Environ. Sci. Technol. 2000, 34, 3337-3345
Mercury in the Soil-Plant-Deer-Predator Food Chain of a Temperate Forest in Slovenia A L E Sˇ G N A M U Sˇ , * ANTHONY R. BYRNE, AND MILENA HORVAT Department of Environmental Sciences, Jozˇef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
Total mercury and methylmercury concentrations from long-term monitoring of the terrestrial soil-vegetationherbivore-carnivore food chain with regard to accumulation and transformation processes were studied in areas of Slovenia contaminated with mercury to differing degrees, as well as uncontaminated areas. Assessment of the inhaled and ingested contribution of mercury from the environment in roe deer (Capreolus capreolus L.), the selected wild mammal species living in these areas, showed that while the ratio between these two routes of uptake is relatively constant, food intake of mercury in roe deer is much more important than inhaled mercury, which represents only up to 0.2% of ingested Hg. Although the plant species comprising roe deer foodstuffs were not active accumulators of mercury from soil or air, vegetation mediates significant transfer of Me-Hg to herbivores, and this becomes subject to further accumulation in the higher trophic levels of this food chain. Besides roe deer other bioindicators such as chamois (Rupicapra rupicapra L.) were selected to confirm the uptake of mercury from plants. Though the conclusions drawn from the carnivorous predators lynx (Felis lynx L.) and wolves (Canis lupus L.) are limited due to the limited number of subjects (8 and 2, respectively), the results and their comparison to other environmental data showed the transfer of Hg from soil (and air) to vegetation, herbivores and carnivores further up the food chain. The results of the measurements as well as concentration factors (CF) and bio-accumulation factors (BAF) show appreciable accumulation of Me-Hg and less marked accumulation of T-Hg at higher trophic levels of this terrestrial food chain. Interestingly, higher accumulation of Me-Hg was observed in those environments polluted with high concentrations of inorganic mercury compared to less contaminated and control areas.
Introduction The role of mercury accumulation in terrestrial food webs is often neglected. The many reviews in the past clearly show a preponderance of studies from aquatic and marine environments compared to terrestrial ecosystems, where relatively few evaluations exist (1-5). Recent estimates of mercury cycling feature the deposition of the majority (approximately 60%) of total mercury (T-Hg) on terrestrial environments (6). A substantial portion (about half) of the emission of mercury is predicted to be deposited locally i.e., close to contamination sources, which usually results in * Corresponding author phone: +386/1/4784680; e-mail:
[email protected]. Present address: Ministry of Science and Technology, Trg OF 13, SI-1000 Ljubljana, Slovenia. 10.1021/es991419w CCC: $19.00 Published on Web 07/13/2000
2000 American Chemical Society
marked accumulation in local food webs. Due to its toxicity, persistence in biogeochemical cycles, widespread distribution and tendency to bio-accumulate, it is important to focus on mercury accumulation and the transfer of its compounds in terrestrial food chains, especially since major differences exist compared to aquatic or marine food chains (5). Most importantly, the number of trophic levels in terrestrial food chains is greatly reduced. One must also be aware of the low efficiency of converting plant biomass into protein by terrestrial herbivores due to the large percentage of indigestible structural material containing high levels of cellulose and lignin in the most abundant terrestrial primary producerssvascular plants. However, the lower energy efficiency causes a higher consumption rate of foodstuffs in herbivores which can effectively intensify metal intake in the food webs within contaminated areas. For studies of mercury distribution in the terrestrial environment and the accumulation process within the selected food chain, the area of the recently closed Idrija mercury mine (which had been in operation for 500 years) was selected, where distinctive local Hg deposition patterns were established (6-10). For comparison similar samples were collected from the Podljubelj area, where mercury mining was abandoned approximately a century ago, and control localities from Hg uncontaminated areas in Slovenia. Thus in our study focused on mercury accumulation, transfer and transformation in a temperate forest ecosystem, the selected food chain comprised composite plant foodstuff samples composed of vascular plants (42 species) as primary producers; roe deer (Capreolus capreolus L.) and chamois (Rupicapra rupicapra L.) as herbivores (first level consumers); and their predators lynx (Felis lynx L.) and wolf (Canis lupus L.) as the top carnivores. Soil and ambient air mercury concentrations were also measured, since these two media were considered as mediators of mercury to the studied terrestrial food chain. The study is based on the results of systematic long-term analyses of T-Hg and methylmercury (Me-Hg) in soil profile samples, higher plants, body tissues of their herbivore consumers, their predators and occasional measurements of mercury in the air. In view of the abovementioned higher consumption rate in terrestrial consumers (especially homeothermic mammals), the aims of the present study were as follows: 1. to assess the prevalent route of metal uptake into terrestrial organisms and its transfer into the selected food chain of environments of varying degrees of pollution and 2. to establish the level of T-Hg and Me-Hg accumulation at different trophic levels of the selected food chain.
