Tritium Migration at the Gasbuggy Site: Evaluation of Possible Hydrologic Pathways

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University and Community College System of Nevada Water Resources Center TRITIUM MIGRATION AT THE GASBUGGY SITE: POSSIBLE HYDROLOGIC PATHWAYS Jenny Chapman, Todd Mihevc, and Brad Lyles Nevada Operations Office U.S. Department of Energy

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This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, mark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. This report has been reproduced directly from the best available copy. Available to DOE and DOE contractors from the Office of Scientific and Technical Information, P.O. Box 62, Oak Ridge, TN 37831; prices available from (423) 576-8401. Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161.

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TRITIUM MIGRATION AT THE GASBUGGY SITE: Evaluation of Possible Hydrologic Pathways Jenny Chapman, Todd Mihevc, and Brad Lyles Water Resources Center Desert Research Institute University and Community College System of Nevada Nevada Operations Office U.S. Department of Energy Las Vegas, Nevada The work upon which this report is based was supported by the U.S. Department of Energy

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An underground nuclear test named Gasbuggy was conducted in northwestern New Mexico in 1967. Subsequent groundwater monitoring in an overlying aquifer by the U.S. Environmental Protection Agency revealed increasing levels of tritium in monitoring well EPNG 10-36, located 132 m from the test, suggesting migration of contaminants from the nuclear cavity. There are three basic scenarios that could explain the occurrence of tritium in well 10-36:1) introduction of tritium into the well from the land surface, 2) migration of tritium through the Ojo Alamo Formation, and 3) migration through the Pictured Cliffs Formation. The two subsurface transport scenarios were evaluated with a travel time analysis. In one, transport occurs to the Ojo Alamo sandstone either up the emplacement hole or through fractures created by the blast, and then laterally through the aquifer to the monitoring well. In the other, lateral transport occurs through fractures in the underlying Pictured Cliffs detonation horizon and then migrates up the monitoring well through plugged casing connecting the two formations. The travel time analysis indicates that the hydraulic conductivity measured in the Ojo Alamo Formation is too low for lateral transport to account for the observed arrival of tritium at the monitoring well. This suggests transport either through fractures intersecting the Ojo Alamo close to well EPNG 10-36, or through fractures in the Pictured Cliffs and up through the bottom plug in the well. The transport scenarios were investigated using hydrologic logging techniques and sampling at the monitoring well, with the fieldwork conducted after the removal of a string of 0.05-m-diameter tubing that had previously provided the only monitoring access. No vertical movement of water was detected with a thermal flowmeter through the bottom cement plug and the water chemistry is consistent with the known characteristics of the Ojo Alamo, suggesting no leakage from the Pictured Cliffs through the bottom plug in the well. Though relatively fresh formation water was present across from the perforations in the Ojo Alamo as the well recovered from the tubing removal, the water did not contain measurable tritium in either May 1994 or May 1995. The previously measured tritium was highly variable from one sample to the next and did not display typical breakthrough behavior for a migrating contaminant. Thus, the earlier monitoring samples may not have been representative of Ojo Alamo Formation water through time, because sampling was restricted inside the 0.05-m tubing. The tritium previously sampled from the tubing may have been dispersed to non-detectable concentrations when the tubing was pulled prior to DRI’s sampling. Though no tritium was detected in water samples consistent with the Ojo Alamo Formation, low levels of tritium were found higher in the borehole in a zone of chemically and isotopically distinct water. This lower salinity water has an evaporative isotopic signature, suggesting exposure in a surface pond. The tritium content could be related to gas flaring operations during Gasbuggy production testing, with the water introduced to the well through leakage or

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Important Observations during Gasbuggy Activities ........................................................... 5 CONTAMINANT TRANSPORT SCENARIOS........................................................................... 5 A Surface Origin for the Tritium in 10-36............................................................................... 5 Transport Through the Ojo Alamo .......................................................................................... 7 Transport Through the Pictured CliffsFormation................................................................... 8 Travel Time Analysis......................................................................................................................... 8 Results of Travel Time Analysis............................................................................................... 12 FIELD INVESTIGATION OF TRANSPORT SCENARIOS..................................................... 14 Thermal Flow Logging ................................................................................................. 19 Thermal Flow Logging ................................................................................................. 21 Interpretation of Logging, Chemistry,and Radiochemistry Data......................................... 24

