Ground conductivity survey of a septic system during and after failure

Abstract

Editor's note: The use of trade names is for descriptive purposes only, and does not imply particular endorsement by the authors or SEG. Many people (e.g., 25% of the U.S. population) use septic systems for wastewater disposal. In the United States, county health departments typically are responsible for overseeing the installation and operation of these systems. Because most systems rely on the soil to absorb the wastewater, special attention is required where the hydraulic permeability of soils may allow contamination to reach either the surface or a nearby aquifer. Electromagnetic induction (EMI) provides an efficient means for observing septic system performance and for guiding costlier onsite investigations. In 2001, the Ft. Wayne–Allen County Health Department identified several failing septic systems in the suburbs of Ft. Wayne, Indiana, U.S., through field observation and water-quality monitoring of nearby streams. One such system had been installed in a fine-textured (35–50% clay) soil of low permeability, and serviced a residence on a two-acre lot. The system was mapped in December 2001 and again in July 2002 with the DUALEM-2 and several other EMI instruments. The DUALEM-2 is designed to measure ground conductivity at low-induction-number (LIN). The instrument contains coils that operate in both the horizontal coplanar (HCP) and perpendicular (PRP) geometries. Transmitter-receiver separation is about 2 m. At LIN, the PRP geometry is sensitive to conductivity fluctuations to a depth of about 1.2 m beneath the instrument, and the HCP geometry is sensitive to a depth of about 3 m. To evaluate the location of the septic system absorption field and contaminant distribution, the instrument was carried at low ground-clearance aligned with north-south traverses spaced at 2-m intervals (Figure 1). Although the surveys progressed along serpentine paths, the transmitter-receiver orientation of the instrument was kept consistent. Continuous measurements were recorded at walking speed (about …