Use of Rodeo® and X‐77® spreader to control smooth cordgrass (Spartina alterniflora) in a southwestern washington estuary: 1. Environmental fate

Abstract

AbstractA 1‐ha plot with smooth cordgrass (Spartina alterniflora) at three locations in Willapa Bay, a southwestern Washington estuary, was aerially treated with 4.7 L/ha Rodeo® and 0.9 L/ha X‐77® Spreader to determine the fate of the herbicide formulation. Rates of spray deposit on filter pads placed over treated intertidal mudflats did not differ among locations for glyphosate and aminomethylphosphonic acid (AMPA); however, deposit rates for nonylphenol polyethoxylates (NPEO) did differ among locations. Glyphosate concentrations in sediment cores from treated mudflats declined 51 to 72% during 119 days posttreatment (DPT); NPEO concentrations in sediment declined (42%) between spray day and 14 DPT. The highest concentrations of glyphosate, AMPA, and NPEO in seawater were found in off‐site samples collected from the leading edge of the first high tide after application. Glyphosate and AMPA concentrations in depth‐integrated seawater samples declined 73 and 42%, respectively, between the first high tide immediately following application and the second high tide at 1 DPT. Glyphosate concentrations in Spartina stems from treated plots declined 91 to 99% between 1 and 28 DPT; whereas, AMPA declined (86 and >96%) during this period. Comparison of maximum concentrations for glyphosate in seawater from this study with acute toxicity values in the literature indicates that under worst‐case conditions direct effects to aquatic organisms would not be likely.