High-frequency sampling and analysis was conducted for the broad-spectrum herbicide glyphosate (Roundup) in ephemeral and perennial streams receiving storm water runoff generated within agricultural, urban and mixed-used watersheds on the island of Oahu, Hawaii. Glyphosate was selected for analysis since it is the most widely used herbicide in the world, and as a result, tends to be ubiquitous in the environment and our food supply. Samples were collected under both baseflow and storm conditions from five streams. The pervasiveness and maximum concentration levels of glyphosate detected in these streams are greater than any other pesticide currently present in Hawaiian streams. Glyphosate was detected in 96% and 65% of the stream samples collected during storm events (53 samples) and under baseflow conditions (34 samples), respectively (detection limit = 0.05 µg/L). The mean glyphosate concentrations measured in stream samples collected under storm conditions were between five to fifty times higher than mean glyphosate levels measured in the same stream under groundwater dominant baseflow conditions. The highest glyphosate concentrations were measured during a small runoff event in Manoa stream which flows through residential communities in urban Honolulu. The mass of dissolved phase glyphosate measured in stream water during the individual storm events monitored ranged from 0.5 to 18 grams. Between 11% and 23% of the total glyphosate load was present in suspended sediment during three sampled storm events in Honouliuli, Waimanalo and Kawa streams. The estimated total mass of dissolved phase glyphosate that discharged into Kaneohe Bay from Kawa Stream over a four-month monitoring period from December 2017 to March 2018 was 987 grams, with 92% of the pesticide load entering under storm conditions.
Occurrence and persistence Of the herbicide glyphOsate in a suburban trOpical watershed steVen r. spengler 1 , MarVin d. hesKett 1 , saMuel c. spengler 2 1 element environmental, 2 united states forest service 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.
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