5.2. Airborne particulates and filters (IV–VI)
5.2.1. Concentrations of TSP, Cr, Ni and Pb in the ambient air in the mine area (IV)
The results in Paper IV showed that opencast chromium mining operations by AvestaPolarit Chrome Oy Kemi Mine have had an adverse impact on the environment in the form of total suspended particulate (TSP) matter, referred to as “dust”.
In 2000 all the 95th percentile values for TSP in the mine area were below the current Finnish air quality limit value of 300 g/m3 (73) at monitoring sites MA1, MA2 and MA3, but the 98th percentile value of 411.6 g/m3 at monitoring station MA2 exceeded the Finnish air quality guideline value of 120 g/m3 (73).
Our results show (see Paper IV, Table III and Fig. 2 and Fig. 3) that the highest daily TSP concentrations can occasionally reach relatively high values, since the highest individual daily TSP concentration was 1481 g/m3 at monitoring station MA2 in October. This value is exceptionally high compared to the other measurements, and was due a storm that lifted a substantial amount of particles into the air. Thus, our results correspond well with those of other studies carried out at the Jharia Coalfield mining area in India (61) and at the Donana mining area in Spain (50), where the highest TSP concentrations were between 900–1200 g/m3.
According to the our results (see Paper IV, Fig. 2 and Fig. 3), the TSP concentrations in the air varied considerably within short time intervals, i.e. day-to-day fluctuation (3–28.9.2000), as well as by the month (January – December in 2000).
The most reasonable explanation for the high summertime TSP values are the dust and particulate material emissions from the surrounding land, piling stores, and during loading; during the summer the land, piling stores and ore are not frozen, but they are in the winter. Because TSP and heavy metal concentrations in the air can vary considerably over short time intervals (day-to-day fluctuation), as well as monthly, TSP measurements should be carried out as a long-term study in order to obtain reliable data about TSP concentrations in the air.
According to our results, the TSP concentrations in the air at monitoring stations MA1, MA2 and MA3 also seemed to be strongly dependent on the wind direction. According to Fig. 5, the TSP concentrations were higher than the annual means at monitoring station MA1 (mean 15.1 g/m3) when the wind direction was between 115–270°, at monitoring station MA2 (mean 80.2 g/m3) when the wind direction was between 30–240°, and at monitoring station MA3 (mean 11.0 g/m3) between 0–45° and 100–330°.
Figure 5. The correlation between TSP (g/m3) and wind direction at monitoring stations MA1 (Porasydänvarasto), MA2 (Konttori) and MA3 (Selkeytysallas) in 2000. (Mean = the annual mean). (IV)
These results indicate that the dust emissions at monitoring station MA1 are most likely derived from a heap of mining spoil located at a distance of 300 meters from the station. The most likely dust source for TSP at monitoring station MA2 are the mineral piles at the enrichment plant and activities at the railway line where trains are loaded.
The heavy metal (Cr, Ni and Pb) concentrations were analysed only at monitoring station MA2 because, according to the TSP measurements (Paper IV, Table III and Fig. 2), this was the most polluted area and most of the anthropogenic activities were situated in the vicinity. In 2000, the Cr concentrations in ambient air varied between 0.65–48.2 g/m3 (average 17.9), the Ni concentrations between 0.05–1.4 g/m3 (average 0.52), and the Pb concentrations were all under 0.1 g/m3. It is not likely that that the Finnish air quality limit value of 0.5 g/m3 (73) for Pb will be exceeded in the mine area at the current level of operations, because the highest Pb concentration in the air in mine area reached a maximum value of 0.09 g/m3. This is only 18 % of the air quality limit value.
Fig. 6 in paper IV shows the cross-correlation plots between the TSP and Cr, Ni and Pb concentrations in ambient air at monitoring station MA2 in 2000. The poorest correlation was between Pb and TSP (R2 = 0.394). The data points indicate, that Pb in total suspended particles may have originated from at least two different emission sources, i.e. emissions from the quarrying and other mining operations, and emissions from dust caused by road traffic. However, it was not possible to distinguish between these two possible sources in this study. However, exhaust emissions from road traffic no longer contain Pb because the petrol used by automobiles and vehicles in Finland is unleaded (188). The correlations between the TSP and Cr and Ni clearly indicate that the Cr and Ni concentrations in the air are strongly correlated with the TSP concentration (R2 = 0.842 and R2 = 0.862, respectively).
