Hydraulic Tomography Surveys Research Articles

Optimal strategies for assigning prior boundary settings in Hydraulic Tomography analysis

Hydraulic Tomography (HT) is a high-resolution method for identifying aquifer hydraulic property heterogeneity. Despite decades of development leading to a relative maturation of HT, the influence of prior boundary information on HT estimations warrants further investigation. In this study, HT methods were applied to a synthetic heterogeneous aquifer to examine the impact of prior boundary types, sizes, and head values on parameter estimations. Results indicate that while boundary size has limited impact on HT outcomes regardless of data adequacy, the boundary condition type is a significant factor in HT inversion. Increasing boundary size can mitigate errors from incorrect boundary type assumptions. The SimSLE-HCA iterative algorithm effectively estimates optimal boundary head values given sufficient HT survey data. For practical application, in cases where the boundary size is unknown, a moderate expansion of the simulation area is recommended. In situations where the boundary type is uncertain, designating it as a constant head boundary proves practical.

Geostatistical analysis of high-resolution hydraulic conductivity estimates from the hydraulic profiling tool and integration with hydraulic tomography at a highly heterogeneous field site

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.

Large-scale three-dimensional hydraulic tomography analyses of long-term municipal wellfield operations

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.

Aquifer Characterization Using Fiber Bragg Grating Multi-Level Monitoring System.

Groundwater responses measured from multiple wells at different depths are essential for delineating the aquifer heterogeneity using hydraulic tomography (HT). In general, conducting HT requires many wells because traditional well monitoring is usually partially open at a specific depth interval or is fully penetrating. Accordingly, conducting an HT survey is typically costly and time-consuming. To tackle these issues, a new multi-level monitoring system (MLMS) for the HT survey was developed using the fiber Bragg grating (FBG) technique. This FBG MLMS could collect the depth-discrete groundwater observations from a fully penetrated 2-inch well. Three field campaigns were conducted to validate the capability of the FBG MLMS for HT surveys. The results show that the accuracy and stability of this MLMS are reliable and that FBG MLMS is beneficial for conducting an HT survey. Specifically, compared to the traditional monitoring well in an injection event, this FBG MLMS can concurrently cause an increase in the number of cross-hole tests several times and collect many more head observations than the standard methods, resulting in the observed flow fields efficiently reaching ergodic conditions and effectively improving the accuracy of the estimated hydraulic heterogeneity. Therefore, the FBG MLMS could be an alternative MLMS for efficiently and economically conducting an HT survey.

Estimation of the hydraulic properties of a fractured aquifer using numerical experiments with the discrete fracture network model

ABSTRACT Inversion simulations have been applied to reconstruct the spatial variability of the hydraulic characteristics of fractured aquifers. Discrete fracture network (DFN) models exhibit good performances for complex fractured systems, whereas few hydraulic tomography (HT)-based inversion studies of DFN models provide new insights into data selection. The study objective is to find reasonable data selection strategies that accurately estimate the hydraulic properties in a DFN model. A two-dimensional DFN model was constructed, and nine pumping tests were designed for forward simulations. Three cases were designed considering different conditions during inversion simulations. Two hypotheses were verified. First, the feasibility of using HT to identify the hydraulic properties in DFN models with high heterogeneity was demonstrated. Second, the flow behaves as quasi-steady flow because of the minimal effects of storage, and the inverse model can use steady-state conditions to estimate hydraulic conductivity. This study promotes the use of HT surveys for mapping fracture zones.