Experimental Section Sampling Sites and Sample Preparation. Equal-sized forest sampling areas of approximately 1.5 km2 each were selected as study areas and were practically defined by the sites where roe deer were culled in the first phase of the study. An exception was the mine-smelter area, which has no permanent deer population (nor were any animals obtained there due to inclusion of the area within the town suburbs). Eight forest sampling areas were in the Idrija region, three in the Podljubelj area, and one control area near Ljubljana. Roe deer and their carnivorous predators lynx and wolf were obtained from an additional eight uncontaminated locations in Slovenia. For the purpose of this study analytical data from similarly contaminated sampling areas were grouped into distinctive contaminated zones. The areas in the Idrija region consisted of the immediate vicinity of the mine-smelter complex, then four areas around the town of Idrija (grouped VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Sampling sites in a) Slovenia; b) locations where roe deer (R), chamois (C), lynx (L) and wolves (W) were culled; c) detail of the Idrija area. as Zone A), and 3 others at greater distance in the wider Idrija area (grouped as Zone B). Thus the mine-smelter area and Zone A are characterized by heavy deposition of mercury vapor and airborne particles from mercury sources. The sampling sites are presented in Figure 1. Soil Samples. A standard soil sampling procedure (10) was used. Since forest soils in the studied areas are rather shallow, only soil profile cores of an average depth from the surface to approximately -15 cm (alterations in depth were approximately 2-3 cm in various soil types or sub-samples) could be used. To obtain representative samples from each sampling area, forest soils were collected from 25 randomly chosen locations within the sampling area twice a year during a seven year period, in summer and in late autumn. Subsamples were collected with undisturbed stratification, packed and sealed in polyethylene bags as soil profile cores including litter (A-horizon and O-horizon). Composite samples were prepared in the laboratory. Due to the expected high concentrations of Hg in soil, samples were prepared in a separate laboratory, away from that used for preparation of plant and animal samples. Samples were air-dried for 4 days at approximately 20 °C. Bigger particles e.g. stones, were removed, and then the samples were homogenized and 3338
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ground in an agate mortar. During the whole sample preparation procedure, special precautions were necessary to avoid cross-contamination of samples, including the sequence of sample treatment. Therefore, homogenization started with the control samples, followed by less contaminated and continued with more contaminated samples. The homogenizer was thoroughly cleaned after each sample. For each sample, the first aliquot ground was discarded, and then the bulk of the sample was ground. Homogenized samples were stored in plastic containers, sealed in polyethylene bags, and stored in a refrigerator until further processing. An aliquot taken for determination of moisture content was dried at 85 °C to constant weight. All data are expressed on a dry weight basis. Composite Plant Samples Corresponding to Roe Deer Diet. Vegetation samples consisted of 42 plant species and were collected twice a year during a seven year period simultaneously with the soil sampling exercises. The composition was based on previous work examining the seasonal stomach contents of roe deer in Slovenia (11). The plants collected were sorted and combined so as to correspond to the determined roe deer diet (Table 1). The portion of each species slightly differed between seasonal sampling exercises,
TABLE 1: List of Plant Species Included in the Composite Plant Samples Representing Roe Deer Diet Collected through a Number of Sampling Exercises in the Period 1991-1997, with the Percentages of Each Particular Species in the Whole Sample
main species in roe deer diet (as collected)
Acer campestre + A. pseudoplatanus Salix caprea + Salix spp. Quercus sessiliflora Carpinus betulus Populus tremula Fagus sylvatica Sorbus aria + S. aucuparia + S. torminalis Ulmus glabra Corylus avellana Others: Pyrus, Prunus, Rosa, Rhamnus, Tilia, ... Rubus idaeus + R. saxatilis agg. Vaccinium myrtillus Picea abies + Abies alba Fragaria vesca Oxalis acetosella Trifolium spp. Gentiana asclepiadea Mercurialis perennis Cyclamen purpurascens Omphalodes verna Salvia glutinosa Others: Potentilla, Euphorbia, Mellitis, Viccia, ... a
summer diet (% f.w.a)
autumn diet (% f.w.a)
7 5 4 5 5 5 6
9 8 6 3 2 6 8
3 2 5
2 3 5
9 2 0 6 5 7 5 4 3 2 5 5
12 3 10 4 4 8 1 1 2 2 1 0
f.w. ) fresh weight.
