H31F-01
Estimating Vertical Groundwater Velocities Using Groundwater Thermal Gradients
An understanding of vertical groundwater flow through unconsolidated deposits is a component for predicting fate and transport of contaminants in the saturated zone. Groundwater movement through heterogeneous glacial deposits common to northern Indiana (USA) provided a test setting for determining if measured vertical groundwater thermal gradients could aid in calculating vertical groundwater velocity estimates. Field procedure was conducted by collecting stratified groundwater temperatures from a series of cased monitoring wells previously advanced through glacial till and outwash sedimentary sequences. Groundwater thermal gradients (temperature-depth profiles) were plotted and matched using automated computer modeling software (Microsoft Excel Solver) with published type curves to derive a dimensionless parameter for estimating vertical groundwater velocities. Data results matched predictions, to include an increase in vertical groundwater velocities during the seasonally wetter Spring; and, higher calculated vertical groundwater velocities for the finer-grained till aquitards when compared to aquifers comprised of coarser-grained outwash deposits. This study shows promise and has gathered interest both in the scientific community and environmental consulting practice for estimating vertical migration rates of contaminants (specifically those affected by advection) within the saturated zone. Government agencies or consultants, for instance, could also potentially apply this estimation technique to measure and map localized recharge rates for developing more accurate wellhead protection zones.
H31F-02 INVITED
An Assessment of Current Concepts for Hydrogeological Site Characterization, and Alternatives
Characterization of the geological and hydrogeological conditions of contaminated sites is a critical element of risk management. Characterization includes data acquisition and interpretation that intends to provide the analytical tools needed for decision making related to transport of contaminants and for remediation. Despite many years of experience, site characterization is still not as well understood and regulated as it should be. The chasm between some of the newer concepts developed by researchers on the one hand, and the technology used in the field by service providers on the other, has never been wider than it is today. Consequently, questions that should be addressed using well-documented rational tools may continue to be addressed based on intuition and experience. Instead of coming up with defensible action plans and implementing them rapidly, such action plans are often a source of contention and end up in the courts. This paper evaluates concepts often used in the practice of hydrogeological site characterization, and tries to distinguish between myth and reality. It explores concepts such as: 1. Accurate descriptions of geological and hydrogeological conditions are attainable; 2. Sound planning and action plans in response to accidents require a large amount of data; 3. Investing in site characterization is a sound investment; 4. Experience gained in decontamination and decommissioning in the field is a sound basis for planning future efforts; 5. If you do not find the contaminants, they do not exist; 6. Numerical models for flow and transport processes in the subsurface are beneficial; 7. Current regulations in the area of hydrogeology are helpful. These concepts, while often invoked in applications, are often wrong and misleading or applied incorrectly, and reflect the many ambiguities prevailing in this area. This paper explores the issues raised above in detail. It also presents the elements of a consistent approach for site characterization, and demonstrates it through a case study that involves uncertainty in the hydrogeology as well as in human physiology.
H31F-03
GEOSTATISTICAL CHARACTERIZATION OF MULTISCALE HYDRAULIC CONDUCTIVITIES AND TRANSMISSIVITIES
The subsurface consists of porous and fractured materials exhibiting systematic and random spatial and directional variations in hydraulic and transport properties on a multiplicity of scales. Traditional geostatistical moment analysis allows one to infer the spatial structure of a subsurface flow property, such as log hydraulic conductivity or transmissivity, on the basis of numerous values measured on a given support scale across a domain or "window" of a given length scale. There is growing evidence that geostatistical parameters one obtains in this manner vary systematically with support and window scales. This observed scale variation is captured quite faithfully upon considering log hydraulic conductivity or transmissivity to be a random fractal. Doing so allows representing measurements, having a common support scale and scattered across a given window, by a truncated power variogram having at most four parameters when the underlying fractal field is statistically isotropic and six parameters when it is anisotropic. One parameter is proportional to the length scale of the data support and another to that of the window. This allows predicting the truncated power variogram that one would obtain for similar data on other support and/or window scales within the same hydrogeologic unit. Such ability to bridge across support and window scales renders the fractal approach capable of (a) characterizing the spatial variability of multiscale hydraulic conductivities and transmissivities jointly by a single family of truncated power variograms, that is fully defined by at most four (in the isotropic case) or six (in the anisotropic case) parameters, and (b) conditioning this characterization on corresponding multiscale measurements via co-kriging. If the measurements represent support-scale mean field values, they can be used for this purpose directly. If (as is more commonly the case) they represent equivalent or effective support-scale values, it is first necessary to translate them into corresponding mean field data. A theoretical basis for doing so in the case of box-shaped support volumes, embedded in a mean uniform flow field parallel to a principal direction of statistical anisotropy, is available. Direct estimates of mean field values, as well as some key variogram parameters, can be obtained via stochastic interpretation of pumping tests in a manner proposed by Neuman et al. (2004) and illustrated by Blattstein et al. (2006). Mean field values obtained in this way represent a support scale proportional to the characteristic distance spanned by all participating wells and piezometers. Regardless of what variogram model one uses to interpret a pumping test, its parameters can always be translated into those of a corresponding truncated power variogram which (other than for support- and window-specific parameters) are representative of all scales. We illustrate some aspects of our ideas and proposed methodology on multiscale hydraulic data from an unconfined aquifer near Tubingen, Germany.
