Sources of groundwater contamination are usually associated with human activities (Fig. 1.1).
Through these activities, materials or waste may come into contact with groundwater, which may become contaminated. It is true that the majority of sources of groundwater contamination are of human origin; however, contamination may also have a natural origin. Groundwater with its large variations in chemical composition contains elements that can be considered as nutrients or substances essential to human and animal life and plants, but in places it may also contain natural substances that can be harmful to human health and the ecosystem.
Contaminating natural substances usually enter the groundwater cycle through a combination of processes (Chapter 2). On one hand, there is the dissolution of minerals containing toxic elements. Dissolution usually does not act alone, but is assisted by oxidation and reduction processes. Ion exchange, acid-base reactions, precipitation, and complexation are other chemical processes that may influence the quantities of those dissolved constituents that may contribute to, or reduce, contamination. On the other hand, there is also the interaction of infiltrating groundwater with plant remains, peat, and humus in the unsaturated zone. Processes like cell synthesis, organic decomposition, and microbiological growth may also lead to toxicity of groundwater.
The risk of groundwater contamination from harmful natural sources may be described by a further subclassification by type. The natural contamination sources are discussed below according to their chemical and bacteriological type. Throughout the discussion it should be kept in mind that, more so than for anthropogenic contamination sources, the contaminant concentration determines whether a natural source can be considered as threatening groundwater
Groundwater contamination inventory
safety. Most natural, potentially hazardous, sources produce contaminant concentrations that are well below the maximum permissible levels listed in Table 3.4. And some naturally-occurring contaminants are quite innocuous, causing only aesthetic, taste, or odour problems (e.g. iron and manganese).
TABLE3.3 Summary of groundwater contamination sources by origin Category
3.2.1 Inorganic substances
The major inorganic ions in groundwater include sodium, potassium, calcium, magnesium, silica, bicarbonate, sulfate, and chloride (Table 2.1). The distribution of these constituents largely depends on the type of geological formations in contact with the groundwater flowing through.
Most of them are rarely harmful to health but some may cause physical inconveniences if digested in large concentrations (e.g. sulfate). High concentrations of calcium and magnesium compounds cause hardness of water. For people suffering from diseases of the heart or kidneys, it is recommended to avoid drinking water with high concentrations of sodium.
The major ions form the majority of chemical compounds found in groundwater. The summed ion concentration of minerals dissolved in water is referred to as total dissolved solids (TDS). There is no evidence of adverse health effects at TDS levels over 1,000 mg/l, although at about 1,200 mg/l taste problems are likely to arise, and at levels over 1,500 mg/l, gastrointestinal irritation may occur.
Certain minor inorganic constituents may be present in groundwater and may render it unfit for human consumption. Perhaps the two best known examples of such constituents are arsenic and fluoride. Arsenic may be released into groundwater through the reduction of arsenic-containing iron hydroxide coatings on sand grains, which are present in some fluviatile and deltaic river sediments. A case at hand is the release of arsenic in the sedimentary basin of Bangladesh and neighbouring India (see Chapter 8, case study 8.1). The consumption of ground-water with excessive arsenic levels is toxic and may eventually lead to the loss of limbs, cancer, or death. Fluoride may be present in groundwater by the disintegration and dissolution of igneous and metamorphic rocks containing minerals such as amphiboles and micas (Hem, 1970). The drinking of groundwater with high concentrations of fluoride may cause mottled teeth and disturb the growth of bones in children. Extremely high concentrations of fluoride are toxic and could lead to death. The recommended limits for arsenic and fluoride are listed in Table 3.4.
3.2.2 Trace metals
Concentrations of trace metals, including aluminium, cadmium, chromium, cobalt, copper, lead, nickel, silver, zinc, etc., are usually extremely small in groundwater. Trace metals are normally associated with igneous and metamorphic rocks, and in particular, with ore bodies. Weathering of these rocks, including oxidation and leaching, may give rise to elevated trace metal levels in groundwater. However, trace metals may also be brought into the groundwater system by human activities. Naturally-occurring elevated concentrations of trace metals in groundwater (maximum limits are listed in Table 3.4) may locally make this resource unfit for human consumption or affect the natural ecosystem.
Groundwater contamination inventory
TABLE3.4 Maximum permissible concentrations of potentially harmful or objectionable substances in drinking water, in mg/l (Source:WHO Guidelines, 1993)
Constituent Concentration Constituent Concentration
Total dissolved solids 1,500 Iron 0.3
Arsenic 0.01 Lead 0.01
Cadmium 0.003 Manganese 0.05
Chloride 250 Nitrate 50
Chromium 0.05 Selenium 0.01
Copper 2 Sulfate 250
Fluoride 1.5 Zinc 3
3.2.3 Radioactive elements
Groundwater contaminated by radioactive elements is usually associated with human activities such as nuclear power generation and the disposal of nuclear waste. Nevertheless, naturally-occurring radioactive substances are by no means exceptional. Radon, radium, and uranium are found as trace elements in most soils and rocks. They are formed principally by radioactive decay of uranium and thorium isotopes. Sedimentary deposits like shales and clays are known to contain radioactive potassium, and, in places, relatively young intrusive granites, pegmatites, and syenites have elevated levels of radon. Also, various ore bodies contain radioactive minerals. For example, the Morro do Ferro ore body in Brasil contains the radioactive uranium isotopes 234U and 238U, resulting in the presence of the thorium isotope 232Th and the radon isotope 228Ra (Bonotto, 1991).
Disintegration and dissolution of the isotope-containing minerals in the Morro do Ferro ore body has led to groundwater containing excessive levels of radioactive isotopes, in particular radon.
Although excessive levels of carcinogenous radioactive isotopes may be present in groundwater, the normal isotope concentrations in groundwater percolating through sediments and ore bodies are low and cannot be considered as toxic.
3.2.4 Organic compounds
In addition to resulting from human activities, organic compounds may be released in ground-water as a result of natural processes. These processes usually take place near the surface in the humus-containing soil, but may also be present in deeper layers where peat, lignite, coal, or even shallow oil deposits are present and in contact with groundwater. Through metabolism, decay, dissolution, advection, and other processes, organic compounds are formed and become part of the groundwater system. Humic acids, pectins, and hydrocarbons, are amongst the natural organic substances occurring in groundwater (Rail, 1989). In most places, the concentrations of naturally-occurring organic compounds are low and cannot be considered as contaminating groundwater.
3.2.5 Microorganisms
Naturally-occurring microorganisms in groundwater rapidly decrease with depth. There is usually a correlation with the concentrations of organic compounds in groundwater, which also tend to decrease with depth. The few microorganisms found in groundwater pumped from deep wells are usually the so-called chemolithotropic bacteria. Nevertheless, groundwater may contain bacteria that play vital roles in oxidation-reduction processes. Well-known examples are: 1) the bacteria-assisted reduction of carbon dioxide and organic acids into methane, 2) the bacteria-aided reduction of sulfate into hydrogen sulfide, and 3) the bacteria-based oxidation of dissolved iron and manganese into iron and manganese oxides and hydroxides. Especially, the last two reactions are known to produce bacterial slimes, which make groundwater less suitable as drinking water, and which also clog well screens and pumping mechanisms.