VZJ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 16 August 2005
Published in Vadose Zone J 4:774-788 (2005)
DOI: 10.2136/vzj2004.0106
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (2)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by McLin, S. G.
Right arrow Articles by Broxton, D. E.
Right arrow Search for Related Content
PubMed
Right arrow Articles by McLin, S. G.
Right arrow Articles by Broxton, D. E.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by McLin, S. G.
Right arrow Articles by Broxton, D. E.
Related Collections
Right arrow Radionuclides
Right arrow Downhole/Borehole Methods
Right arrow Vadose Zone Processes and Chemical Transport

SPECIAL SECTION: LOS ALAMOS NATIONAL LABORATORY

Vadose Zone Characterization and Monitoring Beneath Waste Disposal Pits Using Horizontal Boreholes

Stephen G. McLina,*, Brent D. Newmanb and David E. Broxtonc

a Water Quality and Hydrology Group, P.O. Box 1663, Mail Stop K497, Los Alamos National Laboratory, Los Alamos, NM 87544
b Atmospheric, Climate, and Environmental Dynamics Group, P.O. Box 1663, Mail Stop J495, Los Alamos National Laboratory, Los Alamos, NM 87544
c Hydrology, Geochemistry, and Geology Group, P.O. Box 1663, Mail Stop T003, Los Alamos National Laboratory, Los Alamos, NM 87544
* Corresponding author (sgm{at}lanl.gov)

Received 15 July 2004.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Background and Site Description
 Hydrogeologic Setting at MDA...
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Vadose zone characterization and monitoring immediately below landfills using horizontal boreholes is an emerging technology. However, this topic has received little attention in the peer-reviewed literature. The value of this approach is that activities are conducted below the waste, providing clear and rapid verification of containment. Here we report on two studies that examined the utility of horizontal boreholes for environmental characterization and monitoring under radioactive waste disposal pits. Both studies used core sample analyses to determine the presence of various radionuclides, organic compounds, or metals. At one borehole site, water content and pore-water chloride concentrations were also used to interpret vadose zone behavior. At another site, we examined the feasibility of using flexible membrane liners in uncased boreholes for periodic monitoring. For this demonstration, these retrievable liners were air-injected into boreholes on multiple occasions carrying different instrument packages for environmental surveillance. These included a neutron logging device to measure volumetric water at regular intervals, high-absorbency collectors that wicked available water from borehole walls, or vent tubes that were used to measure air permeability and collect air samples. The flexible and retrievable liner system was an effective way to monitor water content and measure in situ air permeability. The high-absorbency collectors were efficient at extracting liquid water for contaminant analyses even at volumetric water contents below 10%, and revealed tritium migration below one disposal pit. Both demonstration studies proved that effective characterization and periodic monitoring in horizontal boreholes is both feasible and adaptable to many waste disposal problems and locations.

Abbreviations: MDA G, Material Disposal Area G • TA-54, Technical Area 54


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Background and Site Description
 Hydrogeologic Setting at MDA...
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
ACCORDING TO CURRENT USDOE orders and directives (DOE O-435.1 and DOE M-435.1-1, as amended, available online at www.directives.doe.gov; USDOE, 2001a, 2001b), monitoring requirements at all radioactive waste treatment and storage facilities must include a capability for rapid identification of failed confinement, leak detection, or abnormal conditions. Hence, the requirement for periodic vadose zone monitoring immediately below these facilities should be a part of routine environmental surveillance activities. But how can surveillance monitoring be accomplished directly below existing waste sites before costly or complex remediation efforts are required (e.g., before groundwater is contaminated)? Semiarid sites such as those at Los Alamos National Laboratory (the Laboratory) face additional challenges because typical vadose zone water contents are so low that routine sampling below waste sites is extremely difficult. Early studies at hazardous and low-level waste disposal facilities emphasized physical and chemical characterization processes that were active in the vadose zone (e.g., Mercer et al., 1983). Although field-scale demonstration studies of waste disposal pits and covers were often successful (e.g., Abeele and Nyhan, 1987; Nyhan and Duffy, 1999), they produced mixed results because they were both time consuming and not easily adapted to repetitive monitoring activities. More recent efforts have focused on a variety of approaches that range from special, nonrepetitive monitoring studies (e.g., Kersting et al., 1999; Zachara et al., 2004), theoretical numerical simulations of radionuclide transport (e.g., Moridis et al., 2002; Hull et al., 2004), evaluation of historical groundwater recharge at geological time scales (e.g., Dublyansky et al., 2001), and combined approaches using performance assessment criteria or pathway analysis (e.g., Birdsell et al., 2000). However, a disadvantage of these recent studies is that periodic surveillance monitoring is often not directly used to verify theoretical efforts.

To overcome some of the difficulties mentioned above, this paper describes the effectiveness of using horizontal boreholes to characterize the hydrologic and contaminant conditions underneath existing waste disposal areas.

In the peer-reviewed literature there are relatively few discussions about the utility of horizontal boreholes. Most of these focus on geotechnical investigations (especially fracturing and geophysical imaging of structure) and enhanced oil and gas recovery (e.g., Belfield, 1994; Gilman et al., 1995; Warren and Smith, 1985). There are even fewer studies that discuss aspects of waste-site characterization and/or environmental monitoring using horizontal borehole technology. Weisbrod et al. (2002) used a conceptually similar approach to our study to characterize particle detachment and transport from fractures in unsaturated chalk. Their field experiments used a compartmental sampler inside a horizontal borehole that was subjected to intermittent flows. A second field test and computer simulation model also used a conceptually similar approach to our study (Tseng et al., 2003). Here horizontal boreholes were used to conduct injection tests and monitor conservative contaminant flow and transport in fractured tuff at the Nevada Test Site. Legotin et al. (1995) described the development and use of a neutron-logging sonde for determination of rock type and porosity in horizontal boreholes, and Epov et al. (1998) described a geophysical induction logging method in horizontal boreholes for anisotropy characterization. Parkin et al. (2000) tested a ground penetrating radar approach to assess water contents below a wastewater trench and found the approach was comparable to results using time domain reflectometry. Finally, Aikman et al. (2002) suggested that horizontal boreholes can be used to optimize recovery of hydrocarbon contamination from underground storage tanks.

