Developments Of Methods Tools And Solutions For Unsaturated Flow 3 Well Pumping In Unconfined Aquifers The Influence Of The Unsaturated Zone
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Developments Of Methods Tools And Solutions For Unsaturated Flow 3 Well Pumping In Unconfined Aquifers The Influence Of The Unsaturated Zone
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Author : U.I. Kroszynski
language : en
Publisher:
Release Date : 1974
Developments Of Methods Tools And Solutions For Unsaturated Flow 3 Well Pumping In Unconfined Aquifers The Influence Of The Unsaturated Zone written by U.I. Kroszynski and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1974 with categories.
Selected Water Resources Abstracts
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Author :
language : en
Publisher:
Release Date : 1991
Selected Water Resources Abstracts written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1991 with Hydrology categories.
Vadose Zone Response To Pumping In Unconfined Aquifers
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Author : Melissa Irene Bunn
language : en
Publisher:
Release Date : 2011
Vadose Zone Response To Pumping In Unconfined Aquifers written by Melissa Irene Bunn and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2011 with categories.
The interaction between drainage from the variably saturated zone above the water table, and the response of an unconfined aquifer to pumping has been the source of debate for many decades. While various field tests (Nwankwor et al., 1992 and Moench et al., 2001) have supported the concept that variably saturated flow processes delay drainage above a falling water table, Neuman (1972, 1974, 1975), has asserted that the impact is minimal, delay in response of the water table is due to elastic storage effects, and instantaneous yield above the water table is a reasonable assumption in unconfined aquifer analysis. This assumption results in exceedingly low estimates of specific yield in comparison to other analysis techniques (Neuman, 1987). A 7-day pumping test by Bevan et al. (2005) in the unconfined aquifer at Canadian Forces Base Borden has highlighted the complexity in drainage from above the water table during pumping, as the tension saturated zone was found to increase in thickness as a function of both proximity to the pumping well, and elapsed pumping time. This extended thickness persisted for the 7-day pumping duration. Analytical analysis of the test by Endres et al. (2007) resulted in significant underestimates of specific yield in comparison to laboratory values for most solutions. Narasimham (2007) suggested that the use of numerical simulators which include variably saturated flow may provide the most accurate representation of the test results. An attempt to replicate test results using a numerical simulation of variably saturated flow by Moench (2008) could not provide a complete physical mechanism for the extension observed by Bevan et al. (2005). This study provides a detailed investigation on the effect of heterogeneity, hysteresis, and entrapped air on drainage during unconfined pumping tests using numerical simulations, field experiments, and laboratory observations. The results of the Bevan et al. (2005) pumping test are used as a standard for comparison. Three variably-saturated groundwater flow numerical codes were evaluated for their ability to replicate the variations in soil moisture content observed during pumping by Bevan et al. (2005). Results of the numerical simulations were also analyzed for their similarity to the peak and subsequent decrease in vertical gradients observed during pumping in the Borden aquifer. While the models generated vertical gradients through the capillary fringe during pumping, these gradients dissipated significantly before 1000 min. of pumping. No gradients in the saturated zone generated by the numerical model would be capable of shifting the pressure head sufficiently to cause an apparent capillary fringe extension following the first few hours of pumping. Significant gradients were persistent throughout the test at locations where saturation was less than 85%. Accounting for the formation of vertical gradients, no simulation was able to replicate the soil moisture distributions observed by Bevan et al. (2005). Based on these results, heterogeneity, hysteresis, and entrapped air were proposed as processes with the potential to significantly affect drainage from above the water table during pumping, as their investigation may provide the physical mechanism for the observed capillary fringe extension. Compaction of the aquifer material was dismissed as a potential mechanism based on the results of a proctor test. The effect of heterogeneity on drainage from the Borden aquifer during pumping was investigated numerically using geostatistical methods. A log-normal saturated hydraulic conductivity distribution was used to represent the Borden aquifer. Brooks and Corey parameters were used to describe the pressure-saturation-relative conductivity relationships. The air-entry pressure parameter was scaled to the saturated conductivity using the scaling relationship for Borden sand proposed by Keuper and Frind (1991). The Brooks and Corey lambda parameter was kept constant. A Monte Carlo analysis was performed on the results. While several realizations of the hydraulic conductivity distribution resulted in the formation of perched water during drainage, the ensemble capillary fringe thickness was unchanged from the thickness generated using a homogeneous conceptual aquifer model. No single realization produced a capillary fringe extension in which the magnitude was a function of elapsed pumping time, or distance from the pumping well. Approximation of the effect of air-entry barriers on drainage did not increase the estimated capillary fringe thickness. The