Main sources of contaminants in the environment
Table 1 summarizes the main sources of some priority environmental contaminants.
Table 1 - Sources of environmental contaminants.
| Environmental contaminant | Source |
| Oil and petroleum and PAHS | Oil drilling, petroleum industry, metallurgical industries, combustion processes, ship traffic and tanker accidents |
| Heavy metals | Mineral extraction, metallurgical industries, agriculture |
| Pesticides | Agriculture, industrial areas, recreational areas |
| Organochlorines (PCBs, dioxins, etc.) | Industrial activities, transformers, combustion, toxic waste, etc. |
| Organotins | Antifouling paint, shipyards, marinas, harbours, etc. |
|
Alkylphenols, etc. Estrogenic chemicals |
Industrial detergents, plastics, etc. Sewage and industrial effluents |
Environmental sources of contaminants can be categorized as either:
- Point sources - are discrete discharges of chemicals that are generally measurable and identifiable, such as industrial or municipal effluent outfalls, chemical or petroleum spills and dumps, smokestacks, and other stationary atmospheric discharges.
- Nonpoint sources - are more diffuse inputs over large areas with no identifiable single point of entry such as mobile source emissions (automobiles), agrochemical (fertilizer and pesticide) runoff, atmospheric deposition, desorption or leaching from very large areas (contaminated sediments or mine tailings), and groundwater inflow. Nonpoint sources often include multiple smaller point sources, such as septic tanks, which are impractical to consider on an individual basis.
Other examples (more specific) of point and nonpoint sources of contaminants are shown in Table 2.
Table 2 - Sources (point and nonpoint) of contaminants.
| Point sources | Nonpoint sources |
| Discharges from sewage treatment works to rivers | Runoff and underdrainage from agricultural land into rivers |
| Discharges of industrial wastewaters to rivers | General contamination of recharge rainfall to outcropping aquifers |
| Discharges of farm effluents to rivers | Septic tank soakaways into permeable strata |
| Discharges from small domestic sewage treatment plants to rivers | Washoff of litter, dust, and dry fallout from urban roads to rivers |
| Discharges by means of wells or boreholes into underground strata | General entry of sporadic and widespread losses of contaminants to rivers |
| Discharges of collected landfill leachate to rivers | Seepage of landfill leachate to underground strata and rivers |
Data derived from Lester and Birkett (1999).
Therefore, the characterization and identification of a source is relative to the environmental compartment or system being considered. For example, there may be dozens of important contaminant sources to a river. All these sources must be considered when assessing the hazards of contaminants to aquatic life in the river or to humans who might consume the fish and shellfish or drink the water. Nevertheless, these contaminant sources can be well mixed in the river resulting in a rather homogeneous and large point source to a downstream lake or estuary (Figure 1).
Figure 1 - Contaminants enter the environment through many point and nonpoint sources. Source: Hodgson (2010).
The RATE (units of gram per hour) AT WHICH A CONTAMINANT IS EMITTED BY A SOURCE (mass emission rate) can be estimated from the product of the contaminant concentration in the medium (gram per cubic meter) and the flow rate of the medium (cubic meter per hour). This would appear to be relatively simple for point sources, particularly the ones that are routinely monitored to meet environmental regulations. However, the measurement of trace concentrations of chemicals in complex effluent matrices is not a trivial task. Often the analytical methods prescribed by environmental agencies for monitoring are not sensitive or selective enough to measure important contaminants or their reactive metabolites. Estimating the mass emission rates for nonpoint sources is usually extremely difficult. For example, atmospheric deposition of contaminants to a body of water can be highly dependent on both time and space, and high annual loads can result from continuous deposition of trace concentrations that are difficult to measure. The loading of pesticides from an agricultural field to an adjacent body of water also varies with time and space as shown in figure 2 for the herbicide atrazine. Rainfall following the application of atrazine results in drainage ditch loadings more than 100-fold higher than just 2 weeks following the rain. A much smaller but longer-lasting increase in atrazine loading occurs at the edge of the field following the rain (Figure 2). Hence, there is a need to define the spatial scale of concern when identifying and characterizing a source. If the concern is the fate of atrazine within a field, the source is defined by the application rate. On the other hand, if the concern is the fate and exposure of atrazine in an adjacent body of water, the source may be defined as the drainage ditch and/or as runoff from the edge of field. In the latter case, it is necessary to take appropriate measurements in the field or to model the transport of atrazine from the field.
Figure 2 - The loading of atrazine from an agricultural field to an adjacent body of water is highly dependent on rainfall and on the presence of drainage ditches that collect the chemical and focus its movement in the environment. Source: Hodgson (2010).
