Thomas E. Lachmar

Hydrogeochemical Characterization of Leaking, Carbon Dioxide-Charged Fault Zones in East-Central Utah, With Implications for Geologic Carbon Storage

We examined a natural, CO 2 -charged subsurface system located near two fault zones in East-Central Utah that is analogous to engineered sequestration sites. Geologic information and geochemical and isotopic data from water and gas samples were used to develop a conceptual model of the flow system. This flow-system description indicates that CO 2 from a depth >800 m migrates upward through a system of shallower, stacked aquifers. The geologic structure in the area serves to focus the CO 2 -rich waters at the location of a faulted, anticlinal trap. The faults in the area impede horizontal flow but allow vertical leakage through thick, low-permeability formations. An important implication from this CO 2 -sequestration analog is that leakages occur along discrete flow paths in the subsurface; thus, in sequestration scenarios, detailed understanding of discrete flow paths will be necessary. Another implication is that groundwater can transport a significant amount of CO 2 , and thus, sampling groundwater chemistries from wells may be a better way to identify leakages than using monitoring techniques at the surface. Finally, even though mineralization occurs during CO 2 leakage to the surface, self-sealing has not occurred at this natural analog and may not occur at engineered sequestration sites.

Field monitoring and performance evaluation of an in situ air sparging system at a gasoline-contaminated site

In situ air sparging (IAS) has been used since the mid-1980s, but few carefully designed field studies have been performed to evaluate its effectiveness. In this study, 27 discrete monitoring points (MPs) were installed at a gasoline-contaminated site to investigate the efficacy of IAS. Each MP was instrumented with a pressure transducer and a Technalithics dissolved oxygen (DO) probe, and located so they could be used to characterize subsurface changes in total head and DO with depth, distance and orientation around a central injection well. Because the blower over-heated and automatically shut down after approximately 30 min and short-circuiting of air into two MPs occurred within 2 min, the study was designed as three sets of three 30-min trials. Longer trials would not have yielded different nor more insightful results. A volume of soil was not oxygenated during any injection. Instead, air traveled directly to at least four of seven different MPs during eight of the nine trials, probably as a result of an air bubble forming beneath a confining layer. The order of air arrival at the MPs varied during the first few trials, but once a preferential pathway was established, it did not collapse between trials and provided the shortest distance to the vadose zone during subsequent trials. Oxygen uptake rates estimated for MPs that received air during any trial exceeded the consumption rates of the Technalithics DO probes, and indicate that the probes could be used for estimating oxygen transfer during system operation or for oxygen uptake measurements during shut-down tests. The data from the monitoring system indicate that IAS is infeasible for remediation of soil and groundwater at this site due to its low horizontal hydraulic conductivity. Similar behavior is anticipated when IAS is applied at other sites with low hydraulic conductivity materials.

Tracer studies for evaluation of in situ air sparging and in-well aeration system performance at a gasoline-contaminated site

Field-scale tracer studies were conducted at a gasoline-contaminated site in order to evaluate the effectiveness of in situ air sparging (IAS) and in-well aeration (IWA) in controlling the movement of soil gas and groundwater in the subsurface. The field site was comprised of silty sand (SM) and silty clay (CL), underlain by a clay layer at approximately 7.6 m. Depth to groundwater ranged from 2.4 to 3 m. Soil permeability and the natural hydraulic gradient were both low. Helium was used to trace the movement of soil gas in the unsaturated zone during the IAS field study, and successfully confirmed short-circuit pathways for injected air and demonstrated the limited distribution of injected gases at this site. Fluorescein, bromide, and rhodamine were used to trace the movement of groundwater during the IWA system field study, and successfully documented the inability of the IWA system to recirculate enough groundwater to enhance subsurface dissolved oxygen levels or to remediate groundwater by air stripping at this site. The inability of the systems to remediate the site was likely due to site conditions which consist of low-permeability soils and decreasing permeability with depth. As a result, relatively impermeable layers exist at the depth of the IAS screen and the lower IWA screen. These site conditions are not conducive to successful performance of either remediation system.

Field monitoring and performance evaluation of a field-scale in-well aeration system at a gasoline-contaminated site.

