Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2001
Publication Date: 4/20/2001
Citation: Potter, T.L., Wauchope, R.D., Culbreath, A.K. 2001. Accumulation and decay of chlorothalonil and selected metabolites in surface soil following foliar application to peanuts. Environ. Sci. Technol. 35:2634-2639. Interpretive Summary: Calendar-based fungicide application is the most effective disease management strategy available to peanut producers. Chlorothalonil is by far the most widely used for this purpose. Risk assessments conducted by U.S. EPA have indicated that runoff may carry the compound to surface waters and cause adverse impacts. Our study focused on evaluating chlorothalonil accumulation and degradation in soil during the growing season. Rates of these processes control the amount available for runoff. Soil samples were collected following each chlorothalonil spray. They were analyzed for chlorothalonil and incubated in the laboratory and periodically analyzed to determine soil degradation rates. As the plant canopy increased more and more of the chemical was intercepted by the leaves and less reached the soil. This indicated that using directed spray technology when plants are small could decrease soil loading and reduce environmental risks. Study results also showed that degradation rates in peanut soils were much faste than reported in other studies. Use of more rapid degradation rates in risk assessments should reduce overall estimates of risk. During the degradation studies, a series of degradation products were detected which were more stable than the parent compound. The stability of these compounds has indicated that an evaluation of their environmental fate is needed.
Technical Abstract: Peanuts were grown following common agronomic practices in a major peanut production area in south central Georgia. Composite surface soil samples were collected after each of 7 chlorothalonil sprays made at 14-day intervals. At the start of and during laboratory incubations the soil was analyzed for the parent compound and degradates by HPLC-PDA-APCI-MS. After the first two sprays 30 to 35 % of the active ingredient applied was detected in the soil. Levels decreased 10 to 20 times after plant canopy closure. After the last spray, <5 % of the cumulative chlorothalonil applied was detected in the soil. Foliar interception and dissipation and rapid soil degradation contributed to low residue levels. Soil half-lives were <1 to 3.5 days. Values for the principal degradate, 4-hydroxy- chlorothalonil, were 10 to 22 days. Daughter products of this compound were also detected. Levels were highest in the last sample collected indicating temporal accumulation. Results emphasized the plant canopy's role in controlling the amount of residue, which reaches the soil surface. Soil incubations suggested concentration dependent chlorothalonil degradation with degradation rates increasing as soil loading decreased. The study has indicated that the 30-day field dissipation half-life used in recent chlorothalonil risk assessments was highly conservative in the context of peanut production in the Southeastern USA and that monitoring programs should place more emphasis on degradates. They are much more stable than the parent compound.