Temporal sampling and role of flux measurements for subsurface heterogeneous characterization in groundwater basins using hydraulic tomography

AbstractAccurate characterization of heterogeneity in groundwater basins is crucial to the sustainable management of groundwater resources. This study explores the temporal sampling issues and the role of flux measurements in the characterization of heterogeneity in groundwater basins using numerical experiments. The experiments involve a digital basin imitating the groundwater basin of the North China Plain (NCP), where the groundwater exploitation reduction program is ongoing. Using the experiments, we champion that the reduction program could collect groundwater level information induced by operational variations of existing pumping wells at different locations in the basin. Such a dataset could serve as a basin‐scale hydraulic tomography (HT) to characterize the basin‐scale heterogeneity cost‐effectively. Both steady‐state and transient‐state inversion experiments demonstrate the advantage of HT surveys in characterizing basin‐scale heterogeneity over conventional pumping tests at fixed well locations. Additionally, head data at the early, intermediate, and late time from well hydrographs should be selected for the HT analysis to maximize HT's power and save computational costs. When accurate geological zones are incorporated in prior information, flux measurements significantly improve parameter estimates based on conventional pumping tests. However, their effects are less noticeable for long‐term HT surveys in such basin‐scale aquifers without fissures or fractures. This basin‐scale tomographic survey example serves a guide for field data collection and optimization of the analysis of future basin‐scale HT.

Potential of Hydraulic Tomography in Identifying Boundary Conditions of Groundwater Basins

AbstractThis study investigates the potential of hydraulic tomography (HT) in identifying the boundary conditions of groundwater basins using numerical experiments. The experiment mimics the scenario of groundwater exploitation reduction in a pilot area of groundwater overexploitation control in the North China Plain. In this study, we propose an approach that integrates the HT concept and readily available groundwater monitoring data to identify the constant head and impermeable boundaries by mapping anomalously high‐ and low‐permeability zones from HT surveys in a large‐scale domain that encompasses the true groundwater basin. The resulting boundaries and conditions were then used in inversion of steady‐state and transient‐state simultaneous pumping tests and HT surveys of heterogeneity within the groundwater basin. The inversion results demonstrated significant advantages of HT surveys over multiple simultaneous pumping tests to identify boundary conditions and heterogeneity in the groundwater basin. Moreover, steady HT inversion outperforms transient HT inversion in capturing the true boundary conditions, leading to the better T estimates from steady HT inversion than those from transient HT inversion. Additionally, the study shows that accurate geological zonation information can significantly improve HT parameter estimations.

Characterization of basin-scale aquifer heterogeneity using transient hydraulic tomography with aquifer responses induced by groundwater exploitation reduction

This study exploits aquifer responses to the reduction of pumping rates at different locations in a synthetic groundwater basin as basin-scale hydraulic tomography (HT) surveys to estimate transmissivity (T) and storage coefficient (S) fields. This experiment mimics the situation of groundwater exploitation reduction in a pilot area of groundwater-overexploitation control in the North China Plain. The results of the study show that taking advantage of the groundwater exploitation reduction as HT surveys is a viable approach for basin-scale parameter estimations. Results also suggest that HT analysis should use accurate mean values of T and S for geological zones as initial guesses for the inversion of parameters. Further, we show that the T and S fields estimated from HT yield accurate predictions of the groundwater flow velocities and breakthrough curves (BTCs). However, the BTCs based on kriged and zonal mean fields are inaccurate. The predicted BTCs using homogeneous fields fail to capture the true trend of solute concentration over time. We advocate that utilizing aquifer responses induced by groundwater exploitation reduction could be a new paradigm for basin-scale aquifer characterization.

Identification of groundwater basin shape and boundary using hydraulic tomography

Shapes and boundary types of a groundwater basin play essential roles in the analysis of groundwater management and contaminant migration. Hydraulic tomography (HT), a recently developed new approach for high-resolution characterization of aquifers, is not only an inverse method but a logical strategy for collecting non-redundant hydraulic information. In this study, HT was applied to synthetic 2-D aquifers to investigate its feasibility to map the irregular shapes and types of the aquifer boundaries. We first used the forward model of VSAFT2 to simulate hydraulic responses due to HT surveys in the aquifer with irregular geometry and predetermined constant head conditions at some boundaries, and no-flow conditions at others. The SimSLE (Simultaneous Successive Linear Estimator) inverse model in VSAFT2 was then used to interpret the simulated HT data to estimate the spatial distribution of hydraulic properties of the aquifer using a domain with a wrong geometry surrounded by boundaries of a constant head condition. The inverse modeling experiment used steady-state and transient-states data from the HT forward simulations, and it used the same monitoring network as in the aquifer with irregular geometry to assess the ability of HT for detecting types and shapes of the boundary as well as heterogeneity in the aquifer. Results of the experiment show that no-flow boundaries, which were incorrectly treated as constant head boundaries in inverse models, were portrayed as low permeable zones of the aquifer near the boundaries. Overall, the results show that HT could delineate not only the irregular shape of the aquifer in general but also heterogeneity in the aquifer. Improvements of the estimation with prior information of transmissivity and storage coefficient was also investigated. The study shows that using homogeneous initial guess parameters resulted in a slightly better estimate than others. Moreover, this study employs Monte Carlo simulations to ensure statistically meaningful conclusions.

