Many conceptual models have been proposed to explain the fluid-flow mechanism responsible for the origin of carbonate-hosted lead-zinc deposits such as those in the Mississippi Valley and at Pine Point. This study is devoted to the quantitative investigation of one ore-genesis mechanism: gravity-driven groundwater-flow systems. Numerical modeling techniques are used to develop a self-contained computer code for two-dimensional simulation of regional transport processes along cross sections through sedimentary basins. The finite-element method is applied to solve the steady-state, fluid-flow and heat-transport equations, and a moving-particle random-walk model is developed to predict the dispersion and advection of aqueous components. The program EQ3/EQ6 is used to compute possible reaction-path scenarios at the ore-forming site. Full integration of geochemical calculations into the transport model is currently impractical because of computer-time limitations. Results of a sensitivity analysis indicate that gravity-driven ground-water-flow systems are capable of sustaining favorable fluid-flow rates, temperatures, and metal concentrations, for ore formation near the thin edge of a basin. Dispersive processes render long-distance transport of metal and sulfide in the same fluid an unlikely process in the genesis of large ore deposits, unless metal and sulfide are being added to the fluid along the flow path. The transport of metal in sulfate-type brines is a more defensible model, in which case the presence of reducing agents control the location of ore deposition. Hydrodynamic conditions that could result.--in ore formation through mixing of two fluids are rare. The theoretical approach is a powerful tool for gaining insight into the role of fluid flow in ore genesis and in the study of specific ore districts. A preliminary model of the Pine Point deposit suggests paleoflow rates on the order of 1.0 to 5.0 m³/m² yr, paleoconcentrations of zinc on the order of 1.0 to 5.0 mg/kg • H₂O, and paleotemperatures in the range 60°C to 100°C. Under these conditions, the time required for the formation of Pine Point would be on the order of 0.5 to 5.0 million years. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Contaminant fate and transport are two of the most important issues project officers have to deal with when developing a sound remediation strategy for a subsurface contamination site. To accurately assess these issues, knowledge of possible pathways, travel times and groundwater receptors are required. A groundwater flow model of a site facilitates the assessment process by determining flow paths, discharge areas and travel time from a contaminant source to a potential receptor. The resulting model can also be used to show potential impacts on drinking water sources and surface habitats. This project is Phase II of the proposed three phase project, Groundwater Flow Modeling of the Aquifer System at Ft. Belvoir (Widdowson, 1998). Phase I consisted of developing a conceptual model of the aquifer system, recommending a modeling strategy and developing a data collection strategy. The objectives of this phase are to design and construct a computer simulation of the groundwater flow system in the aquifers below Ft. Belvoir and to develop a strategy for improved data collection using the results of the model. Steps in this phase included creation of the numerical model, calibration to known water surface elevations, and a sensitivity analysis of the boundary conditions. The numerical model was created in the Department of Defense Groundwater Modeling System environment using MODFLOW. The model was calibrated to pre-1970 wells for the deep aquifer and recent site characterization wells for the upper aquifer. The head distribution was influenced the greatest by topography and the major creeks. Accuracy of the well surface elevations played a major role in the calibration process, as well as tidal influences. A sensitivity analysis showed that adjusting the recharge and the seepage face parameters affected the model results (head and groundwater flow rates) the greatest. While adjusting the constant head and general head boundaries affected the model results the least. / Master of Science

Isotopic investigations combined with geothermal applications represent powerful tools for the exploration of groundwater potential as a drinking or geothermal resource. This Ph.D. Thesis combines both approaches, environmental and radioactive isotopes together with temperature data in deep aquifers, in order to enrich and update the knowledge concerning the aquifer recharge processes in the Aquitaine Basin (France) and the aquifer recharge processes and geothermal potential in the Bohemian Cretaceous Basin (Czech Republic). Stable isotopes (18 O, 2 H, 13 C) combined with radioisotope data (14 C, 3 H) are used to estimate the recharge timing and climatic conditions prevailing during the infiltration from the Late Pleistocene up to modern time. The character of groundwater recharge and regime are necessary to generate relevant source data for the accurate modelling of complex groundwater systems. Three groups of groundwater recharge types can be distinguished throughout Europe - (i) continuous recharge and (ii) interrupted recharge during Last Glacial Maximum and (iii) a group corresponding to particular recharge conditions. The contrasted geographic and climate conditions at both study sites in France and the Czech Republic have entailed a great heterogeneity of the recharge conditions and...

