Published online 26 May 2006
Published in Vadose Zone J 5:757-763 (2006)
DOI: 10.2136/vzj2005.0116
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
ORIGINAL RESEARCH
Electrical Conductivity of a Failed Septic System Soil Absorption Field
Brad D. Leea,*,
Byron J. Jenkinsona,
James A. Doolittleb,
Richard S. Taylorc and
J. Wes Tuttled
a Agronomy Dep., Purdue Univ., West Lafayette, IN 47907
b USDA-NRCS-NSSC, 11 Campus Blvd., Suite 200, Newton Square, PA, 19073
c Dualem Inc., 540 Churchill Ave., Milton, ON, Canada L9T 3A2
d USDA-NRCS-NSSC, P.O. Box 60, Wilkesboro, NC 28697
* Corresponding author (bdlee{at}purdue.edu)
Mention of trade names is for informational purposes only and does not represent author endorsement.
Received 23 September 2005.
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ABSTRACT
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Locating existing septic systems and determining the extent of soil contamination after septic system failure can be destructive, time consuming, and a nuisance to homeowners. The objective of this study was to determine the effectiveness of noninvasive electromagnetic induction (EMI) for locating a failed septic system in fine-textured glacial-till-derived soils. Components of a failed septic system were located with a push probe, georeferenced with a theodolite, and surveyed with a dual receiver EMI sensor (DUALEM-2) in December 2001 (wet soil moisture condition) and July 2002 (dry soil moisture condition). Three transects located perpendicular to the soil absorption field trenches were sampled to a depth of 1.2 m and used to ground reference the EMI survey. Near-surface (1-m depth) apparent conductivity (ECa) was significantly correlated to unweighted average electrical conductivity from soil saturated paste extracts (ECsat; r = 0.79). The ECa below the soil absorption field was higher than the surrounding native soil under both dry and wet soil moisture conditions. Individual soil absorption trenches had a higher ECa than background ECa under both soil moisture conditions. A higher ECa pattern that was apparent in December 2001 associated with discharge of wastewater at shallow depths was not evident in July 2002 after the system had been abandoned for 6 mo. While more research is warranted, results from this study suggest that electromagnetic induction is a promising technique to identify the location of septic system components, failed septic systems, and their associated effluent plumes.
Abbreviations: ECa, apparent conductivity • ECsat, saturated paste extract electrical conductivity • EMI, electromagnetic induction • HCP, horizontal coplanar • PRP, perpendicular
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INTRODUCTION
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ELECTROMAGNETIC induction is a noninvasive geophysical tool that can be used to conduct detailed investigations of the subsurface. The advantages of EMI include its portability, speed of operation, flexible observation depths, and moderate resolution of subsurface features. The large number of measurements that can be collected in a relatively short time provides comprehensive coverage of sites. Maps prepared from EMI data can provide the basis for assessing site conditions, planning further investigations, and locating appropriate sampling or monitoring sites.
Electromagnetic induction uses electromagnetic energy to measure the ECa of earthen materials. Apparent conductivity is a weighted, average electrical conductivity measurement for a column of earth materials to a specific depth (McNeill, 1980; Taylor, 1999). Values of ECa are used to infer changes in soils and specific soil properties or conditions using spatial patterns within data sets. Soil ECa is influenced by soluble salts, water, temperature, and clay contents (Kachanoski et al., 1988; McNeill, 1980; Rhoades et al., 1976; Sheets and Hendricks, 1995). Geostatistical methods and inverse numerical modeling of ECa data are normally used to assist interpretations; however, to verify interpretations, ground-reference measurements are required.
Utility of Electromagnetic Induction
Electromagnetic induction has been used to infer the relative concentration, extent, and movement of animal waste products in soils. Because of its sensitivity to soluble salts, EMI is an effective tool for the assessment of surface and groundwater contamination from animal wastes (Bowling et al., 1997; Brune and Doolittle, 1990; Drommerhausen et al., 1995; Eigenberg and Nienaber, 1998; Eigenberg et al., 1998; Radcliffe et al., 1994; Ranjan and Karthigesu, 1995; Siegrist and Hargett, 1989; Stierman and Ruedisili, 1988). In relation to the parameters we wish to study, EMI measurements are nearly always indirect. Sometimes, however, they correlate nicely with variations in some chemical constituents. For example, ECa has been correlated with concentrations of K, Na, Cl, SO4, NH4, and NO3-N in soils affected by manure wastes (Brune and Doolittle, 1990; Eigenberg and Nienaber, 1998; Eigenberg et al., 1998; Ranjan and Karthigesu, 1995; Stevens et al., 1995).
