BOX 5-4
Microbial Source Tracking Techniques

Water quality monitoring in the Page Brook watershed in Clarke County, Virginia, exhibited use impairment due to excessive concentrations of fecal coliforms. Land uses in the watershed suggested two likely nonpoint sources of fecal contamination: (1) relatively widespread livestock (cattle) farming and (2) widespread use of single home septic systems. Routine water quality surveys in this watershed were unable to determine the relative contributions of these two sources. To field test a microbial source tracking method using patterns of antibiotic resistance in fecal streptococci, Hagedorn et al. (1999) initiated a watershed improvement project in 1996. Application of the antibiotic resistance method at three highly contaminated sites within the watershed identified the dominant source (79 percent of the isolates) as cattle, with small proportions of waterfowl, deer, and unidentified sources. Based on these results, cattle access to the stream was restricted through BMPs (installation of fencing and in-pasture watering stations). Fecal coliforms were subsequently reduced at the three sites by an average of 94 percent, from pre-BMP average populations of 15,900 per 100 mL to post-BMP average populations of 960 per 100 mL. After fencing, less than 45 percent of fecal streptococcus isolates were classified as being from cattle, indicating that the dominant source of contamination had been successfully remediated

Septic systems were suspected as a major source of fecal coliform impairment contamination of shellfish beds at a tidal inlet on Virginia’s Eastern Shore. Using shoreline survey techniques and discrete sampling over small areas, Simmons et al. (1995) tracked nonpoint sources of fecal coliform at tidal inlets on Virginia’s Eastern Shore and at an uninhabited island. Based on DNA fingerprinting methods, they characterized Escherichia coli from raccoon, goose, otter, and muskrat. These results, along with examination of land use and wildlife management patterns in the area, led them to conclude that fecal contamination of tidal inlets, bays and estuaries on Virginia's Eastern Shore could be largely attributed to fur-bearing animals. The populations of these animals have increased over the past several decades due to lack of predation and land development patterns that concentrate their populations in undeveloped shoreline areas. Simmons and colleagues also determined that septic systems, when sufficiently elevated above the water table and not mechanically damaged or overloaded, are effective at removing E. coli, thus avoiding the need for costly replacement systems that would likely have had minimal impacts in reducing fecal contamination.

serious issue in the region, given that resources for water quality protection and other public objectives are scarce. In colloquial terms, cost-effectiveness analysis is about getting the “biggest bang for the buck.” Simply stated, if two projects yield the same outcome, the preferred project is the less expensive one. Cost-effectiveness is particularly useful for optimizing theachievement of a well-defined policy target. In this context it is used to identify a single project or set of projects that minimize the costs of achieving the target. The use of cost-effectiveness analysis for ranking projects becomes limited when the outcomes are not comparable.

Benefit-Cost Analysis

Benefit-cost analysis ranks projects based on the relationship between benefits and costs. The economic benefits of measures to reduce or eliminate discharges of untreated sewage into the region’s source waters would include the reduction in the likelihood of such events and the associated costs of averting activities and disease. Other things being equal, desirable projects yield benefits in excess of costs, with projects being ranked according to their relative net benefits. Benefit-cost analysis is more powerful than cost effectiveness analysis for project evaluation because it allows for comparison of projects with otherwise noncomparable outcomes.



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