Author(s)

Steven R. Spengler, Marvin D. Heskett, Samuel C. Spengler

Abstract

Stream, streambed sediment and suspended sediment sampling for the herbicide Glyphosate was conducted in a small, 4.05-square kilometer suburban watershed on the island of Oahu, Hawaii between December 2017 and April 2020. Over this 2.5-year study period, a total of 188 stream samples (142 runoff conditions, 46 baseflow conditions), 81 streambed sediment samples, and 9 suspended sediment samples were collected and analysed for glyphosate and a subset of sediment samples were analysed for its degradation product aminomethylphosphonic acid (AMPA).

The glyphosate concentration levels measured during stormwater runoff conditions within Kawa stream were significantly higher than levels measured under groundwater dominant baseflow conditions. The mean and median glyphosate concentrations (µg/l) and the frequency of glyphosate detection (reporting limit 0.075 µg/l) measured in Kawa stream under runoff and baseflow conditions were 0.98/0.51/92% and 0.10/0.035/28%, respectively. the glyphosate concentrations measured in this small suburban tropical stream were significantly higher than mean levels measured by the USGS between 2014 and 2020 in streams that drain small urban watersheds throughout the continental United States. The glyphosate concentration levels measured in riverbed and suspended sediments in Kawa stream were generally two to three orders of magnitude higher than levels measured in stream-water.

The  majority of glyphosate (>90%) was transported to Kaneohe Bay in the dissolved phase and originated from residential areas within the contributory watershed. The mean mass flux of glyphosate measured entering the near coastal environment under baseflow conditions was around 0.16 mg/min, while the mean mass flux during runoff conditions was 106 mg/min. The estimated median half-lives of glyphosate and AMPA measured in streambed sediments during this study were 4.7 and 6.2 days, respectively. This short half-life (4.7 days) along with the high-frequency (92%) of glyphosate detection in Hawaiian streams under runoff conditions illustrates the steady, unceasing input of glyphosate to Hawaiian streams.

Keywords

glyphosate, AMPA, environmental half-life, transport mechanism, streams, stream bed sediment, suspended sediment, urban mass flux.