Andy L. Ward

Improved hydrogeophysical characterization and monitoring through parallel modeling and inversion of time-domain resistivity andinduced-polarization data

Electrical geophysical methods have found wide use in the growing discipline of hydrogeophysics for characterizing the electrical properties of the subsurface and for monitoring subsurface processes in terms of the spatiotemporal changes in subsurface conductivity, chargeability, and source currents they govern. Presently, multichannel and multielectrode data collections systems can collect large data sets in relatively short periods of time. Practitioners, however, often are unable to fully utilize these large data sets and the information they contain because of standard desktop-computer processing limitations. These limitations can be addressed by utilizing the storage and processing capabilities of parallel computing environments. We have developed a parallel distributed-memory forward and inverse modeling algorithm for analyzing resistivity and time-domain induced polar-ization (IP) data. The primary components of the parallel computations include distributed computation of the pole solutions in forw...

Vadose Zone Transport Field Study: Summary Report

From FY 2000 through FY 2003, a series of vadose zone transport field experiments were conducted as part of the U.S. Department of Energy’s Groundwater/Vadose Zone Integration Project Science and Technology Project, now known as the Remediation and Closure Science Project, and managed by the Pacific Northwest National Laboratory (PNNL). The series of experiments included two major field campaigns, one at a 299-E24-11 injection test site near PUREX and a second at a clastic dike site off Army Loop Road. The goals of these experiments were to improve our understanding of vadose zone transport processes; to develop data sets to validate and calibrate vadose zone flow and transport models; and to identify advanced monitoring techniques useful for evaluating flow-and-transport mechanisms and delineating contaminant plumes in the vadose zone at the Hanford Site. This report summarizes the key findings from the field studies and demonstrates how data collected from these studies are being used to improve conceptual models and develop numerical models of flow and transport in Hanford’s vadose zone. Results of these tests have led to a better understanding of the vadose zone. Fine-scale geologic heterogeneities, including grain fabric and lamination, were observed to have a strong effect on the large-scale behavior of contaminant plumes, primarily through increased lateral spreading resulting from anisotropy. Conceptual models have been updated to include lateral spreading and numerical models of unsaturated flow and transport have revised accordingly. A new robust model based on the concept of a connectivity tensor was developed to describe saturation-dependent anisotropy in strongly heterogeneous soils and has been incorporated into PNNL’s Subsurface Transport Over Multiple Phases (STOMP) simulator. Application to field-scale transport problems have led to a better understanding plume behavior at a number of sites where lateral spreading may have dominated waste migration (e.g. BC Cribs and Trenches). The improved models have been also coupled with inverse models and newly-developed parameter scaling techniques to allow estimation of field-scale and effective transport parameters for the vadose zone. The development and utility of pedotransfer functions for describing fine-scale hydrogeochemical heterogeneity and for incorporating this heterogeneity into reactive transport models was explored. An approach based on grain-size statistics appears feasible and has been used to describe heterogeneity in hydraulic properties and sorption properties, such as the cation exchange capacity and the specific surface area of Hanford sediments. This work has also led to the development of inverse modeling capabilities for time-dependent, subsurface, reactive transport with transient flow fields using an automated optimization algorithm. In addition, a number of geophysical techniques investigated for their potential to provide detailed information on the subtle changes in lithology and bedding surfaces; plume delineation, leak detection. High-resolution resistivity is now being used for detecting saline plumes at several waste sites at Hanford, including tank farms. Results from the field studies and associated analysis have appeared in more than 46 publications generated over the past 4 years. These publications include test plans and status reports, in addition to numerous technical notes and peer reviewed papers.

