Cross-correlation analysis and information content of observed heads during pumping in unconfined aquifers (original) (raw)
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Joint interpretation of sequential pumping tests in unconfined aquifers
Water Resources Research, 2013
In this study, we developed a stochastic estimator for characterizing the hydraulic heterogeneity in both unsaturated and saturated zones of unconfined aquifers using transient drawdown data from sequential pumping tests. This estimator was built upon the successive linear estimator by Yeh et al. (1996), the simultaneous successive linear estimator by Xiang et al. (2009), and the 3-D finite element program for flow and transport through heterogeneous media by Srivastava and Yeh (1992). The estimator was tested afterward using simulated data sets of sequential pumping tests in a synthetic unconfined aquifer where saturated conductivity, specific storage, saturated water content, and pore-size distribution parameter vary spatially in three dimensions. Test results show that the estimator is able to produce parameter fields that capture the overall 3-D pattern of the true heterogeneous parameter fields. We subsequently validated the estimated parameter fields by assessing their ability to predict drawdowns during an independent pumping test, which was not used during the estimation phase. Results of the validation show that the predicted drawdowns based on the estimated heterogeneous parameter fields are in close agreement with the true drawdowns. In addition, predicted drawdowns based on the parameter fields from the joint interpretation are superior to those based on the parameters estimated from the homogeneous conceptual model. Lastly, while many field experiments are necessary to fully assess the robustness of this estimator and sequential pumping tests, results of this study suggest they are a promising characterization technique for unconfined aquifers.
Capturing aquifer heterogeneity: Comparison of approaches through controlled sandbox experiments
Water Resources Research, 2011
1] Groundwater modeling has become a vital component to water supply and contaminant transport investigations. These models require representative hydraulic conductivity (K) and specific storage (S s ) estimates, or a set of estimates representing subsurface heterogeneity. Currently, there are a number of approaches for characterizing and modeling K and S s heterogeneity in varying degrees of detail, but there is a lack of consensus for an approach that results in the most robust groundwater models with the best predictive ability. The main goal of this study is to compare different heterogeneity modeling approaches (e.g., effective parameters, geostatistics, geological models, and hydraulic tomography) when input into a forward groundwater model and used to predict 16 independent cross-hole pumping tests. We first characterize a sandbox aquifer through single-and cross-hole pumping tests, and then use these data to construct forward groundwater models of various complexities (both homogeneous and heterogeneous distributions). Two effective parameter models are constructed: (1) by taking the geometric mean of single-hole test K and S s estimates and (2) calibrating effective K and S s estimates by simultaneously matching the response at all ports during a cross-hole test. Heterogeneous models consist of spatially variable K and S s fields obtained via (1) kriging single-hole data; (2) calibrating a geological model; and (3) conducting transient hydraulic tomography . The performance of these parameter fields are then tested through the simulation of 16 independent cross-hole pumping tests. Our results convincingly show that transient hydraulic tomography produces the smallest discrepancy between observed and simulated drawdowns.
Accounting for the influence of aquifer heterogeneity on spatial propagation of pumping drawdown
It has been previously observed that during a pumping test in heterogeneous media, drawdown data from different time periods collected at a single location produce different estimates of aquifer properties and that Theis type-curve inferences are more variable than late-time Cooper-Jacob inferences. In order to obtain estimates of aquifer properties from highly transient drawdown data using the Theis solution, it is necessary to account for this behavior. We present an approach that utilizes an exponential functional form to represent Theis parameter behavior resulting from the spatial propagation of a cone of depression. This approach allows the use of transient data consisting of early-time drawdown data to obtain late-time convergent Theis parameters consistent with Cooper-Jacob method inferences. We demonstrate the approach on a multi-year dataset consisting of multi-well transient water-level observations due to transient multi-well water-supply pumping. Based on previous research, transmissivities associated with each of the pumping wells are required to converge to a single value, while storativities are allowed to converge to distinct values.
Water Resources Research, 2016
A framework for interpreting transient pumping tests in heterogeneous transmissivity fields is developed to infer the overall geostatistical parameters of the medium without reconstructing the specific heterogeneous structure point wise. The methodology of Radial Coarse Graining is applied to deduce an effective radial description of multi‐Gaussian transmissivity. It was used to derive an Effective Well Flow Solution for transient flow conditions including not only the storativity, but also the geometric mean, the variance, and the correlation length of log‐transmissivity. This solution is shown to be appropriate to characterize the pumping test drawdown behavior in heterogeneous transmissivity fields making use of ensembles of simulated pumping tests with multiple combinations of statistical parameters. Based on the Effective Well Flow Solution, a method is developed for inferring heterogeneity parameters from transient pumping test drawdown data by inverse estimation. Thereby, the...
