Bacteria

Contamination of drinking water by pathogenic organisms is a major environmental concern. Similar to water pollution by excess nutrients, water pollution by microbial pathogens can also be caused by point and nonpoint sources. Point source water contamination normally results from a direct entry of wastewater from municipal or water treatment systems into a drinking water supply. Nonpoint sources of bacterial pollution can be difficult to identify as they can originate from animal production units, land application of different manure types, and wildlife.

Although there are many potential sources of pathogenic loadings to streams, agronomic practices that utilize animal manures contaminated with pathogenic or parasitic organisms appear to be the major source of nonpoint contamination in watersheds. In recent years, a concentration of animal feeding operations has occurred in the cattle, swine and poultry production industries. These operations generate substantial amounts of animal manure that are normally applied raw to relatively limited land areas. Even though animal manure can be considered a beneficial fertilizer and soil amendment, high rates of land applied raw manure increase the risk of surface or groundwater contamination, both from excess nutrients and pathogenic organisms such as Cryptosporidium, Salmonella, or Escherichia coli 0157:H7.

Fecal coliforms (generic forms of bacteria) have customarily been used as indicators of potential pathogen contamination for both monitoring and modeling purposes (Baudart et al., 2000; Hunter et al., 2000; Pasquarell and Boyer, 1995; Walker et al., 1990; Stoddard et al., 1998; Moore et al., 1988). However, recent studies have documented waterborne disease outbreaks caused by Cryptosporidium, Norwalk and hepatitis A viruses, and salmonella despite acceptably low levels of indicator bacteria (Field et al, 1996).

SWAT+ considers fecal coliform an indicator of pathogenic organism contamination. However, to account for the presence of serious pathogens that may follow different growth/die-off patterns, SWAT+ allows two species or strains of pathogens with distinctly different die-off/re-growth rates to be defined. The two-population modeling approach is used to account for the long-term impacts of persistent bacteria applied to soils, whose population density when initially applied may be insignificant compared to that of less persistent bacteria.

One or two bacteria populations may be introduced into an HRU through one of the three types of fertilizer applications reviewed in Chapter 6:1. When bacteria in manure are applied to an HRU, some fraction is intercepted by plant foliage with the remainder reaching the soil. SWAT+ monitors the two bacteria populations on foliage and in the top 10 mm of soil that interacts with surface runoff. Bacteria in the surface soil layer may be in solution or associated with the solid phase. Bacteria incorporated deeper into the soil through tillage or transport with percolating water is assumed to die.