presence and location of finer grained layers appeared to have a much greater impact on the thickness of the capillary fringe than the drawdown induced by pumping. Ensemble results for the hydraulic head drawdown provided improved matches to the field observations in comparison to the homogeneous numerical model during intermediate and late times in the pumping test. A mild degree of heterogeneity appears to have sufficient effect on drainage from above the water table during pumping to impact hydraulic drawdown. The effect would be magnified with the greater degree of heterogeneity that is more typical of natural aquifer systems. A 24-hour pumping test was conducted at CFB Borden to gain a better understanding of the nature of drainage during a pumping test. Due to the wet site conditions prior to the test, the moisture profile during pumping was significantly influenced by hysteresis. The hydraulic head drawdown generated during the test was insufficient to generate any drainage due to the lowering of the top of the saturated zone, and the formation of perched lenses could not occur. Hysteresis in the moisture profile was a controlling factor in this result. Although there was no significant drainage initiated due to the lowering of the top of the saturated zone, an inflection point was still apparent in the time-drawdown curve for the four monitoring wells observed. Vertical gradients measured throughout the saturated zone, including the capillary fringe, remained low throughout the duration of pumping, and no significant increase was apparent in the transition from saturated to tension-saturated conditions. Hysteresis has the potential to increase the delay in drainage as the water table falls during pumping. A laboratory tank apparatus was used to determine the effect of entrapped air, grain size distribution, and horizontal gradient on drainage in a primarily horizontal flow regime. The tank was packed on three separate occasions, once with a coarse well sorted silica sand, and twice with sand from the Borden aquifer. For each packing, the tank was drained twice, using two different horizontal gradient magnitudes. Results show that horizontal gradient magnitude has no impact on soil moisture distributions during drainage. Air-entry pressure was elevated in comparison to gravity drainage derived pressure head - saturation curves. This elevation was not transient, nor dependant on gradient or grain size distribution. The increase in air-entry pressure does not appear to be due to insufficient equilibration time between water level drops or flow redistribution around the TDR Rods. Results of this study support a conceptual model of unconfined aquifer response in which drainage from above the water table is a complex and time dependant process. Individually, heterogeneity and hysteresis have been shown to cause a time delay between the lowering of the water table and the subsequent drainage of the tension saturated zone during intermediate to late pumping times. The magnitude and duration of this delay varies by process and is a function of the degree of heterogeneity, moisture conditions in the aquifer prior to pumping, and the drawdown rate of the water table. While no individual process tested could produce the capillary fringe extension observed by Bevan et al. (2005), the investigation of each has led to an improved conceptual understanding of the response to pumping in unconfined aquifers. Due to the complex interaction of these processes it is unlikely that pumping test results, even those which include moisture content observations, could be used to accurately predict unsaturated flow parameters. Storage parameter (i.e. specific yield) estimates made using analytical solutions may not be appropriate unless delayed drainage from above the water table is properly accounted for.
Selected Water Resources Abstracts
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Author :
language : en
Publisher:
Release Date : 1987
Selected Water Resources Abstracts written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1987 with Water categories.
Soil Mechanics For Unsaturated Soils
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Author : Delwyn G. Fredlund
language : en
Publisher: John Wiley & Sons
Release Date : 1993-09-06
Soil Mechanics For Unsaturated Soils written by Delwyn G. Fredlund and has been published by John Wiley & Sons this book supported file pdf, txt, epub, kindle and other format this book has been release on 1993-09-06 with Technology & Engineering categories.
The principles and concepts for unsaturated soils are developed as extensions of saturated soils. Addresses problems where soils have a matric suction or where pore-water pressure is negative. Covers theory, measurement and use of the fundamental properties of unsaturated soils--permeability, shear strength and volume change. Includes a significant amount of case studies.
Streamflow Depletion By Wells
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Author : Paul M. Barlow
language : en
Publisher:
Release Date : 2012
Streamflow Depletion By Wells written by Paul M. Barlow and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2012 with Groundwater categories.
U S Geological Survey Water Supply Paper
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Author :
language : en
Publisher:
Release Date : 1982
U S Geological Survey Water Supply Paper written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1982 with Water-supply categories.
Using Numerical Models For Managing Water Quality In Public Supply Wells
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Author : Marcelo Sousa
language : en
Publisher:
Release Date : 2013
Using Numerical Models For Managing Water Quality In Public Supply Wells written by Marcelo Sousa and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2013 with categories.