Environmental Science & Technology, 2009
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 K fields estimated by geometric mean, kriging, and HTS. Such K fields represent different degrees of flow resolutions as compared with the synthetically generated one. Without intensive experiments to calibrate accurate dispersivities at sites, the flow field based on the HTS K field 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 K fields, and the pump locations need to be installed in high K zones 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.
Accounting for the inuence of aquifer heterogeneity on spatial propagation of pumping drawdown
arXiv: Geophysics, 2011
It has been previously observed that during a pumping test in heterogeneous media, drawdown data from dierent time periods collected at a single location produce dierent estimates of aquifer properties and that Theis type-curve inferences are more variable than late-time Cooper-Jacob inferences. In order to obtain estimates of aquifer properties from highly transient drawdown data using the Theis solution, it is necessary to account for this behavior. We present an approach that utilizes an exponential functional form to represent Theis parameter behavior resulting from the spatial propagation of a cone of depression. This approach allows the use of transient data consisting of early-time drawdown data to obtain late-time convergent Theis parameters consistent with Cooper-Jacob method inferences. We demonstrate the approach on a multi-year dataset consisting of multi-well transient water-level observations due to transient multi-well water-supply pumping. Based on previous research,...
Sensitivity analysis of pumping tests in non-uniform aquifers
Hydrological Sciences Journal, 1995
An approach for better understanding of the physical implication of estimated aquifer parameters is demonstrated by analysing the pumping test data at Cottam in the Nottingham aquifer, UK. A sensitivity analysis showed that the area represented by the estimated parameters was much smaller than the area covered by the depression cone. After parameters are estimated, further research should be carried out to understand what portions of the aquifer the parameters represent. The parameters estimated at Cottam represented mainly aquifer features between roughly 100 and 2000 m. The sensitivity analysis also showed that the observed drawdown being satisfactorily matched by a model with uniform parameters does not prove that the aquifer is homogeneous. Slightly anomalous data may imply the existence of large anomalous zones. Although the drawdowns at Cottam could be 'satisfactorily' fitted by a model with uniform parameters, the fit could be improved by a model using a more permeable aquifer but with a zone about 700 m wide and with 42% less transmissivity.
A simple procedure for the identification of aquifer parameters based on steady-state pumping tests
IAHS-AISH publication, 2006
Résumé/Abstract Three-dimensional steady flow towards a fully penetrating well of radius rw, in a confined aquifer, is studied by means of a finite-volume numerical scheme. The hydraulic conductivity K is modelled as an axisymmetric, stationary random space function mimicking hydraulic property variations at the local scale. We develop a new methodology for the identification of the geostatistical model of variability from a steady-state pumping test involving a few wells. A constant water discharge is extracted from each well in sequence ...
Journal of Hydrology, 2014
We present a numerical study keyed to the analysis of the impact on hydraulic head statistics of two selected methodologies for the stochastic simulation of hydro-facies spatial arrangement. We analyze the distribution of hydraulic heads in a confined aquifer under steady-state convergent three-dimensional flow to a fully penetrating well, superimposed to a mean uniform regional gradient. The heterogeneous structure of the system is modeled on the basis of available field information comprising detailed lithological data collected within an aquifer system located in northern Italy. These data are grouped into five litho-type categories and the aquifer system is modeled as a random composite medium. Monte Carlo realizations of the three-dimensional geo-material distributions are generated through the Sequential Indicator and the Truncated Plurigaussian Simulation methods. The latter enables one to integrate geological conceptual information in the simulation procedure, while the former relies mainly on a variogram-based analysis. Point and vertically averaged hydraulic heads, corresponding to typical observations collected within screened boreholes, are analyzed by evaluating the dependence of their sample probability distributions on (i) the hydro-facies generation scheme, (ii) the extent of the vertical averaging interval and (iii) the relative distance between the location of observation boreholes, hydrological boundaries and the source term. Theoretical probability density function models are fitted against numerically simulated distributions within a maximum likelihood context. Our results indicate that hydraulic heads associated with the Truncated Plurigaussian Simulation method exhibit increased variability when compared to their counterparts evaluated upon relying on a Sequential Indicator based modeling strategy of the system heterogeneity. Covariance matrices and probability distributions of point and vertically averaged hydraulic heads display similar key representative features and patterns. This suggests that typical measurements collected in screened boreholes can be used to infer qualitative information about the correlation structure and the statistical properties of heads.