Volatilization, Off‐Site Deposition, and Dissipation of DCPA in the Field

Residues of dimethyl, 2,3,5,6‐tetrachloroterephthalate (DCPA) have been found repeatedly in recent years on a variety of produce grown in California to which this herbicide had not been applied. In response to this problem, drift, volatilization, and dissipation of DCPA were investigated in a circular plot seeded with bunch onions [ Allium cepa (L.) var. White Lisbon], and off‐site deposition was measured in a surrounding area seeded with parsley [ Petroselinum crispum (Mill.) Mansf. var. Green Modified Curl Leaf]. Contamination by residual soil residues remaining on field after harvest was also examined by reseeding the circular plot with parsley. Atmospheric residues detected downwind on resin and filters indicated DCPA moved off‐target as a vapor as well as on particles both daring and up to 21 d after application. Volatilization flux, measured using the aerodynamic method, reached a maximum rate of 5.6 g ha −1 h −1 . An estimated 10% of the DCPA applied was lost to the atmosphere within 21 d of application. Deposition of DCPA outside the circular plot was evidenced by residues found on potted parsley plants and soil set out up to 23 m from the treated area. Parsley, seeded around the circular plot at the time of application and sampled 63 d later, contained residues ranging from 51 to 250 µ g kg −1 indicating DCPA continued to move off‐target in air up to 60 d after application. The mass of DCPA in soil exhibited a log‐linear decline from which a 50 d dissipation half‐life was calculated. Parsley planted in the circular plot after the onion harvest did not contain DCPA residues when sampled 217 and 336 d after the original application. Results indicated that drift during, and volatilization and subsequent atmospheric transport after, application are potentially important sources of DCPA contamination occurring on nontarget crops in California.