Groundwater models can be useful tools to support decisions regarding the management of public water supply wells. Scientific progress and the availability of increasingly powerful computer resources provide a continuous opportunity for improving the way numerical models are applied for this purpose. In this thesis, numerical groundwater models were applied to address relevant questions regarding the management of water supply wells in two distinct glacial aquifers in southern Ontario, Canada. The objective is to propose science-based methods that can be applied in day-to-day practice in the context of source water protection. Three specific issues were addressed: (1) Time lag in the unsaturated zone: A simplified method was proposed to assess the importance of the unsaturated zone in delaying the effects of changes at ground surface on water supply wells (i.e., unsaturated zone time lag). This assessment is important because it influences field and modelling efforts to estimate well vulnerability to contamination and impacts due to changes in land use. The proposed method is based on estimations of travel time in the saturated and unsaturated zones, and provides a formal framework for an intuitive approach. For the studied case, the delay in the unsaturated zone was deemed to be significant, representing on average ~ 11 years or ~ 53% of the total travel time from ground surface to receptor. Travel times were estimated using approaches with different levels of sophistication, to evaluate the usefulness of simplified calculations. Such calculations lead to the same overall conclusion as more sophisticated and time-consuming approaches. However, when assuming limited knowledge of soil properties, common at earlier stages of most investigations, these simplified techniques generated inconclusive results. (2) Uncertainty in capture zone delineation: A simple method was proposed to address the issue of uncertainty in capture zone delineation. This method considers uncertainty at two different scales: local (parametric) and global (conceptual). Local-scale uncertainty is addressed by using backward transport simulation to create capture probability plumes, with probabilities ranging from 0 to 100%. Global-scale uncertainty is addressed by considering more than one possible representation of the groundwater system (i.e., multiple model scenarios). Multiple scenario analysis accounts for more than one possible representation of the groundwater system, and it incorporates types of uncertainty that are not amenable to stochastic treatment (e.g., uncertainty due to conceptual model, to different model codes and boundary condition types). Finally, the precautionary principle is used to combine capture probability plumes generated by different scenarios. As a result, two maps are generated: One for wellhead protection, and another for selection of priority areas for implementation of measures to improve water quality at the supply well. For the studied case, three models with different spatial distributions of recharge but with similar calibrations were considered, exemplifying the issue of non-uniqueness. The two maps obtained by the proposed method were significantly different, indicating that recharge distribution represents a major source of uncertainty in capture zone delineation. (3) Effects of agricultural Beneficial Management Practices (BMPs) in supply wells: A numerical model framework was used to estimate the effects of measures to reduce nitrate leaching to groundwater from agricultural activities (i.e., Beneficial Management Practices, or BMPs). These measures were implemented in 2003 (~ 10 years ago) at the Thornton well field (Woodstock, Ontario, Canada) to improve well water quality. This case study is based on extensive field work characterization from previous research, and allows the discussion of practical issues related with data collection and interpretation (e.g., different techniques for generating mass loading distributions were compared). Regional flow was simulated in a 3D larger-scale saturated flow model, while variably-saturated flow and transport were simulated in a 3D smaller-scale, more refined grid. A vertical 1D model was used to define the discretization in the unsaturated zone of the variably-saturated flow and transport model. Results indicate that the adoption of BMPs in selected areas can be an effective strategy to improve water quality in supply wells impacted by non-point source contaminants. For the Thornton well field, the currently adopted BMPs are estimated to reduce concentrations from ~ 9.5 to ~ 7.5 mg NO3-N/L. Water quality at the wells are predicted to respond after 5 to 10 years after implementation of BMPs, and are expected to stabilize after 20 to 30 years Management scenarios with further reductions in nitrate concentration are expected to further reduce concentrations by ~ 0.4 to ~ 0.8 mg NO3-N/L. The proposed framework can be adapted to design and evaluate BMPs for similar problems and for other non-point source contaminants. Some insights were common to all three issues discussed and can be useful to practitioners involved in source water protection studies: (1) Reliable recharge estimations are essential for the management of water supply wells; and (2) The use of multiple models should be encouraged to increase the understanding of different aspects of the system, assess uncertainty and provide independent checks for model predictions.
Numerical Methods For Engineers And Scientists
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Author : Amos Gilat
language : en
Publisher: John Wiley & Sons
Release Date : 2008
Numerical Methods For Engineers And Scientists written by Amos Gilat and has been published by John Wiley & Sons this book supported file pdf, txt, epub, kindle and other format this book has been release on 2008 with Computers categories.
Following a unique approach, this innovative book integrates the learning of numerical methods with practicing computer programming and using software tools in applications. It covers the fundamentals while emphasizing the most essential methods throughout the pages. Readers are also given the opportunity to enhance their programming skills using MATLAB to implement algorithms. They'll discover how to use this tool to solve problems in science and engineering.
Channel
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Author :
language : en
Publisher:
Release Date : 1970
Channel written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1970 with Civil engineering categories.