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Estimating distribution and retention of mercury in three different soils contaminated by emissions from chlor-alkali plants: part I

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Estimating distribution and retention of mercury in three different soils contaminated by emissions from chlor-alkali plants: part I
  Ž . The Science of the Total Environment 284 2002 177  189 Estimating distribution and retention of mercury inthree different soils contaminated by emissions fromchlor-alkali plants: part I H. Biester  , G. Muller, H.F. Scholer ¨ ¨  Institute of En   ironmentalGeochemistry, Uni    ersity of Heidelberg, INF 236, D-69120 Heidelberg, Germany Received 15 January 2001; accepted 12 May 2001  Abstract Mercury emissions from chlor-alkali plants have been past and present sources of soil contamination with Hg. Ž . Ž . Here we calculate net mercury Hg deposition to soils in the vicinity 100  1000-m downwind of three-chlor alkaliplants. Calculations were based on spatial distribution patterns of Hg concentrations in soils, which were extrap-olated by kriging. Moreover, we investigated to what extent Hg deposition depends on the elevation of receptors andcanopy throughfall. Mercury concentrations in soil exceed background values up to a factor of 56 and showenrichment factors between 2 and 5.8 calculated from the median Hg concentration. Net deposition rates rangebetween 2356 and 8952   g m  2  year  1 , which is up to 224-fold the background values. Net deposition of Hg to soilsat the three sites varies between 1.2 and 2.4% of total emitted Hg. Highest deposition rates were found at sites withextended elevated or forested areas. Here, Hg concentrations in soils increased by a factor of up to 7.3 in elevated Ž .  180 m forest areas compared to non-elevated grassland soils.    2002 Elsevier Science B.V. All rights reserved.  Keywords:  Chlor alkali plants; Mercury; Soils; Hg deposition rates; Hg retention 1. Introduction Ž . The amount of mercury Hg which mobilizedand released into the biosphere has increasedsince the beginning of the industrial age. Esti-mates of Hg fluxes from natural and anthro-  Corresponding author. Tel.:   49-6221-544819; fax:   49-6221-545228. Ž  E-mail address: H. . Biester . pogenic sources indicate that emissions from an-thropogenic sources have exceeded those of natu- Ž . ral sources Mason et al., 1994 . Most mercury inthe atmosphere is elemental Hg, which circulatesin the atmosphere and hence can be widely dis-persed and transported over thousands of kilome-ters affecting soils and aquatic environments in Ž . remote areas Fitzgerald et al., 1998 . As it cyclesbetween atmosphere, pedosphere and hydro-sphere, mercury undergoes a series of complex chemical and physical transformations such as 0048-9697  02  $ - see front matter    2002 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 4 8 - 9 6 9 7 0 1 0 0 8 8 4 - 1  ( ) H. Biester et al.  The Science of the Total En   ironment 284 2002 177   189 178 Ž . biotic and abiotic methylation or Hg 0 oxidation which may increase toxicity and bioavailability of  Ž the metal Gavis and Ferguson, 1972; Andren andNriagu, 1979; Lindqvist et al., 1991; Stein et al., . 1996 .Due to the removal of Hg from many industrialprocesses and the reduction of Hg emission frompower plants in the past decades, the amount of Hg released from anthropogenic sources steadily Ž . decreases Pacyna and Keeler, 1995 . Next to longdistance transport and deposition of atmospheric-derived Hg in remote areas, there are still numer-ous point sources of Hg emissions which havecaused elevated Hg levels in soil, water and sedi-ments in their vicinity. Besides coal burning and waste incineration, one important source of Hg Ž . emissions are chlor-alkali plants CAP usingmetallic Hg for the electrolytical production of chlorine. The environmental impact of Hg emis-sions from CAPs has been demonstrated in sev- Ž eral studies e.g. Lodenius and Tulisalo, 1983;Baldi and Bargagli, 1984; Maserti and Ferrara, . 1991; Gonzales, 1991 . Several studies have shownthat most Hg emitted from chlor-alkali plants isdispersed over long distances, and only smallamounts are deposited in the vicinity of the sites Ž . Jernelov and Wallin, 1973; Hogstrom et al., 1979 . ¨ ¨ ¨ Nevertheless, it has been reported that soils sur-rounding chlor-alkali plants show Hg concentra- Ž . tions up to 75 times the background EPA, 1997 .In the present study, which consists of twoparts, we investigated accumulation and bindingof Hg in soils in the vicinity of three chlor-alkaliplants. In this first part of the study, we analyzedtotal Hg concentrations in surface soils and inter-polated the obtained Hg distribution to an areaextending from 100 m to 1 km in the main down- wind direction from the plants. Based on this Hgdistribution and the vertical distribution of atmo-spheric-derived Hg in those soils we calculatedhow much of the Hg emitted from the plants hasbeen retained in the soils and determined meandeposition rates. Moreover, we calculated differ-ences in Hg deposition to grassland and to forestareas and to which extent Hg deposition is influ-enced by elevation of receptors. 