Field Investigation Of Salt Transport Processes Using .Resistivity/Ip Imaging

Two examples of the use of combined resistivity/induced polarization (IP) monitoring of salt<br>transport under natural hydraulic loads are presented. Previous workers have performed electrical<br>monitoring of the transport of a tracer artificially injected into the subsurface. The detection of<br>tracer transport due to natural hydraulic processes is a more difficult goal as neither the hydraulic<br>load nor the resistivity contrast can be controlled. Tide-induced salt transport at the saltwaterfreshwater<br>interface was investigated in the first study. 2D resistivity and IP models resolved the<br>structure of the interface. The IP modeling provided greater clarity of the subsurface<br>hydrology/geology than the resistivity modeling alone. Modeling of electrical changes over a<br>tidal cycle revealed resistivity increases in the near surface and at depth, 5-l 5 m up-beach of the<br>high tide mark. The results indicate a phase lag between the subsurface hydrological response<br>and the tidal cycle. In the second study, salt transport from the site of an old road-salt storage pile<br>was investigated. The hydraulic mechanism was natural recharge events that occurred over a<br>four-month period. 2D conductivity and IP models resolved the subsurface distribution of salt,<br>initially identified from an EM3 1 survey. Modeling of resistivity change on lines spaced 33.5 m<br>apart revealed changes at the location of salt contamination. The changes on each line appear<br>closely coupled and correlate with the recharge pattern over the monitoring period. The studies<br>indicate that salt transport occurring under natural hydraulic conditions can be detected with<br>resistivity, and to some extent, IP.