H31F-04
Hydraulic and tracer tomography for the characterization of DNAPL source zones: A laboratory sandbox study
Contaminant source zones of Dense Nonaqueous Phase Liquids (DNAPLs) are prevalent at a large number of
industrial sites throughout the world. Existing direct approaches of site characterization involving coring and
interpolation or geostatistical analysis of DNAPL saturation are considered to be invasive and costly. We present
here a laboratory study of an alternative to traditional characterization approaches based on hydraulic and tracer
tomography. The main advantage of this new approach is that it requires far fewer boreholes than the traditional
coring techniques to image the DNAPL source zones. Specifically, it first analyzes the information derived from
hydraulic tomography, a new type of aquifer test, to identify the three-dimensional hydraulic heterogeneity in
hydraulic conductivity and specific storage of the aquifer. The newly derived knowledge of heterogeneity is then
used to design partitioning tracer tests conducted in a tomographic manner to accurately depict the spatial
distribution of DNAPL saturations in source zones. We conducted several laboratory sandbox studies which
involved the packing of aquifers with a deterministic heterogeneity pattern, spilling of a known volume of
Trichloroethene (TCE) and its subsequent characterization through hydraulic tomography, tracer tomography, and
coring at the conclusion of all other tests. For purposes of interpreting the hydraulic and tracer tomography data,
we utilize the algorithms developed by Yeh and Zhu (2007) to conduct the analysis of our laboratory results. Our
laboratory results to date show that the imaging of DNAPL saturations and estimating their corresponding
uncertainties is viable through this approach. In addition, we found that the proper estimation of hydraulic
heterogeneity first is of paramount importance in designing, conducting, and analyzing partitioning tracer tests.
The proper design of partitioning tracer tests can increase the accuracy of DNAPL saturation estimates in
heterogeneous aquifers as it will better sweep through suspected contaminated areas including DNAPL ganglia
and pools. Characterization of subsurface heterogeneity has an important additional benefit as it contributes to
the lowering of site investigation costs.
http:www.iihr.uiowa.edu/~illman
H31F-05 INVITED
Hydrogeophysical Field Characterization at the DOE Old Rifle Site, CO
Ongoing work within the U(VI)-contaminated alluvial aquifer at the Rifle, CO, Integrated Field Challenge Site (IFC) focuses on investigating the efficacy of biostimulation for facilitating microbial reduction of U(VI) to U(IV). At the site, a series of biostimulation experiments have been conducted within different flow cells by controlled injection of acetate and bromide. Changes in sulfate and dissolved iron and U(VI) concentrations suggested that the system transformations are governed by both chemical and physical heterogeneity of the aquifer at the IFC. Here, we describe the analyses of slug test, flow-meter test, electrical well log, crosshole seismic, radar, and complex electrical resistivity datasets to characterize the hydrogeology of the IFC subsurface. Using a Bayesian approach to integrate the hydrogeological and geophysical wellbore and crosshole datasets, we estimate hydrogeological heterogeneity within, as well as across the different flow cells. The posterior model provides estimates of hydrogeological properties and their associated uncertainties. Moment analyses of the bromide breakthrough data and changes in aqueous sulfate and iron concentrations are used to quantitatively characterize the transport and system transformations as a function of the heterogeneity, both within and across flow cells. The integrated analysis of both estimated hydraulic conductivity field and concentration changes will allow for better understanding of the influence of heterogeneity on stimulated in-situ bioremediation of U(VI) at the IFC.
H31F-06 INVITED
Can a Ground Water Flow Model be Validated?
Multi-scale spatial and temporal variability of inflow and outflow of groundwater basins are well-known facts. Multi- scale aquifer heterogeneity is a reality. Traditional in-situ borehole characterization and monitoring methods can cover only a fraction of a groundwater basin. Consequently, our knowledge of a groundwater basin is limited and uncertain. Our lack of knowledge and information about groundwater basins has led to grossly misleading predictions of groundwater flow and contaminant migration. Validity of our subsurface model as such has been seriously questioned, as has our ability to predict flow and solute migration in aquifers. Groundwater resources management virtually becomes a matter of political debate without much scientific basis. Recent advances in hydrologic and geophysical tomographic survey technologies have brought forth cost- effective means to characterize aquifer spatial heterogeneity. This paper discusses an application of hydraulic tomographic survey to characterization of heterogeneous sandboxes. It demonstrates that detailed characterization can lead to satisfactory predictions, using a ground water flow model, of drawdown evolution induced by pumping tests. We thereby advocate high-resolution characterization and monitoring of the subsurface such that reliable assessment and proper management of our groundwater resources is possible.