Time-lapse ERT monitoring of an injection/withdrawal experiment in a shallow unconfined aquifer

Toquantifyperformanceof3Dtime-lapseelectricalresistivity tomography ERT, a sequential injection/withdrawal experiment was designed for monitoring the pump-and-capture remediation of a conductive solute in an unconfined alluvial aquifer. Prior information is incorporated into the inversion procedure via regularization with respect to a reference model according to threeprotocols:1independentregularizationinvolvingasingle reference model, 2 background regularization involving a referencemodelobtainedviainversionofpreinjectiondata,and3 time-lapseregularizationinvolvinganevolvingreferencemodel obtained via inversion of data from previous experimental stages.Emplacementandsequentialwithdrawalofthesoluteisclearly imaged for all protocols. Time-lapse regularization results in greater amounts of model structure, while providing significantcomputationalsavings.ERT-estimatedelectricalconductivity is used to predict solute concentration and solute mass in the aquifer. At any experimental stage, we are able to estimate total solute mass in the aquifer with a maximum accuracy of 60%‐85% depending on regularization protocol and survey geometry.Wealsoestimatethewithdrawnsolutemassforeveryexperimentalstagethechangeinmassbetweenexperimentalstages. Withdrawn mass estimates are more reliable than total mass estimates and do not exhibit systematic underprediction or dependence on regularization protocol. Withdrawn mass estimates areaccurateforchangesinmassbelow2‐4 kgofpotassiumbromide KBr for horizontal and vertical dipole-dipole surveys, respectively. Estimating the withdrawn solute mass does not require background subtraction and, thus, does not require backgrounddata.