but was stable from year to year within the same sampling period. The summer samples were taken during the period of high airborne dry deposition of Hg onto plant tissues and high leaf growth rate which, in roe deer, coincides with the maximal rate of food consumption. The late autumn samples were collected at the end of the plant vegetation season when, due to the high autumn precipitation rate, the washout of Hg from the air onto leaves is high. Composite plant samples of 2 to 3 kg (fresh weight) were stored in cloth bags to reduce the possibility of bio-transformation of mercury during transfer to the laboratory. All plant samples were collected on the same day to avoid the influence of weather changes during sampling at different locations. All samples were collected at least 5 days after the last appreciable rainfall. Only those twigs and leaves of woody plants which could be reached by roe deer (0 to 1.8 m height) were collected. The samples were transported to the analytical laboratory on the day of collection. To determine the species composition the samples were immediately sorted out and the fresh weight of each species material determined. Later, the plant material was cut into smaller pieces which were combined appropriately into composite samples. The composite plant samples were lyophilized in a Christ alpha 1-4 freeze-drier at -50 C and at 1.5 Pa for 3 days. The lyophilized samples were weighed and ground in a Brabender grinder. The grinder was thoroughly cleaned after each sample procedure. The first aliquot of ground material was discarded and then the bulk of each sample ground. Powdered samples were subsequently packed into plastic dishes, sealed in polyethylene bags and stored in a refrigerator. The results are expressed on a dry weight basis. Animal Samples. Twenty-one roe deer (R1-R21), two chamois (C1, C2), eight lynx (L1-L8) and two wolves (W1, W2) of different age and sex were sampled from the regular shooting allocations during the hunting seasons 1990 to 1998 (Figure 1). Nine roe deer (R1-R9) and one chamois (C1)
originated from relatively highly contaminated Zone A in the vicinity of the mercury mine-smelter complex in Idrija, four roe deer (R10-R13), a chamois (C2) as well as two lynx (L1, L2) and one wolf (W1) originated from localities in the wider Idrija mining area (Zone B). Two roe deer were collected from the abandoned Podljubelj mining area (R14, R15), where predators such as lynx and wolf are not present. Six roe deer (R17-R21), six lynx (L3-L8) and one wolf (W2) were collected at control localities without known mercury sources elsewhere in Slovenia. Tissue samples were carefully removed from the body and stored in separate polyethylene bags less than 20 min after the animal died. In the field samples were labeled, placed in polyethylene bags and temporarily stored at +4 °C in a refrigerator, transferred to the laboratory and stored deep frozen (-25 °C). 27 different animal tissues were selected to establish the Hg distribution and accumulation in the body. Before further processing, brain, eyes, pituitary, pineal gland, suprarenal gland, masseter muscle, myocardium, lungs, liver, spleen, stomach, kidney and urinary bladder were dissected and fur samples were prepared. Jaws were dissected for subsequent age determination by comparison of the dental condition of our samples with a jaw sample collection. Results for 10 tissues were reported recently (12), from which three tissues are selected for the present evaluation of mercury accumulation in the selected food chain, namely masseter muscle, liver and kidney. Soft tissues were macerated on sterilized glass plates using a clean scalpel. A homogeneous mass was obtained and aliquots of the mass sampled for total and Me-Hg determination. Laboratory tools and ware needed for preparation of samples were washed with an aqueous detergent solution, steamed in hot nitric acid vapor, rinsed with double distilled Hg-free water and dried. To avoid changes in the tissue water content, the processing of the homogenized animal tissues was carried out immediately after maceration. Determination of Total Mercury (T-Hg). Soil Samples. T-Hg in soil profile samples was determined by cold vapor atomic fluorescence spectrophotometry - CV AFS. An aliquot of the homogenized soil profile sample (100 to 300 mg) was digested with 10 mL of an acid mixture (HNO3:H2SO4, 7:3 v/v) in a closed Teflon digestion vessel (30 mL) at 70 °C for 4 h. After the digestion 1 mL of strong oxidizing solution was added (1 mL of BrCl) and the sample was diluted to 25 mL with MilliQ water. An aliquot of the digested sample was then transferred to the reaction vessel containing 20-30 mL of MilliQ water with 5 mL of HCl and 0.5 mL of SnCl2 to reduce Hg2+ to Hg0. The Hg vapor was then swept from the solution with nitrogen at 450 mL min-1 through a soda lime trap onto gold coated silica sand. The mercury collected on the trap was then released into the CV AFS analyzer (Brooks Rand: Model-2) by heating (600 °C) the trap for 2 min. Peak area integration was used for quantification of Hg (13). Biological Samples. An aliquot of homogenized tissue samples, 150 to 300 mg (wet weight for animal and dry weight for plant tissues), was carefully weighed and introduced into a 15 mL Pyrex ampule and 2 mL of 70% HNO3 (Merck-MOS Selectipur) added. The ampules were sealed and heated at 90 C for 24 h. The mineralized samples were diluted with double distilled water (DDW) and total mercury (T-Hg) determined by gold amalgamation cold vapor atomic absorption spectrometry (CV AAS) (14, 15) using an LCD Milton Roy Mercury Detector (Model 1255) and computer-controlled reduction/amalgamation system. Some soil and biological samples were also analyzed by radiochemical neutron activation analysis (RNAA). For determination of mercury via the radionuclide 197Hg the standard and sample in quartz ampules were irradiated for 16-20 h in the rotating specimen rack of our TRIGA research reactor at a thermal neutron fluence rate of 1.0 × 1012 n cm-2 VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2: Total and Methylmercury Concentrations in Soil Samples, Dry Weight Basis concentration × h (range)
soil samples 1991-1997 sampling area Idrija near smelter Zone A Zone B
Podljubelj controls
horizon (see legenda)
depth (cm)
samples analyzed (n)
T-Hg (µg/g)
Rendzina Oh, A Rendzina OhAh, A Cambisol OfOh, Ah Rendzina OhAh, A Rendzina OhAh, A Litosol (A)C Cambisol OhAh,A Rendzina OhAh, A
0-10
14 96
0-15
72
65.1 (49.5-77.5) 43.9 (23.7-68.9) 16.7 (1.33-42.8)
0.003
0-10
2456 (1734-2759) 288 (14.2-886) 2.44 (0.385-7.97)
0-15
20
12.9 (12.5-27.0)
22.0 (5.80-80.2)
0.05
0-15
24
0.612 (0.321-0.998)
0.17
Me-Hg (ng/g)
0.368 (0.182-0.680)
proportion Me-Hg (%)
0.07 0.57
a Legend - soil horizon classification (ISSS, ISRIC and FAO 1998): Of - Organic fermentative - surface accumulation of organic matter, undecomposed or partly decomposed litter dominate - plant or animal matter recognizable; Oh - Organic humified - plant or animal matter not recognizable; A - Accumulation of humified organic matter mixed with mineral fraction; Ah - dark colored sublayer of horizon A - organic matter content not exceeding 35%; (A) - initial A horizon - humus accumulative soil layer very thin (max. 2 cm) or not continuously covered; C - a subsurface horizon excluding hard bedrock that is little affected by the soil forming processes.
s-1. For analysis of Hg pyrolysis of the irradiated sample was applied, resulting in volatilization of Hg which was trapped on selenium impregnated paper (16, 17). The gamma activity of the isolated radionuclide was measured with a 3′′ x 3′′ NaI(Tl) well-type detector. The chemical yield determined by tracer experiments was shown to be virtually quantitative. Determination of Monomethylmercury Compounds (Me-Hg). Soil Samples. The technique for determination of Me-Hg in soil samples consisted of water vapor distillation, followed by aqueous phase ethylation, precollection on Tenax at room temperature and separation of ethylated species on a GC column (18-20). Separated mercury species were then transformed into elemental Hg by heating at 600 °C and swept into an atomic fluorescence detector (CV AFS, Brooks Rand: Model-2). Peak heights were used for calculations. Biological Samples. Me-Hg from up to 500 mg of sample was separated by leaching for 12 h with 10 cm-3 of 6 M HCl. Organic and inorganic mercury were then separated on an ion exchange column (Dowex 1xW8 resin, Cl- form, 100200 mesh) which retains inorganic forms of mercury only. The organic mercury in the eluate was measured by CV AAS after decomposition to inorganic mercury forms by UVirradiation. The same instrument was used as for determination of total mercury described above (14, 15). A detailed description of the procedure has been given elsewhere (21, 22). This method quantifies total organic mercury compounds, which