bluuebunny/crossref_metadata_2025_split huggingface dataset:
Sensitivity Analysis of Three‐Dimensional Steady‐State and Transient Spray Irrigation Models
Authors: Heebner David , Toran Laura 
Published: August 2005

AbstractWe conducted a sensitivity analysis of ground water flow on a spray irrigation site to evaluate key parameters affecting use of spray irrigation for sewage disposal. Spray irrigation is considered successful when water recharges the aquifer (low runoff) and mounding of the water table is below set limits. It can be difficult to predict success and allowable irrigation rates given uncertainty in parameters.The U.S. Geological Survey (USGS) code MODFLOW was used to compare two‐ and three‐dimensional steady‐state simulations, and three‐dimensional transient simulations. We compared Latin hypercube sampling and systematic adjustment of parameters as methods of sensitivity analysis, transient versus steady‐state sensitivity, and variations in layer configurations. These models were not calibrated because they were created during planning stages before field data became available.The Latin hypercube sampling effectively tested a large number of parameters simultaneously, and can be used as a screening tool for systematic adjustment of parameters. Transient and steady‐state simulations predicted similar sensitivity to hydraulic conductivity, but steady‐state modeling produced higher peak heads and discharges. Again, we recommend that the steady‐state simulations could be used as a screening tool to evaluate trends. Transient modeling is likely to provide more realistic predictions of heads and discharges. When different layer configurations were compared, we found that two‐dimensional modeling overpredicted mounding and drain discharge because flow in a permeable soil layer is neglected. Two‐dimensional modeling would not be effective for predicting allowable irrigation rates. These results provide modelers an indication of the effects of model simplification, which can be used to plan modeling strategies.
Analytic Determination of Hydrocarbon Transmissivity from Baildown Tests
Authors: Huntley David 
Published: August 2005

AbstractHydrocarbon baildown tests involve the rapid removal of floating hydrocarbon from an observation or production well, followed by monitoring the rate of recovery of both the oil/air and oil/water interfaces. This test has been used erroneously for several years to calculate the “true thickness” of hydrocarbon in the adjacent formation. More recent analysis of hydrocarbon distribution by Farr et al. (1990), Lenhard and Parker (1990), Huntley et al. (1994), and others have shown that, under vertical equilibrium conditions, there is no thickness exaggeration of hydrocarbon in a monitoring well, though there is a significant volume exaggeration. This body of work can be used to demonstrate that the calculation of a “true hydrocarbon thickness” using a baildown test has no basis in theory. The same body of work, however, also demonstrates that hydrocarbon saturations are typically much less than one, and are often below 0.5. Because the relative permeability decreases as hydrocarbon saturation decreases, the effective conductivity and mobility of the hydrocarbon is much less than that of water, even ignoring the effects of increased viscosity and decreased density.It is important to evaluate this decreased mobility of hydrocarbon due to partial pore saturation, as it has substantial impacts on both risk and remediation. This paper presents two analytic approaches to the analysis of hydrocarbon baildown test results to determine hydrocarbon transmissivity. The first approach is based on a modification of the Bouwer and Rice (1976) analysis of slug withdrawal test data. The second approach is based on a modification of Jacob and Lohman's (1952) constant drawdown—variable discharge aquifer test approach. The first approach can be applied only when the effective water transmissivity across the screened interval to water is much greater than the effective hydrocarbon transmissivity. When this condition is met, the two approaches give effectively identical results.
Modeling of Ground Water Flow to Adits
Authors: Zhang Beiyan , Lerner David N. 
Published: August 2005

AbstractMany of the large ground water sources in the Chalk Aquifer have adits, horizontal tunnels below the ground water table, which are connected to pumped wells. The flow in an adit may be pipe or open channel flow. Adits in the United Kingdom are normally full of water, so that the adit flow is pressurized. Darcy's formula is not applicable to the adit, and conventional ground water models are inappropriate to model the aquifer‐adit system. The U.S. Geological Survey (USGS) model BRANCH simulates one‐dimensional unsteady, nonuniform, multiple‐branch interconnected open channel flow. MODBRNCH incorporates BRANCH into MODFLOW simulating open channel and aquifer interaction using deterministic responses of both systems. An aquifer‐adit model can be created by two steps. First, an integrated surface‐ground water model (MODBRNCH) enables open channel flow to be simulated. Second, introducing a fictitious narrow slot (Preissmann slot) above the adit allows pipe flow to be simulated by open channel flow equations. The slot does not affect the adit cross‐section area, and the water level in the slot represents the pressurized adit head, which can be used by MODBRNCH to calculate the water exchange between aquifer and adit according to their head difference. The approach has been tested on the Wilmington public supply source in southeast England.
Mechanisms Controlling the Distribution and Transport of 14C in a Clay‐Rich Till Aquitard
Authors: Wassenaar L.I. , Hendry M.J. 
Published: August 2005

