Monitoring Septic Effluent Transport and Attenuation using Geophysical Methods

In rural and suburban areas, many people are served by private wells and septic systems. The proximity of septic systems to private wells and sensitive surface water bodies presents a potential for contamination with human pathogens and excessive nutrients. This study uses geophysical methods to investigate how mounded soil absorption systems function in practice at two sites, a single-family residence and a small county park. Direct current electrical resistivity, capacitive-coupled electrical resistivity, electromagnetic induction, self-potential and ground penetrating radar were used to monitor displacements and concentration changes in the effluent plumes. Geophysical surveys were supplemented with GeoprobeTM investigations in which soil and groundwater samples taken to groundtruth the collected geophysical data.
All of the geophysical methods with the exception of ground penetrating radar were successful in imaging the septic effluent plumes at both sites as a low-resistivity anomaly. Borehole to borehole GPR was successfully used to image heterogeneities in the soil and the depth to the water table and capillary fringe. The plume at both sites was imaged in 3 dimensions using capacitive-coupled electrical resistivity. EM induction and DC resistivity show good quantitative agreement between the geophysically measured resistivity and the modeled resistivity of direct groundwater samples. SP is useful as a means of identify the general location and shape of the effluent plume but does not provide the resolution to accurately define the boundaries of the plume.
Both plumes remained approximately the same size and in roughly the same position. The concentration of the plume showed a strong response to the precipitation at the site. When the precipitation was high relative to the effluent dosing the plume shrank slightly and became less resistive. However, when the dosing was large relative to the precipitation, either from low precipitation at the single-family residence or high usage at the county park, the plume became more concentrated and grew.
In all of the geophysical investigations at both sites, the plume was identified along only about half of the mound, showing effluent is being discharged from only approximately 50% of the leach field. This means that the soil is receiving roughly twice the contaminant loading that it was designed to.