In addition to the seasonal variation in TSP concentration, the seasonal variation in Cr concentrations (g/m3) in the air at monitoring station MA2 between January–December in 2000 also varied considerably from month to month (see Fig. 6); the lowest monthly value in 2000 was the Cr concentration of 0.65 g/m3 in January, and the highest the Cr concentration of 48.2 g/m3 in July.
Figure 6. The ambient air concentration of Cr (g/m3) at monitoring station MA2 (Konttori) during January–December in 2000. (IV)
If we compare the average Cr and Ni concentrations in ambient air (i.e. 17.9 g/m3 for Cr, and 0.52 g/m3 for Ni) at the mine area of AvestaPolarit Chrome Oy Kemi Mine with the forthcoming EU directive (50, 52–53) on PM10 (20 g/m3 for Cr and 0.02 g/m3 for Ni, respectively) our results are very high. However, it should be noted that PM10 is only a fraction of TSP.
Querol et al. (50) has also reported high ambient air, heavy metal concentrations for As, Pb and Cu during summertime especially (i.e. July–August). According to Infante et al. (84), high summertime values for ambient air heavy metals may be partly explained by climatic factors such as sea breezes, in which air particulates are thought to be rapidly mixed by wind action; this meteorological phenomenon is especially common during the summer on the coast of the Gulf of Bothnia. During this phenomenon the atmosphere is unstable, and it has been reported to cause rapid vertical air movements. According to Ekholm et al. (165), this phenomenon is one cause of the rapid increase in the dust concentration in the ambient area in the mine area. The Finnish Meteorological Institute has also observed this phenomenon in studies on the aerial distribution pattern of malodorous sulphur compounds emitted from the pulp and paper mills located at Kemi (198).
In this context it is worth noting that the heavy metals concentrations in ambient air vary in different mine areas depending on the type of ore, operations, processes, weather conditions, and the location of the samplers. Therefore the results of different studies are not necessarily comparable with each other. Ghose et al. (146) reported Pb concentrations of between 0.4–6.6 g/m3, and Cr concentrations of between 0.004–0.08 g/m3 in ambient air near to a coal mine in India. Very high metal concentrations can also occur in the ambient air in mine areas. Querol et al. (50) reported exceptionally high maximum concentration for Pb (4.4 g/m3) in the Donana area (sulphide ore) in Spain. However, the maximum Cr concentration in their study was 0.07 g/m3, and the maximum Ni concentration 0.04 g/m3.
5.2.2. Estimation of the bioavailability and environmental mobility of heavy metals in TSP material (V)
According to the results in Paper V (Table 2), Cr and Fe were the main leachable components of the heavy metals in TSP material emitted from the mining area of the AvestaPolarit Chrome Oy Kemi Mine. This result is reasonable considering that ((Fe,Mg))(Cr,Al)2O4) is the chief mineral in the ore. However, Cr and Fe were clearly restricted to the environmentally immobile fraction (leaching stage IV), and thus do not have any direct impact on the environment. The very acidic mixture of hydroxylamine hydrochloride and acetic acid, (HONH3Cl + CH3COOH; pH = 1.16), in leaching stage III dissolved only a small proportion of the Cr, while a high proportion of the Fe, Cu and Ni were leached. The heavy metal concentrations obtained in fraction IV (HNO3 + HF + HCl) were higher than those in fractions I–III for all the metals except Cu. Cd and Ni were mainly (50 % and 98 %, respectively) partitioned in the environmentally immobile fraction (leaching stage IV), although part of the Cd (29 %) also dissolved in ammonium acetate (leaching stage II).
The precision of the whole extraction procedure can be evaluated by verifying the results of replicate measurements. The data in Paper V (Table 2) show the results for two discs (d = 50 mm) cut from the same glass fibre filter (203*254 mm). The relative standard deviations were lowest for all the heavy metals in leaching stage IV: 0.54 % for Cr, 0.90 % for Fe, 4.96 % for Cu, 4.40 % for Ni and 1.66 % for Cd. Thus the reproducibility of the acid leaching procedure (HNO3 + HF + HCl) was good for all the metals. The relative standard deviations were generally acceptable (< 13 %) for all the heavy metals in leaching stage I. The poorest reproducibilities occurred in leaching stage I (H2O) for Cr (RSD of 35.5 %), and in leaching stage II (CH3COONH4) for Fe and Ni (RSDs of 54.2 % and 30.1 %, respectively). Lindberg et al. (79) reported similar problems with poor reproducibility in the leaching of atmospheric particles, and Tessier et al. (143) in the extraction of sediments. In both studies this was attributed to differences in the particle size distribution of the samples. Lindberg et al. (79) also suggested that there are problems in maintaining a constant leachate chemistry from sample to sample.
