|Banks, Philip - MARATHON AG ENV CONSULT|
Submitted to: Herbicides
Publication Type: Book / Chapter
Publication Acceptance Date: September 25, 1998
Publication Date: N/A
Technical Abstract: Although the majority of an applied triazine herbicide remains in the surface soil where it can control weeds while it degrades, offsite movement has been well documented. This chapter reviews soil movement and environmental fate of triazine herbicides. Less than 3 percent of applied triazines are lost by spray drift during application, volatilization from the field, and on suspended sediments that are blown from treated fields, which can be deposited off-site in dust or rainfall. The cumulative effects of many small runoff events (less than 2 percent of applied chemical) can result in a large total amount of triazine entering a surface water body; for instance, an estimated 429 metric t of atrazine annually enters into the Gulf of Mexico from the Mississippi River. In various surveys, triazines were detected in 4.9 percent of the wells sampled, but the concentrations were generally low (MCL was exceeded in 0.1 percent of samples). Contamination of groundwater is the result of both point and nonpoint source pollution. Numerous field studies have shown that triazines leach into the vadose zone, tile drains (0.1 percent of applied triazine), or groundwater, however, the majority of the triazine is retained bound and degraded in the surface 50 cm of soil. Triazine persistence is usually characterized by calculating a half-life (t1/2). Half-life values range from 14 to 112 d with a mean t1/2 equal to 36 plus or minus 25 d. Triazines can persist for long periods after application at low levels; however, it appears that they do not accumulate in soil after long-term use. Triazine persistence has been shown to be affected by many factors, including concentration, physical, chemical, and biological properties of soil, and climatic variation from year to year.