Transient Hydraulic Tomography Analysis of Fourteen Pumping Tests at a Highly Heterogeneous Multiple Aquifer–Aquitard System

Hydraulic tomography based on geostatistics has proven to be robust in characterizing subsurface heterogeneity in hydraulic conductivity (K) and specific storage (Ss) through the joint inversion of drawdown records from multiple pumping tests. However, the spatially variable estimates can be smooth or even erroneous for areas where pumping/observation data densities are not high. Previous hydraulic tomography surveys conducted at the North Campus Research Site (NCRS) on the University of Waterloo campus in Waterloo, Canada, revealed that the estimated hydraulic parameters were smooth and the known aquitard was erroneously identified as a high K zone. This was likely the consequence of the site being highly heterogeneous, while only utilizing four pumping tests and not having measurable drawdowns in the low K aquitard for inverse modeling. Here, we investigate whether improved K and Ss estimates could be obtained through the inclusion of additional pumping test data by stressing both aquifer and aquitard zones for a sufficiently long period. Specifically, six additional pumping/injection tests were conducted at the site, and a transient hydraulic tomography analysis with 14 tests was completed. Results reveal that there is a significant improvement to the K and Ss tomograms in terms of the visual correspondence with various geologic units, including its connectivity. More importantly, with the availability of additional data, we found that the inverse model now can better capture the high and low K features for nine boreholes when compared with K values obtained from permeameter tests. The estimated K and Ss tomograms are then used for the forward simulation of one additional pumping test not used for model calibration, revealing reasonable predictions. While encouraging results are obtained by including a large number of pumping tests to the transient hydraulic tomography analysis, stratigraphic boundaries are still smoothed, which is a direct consequence of utilizing a geostatistics-based inversion approach that assumes stationarity in statistical properties. To capture such sharp boundaries, incorporation of additional data types, such as geological and geophysical information, may be necessary when data densities are not sufficiently high.

Exploitation of pump-and-treat remediation systems for characterization of hydraulic heterogeneity

Pump-and-treat (P&T) is a widely adopted technology for groundwater containment/remediation. Over the past decades, we have recognized that mapping aquifer heterogeneity, especially the low hydraulic conductivity (K) zones, has become an urgent need in remediation efforts. Meanwhile, many studies have proved that hydraulic tomography (HT) utilizing groundwater responses from a series of cross-hole pumping tests is a viable technology for detailing hydraulic heterogeneity (high and low K zones) of aquifers. At a P&T field site, operation of pumping and injection wells often vary, creating changes in flow fields analogous to those in HT surveys. Therefore, collecting groundwater monitoring data and analyzing them is tantamount to a cost-effective large-scale HT survey during P&T operations. To demonstrate this idea, water level data was collected at 41 wells for one year at the U.S. Air Force Plant 44 (AFP44) P&T site in Tucson, Arizona, where the pumping and injection rates of the P&T system were changed many times during the year. This dataset was analyzed with a geostatistical inverse model to estimate the hydraulic property distribution, which can reasonably predict the pressure responses of the site under different conditions. Moreover, we showed that large-scale geological information, built based on 245 borehole logs at the site, could improve the model calibration performance through a two-step calibration procedure. Additionally, we used synthetic experiments to verify that the use of a larger variety of P&T configurations can yield non-redundant HT information, and enhance site characterization. Overall, we demonstrated that the proper scheduling of pumping and injection operations at a P&T site and collecting corresponding groundwater responses are equivalent to conducting a large-scale HT survey, with minimal additional costs.