Establishing a geochemical heterogeneity model for a contaminated vadose zone — Aquifer system

A large set of sediment samples from a 1600 m² experimental plot within a 2.2 km² vadose zone and groundwater uranium (VI) plume was subject to physical, chemical, and mineralogic characterization. The plot is being used for field experimentation on U(VI) recharge and transport processes within a persistent groundwater plume that exists in the groundwater-river interaction zone of the Columbia River at the U.S. DOE Hanford site. The samples were obtained during the installation of 35 tightly spaced (10 m separation) groundwater monitoring wells. The characterization measurements for each sample included total contaminant concentrations (U and Cu primarily), bicarbonate extractable U(VI), sequential ²³⁸U(VI) contaminant desorption Kd, ²³³U(VI) adsorption K(d), grain size distribution, surface area, extractable poorly crystalline Fe(III) oxides, and mineralogy. The characterization objective was to inform a conceptual model of coupled processes controlling the anomalous longevity of the plume, and to quantify the spatial heterogeneity of the contaminant inventory and the primary properties effecting reactive transport. Correlations were drawn between chemical, physical, and reaction properties, and Gaussian simulation was used to compute multiple 3-D realizations of extractable U(VI), the ²³³U(VI) adsorption K(d), and the distribution of the reactive <2 mm fraction. Adsorbed contaminant U(VI) was highest in the vadose zone and the zone of seasonal water table fluctuation lying at its base. Adsorbed U(VI) was measureable, but low, in the groundwater plume region where very high hydraulic conductivities existed. The distribution of adsorbed U(VI) displayed no apparent correlation with sediment physical or chemical properties. Desorption [²³⁸U(IV)] and adsorption [²³³U(VI)] K(d) values showed appreciable differences due to mass transfer controlled surface complexation and the effects of long subsurface residence times. The ²³³U(VI) adsorption K(d), a combined measure of surface complexation strength and site concentration, was relatively uniform throughout the domain, displaying correlation with fines distribution and surface area. The characterization results revealed U(VI) supplied to the groundwater plume through spatially heterogeneous recharge from residual contamination in the zone of seasonal water table fluctuation, and transport of U(VI) controlled by weak, kinetically-controlled surface complexation in the coarse-textured saturated zone. Geostatistical relationships for the adsorbed contaminant U distribution in the characterization domain allow an extrapolation to inventory at the plume scale, a critical unknown for remedial action.

A parameter scaling concept for estimating field-scale hydraulic functions of layered soils

Predicting flow and transport in unsaturated porous media is often hampered by limited data and the uncertainties in constitutive property information at the appropriate spatial scales. Some studies have used inverse flow modeling for parameter estimation to overcome these limitations. However, determination of the soil hydraulic parameters of layered soils remains a challenge since inverting for too many parameters can lead to the nonuniqueness of parameter values. Here we propose a parameter scaling method that reduces the number of parameters to be estimated. First, parameter scaling factors are determined using local-scale parameter values. After assigning scaling factors to the corresponding soil textures in the field, the reference hydraulic parameter values at the field scale can be estimated through inverse modeling of well-designed field experiments. Finally, parameters for individual textures are obtained through inverse scaling of the reference values. The number of unknown variables is reduced by a factor equal to the number of textures (M) and the simulation time is reduced by the square of the number of textures (M 2). The proposed method was tested using two infiltration-drainage experiments in layered soils. The STOMP numerical simulator was combined with the inverse modeling program, UCODE, to estimate the hydraulic parameters. Simulation errors were significantly reduced after applying parameter scaling and inverse modeling. When compared to the use of local-scale parameters, parameter scaling reduced the sum of squared weighted residual by 93-96%.

Experimental and Numerical Investigations of Soil Desiccation for Vadose Zone Remediation: Report for Fiscal Year 2007

Apart from source excavation, the options available for the remediation of vadose zone metal and radionuclide contaminants beyond the practical excavation depth (0 to 15 m) are quite limited. Of the available technologies, very few are applicable to the deep vadose zone with the top-ranked candidate being soil desiccation. An expert panel review of the work on infiltration control and supplemental technologies identified a number of knowledge gaps that would need to be overcome before soil desiccation could be deployed. The report documents some of the research conducted in the last year to fill these knowledge gaps. This work included 1) performing intermediate-scale laboratory flow cell experiments to demonstrate the desiccation process, 2) implementing a scalable version of Subsurface Transport Over Multiple Phases–Water-Air-Energy (STOMP-WAE), and 3) performing numerical experiments to identify the factors controlling the performance of a desiccation system.

