These pesticides are insecticides and have rarely contaminated groundwater. Originally, they were thought to be safe to the environment but later were discovered to accumulate in the environment and build up to toxic concentrations in the food chain. The use of many of these pesticides has been restricted, suspended or canceled. Organophosphorus compounds such as malathion, diazinon and parathion are one group of pesticides replacing chlorinated hydrocarbons. Although some are highly toxic to humans, they generally break down rapidly in the environment and are rarely found in groundwater.
Carbamates are a fourth group of pesticides.
They include aldicarb, carbofuran and oxamyl. These compounds tend to be soluble in water and weakly absorbed to soil. Thus, if they are not degraded in the upper soil layer, they have a tendency to migrate t groundwater. The most significant occurrences of groundwater contamination have been with carbamate pesticides. There are many other groups of pesticides too numerous to describe in detail.