Analytical Solution of Advective Mixing in a Conduit     

by Guangquan Li 《Ground water》2009,47(5):714-722

The permeable conduit wall in a karst aquifer allows for water and solute to be exchanged between conduits and the limestone matrix. Contaminant sequestered in the limestone matrix is flushed into conduits following flood events. The contaminant released from the permeable wall will then mix with conduit water and will be transported downgradient in the conduit. A one-dimensional advection-dispersion equation is presented to describe this mixing-transport incorporating water flow and solute flux through the conduit wall. An analytical solution ignoring conduit dispersion is derived using the method of characteristics. Scale analysis is performed to provide a general guideline to estimate when conduit dispersion can be neglected. The solution also can be used to compute the distribution of solute in the matrix before flushing.  相似文献   

On Constraining Pilot Point Calibration with Regularization in PEST   总被引：2，自引：0，他引：2  

by Michael N. Fienen  Christopher T. Muffels  Randall J. Hunt 《Ground water》2009,47(6):835-844

Ground water model calibration has made great advances in recent years with practical tools such as PEST being instrumental for making the latest techniques available to practitioners. As models and calibration tools get more sophisticated, however, the power of these tools can be misapplied, resulting in poor parameter estimates and/or nonoptimally calibrated models that do not suit their intended purpose. Here, we focus on an increasingly common technique for calibrating highly parameterized numerical models—pilot point parameterization with Tikhonov regularization. Pilot points are a popular method for spatially parameterizing complex hydrogeologic systems; however, additional flexibility offered by pilot points can become problematic if not constrained by Tikhonov regularization. The objective of this work is to explain and illustrate the specific roles played by control variables in the PEST software for Tikhonov regularization applied to pilot points. A recent study encountered difficulties implementing this approach, but through examination of that analysis, insight into underlying sources of potential misapplication can be gained and some guidelines for overcoming them developed.  相似文献   

Sampling Strategies for Estimation of Parameters Related to Ground Water Quality     

by Pamela E. Crane  Stephen E. Silliman 《Ground water》2009,47(5):699-708

Multiple theoretical sampling designs are studied to determine whether sampling designs can be identified that will provide for characterization of ground water quality in rural regions of developing nations. Sampling design in this work includes assessing sampling frequency, analytical methods, length of sampling period, and requirements of sampling personnel. The results answer a set of questions regarding whether using innovative sampling designs can allow hydrogeologists to take advantage of a range of characterization technologies, sampling strategies, and available personnel to develop high-value, water-quality data sets. Monte Carlo studies are used to assess different sampling strategies in the estimation of three parameters related to a hypothetical chemical observed in a ground water well: mean concentration (MeanC), maximum concentration (MaxC), and total mass load (TML). Five different scenarios are simulated. These scenarios are then subsampled using multiple simulated sampling instruments, time periods (ranging from 1 to 10 years), and sampling frequencies (ranging from weekly to semiannually to parameter dependent). Results are analyzed via the statistics of the resulting estimates, including mean square error, bias, bias squared, and precision. Results suggest that developing a sampling strategy based on what may be considered lower quality instruments can represent a powerful field research approach for estimating select parameters when applied at high frequency. This result suggests the potential utility of using a combination of lower quality instrument and local populations to obtain high frequency data sets in regions where regular monitoring by technicians is not practical.  相似文献   

Recent Developments and Future Directions for MT3DMS and Related Transport Codes   总被引：1，自引：0，他引：1  

reviewed by Chunmiao Zheng 《Ground water》2009,47(5):620-625

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Energy Security—Addressing the Water Footprint     

by Michael Hightower 《Ground water》2009,47(6):765-766

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Practical Postcalibration Uncertainty Analysis: Yucca Mountain, Nevada     

by Scott C. James  John E. Doherty  Al-Aziz Eddebbarh 《Ground water》2009,47(6):851-869

