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Soil Contamination and Pollution (Eric Mibuari)



Note: Sources are fothcoming!


Introduction:

Soil contamination is a major inhibitor to the health of the Amazon Basin rainforest. It not only directly affects flora but also the fauna in several ways.  Soil contamination is the mixing of hazardous substances with the soil. The contaminants, liquid or solid, are physically or chemically attached to the soils or trapped within its particles.

Contaminants mainly get to the soil through spills, direct burying and/or migration from other spills.

Generally soil contamination effects are felt in a chain effect. Growing plants absorb the contaminants through their roots; the plants are consumed by animals that then get poisoned by the contaminants. Animals may also get affected when they inhale ingest or touch the contaminated soil in the processes of burrowing and feeding .

The major contaminants that are affecting the Amazon Rainforest are shown in the table below:

Contaminant
 Areas most prevalent
 Effects on the Amazon
Mercury
 Tapajos river, Grand Carajas mining scheme
Deforested areas
 Inhibits some plant growth
Negatively affects animal immunity
Cyanide
 Omai Gold Mines
 Kills plant and animal life
Pesticides / Herbicides
 Agricultural regions
 Destabilize soil pH
Acid Rain To be researched...
  


Soil is important because it affects the amount of water available for plants, forms the basis for root support and provides the nutrients for plants. It is also the surface on which many animals move about, and is the dwelling place and food store for burrowers.

Mercury Contamination

This is most prevalent in the Tapajos river system.  The Tapajos river is a major tributary of the Amazon river that flows through the heart of the rainforest in northern Brazil. Its banks mark the site of a major gold rush. About by 1 million miners are extracting gold from river sediment by panning. The mercury used to extract the ore from the river's sediment creates about 130 tonnes of mercury waste every year. There are also isolated gold mines in Mato Grosso, Roraima, Rondonia, Amazonas, and Acre regions where mercury is commonly used to extract raw gold from mine tailings. Between 3 to 5 kilograms of mercury are used to extract 1 kg of gold (8) in these mining regions.

Based on gold production, one estimate has been made that 170 tonnes of Hg enter "garimpos" - informal mining operations - annually. Mercury losses of 40% are typical when amalgam is distilled in open pans, so about 70 tonnes of Hg are lost if retorts are not used at all by miners. About 80% of these losses are emitted directly to the atmosphere while the remainder is discharged with amalgamation tailings to watercourses.

However it has also been observed that a lot of mercury that comes from naturally occurring deposits. This has been washed away by erosion into streams and waterways. The main cause of this erosion is the high level of deforestation in the rain forest.  Evidence for this discovery in recent years is the fact that there is essentially no difference in mercury exposure levels between villagers living 100 kilometers downstream and those living 300 or so kilometers away from the gold mining area in such areas as Jacareacanga and Brasilia Legal.

Natural forest fires were also attributed as a source but the amount released annually has been calculated at 20 g/ha ( less than 0.02% of provincial annual natural emissions), creating a high short-term emission pulse. This evaluation assumes 0.4 ppm as the mercury level in timber, but only 0.08 ppm is thought to be lost during fires.

Worldwide, wild forest fires are estimated to release 20 tonnes of mercury to the atmosphere, which is less than 1% of natural emissions. Intentional wood combustion represents 60 to 300 tonnes of Mercury (about 5% of man-made emissions in 1983) an estimate based on a range of 0.1 to 0.5 ppm mercury in wood. Today, given high rate of deforestation by fires in the Amazon, mercury emissions derived from wood combustion must be a more important source. Deforestation in the Amazon is between 35,000 (1987) and 50,000 (1988) square kilometres per year.

Natural mercury levels in plants range from 0.001 to 0.1 ppm (dry weight). In forest ecosystems, this increases to 0.01 to 0.3 ppm, while crops grown in soils containing less than 0.04 ppm Mercury vary from 0.004 to 0.09 ppm. Little is known about Mercury distribution in the Amazon flora but aquatic macrophytes show levels between 0.1 to 1 ppm.

The temperature range encountered in vegetation fires is between 650 and 1100°C. At 200 - 300°C, destructive distillation of about 85% of organics occurs. Mercury compounds are volatile between 25 - 450°C, and organic mercurials usually have lower boiling points than inorganic ones. When fossil fuel is burnt, more than 90 % Mercury is lost. Because most mercury in wood is present in organic form, it can be assumed that about 90% of Mercury is lost from above-ground biomass. It is also reasonable to infer that as with other metals, the remainder becomes weakly bound to the ash to be leached by runoff water.

