Browsing by Author "Pfannkuch, Hans-Olaf"

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    An Approach to Groundwater Monitoring Well Network Design Relying on Numerical Techniques and Public-Domain Information
    (Water Resources Research Center, University of Minnesota, 1994-05) Mooers, Howard D.; Pfannkuch, Hans-Olaf; Quinn, John J.
    Uncertainties in the hydrogeology of a study area and in the transport properties of potential contaminants challenge the designers of groundwater monitoring well networks- Numerical flow modeling is a useful tool for guiding the placement of wells, but it requires justified boundary conditions and sufficient knowledge of aquifer parameters. In a graciated terrain, additional problems arise because of complex spatial arrangements of aquifers and aquitards. The transport of conservation contaminant tracers is normally calculated as a simple function of average values of hydraulic conductivity, hydraulic gradient, and effective porosity. But are the results of this straightforward method always valid? In order to address these topics, this investigation focused on a landfill on the Anoka sandplain of east-central Minnesota. The purpose of this study was to determine the proper placement of an initial group of monitoring wells at the landfill using only offsite public-domain data. The results may then be applied to the siting of other wells. Finite-difference flow modeling was supported by an abundance of inexpensive public-domain information and by the construction of a detailed, sub-regional glacial geologic map. A two-dimensional kriging analysis refined the model by determining the cell-by-cell best estimates of the basal elevation of the surficial aquifer. Particle tracking results indicated the expected pathway of landfill leachate. Based on the results, one well upgradient of the landfill and several downgradient wells were selected from the database of actual monitoring wells, and the head data from these shallow wells were used to calibrate the model. The calibrated hydraulic conductivity of the sandplain aquifer agrees closely with values obtained through grain-size analyses and pump tests. Numerical analyses of boundary conditions support the validity of the flow model. Other case studies of unconfined outwash aquifers suggest that predicted plumes of conservative tracers are often greater than the actual extents. Compared to the chloride data for monitoring wells at the Anoka site, particle tracking results have an accurate orientation but a length at least two times too long. Uncertainties, such as the effect of longitudinal dispersion and the transient nature of the leachate's initial concentration and source area, suggest an even greater difference' This conceptual understanding of plume migration provides guidance for the placement of additional downgradient wells. The described application of models and inexpensive offsite data to monitoring well network design is a methodology that may be effective for the monitoring of solutes from existing or proposed potential contamination sources.
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    Hydrocarbon Spills, Their Retention in the Subsurface and Propagation into Shallow Aquifers
    (Water Resources Research Center, University of Minnesota, 1983-04) Pfannkuch, Hans-Olaf
    The two most important phases in the propagation of a hydrocarbon spill into the subsurface occur in the vadose or unsaturated zone, between the land surface and the water table, and in the groundwater flow zone at its contact with the free oil phase. The last determines the dissolution and spreading of contaminants through the most mobile flow zone. The first determines the spill geometry, the amount of free phase oil that reaches the groundwater body and the total retention of spilled oil. Oil retention studies carried out for this project show whether the volume of soil within a spill site is sufficient to retain the oil without it escaping into the groundwater and also gives a rough approximation of how' long it takes for the oil phase to reach pendular saturation. Grain size and sorting distribution and are useful factors governing retention, and are useful parameters in evaluating the potential damage of an oil spill and the immediate danger of groundwater contamination. Centrifuge air-oil-water displacement provides a quick and first order approximate means to measure site-specific oil and water retention within the unsaturated zone of a porous media. Results show a semi log relationship between total primary retention and grain size (in phi units) of laboratory samples with oil-water-air systems run under 38 g in the centrifuge. A correction factor adjusts retentions to gravity drainage column pack saturations after gravity drainage used as a standard of comparison. Field samples from an oil spill site assume the same relationship when plotted against effective particle size versus total primary retention. Effective particle size has a finer resolution than mean grain size because it emphasizes the role of the fine fraction and sorting characteristics that tend to increase retention capacity.
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    Study of Criteria and Models Establishing Optimum Level of Hydrogeologic Information for Groundwater Basin Management
    (Water Resources Research Center, University of Minnesota, 1975-04) Pfannkuch, Hans-Olaf
    It is shown that information is a structural part of the decision making process. The role of the information gathering process can best be described as Hayes' process to improve preposterior distributions in probability assignments that will serve as basis for decision making processes. Hydrogeologic information is characterized by a low degree of predictability and high variability. The general cost structure of the hydrologic information gathering process is represented in information cost curves which are combined with production cost or opportunity loss curves to give total cost curves. These may display a minimum which is the point of diminishing returns at which exploration activities should be ceased. One of the main unsolved problems is the proper definition of a universally applicable unit of information content. Optimum levels of information can only be established when utility functions can be expressed in monetary terms and give rise to clear objective functions. Three case histories were used to develop guidelines for the design and operation of monitoring systems for special wastes, minimum hydrogeologic information needed in watershed district management, and the need and role of information in the choice and construction of a proposed simulation model for a regional system under complex decision rules.