Transport modelling of biological tracers from septic systems

Abstract

The potential of contamination of individual ground water supplies from septic systems was investigated. Chloride (Cl) and electrical conductivity ( C e ) as tracers of chemicals and indicator bacteria—total coliforms (TC), fecal coliforms (FC) and fecal streptococci (FS)—were determined for 7 months in septic tank effluent and ground water samples collected downgradient from 17 new or replacement septic systems. Indicator bacteria were also determined occasionally in ground water samples from wells near the systems for a continuing period of 2 years. Following 5 years of system operation a last round of analyses was conducted. Water table fluctuations in the vicinity of the septic systems were monitored for 2 years. As a model for viruses, poliovirus in feces from babies who had received the trivalent Sabin oral poliovirus vaccine was introduced into one septic system as a single dose by flushing stools down the toilet. Effluent and ground water were assayed for poliovirus over a period of approx. 6 months. The data were evaluated statistically and a biological contaminant transport model was tested to simulate ground water contamination by the tracers of pathogens. No tracer bacteria reached ground water from any of the 17 septic systems. Presumably, the bacteria were removed by the soil under the seepage bed; mean transport was < 1 count/100 ml for each bacterial type, in keeping with U.S. drinking water regulations. Poliovirus entered and spread in ground water from the one septic system tested, even though this system was functioning properly. Of 3.3 × 10 8 PFU of poliovirus inoculated into the septic tank, a mean of 70 PFU/100 ml escaped from the tank with the effluent and an overall mean of 62 PFU/100 ml were transported to ground water. Septic systems had little influence on water table fluctuations in their vicinity, compared to the seasonal climatic factors.