Author: Sheridan Kidd Haack, Research Hydrologist/Microbiologist, U.S. Geological Survey, Michigan Water Science Center
Microbial pollutants may enter surface water or groundwater from a variety of sources (see FAQs section, “Fate and Transport of Pathogens”). The fecal indicator bacteria Escherichia coli and enterococci are found in all animals, including humans, livestock, and wildlife. When high numbers of fecal indicator bacteria are detected in surface water or groundwater, it suggests that microbial pathogens from fecal sources, which might be humans, livestock, or wildlife, may also be present. Therefore, it is important to human health to determine and mitigate the source. However, the science of determining these sources is in its infancy, and it remains a challenging task to determine the source of fecal pollution in open environments, subject to many, and constantly changing inputs from humans, livestock, and wildlife.
There are three general ways to determine the sources of microbial contamination of water. The first, and most obvious, is to search the landscape for direct contributions and potential sources and to establish that microorganisms (or the source material) could move from the source area to water. There are several methods, ranging from dye-tracing studies to sophisticated hydrologic modeling, that can establish these connections. In addition, estimates of animal densities and the relative numbers of fecal indicator bacteria in their wastes may be informative in targeting particular sources.
The second method is to examine the affected water for changes in water quality that might arise from the potential source. Nutrients (nitrogen or phosphorous), certain chemicals such as chloride, or the ratios of chemicals such as bromide and chloride, have been used to indicate sources such as septic effluents or manure. More recently, sophisticated analyses have shown that chemicals such as human-use pharmaceuticals or personal care products may be useful in tracking fecal pollution from wastewater treatment plant effluents. Although detecting wildlife contamination by chemical means would be unlikely (since there are few chemicals to associate uniquely with wildlife sources), it is possible that chemicals used in different animal production practices might serve as markers of these manure sources in the future.
Finally, a logical (if not simple or inexpensive) approach is to evaluate whether the fecal indicator bacteria (or pathogens) themselves indicate the source, which is termed "microbial or bacterial source tracking" (MST or BST). In the last five years, several reviews of the state of this science have been produced (see references). In general, these reviews indicate that each method can produce some useful results for distinguishing between human, livestock, and wildlife sources of fecal pollution, especially for small-scale studies with limited source inputs. However, all these methods have technical difficulties, and most are not ready to be broadly used in support of management decisions.
The best approach to source determination is to acquire multiple lines of evidence using several techniques. An example of a study that documents how several approaches were combined is Jackson et al. 1998. In this case, a child living on an Ontario, Canada, dairy farm was determined to have an E. coli O157:H7 infection (ref FAQ I), by isolation of the bacteria from her stool. E. coli O157:H7 were isolated from the farm's well water and from 63% of the cattle on the farm. The E. coli O157:H7 isolates obtained from the child, the well water, and the cattle were determined to be all the same by three different typing methods. Hydrologic investigation of the septic system, manure lagoon, and farm well demonstrated defects that would allow manure-contaminated surface water to enter the well. Chloride and nitrate concentrations in the well water did not indicate contamination when they were sampled some time after the child fell ill, indicating that contamination of the well likely occurred sporadically and was not directly the result of linkage to the septic system or manure lagoon.
Additional reading:
Stoeckel, DM, Mathes, MV, Hyer, KE, Hagedorn, Charles, Kator, Howard, Lukasik, Jerzy, T O'Brien, TL, Fenger, TW, Samadpour, Mansour, Strickler, KM, and Wiggins, BA. 2004. Comparison of Seven Protocols to Identify Fecal Contamination Sources Using Escherichia coli. Environ. Sci. Technol. 38(22):6109-6117.
Stoeckel, DM. 2005. Selection and application of microbial source tracking tools for water-quality investigations. Techniques and Methods 2-A3: U.S. Geological Survey, Reston, VA. 43 pp.
USEPA (U.S. Environmental Protection Agency). 2005. Microbial Source Tracking Guide Document. Office of Research and Development, Washington, D.C. EPA-600/R-05/064. 131 pp.
Jackson, SG, Goodbrand, RB, Johnson, RP, Odorico, VG, Alves, D., Rahn, K, Wilson, JB, Welch, MK, Khakhria, R. 1998. Escherichia coli O157:H7 diarrhoea associated with well water and infected cattle on an Ontario farm. Epidemiology and Infection 120:17-20.