Numerical assessments of recharge-dominated groundwater flow and transport in the nearshore reclamation area in western Taiwan.

This study employed experimental and numerical methods to assess the behavior of conservative solute transport for a selected temporary solid waste site in a reclamation area in western Taiwan. Calibrating a site-specific numerical model, finite element model of water flow through saturated-unsaturated media (FEMWATER), relies on observations from field- and laboratory-scale hydraulic tests and spatial-temporal monitoring. The field-scale experiment used a modified hydraulic tomography survey (MHTS) to identify near surface aquifer stratifications and estimate the distribution of saturated hydraulic conductivity. The pressure plate experiments provided parameters for the van Genuchten soil characteristic model. Sensitivity analyses were then conducted based on varied recharge rates and dispersivities applied to the calibrated model. Observations of groundwater levels and salinity in the wells indicated that the regional groundwater flow was from southeast to northwest. In addition, a shallow freshwater layer was noted in the study area. The tidal-induced amplitudes for water level fluctuation in the wells ranged from 2 to 20cm, depending on their distance from the seawater body. MHTS showed clear stratification, similar to that of well loggings at the storage site. The hydraulic conductivity at the test site ranged from 8 to 10m/day, which is close to that obtained from the laboratory falling head test. The results of particle-tracking modeling showed that the critical recharge rate for the site needed to enhance plume traveling is 1000mm/year. The increase in dispersivity values induced a decrease in plume travel time of up to 1000days from the site to the coastal line. A special case for pulse releasing solute at the site shows that the key factor in controlling plume migration is the recharge rate. This is due to the low natural head gradient in the reclamation area. The results therefore suggest that a land drainage system near the site can play an important role in contaminant transport in the reclamation area.

An application of hydraulic tomography to a deep coal mine: Combining traditional pumping tests with water inrush incidents

Water inrush incidents threaten the safety of coal mining. Understanding of hydrogeologic parameter distributions is critical for preventing water-related hazards in coal mines. During the deep mining (>1000 m) under the North China Plain, water from water-bearing strata discharges into coal seams through geologic conduits (i.e. water inrush) due to the fractured zone under the floor of working faces. In this study, a water inrush incident was exploited as an active stimulus. A 3D groundwater flow model was built for the eighth member of the Middle Ordovician system in Xingdong coal mine. Using this model and an inverse approach, we first checked if the data from the incident and an independent pumping test carry non-redundant information about the heterogeneity of the mine. Afterward, we combined these datasets to conduct a large-scale (approximately 10 km) hydraulic tomography (HT) analysis. The estimated hydraulic conductivity distribution from the HT analysis is found consistent with the distribution of known geologic faults. That is, a cluster of faults is characterized as a high-conductivity zone. A high conductivity zone is identified at locations close to the water inrush location, which is the high cement consumption zone during the grouting project. Finally, results of this study promote exploiting the water inrush events as a HT survey for mapping geologic structures over a large area.

On the importance of geological data for hydraulic tomography analysis: Laboratory sandbox study

This paper investigates the importance of geological data in Hydraulic Tomography (HT) through sandbox experiments. In particular, four groundwater models with homogeneous geological units constructed with borehole data of varying accuracy are jointly calibrated with multiple pumping test data of two different pumping and observation densities. The results are compared to those from a geostatistical inverse model. Model calibration and validation performances are quantitatively assessed using drawdown scatterplots. We find that accurate and inaccurate geological models can be well calibrated, despite the estimated K values for the poor geological models being quite different from the actual values. Model validation results reveal that inaccurate geological models yield poor drawdown predictions, but using more calibration data improves its predictive capability. Moreover, model comparisons among a highly parameterized geostatistical and layer-based geological models show that, (1) as the number of pumping tests and monitoring locations are reduced, the performance gap between the approaches decreases, and (2) a simplified geological model with a fewer number of layers is more reliable than the one based on the wrong description of stratigraphy. Finally, using a geological model as prior information in geostatistical inverse models results in the preservation of geological features, especially in areas where drawdown data are not available. Overall, our sandbox results emphasize the importance of incorporating geological data in HT surveys when data from pumping tests is sparse. These findings have important implications for field applications of HT where well distances are large.

