Characterization of selected organo-nitrogen herbicides in south florida canals: Exposure and risk assessments Original Research Article

Much uncertainty exists regarding the discharge characteristics of terrestrial-use herbicides into aquatic systems. This study evaluated the temporal distribution and concentrations of five commonly used herbicides (atrazine, bromacil, metolachlor, norflurazon, and simazine) in a typical South Florida watershed. Surface water samples were collected weekly over a 3-yr period from four canals and Ten Mile Creek. These systems received drainage water from a variety of land-uses, including residential, pastures, and citrus production. Herbicides were extracted and analyzed by GC-MS/SIM. Atrazine was most frequently detected (87% of samples) in the canal serving the residentially developed sub-basin, with median and maximum concentrations of 0.43 and 6.67 μg L−1, respectively. Norflurazon was most frequently detected (90–95% of samples) in the systems serving agricultural production areas, with median and maximum concentrations ranging from 0.37–0.63 μg L−1 and 1.98–6.97 μg L−1, respectively. Bromacil was detected in 14–36% of samples with median and maximum concentrations ranging from 0.50–0.67 μg L−1 and 2.31–4.96 μg L−1, respectively. Metolachlor was detected in 1.8–10% of the samples, with median and maximum concentrations ranging from 0.16–0.2 μg L−1 and 0.17–1.55 μg L−1, respectively. Simazine was detected in 10–35% of the samples, with median and maximum concentrations ranging from 0.18–0.28 μg L−1 and 0.37–1.35 μg L−1, respectively. Bromacil + norflurazon was the most commonly detected (240 samples of 1060 total) binary combination of herbicides; whereas bromacil + norflurazon + simazine was the most frequently detected tertiary combination (58 samples). While detectable concentrations were present for significant periods of time, risks of acute toxicity were relatively low; affecting < 1% of the potentially affected fraction (PAF) of plant species based on 90th centile exposure concentrations and 10th centile effects concentrations from species sensitivity distributions.