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 of 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 and procedures used to determine fecal 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.
Bacterial densities (total coliform, fecal coliform, and fecal streptococci) and suspended solids in runoff from a feedlot, pasture, and corn field were measured. Densities of fecal coliform were highest from the feedlot but were 1000 to 10,000 times greater than the water quality standard for swimmable waters from all three land uses. Densities of fecal streptococci were highest from the corn field, which suggests that wildlife are the source of bacteria. Fecal coliform/fecal streptococci ratios distinguished cattle from wildlife as the source of bacterial pollution both among land uses and among seasons of the year. Suspended solids concentrations in runoff ranged from 423 to 925 mg/l and were highest from the corn field.
A Geographic Information System (GIS), which utilizes a raster or grid‐cell format, was developed to include algorithms associated with non‐point source pollution. The system accepts digitally mapped information on soil type, topography, and land use. It calculates characteristics such as slope and slope length, and relates these characteristics to soils and land use parameters in order to produce three dimensional maps of runoff potential, sediment pollution potential, and bacterial pollution potential. It offers the advantages of retaining the geographic character of pollution potential information and of conveying in three‐dimensional graphical terms the effects of topography, soil type, land use, and land management practices.
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