Pesticides are poisons designed to destroy unwanted life forms. Used properly, modern pesticides can perform their functions without causing significant hazards to humans or the environment. Pesticides have many uses in homes, gardens, farms, forests and public health. It is difficult to imagine what life would be like without modern pesticides. This discussion is continued from BioTalk 15.

Herbicides in Surface and Ground Water
Herbicides vary widely. Some are water soluble enough to enter lakes or streams with rainfall or runoff irrigation water, but the hazard they represent depends on their persistence and interaction with the soil. They can also leach into ground water or move with eroding soil. Many herbicides designed to be applied to emerged plants are inactivated once they reach the soil surface. Soil-applied herbicides, however, must be soluble in soil water in order to move into the root zones of target weeds. Some move deeply into the ground to kill deep-rooted perennials. Others don't move as deeply in order to kill shallow-rooted weeds and spare a deeper rooted crop.

Among the soil-applied herbicides that are taken up by plant roots are the triazines. Several of these have been detected in surface and ground waters. Some triazines are very stable in the environment and may persist for long periods in the soil.

Fungicides

Fungicides are used to control microorganisms. We could not feed this country without modern fungicides to control plant diseases. Moreover, toxic plant disease organisms would make food far more dangerous than fungicide residues at the maximum levels prescribed. If you want to save your lawn, crops, garden or ornamental trees and shrubs, you must use fungicides.

Fungicides are of small concern in protecting water quality. They are used less frequently than other pesticides, and most are not persistent. However, they can be a possible source of pollution if applied, stored or disposed of improperly. Even when applied correctly, these substances can drift away from the application area, leach into ground water and be carried away by runoff. The table below lists some fungicides commonly used by homeowners and farmers, and in industry. Fungicides are seldom found in water, with the exception of some of the heavy metal fungicides that contained mercury. The EPA has cancelled most uses of these fungicides. When using a fungicide, always follow instructions on the label to minimize the risk of water pollution.

Fungicide / Hazards: Mancozeb / Cancer (Ethylenethiourea); Thiram / Nerve poison, birth defects; Benomyl / Birth defects; Thiophanate / Mutations, birth defects; Pentachloronitrobenzine / Accumulates in food chains, hormone effects; Phenyl mercuric acetate / Heavy metal poisoning; Fixed Copper / Toxic to plants and phytoplankton; Kitazin-P / Nerve poison; and  Streptomycin / Allergic reaction.

This completes the discussion of herbicides and water quality. The editor would like to thank Dr. John A. Jackman, Professor and Extension Entomologist from the Texas A&M University, who has very kindly allowed that use of his excellent paper.