An Application of Hydraulic Tomography to a Large-Scale Fractured Granite Site, Mizunami, Japan.

While hydraulic tomography (HT) is a mature aquifer characterization technology, its applications to characterize hydrogeology of kilometer-scale fault and fracture zones are rare. This paper sequentially analyzes datasets from two new pumping tests as well as those from two previous pumping tests analyzed by Illman et al. (2009) at a fractured granite site in Mizunami, Japan. Results of this analysis show that datasets from two previous pumping tests at one side of a fault zone as used in the previous study led to inaccurate mapping of fracture and fault zones. Inclusion of the datasets from the two new pumping tests (one of which was conducted on the other side of the fault) yields locations of the fault zone consistent with those based on geological mapping. The new datasets also produce a detailed image of the irregular fault zone, which is not available from geological investigation alone and the previous study. As a result, we conclude that if prior knowledge about geological structures at a field site is considered during the design of HT surveys, valuable non-redundant datasets about the fracture and fault zones can be collected. Only with these non-redundant data sets, can HT then be a viable and robust tool for delineating fracture and fault distributions over kilometer scales, even when only a limited number of boreholes are available. In essence, this paper proves that HT is a new tool for geologists, geophysicists, and engineers for mapping large-scale fracture and fault zone distributions.

The relative importance of head, flux, and prior information in hydraulic tomography analysis

AbstractUsing cross‐correlation analysis, we demonstrate that flux measurements at observation locations during hydraulic tomography (HT) surveys carry nonredundant information about heterogeneity that are complementary to head measurements at the same locations. We then hypothesize that a joint interpretation of head and flux data, even when the same observation network as head has been used, can enhance the resolution of HT estimates. Subsequently, we use numerical experiments to test this hypothesis and investigate the impact of flux conditioning and prior information (such as correlation lengths and initial mean models (i.e., uniform mean or distributed means)) on the HT estimates of a nonstationary, layered medium. We find that the addition of flux conditioning to HT analysis improves the estimates in all of the prior models tested. While prior information on geologic structures could be useful, its influence on the estimates reduces as more nonredundant data (i.e., flux) are used in the HT analysis. Lastly, recommendations for conducting HT surveys and analysis are presented.

A temporal sampling strategy for hydraulic tomography analysis

This paper investigates optimal sampling times of drawdowns for the analysis of hydraulic tomography (HT) survey. The investigation was carried out by analyzing the spatial and temporal evolution of cross‐correlations between the head responses at an observation well and transmissivity (T) and storage coefficient (S) properties during a pumping test in homogeneous and heterogeneous aquifers. The analysis shows that the cross‐correlation between the head and S values is limited to the region between the observation and the pumping well in the aquifers: It reaches the highest value near the early time (tm), and decays to zero afterwards. The time tm is approximately equal to the time t0 at which the extrapolated drawdown from the first straight line portion of an observed drawdown‐log time plot becomes zero. At early times, the high cross‐correlation between the head and T is confined to the region between the observation and the pumping well. This region then evolves into two humps: One on each side of the circular region encompassing the observation well and the pumping well. The size of the two humps expands and their values reach the maximum as flow reach steady‐state. As a consequence, we hypothesize that pairs of head data at t0 and those at either the steady‐state or a late time during an HT survey could yield the best estimates of the heterogeneous T and S fields. Results from numerical experiments have verified this hypothesis and demonstrated that this sampling strategy is generally applicable even when the boundary condition is unknown. We, therefore, recommend in principle that (1) carrying out pumping tests of HT surveys for sufficiently long period of time such that drawdown reaches the entire area of interest and (2) using a constant head or zero drawdown for all boundaries during the inverse modeling analysis.

