Off‐Field Movement and Dissipation of Soil‐Incorporated Carbofuran from Three Commercial Rice Fields

Residues of curbofuran (2,3‐dihydro‐2,2‐dimethyl‐7‐benzofuranyl methylcarbamate) were detected in agricultural drain water collected in the Sacramento Valley, a major rice ( Oryza sativa L.) growing region of California. Runoff water from rice fields was suspected as the major source for these residues. In response to this problem we measured soil‐incorporated carbofuran; (i) discharged in runoff, (ii) dissipated from paddy soil and water, and (iii) recovered in paddy soil vs. paddy water over time. This work was conducted in three commercial rice fields in Colusa and Glenn Counties in California. A total of 1.72, 5.40 and 11.03% of carbofuran mass applied was discharged in runoff water from Fields 1, 2, and 3, respectively, during a 54‐ to 80‐d period after flooding. Downstream concentrations, up to 4.4 µ g L −1 , indicated the need for reduced carbofuran discharges to meet a 1991 regional standard of 0.4 µ g L −1 . Most of the carbofuran lost in runoff was discharged during the first 30‐d period after initial flooding in Field 2, and during the 31‐ to 60‐d period in Fields 1 and 3. Dissipation of carbofuran mass, measured separately in rice paddy soft and rice paddy water, was a log‐linear function of time in both media. An exception occurred in paddy soil of Field 3 where carbofuran mass did not decline significantly over a 70‐d sampling period. Soil half‐lives were 58 and 43 d after flooding for Fields 1 and 2, respectively. Water half‐lives for Fields 1, 2, and 3 were 22, 26, and 18 d, respectively. Most of the carbofuran mass applied to the fields remained in paddy soil and, on average, no more than 27% of the applied mass was found in paddy water on any single day during the study. Soil‐incorporation may reduce the amount of carbofuran released into paddy water, and subsequently moved off‐field in runoff.