The values of parameters in a groundwater flow model govern the precision of predictions of future system behavior. Predictive precision, thus, typically depends on an ability to infer values of system properties from historical measurements through calibration. When such data are scarce, or when their information content with respect to parameters that are most relevant to predictions of interest is weak, predictive uncertainty may be high, even if the model is "calibrated." Recent advances help recognize this condition, quantitatively evaluate predictive uncertainty, and suggest a path toward improved predictive accuracy by identifying sources of predictive uncertainty and by determining what observations will most effectively reduce this uncertainty. We demonstrate linear and nonlinear predictive error/uncertainty analyses as applied to a groundwater flow model of Yucca Mountain, Nevada, the United States' proposed site for disposal of high-level radioactive waste. Linear and nonlinear uncertainty analyses are readily implemented as an adjunct to model calibration with medium to high parameterization density. Linear analysis yields contributions made by each parameter to a prediction's uncertainty and the worth of different observations, both existing and yet-to-be-gathered, toward reducing this uncertainty. Nonlinear analysis provides more accurate characterization of the uncertainty of model predictions while yielding their (approximate) probability distribution functions. This article applies the above methods to a prediction of specific discharge and confirms the uncertainty bounds on specific discharge supplied in the Yucca Mountain Project License Application.  相似文献   

Remediation Hydraulics     

reviewed by James A. Jacobs 《Ground water》2009,47(2):171-171

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MODFLOW-CFP: A New Conduit Flow Process for MODFLOW–2005     

reviewed by Thomas Reimann  and Melissa E. Hill 《Ground water》2009,47(3):321-325

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Evaluation of Combined Direct-Push Methods Used for Aquifer Model Generation   总被引：4，自引：0，他引：4  

by Ralf Köber  Götz Hornbruch  Carsten Leven  Lars Tischer  Jochen Großmann  Peter Dietrich  Holger Weiß  Andreas Dahmke 《Ground water》2009,47(4):536-546

Most established methods to characterize aquifer structure and hydraulic conductivities of hydrostratigraphical units are not capable of delivering sufficient information in the spatial resolution that is desired for sophisticated numerical contaminant transport modeling and adapted remediation design. With hydraulic investigation methods based on the direct-push (DP) technology such as DP slug tests, DP injection logging, and the hydraulic profiling tool, it is possible to rapidly delineate hydrogeological structures and estimate their hydraulic conductivity in shallow unconsolidated aquifers without the need for wells. A combined application of these tools was used for the investigation of a contaminated German refinery site and for the setup of hydraulic aquifer models. The quality of DP investigation and the models was evaluated by comparisons of tracer transport simulations using these models and measured breakthroughs of two natural gradient tracer tests. Model scenarios considering the information of all tools together showed good reproduction of the measured breakthroughs, indicating the suitability of the approach and a minor impact of potential technical limitations. Using the DP slug tests alone yielded significantly higher deviations for the determined hydraulic conductivities compared to considering two or three of the tools. Realistic aquifer models developed on basis of such combined DP investigation approaches can help optimize remediation concepts or identify flow regimes for aquifers with a complex structure.  相似文献   

Numerical Modeling of Atoll Island Hydrogeology   总被引：3，自引：0，他引：3  

by R.T. Bailey  J.W. Jenson  and A.E. Olsen 《Ground water》2009,47(2):184-196

We implemented Ayers and Vachers' (1986) inclusive conceptual model for atoll island aquifers in a comprehensive numerical modeling study to evaluate the response of the fresh water lens to selected controlling climatic and geologic variables. Climatic factors include both constant and time-varying recharge rates, with particular attention paid to the effects of El Niño and the associated drought it brings to the western Pacific. Geologic factors include island width; hydraulic conductivity of the uppermost Holocene-age aquifer, which contains the fresh water lens; the depth to the contact with the underlying, and much more conductive, Pleistocene karst aquifer, which transmits tidal signals to the base of the lens; and the presence or absence of a semiconfining reef flat plate on the ocean side. Sensitivity analyses of steady-steady simulations show that lens thickness is most strongly sensitive to the depth to the Holocene-Pleistocene contact and to the hydraulic conductivity of the Holocene aquifer, respectively. Comparisons between modeling results and published observations of atoll island lens thicknesses suggest a hydraulic conductivity of approximately 50 m/d for leeward islands and approximately 400 m/d for windward islands. Results of transient simulations show that lens thickness fluctuations during average seasonal conditions and El Niño events are quite sensitive to island width, recharge rate, and hydraulic conductivity of the Holocene aquifer. In general, the depletion of the lens during drought conditions is most drastic for small, windward islands. Simulation results suggest that recovery from a 6-month drought requires about 1.5 years.  相似文献