This shows that amount of Mercury emitted by deforestation from estimates of biomass distribution in the Amazon.  It can then be assumed that most Mercury compounds are released from above-ground biomass even without complete combustion, whereas only a minor amount volatilizes from surface soil.  Using a conservative estimate of Mercury levels in plants and organic matter of 0.05 and 0.3 ppm respectively, the unit Mercury emission is 17.6 g/ha (1.76 kg/km²). Considering that 50,000 km² was burnt in 1988, 88 tonnes of Mercury likely were emitted to the atmosphere that year, (26 % above the estimate for mining emissions). The total area consumed up to 1991 is estimated at 404,000 square kilometres. So, over 710 tonnes of Mercury have been released from this source. If the mercury wood levels of (0.1 to 0.5 ppm) are used, Mercury emissions from above-ground wood alone would be 117 to 585 tonnes/year.

Release of water-soluble Mercury[II] from burning fossil fuel depends on the presence of chloride and/or active particulate matter but can be as high as 50 % of total mercury. Combustion gases from forest fires would follow the same pattern representing imminent danger as methylation is enhanced in aquatic environments. In contrast, the conditions during amalgam-burning do not favour oxidation of mercury vapour either thermodynamically or kinetically. Deforestation has not been considered in monitoring programs in the Amazon, although fly-ash transport is now being investigated. It is clear that deforestation is a major source of mercury emission in a form more dangerous than that emitted by "garimpos". Mercury emissions from any source must be stopped but all significant villains should be recognized.

Effects of Acid Rain on Land:

The effect of acid rain on soil is dependent on the behaviour of the ions in the soil.  Acid rain mobilizes ions from the soils in two ways:

The H+ ion in the acidic water displaces the other positive ions from their binding sites and increases the concentration of these ions in soil water.

The negatively charged sulphate and nitrate ions in the acid rain act as counter-ions which allow positive ions to be leached from the soil.


Pesticide Contamination

In spite of the good contribution that pesticides have for agriculture, they are essentially poisons and can be potential contaminants to the soil. The main means of pesticide contamination to the water is through leaching.

In leaching, pollutants are flushed through the soil by rain and irrigation as it moves downward. In areas with sandy soils leaching is particularly enhanced. Once in the soil, the pesticide either adheres to particles or dissolves. The pesticide may also vaporize and enter the atmosphere or or break down through microbial and chemical pathways into other, less toxic compounds.  Pesticides may be leached out of the root zone by rain or irrigation water, or wash off the surface of land. The fate of a pesticide applied to soil depends largely on two of its properties: persistence and solubility. Persistence defines the "lasting-power" of a pesticide. Most pesticides break down or "degrade" over time as a result of several chemical and micro-biological reactions in soils. Sunlight breaks down some pesticides. Generally, chemical pathways result in only partial deactivation of pesticides, whereas soil microorganisms can completely break down many pesticides to carbon dioxide, water and other inorganic constituents. Some pesticides produce intermediate substances, called "metabolites" as they degrade. The biological activity of these substances may also have environmental significance. Because populations of microbes decrease rapidly below the root zone, pesticides leached beyond this depth are less likely to be degraded. However, some pesticides will continue to degrade by chemical reactions after they have left the root zone.

Degradation time is measured in "half-life." Each half-life unit measures the amount of time it takes for one-half the original amount of a pesticide in soil to be deactivated. Half-life is sometimes defined as the time required for half the amount of applied pesticide to be completely degraded and released as carbon dioxide. Usually, the half-life of a pesticide measured by the latter basis is longer than that based on deactivation only. This is especially true if toxic or nontoxic metabolites accumulate in the soil during the degradation.

Probably the single most important property influencing a pesticide's movement with water is its solubility. Soil is a complex mixture of solids, liquids and gases that provides the life support system for roots of growing plants and microorganisms such as bacteria. When a pesticide enters soil, some of it will stick to soil particles, particularly organic matter, through a process called adsorption and some will dissolve and mix with the water between soil particles, called "soil-water." As more water enters the soil through rain or irrigation, the adsorbed pesticide molecules may be detached from soil particles through a process called desorption. The solubility of a pesticide and its sorption in soil are inversely related; that is, increased solubility results in less sorption.

One of the most useful indicators for quantifying pesticide adsorption on soils is the "partition coefficient" (PC). The PC value is defined as the ratio of pesticide concentration in the adsorbed-state (that is, bound to soil particles) and the solution-phase (that is, dissolved in the soil-water). Thus, for a given amount of pesticide applied, the smaller the PC value, the greater the concentration of pesticide in solution. Pesticides with small PC values are more likely to be leached compared to those with large PC values.

Cyanide Contamination

This is used in almost the same way that mercury is used. It has been used at a mining operation on the Guyana/Venezuelan border. Though the use of cyanide in gold mining operations is relatively new to Guyana - it is being used in South America's largest gold mine, Omai Gold Mines Ltd, (only such operation in the country which has approval to use cyanide in the processing of gold.) and this means that its amount of waste must a significant quantity.




This page was last updated on October 27, 2002.
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