Groundwater flow in alluvial valleys consists of two components, baseflow and underflow. The baseflow component of the Darcy flux flows normal to the river and contributes to the surface flow. The underflow component moves downstream in the same direction as the river but at a much slower rate. Underflow is important in Texas because the conjunctive use of groundwater and surface water is regulated by controlling the diversion of underflow by wells. Land owners in Texas are legally entitled to unrestricted use of the underground water beneath their property. Stream underflow, however, has been expressly excluded from the definition of underground water. The distinction is important because it allows the State to legally restrict the non-domestic pumpage of groundwater (in an "underflow zone") near streams. Regulators are interested in controlling pumpage near rivers in order to prevent streamflow depletion. Historically, the underflow exemption has not been well recognized by the courts. In large measure, this may be due to the fact that our understanding of underflow in alluvial valleys is incomplete. If the underflow rule is to be successfully implemented, a complete understanding of the nature and occurrence of underflow is imperative. This study was initiated to: 1) determine the hydrogeologic factors that control underflow (and baseflow) in alluvial valley aquifers in Texas and the United States; and 2) to examine the suitability of the underflow criterion as a management tool for the prevention of streamflow depletion by wells. To accomplish this, a data base of 23 alluvial river basins was compiled and a 3-dimensional digital model of a hypothetical alluvial valley aquifer was constructed. Examples from the data base indicate that alluvial aquifers can be classified into three types based on the predominant regional groundwater flow direction: baseflow-dominated, underflow-dominated, and mixed flow. Flow patterns can be transient, however, and respond rapidly to changing river stage if the aquifer and the riverbed are highly permeable. Therefore, the distinction must be made between local, transient underflow and baseflow occurring near the river and regional, steady state underflow and baseflow away from the river. Underflow dominated aquifers are found in classic bedload depositional settings which are characterized by high channel gradient, high width to depth ratio, low channel sinuosity, and low river penetration. Linear regressions performed on the parameter values in the data base verify the validity of the data. The degree of correlation provides the basis for a method of estimating the predominant regional groundwater flow direction in an alluvial aquifer based on geomorphologic and morphometric data. The results from the digital model agree with the findings from the data base. Digital simulations indicate that the amount of underflow is directly related to the channel gradient, the amount of recharge, the aquifer hydraulic conductivity, and the streambed hydraulic conductivity. The riverbed hydraulic conductivity is the most critical hydraulic factor controlling the amount of underflow. The output from the model is 100 percent underflow at low values of riverbed permeability. Both the model results and published field data do not support the existence of a significant local "underflow zone" adjacent to rivers in large alluvial systems. Close to the river, the baseflow component may predominate even in regionally underflow-dominated systems due to the influence of high transverse valley gradients. There are many problems associated with the use of underflow as a management tool. The definition is vague and ambiguous. Underflow can be transient and spatially variable. Texas alluvial systems are baseflow dominated and there is probably no significant "underflow zone" near rivers. Lastly, the presence of underflow has been difficult to prove in court. It is the finding of this study that the underflow criterion is insufficient to prevent streamflow depletion by wells. The underflow rule in the Texas Water Code should be reconsidered, or perhaps abandoned, in favor of criteria that are more justifiable. / text