2. Description of sites The study was carried out at three differentsites within Europe. Eurochlor pre-agreementsrequire that exact locations of the sites have to betreated confidential. Locations of the sites wereselected on the basis of differences in climaticand morphological conditions and in soil type. A more detailed description of the composition of  Ž the soils is given in Part II of this study Biester . et al., 2002, this issue . Ž . Site 1 S1 is located at the seaside and sur-rounded by several other industrial facilities. Thearea is characterized by alternating forests andgrassland. Differences in elevation do not exceed50 m. The main winds are coming from southwestto west and annual mean precipitation rates are600  950 mm. Most samples were taken northeastto east from the plant, which are the main down- wind sectors. Soils are mostly sandy, organic richin the forests and are more clayey in grasslandareas. Approximately half of the area is estimatedto be covered by buildings. Ž . The second site S2 is located within a valleyrunning northeast to southwest. Mountains withinthe study area reach an altitude of approximately450 m. To the west and the north, the plant issurrounded by other chemical sites. A narrow valley extends to the east and a mountain withsteep slopes, densely covered by forest stands issituated southwards of the site in the main winddirection. The valley southwest from the site iscovered by an urban area. Precipitation ratesamount to approximately 900 mm year  1 , themain winds are coming from the north. All sam-ples were taken from soils southwest to southeastfrom the plant, only two samples were takendirectly on the site northeastwards from the plant. Ž . The third site S3 is located on a flat peninsulaand is also surrounded by other chemical andpetro-chemical plants. There is a large coal-firedplant approximately 2-km north of the site, emit-ting high amounts of smoke which produces a visible plume to the south extending all over thepeninsula where the CAP was located. In thenorth and the west, the peninsula is marked by a  ( ) H. Biester et al.  The Science of the Total En   ironment 284 2002 177   189  179 river to the east and the south by the sea. Vegeta-tion in the area is very sparse and consists mostlyof bushes and some small pine trees. High tem- Ž   1 . peratures mean  17.8  C year and low precip- Ž   1 . itation rates mean  472 mm year are typicalfor the climate in this region. Most of the yearthe main winds come from south to south-west, Ž . but also from the landside north during winter. Ž Most samples were taken northeast main wind . direction from the plant alongside the main roadscrossing the peninsula. The average distance of the sampling points from the road was approxi-mately 25 m. Different from the other sites, soilsaround site S3 were not only sampled in the main Ž .  wind direction NE , but also in the oppositesector for two reasons. One is that winds duringthe winter months predominately come from thenorth and that emissions from a coal-fired plantlocated north of the site is assumed to be anadditional source of Hg deposition to the soils. Todetermine the influence of the emissions of thecoal-fired plant on the soils surrounding the CAP we have additionally analyzed Cd and As, whichare typical elements released during coal burning. 3. Methods and materials  3.1. Sampling of soil surface samples Surface soil samples were taken within a dis-tance of 100 to 1000 m from each plant in themain wind direction. The minimum distance of 100 m from the plants was chosen to avoid sam-pling of soil contaminated by direct spillage of Hg. The number of samples ranged between 50and 70 depending on the morphological condi-tions at the site and the size of the area wheresoil could be sampled. The areas around the sitesare partly covered by other industrial plants orurban areas, the distribution of sampling points istherefore, more or less at random. Samples werecut from the soil surface by means of a stainlesssteel soil corer to a depth of 5 cm. Six sub-sam-ples taken from a soil area of approximately 4 m 2  were combined to one sample. Background con-centrations of Hg, including natural and anthro-pogenically elevated Hg in soils, were estimatedfrom three samples taken several kilometers fromthe sampling area opposite to the main winddirection. This estimation of Hg background con-centrations does not consider differences in com-position of soils in the sampling area such asorganic matter or clay content, which might cause variation of Hg background values. The used val-ues are therefore, only a rough estimation of Hgbackground concentrations.  