Degradation of Atrazine, Alachlor, Metolachlor in Soils and Aquifer Materials

Introduction: The United States Environmental Protection Agency (USEPA) provide criteria for identifying and evaluating the source and spread of nonpoint pollution. Nonpoint pollution is the movement of pollutants from diffuse sources into surface or subsurface waterbodies and is among the leading causes of water quality problems (Chesters and Schierow, 1985). Pesticides are among the most important nonpoint pollutants carried from agricultural fields to surface waters or infiltrated into groundwaters. In the last decade, the occurrence of pesticides in ground water has been confirmed (Cohen et al., 1984; Cohen et al., 1986; USEPA, 1986). The USEPA’s Office of Ground Water Protection compiled a list of 17 pesticides in 23 states found in groundwater due to normal land applications (USEPA, 1986). Pesticides in groundwater cause public concern about the quality of drinking water, which has led to renewed pesticide monitoring and research and the call for stricter health advisories and water quality standards (Ehart et al., 1986). Pesticides may also escape to the atmosphere by volatilization (Taylor, 1978; Jury et al., 1980).

In several regions of Wisconsin, evidence shows the presence of a variety of herbicides in groundwater including atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], metolachlor [2-chloro-N-[2-ethyl-6-methylphenyl]-N-2(2-methoxy-1-methyl) acetamide], and alachlor [2-chloro- 2′,6′-diethyl-N-(methoxymethyl) acetanilide]. They are extensively used in corn, potato and soybean production (WDATCP, 1986). A groundwater pesticide monitoring program in Wisconsin has shown detectable amounts of alachlor in 75 of 830 wells sampled, of metolachlor in 59 of 510 wells, and of atrazine in 152 of 752 wells (WDNR, 1988). Under normal agricultural conditions, 4 of 14 wells were contaminated with alachlor, 4 of 10 with metolachlor and 5 of 13 with atrazine (Postle, 1987). Groundwater contamination by these herbicides has led to increased public pressure to regulate their use because of potential adverse effects on human health; alachlor is listed as a probable human carcinogen while metolachlor and atrazine have shown indications of carcinogenicity to rats. Recently, groundwater enforcement standards for alachlor, atrazine, and metolachlor were set for Wisconsin at 0.5, 3.5, and 15 μg/l (ppb), respectively.

Alachlor, metolachlor and atrazine are now under close scrutiny by the state of Wisconsin and federal agencies with regard to their environmental fate and toxicological significance to humans, livestock, and ecosystems. The objective of this investigation is to evaluate transport of the herbicides and include volatilization, sorption, plant uptake, and degradation in a mass balance.