HR: 14:55h
AN: H23I-06    [Abstracts]
TI: Linearised Richards' Equation Approach to Pumping Test Analysis in Compressible Aquifers
AU: * Mathias, S A
EM: simon.mathias@imperial.ac.uk
AF: The Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London, SW7 2BU United Kingdom
AU: Butler, A P
EM: a.butler@imperial.ac.uk
AF: The Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London, SW7 2BU United Kingdom
AB: There is increasing acceptance of the importance of slow drainage from the unsaturated zone (SDUZ) when interpreting drawdown-time curves derived from pumping tests. Previous analytical solutions have assumed instantaneous drainage from the unsaturated zone (Theis, 1935; Neuman, 1972, 1974). Such models typically underestimate the specific yield (Nwankwor et al., 1984; Moench, 1995). Some authors have sought to account for SDUZ by assuming that drainage from the unsaturated zone declines exponentially with time, giving rise to an empirical delay index (Boulton, 1963; Moench, 1995). However, these models tend to overestimate drawdown at early-times and underestimate it during late times. More recently, the superposition of an arbitrary number of exponential models with different delay indices has been advocated, giving rise to an over-parameterised and arbitrarily complicated empirical function (Moench, 2003, 2004). Following the work of Kroszsynski and Dagan (1975) we derive a new drainage function based on a linearised Richards' equation assuming that moisture content and hydraulic conductivity are exponential functions of pressure head. Furthermore, the drainage function can be incorporated into existing analytical solutions (such as that provided by Moench, 1997) with minor adjustment. The resulting model requires an additional three parameters: a moisture retention exponent, a hydraulic conductivity exponent and the initial unsaturated zone thickness. The new drainage function can also be used in an empirical fashion with only one extra parameter (the other two are lost by assuming an infinitely deep unsaturated zone and that the moisture retention and relative permeability exponents are equal). Its applicability is demonstrated using pumping test data sets from Borden (Nwankwor et al., 1984) and Cape Cod (Moench et al., 2004). The results show improved consistency with the experimental data in comparison with previous studies.
References: Boulton, N. S. (1963), Analysis of data from non-equilibrium pumping tests allowing for delayed yield from storage, Proc. Inst. Civ. Eng., 26, 469-482. Kroszsynski, U. I., and G. Dagan (1975), Well pumping in unconfined aquifers: The influence of the unsaturated zone, Water Resour. Res., 11(3), 479-490. Moench, A. F. (1995), Combining the Neuman and Boulton models for flow to a well in an unconfined aquifer, Groundwater, 33(3), 378-384. Moench, A. F. (1996), Flow to a well in a water-table aquifer: An improved Laplace transform solution, Groundwater, 34(4), 593-596. Moench, A. F. (1997), Flow to a well of finite diameter in a homogenous anisotropic water table aquifer, Water Resour. Res., 33(6), 1397-1407. Moench, A. F. (2003), Estimation of hectare-scale soil-moisture characteristics from aquifer-test data, J. Hydrol., 281, 82-95. Moench, A. F. (2004), Importance of the Vadose Zone in Analyses of Unconfined Aquifer Tests, Groundwater, 42(2), 223p-233. Nwankwor, G. I., J. A. Cherry, and R. W. Gillam (1984), A comparative study of specific yield determinations for a shallow sand aquifer, Groundwater, 22(6), 764-772.
DE: 1828 Groundwater hydraulics
DE: 1829 Groundwater hydrology
DE: 1847 Modeling
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: Fall Meeting 2005