Pesticides in Water
If the number of fish killed per year in the streams of the United States were the criteria of water quality problems, then first place on the list of pollutants would be pesticides.
Although pesticides are not usually considered as a chemical characteristic of water, they are discussed briefly here because of their potential as serious water pollutants.
The common pesticide chemicals can be categorized chemically into three groups:
(a) inorganic, the inorganic arsenicals, mercurials, borates and fluorides;
(b) synthetic organic, the synthetic organics include chlorinated hydrocarbons, organic phosphates and thiocarbamates and
(c) natural organic, and natural organic rotonone, pyrethrum and nicotine.
Pesticides may also be classified by their biological usefulness, in other words, algaecides, fungicides, herbicides, etc. Other practical means of classification are also available such as the form of use, attractant, fumigant and repellant, etc.
In general, the inorganic pesticides are toxic to aquatic plants and fish as well as insects. Some formulations are more toxic to mammals than to insects. Inorganic pesticides have been supplanted largely by organics that can be adapted to more specific use. The natural organics are somewhat limited in supply and popularity.
Pesticides may gain access to the ground and surface waters through direct application, through percolation and runoff from treated areas and/or through drift during application.
Gross over application of pesticides is a common practice that augments the pollution affects of pesticides to a considerable extent. Although knowledge is lacking regarding the long-term effects of pesticides on our water resources, it is clear that these substances with their accompanying dilutants and solvents are entering the natural waters in ever increasing amounts. It must be remembered when dealing with pesticides as water pollutants that any specific pesticide will rarely if ever be encountered as the sole, solitary pollutant.
Many solvents, dilutants and other carriers used in the pesticide also have toxic properties. For instance, the light petroleum oils used in household insect sprays contain solvents such as xylene, fuel oil or kerosene which each have some toxicity. This effect is believed to be particularly evident in aquatic environments when solvents have an opportunity to be emulsified by riffles in streams.
When considering pesticide pollution of water, toxicity is not the only aspect of concern. There are also difficult problems posed by tastes and odors. Some synthetic organics cause highly objectionable tastes and odors as do many of their solvents.
When the complexity of various formulations are added to the differing environmental conditions and inherent biological variations, it is not surprising to find that there is a wide variation in reported values of toxicity levels for these compounds to various aquatic animals.
Although recognizing that it is important to duplicate test conditions such as dissolved oxygen, pH, temperature, time, hardness of water, etc., several studies have indicated that:
(a) not enough variables are included;
(b) ecological factors are not considered sufficiently, and
(c) chronic effects of sublethal dosages are not considered.
They conclude, therefore, that a great deal of additional information is necessary in order to assess the impacts of pesticide pollution.