall net acidifying potential (NAP) applied to the terrestrial ecosystem is defined by the value of ([SO₄ ^??] - [Ca⁺⁺ + Mg⁺⁺]) in precipitation plus [NO₃ ^??] in runoff from the watershed.     

Identifying indicators of atmospheric nitrogen deposition impacts in acid grasslands

Atmospheric deposition of nitrogen has become a serious concern for nature conservation managers and policy makers. It has the potential to reduce species richness, increase the graminoid component of the sward, encourage species typical of more fertile conditions and alter the soil biogeochemistry of grasslands. Calcifugous grasslands (grasslands found on acid soils) are among the most sensitive to N deposition due to their poorly buffered soils and species typical of nutrient poor environments.Indicators have an important role to play in detecting the impact of nitrogen deposition on sites of conservation importance and assessing conservation status. This study investigates potential indicators of nitrogen deposition impacts that could be incorporated into site condition monitoring programmes such as the UK Common Standards Monitoring.Using two national surveys of calcifugous grasslands we examined the potential for using: the presence or absence of indicator species, the cover of indicator species, the species richness and richness of functional groups, and the cover of functional groups as indicators of N deposition impacts. Of all the potential indicators investigated, graminoid:forb ratio was found to be the best indicator of N deposition impacts. It showed a significant relationship to N deposition in both data sets and is quick and easy to assess in the field. Vegetation indicators must be used with caution as there is potential for vegetation management regime and nutrients from other sources to cause similar changes in species composition. Consideration must be given to these before attributing changes to nitrogen deposition.     

Measurements of atmospheric deposition and internal circulation of base cations to a forested catchment area

The dry deposition of base cations to a Norway spruce stand was estimated by multiplying the ratio of the ion deposition to the sodium deposition on a surrogate surface with the dry deposition of sodium on the forest stand. The method can in principle only be applied to species that are present only in particles, but the method gives reasonable results when tested on ions that are also dry deposited in other forms (SO ₄ ^2– . NO ₃ ^– and NH ₄ ⁺ ). The atmospheric input and especially the dry deposition of base cations is an important replacement for the loss of base cations from the soil by run-off. The calculated internal circulation of K⁺ and Ca²⁺ showed maxima synchronously with rainfall maxima and constitute 71% and 53%, respectively, of the net throughfall deposition. The internal circulation of Ca²⁺ was almost equal to the SO₂ uptake.     

Detectability of step trends in the rate of atmospheric sulphate deposition

Step trend analyses are performed on acid deposition rates using a combination of monitored sulphate deposition data and a long-range transport of acid precipitation (LRTAP) model. The step trend analyses consider Sudbury's emissions and eastern North America's emissions to explore strategies for reducing SO₂ emissions such that a decrease in sulphate deposition is detected in a specified time period. Results indicate Sudbury's emissions could be reduced with significant detectable reductions in sulphate deposition occurring at the Muskoka region. Significant detectable reductions do not occur elsewhere due to the variability of monitoring data ‘masking’ the small change in sulphate depositions associated with abatement. Reductions in eastern North America's emissions would result in widespread evidence of significant detectable reductions.     

A comparison between wet and bulk deposition at an urban site in the U.K.

As a part of an ongoing investigation into urban acidic deposition in Greater Manchester, a wet-only collector was collocated with a bulk collector in the city centre of Manchester, one of 18 sites of an urban precipitation monitoring network. A comparison of data from these collocated bulk and wet-only collectors allowed estimates of the contribution of dry deposition to the bulk collector to be made. Dry deposition was found to contribute between 15 and 17% to elevated deposition and concentrations of non marine sulphate, nitrate and ammonium in the bulk collector. By a statistical analysis between ion concentrations with gas concentrations and an evaluation of the available literature, the form of the dry deposited material was inferred. The contribution of dry deposited Ca²⁺ to concentration and deposition in the bulk collector was found to be 46%. Calcium species are suggested as being those which reduce acidity most. Statistical analyses revealed significant differences between concentrations of ions found in the bulk and wet-only collectors. An evaluation of the data from the collocated collectors has identified possible sampling artifacts introduced by the use of bulk collectors in urban areas.     

Procedures for estimating atmospheric deposition properties of organic chemicals

A rationale for procedures to estimate the climatological mean dry deposition velocity and precipitation scavenging ratio of organic chemicals is developed. Both deposition parameters depend strongly on vapor pressure, as it affects the partitioning between vapor and sorbed phases. Separate formulae are presented for vapor and sorbed fractions using their mass-weighted average to describe the deposition parameters for the total airborne concentration. Deposition of sorbed fractions is controlled solely by particle size, and methods for estimating the particle size are presented. Dry deposition velocity for the vapor fraction is derived by semiempirical analysis, and is found to depend on Henry's Law constant, molecular diffusion coefficients in air and water, and the chemical's reaction rate in water. Estimates of the dry deposition velocity for 5 chlorinated hydrocarbons are made and compared with observations. Estimates of the precipitation scavenging ratio are compared with observational data for 11 chlorinated or aromatic hydrocarbons. The estimates generally are within an order of magnitude of the measured values.     

