Fate and acute toxicity of bromoxynil esters in an experimental prairie wetland

The partitioning, degradation and toxicity of a single spray application of a 1:1 mixture of bromoxynil octanoate (BO) and bromoxynil butyrate (BB) were studied in fifteen 0.01-ha ponds located in the Delta marsh area of Manitoba. Intensive sarnpling of two ponds treated at 2.5 μg/L and two treated at 50 μg/L (nominal concentrations) showed that BO and BB persisted in the surface microlayer (0–1 mm) of the water at levels 50- to 100-fold higher than the intended treatment level, with half-lives of 0.8 to 2.5 h. In subsurface waters (10–20 cm depth) the major forms of the herbicide were bromoxynil-phenol and its monobromo- analog (3-bromo-4-hydroxybenzonitrile; MBBP). Half-lives of the phenol averaged 9.2 d in ponds treated at 50 μg/L and 14.5 d in those receiving 2.5 μg/L. Low ng/g (dry wt.) levels of BO, BB, MBBP and bromoxynil-phenol were present in bottom sediments (0–2 cm depth) during the first 15 d posttreatment and declined to near detection limits (<0.5 ng/g) by 120 d after spray application. Complete mortality of brook sticklebacks (Culaea inconstans), held in cages in subsurface waters, occurred within 24 h at the two highest treatment levels (nominal concentrations of 100 and 500 μg/L). Mortality of caged Hyalella azteca at the highest dose levels ranged from 85 to 95% at 50 h.