Rainfall runoff and the runs. (Infectious Disease).

In 1993 a period of heavy rainfall and subsequent runoff in Milwaukee, Wisconsin, caused a local water treatment plant to malfunction. Fifty-four people died, and public health officials reported 403,000 cases of intestinal illness in the largest waterborne disease outbreak reported to date. Six years later, rainwater apparently washed cow manure infected with Escherichia coli into a well at a fairground in New York State, causing the largest case of E. coli poisoning ever reported.

"There is increasing evidence that heavy rainfall runoff contributes to the risk of waterborne disease outbreaks," says Jonathan Patz, director of the Program on Health Effects of Global Environmental Change at the Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland. In August 2001, Patz and three colleagues published a study in the American Journal of Public Health documenting that heavy rainfall was associated with more than half of the U.S. waterborne disease outbreaks occurring over the past 50 years. The team found that 24% of the 548 reported outbreaks from 1948 to 1994 resulted from surface water contamination and 36% from groundwater contamination.

The John Hopkins study highlights the importance of rainfall runoff as an environmental factor. "The impact of rainfall runoff is pervasive and affects both rural and urban areas," explains Timothy Downs, a professor in the Department of Development, Community, and Environment at Clark University in Worcester, Massachusetts. "Runoff mobilizes chemical and pathogenic pollutants, often increasing the contact organisms have with them, increasing potential health risks."

When rain falls to the earth, a part of it seeps into the land to replenish the earth's groundwater, but most of it flows downhill as runoff. In urban areas, when a downpour hits the pavement, sheets of rain wash off more than just oil and grease. A mix of metals--cadmium, lead, and copper, among others--is also carried to nearby waters.

Agricultural activity combined with rainfall runoff can also introduce many chemicals into water systems. "Animal feeds often contain hormone supplements and natural plant steroids, and these substances enter the water system as waste, thanks to rainfall runoff," explains Douglas Fort, president of Fort Environmental Laboratories in Stillwater, Oklahoma. "The waste contains endocrine-active chemicals, potential toxic chemicals, and other biological agents."

Rainfall and spring meltwater runoff is furthermore the primary agent of soil erosion, especially in deforested areas. The result can be significant pollutant loads of soil particles and their associated contaminants, which may include heavy metals and toxic organics such as polychiorinated biphenyls.

Brian Oram, director of the Center for Environmental Quality testing lab at Wilkes University in Wilkes-Barre, Pennsylvania, points out that "the potential for the migration of nutrients is a function of watershed characteristics, land use, and engineering controls." Watershed characteristics include rainfall frequency, rate of runoff, ground slope, infiltration capacity of the soil, and the nature and type of soil. "These factors can control or influence the degree to which the rainfall recharges [groundwater] or runs off," Oram explains. "This will also control the rate of soil erosion and sedimentation."

Many researchers believe global warming can increase rainfall and runoff intensity, a phenomenon that could result in more surface water and groundwater contamination and environmental health problems. Writing in the May 2001 issue of EHP Supplements, researchers led by Joan B. Rose of the Department of Marine Science at the University of South Florida in St. Petersburg wrote: "Knowledge about transport processes and the fate of microbial pollutants associated with rainfall and snowmelt is key to predicting risks from a change in weather variability."

The rainfall runoff issue is important for public health officials, given the documented connection between rainfall runoff and disease and the continued major health threat that waterborne diseases pose. As Patz explains, "Many waterborne diseases are sensitive to climatic change, so to prevent further disease outbreaks we need to enhance monitoring and reporting, improve engineering measures, and strengthen watershed protection laws to protect source waters."