Previous studies of cooling igneous plutons did not consider the possible influence of sloping surface topography. Topographically-driven fluids in high relief terrain, however, are thought to interact with deep buoyancy-driven fluids to produce large lateral-flow systems up to 5 km long and 20 km long in silicic and andesitic volcanic terrain, respectively. In this study, a quantitative investigation of the interaction of topographically-driven and buoyancy-driven fluid flow is conducted through the use of a finite element numerical model to simulate the fluid flow and thermal regimes associated with a cooling igneous pluton in the presence of significant topographic relief. The system considered in this study is that of a pluton with dimensions 2 km by 3 km and an initial temperature of 980 °C centered beneath a mountain having relief of 1 km over a horizontal distance of 3 km. Simulation results indicate that the topographic component of flow interacts with buoyancy to produce two separate flow systems, a shallow topographically-driven flow system and a deeper convecting system. The resulting hydrothermal system evolves in a more complicated fashion than in flat topography cases. In addition, the existence of the shallow topographically-driven flow system partially masks the presence of the heat source by preventing fluids having the chemical signature of the deeper, hotter environment from reaching the surface. Cooling rate of the pluton is also increased and boiling is inhibited. These effects, however, are primarily a result of the pluton being injected into a cooler host rock. The host rock is cooler in the sloping topography case due to advective cooling prior to pluton injection. Model results also indicate that temperature beneath the mountain and the position of the zone of mixing remain relatively constant for almost 50,000 years. The stability of the temperature conditions and the position of the zone of mixing may increase the likelihood for the deposition of epithermal ore bodies in this region.

No description available.

The objective of this study is to develop guidelines for the analysis of storm water retention ponds in Central Florida. Development of a combined seepage analysis considering both unsaturated and saturated groundwater flow incorporation into a systematic approach for storm water retention pond design will result in safer and more economical storm water management practices. The research was conducted in four phases: 1) A literature review. 2) A review of mathematical and empirical analysis of transient – unsaturated/saturated groundwater flow. 3) Construction and testing of a downscaled storm water pond model. 4) Incorporation of full scale percolation pond test data from ponds constructed and tested in Central Florida. The literature review was undertaken to research mathematical and empirical equations for both unsaturated and saturated groundwater seepage to be incorporated into the modelling and development of final system analysis. The review was concentrated on seepage studies conducted in subsoil conditions similar to those in Central Florida. It was found in the early review of literature that separate studies and modelling for unsaturated and saturated seepage are widespread and relatively well documented. However, documentation of combined groundwater seepage model, since the majority of storm water retention ponds in Central Florida experience both seepage condition (unsaturated/saturated) during the design storm event. Inspection of operational seepage ponds and interviews with regulatory agencies and consulting firms in Central Florida indicated that successful design of storm water retention ponds greatly depends on the accurate definition of the subsoil conditions and the seepage characteristics during a specific design storm event. Interviews with those responsible for pond design revealed that there is currently no widespread accepted design method, and most of the designs are based on mostly local experience. Also, history indicates that even though the unsteady seepage analysis is a complex phenomenon, there is usually very little money I the budget for its analysis and design. However, due to continuous increase of property values and the need to optimize pond sizing, the trend of under budgeting for seepage bond design is reversing. Application of the research results will yield a more accurate analysis which accounts for unsaturated and saturated seepage for sizing of storm water retention ponds in Central Florida. The analytical solutions developed from this research are a combination of acceptable mathematical and empirical groundwater seepage equations or dimensionless graphs modified for the purpose of storm water retention pond analysis.

