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Intrusion of Soil Water through Pipe Cracks
Citation:
Rossman, L., M. Simon, J. Neal, S. Panguluri, G. Silva, AND R. Krishnan. Intrusion of Soil Water through Pipe Cracks. U.S. EPA Office of Research and Development, Washington, DC, EPA/600/R-14/459, 2014.
Impact/Purpose:
Report of general scientific interest for technical public
Description:
This report describes a series of experiments conducted at U.S. EPA’s Test and Evaluation Facility in 2013-2014 to study the intrusion of contaminated soil water into a pipe crack during simulated backflow events. A test rig was used consisting of a 3’ x 3’ x 3’ acrylic soil box with a one-inch diameter pipe running along 2 inches1 above the bottom of the soil box. The pipe had a 1/16-inch1 hole at its top, positioned in the center of the box. Each experiment consisted of filling the box with soil media, saturating the media with a solution containing both a microbial and chemical tracer, running tap water through the pipe at a specific pressure to represent normal conditions where clean water leaks out into the soil, and then turning off the pipe flow and sampling the water drawn back into the pipe through the crack either by gravity or forced pumping. Ten experimental runs were performed under various conditions – backflow method (gravity drainage or forced pumping); type of soil media (sand or gravel); microbial tracer (B. globigii or E. coli), and leak pressures (20, 40 and 55 psi1). All of the tests indicated that significant levels of microbial tracer re-entered the pipe during the first five minutes of backflow while the chemical tracer remained essentially at background. This behavior can be explained by the displacement of soil water around the hole with clean water during normal operation which removes the dissolved chemical tracer, but allows some microbial particles to remain due to filtration through the soil media. The sand media provided higher filter efficiency than the gravel media resulting in lower numbers of microorganisms entering the pipe during backflow. Lower backflow rates produced lower average concentration of microorganisms in the intruded soil water. As the gravity backflow period extended beyond 5 minutes, microbial levels tended to level out or be reduced while the chemical tracer concentrations began to increase.