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1. Tritium concentration for Well EPNG 10-36.................................................................... 1 2. The upper portion of the figure shows the location of the Gasbuggy site in northwestern New Mexico and the location of the cross section shown in the lower portion................................................................................................................ 2 3. Generalized geologic cross section at the Gasbuggy emplacement hole....................... 3 4. Locations of the boreholes drilled for the Gasbuggy Project and Well 10-36............... 4 5. Two basic scenarios for the transport of tritium from the cavity at GB-ER to the monitoring well, EPNG 10-36................................................................................... 6 6. Probability density function for groundwater velocity in the Ojo Alamo Aquifer. ... 11 7. Cumulative distribution function for groundwater travel time in the Ojo Alamo Aquifer to Well 10-36........................................................................................ 13 8. Cumulative distribution function of travel time to 10-36 through a fractured Pictured Cliffs Formation..................................................................................... 13 9. Vertical distribution of permeability measured in cores from GB-1............................... 14 10. Status of Well 10-36, from Holmes and Narver (1983) prior to the 11. Chemical log and location of thermal flowmeter measurements in 12. Chemical log and results from the thermal flowmeter survey conducted 13. Comparison between temperature logs collected in 1994 and 1995............................... 22 14. Comparison between EC logs collected in 1994 and 1995.............................................. 22 15. Comparison between pH logs collected in 1994 and 1995 ............................................ 23 16. Stable isotopic composition of water samples from well EPNG 10-36 and Bubbling Spring.............................................................................................................. 23 1. Chemical and Isotopic Analyses of Groundwater Samples from the Gasbuggy Site . 17

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A nuclear detonation occurred on December 10,1967 in northwestern New Mexico, for the purpose of testing the effectiveness of such explosives for fracturing tight gas reservoirs to enhance production. This test, named Gasbuggy, was followed by production testing and pressure monitoring through 1976, and site cleanup activities that concluded in 1978. Since that time, groundwater has been monitored in the Gasbuggy area by the U.S. Environmental Protection Agency (EPA) as part of the Long-Term Hydrologic Monitoring Program. A pre-existing gas production well, EPNG 10-36, located 132 m from the test hole, was sealed with sand and cement prior to the nuclear test with the intention that it would be reopened after the test to determine the effectiveness of the experiment. Drilling problems precluded reopening the well to its pretest total depth; however, the well was recompleted for the purpose of hydrologic monitoring, and added to the monitoring program. In 1984, tritium activities well above background began appearing in 10-36 (Figure 1). Though these activities were substantially below the drinking water standard for tritium (20,000 pCi/1), they indicated a possibility of migration from the cavity and warranted further study. This report presents an analysis of the potential migration pathways and the results of a logging and sampling program designed to investigate the Gasbuggy site. It follows the sequence of work performed: background information is presented first, then a conceptual description of possible contaminant release scenarios, followed by a travel-time analysis of those scenarios and, finally, results from the field investigation. Figure 1. Tritium concentration for Well EPNG 10-36. Data are from the U.S. EPA Long-Term Hydrologic Monitoring Program (U.S. EPA, 1992).

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The Gasbuggy site is in the San Juan basin, a large structural basin containing about 3,660 m of sedimentary rocks. Groundwater flow from the site is believed to be to the west-northwest to discharge points along the San Juan River about 40 km away (Mercer, 1970) (Figure 2). At the site, hydraulic head values decrease with depth, indicating a potential for downward flow (Sokol, 1970). The gas producing horizon fractured by the shot is the Pictured Cliffs Formation, a tight sandstone between about 1,189 and 1,280 m below ground surface at the Gasbuggy site (Figure 3). The aquifer where the tritium has been detected is the Ojo Alamo Sandstone, located between about 1,057 and 1,110m below ground surface. The two sandstones are separated by over 60 m of shale and siltstone. Farmington San Juan Gasbuggy Figure 2. The upper portion of the figure shows the location of the Gasbuggy site in northwestern New Mexico and the location of the cross section shown in the lower portion. The cross section was modified by Sokol (1970) from that presented by Peterson et al. (1965).

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Figure 3. Generalized geologic cross section at the Gasbuggy emplacement hole. From U.S. DOE (1986). Four holes were drilled as part of the Gasbuggy project (Figure 4). Hole GB-1 was the first exploratory hole, from which many cores were collected during drilling. These cores provide numerous porosity and permeability measurements in both the Ojo Alamo and Pictured Cliffs Formations. Hole GB-2 was a second exploratory borehole. Hole GB-3 was drilled after the detonation, with the purpose of determining the effect of the blast on the underground environment, including the Ojo Alamo. Single-hole hydraulic tests were performed in each of the three holes. Hole GB-E/GB-ER was the emplacement and reentry hole (this somewhat unusual configuration, drilling the reentry hole through the cement plug in the event hole, was performed to save costs by speeding drilling, eliminating a need to set casing, etc.). All of these holes have been plugged to the surface with cement and are therefore unavailable for additional measurements. The monitoring well, EPNG 10-36 (formerly known as Well 29-4 No. 10 and hereafter referred to as 10-36), was originally a gas production well, producing from the Pictured Cliffs Formation.