Groundwater contamination with organic compounds, especially leaked or spilled fuels, is a serious and growing environmental problem which is difficult to remediate. Alcohol additives in gasoline present more urgent problems because of their relatively high mobility in the subsurface. This study focused on the subsurface biodegradation of two octane-boosting additives, methanol and tertiary butyl alcohol (TBA). A microcosm study was undertaken to determine the <u>in situ</u> biotransformation rates. While both alcohols were found to be amenable to biodegradation in the subsurface systems, the methanol removal rate was much greater than the TBA rate. By using sets of microcosms, several chemical additives were used in an effort to determine their effects on biodegradation. Organic substrates were added to microcosms containing low concentrations of TBA (about 1 mg/L) without evidence of stimulation or inhibition. Variation of pH (from pH 5.1 to 8.8) in microcosms containing methanol and TBA was part of the next experiment. This was combined with the addition of alternative electron acceptors, nitrate and sulfate. Variation of pH in non-amended and sulfate-dosed (100 mg/L) microcosms did not enhance the biodegradation of either alcohol. Nitrate addition inhibited both methanol and TBA biodegradation at lower pH levels, but enhanced methanol degradation when combined with base. TBA degradation was not affected by base addition except in the highest (initial pH 8.8) base dose. These microcosms exhibited TBA removal rates which were uniformly higher than all other treatments. / M.S.

Modeling of groundwater flow and chemical transport calls for a good characterization of heterogeneity of soil properties. The effects of variabilities of soil properties at local scale on the mean behavior of heterogeneous flows were investigated via Monte Carlo method. Assuming log-normal distribution of conductivity and capillary pressure head being a stochastic process, the mean behavior model defined by effective properties was derived to predict the mean behavior of heterogeneous flows. Effective parameters were evaluated numerically and were parameterized in terms of statistics of randomly distributed soil properties and flow characteristics. Monte Carlo simulations were performed to reveal the mean behaviors of steady unsaturated gravity flows, transient areal source infiltration and transient point source infiltration in heterogeneous porous media. The most important findings include (a) the effective conductivity is between the geometric mean and the arithmetic mean, (b) the effective conductivity is time dependent. The derivation of a mean behavior model and the capability of parametrizing the effective properties make it possible to use current deterministic models to predict the mean behavior with minimum changes in the constitutive relations, specifically adding the dependence on the flow characteristic to the K-S-P relations. The mean model derived in this study was applied to a field site, good agreement was found between the model prediction and field data. / Ph. D.