Use of Induced Polarization to Characterize the Hydrogeologic Framework of the Zone of Surface‐Water/Groundwater Exchange at the Hanford 300 Area, WA

An extensive continuous waterborne electrical imaging (CWEI) survey was conducted along the Columbia River corridor adjacent to the U.S. Department of Energy (DOE) Hanford 300 Area, WA, in order to improve the conceptual model for exchange between surface water and U-contaminated groundwater. The primary objective was to determine spatial variability in the depth to the HanfordRingold (H-R) contact, an important lithologic boundary that limits vertical transport of groundwater along the river corridor. Resistivity and induced polarization (IP) measurements were performed along six survey lines parallel to the shore (each greater than 2.5 km in length), with a measurement recorded every 0.5-3.0 m depending on survey speed, resulting in approximately 65,000 measurements. The H-R contact was clearly resolved in images of the normalized chargeability along the river corridor due to the large contrast in surface area (hence polarizability) of the granular material between the two lithologic units. Cross sections of the lithologic structure along the river corridor reveal a large variation in the thickness of the overlying Hanford unit (the aquifer through which contaminated groundwater discharges to the river) and clearly identify locations along the river corridor where the underlying Ringold unit is exposed to the riverbed. Knowing the distribution of the Hanford and Ringold units along the river corridor substantially improves the conceptual model for the hydrogeologic framework regulating U exchange between groundwater and Columbia River water relative to current models based on projections of data from boreholes on land into the river.

USE OF WATER FLUXMETERS TO MEASURE DRAINAGE

Water supplies throughout the world are rapidly diminishing in quantity and quality. Efforts over the next decade must focus on methods that use water more efficiently for agriculture, industry, and recreational purposes and at the same time reduce the potential for groundwater pollution. To assist in this effort, we have developed an improved method to simultaneously measure drainage quantity and quality using a water fluxmeter. Our water fluxmeter is a wick-lysimeter fitted with a small tipping-spoon and a solution-collection system. The only moving part is the tipping spoon. We have tested our fluxmeters under a range of conditions, from non-vegetated desert settings in Washington State USA to irrigated tea plantations in Sri Lanka. Conditions of overirrigation have been documented with our fluxmeters. When 4200 mm of water was applied to sandy soil via drip irrigation at the Washington State site, over 3100 mm of drainage occurred. In contrast, at the same site, in the absence of both irrigation and vegetation, drainage was found to range from 0 mm/yr for a 1-m-deep silt loam soil to more than 100 mm/yr for a coarse-gravel surface. Solute transport, related to nitrate leaching, can also be analyzed using water fluxmeters. Water fluxmeters have provided a reliable and inexpensive method to assess both quantity and quality of drainage waters over a wide range of environmental conditions.

Use of Time‐Domain Induced Polarization for Lithology Identification: A Case Study from the Hanford 300‐Area

Use of the induced polarization (IP) method, as being applied to determine the hydrostratigraphic framework at the Hanford 300-Area, Washington, offers advantages over electrical resistivity (ER) in yielding information about the physicochemical characteristics of the subsurface. The IP response in soils results from surface polarization that is largely determined by lithology. For non-metallic soils, IP typically shows a weak dependence on fluid conductivity, but strong nearlinear relationships to textural parameters (surface area, grain size, etc.). Compared to the ER method, IP has smaller signal-to-noise ratio, requiring additional care during data acquisition and processing. In particular, appropriate data weight needs to be considered for successful inversion of IP datasets.

Nuclear Waste Disposal

Waste disposal is the final step of waste management and ideally comprises placing radioactive waste in a dedicated disposal facility although discharging of effluents into the environment within permitted limits is also a disposal option. Concepts for radioactive waste disposal have developed considerably with time and great consideration is given to the necessary retention times and retention capacities for different types of waste resulting in much improved repositories and planned disposal facilities. Disposal of radioactive wastes in a disposal facility is intended to isolate the waste both from human activity and from natural dynamic processes. Repositories for the storage/disposal of radioactive waste generally rely on a multibarrier system to isolate the waste from the biosphere. This multibarrier system typically comprises the natural geological barrier provided by the host rock and an engineered barrier system (EBS). Furthermore, features and limitations of some disposal/storage options are being discussed, followed by the transport of radionuclides. The flow rate/throughput of water through/around the repository is one of the most important parameters, which must be as low as possible.