Three-dimensional Time-domain Induced Polarization of a Desert Aridisol for Septic Design

Septic leach field design in the arid southwestern US requires direct testing of soils to ensure adequate drainage. We propose that direct testing costs can be reduced if electrical geophysical methods are used to determine soil structure that facilitates proper drainage rates. We demonstrate this concept at a residential site where resistivity and induced polarization data were acquired within a desert soil with variable mixtures of clay (Argid) and calcareous (Orthid) components. Electrode arrays were tested to ensure high data quality and minimal workload during inversion modeling. The resulting resistivity structure identified a sharp boundary between the eastern and western halves of the property. The west was more conductive than the east, which could indicate the presence of higher moisture or higher clay content. Chargeability data gathered from the induced polarization survey showed similar stark patterns between east and west halves of the property. The western portion of the survey area was verified to have some clay based on high chargeability values and direct soil testing from pits. Test pits dug to 3.65 m showed transitions between Argid and Othid conditions; the test pits were in locations of moderate resistivity and low chargeability. From this study, we concluded that resistivity and IP were useful in septic design because they reduce the total amount of direct testing needed when testing locations are carefully chosen based on the geophysical results.