Shellfish beds in the Chesapeake Bay region are being contaminated with fecal coliform bacteria which can cause the beds to be closed to harvesting. The cause of the bacteria contamination is basically unknown but has been hypothesized to be coming from non-point sources of pollution. The suggested source of the bacteria is on-site waste disposal systems (OSWDS) including septic tanks. Septic tank effluent (STE) may be transported through the soil and contaminate the groundwater. Submarine groundwater discharge (SGWD) may then be providing a conduit into the estuary for the bacteria. Data of OSWDS locations was not available, thus the groundwater transport mechanism was the focus of the study. A comprehensive analysis was performed on fecal coliform concentration measurements taken from 1981 through 1992 in the Chesapeake Bay region. In all, over 191,000 bacteria concentration measurements in the main database were considered. An analysis of various hydrological factors was made to determine how much these factors affected the bacteria most probable number (MPN) concentration levels. Subsets of the data consisting of heavy rain events, low tide and high tide were analyzed which had geometric mean bacteria MPN concentrations of 16.1 fecal coliforms (FC) / 100 mL, 9.4 FC / 100 mL, and 6.9 FC / 100 mL respectively compared to an overall geometric mean of 7.8 FC / 100 mL. Of the factors analyzed, rain events and tidal elevation had the most affect on bacteria MPN levels. Other factors analyzed were secchi depth, salinity, season, and water temperature. Additional data was available for Virginia's Eastern Shore including soil permeability, hydraulic gradient and land use which allowed for a more detailed analysis of samples taken in this area. A multiple regression correlation on bacteria MPN measurements with water quality data of the Eastern Shore yielded an R* value of 22.2% which does not statistically link groundwater transport and high bacteria counts. Many factors, which were calculated using a geographic information system (GIS), were examined, including near shore soil permeability, season, and surface area of water within a 400 meter radius, to determine their contribution to high area of water within a 400 meter radius, to determine their contribution to high bacteria concentration measurements. The factors used in the multiple regression were near shore soil permeability, near shore soil Darcy velocity, near shore soil hydraulic gradient, proximity to shoreline, proximity to wastewater treatment plant, rain in the last two days prior to sample collection, runoff event occurrence, salinity, season, urban land use, agricultural land use, pasture land use, surface area of water in a 400 meter radius, tide elevation, and water temperature. Of these factors, distance upstream, rain, tide and season were determined to have the highest effect on geometric mean bacteria concentrations. Groundwater transport could not be statistically linked to high bacteria MPN counts. Thus OSWDS can not be linked as the source of the contamination. Further research on a more local basis is necessary to examine the relationship between OSWDS usage and bacteria MPN levels. A GIS proved to be a very important tool for examining the effects of the different data sets. / Master of Science

Patterns of fossil diagenesis caused by groundwater leaching provide insight into how shells are altered. This study analyzes fossils in unconsolidated terrigenous sediments from the Virginia Coastal Plain, unlike previous studies conducted mostly in carbonate terranes. The vertical and lateral distribution of diagenetic states was mapped in an outcrop (63 m by 2.1 m) of the Yorktown Formation. A paleostream channel located at one end was incised during the Pleistocene and filled with sediments of the Shirley Formation. The Tabb Formation unconformably overlies the outcrop. Acidic groundwater caused the observed patterns of fossil and sediment diagenesis. These patterns include zones of fossil alteration, diagenetic stratification of the sediment, and fossil diagenesis on a microstructural level. Groundwater movement, controlled by the presence of the paleochannel, caused diagenetic alteration or complete dissolution of the fossils, and possibly caused precipitation of fine-grained iron oxyhydroxides. All carbonate material in the vicinity of the paleochannel is completely dissolved away, although iron oxyhydroxide coatings of fossils remain. Away from the paleochannel Crepidula fornicata (gastropod; aragonite), Ostrea sp. (bivalve; calcite), Balanus sp. (barnacle; calcite), and bryozoans (calcite) are found in parallel zones of alteration that dip toward the paleochannel and cut across horizontal sedimentologic and fossiliferous layers. Groundwater also leached the Yorktown sediments. This alteration caused a diagenetic stratification of the sediment, with unaltered greenish-gray silty fine sand along the base of the outcrop, overlain by leached yellowish-brown silty fine sand and areas of concentrated iron oxyhydroxides. The preservation of both aragonitic and calcitic shells was affected by groundwater movement. Original aragonitic shell material is found as chalky, uncrystallized specimens or neomorphosed shells, or is completely dissolved with only molds or ghosts remaining. Neomorphosed specimens typically consist of calcite-replaced shell material with pockets of original aragonite, and sparry calcite filling empty shell cavities. Original calcitic shell material is either chalky or unaltered. Chalky shells range from relatively hard to soft and pasty. Crepidula shells of intermediate chalkiness tend to separate into thin flakes, caused by dissolution along growth surfaces. Chalkiness of pasty shells is caused by dissolution of shell material (without recrystallization) and not simply loss of organic matrix. SEM photos of Crepidula reveal the more porous and leached appearance of chalky shells in contrast with hard; unaltered shells. The presence of chalky aragonitic and calcitic shells indicates that chalky textures are, to some degree, independent of mineralogy and microstructure. / M.S.