Peter Kitanidis, Stanford University Michael Cardiff, University of Wisconsin Warren Barrash, Boise State University
The conventional approach to representing the governing physics of fluid flow in hydrogeology is based on the premises that (a) fluid flow can be treated separately from deformation in the solid matrix and (b) changes in flow conditions are gradual (i.e., allowing simplifying assumptions about momentum changes to be valid). Oscillatory flows may violate these premises in certain ?intermediate? frequency ranges, regardless of whether they are created deliberately at a well for purposes of monitoring or site characterization, or are created accidentally from a natural or man-made source. These violations raise important questions for understanding oscillatory fluid flow in subsurface porous media at core and field scales (i.e., scales of interest in hydrologic applications): (a) must Darcy?s law be modified? and (b) do controlling hydraulic parameters such as permeability and elastic storage change with oscillation frequency rather than remain constant? This research is a comprehensive study of oscillatory flows at intermediate frequencies involving: a careful mathematical study of flow at the pore scale and upscaling to the core scale to generalize Darcy?s law; experiments in a laboratory sandbox to test/validate the theoretical developments and examine heterogeneous cases including infiltration of an immiscible fluid; and upscaling to hydrologic scale with controlled oscillatory flow experiments in the field at a highly characterized research wellfield to test/validate theoretical developments and models. Expected results include a rigorous theoretical treatment of oscillatory flow mechanics leading to predictions and modeling of oscillatory signal propagation characteristics and frequency dependent aquifer parameter behavior at laboratory and field scale for a range of porous media materials.
This project has broad impacts for society overall and for the scientific and engineering communities because it deals with basic hydrologic research in the subsurface, where most of the available freshwater is stored. In the United States, groundwater is the primary source of water for over 50 percent of Americans, and roughly 95 percent for those in rural areas. In the world, many of the most important aquifers are being gradually depleted or contaminated. This research will lead to better methods for the restoration and management of this important resource. In particular, oscillatory flows may become important tools for characterization of subsurface volumes to determine the 3D heterogeneity of aquifer parameters and to monitor for changes in water quality or aquifer status without having to remove water (that may be contaminated and hazardous). Oscillatory flows also have potential applications in enhancing mixing, which can enhance reaction rates and result in more efficient site remediation technologies.
from Boise State University (BSU) for a 2-year collaborative project with BSU, Stanford University, and the University of Wisconsin-Madison to examine if aquifer properties (long understood to be constant based on theory and experience given constant discharge rate or slowly changing pumping or environmental influences) actually vary under conditions of continuously alternating pumping and injection cycles (oscillatory flow). Theoretical considerations allow the possibility that aquifer properties at a given location might change with the frequency of an oscillating pumping test or oscillating natural phenomenon or "disturbance" – and also allow for using information from responses to tests or disturbances at multiple frequencies to improve estimation of aquifer storage properties and identification of spatial variations ("heterogeneity") in aquifer transmissive and storage properties. Understanding of such properties and distributions of properties are valuable for many engineering and environmental applications including groundwater supply, management, clean-up, and monitoring. Furthermore, use of oscillating pumping stimulation can be accomplished with no net withdrawal of water which is beneficial for investigation at contaminated sites where conventional pumping tests remove contaminated water that must be managed, and perhaps treated, at the surface. The BSU portion of the project has completed but the Stanford and Wisconsin portions are extended for a year – and BSU is continuing collaborative participation in analysis, modeling, interpretation, and reporting through publications, professional presentations, and data availability at the project website (under development at the University of Wisconsin-Madison). Important scientific and engineering developments to date with this project include tjhe following. We have shown how signal processing can recover small amplitude oscillatory hydrology (OH) signals from noisy data measured at observation wells, we quantify the uncertainties in the estimates, and we demonstrate results from a joint inversion of transmissive and storage properties. We have developed analytical modeling tools for design of OH tests in the range of natural aquifer conditions. We have developed and demonstrated new technology for generating oscillatory signals for field-scale OH testing at a range of amplitudes and frequencies without removing water from or introducing water into the well. We have conducted field-scale OH tests sufficient to produce 3D hydraulic tomography or 3D "images" of aquifer property distributions (modeling in progress). In particular, for the OH tomography we combine results from OH tests at a range of amplitudes and frequencies from each of three levels in the aquifer with observations in 9-10 isolated zones in each of three observation wells to simultaneously solve for transmissive and storage properties in a highly discretized and parameterized 3D volume. Additional theoretical and modeling work in progress aims at estimating how aquifer transmissive and storage properties may depend on the oscillation frequency by matching the measured data from OH pumping tests in a water table aquifer using a recently-derived analytical solution. Important broader impacts of this project include the following. With regard to impact on the development of human resources, student and post-doc team members from US institutions have received cross-training between field and modeling methods and tools, and we are collaborating with an international post-doc (from Tel Aviv University, in residence at Stanford) on theory for and modeling of oscillatory hydrology testing. With regard to impact on technology transfer, new technology (oscillating signal generator or OSG) was developed for this project (in collaboration with Mt. Sopris Instruments of Denver), tested, modified, and then used during a sustained (3-week) OH field-testing campaign at BSU’s Boise Hydrogeophysical Research Site. During the field tests in 2013, the OSG produced sinusoidal signals at a range of frequencies (spanning an order of magnitude) and discharge volumes (ranging over a factor of 1.6) per injection or withdrawal stroke. This variety of stimulation was conducted at three levels in the 16.5 m thick aquifer with observation in isolated intervals (total of 28) in three observation wells located at distances between 5.6 m and 12.3 m from the OSG-stimulation well. With regard to impact on society beyond science and technology, applications of OH shows potential for significant savings in costs for remediation and monitoring of contaminated groundwater, and attendant beneficial impacts on human health, the environment, and the economy (e.g., through reduction of clean-up and monitoring costs and faster return of contaminated property to productive uses). With regard to information transfer within and between science and engineering disciplines, results have been and will continue to be disseminated through peer-reviewed publications and professional scientific and engineering conferences including international conferences and more-focused specialized discipline conferences. Additionally, data sets from field experiments and models (with tutorials) will be made available on a project website at the University of Wisconsin.
