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.
glyphosate, AMPA, environmental half-life, transport mechanism, streams, stream bed sediment, suspended sediment, urban mass flux
 City and County of Honolulu. 2015. Implementation and Monitoring Plan for Kawa Stream Waste Load Allocation for the City and County of Honolulu Municipal Separate Storm Sewer System, NPDES Permit No. HI S000002, Report dated June 2015.
 Oceanit. 2015. Kawa Stream and Ditch Improvements Project, Kaneohe, Oahu. Draft Environmental Assessment prepared for the Department of Design and Construction, City and County of Honolulu by Oceanit Laboratories Inc. Report dated July 2015.
 Oceanit Laboratories, Inc. 2002. Total Maximum Daily Loads of Total Suspended Solids, Nitrogen and Phosphorous for Kawa Stream, Kaneohe, Hawaii. Report prepared March 2002 in conjunction with AECOS, Inc. and the State of Hawaii Environmental Planning Office.
 Yang, X.M., F. Wang, C.P.M. Bento, S. Xue, L.T. Gai, R. van Dam, H. Mol, C.J. Ritsema, V. Geissen. 2015a. Short-term transport of glyphosate with erosion in Chinese loess soil – a flume experiment. Sci. Total Environ., 512, pp. 406–414.
 Williams GM, Kroes R, Munro IC. 2000. Safety evaluation and risk assessment of the herbicide Roundup and its active ingredient, glyphosate, for humans. Regulatory Toxicology and Pharmacology. 31, pp. 117–165.
 Battaglin, W.A., Meyer, M.T., Kuivila, K.M., Dietze, J.E. 2014. Glyphosate and its degradation product AMPA occur frequently and widely in U.S. soils, surface water, groundwater and precipitation. American Water Resources Association, Am. Water Resour. Assoc. 50, 275–290.
 Borggaard, O.K., Gimsing, A.L. 2008. Review. Fate of glyphosate in soil and the possibility of leaching to ground and surface waters: a review. Pest Management Science, 64:441–456 (2008)
 Giesy, JP, Dobson, S. and Solomon, K.R. 2000. Ecotoxicological Risk Assessment for Roundup Herbicide. Reviews of Environmental Contamination and Toxicology, 167: pp. 35–120.
 Bergstrom, L Borjesson, E., E. Stenstrom, J. 2011. Laboratory and lysimeter studies of glyphosate and aminomethylphosphonic acid in a sand and a clay soil. J. Environ. Qual. 40, 98–108. http://dx.doi.org/10.2134/jeq2010.0179.
 Bento, C.P.M., X.M. Yang, G. Gort, S. Xue, R. van Dam, P. Zomer, H.G.J. Mol, C.J. Ritsema, V. Geissen. 2016. Persistence of glyphosate and aminomethylphosphonic acid in loess soil under different combinations of temperature, soil moisture and light/darkness. Sci. Total Environ., 572, pp. 301–311.
 Benbrook, C.M. 2016. Trends in glyphosate herbicide use in the United States and globally. Environ. Sci Eur. 28:3. DOI 10.1186/s12302-016-0070-0.
 Myers, J.P., M.N. Antoniou, B. Blumberg, L. Carroll, T. Colborn, L.G. Everett, M. Hanen, P.J. Landrigan, B.P. Lanphear, R. Mesnage, L. N. Vandenberg, F.S. vom Sall, W.V. Weishons and C.M. Benbrook. 2016. Concerns over use of Glyphosate-Based herbicides and risks associated with exposures: a consensus statement. Environmental Health, 15:19, DOI: 10.1186/s12940-016-0117-0.
 Hanke, I., I Wittmer, S. Bischofberger, C. Stamm, H. Singer. 2010. Relevance of urban glyphosate use for surface water quality. Chemosphere, Vol. 81, Issue 3, pp. 422–429.
 Tang, T., W. Boenne, N. Desmet, P. Seuntjens, J. Bronders, A. van Griensven. 2015. Quantification and characterization of glyphosate use and loss in a residential area. Science of the Total Environment 517, pp. 207–214
 Spengler, S.R., M.D. Heskett, J.I. Gray. 2019. Pesticide Levels in Streams and Sediments on the Islands of Oahu and Kauai, Hawaii. Int. J. Environ. Impacts, Vol. 10, No. 9, pp. 1–17.
 Johnson, A.G. and Kennedy, J.J., 2018, Summary of dissolved pesticide concentrations in discrete surface-water samples collected on the islands of Kauai and Oahu, Hawaii, November 2016–April 2017: U.S. Geological Survey data release, https://doi.org/10.5066/F7BG2N79.
 Lau, L.S. 1973. Quality of Coastal Waters, Second Annual Progress Report. Water Resources Research Center, Technical Report No. 77, September 1973.
 Pitard, F. F. 1993. Pierre Gy’s Sampling Theory and Sampling Practice. New York: CRC Press.
 Brewer, R., John Peard & Marvin Heskett (2017a) A Critical Review of Discrete Soil Sample Data Reliability: Part 1 – Field Study Results, Soil and Sediment Contamination: An International Journal, 26:1, 1–22, DOI: 10.1080/15320383.2017.1244171
 Brewer, R., John Peard & Marvin Heskett (2017b) A Critical Review of Discrete Soil Sample Data Reliability: Part 2 – Implications, Soil and Sediment Contamination: An International Journal, 26:1, 23–44, DOI: 10.1080/15320383.2017.12441724
 Norman, J.E., Toccalino, P.L., Morman, S.A., 2018. Health-based screening levels for evaluating water-quality data. U.S. Geological Survey Web Page, 2d ed accessed September 23, 2019 at. https://water.usgs.gov/nawqa/HBSL doi:10.5066/F71C1TWP
 Hawaii Department of Health, Environmental Management Division. 2017. Evaluation of Environmental Hazards at Sites with Contaminated Soil and Groundwater. Hawaii Edition, https://health.hawaii.gov/heer/files/2019/11/Volume-1-HDOH-2017.pdf
 Medalie, N., T. Baker, M.E. Shoda, W.W. Stone, M.T. Meyer, E.G. Stets, M. Wilson. 2020. Influence of land use and region on glyphosate and aminomethylphosphonic acid in streams in the USA. Science of the Total Environment, Volume 707, March 10, 2020, 136008. https://doi.org/10.1016/j.scitotenv.2019.136008.