Junfeng Zhu

Analysis of hydraulic tomography using temporal moments of drawdown recovery data

Received 2 June 2005; revised 30 September 2005; accepted 21 October 2005; published 1 February 2006. [1] Transient hydraulic tomography (THT) is a potentially cost-effective and highresolution technique for mapping spatial distributions of the hydraulic conductivity and specific storage in aquifers. Interpretation of abundant well hydrographs of a THT survey, however, is a computational challenge. We take on this challenge by developing an estimation approach that utilizes the zeroth and first temporal moments of well hydrographs, instead of drawdown itself. The governing equations for the temporal moments are Poisson’s equations. These equations demand less computational resources as opposed to the parabolic equation that governs drawdown evolution. Likewise, the adjoint equations for evaluating sensitivities of the moments for parameter estimation also take the same forms. Therefore a temporal moment approach is expected to expedite the interpretation of THT surveys. On the basis of this premise we extend our sequential successive linear estimator to use the zeroth moment and characteristic time of the drawdown-recovery data generated by THT surveys. We subsequently investigate computational efficiency and accuracy of the moment approach. Results of the investigation show that the temporal moment approach yields results similar to those from the approach that uses transient heads but at significantly less computational costs. Limitations using temporal moments are discussed subsequently.

Hydraulic/partitioning tracer tomography for characterization of dense nonaqueous phase liquid source zones

[1] A new technology, hydraulic/partitioning tracer tomography (HPTT), is proposed to survey spatial distributions of hydraulic properties and dense nonaqueous phase liquids (DNAPLs) in the subsurface. HPTT is nothing more than a set of multiple hydraulic/ partitioning tracer tests and synthesis of all the tests to map the spatial distributions. It involves injection of water at one borehole in a source zone to create a steady state forced gradient flow field, then release of conservative/partitioning tracers at the same borehole, and monitoring heads and tracer breakthroughs at the others. The same operation is repeated using different boreholes for the water and the tracer injections. To analyze the head and tracer data obtained from the proposed tomographic survey, a joint stochastic estimator was developed. Numerical experiments were then conducted to evaluate the effectiveness of HPTT as well as the stochastic estimator. Results show that prior knowledge of hydraulic heterogeneity is critical for mapping the distribution of DNAPLs. In addition, the results suggest that the proposed HPTT in conjunction with the stochastic estimator is potentially a viable tool for high-resolution characterization of subsurface heterogeneity and contamination.

Improved Karst Sinkhole Mapping in Kentucky Using Lidar Techniques: A Pilot Study in Floyds

The existing sinkhole database for Kentucky is based on low-resolution topographic maps created more than fifty years ago. LiDAR (Light Detection and Ranging) is a relatively recent technique that rapidly and accurately measures features on earths surface in high-resolution. To test the feasibility of using LiDAR to map sinkholes in Kentucky, we have developed a method of processing LiDAR data to identify sinkholes and tested the method in portions of the Floyds Fork watershed in central Kentucky. The method consisted of four steps, creating a high-resolution digital elevation model (DEM) from LiDAR data, extracting surface depression features from the DEM, inspecting the depression features for probable sinkholes, and verifying the probable sinkholes in the field. A total of 1,683 probable sinkholes were identified in the study area, compared to 383 previously mapped for the same area. We field-checked 121 randomly-selected probable sinkholes and confirmed that 106 of them were karst sinkholes. This method increased the number of sinkholes by a factor of four with a success rate between 80% and 93% for the study area, demonstrating that the LiDAR sinkhole-mapping method is reliable and efficient. This method identified approximately 55% of the previously mapped sinkholes, and approximately 98% of the missed sinkholes appeared to be filled or covered for urban development and agriculture purposes. The next step is to extend this method to provide high-resolution sinkhole maps for other karst areas in Kentucky where LiDAR data become available.