3.2. Sample preparation  All soil material was sampled and stored inpolyethylene bags. Samples were not stored frozenas we assumed that volatile Hg compounds, suchas free or adsorbed elemental Hg, which can exist Ž under open field conditions strong sun light, . strong wind, precipitation etc. would not becomelost during sample transport at ambient tempera-tures. Twenty fresh samples taken nearest to thesites were analyzed by means of a solid phaseHg-thermal-desorption technique for any metallicHg. The detection limit of this technique is 40  50ng if all Hg in the sample is released within a Ž . single peak Biester and Scholz, 1997 . Due tothis comparatively high detection limit this tech-nique does not allow to determine traces of Hg inthe soil gas, which is known to be ubiquitous in Ž . soils due to natural reduction processes of Hg IIspecies. As metallic Hg could not be detected in Ž . any of the samples Biester et al., 2002 , the soils were freeze-dried and sieved to   2 mm to re-move most of the root material.  3.3. Analysis of Hg, Cd and As in soils  Analyses of total Hg were carried out by means Ž . of a cold vapor atomic absorption CV-AAS Hg Ž . analyzer TSP-mercury Monitor 3200 after diges-tion of a 3-g sample in concentrated nitric acidfor 3 h at 160  C. Arsenic in the S3 soil samples was determined by means of a Perkin-Elmer FIAS200 Hydride-Generation system coupled to aPerkin-Elmer 4100 AAS. Cadmium was analyzedfrom the same digests by means of graphite fur-nace atomic absorption spectroscopy. Accuracy of analysis was checked using standard reference Ž soil materials NIST 2710, 2711, RTC-CRM008-  ( ) H. Biester et al.  The Science of the Total En   ironment 284 2002 177   189 180 . 050 . Relative standard deviation of replicate Ž . measurements  n  4 did not exceed 4% for Hg,7% for As and 0.8% for Cd in the referencesamples and 8% for Hg, 9% for As and 3% forCd in the soil samples.  3.4. Calculation of total Hg in soils Spatial distribution of Hg concentrations basedon kriging was calculated using SURFER 6.04 Ž . Golden Software . Kriging was based on a linear variogram model. Based on the surface plots of  Ž . Hg concentrations Figs. 1  3 we calculated theamount of Hg stored in the soils around the sitesand compared the data to the amount of Hgemitted from the plants. For all sites we set anangel of spread of 90   for Hg dispersion from thesite in the main wind direction. The calculatedarea was framed by this angle and the stretchbetween 100 and 1000 m distance from the plant.The amount of Hg in the soils was obtained bysumming up the amount of Hg calculated foreach Hg concentration area separately. Theamount of Hg in each single Hg concentrationarea was calculated using the surface of the areabetween two isolines, the mean concentration of  Ž . Fig. 1. Mean proportional decrease of Hg and organic carbon C concentrations calculated from data of two soil profiles org Ž . Biester et al., 2002 from the sites S1, S2 and S3, respectively. S1 profiles were taken 240 m NNE and 300 m E of the plant. S2profiles were sampled 420 m SSW and 930 SE from the plant. S3 profiles were collected 250 m S and 600 m NNW from the plant.  ( ) H. Biester et al.  The Science of the Total En   ironment 284 2002 177   189  181Fig. 2. Distribution of mercury in surface soils within 1-km distance from plant S1 in the main wind direction. Hg between two isolines, the thickness of the soillayer and the mean soil density. Soil density wascalculated from the median amount of organic Ž   3 . Ž matter    1.4 g cm and mineral matter     3 . 2.65 g cm of 20 samples from each site, respec-tively. This calculation was carried out for each Ž . soil layer 5 cm down to a soil depth of 20 cm.The Hg content and the soil density in sub-surfacesoil layers were calculated for each Hg concentra-tion area based on the proportional decrease of Hg and organic carbon concentrations in the soil Ž . column Fig. 1 which was calculated from the Ž . data given in Biester et al. 2002 . Background Hgconcentrations were set to be constant in theupper 20 cm of the soils. The total amount of atmospheric-derived Hg retained in the soils wasobtained by adding up the amount of Hg calcu-lated for each soil layer and substracting theamount of background Hg. 4. Results and discussion 4.1. Distribution of mercury Soils of all sites show highly elevated Hg con-centrations if compared to local background val-ues. Maximum enrichment factors were 56-foldfor S1, 20-fold for S2, and 40-fold for site S3. SiteS3 shows the most widespread soil contaminationexpressed by a median Hg concentration of 571  1 Ž .  g kg  n  70 which is 5.8-fold the back-ground compared to a median of only 157   g  1 Ž . kg for S1  n  56 which corresponds to anenrichment factor of approximately 2 and a me-  1 Ž . dian of 541   g kg for S2  n  51 which is3.6-fold the background of this site. Samplesshowing isolated high Hg concentrations, which were not attributed to atmospheric Hg emission
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