ELECTRICAL MONITORING OF SOLUTE TRANSPORT AT THE CAPE COD TOXIC SUBSTANCES HYDROLOGY FIELD SITE: A 10-YEAR RETROSPECTIVE ON WHAT CAN BE ACCOMPLISHED WITH 6000 GALLONS OF SODIUM CHLORIDE TRACER AND A CAR BATTERY
The goal of the experiment was to estimate properties of plume movement, such as tracer mass, center of mass, and spatial variance without having the large number of wells often required to calculate these spatial moments. Using modified moment analysis of the electrical conductivity tomograms, these properties were calculated using only four wells. Although the tomograms provide valuable insights into field-scale tracer migration behavior and aquifer heterogeneity, and the center of mass estimated from the ER inversions coincided with that given by migration of the tracer plume using 3-D advective-dispersion simulation, standard tomographic inversion lead to some complicating factors: underestimation of tracer mass and overestimation of spatial variance. Such issues were attributed to (1) reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and (2) spatial smoothing (regularization) from tomographic inversion.
This work prompted the development of new inversion techniques for electrical imaging that focus on inversion for plume moments. Capitalizing on the mathematical analogy between moment-based descriptors of plumes and the moment-based parameters of probability distributions, a new inverse problem was designed that describes plumes by only a few parameters (rather than many thousands, as is used in the state-of-the-practice), producing tomograms consistent with expected plume behavior and requiring comparatively few data to achieve reasonable model estimates.