While information exists on the use of EMI to assess surface and groundwater contamination from animal wastes, few references exist on the use of EMI to detect buried septic tanks, absorption fields, or field drains. Geophysical Survey Systems (1998) mentioned the use of EMI to locate buried utilities, detect leakage from buried pipes, and delineate septic systems. Researchers in Ohio explored the use of ground-penetrating radar, EMI, and geomagnetic methods to locate buried agricultural tile lines and golf course drains (Allred et al., 2000; Allred et al., 2005; Kier, 1989). Electromagnetic induction has been used to locate backfilled disposal trenches, buried landfills, storm sewers, and buried metallic and nonmetallic containers and pipes (Huang and Won, 2000; Jordan and Costantini, 1995; Lanz et al., 1998; Roberts et al., 1989; Won et al., 1996); however, these features (sewer lines, steel pipes, buried metallic containers, and filled trenches) were relatively large or orders of magnitude more conductive than surrounding undisturbed soil materials. Absorption field trenches and pipes are narrow and small, therefore more difficult to detect with EMI. It is unclear whether septic absorption fields and household effluents provide sufficient electrical contrast to be detected with EMI. The purpose of this investigation was to evaluate the utility of EMI to locate septic system components and effluent plumes from failed septic systems.
Septic Systems
The most common septic system used in the USA is a conventional trench septic system, which consists of a septic tank and a 0.10- to 0.15-m-diameter outlet pipe that carries effluent by gravity to a network of distribution pipes. These distribution pipes transport the effluent to an absorption field made up of parallel trenches that are approximately 0.9 m wide and 0.6 to 0.9 m deep. These trenches contain 0.3-m-deep gravel surrounding a 0.10-m-diameter perforated pipe in the bottom of the trench. The upper part of the trench is backfilled with the previously excavated soil materials. The remaining soil is graded over the soil absorption field to increase surface water runoff.
The effluent discharged into absorption fields carries organic matter and dissolved inorganic constituents. Effluents from kitchens, laundry, toilets, and baths and showers contain high levels of nutrients (N and P) and salts from well water and water softener discharge, increasing the ECa relative to the surrounding soil.
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MATERIALS AND METHODS
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Study Site
A failed septic system of a residential three-bedroom home built in 1995 was intensively studied. The septic system components included a 4750-L septic tank, concrete distribution box, and 
167 m2 of absorption field trenches. Six trenches approximately 30.5 m long and 0.9 m wide with 1.2-m-wide native soil between each trench comprised the soil absorption field. An observation port, consisting of a 25-cm-diameter corrugated plastic pipe extending from 5 cm above the trench bottom to 5 cm above the soil surface, was located at the distal end of the soil absorption trench furthest from the septic tank. The observation port is used as an access point - to observe any changes in wastewater level in the soil absorption trenches.
The duration of time the system was in failure is unknown; however, during a routine home inspection while the house sale was pending in spring 2001, effluent was observed on the surface of the lawn downslope of the observation port and attributed to hydraulic overloading of the septic system. An introduction of dye into the plumbing system by the county health department demonstrated the hydraulic overload failure. Wastewater containing dye was observed ponding on the soil surface downslope of the distal end of the soil absorption field trench farthest away from the septic tank. In early 2002, the occupants of the home moved, thus the failed septic system soil absorption field was not used. Construction of a new soil absorption field at another lot location began in May 2002, and was completed before home occupation by the new owners.
The soil map unit where the system is located is in an area of Morley silt loam (fine, illitic, mesic Oxyaquic Hapludalf), on 2 to 6% slopes (Kirschner and Zachary, 1969). Near the soil absorption field, a pedon was described according to NRCS standard procedures (Schoeneberger et al., 2002; Table 1).
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Table 1. Selected morphological, physical and chemical properties of soil located 80 m upslope (north) of the septic system soil absorption field.
