Duplicate sampling of surface films and associated pond water for herbicides

This paper describes the variability of herbicide concentrations in the sur face film and subsurface water of small, artificial prairie ponds (dugouts) as determined by comparing duplicate samples. Duplicate surface film sampl es were collected using a horizontally held glass plate and washed into a c ollection bottle with dichloromethane. Subsurface water samples were collec ted by plunging bottles into the pond to a depth of approximately 0.25 m. T he samples were collected weekly from two dugouts in 1989 and from one dugo ut in 1990. Samples were analyzed for 2,4-D, dicamba, bromoxynil, MCPA, tri allate, trifluralin, and diclofop using a gas chromatograph interfaced with a mass selective detector. The average variability of the surface film sam ples was +/-40% of the average of the pair with a confidence interval of 97 % (p = 0.05). The average variability of the subsurface water samples was /-25% of the average of the pair with a confidence interval of 71% (p = 0.0 5). Possible reasons for the variability, including the nonhomogeneity of t he surface film, are discussed.