Application of a glyphosate-based herbicide to Phragmites australis: Impact on groundwater and near-shore lake water at a beach on Georgian Bay

During the past decade, the invasive Phragmites australis (common reed) has established itself along beaches of the Great Lakes, causing detrimental impacts to both the natural ecological integrity of the shoreline and the recreational value of beaches. The herbicide Roundup®, containing the active ingredient glyphosate, was applied to Phragmites along a beach on the southern shore of Georgian Bay, Canada, to eradicate the Phragmites which was destroying the natural beach ecosystem. Groundwater and lake water were tested to determine if glyphosate enters the groundwater and lake at the beach and how long glyphosate will persist. Two days after application, the geometric mean concentration of glyphosate in the groundwater below the Phragmites was 0.060μg/L with a maximum of 12.50μg/L. Concentrations rapidly declined over the next two to three weeks to below minimum detection limits (<0.020μg/L). Glyphosate was also detected in the nearshore lake water with concentrations peaking at a geometric mean of 0.14μg/L one week after application, and declining to 0.039μg/L four weeks after application. Concentrations of glyphosate never exceeded the Canadian water quality guideline for the protection of aquatic life (65μg/L) in either the groundwater or lake water. An approximate half-life for the dissipation of glyphosate by degradation and dilution/flushing as groundwater flows toward the lake, assuming a first order kinetic reaction, yielded a half-life of 3.5 during the 4weeks after the herbicide was applied. The application of Roundup® resulted in an 90% reduction in the size of the stand of Phragmites.