«Dissertation zur Erlangung des Grades “Doktor der Naturwissenschaften” Im Promotionsfach Geowissenschaften Am Fachbereich Chemie, Pharmazie und ...»
The results suggested that the exchangeable thallium concentration was highest in surface soils and gradually decreasing with increasing sample depth at sampling point 1, 2, 12 and 19 (all were from Greenland) (Fig. 22.214.171.124, Appendix A) (p.219). The high thallium concentration observed on the surface soils was possibly due to anthropogenic emission and not from the natural parental minerals. It has been suggested that the atmospheric Tl pollution can contribute to soil contamination in the vicinity of Tl emission sources. (Brockhaus et al. 1981). Tl soil contamination is mainly caused by dust fall-out from emissions of cement plants (ATSDR, 1992).
In the above figure (20) suggested that the majority of the wind direction in the region was recorded towards north-east direction. Therefore a strong indication was shown due to the sampling site 1 is located at the north-east of the local cement factory. Samples 13, 14 and 18 from Greenland also demonstrated decreased exchangeable thallium concentrations with increasing sample depth (except sample 17) (Fig. 126.96.36.199, Appendix A) (p.219). These four sampling points showed less drastic decrease of the thallium concentrations with increase sampling depth. Overall all of the above Greenland samples showed highest exchangeable thallium concentrations on the surface soils with decrease Tl concentrations by increasing sample depths.
Results and discussions
Aqua Regia acid digestion as a weaker acid extractant in which aimed at dissolving pseudo-total thallium content in the soil samples were carried out. A hydrofluoric acid digestion method could be employed to figure out the so called “real total content”. (Utermann et al. 1999;
2000) suggested a comprehensive analytical investigations to compare the above two elemental fractions. A set of regression functions was derived for transforming element contents between the above two fractions with a given statistical certainty. The Federal Institute for Geosciences and Natural Resources (BGR) and Geological Survey of North Rhine-Westphalia (GD NRW) (http://www.bgr.de/app/FISBoBGR_Stoffhaushalt/index.htm) provides practical transformation for certain elements except for Tl under these circumstances. HF is extremely toxic and corrosive to mankind by absorption through skin contact with 2% of skin exposure of body area can be fatal. HF can penetrate through skin and decalcify bones as the HF molecules can react with calcium and magnesium ions and lead to heart and organ failure. Splashing into eyes can cause irreversible damage to the cornea (Government of Western Australia, Section 8.3, Ref. 19). With the huge safety concerns, it would be preferable to replace HF with less hazardous mineral acid as the above measurements required large amount of sample handling for the two sampling locations (~500 times sample extractions being carried out). Although the applied aqua regia acid digestion was not able to digest the “real total” however the present “pseudo” total thallium content could still serve a good reference in analyzing total metal content. Numerous research in the past suggested the data employed from two above methods are closely resemblance let alone not complete identical.
The thallium contents in aqua regia acid digestion also demonstrated highest thallium concentration in the surface soils and with significant decreasing Tl concentrations of increasing sample depth at all Greenland sampling points (except sample 13 and 14) (Fig. 188.8.131.52, Appendix A) (p.220). Exchangeable and total thallium concentration in agricultural sampling points was shown in graph plot and suggested rather mixed results with increasing sample depth (Fig. 184.108.40.206, Appendix A) (Fig. 220.127.116.11, Appendix A) (p.221-222). This could be due to extensive plowing process from the local farmers and as a result mixing the top and sub-soils thoroughly. Hence, no obvious trends could be revealed with increasing sample depth.
Fig. (21) Isoline plot for thallium top-soils extracted by ammonium nitrate after kriging estimation at location 1 (Blue dot represents corresponding sampling point) The above isoline plot showed concentration profile of exchangeable thallium concentration of top soil. The concentration map suggested the hot spot in which the exchangeable thallium concentration exceeding the suggested safety limit by soil conservation regulation under Federal Minster of Justice (BodenSchutzVerordnung, Bundesministerium der Justiz) were located at the bottom left corner of the entire sampling site. The suggested safety value was below 100 µg/kg for thallium extracted by ammonium nitrate. According to figure (21) areas under sampling point 1 and 2 should require remediation control. The agricultural area was however not revealing exchangeable thallium concentration exceeding the threshold value.