AbstractThe distribution and transport of radiocarbon in dissolved inorganic carbon (DIC) in a thick clay‐rich till aquitard was investigated. Radiocarbon activities decreased with depth from 109 percent modern carbon (pmC) in the surficial oxidized and fractured zone (at 1.5 m BG) through the thick underlying unoxidized and nonfractured aquitard, reaching minimum values of between 3 and 5 pmC at between 29 and 37 m depth. Numerical simulations of diffusive transport of radiocarbon from the oxidized zone into the unoxidized zone during the Holocene closely approximated the measured values and showed that diffusive transport and radioactive decay fully described the transport of radiocarbon in the aquitard. The lack of geochemical reactions involving DIC permitted the age of DIC in the pore water deposited with the till (between 29 and 37 m BG) to be estimated at 25 to 31 ka BP. Results of this study showed that the use of radiocarbon in the study of solute transport in aquitards provides age information that cannot be obtained from natural conservative tracers such δ2H and δ18. Our results also showed that the application of conventional radiocarbon age‐dating models based on piston‐flow transport assumptions, is inappropriate in diffusion‐dominated environments and considerably overestimates the age of the ground water.
Transmissivity Estimates from Well Hydrographs in Karst and Fractured Aquifers
Authors: Powers Jefferey G. , Shevenell Lisa 
Published: August 2005

AbstractHydrograph recessions from rainfall events have been previously analyzed for discharge at springs and streams; however, relatively little quantitative research has been conducted with regard to hydrograph analysis of recessions from monitoring wells screened in karst aquifers. In previous work, a quantitative hydrograph analysis technique has been proposed from which matrix transmissivity (i.e., transmissivity of intergranular porosity) and specific yields of matrix, fracture, and conduit components of the aquifer may be determined from well hydrographs. The technique has yielded realistic results at three sites tested by the authors (Y‐12, Oak Ridge, Tennessee; Crane, Indiana; and Fort Campbell, Kentucky). Observed field data, as well as theoretical considerations, show that karst well hydrographs are valid indicators of hydraulic properties of the associated karst aquifers. Results show matrix transmissivity (T) values to be in good agreement with values calculated using more traditional parameter estimation techniques, such as aquifer pumping tests and slug tests in matrix dominated wells. While the hydrograph analysis technique shows promise for obtaining reliable estimates of karst aquifer T with a simple, relatively inexpensive and passive method, the utility of the technique is limited in its application depending on site‐specific hydrologic conditions, which include shallow, submerged conduit systems located in areas with sufficient rainfall for water levels to respond to precipitation events.
Relation of Pathways and Transit Times of Recharge Water to Nitrate Concentrations Using Stable Isotopes.
Authors: Landon Matthew K. , Delin Geoffrey N. , Komor Stephen C. , Regan Charles P. 
Published: August 2005

AbstractOxygen and hydrogen stable isotope values of precipitation, irrigation water, soil water, and ground water were used with soil‐moisture contents and water levels to estimate transit times and pathways of recharge water in the unsaturated zone of a sand and gravel aquifer. Nitrate‐nitrogen (nitrate) concentrations in ground water were also measured to assess their relation to seasonal recharge. Stable isotope values indicated that recharge water usually had a transit time through the unsaturated zone of several weeks to months. However, wetting fronts usually moved through the unsaturated zone in hours to weeks. The much slower transit of isotopic signals than that of wetting fronts indicates that recharge was predominantly composed of older soil water that was displaced downward by more recent infiltrating water. Comparison of observed and simulated isotopic values from pure‐piston flow and mixing‐cell water and isotope mass balance models indicates that soil water isotopic values were usually highly mixed. Thus, movement of recharge water did not occur following a pure piston‐flow displacement model but rather follows a hydrid model involving displacement of mixed older soil water with new infiltration water. An exception to this model occurred in a topographic depression, where movement of water along prelerential flowpaths to the water table occurred within hours to days following spring thaw as result of depression‐focused infiltration of snow melt. In an adjacent upland area, recharge of snow melt occurred one to two months later. Increases in nitrate concentrations at the water table during April‐May 1993 and 1994 in a topographic low‐land within a cora field were related to recharge of water that had infiltrated the previous summer and was displaced from the unsaturated zone by spring infiltration. Increases in nitrate concentrations also occurred during July‐August 1994 in response to recharge of water that infiltrated during May‐August 1994. These results indicate that the largest ground water nitrate concentrations were associated with recharge of water that infiltrated into the soil during May‐August, when most nitrogen fertilizer was applied.
On Fracture Structure and Preferential Flow in Unsaturated Chalk
Authors: Dahan Ofer , Nativ Ronit , Adar Eilon M. , Berkowitz Brian , WeisbrodB Noam 
Published: August 2005

