Characterisation of a NAPL-contaminated former quarry site using electrical impedance tomography
Electrical impedance tomography (EIT) was used to characterize two tar pits, containing a range of NAPL and aqueous contaminants, located within a former quarry site. The site, which is situated on Coal Measures strata near Belper, Derbyshire, UK, has been the subject of an extensive conventional site investigation. Consequently, the geometry and contents of the pits are relatively well defined. The aims of the survey were to test 3D EIT in field conditions against good ground-truth data and, in combination with intrusive and historical site records, to improve the characterization of the waste pits. The EIT survey employed surface electrodes within a network of linear survey lines. Impedance magnitude and phase data were collected at frequencies of 0.6, 2, 10, 40 and 125 Hz, from which resistivity and phase models were calculated using a 3D smoothness-constrained least-squares inversion technique. Resistivity models displayed only weak frequency-dependent variations. The resistivity images were proved to correlate well with the ground-truth data, and were used to further constrain models of the distribution of waste and bedrock contamination. Furthermore, interrogation of the 0.6 Hz resistivity model permitted estimates of waste and contaminated bedrock volumes for the two pits. A reliable phase model could be produced only from the 0.6 Hz data, due to the increased noise levels affecting the data at higher frequencies. The 0.6 Hz phase model displayed phase variations consistent with the known distribution of the waste. The mechanisms causing the observed phase anomalies could not be conclusively determined; however, it is probable that they are related to the presence of metallic waste. EIT was shown to be a particularly effective tool for site characterization when used in conjunction with intrusive and historical data for the purposes of model calibration and interpretation.