Fig. (22) Isoline plot for thallium sub-soils extracted by ammonium nitrate after kriging estimation at location 1(Blue dot represents corresponding sampling point) Figure (22) showed the sub-soils exchangeable thallium concentration profile at sampling location 1. The map suggested the exchangeable thallium concentration did not exceed the safety limit of the soil conversation regulation at all the sampling points. The sampling points with relatively higher exchangeable thallium concentrations were again mainly located at the bottom left corner of the sampling site.
Fig. (23) Isoline plot for thallium top-soils extracted by aqua regia acid digestion after kriging estimation at location 1(Blue dot represents corresponding sampling point) Fig. (24) Isoline plot for thallium sub-soils extracted by aqua regia acid digestion after kriging estimation at location 1(Blue dot represents corresponding sampling point)
Figure (23) and (24) showed the total thallium concentration of top and sub-soils at the sampling location 1. The results suggested that highest total thallium concentrations were identified at the top-soils with the concentration range between 0.4-2.4 ppm in comparison with 0.3-1.3 ppm concentration range in the sub-soils. Several Tl hot spots in which exceeding the 1 mg/kg value were also found in the figure (24). The following table (18.104.22.168, Appendix A) (p.218) summarized the average Tl content (mg/kg) in different minerals. Soils in which formed by weathering of rocks and by knowing the thallium concentrations in rock-forming minerals could be able to estimate the fate of Tl in soils. Table (22.214.171.124, Appendix A) (p.218) shows thallium concentrations in the upper soil layers at various countries. The results in figure (23) and (24) suggested that the total thallium concentration was higher than common soils Tl concentration in which below 1 mg/kg (Kazantzis, 2000) and exceeded the potential risk for human of excessive Tl contents in soils being set up by the North-Rhine Westphalia (FRG) (1 mg/kg). The sub-sampling points contained relatively high Tl concentrations were cluster around the bottom left corner of the sampling site. The total thallium concentrations of top and sub-soils were revealed. Sub-sampling points in above figure (23) and (24) which demonstrated above 1 mg/kg thallium content should require careful monitoring by the corresponding authority.
y-axis = Semivariogram, x-axis = distance lag (m) Fig. (25) Empirical semivariogram (dots) and the fitted model (line) of LnP (spherical model: Co = 347.1; Co + C1 = 347.1+643.2; a = 507.8 m) (no. of lags = 5; lag tolerance: % of lags = 60%; define maximum distance = 600m)
Semivariance analysis of top-soils ammonium nitrate extracts targeting exchangeable thallium at sampling location 1 (Fig. 21) was carried out. A semivariogram was calculated in order to describe the spatial variation of soil. No evidence of anisotropy in the variogram of LnP data was found. The top soils exchangeable thallium concentrations varied similarly in all directions of the study area and the semivariance depended only on the distance between samples.
The experimental variogram was fitted with a spherical model shown at figure (25). The fitted spherical semivariogram function contained a range of 507.8 m, a sill of 990.3, and nugget of 347.1. The nugget effect was calculated as follows.
= 35.0% The nugget effect accounted for 35.0% of the total sill. A rough guideline suggested the variable is considered to have a strong spatial dependence if the nugget-to-sill ratio is 25%, a moderate spatial dependence if the ratio is between 25% and 75%, and a weak spatial dependence if the ratio is 75% (Cambardella et al., 1994). The above findings suggested exchangeable thallium in top-soils was moderately spatially dependent. Variables which are strongly spatially dependent may be controlled by intrinsic variations whereas weak spatial dependence may indicate that variability is controlled more by extrinsic variations. The definition of the range of the semivariogram is the maximum distance between correlated measurements and this factor is effective in terms of selecting a sampling design to map soil properties (Utset et al., 1998).
(5.5) Thallium concentration profiles of top and sub soils at sampling location Table (20): Table shows soil thallium (Tl) concentrations extracted by ammounium nitrate and aqua regia total acid digestion at location 2
The above table (20) showed the exchangeable thallium (extracted by ammonium nitrated) and total thallium concentrations in different sampling points and depths.