This dissertation considers the effect of measuring randomly varying local hydraulic conductivities K(x) on one's ability to predict steady state flow within a bounded domain, driven by random source and boundary functions. That is, the work concerns the prediction of local hydraulic head h(x) and Darcy flux q(x) by means of their unbiased ensemble moments (h(x))(κ) and (q(x))(κ) conditioned on measurements of K(x). These predictors satisfy a deterministic flow equation in which (q(x))(κ) = -(κ)(x)∇(h(x))(κ) + r(κ)(x) where κ(x) is a relatively smooth unbiased estimate of K(x) and r(κ)(x) is a "residual flux." A compact integral expression is derived for r(κ)(x) which is rigorously valid for a broad class of K(x) fields, including fractals. It demonstrates that (q(x))(κ) is nonlocal and non-Darcian so that an effective hydraulic conductivity does not generally exist. It is shown analytically that under uniform mean flow the effective conductivity may be a scalar, a symmetric or a nonsymmetric tensor, or a set of directional scalars which do not form a tensor. For cases where r(κ)(x) can neither be expressed nor approximated by a local expression, a weak (integral) approximation (closure) is proposed, which appears to work well in media with pronounced heterogeneity and improves as the quantity and quality of K(x) measurements increase. The nonlocal deterministic flow equation can be solved numerically by standard methods; the theory here shows clearly how the scale of grid discretization should relate to the scale, quantity and quality of available data. After providing explicit approximations for the prediction error moments of head and flux, some practical methods are discussed to compute κ(x) from noisy measurements of K(x) and to calculate required second moments of the associated estimation errors when K(x) is log normal. Nonuniform mean flow is studied by conducting high resolution Monte Carlo simulations of two dimensional seepage to a point sink in statistically homogeneous and isotropic log normal K(x) fields. These reveal the existence of radial effective hydraulic conductivity which increases from the harmonic mean of K(x) near interior and boundary sources to geometric mean far from such sources for σ^2/Υ (the variance of ln K) at least as large as 4. They suggest the possibility of replacing r(κ)(x) by a local expression at distances of few conditional integral scales from the interior and boundary sources. Special attention is paid to the "art" of random field generation, and comparisons are made between four alternative methods with five different random number generators.

This dissertation considers modeling groundwater flow under imprecisely known parameters and managing a plume of contaminant. A new approach has been developed to study the effects of parameters uncertainty on the dependent variable, here the head. The proposed approach is developed based on fuzzy set theory combined with interval analysis. The kind of uncertainty modeled here is the imprecision associated with model parameters as a result of machine or human imprecision or lack of information. In this technique each parameter is described by a membership function. The fuzzy inputs into the model are in the form of intervals so are the outputs. The resulting head interval represents the change in the output due to interval inputs of model parameters. The proposed technique is illustrated using a two dimensional flow problem solved with a finite element technique. Three different cases are studied: homogeneous, mildly heterogeneous and highly heterogeneous transmissivity field. The groundwater flow problem analysis requires interval input values for the parameters, the output may be presented in terms of mean value, upper and lower bounds of the hydraulic head. The width of the resulting head interval can be used as a measure of uncertainty due to imprecise inputs. The degree of uncertainty associated with the predicted hydraulic head is found to increase as the width of the input parameters interval increases. Compared to Monte Carlo simulation approach, the proposed technique requires less computer storage and CPU time, however at this stage autocorrelation and crosscorolation are not configured in the presented formulation. In the plume containment problem two formulations are presented using the hydraulic gradient technique to control the movement of the contaminants. The first one is based on multiobjective analysis and the second, on fuzzy set theory. Multiobjective analysis yields a set of alternative strategies each of which satisfies the multiple objectives to a certain degree. Three different techniques have been used to choose a compromise strategy. Although they follow different principles, the same preferred strategies are selected. It is also noticed that rapid restoration results in a large pumping volumes and high costs. Using a fuzzy formulation for plume containment yields the optimum pumping rates and locations in addition to the membership function at each pumping location. The resulting membership functions at these pumping locations can be used to study the sensitivity of each location to a change in objective function and constraints bounds. Overall, both the fuzzy and multiobjective methodologies, presented in this dissertation, provide new and encouraging approaches to groundwater quality management.

No description available.

A hydrological modelling effort using MODFLOW was undertaken in order to determine the relative importance of some of the factors influencing hydraulic gradient reversals in peatlands. Model domains were of two types, large raised bog type (LRBT) and kettle bog type (KBT), and were made to undergo various levels of meteorological forcing (water deficit). Substrate, too, was varied in order to determine its importance on reversals. Domain-wide reversals were successfully simulated in LRBT systems, but not in KBT systems. Although simulated flow patterns matched field-observed patterns, both pre- and post-drought, simulated reversals occurred more quickly than in the field. This may be due to insufficiently distributed parameters, such as hydraulic conductivity. Reversals were easily terminated by simulating non-drought conditions. In the LRBT system, reversal duration decreased, and time-to-reversal increased, with a decrease in drought severity. Increasing drought severity in KBT systems had the opposite effect on the duration of semi-reversed flow patterns, suggesting a possibly different/additional mechanism for flow reversals in KBT systems. Hydraulic conductivity had an appreciable effect on flow reversal evolution, though neither changing porosity, nor differences in catotelm layering had a great effect.

