Evaluation and use of a biotracer to study ground water contamination by leaching bed systems

There is an increasing concern about dangerous levels of bacterial contamination of rural ground water resources in Ontario and throughout the world. Recent studies in rural parts of Canada have identified leaching bed systems as one of the major sources of this contamination. Field studies were undertaken to evaluate bacterial contamination from three different types of leaching bed designs, using nalidixic acid-resistant Escherichia coli (E. coli NAR) as a biotracer. This biotracer was used rather than passive ground water sampling to clearly identify the source of the contamination and also to allow the determination of travel times and distances more clearly. While this biotracer has been used for other studies its use in actual working septic systems has not yet been reported.This work has also shown that E. coli NAR is an excellent biotracer and can be used to give an accurate assessment of a leaching bed's performance provided it is introduced into the system over a reasonable period of time. Results also show that bacteria are not necessarily removed before the effluent reaches the ground water. The speed, distance of travel and attenuation of biotracer concentrations was found to be highly related to precipitation events, age of system and depth of unsaturated zone below the bed.