Hydraulic tomography (HT) has been proven to be an effective approach in mapping the heterogeneity of hydraulic parameters. The travel-time based inversion (TTI) and geostatistical inversion (GI) approaches are two of several HT methods. In particular, the GI approach is used to compute heterogeneous hydraulic conductivity (K) and specific storage (Ss) tomograms, while the TTI approach yields diffusivity (D = K/Ss) tomograms. The main objective of this paper is to evaluate these two methods through a synthetic study. Two cases are designed based on different monitoring configurations. Two independent scenarios are designed by: providing the same data sets and providing all available data for calibration, while data selection follows recommended strategies utilized by the two approaches. Then, the estimated tomograms are evaluated by visual comparison of estimated parameter distributions and assessments of model calibration and validation results. Results show that the advantages of TTI are: (1) imaging of structural features representing high D zones; (2) requirement of less data for inverse modeling; and (3) rapid computational times. In contrast, the advantages of the GI approach are: (1) the direct characterization of both K and Ss distributions; (2) better drawdown predictions; and (3) a larger estimated area. Our study suggests that the TTI approach is suitable for rapid, coarse characterization of heterogeneity that could potentially be utilized for providing hydrogeological structures for an initial model for the GI approach. The GI approach, although significantly more computationally intensive, is more robust and preferable to applications that require higher accuracy in parameter estimation.

This study proposes the utilization of municipal well records as an alternative dataset for large-scale heterogeneity characterization of hydraulic conductivity () and specific storage () using hydraulic tomography (HT). To investigate the performance of HT and the feasibility of utilizing municipal well records, a three-dimensional aquifer/aquitard system is constructed and synthetic groundwater flow and solute transport experiments are conducted to generate data for inverse modeling and validation of results. In particular, we simultaneously calibrate four groundwater models with varying parameterization complexity using five datasets consisting of different time durations and periods. Calibration and validation results are qualitatively and quantitatively assessed to evaluate the performance of investigated models. The estimated and tomograms from different model cases are also validated through the simulation of independently conducted pumping tests and conservative solute transport. Our study reveals that: 1) the HT analysis of municipal well records is feasible and yields reliable heterogeneous and distributions where drawdown records are available; 2) accurate geological information is of critical importance when data density is low and should be incorporated for geostatistical inversions; 3) the estimated and tomograms from the geostatistical model with geological information are capable in providing robust predictions of both groundwater flow and solute transport. Overall, this synthetic study provides a general framework for large-scale heterogeneity characterization using HT through the interpretation of municipal well records, and provides guidance for applying this concept to field problems.