AbstractThe mechanisms controlling fluid flow through fractures intersecting chalk in the vadose zone were studied through water percolation experiments in natural discrete fractures and by close examination of the inner Structure of fracture voids. The percolation experiments showed that the flow is focused in dissolution channels along the fracture plane, and that fluxes and flow trajectories within that net vary in both time and space. The locations of the dissolution channels, the main potential flow paths within the fracture plane, were generally associated with fracture Intersections. The flow through these channels was governed primarily by the mineralogical composition of the filling material and the inner structure of the fracture voids. Salt dissolution, solid‐particle migration, and clay swelling were found to be the predominant processes controlling flow through the dissolution channels. These physical changes in the structure of the filling material in the dissolution channels accounted for the observed unstable flow behavior. Our results suggest that models aimed at simulating water percolation through fractures in unsaturated chalk should consider the mapping of fracture intersections in addition to the commonly used mapping of fracture lineaments. Moreover, the detailed characterization of fracture apertures may not be the key parameter determining fracture flow, because in such formations the flow is controlled primarily by filling material. These materials undergo significant physical variations during wetting and drying cycles.
Salt Accumulation and Flushing in Unsaturated Fractures in an Arid Environment
Authors: Weisbrod Noam , Nativh Ronit , Adar Kilon M. , Ronend Daniel 
Published: August 2005

AbstractSalt precipitation and dissolution in fractured chalk was investigated through flow experiments in both the laboratory and the field (in the Negev Desert, Israel). In the laboratory, a flow‐cell experimental setup was used to simulate intermittent Infiltration and drying periods along coated and uncoated fracture surfaces. Three Infiltration events, lasting 24, 8, and 8 hours, were carried out with long drying periods of 82 and 44 days between them. In the field, two flow experiments were conducted through a single fracture. Water was percolated from land surface through a discrete fracture into a compartmental sampler. The duration of the two field experiments was 5 and 119 hours, respectively, with a drying period of seven months between them. The percolating outflows in both the laboratory and field experiments were collected and analyzed for electrical conductivity.The electrical conductivity of the outflows and its temporal variations during the experiments suggest that evaporation triggered capillary forces that mobilized water and solutes from the bulk matrix toward the fracture surface. As the water evaporated, the solutes precipitated on the fracture surface. The precipitated soluble salts were dissolved during the first few hours of the sub‐sequent flow event that followed the drying period. This mechanism, enhanced in arid environments, may result in the transport of salts that accumulate in the upper few meters of the unsaturated zone to the ground water, bypassing the low‐permeability matrix. The calculated amount of solutes transported by back‐diffusion from the chalk matrix (190 g) could not account for the large (1200 g) amount of salts released from the fracture void during the experiments.
A Tracer Method to Determine Hydraulic Conductivity and Effective Porosity of Saturated Clays Under Low Gradients
Authors: Yeh Yi‐Jang , Lee Cheng‐Haw , Chen Shih‐Tsu 
Published: August 2005

AbstractBased on the column tracer test, we developed a research method to determine the hydraulic conductivity and effective porosity of saturated clays under low hydraulic gradients or small flow rates. Derived from Darcy's law and the solute transport equation, this method evaluates the hydraulic conductivity through measuring solute concentrations rather than by measuring flow rates. And the effective porosity is determined by applying an analytical solution of the one‐dimensional uniform flow equation.Two types of experimental data drawn from a review of the literature and four sorts of accuracy test data carried out in the laboratory are used to examine the proposed method. The reproducibilities of accuracy tests for hydraulic conductivity determination indicate a consistency within a 5.5% error margin. The experimental results further indicate that hydraulic conductivities determined using the tracer method are more precise than those from the conventional flowmeter method. In addition, with the assistance of the proposed tracer method, we argue that the effective porosities may be overestimated as shown in the drawn example cases due to mistaking total pore volumes as the effective pores. The results of the accuracy tests further indicate that the effective porosities of saturated clay specimens are significantly smaller than the total porosities, at least when the samples are allowed to swell freely.
The Fate of Haloacetic Acids and Trihalomethanes in an Aquifer Storage and Recovery Program, Las Vegas, Nevada
Authors: Thomas James M. , McKay W. Alan , Cole Erin , Landmeyer James E. , Bradley Paul M. 
Published: August 2005