Field Study of Subsurface Heterogeneity with Steady‐State Hydraulic Tomography

Remediation of subsurface contamination requires an understanding of the contaminant (history, source location, plume extent and concentration, etc.), and, knowledge of the spatial distribution of hydraulic conductivity (K) that governs groundwater flow and solute transport. Many methods exist for characterizing K heterogeneity, but most if not all methods require the collection of a large number of small-scale data and its interpolation. In this study, we conduct a hydraulic tomography survey at a highly heterogeneous glaciofluvial deposit at the North Campus Research Site (NCRS) located at the University of Waterloo, Waterloo, Ontario, Canada to sequentially interpret four pumping tests using the steady-state form of the Sequential Successive Linear Estimator (SSLE) (Yeh and Liu 2000). The resulting three-dimensional (3D) K distribution (or K-tomogram) is compared against: (1) K distributions obtained through the inverse modeling of individual pumping tests using SSLE, and (2) effective hydraulic conductivity (K(eff) ) estimates obtained by automatically calibrating a groundwater flow model while treating the medium to be homogeneous. Such a K(eff) is often used for designing remediation operations, and thus is used as the basis for comparison with the K-tomogram. Our results clearly show that hydraulic tomography is superior to the inversions of single pumping tests or K(eff) estimates. This is particularly significant for contaminated sites where an accurate representation of the flow field is critical for simulating contaminant transport and injection of chemical and biological agents used for active remediation of contaminant source zones and plumes.

Comparison of Approaches for Predicting Solute Transport: Sandbox Experiments

The main purpose of this paper was to compare three approaches for predicting solute transport. The approaches include: (1) an effective parameter/macrodispersion approach (Gelhar and Axness 1983); (2) a heterogeneous approach using ordinary kriging based on core samples; and (3) a heterogeneous approach based on hydraulic tomography. We conducted our comparison in a heterogeneous sandbox aquifer. The aquifer was first characterized by taking 48 core samples to obtain local-scale hydraulic conductivity (K). The spatial statistics of these K values were then used to calculate the effective parameters. These K values and their statistics were also used for kriging to obtain a heterogeneous K field. In parallel, we performed a hydraulic tomography survey using hydraulic tests conducted in a dipole fashion with the drawdown data analyzed using the sequential successive linear estimator code (Yeh and Liu 2000) to obtain a K distribution (or K tomogram). The effective parameters and the heterogeneous K fields from kriging and hydraulic tomography were used in forward simulations of a dipole conservative tracer test. The simulated and observed breakthrough curves and their temporal moments were compared. Results show an improvement in predictions of drawdown behavior and tracer transport when the K tomogram from hydraulic tomography was used. This suggests that the high-resolution prediction of solute transport is possible without collecting a large number of small-scale samples to estimate flow and transport properties that are costly to obtain at the field scale.

Cost-Effective Hydraulic Tomography Surveys for Predicting Flow and Transport in Heterogeneous Aquifers

This study shows how a cost-effective hydraulic tomography survey (HTS) and the associated data estimator can be used to characterize flow and transport in heterogeneous aquifers. The HTS is an improved field hydraulic test that accounts for responses of hydraulic stresses caused by pumping or injection events at different locations of an aquifer. A sequential data assimilation method based on a cokriging algorithm is then used to map the aquifer hydraulic conductivity (K). This study uses a synthetic two-dimensional aquifer to assess the accuracy of predicted concentration breakthrough curves (BTCs) on the basis of the Kfields estimated by geometric mean, kriging, and HTS. Such Kfields represent different degrees of flow resolutions as compared with the synthetically generated one. Without intensive experimentsto calibrate accurate dispersivities at sites, the flow field based on the HTS Kfield can yield accurate predictions of BTC peaks and phases. On the basis of calculating mean absolute and square errors for estimated K fields, numerical assessments on the HTS operation strategy show that more pumping events will generally lead to more accurate estimations of Kfields, and the pump locations need to be installed in high Kzones to maximize the delivery of head information from pumps to measurement points. Additionally, the appropriate distances of installed wells are suggested to be less than one-third of the ln(K) correlation length in x direction.