was shown to correspond to Me-Hg in biological samples (22). Analytical Quality Control. To control the accuracy of T-Hg and Me-Hg determinations certified reference materials (CRMs) obtained from the National Research Council of Canada (NRCC), namely DOLT-1, Dogfish liver; DORM-1, Dogfish muscle; and TORT-1, Lobster hepatopancreas were used. To control the data for soil samples IAEA-356 Polluted Marine Sediment was used because of the absence of any other CRMs certified for both T-Hg and Me-Hg. All samples in a given batch, together with blanks and CRMs were prepared in duplicate (or triplicate) and every measurement for each replicate was repeated 2-3 times. The values obtained for the NRCC CRMs for T-Hg and MeHg, respectively, were as follows: DOLT-1 233 ( 22 (12), 82 ( 7 (12); DORM-1 806 ( 45 (23), 731 ( 26 (22); TORT-1 332 ( 26 (28), 127 ( 8 (28); and IAEA-356 7580 ( 312 (23), 5.2 ( 0.24 (18). All values are given as means ( standard 3340
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deviation, number of sample aliquots in parentheses (italic), in ng g-1 d.w. These results are in good agreement with the recommended certified values with uncertainties: DOLT-1 225 ( 37, 80 ( 11; DORM-1 798 ( 74, 731 ( 60; TORT-1 330 ( 60, 128 ( 14; and IAEA-356 7620 ( 650, 5.4 ( 0.89. Data quality control charts were also systematically recorded throughout the study period. In addition, T-Hg in some samples was analyzed by both CV AAS and RNAA. The excellent agreement of the results obtained by the two techniques confirm the quality of the analytical data.
Results and Discussion T-Hg and Me-Hg in Soil and Plant Samples. The results for T-Hg and Me-Hg in soil samples from the various sampling areas are summarized in Table 2. The corresponding results for composite plant samples representing roe deer diet are shown in Table 3. Although separate sets of data were obtained from the summer and autumn sampling exercises each year, since only small seasonal differences were observed, the data have been combined. These results are in accord with the other observations (5, 12, 23, 24) that levels of mercury in soil are much higher than in most terrestrial plants, which are believed to have a barrier to effectively block Hg intake through the root system. Intake of Hg from the air to plants and its re-emission has been the subject of recent studies (25-27), though its importance is still difficult to assess properly. Although measurements of total mercury concentrations in the air were performed in the surveyed areas during the study (see Table 4), direct aerial intake of Hg (as compared to root uptake) was not taken into consideration separately due to difficulties in the assessment of the rate of Hg intake from the air into plant leaves through stomata in the variable field conditions and changeable air Hg concentrations. The ratio of T-Hg for plants from the contaminated areas of Zone A and Zone B of 14.4:0.21 (see Table 3) or about 70 differs significantly from the ratio of the air concentrations of Hg for the same areas (Zone A: Zone B ) 75:25; see Table 4), with a ratio of only 3, but is similar to the ratio of the T-Hg soil concentrations (Zone A: Zone B ) 288:2.44, Table 2) of about 120. Therefore, the results from the field measurements suggest that the T-Hg concentrations in plants reflect soil contents and not air concentrations. However, recently published work by Wyttenbach and Tobler (28) emphasizes the importance of the external loading
TABLE 3: Total and Methylmercury Concentrations (Dry Weight Basis) in the Composite Plant Samples Representing Roe Deer Diet (Composed of 42 Plant Species) sampling area
composite plant samples 1991-1997 samples analyzed (n)
T-Hg (µg/g)
Me-Hg (ng/g)
proportion Me-Hg (%)
Idrija near smelter Zone A Zone B Podljubelj controls
12 48 36 10 12
51.8 (39.9-58.9) 14.4 (1.55-42.9) 0.21 (0.039-0.595) 0.58 (0.11-1.74) 0.080 (0.056-0.222)
118 (67.4-172) 24.2 (6.91-90.3) 3.07 (0.84-8.68) 4.04 (1.66-12.8) 1.64 (0.91-2.45)
0.23 0.55 1.77 1.59 2.05
concentration × h (range)
TABLE 4: Total Mercury Concentrations in Air of the Surveyed Areasa location
no. of analyses (n) 1991-1997
T-Hg concentration (ng/m3) (range)
Idrija near smelter Zone A Zone B Podljubelj controls
20 90 110 15 12
1000 (410-5000) 75 (50-100) 25 (10-50) 18 (1.5-38) 1.8 (1.5-2.1)
a References 9, 37 and unpublished results. Last measurements: Ferrara, Mazzolai and Gnamusˇ 1996-1997, unpublished data.