A SUCCESSIVE LINEAR ESTIMATOR FOR ELECTRICAL RESISTIVITY TOMOGRAPHY

A dc resistivity survey is an inexpensive and widely used technique for investigation of near surface resistivity anomalies. It recently has become popular for the investigation of subsurface pollution problems (NRC, 2000). In principle, it measures the electric potential field generated by a transmission of dc electric current between electrodes implanted at the ground surface. Then, an apparent (bulk or effective) electrical resistivity for a particular set of measurement electrodes is calculated using formulas that assume homogeneous earth. Many pairs of current transmission and electric potential measurements are used to “map” subsurface electrical resistivity anomalies. This conventional resistivity survey is analogous to classical aquifer test in which an aquifer is excited by pumping at one well and the response of the aquifer (e.g. drawdown-time relation or well hydrograph) is observed at another well. The theoretical well hydrograph from an analytical solution that assumes aquifer homogeneity and infinite domain (e.g. Theis’ solution, 1935) is then used to match the observed hydrograph to obtain apparent or effective aquifer transmissivity and storage coefficient. Due to the homogeneity assumption, the theoretical drawdown represents a spatially averaged drawdown in a heterogeneous aquifer. This average drawdown is unequivocally different from the one observed at a well in a heterogeneous aquifer, although the difference may be small due to diffusive nature of the flow process. Thus, applying Theis’ solution to aquifer tests in a heterogeneous aquifer is tantamount to comparing apples to oranges (Wu et al., 2005). They suggested that the apparent transmissivity represents a weighted average of transmissivity anomalies over the cone of depression. High weights are given to transmissivity anomalies near the observation and the pumping well. The apparent transmissivity reflects, as a consequence, local geology but it can also be affected by significant geologic anomalies within the cone of depression. In other words, the physical meaning of the apparent transmissivity can be highly dubious. The strong similarity between traditional aquifer and apparent resistivity analysis leads us to conclude that conventional analysis of electrical resistivity

Assessing Methane in Shallow Groundwater in Unconventional Oil and Gas Play Areas, Eastern Kentucky

The expanding use of horizontal drilling and hydraulic fracturing technology to produce oil and gas from tight rock formations has increased public concern about potential impacts on the environment, especially on shallow drinking water aquifers. In eastern Kentucky, horizontal drilling and hydraulic fracturing have been used to develop the Berea Sandstone and the Rogersville Shale. To assess baseline groundwater chemistry and evaluate methane detected in groundwater overlying the Berea and Rogersville plays, we sampled 51 water wells and analyzed the samples for concentrations of major cations and anions, metals, dissolved methane, and other light hydrocarbon gases. In addition, the stable carbon and hydrogen isotopic composition of methane (δ13 C-CH4 and δ2 H-CH4 ) was analyzed for samples with methane concentration exceeding 1 mg/L. Our study indicates that methane is a relatively common constituent in shallow groundwater in eastern Kentucky, where methane was detected in 78% of the sampled wells (40 of 51 wells) with 51% of wells (26 of 51 wells) exhibiting methane concentrations above 1 mg/L. The δ13 C-CH4 and δ2 H-CH4 ranged from -84.0‰ to -58.3‰ and from -246.5‰ to -146.0‰, respectively. Isotopic analysis indicated that dissolved methane was primarily microbial in origin formed through CO2 reduction pathway. Results from this study provide a first assessment of methane in the shallow aquifers in the Berea and Rogersville play areas and can be used as a reference to evaluate potential impacts of future horizontal drilling and hydraulic fracturing activities on groundwater quality in the region.

Hydrogeologic Investigations of Pavement Subsidence in the Cumberland Gap Tunnel

Cumberland Gap Tunnel was constructed under Cumberland Gap National Historical Park in 1996 to improve transportation on a segment of U.S. 25E, connecting Kentucky and Tennessee and restoring Cumberland Gap to its historical appearance. The concrete pavement in the tunnel started to subside in 2001. Ground penetrating radar surveys revealed voids in many areas of the limestone roadbed aggregate beneath the pavement. To investigate possible hydrogeologic processes that may have caused favorable conditions for voids to form in the aggregate, we studied geology, groundwater flow, and groundwater chemistry in the tunnel using a variety of methods, including bore drilling, packer test, dye tracing, groundwaterand surface-flow monitoring, waterchemistry modeling, and an aggregate dissolution experiment. The study revealed that the aggregate receives a large volume of groundwater from much of the bedrock invert, but the flow velocity is too slow to transport small particles out of the aggregate. Calcite saturation indices calculated from water-chemistry data suggest that the groundwater was capable of continuously dissolving calcite, the primary mineral in the limestone aggregate. Water samples taken during different flow conditions indicate that groundwater under high-flow conditions could dissolve calcite more quickly than groundwater under low-flow conditions. The dissolution experiment showed that all the limestone aggregate placed beneath the roadbed and in contact with groundwater lost mass; the highest mass loss was 3.4 percent during a 178-day period. The experiment also suggested that water with higher calcite-dissolving potential removed limestone mass quicker than water with low calcite-dissolving potential. We recommend that the limestone aggregate be replaced with noncarbonate aggregate, such as granite, to prevent dissolution and future road subsidence. Introduction On October 18, 1996, a segment of U.S. 25E from Middlesboro, Ky., to Harrogate, Tenn., was relocated into a newly constructed tunnel beneath Cumberland Mountain, to improve transportation efficiency and safety as well as help restore Cumberland Gap to its appearance when Daniel Boone brought the first settlers to Kentucky in the mid1Kentucky Geological Survey, University of Kentucky 2Kentucky Transportation Center, University of Kentucky