Search

Transient hydraulic tomography (THT) is a robust method of aquifer characterization to estimate the spatial distributions (or tomograms) of both hydraulic conductivity ( K ) and specific storage ( S s ). However, the highly-parameterized nature of the geostatistical inversion approach renders it computationally intensive for large-scale investigations. In addition, geostatistics-based THT may produce overly smooth tomograms when head data used to constrain the inversion is limited. Therefore, alternative model conceptualizations for THT need to be examined. To investigate this, we simultaneously calibrated different groundwater models with varying parameterizations and zonations using two cases of different pumping and monitoring data densities from a laboratory sandbox. Specifically, one effective parameter model, four geology-based zonation models with varying accuracy and resolution, and five geostatistical models with different prior information are calibrated. Model performance is quantitatively assessed by examining the calibration and validation results. Our study reveals that highly parameterized geostatistical models perform the best among the models compared, while the zonation model with excellent knowledge of stratigraphy also yields comparable results. When few pumping tests with sparse monitoring intervals are available, the incorporation of accurate or simplified geological information into geostatistical models reveals more details in heterogeneity and yields more robust validation results. However, results deteriorate when inaccurate geological information are incorporated. Finally, our study reveals that transient inversions are necessary to obtain reliable K and S s estimates for making accurate predictions of transient drawdown events. [ABSTRACT FROM AUTHOR]

• HPT surveys at a site with K spanning nearly-seven orders of magnitude. • HPT reveals both sharp and gradual changes in K values across layer boundaries. • Integration of HPT-inverted K into HT yields better matches with permeameter K. • HPT K can help HT refine layer boundaries and capture intralayer K variability. Hydraulic profiling tool (HPT) and hydraulic tomography (HT) have been developed as promising techniques for the high-resolution characterization of surficial aquifer systems. HPT surveys can be rapidly conducted at a resolution of 1.5-cm, but provide only one-dimensional vertical profiles and require site-specific formulae to relate HPT measurements to K. Geostatistics-based HT can estimate three-dimensional distributions of hydraulic parameters, but may fail to provide detailed information and can be smooth when pumping/observation data density is sparse and usually are constrained to the area enclosed by the pumping and observation network. In this study, HPT and HT surveys were conducted at the North Campus Research Site (NCRS) in Waterloo, Ontario, Canada to characterize a glaciofluvial multi-aquifer-aquitard system with permeameter K values spanning nearly seven orders of magnitude. We first performed the geostatistical analysis of K values derived from 11 HPT surveys using the power-law model developed for the NCRS by Zhao and Illman (2022b). Then, the benefits of incorporating HPT K profiles into HT were evaluated for the reconstruction of hydraulic parameter fields. Results showed that the arithmetic mean of 11 HPT K profiles was nearly one order of magnitude higher than that of the 544 permeameter test K measurements. The K fields interpolated by ordinary kriging captured the vertical alternating layering patterns of aquifer and aquitard layers, while they over-predicted the values by nearly one order of magnitude than permeameter K for the top part of the aquifer system. By incorporating the kriged K values as prior means for the geostatistics-based HT, improvements were found in capturing spatial heterogeneity of K fields for areas both inside and outside the well cluster and in drawdown predictions than the inversion case using only pressure head data. The inclusion of vertical variation information of K derived from HPT into HT was also helpful in refining the vertical locations of estimated layer boundaries and introducing intralayer variation patterns of K for both aquifer and aquitard layers. This study demonstrates the potential ability of HT and HPT and advocates the joint use of both techniques for the characterization of subsurface heterogeneity at highly heterogeneous sites such as the NCRS. [ABSTRACT FROM AUTHOR]

• HT surveys in 2019 and 2020 to test the reproducibility of estimated K and S s. • 3D K and S s fields are reproducible even when boundary conditions are varied. • Improved reproducibility observed when inverting with higher data density. • Accurate geological information included as initial guess improved reproducibility. • HT has the potential for mapping changes in K and S s heterogeneity over time. Over the past two decades, geostatistics-based hydraulic tomography (HT) has been proven to be a robust method for subsurface heterogeneity characterization. However, the reproducibility of estimated parameter fields, namely the hydraulic conductivity (K) and specific storage (S s), obtained over different data collection periods and under different flow scenarios have been rarely studied. In this study, we investigate the reproducibility and predictivity of HT estimates using data collected in 2019 and 2020 at a field site of varying boundary conditions underlain by sedimentary units in Japan, with different data density and prior geological information. Model performance of each case is evaluated qualitatively and quantitatively by examining results from model calibration, self-validation and cross-validation, in terms of parameter fields comparison, drawdown matches and cross-correlation distributions. Results of this study reveal that: 1) overall, the parameter fields for 2019 and 2020 HT estimates are generally reproducible, while fine-scale discrepancy exists; 2) they are also found to be reproducible in terms of consistency of estimated geostatistical parameters; 3) higher levels of reproducibility and predictivity associated with estimated K and S s fields are obtained when higher data density is included for geostatistical inverse modeling with more accurate prior geological information; and 4) the different spatial distributions of cross-correlation between head and parameter fields induced through the significant variation of the groundwater flow field from 2019 to 2020 strengthen the notion of reproducibility of HT results. These results collectively suggest that HT has the potential for mapping changes to K and S s heterogeneity over time broadening its future applications to various practical problems. [ABSTRACT FROM AUTHOR]

