DDT Residues in Mountain Stream Water as Influenced by Treatment Practices1

Abstract

During 1961 and 1962, a mountainous 4000-acre oakhickory forest watershed was sprayed with DDT to control the elm spanworm, Ennomos subsignarius (Hübner). The sprayings were made over the entire basin by fixedwinged aircraft in 1961. The next year sprayings were made by helicopter and were restricted to 2000 acres of upper slopes and ridges, DDT residues were noted in stream water when the entire basin was sprayed in 1961. The highest concentration of DDT recovered (0.346 parts per billion) was observed in a sample taken during treatment. The minimum concentration (0.005 parts per billion) was noted 2 months after application, in the last sample collected. Thc fol-lowing year, when only 49% of the basin was sprayed, no DDT residues were noted in stream water and suspended sediment samples collected over a 7-month period. The results of this study and other investigations indicate that the probability of DDT occurring as stream contaminant is greatly reduced by its use in controlled spot-applications.