Group 2

4.121 - Lead

Lead is an example of an element with group two element release behaviour. Leaching data of lead (in cumulative and flux forms) against equivalent time for the Mt. Ruapehu samples are shown in Figure 26. A high initial flux for lead continues for c. 260 days, where the maximum acceptable value for drinking water (0.07 mg/kg) is exceeded almost immediately, (MoH, 2008). After this initial period, there is a relatively abrupt transition into a second period of increasing cumulative concentration.

Figure 26: Lead is an example of the group 1 element release behaviour, (A) shows the cumulative

concentration for the Mt. Ruapehu 1995-96 volcanic ash samples (MAV) is the maximum acceptable potable drinking water value for lead, (B) shows the flux (q) non-norm (mg/kg/min).

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During these next equivalent c. 17.3 years, the flux of Pb into brine is exponentially declining. All of the Mt. Ruapehu volcanic ash samples follow a remarkably similar flux for the duration of the soxhlet reactor experiment.

The behaviour of leached lead in the soil profile is dependent on the abundance of clay and iron-manganese hydroxides (Kabata-Pendias, 2010). Furthermore, soils with a high content of soil organic matter such as in well-drained allophanic soils have been shown to have a

significant role in the adsorption of lead near the soil surface. Another factor that influences the solubility of lead in the soil is the soil acidity. High soil pH decrease the solubility of lead significantly. However, the scientific literature is dominated by the impacts on plants by toxic levels of lead pollution from contaminated sites (Sabti, Hossain, Brooks, & Stewart, 2000).

Lead in its natural state occurs in plants, though it is not known for its importance to the functioning of any plant species (Kabata-Pendias, 2010). However, there have been some studies that have highlighted the beneficial effects on the amount of plant growth with the supplication of Pb salts. However, these studies have not isolated all of the potential factors that may result in this plant growth. Lead in plants inhibits several important enzymes that govern plant growth, as well as damage the soil biological functions.

There is a high flux for the first 260 days. Figure 26, shows that the long-term impacts of lead from the Mt. Ruapehu volcanic ash is going to be relatively constant from c. 3 to 18 years of estimated real world time. However, lead is not very mobile in the soil and is often held in organic matter and iron hydroxide complexes irrespective of the drainage profile of a soil.

4.122 – Zinc

Figure 27, presents leaching data for zinc, another example of an element with group two release behaviour, against equivalent time. There is a high initial flux for zinc until c. 0.9 years, it is after this time that the flux exceeds the (5 mg/kg) EPA secondary standard (EPA, 2002) for zinc in drinking water. Zinc changes into a period of decreasing flux over time until c. 8 years. From c. 8 years onwards, the flux changes into an approximately constant and low release over time. Time-variant cumulative concentration data for zinc shows an interesting relationship with the style of eruption. The sample with the highest flux is the phreatomagmatic sample of 96/7 that is followed by the 95/5 lower layer. The other three samples, dry magmatic in eruption style except for the upper layer of 95/5 sample are all clustered together. The 96/46 strombolian style sample shows the lowest cumulative concentrations (and fluxes) and does not exceed the EPA value at any time.

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In the soil profile the most common form of dissolved zinc is in the form of free ions (Zn2+_{). The}

behaviour and the storage of zinc in controlled by the soil pH as well as the forms in which the metal is added. Although zinc is known to be mobile on most soil profiles, soils with high clay content and organic matter such as a well-drained allophanic soil are the most efficient at holding zinc. Within the soil profile there are two main mechanisms of zinc adsorption. The first is the cation exchange sites similar to the way in which calcium is stored in the soil. The second way is the adsorption by organic ligands in the soil. There are also the iron, aluminium as well as manganese hydroxides in the soil profile that adsorb zinc within the soil profile (Kabata-Pendias, 2010).

In plants, zinc has an important function in a variety of enzymes. The basic function of zinc in a plant is the metabolism of carbohydrates. In the scientific literature, there have been some studies expressing concern at the use of zinc based fertilisers that are increasing the toxicity of the soil profile. Zinc is also used extensively for the protection of livestock against biofortified Zn willow may have be a potential method to reduce fungal infection (Anderson et al., 2012). In the aftermath of the Mt. Ruapehu eruptions zinc was also found to be supplied to plants in small concentrations partially by the deposition of volcanic ash (Cronin et al., 1998).

As illustrated in Figure 27, the impacts of zinc are high in the short-term until c. 0.9 years. Then the flux of zinc will increase further in the long-term for 8 years. After 8 years the flux of zinc will be relatively constant and low. This could be an explanation for the findings of Cronin et al.

Figure 27: Zinc is an example of the group 2 element release behaviour, (A) shows the cumulative concentration for the Mt. Ruapehu 1995-96 volcanic ash samples (EPA secondary) is the maximum acceptable potable drinking water value for zinc, (B) shows the flux (q) non-norm (mg/kg/min).

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(1998), inferring that there were small amounts of zinc supplied by input from volcanic ash from the 1995-96 eruption sequence.