TABLE 5: Surface Contamination of Leaves by Mercury Loading from Soila
sampling area
leaf Hg contribution estimated from soil total leaf leaf loadingb Hg (µg/g contamination (µg/g d.w.c) d.w.c) (%)
soil Hg (µg/g d.w.c)
Idrija near smelter 2456 Zone A 288 Zone B 2.44 Podljubelj 129 controls 0.37 a Hypothetical - see text. - dry weight.
1.2 0.14 0.0012 0.06 0.0002 b
51.8 14.4 0.21 0.58 0.080
2 1 0.5 10 0.25
Assuming loading of 0.5 mg g-1. c d.w.
(aerosols, dust, soil) on plant leaf surfaces on analytical results, which for certain elements can contribute to the plant content significantly. Typical values for this loading are around 0.5 mg g-1 (28). Therefore, one could suspect that in the heavily contaminated areas such a loading might contribute significantly to the mercury content. In this regard, the soil and plant data were used to calculate the hypothetical contribution of external Hg loading for the worst case scenario, assuming all the leaf loading originated from soil (Table 5). The results show that only a minor contribution from external contamination from 0.5 to 2% of the mercury content of the untreated leaves would occur (the estimated fraction is larger in the Podljubelj area, being up to 10%). Thus, the concentration ratios (CR or CF0) can be used for the particular areas surveyed in this study with satisfactory accuracy regardless of surface contamination even in the most contaminated areas. In any case, the object of this work was not to study the source of Hg uptake into plants but rather Hg transfer within the food chain itself, and thus the ratios between soil and vegetation mentioned are merely indicative. Compared to the concentrations of total mercury in soil (Table 2), concentrations of T-Hg in plants (Table 3) of the polluted areas are low (plant-to-soil ratios about 1:50, 1:20, 1:12, and 1:20 for the smelter area, Zone A, Zone B and Podljubelj, respectively), while control areas with low soil
mercury contents have a much higher ratio (1:5). This confirms that plant Hg intake from the soil is limited due to the physicochemical binding characteristics of the soil and the insolubility of some chemical forms of Hg (29) and also physiologically by the plant’s roots (5). Plant to soil concentration ratios CR for T-Hg concentrations are lower than 0.5 in all surveyed areas (Idrija 0.02-0.1; Podljubelj 0.010.06; and controls approximately 0.3). This shows that the species comprised in the composite plant samples representing roe deer foodstuffs in the studied areas are not active accumulators of mercury from the ground (or the air). CRs for the polluted Idrija region are similar to those calculated for the Mt. Amiata mercury mine area in Italy (24). Concentration factors for the vast majority of plant species are known to be lower than 0.5; values closer to 1 are only found in plants which are known as intensive bio-accumulators of mercury (24). Interestingly, CR’s for Me-Hg concentrations are higher than CR’s for T-Hg in all surveyed areas (Idrija: near smelter ) 1.8, Zone A ) 0.55, Zone B ) 0.18; Podljubelj ) 0.18; and Controls ) 2.7; all values averages). That the CR for Me-Hg between vegetation and soil samples can exceed unity in uncontaminated areas shows the bio-accumulative potential of this toxic organic form. Concentration ratios for Me-Hg reflect enhanced intake of the organic form from the soil to terrestrial higher plants as a consequence of the different biochemical characteristics of Me-Hg compared to inorganic Hg. The concentrations of Me-Hg measured in vegetation often exceeded the soil Me-Hg content, despite T-Hg values in soil being 50 or more times higher (Tables 2 and 3). Such a distribution indicates marked accumulation and/or retention of the organic form of mercury in vegetation; however it could also suggest the existence of a methylation process in the plant or its leaf surface. It is not possible to distinguish between accumulation and plant induced methylation in this phase of our environmental study. Both processes should be scrutinized in further specific studies. Since the concentrations of Me-Hg in the air of cinnabar mining areas (30) and urban areas (31) are negligible, higher values in plants may be attributed to intake from the soil (mechanisms still unknown) and/or methylation of adsorbed Hg0 or inorganic Hg as soil/dust particles on plant assimilation surfaces. In any case, vegetation enables transfer of Me-Hg into food webs which is especially important for the food chain burden in heavily polluted areas. Despite Me-Hg representing only a small portion of T-Hg in vegetation (
