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The sample site has been subject to heavy metal contamination by various mining operations typical of many industrialised areas of Derbyshire. The site is particularly contaminated by the elements zinc and lead. Principal sources of contamination include: ore extraction, disposal of the soil, smelting and re-working of the soil for fluorspar. The results observed in the samples taken from the site were analysed for geochemical association, in particular the geochemical associations between zinc/lead and lead/cadmium. There has been a large body of research on the cycling of zinc, lead and cadmium originating from industrial ad biological sources but less is known about their mobility (Olade, 1987). The data was also analysed to reveal whether geochemical associations found in the soil profile translated to the water profile or vice versa. The land sampled is also the location of adjoining farms. The data was analysed for 'agricultural indicators.' The presence of nitrate, phosphate and potassium are strong signs of agricultural activity. These elements are used in fertilisers, animal manure and occasionally irrigation water. The alkalinity and charge balances in the water were investigated. To determine whether all elements that needed analysis and collection have been accounted for. The pH data of the soil and water profiles was investigated. The site was contaminated with smelter fallout and metal-rich dust resulting in soils with a pH value of <5. The report will determine whether the contamination of the soil and water profiles exceed the drinking water (DW) standards, environmental quality standards EQS or soil guideline values (SGV) set by various standards agencies, i.e. DEFRA and the WHO. These factors will be studied to establish possible future land uses of the site. High levels of contaminants that exceed these values severely restrict the options for land use.
3.1 Geochemical associations
3.1.1 Geochemical association between calcium and zinc
Across the Clough Wood site a strong correlation of 0.778 was recorded between calcium and zinc in the soil. However, a weak correlation of 0.345 was recorded between calcium and zinc in the water.
Figure 3.1. The correlation between Zn2+ and Ca2+ cations in the soil profile across the Clough Wood site.
Zinc and calcium have a strong positive correlation in soil. Zinc was mined at the site and calcium is present as calcium carbonate (CaCO3), calcium carbonate being the major constituent of limestone. It is thought that the divalent cation zinc reacts with the carbonate (CO32-) in limestone to form zinc carbonate (ZnCO3). This is due to its like for like charge with calcium. Therefore, the more calcium the more lead. The Zn-Ca association does not continue into the aquatic environment, which is surprising as the streams should receive acid mine drainage. One explanation may be that discharge of chemically distinct groundwater to small stream water increases the pH. An inflow of chemically distinct groundwater with a high dissolved inorganic carbon content into a small stream system can increase the pH. This process in addition to biological activity, carbonate precipitation and an increase in pH, governs chemical transformations of solutes, such as metals. Metal mobility in the water is affected by the rise in pH (Choi et al., 1998). A greater concentration of zinc was found in the soil downstream (0.33 mol kg-1) at site S9 and (0.70 mol kg-1) at site S8, than upstream at site S1 (negligible). This is due to the main mining activities being found downstream and minespoil dumping downstream.
3.1.2 Geochemical association between zinc/lead and cadmium/lead
A strong correlation of 0.976 and 0.931 was observed in the data between lead/zinc and lead/cadmium in the soil respectively. However, there was no corresponding correlation between these elements observed in the water.
Figure 3.2 (A) the correlation between Pb2+ and Zn2+ cations in the soil profile across the Clough Wood site. (B) the correlation between Cd2+ and Pb2+ cations in the soil profile across the Clough Wood site.
The strong correlation between cadmium and zinc in the soil profile was to be expected. Cadmium is extremely rare in its pure form. Cadmium substitutes directly for zinc in the sphalerite mineral via isomorphous substitution. The mineral formed is CdS. Cadmium is far more abundant in this mineral form. These elements are bonded together on an atomic scale, they are very difficult to separate and it was very likely that there would be an association between these two elements. Surprisingly, there was also a strong correlation between zinc and lead. Zinc and lead were expected to be present in large concentrations across the site due to the heavy lead and zinc mining in the past. However, it was thought that they would have separated due to mine spoil sorting. Lead is also a heavy element and it was thought it would easily separate from zinc and a correlation less pronounced than 0.976 would be observed. The correlation between lead and zinc in the water profile was 0.008 and the correlation between lead and cadmium in the water profile was 0.361. The greater association between cadmium and lead than zinc and lead in the water profile could be evidence of zinc and lead separation in the water profile. Whereas, cadmium and lead showed a greater association as they were more likely to remain associated due to their bonding on an atomic scale.
