Control of variable watermilfoil in Bashan Lake, CT with 2,4-D: Monitoring of lake and well water

Variable watermilfoil (Myriophyllum heterophyllum Michx.) has recently become a problem in Bashan Lake, East Haddam, CT, USA. By 1998, approximately 4 ha of the I 10 ha lake was covered wish variable watermilfoil. In 1999, the milfoil was spot treated with Aquacide(R), an 18% active ingredient of the sodium salt of 2,4-D [ (2,4-dichlorophenoxy) acetic acid], applied at a rate of 114 kg/ha. Aquacide(R) was used because labeling regarding domestic water intakes and irrigation limitations prevented the use of Navigate(R) or AquaKleen(R), a 19% active ingredient of the butoxyethyl ester of 2,4-D. Variable watermilfoil was partially controlled in shallow protected coves but little control occurred in deeper more exposed locations. 2,4-D levels in the treatment sites were lower than desired and offsite dilution was rapid. In 2000, the United States Environmental Protection Agency (USEPA) issued a special local need (SLN) registration to allow the use of Navigate(R) or AquaKleen(R) in lakes with potable and irrigation water intakes. Navigate(R) was applied at a rate of 227 kg/ha to the same areas as treated in 1999. An additional 2 ha of variable watermilfoil was treated with Navigate(R) in 2001, and 0.4 ha was treated in mid-September. Dilution of the 2,4-D ester formulation to untreated areas was slower than with the salt formulation. Concentrations of 2,4-D exceeded 1000 mug/L in several lake water samples in 2000 but not 2001. Nearly all of the treated variable watermilfoil was controlled in both years. The mid-September treatment appeared as effective as the spring and early summer treatments. Testing of homeowner wells in all 3 years found no detectable levels of 2,4-D.