Dec. 6, 1999 Americans' love affair with their pets may be a major cause of water pollution in urban areas, particularly following periods of heavy rain.
That is one of the implications of an investigation into the source of bacterial contamination in the streams and tributaries in the Nashville area conducted by doctoral student Katherine D. Young and Professor Edward L. Thackston in the civil and environmental engineering department at Vanderbilt University. It is the latest in a series of studies conducted in various parts of the country that suggest pet wastes may be a significant cause of bacterial pollution.
The research project, described in a paper published in the December issue of the Journal of Environmental Engineering, measured levels of fecal coliform bacteria in four neighborhoods in north Nashville. The object of the study was to determine if septic systems could be the cause of unexpectedly high bacterial levels that had been detected previously in local streams and tributaries. Two of the Nashville neighborhoods that the researchers studied were sewered and two relied on septic systems. The researchers could not find any evidence of leaking septic or sewer systems, but they did find high bacterial levels in runoff from streets and lawns.
"We can't say with absolute certainty that pets, along with other urban wildlife, are the cause of this bacterial pollution," says Thackston, "but all the signs point in that direction."
If pets and urban wildlife are indeed major sources of water contamination in many cities, it raises an important public policy issue, Thackston says. In urban areas, environmental regulators generally assume that bacterial contamination comes from human wastes, such as leaky sewers and septic tanks, rather than from animals. Many states and cities are currently struggling to establish maximum daily loads for bacterial pollutants in local streams. If the basic assumption about the cause of this contamination is incorrect, then the decisions that they make are likely to be inappropriate, Thackston argues. So he urges regulators to do the additional testing required to differentiate between the two causes of pollution so that they can make informed decisions on the matter.
Animal wastes are not considered to be as dangerous as human wastes, because the bacteria they contain are not as likely to attack humans. However, exposure to animal feces can cause gastric distress and, along with other organic wastes like grass clippings, leaves and garbage, they can taint fresh streams and rivers, robbing them of oxygen and killing aquatic life.
Fecal coliform bacteria, the indicator of fecal matter contamination used in state and federal water quality regulations, are present in both human and animal waste; so, routine tests cannot tell the difference between the two, Thackston explains. Animal wastes contain higher levels of another bacteria, fecal streptococci. As a result, when the ratio of coliform to streptococci bacteria is low, researchers consider it to be a strong indication that the source is primarily animal rather than human.
In the case of the Nashville study, Thackston and Young confirmed previous studies that found the coliform/streptococci ratios in the Nashville runoff to be extremely low. The researchers also found that bacterial levels in local runoff were 10 times higher in the sewered areas than it was in the less heavily developed areas with septic tanks. In fact, the researchers found that the higher the housing density in a neighborhood, the higher the level of contamination. "And one of the things associated with housing density is the number of pets per acre," Thackston says.
Two factors contribute to the pet pollution problem, the water pollution expert says. One is the sheer number of pets in urban areas. "The density of pets in urban neighborhoods is far greater than the number of similarly sized animals in a wild setting," he points out.
The second is the nature of the urban environment itself. In the wild, animal droppings are generally held in place where they fall by long grass or bushes. That allows them to decompose in place. By comparison, streets, parking lots and even lawns are hard, flat surfaces. So animal wastes deposited there are less likely to decompose and much more likely to get washed into drains and ditches and carried into nearby streams.
New methods have been developed to test for bacteria that are unique to animal and human feces. Thackston's research group is attempting to apply some of these techniques to local runoff in order to provide more direct evidence for the source of the contamination, but they have been hampered by lack of local rainfall. The Vanderbilt research is consistent with a number of other studies:
* Environmental Protection Agency studies conducted in 1983 and 1989 at the University of Michigan found that high levels of bacteria were present in urban runoff, especially during and immediately after heavy rains.
* A 1995 study in Wilmington, N.C., found that the bacterial levels in tidewater creeks in the area were positively correlated with the urban density in the drainage areas.
* An extensive 1996 study undertaken by the Boston Water & Power Company attributed bacterial contamination in the storm drains covering a 200-acre area of the city to dog feces.
* In 1998 Mansour Samadpour, a microbiologist at the University of Washington, applied a "microbial source tracking" method that he developed to identify the source of bacterial contamination that closed a swimming beach on Lake Washington. Although local officials thought that leaky toilets at the beach might have been the source, Samadpour's result implicated dogs, cats and wild birds.
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