Duplicate Sampling of Surface Films and Associated Pond Water for Herbicides

Abstract:
This paper describes the variability of herbicide concentrations in the surface film and subsurface water of small, artificial, prairie ponds (dugouts) as determined by comparing duplicate samples. Duplicate surface film samples were collected using a horizontally held glass plate which was washed into a collection bottle with dichloromethane. Subsurface water samples were collected by plunging bottles into the pond to a depth of approximately 0.25 m. The samples were collected from two dugouts in 1989 and from one dugout in 1990. Samples were analyzed, by gas chromatograph/mass selective detector, for 2,4-D, dicamba, bromoxynil, MCPA, triallate, trifluralin and diclofop. The average variability of the surface film samples was *40% of the average of the pair with a confidence interval of 87% (p = 0.05). The average variability of the subsurface water samples was *25% of the average of the pair with a confidence interval of 68% (p = 0.05). Possible reasons for the variability, including the non-homogenicity of the surface film, are discussed.