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Ground Reference
A topographic survey of the septic system components (septic tank, distribution box, trenches, and observation port) and sampling locations was conducted using a theodolite (Fig. 1
). Six to eight, 5-cm-diameter soil cores to a depth of 1.2 m were collected with a pneumatic probe along three transects, which were orientated perpendicular to the absorption field trenches. Soil cores were subsampled in 0.3-m increments. Soil samples were ground and sieved to separate the <2-mm fraction. Saturated pastes were made from 200 g of soil and distilled, deionized water (Rhoades, 1996). Electrical conductivity from saturated pastes was determined using a 0.01-m conductivity cell.
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Fig. 1. Topographic survey of study site. Vertical exaggeration is 10-fold. Septic system components were identified on soil surface or by tile probe.
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Apparent Conductivity Surveys
The ECa was measured using DUALEM-2 (Taylor, 1999). The DUALEM-2 incorporates one transmitting coil and two receiving coils that combine to provide two geometrical arrays with distinct depths of sounding. The transmitting coil and one of the receiving coils have windings that are horizontal and coplanar, hence the name, horizontal coplanar (HCP), for this array. The axis of the other receiving coil intersects that of the transmitting coil. The perpendicularity of these axes suggests the name, perpendicular (PRP), for this array. Transmitter–receiver separation is 2 m, and operating frequency is 9 kHz. Thus, the DUALEM-2 operates at the low-frequency approximation (Wait, 1962) where the overall conductivity of the ground does not exceed 100 mS m–1 through the depth of sensitivity for the HCP geometry, and 800 mS m–1 through the depth of sensitivity for the PRP geometry. At the low-frequency approximation (i.e., low induction number), the amplitude of response of an EMI sensor is essentially in quadrature to the transmitted field, and is linearly proportional to the conductivity of the ground (Doll, 1949). Sensors such as the DUALEM-2 that are intended to operate at low induction numbers are calibrated to provide measurements of ECa that are scaled linearly from the amplitude of the quadrature component of response. Apparent conductivity is measured throughout depths of 1 and 3 m beneath the DUALEM-2 sensor in the PRP and HCP geometries, respectively. No ground contact is required with this or similarly designed EMI sensors.
In December of 2001 a 40- by 60-m grid (0.24 ha) was established over the known locations of the absorption field and the septic tank. In July of 2002, a 35- by 45-m grid (0.16 ha) was established over the soil absorption field. Survey flags were inserted in the ground at 2-m intervals around the perimeter of the grid, and served as grid line endpoints. Surveys were completed along both north–south and east–west trending grid lines. For the surveys, the DUALEM-2 was held 5 cm above the ground surface with the long axis parallel to the direction of traverse. Surveys were conducted with the EMI sensor operating in continuous mode while walking at a uniform pace. Measurements were recorded at 1-s intervals for the December survey and 0.5-s intervals for the July survey. The data processor is slower than walking speed, therefore post processing of data is necessary to adjust line positions and minimize the apparent lag of data entry. Grids of ECa were generated using kriging interpolation, based on a linear variogram with slope and anisotropy parameters both equal to 1. Plots of ECa were generated using Surfer Version 7 (Golden Software, 2001).
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RESULTS AND DISCUSSION
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Apparent Conductivity Surveys
The plots of the EMI December and July surveys in the PRP and HCP geometry are shown in Fig. 2
. In each plot, the location of the absorption field is evident and marked by a well-defined rectangular area of higher ECa. Spatial patterns in the data sets suggest the presence of linear features within the absorption field. The parallel, linear patterns within the absorption field suggest the orientation and locations of trenches and pipes. The individual absorption field trenches are best expressed in the data plot collected in the deeper sensing, HCP geometry (Fig. 2b and 2d). In the December 2001 plot of the shallower sensing PRP (Fig. 2a), an area of slightly higher ECa extends southward from the southwest corner of the absorption field. This area of slightly higher ECa is not as prominent in the June 2002 PRP plot (Fig. 2c). This pattern represents the discharge of wastewater from the observation port before the soil absorption field was abandoned. Where surveyed, the septic tank produces a most conspicuous and anomalous response.