The average amounts of metals in the water soluble fraction (i.e. leaching stage I) leached from a TSP sample with an average mass of 173 mg were Cr 3.6 g, Fe 55 g, Cu 19 g, Ni 3 g and Cd 0.1 g. The corresponding values for the environmentally mobile fraction (i.e. leaching state II) were Cr 1 g, Fe 53 g, Cu 207 g, Ni 6 g and Cd 0.2 g. The sum of water-soluble (H2O) and environmentally mobile (CH3COONH4) metals was Cr 4.6 g, Fe 103 g, Cu 226 g, Ni 9 g and Cd 0.3 g. Based on the total annual particle emissions from the AvestaPolarit Chrome Oy Kemi Mine (48 t), the calculated annual airborne pollution impact of water-soluble and environmentally mobile heavy metals is therefore Cr 1.2 kg (r.s. 1.2 %), Fe 29 kg (r.s. 30 %), Cu 63 kg (r.s. 66 %), Ni 2.5 kg (r.s. 2.6 %) and Cd < 100 mg (r.s. < 0.1 %). These amounts are theoretically released after deposition of the aerosols (particulate material) on the surface of lakes, rives, soil and plants, and are thus potentially bioavailable. However, the magnitude of the “bioavailable” and “environmentally mobile” fractions are strongly dependent on the physical and chemical properties of the “dust”, e.g. the solubility, and chromite especially is known to be difficult to dissolve. The leaching studies represent an assessment of the worst case environmental scenarios in which the components of the sample become soluble and mobile (135).
Thus, the results of our leaching studies were in agreement with those obtained by Hlavay et al. (48) for the fractionation of Cr in urban particles at Tihany in Hungary, and by Dreetz et al. (47) at Oslo in Norway. Querol et al. (50) reported that only a minor portion of the Cr in atmospheric particulates collected in a sulphide ore mining area was leachable by H2O or by CH3COONH4. However, in another study carried out by Hlavay et al. (74) on the distribution of Cr in aerosol samples collected in the moderately polluted cities of Veszprém and Kabhegy in Hungary, Cr was also clearly partitioned into the environmentally mobile fraction (leaching state II; CH3COOHN4); the proportion of Cr in the environmentally mobile fraction in the two cities was 51 % and 44 %, respectively.
5.2.3. The homogeneity of heavy metal deposition on TSP filters (VI)
The rsd values for Cr, Ni, Cu and Fe in paper VI (table 7) suggest that these heavy metals are not necessary uniformly distributed over the glass fibre filters. The rsd values between 9 discs cut from the same filter varied between 5.4–33.9 % for Cr, between 7.50–35.0 % for Ni, between 3.6–25.9 % for Cu, and between 6.6–19.9 % for Fe, depending on the sampling month. All the Cd concentrations in the TSP filters were very low, and it was therefore not possible to determinate the homogeneity or un-homogeneity of the deposition of Cd on the glass fibre filters.
Although the cause of the non-uniform distribution of elements on the filters was not determined, Zdrojewski et al. (199–200) reported that one explanation for this phenomenon in a study on Pb and Cd was that a high-volume sampler draws in air non-uniformly. Another reason might be the differences in the particle size distribution of the metals. According to Zdrojewski et al. (199), one explanation could be the detachment of particulate matter during transit and storage.
However, although there is no exact criterion for the degree of homogeneity of heavy metal deposition on filters, Low et al. (201) and Dreetz et al. (47) considered heavy metal deposition to be homogeneous when the rsd for the element, calculated from different discs cut from the same filter, was < 15 %. The corresponding criterion value for rsd according to Wang et al. (69) was < 10%. Thus, if we compare the results in Paper VI (table 7) with the criteria of Low et al. (201), Dreetz et al. (47) and Wang et al. (69), it is apparent that the distributions of Cr, Ni, Cu and Fe across the glass fibre filters collected using a high-volume sampler were non-uniform. In conclusion, the non-uniform deposition of heavy metals in filters causes error in the calculated heavy metals concentrations of ambient air concentrations, and thus gives underestimated results for the impact monitoring (i.e. the quality of air) of point sources.