Combating bacterial contamination is often an intimidating task, especially when the source is ambiguous. Scientists at the State of Washington Department of Ecology have developed a new tool to help make it a little easier. After a comprehensive literature review, the Department published Fecal Contamination Source Identification Methods in Surface Water.
"We work with local groups a lot with their monitoring efforts - many of them asked about ways to differentiate between sources of bacteria," explains Debby Sargeant, the document's primary author. "We realized that we didn't have all of the answers, so we decided to develop a reference document for us and others to use. In the past, bacterial contamination of water was often blamed on sewage treatment plants, but today we know that many different sources can be to blame, including runoff from livestock operations, failing septic tanks, an overabundance of geese or deer, or other nonpoint sources."
Sargeant compiled information on the available methods for identifying human versus nonhuman sources of fecal contamination in surface waters. The document describes each method, lists the advantages and disadvantages of each, and provides examples of application. The methods detailed in the document include the following:
Microbiological Methods
Fecal Coliform-to-Fecal Streptococci Ratios. Different ratios indicate the presence of human or nonhuman bacteria.
Streptococcal Population Profiles. Species composition is used to indicate host animals. Composition varies among types of animals and has been quantified for human, nonhuman, and dairy.
Species-specific Indicators. Some bacteria strains are specific to certain animal species and can be identified as originating within that animal group. For example, Rhodococcus coprophilus is primarily associated with domestic farm animal fecal pollution.
Bacteriophages/Coliphages and Viruses. Phages are viruses that infect bacteria-they can multiply in sewage and therefore can be used to indicate the presence of human fecal pollution.
Multiple Antibiotic Resistance. Bacteria are analyzed for their resistance to various antibiotics. Bacteria from humans would be expected to show resistance to the antibiotics typically used in humans, whereas bacteria from dairy would show resistance only to antibiotics used in dairy operations.
DNA Ribotyping/Genetic Fingerprinting. Ribosomal RNA (rRNA) genetic information is isolated from a pure bacteria culture grown from bacteria found in a water sample. This information is compared to a known sample of bacteria rRNA, such as one from a human. If the rRNA information for the unknown bacteria matches that for the known sample, the researcher knows the bacteria are derived from the same strain (in this example, a human source).
Chemical Methods
Detergents/Optical Brighteners. These chemicals are associated with laundry discharge and might indicate human waste discharge from leaky septic tanks. The chemicals are absorbed by cotton in surface water and can be seen when viewed under ultraviolet light.
Caffeine. Has been proposed as an indicator of human fecal pollution.
Coprostanol. Coprostanol forms as a byproduct of the bacterial breakdown of cholesterol in the body and is proposed as an indicator of human fecal pollution.
Other Methods
Fluorescent Dye Tracing. If dye is introduced into an onsite septic system that is suspected of leaking, charcoal in the receiving water body absorbs the dye, indicating that a leak is present.
Land-use-based Site Selection. Land-use information can be used to select monitoring sites that can help pinpoint potential bacterial sources. For example, GIS overlays of specific sources can be used to select monitoring sites to determine relative pollutant contributions from each source.
Each method has advantages and disadvantages. "Unfortunately, we discovered that there is no sure-fire, inexpensive way to go. You have to keep track of the new methods that become available," notes Sargeant. "In fact, it is likely that new information has been published since our document was printed in October 1999." When choosing a method, Sargeant recommends researchers consider the types of sources likely to be in the water, the pollutant loading mechanisms and time frame, the type of medium being sampled (fresh water, sea water), and the budget. To date, the DNA ribotyping/genetic fingerprinting method has been used most often. Sargeant notes, however, that other promising techniques on the horizon include improved DNA techniques, multiple antibiotic resistance, bacteriophages, and method combinations.
The document, available on the Internet at www.ecy.wa.gov/biblio/99345.html, is organized by method type and includes an appendix summarizing the typical microbiological indicators used in bacterial assessments.
[For more information, contact Debby Sargeant, Washington State Department of Ecology, Environmental Assessment Program, P.O. Box 47600, Olympia, WA 98504-7600. Phone (360) 407-6684; e-mail: dsar461@ecy.wa.gov.]