The Effect of Cattle Grazing on Indicator Bacteria in Runoff From a Pacific Northwest Watershed ~

Total coliform (TC), fecal coliform (FC), and fecal streptococcal (FS) numbers were monitored for 3 years to determine the effect of grazing on the presence of these organisms in runoff from a cattlegrazed and a nongrazed watershed in the Pacific Northwest. The watersheds were characterized by winter precipitation and summer grazing. Weighted-average numbers of TC and FS in runoff did not appear to be appreciably different between the two watersheds during the study. Numbers of TC in runoff from both watersheds routinely exceeded 10,000/100 mL. Prolonged absence of grazing animals did not seem to affect number of TC and FS in runoff from the check watershed. Each spring after a period of warm weather and prolonged absence of animals, there were increases in numbers of TC, FC, and FS in the runoff. There was some correlation between recentness of grazing and numbers of indicator bacteria in runoff. However, more than a year after animals were removed from the nongrazed check watershed FC numbers in runoff still exceeded 200/100 mL in many samples, and not until the following year did they drop to < 10/100 mL. Sampling at several locations within the grazed watershed showed that sources of indicator bacteria were well distributed, and as a result were nonpoint after the initial runoff events. Thus, present FC recommendations developed for point-sources would not apply adequately to grazed land in the Pacific Northwest. Indicator bacteria as presently analyzed would not provide a basis for developing best management practices. Additional Index Words: fecal coliforms, fecal streptococci, total coliforms, pollution potential, best managemeut practices. Jawson, M. D., L. F. Elliott, K. E. Saxton, and D. H. Fortier. 1982. The effect of cattle grazing on indicator bacteria in runoff from a Pacific Northwest watershed. J. Environ. Qual. 11:621-627. Over 300 million ha of land in the United States is grazed annually (USDA, 1969, 1979). The influence grazing on bacterial quality of downstream waters is not completely understood. Protection of downstream waters requires indicator bacterial data from contributing areas to evaluate best management practices. However, bacterial water quality standards were developed for point-sources (Table 1) and may not be applicable nonpoint-source situations. Several reports have provided some insight on bacterial water quality from grazed land. Runoff from a grazed area usually contains more sediment and indicator bacteria than a nongrazed area, but runoff from both areas may exceed recommended bacterial water quality standards (Doran and Linn, 1979; Schepers et al., 1980; Kunkle, 1970; Harms et al., 1975; Milne, ~Contribution of USDA-ARS in cooperation with the College of Agricultural Research Center, Washington State University, Pullman, WA 99164, and College of Agriculture, University of Idaho, Moscow, ID 83843. Scientific Paper no. 6070. Work partially supported under interagency agreement no. EPA-78-D-X0249 between the USDA-ARS and U.S. EPA. Received 4 Jan. 1981. 2Graduate Research Assistant, Washington State Univ. (formerly Soil Scientist, USDA-ARS); Microbiologist and Hydrologist, USDAARS, Pullman; and Agricultural Engineer, USDA-Bureau of Land Management, Coeur D’Alene, ID 83814 (formerly Agricultural Engineer, Univ. of Idaho); respectively. 1976; Robbins et al., 1972). Stephenson and Street (1978) found maximum fecal coliform (FC) numbers a southern Idaho stream occurred shortly after cattle were moved onto the grazing area. Fecal coliform counts in the stream increased from 0 to 2,500/100 mL and remained high for up to 3 months after the animals were removed. On the other hand, Buckhouse and Gifford (1976) simulated rain on small cattle-grazed and nongrazed plots in southern Utah and were unable to find significant differences in numbers of indicator bacteria in the runoff, although FC survived in cow dung longer than 18 weeks. Doran and Linn (1979) concluded from a 3-year grazing study in Nebraska that the FC content of runoff from grazed land was the best indicator of bacterial quality. However, they showed that FC in runoff from the grazed and nongrazed areas usually exceeded recommended primary contact water quality standards. They showed that the FC/FS (fecal streptococcal) ratio and percentage of Streptococcus bovis were useful for evaluating livestock management practices to minimize bacterial contamination of pasture runoff, but concluded that recommended bacterial water quality standards developed for point-sources may be unsuitable for nonpoint sources. Rychert and Stephenson (1981) found that many Eschericia coli isolated from an Idaho rangeland stream were atypical. They also found that E. coli concentrations in stream-bottom sediments were 2-760 times greater than in the overlying water. In an earlier study, Hendricks and Morrison (1967) showed that enteric bacteria may multiply in relatively clean water. These reports indicate there are questions about testing methods and interpreting results concerning indicator bacteria when dealing with nonpoint sources. Geldreich (1976) discussed the behavior of indicator organisms from point-sources and the collection and use of FC and FS data. He concluded that FC/FS ratios > 1 in water indicate human fecal contamination, while ratios of 1.0 or less indicate domestic animal fecal contamination and those < 0.1, wild aimals. Geldreich cautioned that although fresh cattle manure has an average FC/FS ratio of about 0.2, prolonged sample storage or stream residence time can cause the ratio from this source to increase to as high as 3.0, since S. bovis and S. equinus die off rapidly under these conditions. He believes that FC/FS ratios developed from FC numbers below 100/100 mL are of limited usefulness. Geldreich also stated that S. faecalis var. liquifaciens should be Table 1--Bacteriological water quality standards~ (surface waters).