Comparison of Hydraulic Tomography with Traditional Methods at a Highly Heterogeneous Site by Steven J. Berg 1,2 and Walter A. Illman 3 Abstract Over the past several decades, different groundwater modeling approaches of various complexities and data use have been developed. A recently developed approach for mapping hydraulic conductivity (K ) and specific storage (S s ) heterogeneity is hydraulic tomography, the performance of which has not been compared to other more ‘‘traditional’’ methods that have been utilized over the past several decades. In this study, we compare seven methods of modeling heterogeneity which are (1) kriging, (2) effective parameter models, (3) transition probability/Markov Chain geostatistics models, (4) geological models, (5) stochastic inverse models conditioned to local K data, (6) hydraulic tomography, and (7) hydraulic tomography conditioned to local K data using data collected in five boreholes at a field site on the University of Waterloo (UW) campus, in Waterloo, Ontario, Canada. The performance of each heterogeneity model is first assessed during model calibration. In particular, the correspondence between simulated and observed drawdowns is assessed using the mean absolute error norm, (L 1 ), mean square error norm (L 2 ), and correlation coefficient (R) as well as through scatterplots. We also assess the various models on their ability to predict drawdown data not used in the calibration effort from nine pumping tests. Results reveal that hydraulic tomography is best able to reproduce these tests in terms of the smallest discrepancy and highest correlation between simulated and observed drawdowns. However, conditioning of hydraulic tomography results with permeameter K data caused a slight deterioration in accuracy of drawdown predictions which suggests that data integration may need to be conducted carefully. Introduction Groundwater investigations require estimates of hydraulic parameters, namely hydraulic conductivity (K ) and specific storage (S s ). For relatively homogeneous sys- tems, it may be possible to characterize the aquifer using traditional type curve (e.g., Theis 1935) or straight line (Cooper and Jacob 1946; Illman and Tartakovsky 2006) methods that treat the medium to be uniform. However, many aquifers cannot be treated to be uniform because of significant variability in geology which translates to large heterogeneities in hydraulic parameters. This has led to decades of research in characterizing and modeling sub- surface heterogeneity of hydraulic parameters (de Marsily et al. 2005). 1 Department of Earth & Environmental Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. 2 Aquanty Inc., Waterloo, Ontario N2L 5C6, Canada. 3 Corresponding author: Department of Earth & Environ- mental Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada; 519-888-4567; fax: 519-746-7484; willman@uwaterloo.ca Received August 2013, accepted December 2013.  2014, National Ground Water Association. doi: 10.1111/gwat.12159 Representative methods for capturing heterogeneity of hydraulic parameters include those that (1) upscale point-scale hydraulic parameter estimates (Renard and de Marsily 1997; Sanchez-Vila et al. 2006 and references therein); (2) interpolate point-scale estimates between sampling points through geostatistics (de Marsily et al. 2005); (3) create realizations of heterogeneity fields through random field simulations (Deutsch and Journel 1998); (4) estimate parameters via deterministic or stochastic inverse modeling of hydraulic head and/or other data types such as solute concentration or geophysical data (Harvey and Gorelick 1995; Kitanidis 1995; Yeh et al. 1996); and (5) create stratigraphic or geological models in which the layers are calibrated to pumping tests or other data (Martin and Frind 1998; Jones et al. 2008). Despite the wide range of approaches available, 4 the selection of an appropriate characterization and modeling method, as well as choosing data to create these models is an ever present challenge at any given site. A new method for characterizing and modeling aquifer heterogeneity that has received considerable 4 A more detailed review of these methods is provided in Appendix S1. NGWA.org Groundwater 1