Soil Contamination/Pollution

Introduction: Soil contamination is a major inhibitor to to the good health of the Amazon basin rain forest. It not only affetcs the flora in a direct way but also the fauna in several diverse ways.  Soil contamination is the mixing of hazardous substances with the soil. The contaminants, liquids or solids, are physically or chemically attached to the soils or trapped within its particles.

Contaminansts mainly get to the soil through spills, direct burying and  migration from othe spills.

Generally soil contamination effects are felt in a chain effect: Growing plants absorb the contaminats through their roots, the plants are cosumed 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 Rain forest are shown in the table below

Soil  is important because: it affects the amount of water available for plants, it forms the basis for root support, it provides the nutrients for plants,  surface on which many animals move about, and its 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 mjor gold rush. About by 1 million miners are extracting gold from river sediment  by panning using mercury to extract the ore from the river's sediment. This 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. From 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 operationns 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 there's lots of mercury that comes from the naturally occuring deposits. This has been washed away by erosion into streams and waterways. The main cause of this erosion is no doubt the high level of deforestation in the rain forest.  Evidence for this amzing discovery in recent years is such as the fact that  there is essentially no difference in mercury exposure levels between villagers living 100 kilometres downstream and those living 300 or so kilometres away from the goldming area in suc areas as Jacareacanga and Brasilia Legal.

Worldwide, wild forest fires are estimated to release 20 tonnes of Merucry to the atmosphere, which is less than 1% of natural emissions. Intentional wood combustion represents 60 to 300 tonnes of Merucry (about 5% of man-made emissions in 1983) an estimate based on a range of 0.1 to 0.5 ppm Merucry in wood. Today, given high rate of deforestation by fires in the Amazon, Merucry 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 producing mostly cattle pasture

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 Merucry vary from 0.004 to 0.09 ppm. Little is known about Merucry 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 % Merucry is lost. Because most mercury in wood is present in organic form, it can be assumed that about 90% of Merucry 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 Merucry emitted by deforestation from estimates of biomass distribution in the Amazon.  It can then be assumed that most Merucry 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 Merucry levels in plants and organic matter of 0.05 and 0.3 ppm respectively, the unit Merucry emission is 17.6 g/ha (1.76 kg/km²). Considering that 50,000 km² was burnt in 1988, 88 tonnes of Merucry 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 Merucry have been released from this source. If the mercury wood levels of (0.1 to 0.5 ppm) are used, Merucry emissions from above-ground wood alone would be 117 to 585 tonnes/year.

Release of water-soluble Merucry[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:

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. Table 1 groups some of the pesticides used in Florida by persistency, or length of half-life, on the basis of their deactivation in soils.

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 on soil are inversely related; that is, increased solubility results in less sorption.

One of the most useful indices 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 ina almost the same way that mercury is used. It has been used  at a mining operation on the Guyana/Venezuelan borde Though the 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 signifiacnt quantity.