14.5 and 126.96.36.199 in Appendix A (p. 223-224) showed the general trends of exchangeable thallium and total thallium concentrations at each sub-samples with increasing sample depth. Both of the figures demonstrated similar overall trends. All of the sub-samples revealed highest thallium concentrations (except sub-samples 4, 5 and 15) on the top-soils with decreasing Tl concentrations at increasing sample depth. 9 out of 13 sub-samples measured with exchangeable thallium concentrations were bigger than 100 µg/kg safety limit for mobile thallium. Sub-sample 2 was identified with highest mobile thallium concentration of thallium in the surface soil with 554.50 ± 2.22 µg/kg in which was approximately over 5-folds higher than the safety limit set by the German Federal. The trend showed mobile thallium concentration returned to below safety limit with the sample depth greater than 30 cm. Similar trend was also found in total thallium with 12 out of 13 sub-samples contained less than 1 mg/kg total thallium with sample depth above 30 cm. 4 sub-samples on the surface soils showed total thallium content above 2.5 mg/kg in the trend. The above table (20) indicated the high thallium concentrations could be also due to anthropogenic emission in similar with the results obtained at location 1 albeit with much higher mobile thallium concentration being observed in top-soils.
The exchangeable thallium t-test value was 3.2036 with degree of freedom equal to 38 and p = 0.0027. The p-value associated with t was low ( 0.05). The small p-value indicated rejection of the null hypothesis. The paired exchangeable thallium concentration between top and sub-soils was considered to be statistically significant difference. The t-test was also applied for total thallium extracted by aqua regia acid digestion. T-test value was 4.9004 with degree of freedom equal to 38 and p = 0.0001. The paired total thallium concentration between top and sub-soils was also considered to be statistically significant difference.
Fig. (26) Isoline plot for thallium top-soils extracted by ammonium nitrate after kriging estimation at location 2 (Blue dot represents corresponding sampling point) The above figure (26) showed the mobile thallium concentration map at the sampling location 2. The highest thallium concentration was located at bottom left corner forest region (sub-sample 2) and the hot spot area was also identified at the same region in the isoline plot.
The maximum mobile thallium concentration was found to be ≈ 540 µg/kg in comparison with ≈ 130 µg/kg at location 1 for mobile thallium. In the isoline plot above 50% of the total area showed mobile thallium concentration exceeding the safety limit. Larger area in location 2 showed mobile thallium concentration exceeding the safety limit than location 1 (Figure 21 and 26). The kriging estimation in above suggested the area next to sampling points 1-6 if in which being used for agricultural purpose could be potentially not suitable for consumption.
The concentration map suggested the hot spot in which the exchangeable thallium concentration exceeding the suggested safety limit by soil conservation regulation under Federal Minster of Justice (BodenSchutzVerordnung, Bundesministerium der Justiz) were located at subsampling point 2. The suggested safety value was below 100 µg/kg for thallium extracted by ammonium nitrate. Most of the area in above figure 26 demonstrated mobile thallium concentration much higher than the safety value. Remediation control should be implemented under the above circumstances.
Results and discussions Fig. (27) Isoline plot for thallium sub-soils extracted by ammonium nitrate after kriging estimation at location 2 (Blue dot represents corresponding sampling point) The above figure (27) showed exchangeable thallium concentration in the sub-soils at location 2. The maximum mobile thallium concentration was found to be ≈ 170 µg/kg in comparison with ≈ 100 µg/kg at location 1. The sub-soils demonstrated mobile thallium in the majority of the area was below the safety limit and the hot spot was found at sub-sample 5. Only an approximately 50 m from the centre of radius surrounding the sub-sample 5 showed mobile thallium content above the safety limit. Overall the mobile thallium content in the sub-soils showed significantly less than the top-soils.
Fig. (28) Isoline plot for thallium top-soils extracted by aqua regia acid digestion after kriging estimation at location 2 (Blue dot represents corresponding sampling point) Fig. (29) Isoline plot for thallium sub-soils extracted by aqua regia acid digestion after kriging estimation at location 2 (Blue dot represents corresponding sampling point)