Evidence of deposition of anthropogenic pollutants in remote rocky mountain lakes

Thirteen trace elements, the PAH fluoranthene and PCBs were measured in the sediments of four remote high altitude lakes in Rocky Mountain National Park. PCBs were found in the sediments of all four lakes providing substantial evidence of anthropogenic inputs, most likely through atmospheric transport. The results were less certain, but still suggestive as to the anthropogenic origins for the elements As, Cd, Pb, and Sb and the PAH fluoranthene. Several of these elements, as well as fluoranthene and the PCBs, were found to be elevated in surface sediments compared to samples taken at greater depths, inferring that loadings of these substances into Rocky Mountain National Park region have increased with time.     

Bioavailability of heavy metals and abundance of arbuscular mycorrhiza in a soil polluted by atmospheric deposition from a smelter

The bioavailability of heavy metals (Cd, Zn, Pb, Cu) and the abundance of arbuscular mycorrhiza (AM) were studied in two agricultural fields close to a Pb-Zn smelter and three fields outside the pollution zone all cultivated with maize (Zea mays L.). Metal extractability with ethylenediaminetetraacetic acid (EDTA)-NH₄OAc and Ca(NO₃)₂, plant metal uptake, and mycorrhizal parameters (spore number, root colonization) were assessed at two growth stages (six-leaf and maturity). Despite regular liming, the availability of Cd, Zn, and Pb was markedly higher in the two metal-polluted fields than in the three uncontaminated fields. However, the AM abundance was not correlated with metal availability. Root colonization and spore numbers in the metal polluted fields were relatively high, though at plant maturity the former was significantly lower than in one of the uncontaminated fields. The very low AM abundance in the two other unpolluted fields was related to other factors, particular soil and plant P status and soil pH. AM root colonization did not substantially prevent plant metal accumulation, since the metal concentrations in maize grown on the polluted fields strongly exceeded normal values, and for Cd and Pb reached the limits of toxicity for animal feed.     

Historical atmospheric deposition in a Swedish mining area traced by S isotope ratios in soils

Mining activities for almost thousand years have caused large S emissions in the Falun area, central Sweden. Since the beginning of the 20th century, the S deposition has decreased considerably. The soil S concentrations and S isotope compositions were analyzed for ΣS and adsorbed SO42- for three soil profiles close to the mining area in order to identify the soil S sources. The δ34S values were found to be different for ΣS and adsorbed SO42- and ranged from –3.7‰ to +2.6‰. In the B-horizon, the observed ΣS δ34S values (and hence calculated δ34S values for organic S) were mostly lower than those of the adsorbed SO42-. In the O-horizon, ΣS showed similar δ34S values as the adsorbed SO42- in the mineral soil. The adsorbed SO42- showed nearly constant δ34S values with depth. The δ34S values in the soils are interpreted to reflect a mixture of historical and modern deposition due to soil S circulation with no or negligible fractionation. The lower δ34S values of organic S in the B-horizon suggests preservation of acid deposition originating from mining activities back in time. The adsorbed SO42- in the mineral soil and the organic S in the O-horizon reflect a response to a new δ34S composition in the atmosphere due to fossil fuel burning during the 20th century.     

Mercury in forest canopy throughfall water and its relation to atmospheric deposition

A seasonal variation of both particle and gaseous Hg concentrations in the atmosphere is present in south-western Sweden. An average gaseous Hg level of 3.7 ng m⁻³ is found in winter, compared to 2.8 ng m⁻³ in summer. A weak decreasing south-north gradient for gaseous Hg in air over the Nordic countries is also present, with yearly average values from 3.2 to 2.8 ng m⁻³. A gradient for particulate Hg is less clear. An air parcel trajectory sector classification of gaseous Hg levels in air, and to some extent the particulate associated Hg, clearly demonstrates the increased concentrations in the southern sectors, especially in south-western Sweden where the gaseous Hg increase is about I ng m⁻³. These observations are consistent with an influence from the European continent. The average concentrations of Hg in precipitation at the various stations show a pronounced decreasing south-north gradient. A major portion of the total Hg present in precipitation is associated with particles. For the southern stations, a strong correlation between Hg and sulfate, or pH, is present suggesting a connection between Hg in precipitation and anthropogenic activities.     

