Adelana reported in 2005 that the word contaminant is a general term to describe dissolved constituents, nonaqueous phase liquids (NAPLs), or industrial solvents which, when added to water as a result of human activities, impair groundwater quality. To an important extent, the complexity of a groundwater contamination problem can be determined by whether or not NAPLs are present. Obviously, contaminants related to anthropogenic activities are one of the major threats facing groundwater resources today.
Groundwater contamination can have a significant impact on the present and future use of groundwater resources as a source of drinking water for an
increasing population living in cities and rural areas. In the investigation of problems posed by contaminants in the hydrologic environment, the modern
approach is to use a combination of tracers. These tracers include radioactive isotopes (such as ³H, ¹³C, ¹⁴C,), stable isotopes (¹⁸O, ²H), and dissolved geochemical constituents (Cl−, SO₄²−, NO₃− and some cations).
An important facet of contaminant hydrogeology is the movement of liquids that are immiscible with water through the vadose zone as well as below the water table. Such liquids are usually called nonaqueous phase liquids. There are numerous cases of groundwater contamination involving organic liquids that are either insoluble or partially soluble in water. When the densities of these liquids are greater than that of water, they are called dense nonaqueous phase liquids (DNAPLs). Those whose densities are less than that of water are light nonaqueous phase liquids (LNAPLs). They could also be partially soluble in water, such that both dissolved and nonaqueous phases may be present.
The major groups of contaminants in hydrogeology include radionuclides, trace metals, and nutrients. They could also be classified as inorganic and organic contaminants. These include synthetic organic compounds, hydrocarbons, and pathogens (7). Some types of these contaminants are inorganic chemicals that are either metallic or nonmetallic. Included in the group of organic contaminants are NAPLs, which naturally subdivide into LNAPLs and DNAPLs.
A wide variety of materials have been identified as contaminants found in groundwater. An extensive listing of these compounds is tabulated in (Reference 7, pp. 3– 10). Many of these materials will dissolve in water to different degrees. Some of the organic compounds are only slightly soluble and will exist in both dissolved form and as an insoluble phase, which can also migrate through the ground. According to (2), the main relevant organic compounds of the LNAPLs are benzene, toluene, ethyl-benzene, and xylenes (BTEX) from spills at gas stations and industrial facilities. The most common compounds of the group found in groundwater are perchloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cis- 1,2-DCE), and dichloromethane (DCA) (9). In terms of environmental contamination, the major concern is focused on those organic compounds that are not readily degraded by bacteria, either in the soil zone or in sewage treatment facilities.
The presence of these substances in the surface environment is becoming increasingly pervasive. The sources that contribute to groundwater contamination are varied. Among these are land disposal of solid wastes, inorganic substances, metal and nonmetal trace elements. Other sources of groundwater contamination are from introducing of salts on roads to combat ice in winter, the infiltration of leachate from tailings in the mining industry, seepage from industrial water lagoons and septic tanks of urban waste disposal systems, wastewater discharge into rivers and lakes, agricultural wastes and fertilizers, saline or geogenic polluted water attracted by overexploitation of groundwater, and fossil fuels. Inorganic cations and anions occur in nature and may come from natural as well as anthropogenic sources.
Some of the radionuclides are naturally occurring; others are man-made and come from the pro- duction and testing of nuclear weapons. There are three important attributes that distinguish sources of groundwater contamination:
1) their degree of localization,
2) their loading history, and
3) the kinds of contaminants emanating from them.
From the work of Domenico and Schwartz contaminants are organized by groups according to reaction type and mode of occurrence. Six categories have been tabulated:
Category 1—Sources designed to discharge substances, for example, subsurface percolation (from septic tanks and cesspools), injection wells (hazardous and nonhazardous wastes), and land application.
Category 2—Sources designed to store, treat, and/or dispose of substances; discharge through unplanned release, for example, landfills, open dumps (including illegal dumping waste), residential or local disposal wastes, waste tailings, material stockpiles (non-waste), graveyards, animal burial, surface and underground storage tanks, and containers.
Category 3—Sources designed to retain substances during transport or transmission, for example,
pipelines and materials transport and transfer operations.
Category 4—Sources discharging substances as a con- sequence of other planned activities, for example, irrigation practices, pesticide applications, fertilizer applications, animal feeding operations, deicing salt applications, urban runoff, percolation of atmospheric pollutants, and mining and mining drainage.
Category 5—Sources providing conduits or inducing discharge through altered flow patterns, for example, production wells (oil/gas wells, geothermal and heat recovery wells, water supply wells), other non-waste wells (monitoring and exploration wells), and construction excavation.
Category 6—Naturally occurring sources whose discharge is created and/or exacerbated by human activity, for example, groundwater–surface water interactions, natural leaching, saltwater intrusions and brackish water upconing, or intrusion of other poor-quality natural water.
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