DDT DISSIPATION IN HAWAIIAN IN-SITU SOIL COLUMNS

[C-14]DDT persistence and movement was monitored for 25 months after t reatment of in-situ soil columns in Hawaii, USA. This was part of an i nternational collaborative protocol coordinated by the International A tomic Energy Agency and carried out at 13 tropical locations. DDT meta bolite DDE, and herbicide tebuthiuron, were included in the Hawaiian t est for comparison with DDT. Persistence based on overall loss of radi oactivity was described by first-order and two-compartment models: hal f-dissipation times were 53 and 25 weeks, respectively. The second mod el represented more accurately the rapid loss of C-14 during the first month after treatment. Time to 50% loss based on analysis of methanol -extractable C-14 was 66-68 weeks (both models), or somewhat shorter ( 40-53 weeks) when C-14 was measured in the final hexane solution befor e gas chromatographic (GC) analysis. Direct GC analysis of that soluti on gave 24 weeks (first-order) and 13 weeks (two-compartment) for DDT alone, and 37 weeks and 27 weeks, respectively, for the sum of DDT + D DE + DDD residues. After 10 months, further dissipation of either radi oactivity or of DDT itself was very slow; the two-compartment model pr edicted 90% loss of methanol-extractable C-14 in 11 years and of DDT, in 3 years. Within 18 weeks post-DDT-treatment, metabolite DDE had rea ched a stable concentration of ca. 7% of added DDT. No DDD accumulated . Bound residue content was 10-15% by 2 years. [C-14]DDT and other rad ioactivity moved below the 10-cm leaching column by 3-4 months. By 25 months, 3% of the originally applied C-14 occurred deeper than 10 cm, or about 8% of the total C-14 then recovered. Little or no radioactivi ty migrated below 17.5 cm. Rainfall at the site averaged ca. 2500 mm y (-1). Tebuthiuron was less persistent than DDT; estimated half-life wa s ca. 12 weeks and the herbicide was much more mobile.