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Fig. 2. Spatial distribution of ECa collected with the DUALEM-2 along a north–south traverse under the following conditions: (A) December 2001 (wet soil moisture condition), perpendicular (PRP) geometry; (B) December 2001, horizontal coplanar (HCP) geometry; (C) July 2002 (dry soil moisture condition), PRP geometry; and (D) July 2002, HCP geometry.
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A north–south transect of ECa data is shown in Fig. 3
. In general, ECa increased with increased soil depth (measurements obtained in the deeper sensing, HCP geometry were higher than those obtained in the shallower sensing, PRP geometry). This can be attributed to increased soil moisture and soluble salts at lower soil depths. Temporal variations in ECa between the two surveys (December 2001 and July 2002) are not manifested in the data. Along other transect lines, a few anomalously high or negative measurements were recorded. These measurements principally reflect interference from the septic tank, which contains a considerable amount of rebar used in concrete septic tank construction (see Fig. 2).
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Fig. 3. Two-dimensional depiction of apparent electrical conductivity (ECa) along transect x–x' (line 20 east, south to north). The septic system absorption field had a greater ECa under both the wet soil moisture conditions of the December 2001 survey and dry soil moisture conditions of the July 2002 survey.
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The ECa of the absorption field was considerably higher than the surrounding soil in both the wet (December 2001) and the dry (July 2002) soil moisture condition surveys (Fig. 3). The similar ECa patterns obtained at different times of the year suggest that temporal variations in soil moisture will not interfere with the use of EMI to locate soil absorption fields.
The close agreement of the HCP conductivities measured under different soil moisture conditions suggests that, in combination, the conductive effects of moisture content and dissolved solids remained fairly constant to the exploration depth of 3 m. The PRP conductivities measured in July, however, are generally lower than those measured in December. The reduction in conductivity varies from about 2 mS m–1 south (downslope) from the absorption field, to negligible over the field, to about 5 mS m–1 north (upslope) from the field. The seasonal fluctuations may reflect changes in moisture content to the exploration depth of 1 m. (Base-level drift caused by thermal distortion or component changes within the DUALEM-2 is typically <1 mS m–1 per 10°C.)
Ground Reference
Unweighted average ECsat from 0- to 120-cm depth as well as ECa measurements (collected in the PRP coil geometry) at corresponding locations are shown in Fig. 4
. Saturated paste electrical conductivity measurements of sample points along the transect show that the soil between the soil absorption field trenches had a higher conductivity than soil ECsat outside of the soil absorption field. A similar trend was found between ECa measurements. This suggests that wastewater effluent was migrating out from the trenches and into the native soil between trenches within the soil absorption field.
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Fig. 4. Unweighted average saturated paste electrical conductivity (ECsat) to 1.2-m depth relative to the measured apparent electrical conductivity (ECa) collected in the perpendicular orientation (1-m depth) along three transect lines. In all transects, ECa was lower than ECsat.
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The ECsat and ECa were significantly correlated (r = 0.79; Fig. 5
). The significant correlation between the ECa of the shallow depth and the ECsat suggests that this technique can be used to infer changes in electrical conductivity over soil absorption fields.
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Fig. 5. There was a significant correlation (r = 0.79, P < 0.01) between the apparent electrical conductivity measured in the perpendicular geometry (1-m depth) relative to the average saturated paste electrical conductivity measured to a depth of 1.2 m.
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CONCLUSIONS
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Within the fine-textured, glacial-till-derived soils of northeastern Indiana, results suggest that EMI is a promising technique to assist regulatory personnel and other interested parties in locating septic system components and discharging effluent plumes from failed septic systems. Ground referencing and soil analysis confirmed that the location and extent of septic system components and effluent plumes were correctly identified using EMI. Septic system components identified using EMI included the septic tank and soil absorption field trenches. Components that could not be clearly identified include the distribution box, observation port, and perimeter drain. Further work is necessary to determine the utility of this technique in other soils and under other environmental conditions.
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ACKNOWLEDGMENTS
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Support for this study was provided by the Indiana Water Resources Research Center and the Fort Wayne– Allen County Health Department. We thank Gary Chapple, Fort Wayne–Allen County Health Department, for providing system and site information, and an anonymous homeowner for providing access to the property.
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ABSTRACT
INTRODUCTION
