ACUTE AND CHRONIC TOXICITY OF THE HERBICIDE STAM(R)M-4 IN-FIELD AND LABORATORY EXPOSURES

Propanil (3',4'-dichloropropionanilide), the active ingredient in the herbicide Stam(R)M-4, is possibly the most extensively used herbicide for rice production in the world. Propanil and its metabolites are tra nsported within characteristic ditch ecosystems in the production land scape of northeast Arkansas. Runoff from these ditch ecosystems is fur ther transported to a river or other water body supplied by the Missis sippi River Alluvial aquifer. Forty-eight-hour acute toxicity tests wi th Ceriodaphnia dubia (cladoceran) and Pimphales promelas (fathead min now) were conducted on stormwater runoff, laboratory synthetic water, and irrigation (ground) water. No effects on survival were observed in this study following 48-h toxicity testing with the stormwater. Survi val studies indicated assimilative capacity in irrigation (ground) wat er as opposed to laboratory synthetic water. Mean 48-h LC(50)s of C. d ubia increased from 2.94 mg/L Stam(R)M-4 in laboratory synthetic water to 8.01 mg/L Stam(R)M-4 in irrigation water. Likewise, P. promelas me an 48-h LC(50)s increased from 23.76 (laboratory synthetic water) to 3 3.52 mg/L Stam(R)M-4 (ground water). In 7-d chronic tests, there was a n increase in mean LC(50)s of C. dubia when comparing synthetic water to irrigation water (0.48 to 1.24 mg/L Stam(R)M-4, respectively). P. p romelas, however, had less tolerance for Stam(R)M-4 in irrigation wate r (4.45 mg/L) than in synthetic water (5.93 mg/L) in 7-d chronic toxic ity tests. Forty-eight-hour toxicity tests indicate that ground water affords organisms some assimilative capacity that laboratory synthetic water does not. Since herbicides and most other pesticides are manufa ctured to elicit rapid responses, 48-h toxicity results best describe potential nontarget organism effects in aquatic ecosystems.