We will conduct research on quantification of hydraulic conductivity (K) from complex conductivity (sigma*) measurements. We will study (1) coarse alluvial deposits of the Boise Hydrogeophysical Research Site (BHRS), and (2) finer glacial melt deposits. The (sigma*) contains information on (a) the interconnected pore volume, (b) the interconnected pore surface area, and (c) the pore throat size controlling flow. We will explore whether low frequency electrical parameters can provide proxies of these pore geometrical parameters used in K prediction based on percolation theory, as well as capillary tube models. Soils with a narrow grain size distribution exhibit a low-frequency peak in polarization theoretically related to a pore length scale. Models for K prediction based on percolation theory utilize a characteristic length scale. Our work will explore the effectiveness of K prediction based on percolation type theory using the pore length scale given by (sigma*) measurements. Soils that exhibit a broad grain size distribution are typically devoid of a polarization peak and instead exhibit a constant polarization over the frequency range of (sigma*) measurements. Models for K prediction based on capillary tube models rely on a proxy measure of the hydraulic radius of tubes, usually the measurable specific surface area per unit pore volume (Spor). Our research will explore whether the magnitude of the polarization can also be used to develop electrical models of K prediction.
Laboratory studies will examine candidate petrophysical relationships linking (sigma*) to measures of the effective pore radius and (Spor). We will examine (1) the (r-) pore radius relation, where (r) is a relaxation time related to the peak in frequency (w) of the (sigma*(w)) polarization, and (2) the single frequency (sigma')-(Spor) relation. A theoretical framework for interpretation of (sigma*(w)) in terms of a complex surface conductivity ((sigma*)surf(w)), will be derived and its predictive capability evaluated by comparison with Darcy flow tests. Upscaling will be examined at the BHRS. Hydraulic conductivity estimates based on borehole (sigma*) profiles will be compared with K estimates from multi-level slug tests. Two strategies for inverting (sigma*) datasets for tomographic estimates of K are: (1) direct conversion of (sigma*) images to K images assuming a stationary K prediction equation, and (2) a structural inversion whereby the K zonation is estimated. These strategies will be assessed via comparison with spatial K distribution at the BHRS estimated from kriging of borehole-based K measurements, and available hydraulic tomography datasets.
A Hydrogeophysics Workshop will be offered to Ph.D. students during Yr 3 of this project. We will also develop Honors student UnderGraduate (HUG) research experiences in Hydrogeophysics on the Rutgers-Newark (R-N) campus. This initiative, run in collaboration with the R-N Honors College, will provide 2-3 HUG stipends per semester. We will selectively target the unique minority population of the R-N campus. We will also accelerate ongoing efforts to make the BHRS a test bed for hydrogeophysics. Equipment purchased to conduct this research will be made available to the hydrological community via the Hydrologic Measurement Facility (HMF)-Geophysics module of the Consortium of Universities for the Advancement of Hydrologic Science (CUAHSI).
This report covers research results on Boise State University’s contributions to this collaborative project "Hydrogeophysical Quantification of Hydraulic Conductivity from Electrical Measurements of the Effective Properties of Porous Media" (including Boise State University, Rutgers University-Newark, Colorado School of Mines, and Lancaster University-England) addressing fundamental understanding of the relationship between hydraulic conductivity (K) and electrical properties in a common type of aquifer that (a) is used broadly for water supplies world-wide and (b) is easily contaminated but difficult to clean-up. The research approach included controlled laboratory and field measurements, including measurements at the Boise Hydrogeophysical Research Site (BHRS) which now provide high-quality high-resolution 3-dimensional K and electrical data sets in combination with additional supporting data that are not available at field sites elsewhere. These measurements, and analysis and modeling of them, not only lead to new scientific tools and understanding (described below) but they enhance the value of the BHRS which, beyond this project, serves as a unique resource to the research and education communities for field-scale study of natural variations in aquifers and for developing "faster, cheaper, better" methods and models to determine subsurface properties with hydrologic and geophysical methods. In particular, important outcomes from this project include: (a) development and demonstration of improved modeling methods to include realistic field conditions of water table and wellbore skin in both low and high K zones for slug testing, one of most-used methods for measuring K; (b) analysis of the extensive K data set at the BHRS from slug tests which has led to unexpected and significant results that K is not highly or consistently correlated with porosity, and (using available supplementary porosity and electrical conductivity data sets from the BHRS) K zones and lenses in the aquifer can be recognized and mapped that are associated with three types of petrophysics rather than the one type that is almost universally assumed; (c) lab-scale measurements (conducted by Dr. Slater’s group at Rutgers-Newark on prepared BHRS samples) to help understand sedimentary influences on electrical responses to relate to K in the very coarse sediment mixtures of cobbles and sand (which are important in many river valley, mountain, and mountain margin settings, especially in semi-arid to arid environments) which has led to the need to modify theory to account for the presence of cobbles which are significantly less electrically responsive than the sands which occur between the cobbles and where nearly all of the water and electrical current flow occurs; (d) collecting and modeling independent field-scale electrical geophysical data to test theory for relating electrical measurements to K (led by Dr. Binley’s group at Lancaster University, England); initial modeling shows strong relationship between the distributions of electrical resistivity and porosity, and follow-up analyses (in progress) will examine the relationship with K at the field scale using K data generated in this project; and (e) development and demonstration of 3D hydraulic tomography with tractable field and modeling systems that account for realistic aquifer conditions to make this measurement method for "imaging" the 3D distribution of K directly accessible to practitioners and researchers, including initial results that provide independent corroboration for slug test results at common locations and will provide the 3D distribution for comparison with the 3D electrical geophisical data sets col;lected and modeled in this project. Broader Impacts This project advanced the development of two hydrologic testing methods and associated modeling capabilities for improved direct measurement of K in wells and between wells – what site managers and regulators need to evaluate risk and aquifer management choices and then what they need to efficiently design and operate groundwater treatments or developments. These K measurements, including the data sets and analysis and modeling of them, not only lead to the understanding derived from them (described more fully above) but they enhance the value of the BHRS which, beyond this project, serves as a unique resource to the research and education communities for education and training, and for field-scale study of variations in natural aquifers with geologic, hydrologic, and combined methods to understand fundamental controls on groundwater flow and contaminant transport. Furthermore, findings from this study on the value and limitations of laboratory and field methods and models for indirectly relating electrical geophysical measurements to hydraulic conductivity will make future use of these types of data more accurate and efficient to support better water resources management overall.
