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The Movement of Fecal Coliforms and Coliphages below Septic Lines¹

ABSTRACT

A 2-year lysimetric study utilizing three undisturbed soils was conducted to investigate the movement of fecal coliforms and coliphages to the ground water. Septic tank effluent was applied to each of the three soils at appropriate design rates via subsurface septic lines. The soils included had sand contents of 80, 41, and 7.6%. Indigenous concentrations of fecal coliforms in the effluent were more than sufficient to assure detectability. During the winter the levels of indigenous coliphages decreased, and on several occasions the septic effluent was spiked with cultured coliphages. The remainder of the year, indigenous levels were sufficient to allow adequate detection. Leachate samples were analyzed on a continuous basis, and at the end of the study the soils below the septic lines were dissected and sampled on a grid pattern. They were analyzed for both fecal coliforms and coliphages.

On only a few occasions were fecal coliforms present in leachate collected 120 cm below the septic lines. Subsequent samples from the same locations did not indicate the presence of fecal coliforms so that the few samples that were collected shortly after application began may have been a result of contamination, or they may be indicative of greater mobility before organic residue built up in the soil. Soil samples taken 1 and 2 years after application began indicated limited mobility and survival of fecal coliforms in all three soils.

Coliphages were present in the leachate only in very low concentrations immediately after spiking of the applied sewage with 10³ times more organisms than were applied. Soil samples also confirmed the limited mobility of coliphages. Thus, 120 cm of any of the soils tested appeared to be sufficient to minimize the possibility of ground waterpollution by fecal coliform or coliphages from septic effluent disposal.

Key Words: bacteria • virus • leaching fields

¹ Contribution of the Texas Agric. Exp. Stn. and the Texas Eng. Exp. Stn. This work was funded partially by Grant no. R801955-02-0 from the USEPA.

² Associate Professor, Soil and Crop Sci. Dep.; Professor, Civil Eng. Dep.; Technican, Soil and Crop Sci. Dep.; and Lecturer, Civil Eng. Dep., respectively, Texas A&M Univ., College Station, TX 77843.

Received for publication February 27, 1978.