COMBINED GEOPHYSICAL METHODS FOR MAPPING INFILTRATION PATHWAYS AT AN AQUIFER RECHARGE AND RECOVERY SITE

Although aquifer recharge and recovery systems offer a sustainable approach for the reclamation, treatment, and storage of post-treatment wastewater, infiltration ponds can suffer from poor hydraulic loading rates due to clogging layer development and low permeability floodplain deposits. A method for underwater self-potential monitoring was developed to survey self-potential signals at the ground surface in a flooded infiltration pond for mapping infiltration pathways and permeability at the Aurora Water aquifer recharge and recovery site in Fort Lupton, Colorado, U.S.A. In order to evaluate electrokinetic contributions to the measured self-potential anomalies, a method for using heat as a groundwater tracer was implemented with an array of in situ high-resolution temperature sensing probes. Both relatively positive and negative self-potential anomalies with respect to the background signals are consistent with specific discharge estimates from the temperature data. Results from electrical resistivity tomography and electromagnetics surveys provide electrical conductivity distributions that indicate finer sediment textures over positive self-potential anomalies and coarser sediment textures over negative self-potential anomalies. A laboratory method was developed for saturated electrical resistivity tests of near-surface sediment samples from the clogging layer and below. Measured electrical conductivity distributions are utilized to improve the inversion of self-potential measurements to permeability in 3D using a finite element code. Preliminary forward modeling suggests that the underwater self-potential measurements were more sensitive to differences in specific discharge and permeability than electrical conductivity distributions. Aquifer recharge and recovery system design and capacity could be optimized from such combined geophysical methods by mapping permeability and primary infiltration pathways to guide pond and well locations away from less permeable floodplain deposits.