3.1.3 Geochemical association between potassium and caesium
Across the Clough Wood site one of the strongest correlations of 0.924 was recorded between potassium and caesium in the soil. In addition, a strong correlation of 0.983 was recorded between potassium and caesium in the water.
Figure 3.3. The correlation between Cs+ and K+ cations in the soil profile across the Clough Wood site.
Figure 3.4. X, Y scatter graph showing the correlation between Cs+ and Zn+ cations in the water profile across the Clough Wood site.
The association between caesium and zinc in both the soil and water profiles is due to the presence of clay minerals. The weathering of shale is responsible for the formation of the clay minerals. For example, mica is in the group of sheet silicate minerals known as the alumino-silicates. Chemically micas can be given the general formula:
In which X is K, Na, or Ca or less commonly Cs. These micas also contain potassium within their interlayer spaces. One explanation for the good correlation between Cs+ and K+ in both the soil and water profiles is the solubility of clay in water. Clays are not readily soluble in water. Silicate phases dissolve slower than carbonate phases. Silicate minerals contribute a high proportion of total dissolved solids in river systems. It is likely that the clay minerals have remained intact within the samples and not diluted or influenced by the increase in pH found in the water samples. These silicates could also be responsible for buffering the pH increase in the water.
3.2 pH of water and soil
The mean pH of the soil across the Clough Wood site was 5.81. This value is lower than the more neutral mean value of 7.71 found at the 15 nearest water sites.
Figure 3.5. The mean pH at the soil sampling sites compared to the mean pH of the nearest corresponding water sampling site. For example the pH at soil sampling site S1 is compared to the pH of water sampling site W6, as it is closest. See figure 2.1 for the soil sampling sites and their nearest water sampling location.
The pH of the soil is lower (more acidic) upstream due to the location of the acidic woodlands. The pH of the soil is lower in the soil than in the water. The causal agent is a change in equilibrium. Microbial activity and root respiration within the soil creates a build up of CO2. The build up of CO2 creates very acidic soil conditions as low as 4.19 at S3. However, as water moves through the soil profile it mixes with CO2, producing carbonic acid, and a high partial pressure of CO2 in the soil. The water exits the soil into the surface stream. The exit point into the surface stream causes degassing. The degassing leads to a loss of CO2 and a rise in pH. Hence, the pH at the comparable water site W7 is 6.65. It would be appropriate to return to the site and sample the CO2 content of both the soil and water profiles. These readings could provide clear evidence of the scale of the loss of CO2 and back up the degassing theory. Another explanation is the occurrence of rainfall the night before the sample. The rainfall was likely to increase the swell of the river significantly enough to dilute the samples and increase the pH of the water samples.
3.3 Alkalinity and charge balances in water
Carbonates are a good measure of alkalinity. The bicarbonate CaCO3- is a good measure of alkalinity. Dissolved organic carbon is an indicator of acidification of a sample. It drives the pH driven chemical equilibria of the water to CO2.
Figure 3.5. The correlation between CaCO3 and dissolved inorganic carbon (DIC).
Figure 3.6. The correlation between DIC and pH across the Clough Wood field site.
Figure 3.5 shows an increase in calcium carbonate with increasing DIC. This is expected, as an increase in CaCO3 in the water samples would naturally increase the DIC content of the water samples. The increase in DIC has lead to a rise in pH seen in figure 3.6. The increase in dissolved inorganic carbon in the water influences the pH of the aqueous system. CO2, HCO3- and CO32- are related by the following pH driven chemical equilibria.
CO2 + H2O H2CO3 H+ + HCO3− 2H+ + CO32-
An increase in DIC shifts the equilibria in the CO2 direction. As a result the acidification of the water body is reduced and the pH rises.
Carbonate solubility also depends strongly on pH. At lower pH values (more acidic), calcium carbonate is more soluble and at higher pH values it precipitates. As the pH is increased the alkalinity of the solution decreases.
Figure 3.7. The loss of hydrogen ions with increasing pH value. The rise in pH allows a greater portion of total ions to be present in carbonate form.
A full pH unit drop corresponds to a ten-fold decrease in carbonate concentration.
An equation of electro-neutrality details the balance of ions within a water sample. The equation is given:
Total cations + Total anions x 103
Total cations - Total anions
The result of the equation should yield a percentage value. The closer the value is to 0% the greater the balance between cations and anions within the sample.