Time-Dependent Toxicity Assessment of Herbicide Contaminated Soil Using the Green Alga Selenastrum capricornutum

Bioassays utilizing the green alga Selenastrum capricornutum were performed on filtered eluates from soil treated with six commonly used forestry herbicides applied at labelrecommended rates. The bioassays were conducted at three time periods after herbicide application—one hour, five days, and ten days. The 96-h EC₅₀ values indicated growth inhibition (relative to control sample) for all treatments when assayed 1 h after herbicide application. Algal EC₅₀ values of +100 (Control), +27.3 (Roundup™), -20.4 (Arsenal™ [2 lb Acid Equivalent] [AE]/gal), -22.4 (Garlon™ 4), -49.4 (Tordon 101M™), -100 (Velpar L™), and -100 (Velpar ULW™) were obtained. Assays conducted ten days after herbicide application to soil revealed substantially reduced toxicity of two herbicides. The 96-h EC₅₀ values for Roundup and Arsenal were both +100. There was a significant enhancement effect observed with Roundup. A slight reduction in toxicity was noted for Garlon 4 (-15.9) and Tordon 101M (+9.9). No change in toxicity occurred for Velpar L or Velpar ULW. The herbicides were also applied to water and the following 96-h EC₅₀ values in μg/ml were obtained: 5500 (Arsenal [2 lb AE]), 5300 (Arsenal [4 lb AE]), 5000 (Tordon 101M), 5000 (Garlon 4), 2600 (Roundup), 2.5 (Velpar L), and 1.2 (Velpar ULW).