No description available.

Natural attenuation of trichloroethene (TCE) and technetium (99Tc) was studied for five consecutive seasons (from January 2002 to January 2003) in Little Bayou Creek. The stream receives ground water discharge from an aquifer contaminated by past waste disposal activities at the Paducah Gaseous Diffusion Plant (PGDP), a uranium enrichment facility near Paducah, Kentucky. Results from stream gaging, contaminant monitoring, tracer tests (with bromide, nitrate, rhodamine WT and propane) and simulation modeling indicate the TCE is naturally attenuated by volatilization and dilution, with volatilization rates related to the ambient temperature and surface discharge rate. The only apparent mechanism of 99Tc attenuation is dilution. Travel times of non-gaseous tracers were found to be similar and have highest values in October and lowest in June. It was also estimated from modeling that the transport of the solutes in the stream was mostly one-dimensional with insignificant secondary storage.

The impact of septic tank effluent disposal on groundwater quality was investigated at Oxford. The Oxford township can be regarded as typical of many small communities on the Canterbury Plains which have a high density of septic systems serviced by soakage pit drainage. The primary concern with grouped septic systems is the potential for both chemical and microbial groundwater contamination. The alluvial gravel aquifers of the Canterbury Plains are especially susceptible to microbial contamination due to the high rates of groundwater flow which may transport both bacteria and viral contaminants over large distances. Geological investigations established the presence of an areally extensive, tuff derived, clay unit which forms an aquitard beneath the unconfined aquifer in the north of the Oxford area. Recharge of the unconfined aquifer above the clay unit is exclusively from rainfall infiltration while to the south, groundwater levels respond to rainfall infiltration and influent seepage from the Eyre River. The presence of two hydrogeologically distinct gravel units within the unconfined aquifer was determined by application of the column dilution technique. Point dilution tests showed the average groundwater velocity of 130 m/day in the upper gravel unit to be significantly higher than the 40 m/day measured in the underlying gravels. The presence of discrete channels of preferred flow within the unconfined aquifer system was also established by point dilution tests, intrachannel velocities ranging from 210 to 400 m/day. A resistivity salt tracing test indicated groundwater flow in an easterly direction with a velocity of 250 m/day through an observed channel feature. Groundwater quality monitoring showed a significant degree of groundwater contamination close to the Oxford township. Concentrations of faecal coliform bacteria in excess of drinking water standards were detected up to 900 m downgradient of the nearest septic tank. Elevated levels of chemical indicators (N03-N, CL-) were also detected in all monitoring wells. The pattern of groundwater contamination was complex, reflecting both the heterogeneity of groundwater flow through the unconfined aquifer system and the influence of monitoring well location. Predictive modelling indicated the potential for the transport of faecal coliform bacteria up to 2.6 km downgradient of Oxford. Modelling also suggested increased urban development within Oxford to have a relatively minor effect on the overall extent of groundwater contamination. Additional hydrogeological and water quality data, aided by the application of numerical solute transport modelling techniques, may provide a more accurate estimate of the impact of septic tank effluent disposal on groundwater quality. Future sewage disposal options for Oxford have to balance the low potential for microbial contamination of drinking water supplies outside the groundwater zone delineated by this study, against the environmental acceptability continuing contamination of this zone. This study has identified the need for further research into the effects of septic tank effluent discharge on groundwater quality in the Canterbury region, to provide a sound base for future resource management decisions.

