The global distribution of fluorocarbon-12 and fluorocarbon-11 is used to establish a relatively fast interhemispheric exchange rate of 1 to 1.2 years. Atmospheric residence times of 65 to 70 years for fluorocarbon-12 and 40 to 45 years for fluorocarbon-l1 best fit the observational data. These residence times rule out the possibility of any significant missing sinks that may prevent these fluorocarbons from entering the stratosphere. Atmospheric measurements of methyl chloroform support an 8-to 10-year residence time and suggest global average hydroxyl radical (HO) concentrations of 3 x 10(5) to 4 x 10(5) molecules per cubic centimeter. These are a factor of 5 lower than predicted by models. Additionally, methyl chloroform global distribution supports Southern Hemispheric HO levels that are a factor of 1.5 or more larger than the Northern Hemispheric values. The long residence time and the rapid growth of methyl chloroform cause it to be a potentially significant depleter of stratospheric ozone. The oceanic sink for atmospheric carbon tetrachloride is about half as important as the stratospheric sink. A major source of methyl chloride (3 x 10(12)grams per year), sufficient to account for nearly all the atmospheric methyl chloride, has been identified in the ocean.

Map showing location of streamflow-gaging stations where samples were collected for analysis of glyphosate, its degradation product, aminomethylphosphonic acid, and glufosinate as part of the U.

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Water samples were collected from nine streams in five Midwestern States during 1990 and from two of the nine streams during 1991-92 to determine the concentration, temporal distribution, and transport of 11 agricultural herbicides, two triazine metabolites, and nutrients during storm runoff. This report presents a description of the sampling plan, data-collection techniques, laboratory methods, and a compilation of herbicide and nutrient concentration and quality-assurance data. Samples were collected at nine sites during April through August 1990 by a combination of manual sampling and automatic samplers. Automatic samplers were programmed to collect samples during baseflow conditions and when runoff from precipitation occurred. Sampling was continued at two sites during March 1991 through April 1992 using the same sampling procedures used in 1990. Approximately 2,000 samples were collected from the nine sites in 1990, and about 400 samples were collected from two sites in 1991-92. Laboratory analyses consisted of microtitre-plate enzyme-linked immunosorbent assay for triazine and chloroacetanilide herbicides and confirmation of selected samples by gas chromatography/mass spectrometry. The data have been useful in studying herbicide and nutrient transport, in showing that atrazine and cyanazine are the major triazine herbicides and that alachlor and metolachlor are the primary chloroacetanilide herbicides present in water samples, and in examining the annual persistence of herbicides in surface water. Introduction 1 Sample Preparation All herbicides and nutrient samples were filtered to remove particulate and colloidal matter. During 1990, samples for the analysis of herbicide and nitrite-plus-nitrate as nitrogen were filtered through a l-(im glass-fiber filter using a peristaltic 1 The use of brand names is for identification purposes only and does not constitute endorsement by the U.