AbstractThe fate of disinfection byproducts during aquifer storage and recovery (ASR) is evaluated for aquifers in southern Nevada. Rapid declines of haloacetic acid (HAA) concentrations during ASR, with associated little change in Cl concentration, indicate that (HAAs) decline primarily by in situ microbial oxidation. Dilution is only a minor contributor to HAA concentration declines during ASR. Trihalomethane (THM) concentrations generally increased during storage of artificial recharge (AR) water and then declined during recovery. The decline of THM concentrations during recovery was primarily from dilution of current season AR water with residual AR water remaining in the aquifer from previous ASR seasons and native ground water. In more recent ASR seasons, for wells with the longest history of ASR, brominated THMs declined during storage and recovery by processes in addition to dilution. These conclusions about THMs are indicated by THM/Cl values and supported by a comparison of measured and model predicted THM concentrations. Geochemical mixing models were constructed using major‐ion chemistry of the three end‐member waters to calculate predicted THM concentrations. The decline in brominated THM concentrations in addition to that from dilution may result from biotransformation processes.
Fractured‐Aquifer Hydrogeology from Geophysical Logs: Brunswick Group and Lockatong Formation, Pennsylvania
Authors: Morin Roger H. , Senior Lisa A. , Decker Edward R. 
Published: August 2005

AbstractThe Brunswick Group and the underlying Lockatong Formation are composed of lithified Mesozoic sediments that constitute part of the Newark Basin in southeastern Pennsylvania. These fractured rocks form an important regional aquifer that consists of gradational sequences of shale, siltstone, and sandstone, with fluid transport occurring primarily in fractures. An extensive suite of geophysical logs was obtained in seven wells located at the borough of Lansdale, Pennsylvania, in order to better characterize the areal hydrogeologic system and provide guidelines for the refinement of numerical ground water models. Six of the seven wells are approximately 120 m deep and the seventh extends to a depth of 335 m. Temperature, fluid conductivity, and flowmeter logs are used to locate zones of fluid exchange and to quantify transmissivities. Electrical resistivity and natural gamma logs together yield detailed stratigraphic information, and digital acoustic televiewer data provide magnetically oriented images of the borehole wall from which almost 900 fractures are identified.Analyses of the geophysical data indicate that the aquifer penetrated by the deep well can be separated into two distinct structural domains, which may, in turn, reflect different mechanical responses to basin extension by different sedimentary units:1. In the shallow zone (above 125 m), the dominant fracture population consists of gently dipping bedding plane partings that strike N46°E and dip to the northwest at about 11 degrees. Fluid flow is concentrated in the upper 80 m along these subhorizontal fractures, with transmissivities rapidly diminishing in magnitude with depth.2. The zone below 125 m marks the appearance of numerous high‐angle fractures that are orthogonal to the bedding planes, striking parallel but dipping steeply southeast at 77 degrees.This secondary set of fractures is associated with a fairly thick (approximately 60 m) high‐resistivity, low‐transmissivity sandstone unit that is abruptly terminated by a thin shale bed at a depth of 190 m. This lower contact effectively delineates the aquifer's vertical extent at this location because no detectable evidence of ground water movement is found below it. Thus, fluid flow is controlled by fractures, but fracture type and orientation are related to lithology. Finally, a transient thermal‐conduction model is successfully applied to simulate observed temperature logs, thereby confirming the effects of ground‐surface warming that occurred in the area as a result of urbanization at the turn of the century. The systematic warming of the upper 120 m has increased the transmissivity of this aquifer by almost 10%, simply due to changes in fluid viscosity and density.
Empirical Assessment of the Influence of the Unsaturated Zone on Aquifer Vulnerability, Manawatu Region, New Zealand
Authors: Bekesi Gabor , McConchie Jack 
Published: August 2005

AbstractVulnerability assessments provide essential screening tools to focus efforts on prevention of ground water contamination. The unsaturated zone, between the base of the soil and the phreatic surface, has an important role on the fate of contaminants. Yet most vulnerability assessments either do not address the unsaturated zone or overly simplify its effect on ground water contamination.In the Manawatu region of New Zealand, four factors have been found to predominantly control vulnerability: the soil, the unsaturated zone, rainfall recharge, and the aquifer medium. This paper focuses on assessing the spatial variability of the unsaturated zone. The model is based on an assessment of the total surface area available for sorption, or total sorption capacity of the unsaturated zone at each borehole. This assessment uses well drillers’logs, which provide the only available database covering the entire unsaturated zone of the study area, and published sorption values of various sediments to estimate vulnerability resulting from sorption capacity. These “point estimates” are treated as a spatially distributed parameter that can be contoured as a continuous surface. The entire modeling process was computerized in a geographical information system.Although the quality of drillers’logs varies considerably, high and low sorption areas can be distinguished readily, and the final map of unsaturated zone sorption shows clear regional trends. The sorption index generally decreases with distance from the recharge areas. The methods developed here can be applied and modified when necessary at other hydrogeological settings.