The use of temperature logging for identifying water flow through fractures in sedimentary rock has declined since the 1960’s and 70’s primarily because of low sensor resolution and cross-connected flow along the borehole. Although sensor resolution has improved to the order of 10-3 C for several decades, temperature logging has not experienced a notable increase in popularity. This thesis studies these and other fundamental limitations to the application of borehole temperature logging for identifying flow through fractured rock, and tests the hypothesis that the limitations can be overcome, presents new methods for accomplishing that goal, and increases the applicability of the technology. Although some conventional open-hole testing (e.g. flow meters) rely on vertical cross-connected flow in the borehole annulus to identify transmissive fractures, the flow is recognized to both distort open-hole temperature logs and facilitate chemical cross contamination. Removable polyurethane coated nylon liners have recently been developed to seal boreholes and minimize cross-contamination. High sensitivity temperature logs collected in the stagnant water column of lined boreholes under different hydrogeologic conditions herein show the degree to which cross connected flow can mask important flow conduits and thereby distort the interpretation of which fractures control flow. Results from the lined holes consistently lead to identification of more hydraulically active fractures than the open-hole profiles and an improved qualitative ranking of their relative importance to flow consistent with contaminant distributions observed in rock core. The identification of flow in fractures with temperature logs depends on the presence of a temperature contrast between the water and the rock matrix to create an aberration in the otherwise gradually varying profile. Atmospherically driven thermal disequilibrium commonly only extends several tens of meters from surface and dissipates with depth, making temperatures logs a variable assessment of flow that is depth limited to the heterothermic zone. The active line source (ALS) method, a series of temperature logs measured before and within a day after the water column of a lined borehole is placed into thermal disequilibrium with the broader rock mass with a heating cable, is shown to provide two advantages. First, the method eliminates the depth limitation allowing flow zones to be identified below the hetro-homothermic boundary and second, the qualitative assessment of ambient water flow in fractures is improved throughout the test interval. The identification of the flow conduits is supported by the combined evidence from visual inspection of core, rock contamination profiles, acoustic televiewer logs and tests for hydraulic conductivity using straddle packers. A new device, the thermal vector probe (TVP) is presented. It measures the temperature of the borehole fluid with four high sensitivity temperature sensors arranged in a tetrahedral pattern which is orientated using three directional magnetometers. Based on these, the total thermal gradient, its horizontal and vertical components as well as the direction and inclination are determined, typically at less than 0.01m intervals. Comparison of TVP data collected in lined boreholes under ambient conditions (thermal and hydraulic) as well during thermal recovery after ALS heating demonstrate the reproducibility of the results and superior characterization of thermal aberrations indicative of flow relative to single sensor temperature data. A detailed comparison of subdivisions in the thermal field to the vertical changes in the hydraulic gradient measured from three nearby high detail (12-14 port) multi-level installations demonstrates the interrelationship between hydraulic and thermal fields and thereby the potential benefit of the TVP in hydrogeologic investigations. Developing confidence in the use of both the TVP and ALS techniques in lined holes relies on demonstrating the reproducibility of results, consistency with observations from other technologies, and numerical simulation. Comparisons of field data with highly detailed numerical simulations using the program SMOKER shows that the influence of water flow in a fracture around a lined borehole on the temperature patterns is complex and factors such as convection likely influence the shape of the thermal aberrations observed. Model results suggest that the temperature aberrations are related to the volumetric water flow, a distinct lower resolution limit exists (approximately 5.6x10-7 m3/sec per metre across the fracture, m2/s), and although flow above 10-4 m2/s is readily detectable, prospects for quantification of higher flows are poor. Some field data indicate the numerically determined lower limit is conservative and the details of the limit require additional study. The aspects of temperature logging historically limiting applicability for detecting and comparing flow through discrete or groups of fractures in rock are hereby better understood and consistently overcome. The high level of detail achieved in the data highlights the complexity of the system and offers opportunities for further refinement. The TVP and ALS technique applied in a lined borehole promise both new insights into, and potential for quantification of ambient groundwater flow through fractures in rock.

The focus of this study was on recharge mechanisms and recharge estimation within the Table Mountain Group area. The study evaluated recharge processes and recharge estimation methods in the Table Mountain Group aquifer systems.