Ronald T. Green

Modeling Barton Springs Segment of the Edwards Aquifer using MODFLOW-DCM

The Barton Springs segment of the Edwards Aquifer is the sole source of water supply for about 45,000 people in and immediately south of Austin, Texas. For water management purposes, it is important to be able to predict the availability of groundwater in response to future development and potential droughts. The Edwards Aquifer comprises heterogeneous carbonate rock strata that have developed a wellconnected network of karst conduits. This karstic geological structure makes aquifer characterization and groundwater modeling challenging. Previously, Scanlon et al. (2001) developed a two-dimensional groundwater model for the Barton Springs segment using MODFLOW. The karst conduits were not explicitly represented in the model. Instead, the study area was divided into 9 zones for which the transmissivities were obtained through calibration. We revisit Scanlon et al.’s work in this study by using MODFLOW-DCM, a MODFLOW module developed to represent flow through karstic aquifers. MODFLOW-DCM adopts a dual-conductivity approach in which the aquifer is conceptualized as being composed of two interacting flow systems, i.e., the background matrix and the karst conduits. This approach allows karst aquifers to be modeled as coupled systems, and thus allows aquifer dynamics related to karst conduit flows to be accurately simulated. Our preliminary results show improved matching of both water level measurements and spring discharges records.

A test of long-term, predictive, geochemical transport modeling at the Akrotiri archaeological site

Abstract A study of elemental transport at the Akrotiri archeological site on the island of Santorini, Greece, has been conducted to evaluate the use of natural analog data in support of long-term predictive modeling of the performance of a proposed geologic repository for nuclear waste at Yucca Mountain, Nevada. Akrotiri and Yucca Mountain have many analogous features including silicic volcanic rocks, relatively dry climates, and oxidizing, hydrologically unsaturated subsurface conditions. Transport of trace elements from artifacts buried in volcanic ash 3600 years ago at Akrotiri is analogous to transport of radioactive wastes in the proposed repository. Subtle evidence for a plume of Cu, Zn, and Pb has been detected by selective leaching of packed earth and bedrock samples collected immediately beneath the site where bronze and lead artifacts were excavated. The geologic setting of the artifacts and the hydraulic properties of the enclosing media were characterized. A numerical model of the type used in repository performance assessments was developed for elemental transport at the site. Site characterization data were used to build the model but no prior information on the nature of the contaminant plume was provided to the modelers. Some model results are qualitatively consistent with field data, including the small amount of material transported, limited amounts of sorbed material, and relatively elevated sorption on a packed earth layer, However, discrepancies result from incomplete representation of heterogeneity and complexity and poorly constrained model parameters. Identification of such system characteristics and model limitations in relevant systems is a major contribution that analog studies can contribute in support of repository modeling.

Numerical Simulation of Thermal-Hydrological Processes Observed at the Drift-Scale Heater Test at Yucca Mountain, Nevada

Abstract Results from the four-year long heating phase of the Drift-Scale Heater Test at the Exploratory Studies Facility at Yucca Mountain, Nevada, provide a basis to evaluate conceptual and numerical models used to simulate thermal-hydrological coupled processes expected to occur at the proposed repository. A three-dimensional numerical model was built to perform the analyses. All model simulations were predicated on a dual (fracture and matrix) continuum conceptualization. A 20-percent reduction in the canister heat load to account for conduction and radiation heat loss through the bulkhead, a constant pressure boundary condition at the drift wall, and inclusion of the active fracture model to account for a reduction in the number of fractures that were hydraulically active provided the best agreement between model results and observed temperatures. The views expressed herein are preliminary and do not constitute a final judgment of the matter addressed or of the acceptability of its use in a license application

Mapping borehole-accessed karst solutional features and culvert conduits using remote sensor technology

