A small-scale ground water leaching study for a primisulfuron herbicide

Primisulfuron is a sulfonyl urea herbicide designed to control broad-leaf weeds in fields of small grain crops. The pesticide is potent, requiring only a low dosage. It is designed to replace many existing pesticides that are suspected to contaminate the nation's ground water resources. Under the auspices of the Federal Insecticide Fungicide and Rodenticide Act (FIFRA), the pesticides must be demonstrated to be environmentally safe in a field study for registration with the U.S. Environmental Protection Agency (EPA). This paper presents the design and results of a ground water leaching study for the successful registration of primisulfuron. A systematic approach was employed to identify a suitable site for the ground water study. Agricultural, ground water, and soil databases were carefully reviewed to quickly determine, from a national level down to an individual farmer, a suitable candidate site. The site selected had a shallow ground water table and a highly permeable soil matrix. The site ground water was highly vulnerable to pesticide contamination exceeding EPA site requirements. A total of eight monitoring wells, 12 lysimeters, four tensiometers, and four rain gauges were installed in four sampling clusters to collect ground water and soil-pore water samples. Bromide was used as a tracer to follow the movement of infiltrating water. The results showed that the downward migration of primisulfuron was retarded by the site soils. The average breakthrough time for the pesticide at 3-foot lysimeters was equal to 109 +/- 19 days, which was 36 days longer than that of bromide (73 +/- 12 days). The retardation was apparently caused by the adsorption of primisulfuron to the site soils, which could be attributed to the hydrophobicity of the compound and the organic matter of the site soils. The field half-life for primisulfuron was determined to be 5.1 days, which was much shorter than the 14 days required by EPA. It was concluded that primisulfuron did not leach to the ground water table at the site under normal use conditions.