Bacterial contamination of groundwater from private septic systems in Illinois' sinkhole plain: Regulatory considerations

Previous and new investigations in the sinkhole plain of southwestern Illinois by the authors revealed that groundwater collected from 15 springs and 30 of 55 wells drilled into bedrock contained unacceptably high levels of coliform bacteria. It was also found that aeration-type septic-treatment systems used in the sinkhole plain had a high failure rate. The outflow from relatively new, professionally maintained systems were sampled, and of these samples, 63% did not meet minimum requirements for fecal coliform levels of the Illinois Department of Public Health. This, and a 50% to 70% failure rate of seepage-type septic systems reported by others in the midwest, suggest that private septic systems may be inherently unreliable. Data collected by the authors suggest that a significant part of the bacterial contamination of the shallow karst aquifer of southwestern Illinois' sinkhole plain may be attributed to private septic systems. This presents a potentially dangerous situation for those who drink from, or allow an open wound to come in contact with, water from a contaminated well or spring. Because of current well construction regulations, most drilled wells are not cased deeply enough to prevent contact with contaminated groundwater in the shallow karst aquifer. Consequently, the practice of chlorinating wells in an attempt to eradicate bacterial contaminants often does not yield long-term effects. We suggest that the current well code be modified to stipulate that wells drilled in the sinkhole plain should be cased and grouted to at least 20 m below the bedrock-soil interface (based on field observations) in order to avoid intercepting the zone of contaminated groundwater in the shallow karst aquifer. New county regulations no longer allow septic systems with surface discharge to be installed within the sinkhole plain. However, subsurface seepage systems are still allowed, but must be located a minimum of 23 m from the margin of a sinkhole. Based on our investigations and other studies, we suggest that private septic-treatment systems are ineffective for the protection of groundwater quality in the sinkhole plain. Because of this, and the potential danger that human waste presents to residents, we suggest that alternatives to currently used private septic systems be explored. Specifically, only those septic systems that do not rely on regolith as part of the disinfection process should be considered.