Deqiang Mao

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.

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.

Validation of hydraulic tomography in an unconfined aquifer: A controlled sandbox study

In this study, we demonstrate the effectiveness of hydraulic tomography (HT) that considers variably saturated flow processes in mapping the heterogeneity of both the saturated and unsaturated zones in a laboratory unconfined aquifer. The successive linear estimator (SLE) developed by Mao et al. (2013c) for interpreting HT in unconfined aquifers is utilized to obtain tomograms of hydraulic conductivity (K), specific storage (Ss), and the unsaturated zone parameters (pore size parameter (α) and saturated water content (θs)) for the Gardner-Russos model. The estimated tomograms are first evaluated by visually comparing them with stratigraphy visible in the sandbox. Results reveal that the HT analysis is able to accurately capture the location and extent of heterogeneity including high and low K layers within the saturated and unsaturated zones, as well as reasonable distribution patterns of α and θs for the Gardner-Russos model. We then validate the estimated tomograms through predictions of drawdown responses of pumping tests not used during the inverse modeling effort. The strong agreement between simulated and observed drawdown curves obtained by pressure transducers and tensiometers demonstrates the robust performance of HT that considers variably saturated flow processes in unconfined aquifers and the unsaturated zone above it. In addition, compared to the case using the homogeneous assumption, HT results, as expected, yield significantly better predictions of drawdowns in both the saturated and unsaturated zones. This comparison further substantiates the unbiased and minimal variance of HT analysis with the SLE algorithm.

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 ).

Evaluating the potential for quantitative monitoring of in situ chemical oxidation of aqueous‐phase TCE using in‐phase and quadrature electrical conductivity

Electrical resistivity measurements can potentially be used to remotely monitor fate and transport of ionic oxidants such as permanganate ( MnO4−) during in situ chemical oxidation (ISCO) of contaminants like trichloroethene (TCE). Time-lapse two-dimensional bulk conductivity and induced polarization surveys conducted during a sand tank ISCO simulation demonstrated that MnO4− plume movement could be monitored in a qualitative manner using bulk conductivity tomograms, although chargeability was below sensitivity limits. We also examined changes to in-phase and quadrature electrical conductivity resulting from ion injection, MnO2 and Cl− production, and pH change during TCE and humate oxidation by MnO4− in homogeneous aqueous solutions and saturated porous media samples. Data from the homogeneous samples demonstrated that inversion of the sand tank resistivity data using a common Tikhonov regularization approach was insufficient to recover an accurate conductivity distribution within the tank. While changes to in-phase conductivity could be successfully modeled, quadrature conductivity values could not be directly related to TCE oxidation product or MnO4− concentrations at frequencies consistent with field induced polarization surveys, limiting the utility of quadrature conductivity for monitoring ISCO.

Replies to comments on ''A revisit of drawdown behavior during pumping in unconfined aquifers'' by Neuman and Mishra

] Heterogeneity as investigated in our paper, presencesof perched water table aquifers, limited leakage sources,boundary conditions, and many other factors that may mod-ify the shape of a drawdown-time curve. Likewise, ground-water flow is driven by the energy gradient but it can beinfluenced by many different factors. It is not the intent ofour paper to discuss them all. We merely emphasize thefact that the fundamental mechanisms causing the S-shapedcurves are the transition of water release mechanisms andthe vertical flow components. More importantly, our paperadvocates that a multidimensional variably saturated flowmodel, which considers the transition of water releasemechanisms and accounts for heterogeneity, would providea more realistic representation of flow processes in uncon-fined aquifers during a pumping test. These conclusions areindependent from the number of experiments we examined.1.2. Reply to Comments in Paragraph 3[