• Field application of hydraulic tomography using existing municipal well records. • Uncertainties regarding initial and boundary conditions are accounted in analyses. • Geostatistical model is advocated for revealing intralayer heterogeneity patterns. Hydraulic tomography (HT) has been shown to be an effective approach for subsurface heterogeneity characterization. However, only a few HT studies have been performed for large-scale field problems due to the difficulty in conducting dedicated HT surveys at large-scale sites, as well as the uncertainty regarding estimated initial and boundary conditions for inverse modeling. To overcome these issues, this study advocates the utilization of existing long-term municipal well records as alternative datasets for large-scale heterogeneity characterization, along with novel data processing and analyses strategies that are proposed to minimize the effect of uncertain initial and boundary conditions on inverse modeling. Specifically, geology-based zonation and geostatistical models are adopted for site heterogeneity characterization through HT analyses of municipal well records, and the estimated hydraulic parameters from both models are validated through the prediction of head response data that have not been used for calibration efforts. Our results reveal that existing field municipal well records could be utilized for large-scale subsurface heterogeneity characterization using the approach of HT when uncertainties regarding initial and boundary conditions are well addressed for inverse modeling. In comparison to geology-based zonation model, the geostatistical model reveals greater details of intralayer heterogeneity where head response data are available, yielding significantly improved calibration and validation results. Overall, this study provides a general framework of using existing hydrographs for large-scale heterogeneity characterization through HT, and advocates the utilization of geostatistical inverse modeling as the second step over traditional zonation modeling approach to reveal intralayer heterogeneity details of hydraulic parameters. [ABSTRACT FROM AUTHOR]

• Geostatistics-based hydraulic tomography (HT) yields smooth K and S s estimates. • Integration of accurate geologic data significantly improves parameter estimates. • Fracture orientation, connectivity, and matrix data are useful for improving HT. • Inversion results are better with homogeneous values than with bad geologic data. The accurate characterization of hydraulic properties such as hydraulic conductivity (K) and specific storage (S s) for fractured geologic media as well as imaging of fracture patterns and their connectivity are of paramount importance to robust groundwater flow and transport predictions. Recently, transient hydraulic tomography (THT) has been suggested as a promising approach for imaging the K and S s distributions of porous and fractured geologic media. This study investigates the importance of geological information on geostatistics-based THT analysis through synthetic and laboratory rock block experiments. Specifically, fracture geometry, connectivity and matrix hydraulic parameters of varying accuracy are utilized as priori information in THT analysis. Comparison of results from model calibration and validation indicates that: 1) THT analysis can capture the overall fracture pattern and their hydraulic properties (K and S s), but the estimated values are higher within the matrix, where observations are limited when the inversion begins with effective K and S s ; 2) using correct fracture pattern, connectivity and matrix data as prior information can better preserve fracture and matrix hydraulic parameters and their patterns, especially where drawdown data are hard to obtain; and 3) THT analysis starting with homogeneous hydraulic parameter estimates without geologic information is more reliable than the one based on the wrong or incomplete description of geologic features. Overall, results from this study indicate the importance of incorporating accurate geological data in THT analyses when drawdown data are sparse and not available within the matrix. However, existing technology does not allow for the accurate mapping of fracture patterns and their connectivity between boreholes, hence further research is necessary in data fusion of other types of information to improve THT results. [ABSTRACT FROM AUTHOR]

• Hydraulic tomography analysis of long-term municipal wellfield data is feasible. • Selection of data points for inverse modeling requires careful consideration. • Results are validated through synthetic flow and solute transport experiments. This study proposes the utilization of municipal well records as an alternative dataset for large-scale heterogeneity characterization of hydraulic conductivity (K) and specific storage (S s) 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 K and S s 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 K and S s 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 K and S s 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. [ABSTRACT FROM AUTHOR]

• Comparison of tomograms from travel time and geostatistical inversion approaches; • Travel time inversion is suitable for coarse characterization of heterogeneity with low cost; • Geostatistical inversion is suitable for parameter estimation with high accuracy. 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 interpretation methods for HT data. In particular, GI is used to estimate heterogeneous hydraulic conductivity (K) and specific storage (S s) tomograms, while TTI yields the diffusivity (D = K/S s) tomogram. The main objective of this paper is to compare the performances of these two methods through a synthetic study. Two cases are designed based on different monitoring configurations. In each case, two independent scenarios are designed as: (1) providing the same dataset, and (2) providing datasets based on recommended strategies for each inversion approach. Then, the estimated tomograms are evaluated by direct comparison of estimated parameter distributions and assessment of model validation results. Comparison results show that the advantages of TTI are: (1) imaging of structural features representing high-D zones; and (2) requirement of less data for inverse modeling. In contrast, the advantages of GI are: (1) estimation of parameters throughout the simulation domain; (2) better characterization of low-D zones; and (3) generating the best estimated tomogram leading to best drawdown predictions. Collectively, our study suggests that TTI is suitable for rapid, coarse characterization of heterogeneity that could be utilized for providing hydrogeological structures for GI as an initial model. GI is more robust and preferable to applications that require higher accuracy in parameter estimation over a larger volume. [ABSTRACT FROM AUTHOR]