Storage of electrical charge at interfaces and/or proximal to ion-selective membranes in soils and rocks is measured with induced polarization (IP). The three principle physicochemical processes postulated to cause an IP response in porous media are: (1) tangential migration of ions within the electrical double layer (EDL) that develops at a mineral-fluid interface (2) diffusion of redox active ions, coupled with displacement of inactive ions, normal to a metal-fluid interface; (3) ionic charge redistribution due to the formation of ion-selective membranes. The bio-physicochemical properties of porous media related to these IP mechanisms are of relevance to many subsurface physical, chemical and biological processes. Results and interpretation of IP measurements on near-surface environmental systems are presented here.

Metallic mineral polarization results in large IP effects, having implications for investigating heavy metal contaminated soils and the performance of reactive iron wall barriers in groundwater remediation. Measurements on sand- zero valent iron (Fe0) mixtures as a function of metallic surface area and solution chemistry are presented here. The magnitude of the IP response is highly correlated with the surface area of Fe0 per unit sample volume and depends on electrolyte activity and valence. The dominant length-scale of the relaxation mechanism is considered a function of the metallic mineral size and EDL thickness as controlled by electrolyte activity. Precipitation induced on the Fe0 surface results in an increase in the dominant relaxation length scale consistent with an increase in metallic mineral size.

Recent research focuses on the application of electrical methods to investigate microbial activity in porous media. Bacteria are characterized by large surface areas and exhibit an EDL and an ion penetrable outer membrane. IP measurements during natural biodegradation of diesel reveal increases in the surface conductivity attributed to the accumulation of bacteria, perhaps as biofilms, on the mineral surface. Sulfate respiring bacteria in anaerobic environments are responsible for the precipitation of heavy metals on mineral surfaces. IP measurements during microbial induced metal sulfide precipitation show a conclusive correlation with the microbial population and illustrate a remarkable similarity to the rate of lactate consumption with time, suggesting that IP measurements can indirectly track cell metabolism in such a system.

The presented studies clearly illustrate the potential for IP to examine the bio-physicochemical properties of environmental systems. It is important to note that this information obtainable with IP is generally not readily resolved (at least so directly) with other available geophysical measurements.