Pesticides are used on a regular basis for agricultural, commercial, and domestic protection of plants, wood, soil, and many other products from harmful organisms, and to control the growth of certain vegetation. Each of these uses is a possible source of pesticide entry into the environment. Pesticides may enter surface waters as a result of direct application of herbicides to streams or streambanks to control weed growth. Atmospheric deposition results from precipitation scouring contaminants from the air and depositing them on the ground or directly into the streams. Nonpoint-source runoff of pesticides occurs when rain washes over the ground surface, carrying contaminants with it into nearby streams.
Pesticides can also soak into the ground and move into streams where ground water discharges.
No fungicides were reported at detectable concentrations in the SANT study area. Nonagricultural pesticide usage is not as well documented as agricultural use. Domestic pesticide application on lawns, home gardens, or around the home are not included in these estimates unless sold to a registered applicator. Chlorpyrifos is used in urban and agricultural settings for control of a wide variety of insects. Atrazine and alachlor are commonly applied to corn and soybeans during planting, and 2,4-D is applied prior to planting as a general herbicide.