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Figure 4. Locations of the boreholes drilled for the Gasbuggy Project and Well 10-36. The distance from GB-ER (the emplacement and reentry hole) to Well 10-36 is 433 ft. enhance production, hydraulic fracturing was performed at 10-36 at an unknown time in the past. The well had reportedly been producing for about ten years prior to the Gasbuggy test. The original plans had called for postshot production testing at 10-36, so the perforated portion of the well opposite the Pictured Cliffs was filled with sand, then the remainder of the well was plugged with cement prior to the nuclear test. Postshot drilling to clean out the cement encountered damaged casing near the bottom of the Ojo Alamo that prevented penetration to the Pictured Cliffs. The well was then completed in the Ojo Alamo by shot perforating the 0.14-m casing, and left for The Gasbuggy nuclear test was conducted in well GB-E at a depth of 1292 m below ground surface. The detonation occurred in the Lewis Shale Formation (Figure 3), but the test was designed

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to fracture the Pictured Cliffs gas reservoir directly overlying the Lewis Shale. The Ojo Alamo Sandstone is separated from the Pictured Cliffs by the Fruitland Sandstone and Kirtland Shale. The chimney created by the nuclear explosion (a rubble zone of increased porosity caused by fractures and collapse of overlying material into the void left by the test) was estimated to extend about 102 m above the detonation point (Korver and Rawson, 1968), which should be close to the top of the Pictured Cliffs Formation. Gas-production testing took place through the re-entry well, GB-ER, drilled through the plugged well GB-E. Important Observations during Gasbuggy Activities There are a number of observations scattered throughout reports of Gasbuggy activities that arepertinent to the issue of contaminant transport at the Gasbuggy site. These observations are listed here for clarity and are discussed in the context of transport hypotheses in later sections. • Tritium was released to the atmosphere during gas-flaring operations and resulted in locally high tritium concentrations in soil water (Eberline Instrument Corp., 1979) and presumably any surface impoundments. • There is no record of any downhole disposal of fluids into well 10-36. • Apressure connection between GB-1 and well 10-36 in the Pictured Cliffs was noted during testing in GB-1 prior to the Gasbuggy event. • Water collected from the Ojo Alamo during reentry drilling of GB-ER contained tritium at twice background concentrations. • Concentrations of tritium in cavity gas and water could not account for all the tritium produced by the shot. • There was inflow of water from the Ojo Alamo to the cavity during production testing. Both Korver and Rawson (1968) and Power and Bowman (1970) refer to sealing and stemming difficulties in GB-E, both pre- and post-shot, and attribute the water inflow to those poor seals. No clear details are given on the nature of the problems, though the use of thick sections of sand plugs, rather than continuous cement stemming, was noted as a contributing factor. • A pressure connection between the cavity and the Ojo Alamo at Well 10-36 was recorded during production testing of GB-ER. CONTAMINANT TRANSPORT SCENARIOS There are three basic scenarios that could explain the occurrence of tritium in well 10-36 (Figure 5): 1) introduction of tritium into the well from the land surface, 2) migration of tritium through the Ojo Alamo, and 3) migration through the Pictured Cliffs. All three scenarios are A Surface Origin for the Tritium in 10-36 The ultimate source of tritium in this scenario is the Gasbuggy test, but the tritium is first brought to the land surface during drillback and testing of the re-entry well. As stated above, no

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Figure 5. The three basic scenarios for the transport of tritium into the monitoring well, EPNG 10-36. In case 1, tritium is injected or otherwise enters the well from the surface. In the second scenario, transport occurs through the Ojo Alamo Formation after migration up the GB-ER borehole or through a fracture connecting the cavity and the Ojo Alamo. In the third scenario, transport occurs through fractures in the Pictured Cliffs Formation, then up the 10-36 borehole. records have been found of intentional downhole disposal of fluids into well 10-36, so the mechanism by which the tritium could enter the top of the well can only be speculated. There are a number of possibilities, including unauthorized and/or unrecorded disposal down the well bore, and leakage into the well of water accumulated in the below-grade cellar. Though tritium was not detected until 1984, it should be noted that contamination …