Bacterial Pollution in Runoff from Agricultural Lands

A review of the literature reporting the methods and results of studies concerned with the nature of nonpoint source microbial pollution leads to several conclusions: (i) Comparison f data from different studies may be complicated by variation in the choice of indicator organisms, variation in watershed size and homogeneity, and variation in media and procedures for determining fecal streptococcus densities. (ii) The ratio of fecal coliform to fecal streptococcus densities can be used to help identify particular sources of fecal pollution, but its usefulness declines with age. Its usefulness may also be affected by media nd procedures used to determine f cal streptococcus densities, particularly when data derived from different media or procedures are compared. (iii) Bacterial densities in runoff from agricultural lands often exceed water quality standards. This is true of virtually all types of agricultural land. (iv) Although some connection between bacterial densities and stream discharge during storm events is apparent, the relationship is not simple. Factors such as temperature, hydrologic proximity of pollution sources, livestock management practices, wildlife activity, fecal deposit age, and channel and bank storage, all affect bacterial densities in runoff. Of these factors, only the influence of fecal deposit age on bacterial releases has been studied and quantified. (v) There is in the literature no loading function to predict bacterial densities in runoff that satisfactorily considers the factors listed above; however, typical density values may be used in a simple loading function to estimate probable density ranges. Additional Index Words: Coliforms, Fecal streptococcus, Nonpoint source pollution, Water quality, Feedlot management. In the 1970s it became apparent that the streams, lakes, estuaries, and aquifers of our nation were receiving significant pollutant loads from nonpoint sources. Today, nonpoint sources are the principal sources of the conventional pollutants for which the USEPA has established water quality criteria (Gianessi and Peskin, 1981). Agricultural lands are now the principal nonpoint sources of such conventional pollutants as biochemical oxygen demand (BOD), P, suspended solids, and bacteria (Gianessi and Peskin, 1981). In many central and western states, where very high percentages of the land are used for cattle (Bos sp.) production, fecal coliform bacteria are a prominent and troublesome conventional pollutant (Nebraska Dep. of Environmental Control, 1982). Many studies offer insights into the nature of nonpoint source bacterial pollution, but no review of these studies is currently available. This review summarizes the methods and results of such studies. First, factors that must be considered when evaluating or comparing data from different studies are discussed, and the use of the ratio of fecal coliform density to fecal Both authors, Civil Engineering Dep., Univ. of Nebraska-Lincoln, Omaha, NE 68182-0178. W.R. Baxter-Potter, current address: 1532 Rosemary Lane, Apt. D, South Bend, IN 46637. This work was funded through a grant from the Jane Layman Fund of the Univ. of Nebraska-Lincoln. Received 22 Sept. 1986. *Corresponding author. Published in J. Environ. Qual. 17:27-34 (1988). streptococcus density (FC/FS) is described. Next, significant studies of bacteriological characteristics of agricultural runoff are reviewed in roughly chronological order, and feedlots are briefly discussed as a particular agricultural source of bacteria. Finally, a simple method suggested by McElroy (1976) for predicting bacterial densities is described. FACTORS INFLUENCING DATA COMPARISON AND EVALUATION Microbial water quality is not commonly determined by testing for specific microbial pathogens, but by testing for the presence and concentration of some indicator organism. Total coliforms (TC), fecal coliforms (FC), fecal streptococci (FS) are the bacterial groups most commonly used as indicators of the presence of fecal contamination. Study reports may include data on one or all of these bacterial groups. Which bacterial groups are selected depends on the objectives of the research and on when the study was conducted. Fecal coliform densities are nearly always reported since FC are presently the preferred indicator organisms for water quality criteria and standards (Dep. of Interior, 1968; USEPA, 1976). Reports describing older studies tend to include TC data also, since early attempts to establish bacteriological water quality criteria relied on TC as indicator organisms (McKee and Wolf, 1963). Reports describing recent studies often include FS as well as FC data. Specific FS biotypes are sometimes identified in an attempt to identify the source of fecal pollution. This variation in the selection of indicators can complicate the comparison of data from different studies. Some researchers sample established streams that drain large watersheds containing diverse agricultural lands. Others sample direct runoff from small watersheds that are relatively homogeneous in agricultural use and practice. This distinction is not clear-cut, yet it is important when evaluating and comparing the results of different studies. In studies sampling large watersheds the data reflect the combined effect from several indistinguishable sources. In addition, the fecal contamination detected has been subjected to instream conditions for an unknown period of time. The data collected tell more about the affected stream than about the pollution source. In studies sampling small watersheds distinct sources may be identified and samples collected before significant aging or stress occurs. The data collected tell more about the characteristics and possible significance of particular sources than about the condition of the affected stream. Further complicating comparison of data from different sources, there is no general agreement on best laboratory procedures for measuring FS densities. Different media and procedures yield different results, making comparisons difficult, particularly where authors do not clearly state which methods were used. Three media (KF agar, Pfizer’s Selective Enterococcus [PSE] agar, and Environ. Qual., Vol. 17, no. 1, 1988 27 M-enterococcus medium) are commonly used in one of two procedures (membrane filter or pour plate). Pavlova et al. (1972) evaluated five media for the isolation, enumeration, and identification of FS from several natural sources. Neither the membrane filter nor the pour plate procedure was used for comparison. Instead, 0.1 mL of each sample dilution was spread directly on each medium in triplicate plates. Pavlova et al. (1972) found that KF and PSE agars yielded the highest recovery of FS. The KF agar exhibited slightly better recovery of the strains Streptococcus bovis and Streptococcus equinus, the predominant but short-lived streptococcus strains in fresh cattle feces. The KF agar also supported a lower percentage of non-FS colonies (19070) than PSE agar (23°7o). However, Pavlova et al. (1972) pointed out the PSE agar offered the advantage of a shorter incubation period (24 vs. 48 h). In a comparison of media, Geldreich (1976) found that KF agar used in the membrane filter procedure consistently recovered higher densities of FS than M-enterococcus medium used in the membrane filter procedure. At least four of the studies reported by Geldreich (1976) indicated that both KF and PSE agars used in the pour plate procedure were "more sensitive for detecting FS in general, and S. bovis and S. equinus in particular." This conclusion is reasonably consistent with that of Pavlova et al. (1972). Switzer and Evans (1974) found that KF broth used in the membrane filter procedure gave < 1 °7o recovery of S. boris, but that PSE agar gave good recovery. They suggested that the omission of agar from the PSE formulation might improve recovery. This appears to be inconsistent with the conclusions of Pavlova et al. (1972) and Geldreich (1976) regarding the KF medium, but Switzer and Evans (1974) used a different form (broth) of medium in a different procedure. When testing sewage samples, Pavlova et al. (1972) used KF agar in both the membrane filter procedure and the procedure described above. When used in the membrane filter procedure, KF agar recovered only 30 to 40°7o of the number of organisms recovered by the procedure used for comparison of media. Though KF and PSE media appear, in general, to be preferable for recovery of FS, they may not perform identically. Their performance may depend on the predominant FS strains present, the procedure in which they are employed, and whether they are used in the agar or broth form.