Prior papers have described prototype sensors that were developed to autonomously map pathway, flow velocity, and dimensions as they flow through karst conduits. The prototype sensors are equipped with sonar and magnetometers to measure conduit morphology and orientation. The sensors are developed to be approximately neutrally buoyant but have been equipped with a propulsion system to enable the sensors to negotiate around impediments in the flow channel and avoid stalling at the walls of the conduit or cave. Data collected during an excursion are downloaded from the sensor upon completion of the survey mission. An autonomous sensor was successfully used to characterize a segment in Honey Creek Cave, a wet cave in south-central Texas. Sonar proved to be effective in measuring the cave dimensions and the velocity of flow. A magnetometer was used to orient the pathway taken by the sensor. Together, these data provided a representative reproduction of the oriented morphology of the wet cave. Two variations of the initial generation of sensors have been developed to meet the requirements of projects funded by the United States Army Corps of Engineers for mapping borehole-accessed karst solution cavities and by the Federal Highway Administration for mapping, monitoring, and diagnosing damage to roadway culverts. The first variation is tethered to map karst voids intersected by a drill hole but where discharge to a spring is not anticipated. The second features an enhanced sonar scheme to overcome the extreme multipath environments found inside a partially filled metal culvert and to provide localization information in a magnetically shielded environment.

Multi-scale Geophysical Investigations of the Edwards Aquifer and Similar Karst Hydrogeophysics

The Main Barton Springs is a major discharge site for the Edwards Aquifer and is located in Zilker Park in Austin, Texas. The spring discharges into the Barton Springs pool near the diving board at an obvious fault line (Barton Springs fault). The thin bedded unit on the southwest side of the fault is the Regional Dense Member and the lower Georgetown Formation of the Edwards Group is exposed on the northeast side of the fault. The offset of the fault is in between 40 and 70 feet. It was geologically assumed that the groundwater recharged from the Barton Spring Segment, which is located several miles to the south?west of the Barton Springs pool area, follows the Barton Springs Fault strike and empties into the pool. To test this hypothesis geophysical surveys [2D and 3D resistivity imaging, natural potential (NP), seismic refraction tomography, induced polarization, and ground penetrating radar] were performed across the Barton Springs fault and in the southern part of the Zilker Park. Only NP surveys were allowed within the boundaries of the pool because of endangered species of Barton Springs Salamander. The purpose of the surveys was multi?folded: 1) to locate the precise location of the Main Springs on the south banks of the Barton Springs pool; 2) to determine the potential location of caves and active flow paths beneath the spring; 3) to characterize the geophysical signature of the fault crossing the Barton Springs pool. Geophysical results altogether, thus, suggest that significant amount of groundwater flow follows the path to the south of the fault along a fracture/fault zone that appears to coincide with the Springs location emptying into the pool within the Georgetown Formation.

Use of Integrated Geophysics to Characterize Paleo-Fluvial Deposits

Near-surface fluvial depositional environments at two field sites were characterized using integrated geophysical surveying techniques to determine the location and extent of paleo-fluvial deposits. The integrated surveys consisted of ground conductivity and electrical dipole-dipole resistivity measurements. The two techniques were used in combination to map the geoelectrical structure of the subsurface from which correlations to geologic structures were made. The correlations were subsequently confirmed by exploratory drilling and test trench data. Subsurface zones exhibiting high electrical resistivity values were correlated to coarse-grained fluvial deposits (i.e., sands and gravels) and zones exhibiting low electrical resistivity values were correlated with fine-grained fluvial deposits (i.e., clays and silts). Ground conductivity surveys were conducted to generate a map of the near-surface expression of the fluvial depositional environment. Dipole-dipole resistivity surveys were subsequently performed to generate vertical images of the subsurface structure and to map the vertical and lateral extent of the paleo-fluvial depositional environments. The integrated geophysical survey method successfully discriminated between fine-grained and coarse-grained fluvial deposits due to the significant electrical contrast that exists between these types of sediments and allowed for greater insight of the subsurface geologic structure and the nature of the depositional environments.