Page 23 - Soil and water contamination, 2nd edition

10 Soil and Water Contamination

quantities of a pollutant are released from a point source and if the pollutant is contained at
or near its location of release. If the pollutants are released over a larger area (diffuse source )
or if a pollutant disperses rapidly over a larger area or volume, the pollutant is diluted and the
enrichment is accordingly smaller.

1.4 ECOLOGICAL IMPACTS

The presence of pollutants in the environment may have adverse effects on the health of both
humans and ecosystems. The effects on individual organisms (including humans) and whole
ecosystems are studied by scientists from various related fields of natural sciences. Toxicology
is the study of the nature and mechanisms of harmful effects of substances on living
organisms, usually Homo sapiens. Ecotoxicology is the study of the nature and mechanisms
of toxic effects of chemicals on living organisms, especially on populations and communities
within defined ecosystems (Butler, 1978). Radioecology is the study of the transfer of
radionuclides through natural and agricultural ecosystems and the effects of environmental
radioactivity on plants and animals, and humans (Alexakhin et al., 2001). All three
disciplines consider the interactions between chemicals and their environment, their transfer
pathways, and their harmful effects. In addition, they assess the hazards (i.e. the potential to
cause harm) and risks (i.e. the probability that harm will be caused) related to exposure to
toxic chemicals, and they develop methods of diagnosis, prevention, and treatment.
The term toxicity is widely used to denote the capacity to cause harm to a living
organism. There are various ways in which toxicity can be measured, but they are nearly all
assessed relative to a particular outcome or end point. Initially, most toxicity tests measured
the number of organisms killed by a particular dose or concentration of the chemical
being tested. Dose is often used where the dietary dose of a test chemical can be accurately
determined; for example, in the case of terrestrial animals, the dose administered (taken
orally, applied to the skin, or injected) is usually recorded. For aquatic organisms or where
the test chemical is dosed onto the surrounding medium, the tests usually measure the
concentration of chemical in the surrounding water. The median lethal dose (LD50 ) and the
median lethal concentration (LC50) describe the level of exposure (dose or concentration)
that kills 50 percent of the population. In recent years, there has been a move away from
the use of lethal end points in toxicity testing towards the measurement of effects other
than death. Examples of such effects are changes in growth (e.g. biomass or body length),
reproduction (e.g. number of offspring), or biochemical or physiological processes (e.g.
enzyme synthesis or respiration ). The median effect dose/concentration (ED50 /EC50)
describes the level of exposure that causes a defined effect to 50 percent of the population.
The no observed effect level (NOEL ) is the general term for the dose (NOED) or
concentration (NOEC) at which a test chemical does not cause an effect that is statistically
significantly different from the control.
In natural systems, organisms are often exposed to more than one pollutant at the same
time. It has often been assumed that the toxicity of combinations of chemicals is roughly
additive, and in many cases this is true. However, in some cases, the resulting toxicity of
more than one chemical is not additive. The presence of other chemicals may decrease
toxicity (antagonism), or increase the toxicity (synergism or potentiation) of a chemical.
When assessing the ecological impact of a chemical it is not enough to consider its
toxicity ; it is also essential to consider its bioavailability . Bioavailability means the rate
and extent to which a substance can be taken up or absorbed into the tissues of organisms
and so influence their physiology. It is possibly the most important factor determining the
extent to which a contaminant in soil or water will enter the food chain. Chemicals enter the
food chain via a variety of pathways. Primary producers (green plants and algae ) and some

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