Method to estimate atmospheric deposition of base cations in coniferous throughfall

A convenient non-electric method for estimating the dry deposition of base cations on a coniferous forest is presented. The dry deposition is estimated by multiplying the ratio of the base cation deposition to the sodium deposition on a surrogate surface with the dry deposition of sodium on the forest stand (throughfall technique). The surrogate surface is designed to resemble the needles in a coniferous forest with respect to particle deposition. Atmospheric non-marine dry deposition measured using the surrogate surface was compared to model calculated depositions. There was a good agreement for calcium but not for potassium.     

Sudbury smelter impact on atmospheric deposition of acidic substances in Ontario

During the period from June 1982 to March 1983, there was a prolonged shutdown of the INCO and Falconbridge smelters at Sudbury, Ontario. Taking advantage of this opportunity, the Air Resources Branch of the Ontario Ministry of the Environment undertook a detailed analysis of the impact that the resulting emission reductions had on the atmospheric deposition of acidity and SO_X in Ontario. This paper summarizes the various meteorological analyses and mathematical modeling estimates that were carried out in order to determine the contribution of the Sudbury smelters to atmospheric deposition in Ontario. For sulfates, the Sudbury contribution to the total wet deposition was typically less than 15% at the receptors studied. Furthermore, the smelters were found to contribute on the order of 10 to 20% of the total dry deposition of S compounds in central and northeastern Ontario, and less than 10% elsewhere.     

The use of calibrated lakes and watersheds for estimating atmospheric deposition near a large point source

We have measured the input and output rates of substances to and from both lakes and watersheds in the Sudbury and Muskoka-Haliburton areas of Ontario. At the former location, we have conducted mass balance studies on 5 lakes and their watersheds for 2½ yrs. At the latter site, we have measured mass balances for 6 lakes and about 30 individual watersheds for the past 5 yrs. Substances studied included SO₄ ^2?, NO₃ ^?, NH₄ ⁺, H⁺, major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) and HCO₃ ^?. During the course of the investigation at Sudbury we have made several observations that indicate that the inputs of some substances, specifically SO₄ ^2? or SO₄ ^2?-precursors and strong acids, to lakes and watersheds are underestimated when measured as bulk deposition (i.e. by collection in a continuously open container): (a) The output of SO₄ ^2? from the calibrated watersheds was substantially greater than the input measured as bulk deposition. (b) The SO₄ ^2? concentrations of the lakes could not be explained on the basis of the measured inputs. An additional input directly to the lake surface was needed to obtain a mass balance. (c) The net input of acids measured as bulk deposition to the watersheds was much less than the acid consumed, which was estimated by the net output of Ca²⁺, Mg²⁺, Na⁺, K⁺, Al³⁺, and the net retention of NO₃ ^?. (d) The major cation content of the study lakes could be explained on the basis of weathering reactions in the lakes' watersheds only if the input of strong acid had been underestimated. When these observations were quantified, they indicated a major portion of the total input of SO₄ ^2?-precursors and of strong acid was not included in our bulk deposition measurements. Deposition of SO₂ is the most likely explanation for these observations.     

Evidence for acidification of sensitive Scottish soils by atmospheric deposition

The Scottish soils most sensitive to acidification from acid deposition are peats and those derived from quartzite or Devonian and Torridonian sandstones. It is shown that the surface horizons of Calluna moorland podzols derived from these mineral parent materials behave in a similar way to peats, in that their pH depends upon cation exchange equilibria between H⁺ and Ca²⁺. The pH is therefore related, as might be expected from the ratio law, to the ratio [H⁺]:[Ca²⁺], and this relationship may be used to predict the effect of emission reductions or increases upon these soils.     

Linkages between atmospheric mercury deposition and the methylmercury content of marine fish

Enhanced Hg deposition to productive marine systems may result in concurrent increases in monomethyl Hg (MMHg) concentrations of marine fish. Consequently, it is important to understand what effects an increasing Hg supply may have on the marine food chain. A simple ocean model is employed to estimate the fraction of total Hg inputs which is required to sustain “average” marine fish MMHg concentrations annually. Calculations show that upwelling zones require 20% of total annual Hg inputs, coastal zones 5%, and open-ocean regions only 0.02%. The value for coastal areas is similar to that calculated for the acidified basin of Little Rock Lake, Wisconsin, a small fresh water seepage lake. These calculations point to Hg source strength and rates of particle scavenging as being key factors in controlling the rate of transport to sites of methylation (and subsequent entry into the marine food chain). If biological variables (scavenging rates, primary productivity) remain constant while anthropogenically-derived Hg deposition increases, it is likely that concentrations in marine biota (including fish) will rise in accord.