Kuo Chin Hsu

Hydraulic Tomography for Detecting Fracture Zone Connectivity

Fracture zones and their connectivity in geologic media are of great importance to ground water resources management as well as ground water contamination prevention and remediation. In this paper, we applied a recently developed hydraulic tomography (HT) technique and an analysis algorithm (sequential successive linear estimator) to synthetic fractured media. The application aims to explore the potential utility of the technique and the algorithm for characterizing fracture zone distribution and their connectivity. Results of this investigation showed that using HT with a limited number of wells, the fracture zone distribution and its connectivity (general pattern) can be mapped satisfactorily although estimated hydraulic property fields are smooth. As the number of wells and monitoring ports increases, the fracture zone distribution and connectivity become vivid and the estimated hydraulic properties approach true values. We hope that the success of this application may promote the development and application of the new generations of technology (i.e., hydraulic, tracer, pneumatic tomographic surveys) for mapping fractures and other features in geologic media.

A revisit of drawdown behavior during pumping in unconfined aquifers

Received 16 March 2010; revised 7 February 2011; accepted 7 March 2011; published 6 May 2011. [1] In this study, the S‐shaped log‐log drawdown‐time curve typical of pumping tests in unconfined aquifers is reinvestigated via numerical experiments. Like previous investigations, this study attributes the departure of the S shape from the drawdown‐time behavior of the confined aquifer to the presence of an “additional” source of water. Unlike previous studies,this sourceof water isreinvestigated byexaminingthetemporal and spatial evolution of the rate of change in storage in an unconfined aquifer during pumping. This evolution is then related to the transition of water release mechanisms from the expansion of water and compaction of the porous medium to the drainage of water from the unsaturated zone above the initial water table and initially saturated pores as the water table falls during the pumping of the aquifer. Afterward, the 1‐D vertical drainage process in a soil column is simulated. Results of the simulation show that the transition of the water release mechanisms in the 1‐D vertical flow without an initial unsaturated zone can also yield the S‐shaped drawdown‐time curve as in an unconfined aquifer. We therefore conclude that thetransitionofthewaterreleasemechanismsandverticalflowintheaquiferarethecauseof the S‐shaped drawdown‐time curve observed during pumping in an unconfined aquifer. We also find that the moisture retention characteristics of the aquifer material have greater impact than its relative permeability characteristics on the drawdown‐time curve. Furthermore, influences of the spatial variability of saturated hydraulic conductivity, specific storage, and saturated moisture content on the drawdown curve in the saturated zone are found to be more significant than those of other unsaturated properties. Finally, a cross‐correlation analysis reveals that the drawdown at a location in a heterogeneous unconfined aquifer is mainly affected by local heterogeneity near the pumping and observation wells. Applications of a model assuming homogeneity to the estimation of aquifer parameters as such may require a large number of observation wells to obtain representative parameter values. In conclusion, we advocate that the governing equation for variably saturated flow through heterogeneous media is a more appropriate and realistic model that explains the S‐shaped drawdown‐time curves observed in the field.

River stage tomography: A new approach for characterizing groundwater basins

Received 19 June 2008; revised 16 February 2009; accepted 12 March 2009; published 9 May 2009. [1] Data from tomographic surveys make an inverse problem better posed in comparison to the data from a single excitation source. A tomographic survey provides different coverages and perspectives of subsurface heterogeneity: nonfully redundant information of the subsurface. Fusion of these pieces of information expands and enhances the capability of a conventional survey, provides cross validation of inverse solutions, and constrains inherently ill posed field-scale inverse problems. Basin-scale tomography requires energy sources of great strengths. Spatially and temporally varying natural stimuli are ideal energy sources for this purpose. In this study, we explore the possibility of using river stage variations for basin-scale subsurface tomographic surveys. Specifically, we use numerical models to simulate groundwater level changes in response to temporal and spatial variations of the river stage in a hypothetical groundwater basin. We then exploit the relation between temporal and spatial variations of well hydrographs and river stage to image subsurface heterogeneity of the basin. Results of the numerical exercises are encouraging and provide insights into the proposed river stage tomography. Using naturally recurrent stimuli such as river stage variations for characterizing groundwater basins could be the future of geohydrology. However, it calls for implementation of sensor networks that provide long-term and spatially distributed monitoring of excitation as well as response signals on the land surface and in the subsurface.

Why hydraulic tomography works

Head measurements at a single observation well during a cross-hole pumping test carry a great amount of information about aquifer heterogeneity other than the average property of the aquifer as implied in Theis analysis of aquifer test. In this commentary, we use simple examples and a probabilistic reasoning approach based on Darcy’s law to unravel this information, buried in the results of quantitative stochastic analyses of flow in heterogeneous aquifers (Bakr et al. 1978; Dagan 1985, 1989) and vadose zones (Yeh et al. 1985a, 1985b, 1985c; Yeh and Zhang 1996). We subsequently use this information to elucidate the principles of hydraulic tomography (HT), sequential pumping tests, or multi-well interference tests (see Yeh and Liu 2000; Illman et al. 2009; Brauchler et al. 2011; Cardiff and Barrash 2011). Consider a pumping test in a one-dimensional heterogeneous confined aquifer (i.e., a horizontal soil column) which contains a pumping and an observation port. Ends of the aquifer are held at the same prescribed constant head, flow is at steady state, and the pumping rate, Q , is known. We now ask what the pumping rate and the drawdown at the observation port tell us about the spatial variation of the aquifer hydraulic conductivity (K ).

