Survival and Movement of Fecal Indicator Bacteria in Soil under Conditions of Saturated Flow I

Antibiotic-resistant fecal bacteria were used to monitor the degree of movement and subsequent ground water contamination by septictank effluent discharged into a drainfield under saturated conditions. Two pits of different depths were constructed to simulate drainfield beds, and ground water samples were removed uring 32-day sampling intervals from sampling wells installed at set distances from each inoculation pit. The bacteria added to the deep pit were released into a B2t horizon which contained a higher clay content than the A horizon in which the shallower pit was installed. Streptomycin-resistant strains of Escherichia coli and Streptococcusfaecalis amended to each pit site moved in a directional manner, required more time to reach sampling wells when inoculated into the deeper of the two pits, and moved relatively long distances when considering that the area where the sites were located had only a 2o70 slope. Bacterial numbers peaked in the sampling wells in association with major rainfall patterns and the populations required longer periods to peak in the wells furthest from the inoculation pits. The results indicated that antibiotic-resistant bacteria eliminated the problem of differentiating between the amended bacteria and those nonresistant strains already in the soil, and the potential is excellent for including this type of microbiological procedure for assessing the suitability of a soil site for septic-tank and waste water drainfield installations. Additional Index Words: environmental pollution, ground water contamination, septic-tank discharge. Limited information is available on the potential for ground water contamination from recharged sewage plant or septic tank effluents. Site ratings for septic tank and drainfield installations traditionally have been determined through evaluating such factors as soil texture and structure, degree of slope, depth of soil to an impermeable layer, and depth to a water table and filtration rates (Weibel et aI., 1954; Bouma et al., 1972). Research on the environmental effects of septic tanks was reviewed by Patterson et al. (1971). They found that, except for a few studies on the average composition of septic tank effluent (which included fecal bacteria), pollution of ground water by effluent has been monitored largely through the use of fluorescein dyes or halogen salts. Fecal bacteria have received little emphasis as effluent tracers because of the difficulties in differentiating between those which represent recent contamination from septic drainage and those from sources such as livestock or rodents. The problem is further enhanced by the limited survival time of the fecal bacteria in soil. Kaufman and Orlob (1956) evaluated ground water tracers and stated that the ideal tracer should correctly depict ’ Contribution from the Oregon State Agric. Exp. Stn. and the Oregon State Univ. Res. Counc., Corvallis, OR 97331. Tech. Pap. no. 4412. Received ll Apr. 1977. 2Assistant Professor, Dep. of Microbiology, Oregon State Univ.; Soil Conservation Specialist, Soil Conservation Service, Twin Falls, Idaho; and Professor, Dep. of Soil Science, Oregon State Univ., respectively. All correspondence regarding this manuscript should be mailed to Dr. C. Hagedorn, Dep. of Microbiology, Oregon State Univ., Corvallis, OR 9733 I. the movement of water through a porous medium without modifying the transmission characteristics of the system. By these criteria, the fecal bacteria would hardly qualify as ground water tracers. However, there is good evidence to indicate that the fecal bacteria do survive long enough in soil and water (Van Donsel et al,, 1967) to allow their use as a tracer to assess the potential for ground water contamination. Our objectives were to examine the potential use of antibiotic-resistant fecal bacteria to monitor subsurface flow under saturated conditions and to assess health hazard potentials resulting from ground water contamination by septic tank effluents. MATERIALS AND METHODS