Tracing septic system effluent movement through saturated hillslope soils Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rj4306903

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  • Three isolates of Escherichia coli were labeled by their resistance to sodium azide and, separately, to novobiocin, nalidix acid, and tetracycline. The strains exhibited a high degree of persistence in the soil environment and were recoverable on strain specific media at levels within the 95% confidence interval of the numbers recovered on nonselective media. The E. coli strains were subsequently used to evaluate the events which would occur when a septic tank drainfield became submerged in a perched water table and effluent-borne bacteria escaped into the groundwater. Field experiments were conducted in a Dixonville soil and in a soil representing a transition between the Steiner and Hazelair soil series, by introducing the tracer strains into horizontal lines installed into the A., B, and C horizons of the soil profiles. Bacterial transport was evaluated by collecting groundwater samples from rows of piezometers (sampling six separate depth zones/row) located downslope from the injection 1 ines and enumerating the tracer organisms present in the water samples. In addition, surface water samples were collected from the transition site at the furthest upslope location of observed overland flow. The Dixonville series site was located on a uniform 14% slope and bacterial penetration proceeded at a relatively constant velocity within a single layer throughout the site. Also, the maximum bacterial density in the groundwater, observed at each sampling distance downslope, was used to produce a mathematical relationship which described the overall decrease in numbers of organisms with increased distance through the soil. In the transition series, however, bacterial translocation patterns varied as the downslope flow was directed upward in the soil profile by hydraulic gradients and a restrictive clay layer. Also, a transition from predominantly matrix to flow to predominantly "pipe" flow occurred and a large volume of the water passing downslope was intercepted and conducted by these channels. In both experimental sites, subsurface bacterial transport progressed at high apparent velocities and large numbers of organisms were carried substantial distances downslope. These results demonstrate the extent of incomplete septic-effluent treatment as these wastewaters migrate through saturated hillslope soils, and provides a basis for assessing the potential health hazards which were created.
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