Several in-well aeration (IWA) technologies have been used since the early 1990s, but few field studies have been performed to evaluate the extent of water circulation around IWA systems. In this study, 27 discrete monitoring points (MPs) were installed at a gasoline-contaminated site to assess the efficacy of IWA. Pressure transducers and dissolved oxygen (DO) probes were sealed into the MPs, allowing them to be used to characterize subsurface changes in total head and DO with depth, distance and orientation from a central injection well. No change in DO or in hydrocarbon total mass or distribution occurred across the site during two trials (41 and 20 days) of the system. Water level fluctuations during the trials were similar in all MPs, and were due to seasonal water table changes and rainfall events. No circulation cell was established around the IWA well after 41 days of operation, and the impact of the well extended less than 90cm from it. Groundwater only circulated through the sand pack around the well. Little, if any, recharge occurred through the lower screen. Silt accumulated in the well, limiting its operation time, even with a fabric filter sock over the lower screen. Obviously, IWA was ineffective at this site, probably because the horizontal hydraulic conductivity (K(h)) of the soil opposite the lower screen was low (0.09cm per day) and because the distance between the two screens was short relative to the borehole radius. Long remediation times would likely make IWA unattractive at this or other sites where the K(h) of the soil is so low that the air injection rate would have to be low to prevent blowing the well dry.

Application of fracture-flow hydrogeology to acid-mine drainage at the Bunker Hill Mine, Kellogg, Idaho

Abstract The mechanics of groundwater flow through fractured rock has become an object of major research interest during recent years. This project has investigated the flow of groundwater through fractured Precambrian metaquartzite rocks in a portion of the Bunker Hill Mine near Kellogg, Idaho. Groundwater flow through these types of rocks is largely dependent upon the properties of fractures such as faults, joints and relict bedding planes. Groundwater that flows into the mine via the fractures is acidic and is contaminated by heavy metals, which results in a severe acid mine drainage problem. A more complete understanding of how the fractures influence the groundwater flow system is a prerequisite of the evaluation of reclamation alternatives to reduce acid drainage from the mine. Fracture mapping techniques were used to obtain detailed information on the fracture properties observed in the New East Reed drift of the Bunker Hill Mine. The information obtained includes fracture type, orientation, trace length, the number of visible terminations, roughness, waviness, infilling material, and a qualitative measure of the amount of water flowing through each fracture. The hydrogeologic field data collected include routine measurements of the discharge from four individual structural features and four areas where large quantities of water are discharging from vertical rock bolts, the depths to water in three piezometer nests at the ground surface, the pressure variations in four diamond drillholes, and constant discharge flow tests conducted on three of the diamond drillholes. The field data indicate that relict bedding planes are the primary conduits for groundwater flow, and suggest that the two major joint sets that are present connect water flowing through the discontinuous bedding planes. The three minor joint sets that are present do not seem to have a significant impact on groundwater flow, but along with the two major joint sets may store relatively large quantities of water. It appears that rock-bolt holes in the central portion of the drift primarily intersect relict bedding planes, whereas rock-bolt holes in the southeastern portion of the drift primarily intersect joints; this probably is related to the shallower angle of dip of the bedding planes in the central portion of the drift. It also appears that recharge from the surface directly above the mined-out openings is the primary source of water in the upper workings of the mine, and that the large seasonal head variations in the potentiometric surface are primarily responsible for the observed temporal variations in mine inflow. Infilling material may control the hydrogeologic character of the faults, with those filled with gouge having low hydraulic conductivities and those filled with breccia having relatively high hydraulic conductivities. In addition, one of the faults may act as a positive (constant head-recharge) hydrogeologic boundary. A double-porosity approach probably is the most appropriate for simulating the groundwater flow system in the vicinity of the New East Reed drift. Finally, grouting of a combination of breccia-filled faults and relict bedding planes may offer the best hope for minimizing mine-water inflow or recharge.

Structure and Hydrogeology of Deformed Sedimentary Bedrock Aquifers, Western Summit County, Utah

The Snyderville basin, near Park City in western Summit County, Utah, has experienced significant water shortages coupled with a 50 percent growth rate in the past 10 to 15 years. Recent development rests directly on complexly folded and fractured sedimentary bedrock aquifers in the hanging wall of the Mount Raymond thrust. Detailed geologic and fracture scanline mapping coupled with structural analyses in the Pinebrook subdivision, one site within the Snyderville basin demonstrating abrupt hydrogeologic changes, provide a clearer picture of the local hydrogeologic setting. The dominant map-scale structure is the Twomile Canyon anticline. Several macroscopic faults cut this fold, including the Toll Canyon fault, a backthrust off the Mount Raymond thrust. Fracture orientations and densities vary within meters across the Twomile Canyon anticline as a function of lithology and position relative to macroscopic faults. Exposures of the Toll Canyon fault show that the width and lithologic composition of the fault core and related damaged zone are a function of lithology, and the fault strongly controls fracture permeability. Damage zones in limestones and sandstones with high fracture intensities may be regions of enhanced permeability, whereas shale smears and clay gouge adjacent to the fault core act as barriers to fluid flow. A conceptual model of the subsurface in the Pinebrook study area has been developed, and several test well sites have been proposed based on this model and field observations. The target formation, structural position, fracture intensity, local hydrogeology, and accessibility were factors considered in locating these wells.