A study of the aquatic fate of the triethylamine salt of triclopyr (3,5,6‐trichloro‐2‐pyridinyloxyacetic acid) was conducted in three bays of Lake Minnetonka, Minnesota. Triclopyr is under review by the US Environmental Protection Agency as a selective aquatic herbicide. The primary purpose of this study was to determine dissipation rates of the parent active ingredient, triclopyr, and its major metabolites, 3,5,6‐trichloropyridinol (TCP) and 3,5,6‐trichloro‐2‐methoxypyridine (TMP) in selected matrices including water, sediment, plants, finfish and shellfish. Two 6.5‐ha plots dominated by the weedy species Eurasian watermilfoil (Myriophyllum spicatum L) were treated with triclopyr‐triethylammonum at a rate of 2.5 mg AE liter−1 (2.5 ppm) on 21–23 June 1994. A third 6.5‐ha plot was established as an untreated reference. Water and sediment samples were collected from within the plots and at selected locations up to 1600 m outside of the plots through six weeks post‐treatment for chemical residue analysis. In addition, residue samples were collected from the target and non‐target plants and other non‐target matrices, including game and rough fish, clams and crayfish. All test animals were sequestered in cages located in the center of each plot and samples were collected through four weeks post‐treatment. Half‐lives for dissipation of triclopyr and TCP in water ranged from 3.7 to 4.7 days and from 4.2 to 7.9 days, respectively, with trace amounts of TMP found. Peak triclopyr sediment values ranged from 257 to 335 ng gram−1, with a mean half‐life of 5.4 days, while peak TCP sediment levels ranged from 27 to 65 ng gram−1 (mean half−life = 11.0 days). Trace levels of TMP were detected at one treatment site at one sampling event. Triclopyr and TCP accumulated and cleared from animal tissues proportionately to concentrations in the water (triclopyr dissipation half‐lives <11 days, TCP < 14 days). TMP levels were two to three times higher than those of the other compounds, particularly in visceral tissue. In all cases, residues of these compounds were higher in the inedible portions of the animals, and were usually higher in bottom‐feeding fish species. © 2000 Society of Chemical Industry

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In an effort to evaluate the selective control of the exotic weed Eurasian water milfoil (Myriophyllum spicatum L.) and to assess the recovery and restoration of the native submersed plant community, a 6‐ha river and 4‐ha cove plot were treated with the herbicide triclopyr at application rates of 2·5 and 1·75 mg/l, respectively, in the Pend Oreille River, WA, in August 1991. Water exchange half‐lives within the plots were measured using rhodamine WT dye (river, t1/2=20 h; cove, t1/2=52 h), and triclopyr dissipation rates were also calculated (river, t1/2=19 h); cove, t1/2=53 h). Triclopyr concentrations were below the proposed potable water tolerance level (0·5 mg/l) within the river treatment plot by 3 days after treatment (<0·01 to 0·41 mg/l), and 675 m downstream of that plot by 1 day after treatment (<0·01 to 0·47 mg/l). Following the cove treatment, triclopyr residues ranged from 0·12 to 0·29 mg/l by 7 days after treatment, and from<0·01 to 0·06 mg/l as close as 150 m downstream from the plot. Eurasian water milfoil biomass was reduced by 99% in the treated plots at 4 weeks post‐treatment, remained low one year later (river treatment, 28% of pretreat levels; cove treatment 1% of pre‐treat levels) and was still at acceptable levels of control at two years post‐treatment (river treatment, 47% of pre‐treat levels; cove treatment, 24% of pre‐treat levels). The four‐week post‐treatment efficacy results verified triclopyr concentration/exposure time relationships for controlling Eurasian water milfoil developed under laboratory conditions. Non‐target native plant biomass increased 500–1000% by one year post‐treatment, and remained significantly higher in the cove plot at two years after treatment. Native species diversity doubled following herbicide treatment, and the restoration of this robust community delayed the re‐establishment and dominance of Eurasian water milfoil for three growing seasons. ©1997 John Wiley & Sons, Ltd.

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Fluorescent dye was applied concurrently with triclopyr in two 6.5-ha treatment plots in Lake Minnetonka, MN for data collection to support full aquatic registration of this herbicide. The herbicide and dye mixture was applied by airboat to Phelps Bay with weighted, trailing hoses to maximize uniform distribution of these materials in the water column. A surface application was made to the Carsons Bay plot to attain theoretical triclopyr and dye concentrations of 2500 and 10 micrograms litre-1, respectively. Water samples collected at various times following application showed very little movement of the herbicide and dye out of the Carsons Bay plot. Triclopyr residues moved to a greater extent out of the Phelps Bay plot. The dye was easily tracked in real-time using field fluorometers, which allowed new sampling stations to be established to monitor this movement. Dye concentrations were strongly correlated to herbicide concentrations (r2 = 0.97 in both plots) but were less predictive of the triclopyr metabolite 3,5,6-trichloropyridinol (TCP; r2 = 0.82 in Phelps and r2 = 0.73 in Carsons), probably due to its differential metabolism and degradation. The inert dye can be used to compare the dissipation of herbicide residues by dilution versus microbial or other breakdown processes. Vertical sampling of dye in the water column showed that surface applications of aquatic herbicides can delay uniform mixing in the water column by several days. Although the dye aided the tracking of residues outside the treatment areas, predetermined sampling times and stations were still needed if very low concentrations of herbicide were to be detected at times and stations where the dye had been diluted below its limit of detection.