GIS and SBF for estimating groundwater recharge of a mountainous basin in the Wu River watershed, Taiwan

The temporal and spatial distributions of precipitation are extremely uneven; so, careful management of water resources in Taiwan is crucial. The long-term overexploitation of groundwater resources poses a challenge to water resource management in Taiwan. However, assessing groundwater resources in mountainous basins is challenging due to limited information. In this study, a geographic information system (GIS) and stable base-flow (SBF) techniques were used to assess the characteristics of groundwater recharge considering the Wu River watershed in central Taiwan as a study area. First, a GIS approach was used to integrate five contributing factors: lithology, land cover/land use, lineaments, drainage, and slope. The weights of factors contributing to the groundwater recharge were obtained from aerial photos, geological maps, a land use database, and field verification. Second, the SBF was used to estimate the groundwater recharge in a mountainous basin scale. The concept of the SBF technique was to separate the base-flow from the total streamflow discharge in order to obtain a measure of groundwater recharge. The SBF technique has the advantage of integrating groundwater recharge across an entire basin without complex hydro-geologic modelling and detailed knowledge of the soil characteristics. In this study, our approach for estimating recharge provides not only an estimate of how much water becomes groundwater, but also explains the characteristics of a potential groundwater recharge zone.

Estimating poromechanical properties using a nonlinear poroelastic model

AbstractA physics-based method is proposed for simultaneously obtaining the hydraulic conductivity, Young’s modulus, and Poisson’s ratio of soil materials using the uniaxial consolidation test. A nonlinear poroelastic model is presented, and the settlement data from consolidation tests are fitted to the model at each load step with the least-squares error method to inverse the parameters. The model results perfectly fit the experimental data in the initial load steps but slightly deviate from the data in later load steps as a result of secondary settlement and a largely increased Young’s modulus. The inversed parameters are compared with those calculated from the uniaxial consolidation test and those found in the literature. The comparison results demonstrate that the inversed parameters are reasonable. The proposed method provides both an estimation of parameters and the parameter-change information during a consolidation test. The method is simple, efficient, and versatile for obtaining poromechanical p...

Current monsoon conditions of river runoff and groundwater formation in west pacific regions: Kamchatka Peninsula and Taiwan Island

The dynamics of climate and runoff characteristics are studied in the territories of Kamchatka Peninsula and Taiwan Island, which are situated in the influence zone of monsoon circulation. Long-term variations in the temperature, precipitation, and runoff are examined in Kamchatka and Taiwan for the weather and gauging stations with the longest observation series to analyze new climate conditions of water resources generation.

Evaluation of Hydraulic Properties of Aquitards Using Earthquake-Triggered Groundwater Variation

The hydraulic properties of aquitards are not easily obtained because monitoring wells are usually installed in aquifers for groundwater resources management. Earthquake-induced crust stress (strain) triggers groundwater level variations over a short period of time in a large area. These groundwater anomalies can be used to investigate aquifer systems. This study uses a poroelastic model to fit the postseismic variations of groundwater level triggered by the Chi-Chi earthquake to evaluate the hydraulic properties of aquitards in the Jhoushuei River alluvial fan (JRAF), Taiwan. Six of the adopted eight wells with depths of 70 to 130 m showed good agreement with the recovery theory. The mean hydraulic conductivities (K) of the aquifers for the eight wells are 1.62 × 10-4 to 9.06 × 10-4 m/s, and the thicknesses are 18.8 to 46.1 m. The thicknesses of the aquitards are 11.3 to 42.0 m. Under the isotropic assumption for K, the estimated values of K for the aquitards are 3.0 × 10-8 to 2.1 × 10-6 m/s, corresponding to a silty medium. The results match the values obtained for the geological material of the drilling core and those reported in previous studies. The estimated values were combined with those given in previous studies to determine the distribution of K in the first two aquitards in the JRAF. The distribution patterns of the aquitards reflect the sedimentary environments and fit the geological material. The proposed technique can be used to evaluate the K value of aquitards using inverse methods. The inversion results can be used in hydrogeological analyses, contaminant modeling, and subsidence evaluation.

Use of Falling-Head Infiltration to Estimate Hydraulic Conductivity at Various Depths

Abstract Most infiltration methods measure the saturated hydraulic conductivity (Ks) of the unsaturated zone at only one depth at or near the surface. The purpose of this study was to extend the falling-head infiltration theory to a multistep falling-head infiltration test (MSFIT) for estimating Ks at various depths in a one-dimensional saturated vertical flow domain. A general analytical solution for Ks is derived for the wetted region of each step in the MSFIT. The Ks values for individual wetted sub-regions are calculated sequentially from the top down based on the fact that the Ks value of the whole wetted region is the depth-weighted harmonic mean of sub-regions for flow perpendicular to the wetted layers. The resolution of the wetted sub-region is determined by the number of steps and the change of the ponded water head. The MSFIT is performed in three independent laboratory two-layer sand columns. The mean value of Ks for the upper coarse sand is 2.13 × 10−2 cm/sec, and that for the lower fine sand is 3.63 × 10−3 cm/sec. Similar Ks results are obtained from 12 core sample measurements. In a field study with the MSFIT, the mean values of Ks estimated from top down are 8.37 × 10−4, 2.73 × 10−4, and 3.21 × 10−5 cm/sec, respectively. The decreasing Ks with depth is similar to the result of small core samples and is consistent with field observations of the soil texture. The proposed MSFIT is simple, flexible, and versatile for obtaining hydraulic conductivity at various depths for one-dimensional saturated vertical flow domains.