Streambed-sediment samples were collected from 11 sites in the Richland Creek Basin and 3 sites on the Buffalo River and Bear Creek in 1999 for analysis of semivolatile organic compounds. The sites were sampled to better characterize the distribution of semivolatile organic compounds after 23 compounds were detected in a sample collected from Richland Creek in 1992. In 1999, the highest numbers of detected semivolatile organic compounds and the highest total concentrations of semivolatile organic compounds generally occurred in samples collected at sites on Richland Creek and Falling Water Creek just upstream and downstream from the confluence of Falling Water Creek and Richland Creek. Total concentrations in samples from these sites and another site on Falling Water Creek (about 6 river miles upstream) ranged from 115 to 323 jag/kg. Twelve to 22 compounds were detected. The numbers of detected compounds and total concentration of semivolatile organic compounds in the samples from the Buffalo River and Bear Creek generally were lower than in samples from the area near the confluence of Falling Water and Richland Creeks. However, 746 jag/kg of /?-cresol was detected in one sample from the Buffalo River just upstream from the confluence with Richland Creek. Total concentrations of semivolatile organic compounds at the sites on Falling Water Creek, on Richland Creek near the confluence with Falling Water Creek, and on the Buffalo River just upstream from Richland Creek ranged from approximately 1/2 to 1/17 of the concentrations at two sites downstream from major urban areas sampled in 1992. Concentrations did not exceed aquatic-life criteria. However, criteria do not exist for 18 compounds detected in samples from Richland Creek, Falling Water Creek, Buffalo River, and Bear Creek. Three phthalate compounds were found in all samples collected in 1999. These three compounds are commonly detected in laboratory blanks. Most of the detected compounds are polycyclic aromatic hydrocarbons. Sources and uses of detected semivolatile organic compounds include plasticizers, solvents, resins, dyes, oils, coal, and combustion products. Parts of the study area are underlain by geologic formations containing carbonaceous shales and thin coal beds.

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The Buffalo River and its tributary Bear Creek are in the White River Basin in the Ozark Plateaus in north-central Arkansas. Analysis of streamflow measurements and water-quality samples at a site on Bear Creek and a site on the Buffalo River in Searcy County, Arkansas, quantify differences between the two sites during calendar years 1999 and 2000. Streamflow and water quality also vary seasonally at each site. Mean annual streamflow was substantially larger at the Buffalo River site (836 and 719 cubic feet per second in 1999 and 2000) than at the Bear Creek site (56 and 63 cubic feet per second). However, during times of low flow, discharge of Bear Creek comprises a larger proportion of the flow of the Buffalo River. Concentrations of nutrients, fecal-indicator bacteria, dissolved organic carbon, and suspended sediment generally were greater in samples from Bear Creek than in samples from the Buffalo River. Statistically significant differences were detected in concentrations of nitrite plus nitrate, total nitrogen, dissolved phosphorus, orthophosphorus, total phosphorus, fecal coliform bacteria, and suspended sediment. Loads varied between sites, hydrologic conditions, seasons, and years. Loads were substantially higher for the Buffalo River than for Bear Creek (as would be expected because of the Buffalo's higher streamflow). Loads contributed by surface runoff usually comprised more than 85 percent of the annual load. Constituent yields (loads divided by drainage area) were much more similar between sites than were loads. Flow-weighted concentrations and dissolved constituent yields generally were greater for Bear Creek than yields for the Buffalo River and flowweighted concentrations yields were higher than typical flow-weighted concentrations and yields in undeveloped basins, but lower than flow-weighted concentrations and yields at a site in a more developed basin.

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For use of readers who prefer to use metric (International System) units, rather than the inch-pound units used in this report, the following conversion factors may be used: Multiply inch-pound unit By To obtain metric unit cubic foot per second 0.02832 cubic meter per second (ft 3 /s) (m3 /s) square mile (mi^) 2.590 square kilometer (km^)