Septic suburbia: too many tanks increases disease. (Environews Science Selections).

Throughout the U.S. countryside and in many suburban subdivisions--in fact, in one-quarter of U.S. households--septic systems are used for wastewater disposal. Mark A. Borchardt and colleagues from the Marshfield Medical Research Foundation and the Marshfield Clinic now report that septic system density is associated with acute infectious diarrhea in children in a central, rural area of Wisconsin [EHP 111:742-748]. Acute infectious diarrhea remains a common illness in the United States, particularly among young children, the age group most susceptible to gastrointestinal infections.

About one-third of all private septic tanks in the study area--14 contiguous ZIP codes around the town of Marshfield--included holding tanks. Holding tanks are used on sites where the soil is unsuitable for a septic drain field. Unlike a conventional septic system, a holding tank is a sealed concrete vault that stores wastewater until it is removed by a licensed waste hauler. But in central Wisconsin, many homeowners improperly discharge wastewater from holding tanks into nearby open ditches. County officials in central Wisconsin estimate that up to 40% of all holding tanks have some illegal surface discharge. Plus, septic systems can leak if not properly maintained.

The investigators selected 153 case children with a median age of 2.2 years who were seeking medical treatment for acute diarrhea, and 274 randomly selected control children with a median age of 3.7 years. Similar proportions of case and control subjects lived in rural households with private wells and septic systems.

The children's parents answered questions on demographic information, disease symptoms, and potential risk factors or confounders for acute diarrhea. Household wells were sampled for bacterial pathogens and indicators of water sanitary quality. (The investigators did not test well water for viral pathogens during this study.)

The investigators used public property tax records to find the parcel identification number (PIN) for each case and control household. This geographic coding system specifies the location of property by section (640 acres, which is also one square mile), quarter section (160 acres), and quarter-quarter section (40 acres). These square land survey units are defined by fixed lines established under the federal Public Land Survey System used in 30 states.

The county sanitary permit programs for septic systems use the same PIN, so the investigators were able to match household locations with sanitary permits to determine the number and type of septic systems surrounding each household. Septic systems were classified as holding tanks, nonholding tanks (including septic field systems) less than 20 years old, and nonholding tanks more than 20 years old. Twenty years is the functional life of a septic drain field.

The researchers found that diarrheal illness correlated with higher densities of all types of septic systems surrounding homes. Viral diarrhea was associated with holding tank density per section--the largest scale--with the risk increasing by 8% with every additional tank. Bacterial diarrhea was associated with holding tank density only at the quarter-quarter section level, with the risk increasing by 22% with each additional tank. The reason for the geographic scale difference between viral and bacterial diarrhea likely lies in previous research findings showing that viruses move farther than bacteria through the environment.

Yet, surprisingly, bacterial pathogens were not found in the wells of case households. It therefore seems likely that the case children were not infected through contaminated well water, the investigators say. Instead, children in case households could have been exposed to pathogens directly released from septic tanks to the land surface surrounding their homes.

This study is important because it is the first to assess residential proximity to septic systems as a risk factor for intestinal pathogens. The U.S. Environmental Protection Agency is preparing to promulgate the Groundwater Rule, a set of measures intended to reduce disease transmission from the more than 158,000 public groundwater systems in the nation.