While the abovementioned studies support the usefulness of horizontal boreholes for environmental work at waste sites and elsewhere, they do not discuss how horizontal boreholes can be integrated into a combined subsurface characterization strategy to constrain hydrogeology, quantify contaminant nature and extent directly beneath existing waste sites, or verify numerical modeling assumptions. These studies also do not discuss the applicability of periodic monitoring for regulatory compliance using horizontal boreholes. Our objective is to describe the value of horizontal boreholes for waste site characterization and periodic monitoring by way of examples. Specifically, we describe horizontal borehole characterization results from low-level, radioactive, solid-waste disposal pits at Los Alamos. In addition, we describe a demonstration effort to evaluate the effectiveness of using a combination of flexible liner technology and horizontal boreholes for periodic monitoring. Horizontal borehole characterization using core samples was conducted at two locations and the flexible liner demonstration was conducted at one of those sites.


   Background and Site Description
TOP
 ABSTRACT
 INTRODUCTION
 Background and Site Description
Hydrogeologic Setting at MDA...
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Material Disposal Area G (MDA G) is located within Technical Area 54 (TA-54) in the east-central part of the Laboratory (Fig. 1) on Mesita del Buey, a narrow, southeast-sloping mesa that terminates about 1 km west of the town of White Rock, and 5.3 km west-northwest of the Rio Grande. At MDA G, shafts and pits were originally excavated on the mesa top to dispose of contaminated solid wastes, tritiated water (HTO), and contaminated sludges generated from wastewater treatment processes. These sludges also contained transuranic wastes (mainly Pu, Am, U, and fission and activation products). MDA G is the site of two horizontal borehole demonstration studies described in this paper (Fig. 2) . The first site is below Pit 3, where five horizontal boreholes are located (Fig. 3) . The second site is below Pits 36, 37, and 38, where three horizontal boreholes are located (Fig. 4) . Pit 3 was excavated in 1963, and the waste-filled pit was capped with crushed tuff in 1966. Pit 36 was closed and capped with crushed tuff in 1996, while Pit 37 was capped in 2000. Pit 38 is still receiving solid wastes.



View larger version (43K):
[in this window]
[in a new window]
  		Fig. 1. Location map showing Material Disposal Area G (MDA G) near Los Alamos, NM.

 


View larger version (99K):
[in this window]
[in a new window]
  		Fig. 2. Mesita del Buey showing (a) the solid radioactive disposal pits at Material Disposal Area G (MDA G) and the locations of Drill Pads 1 and 2 (or Sites 1 and 2). (b) Pad 1 has five horizontal boreholes below Pit 3. (c) Pad 2 has three horizontal boreholes below Pits 36, 37, and 38.

 


View larger version (31K):
[in this window]
[in a new window]
  		Fig. 3. Cross sections below Pit 3 showing the gentle parabolic trajectory of Boreholes (a) MH-1, (b) MH-2, (c) MH-3, (d) MH-4, and (e) MH-5 from Site 1 that were sampled in this study. Each projection is along an individual bearing shown in Fig. 2b. Seamist labels mark the maximum deployment of dedicated flexible liner systems in each borehole (MH-1 was not sampled). Stations 1 and 2 in Borehole MH-3 depict locations of gas sampling and testing portholes during the first Seamist deployment.

 


View larger version (22K):
[in this window]
[in a new window]
  		Fig. 4. Cross section below Pits 36, 37, and 38 showing the approximate trajectory of core Holes H-1 through H-3 from Site 2 that were sampled in this study.

 
At the present time, 39 pits and approximately 200 shafts fill the 25.5-ha MDA G site to near capacity. Numerous investigations of this waste disposal area have revealed that some surface and subsurface contaminant migration has occurred (e.g., Purtymun, 1973; Purtymun et al., 1978a, 1978b, 1980; Abeele and Nyhan, 1987; Environmental Surveillance Report, 2004; Nyhan et al., 2004). This contamination is associated with surficial soil disturbances resulting from construction and burial activities, and with subsurface 3H diffusion and Pu migration from buried solid wastes. The 3H source is primarily from contaminated cement–HTO mixtures (i.e., concrete) that were poured into numerous shafts and pits while still wet. However, direct liquid injection of HTO into some shafts may also have occurred. One subsurface 3H plume migrated at least 32 m along an unsaturated ash-flow tuff contact (Qbt 2/Qbt 1v contact discussed below) in a 4-yr period (i.e., 1966–1970) according to the 3.7 kBq L–1 isotritium concentration contour line that Purtymun (1973) mapped. Migration processes are further complicated by atmospheric pumping associated with barometric pressure fluctuations that result in additional dispersive mixing in the vadose zone (Auer et al., 1996; Newman, 1996; Neeper, 2001, 2002, 2003). The source of Pu is from concrete mixtures that migrated short distances in open fractures adjacent to disposal shafts because these mixtures were also placed in shafts while still wet (Purtymun et al., 1978b).

Generalized summary assessments of the impact of historical 3H (Rogers, 1998) and 90Sr (Rogers, 2001) disposal operations across the Laboratory facility were also completed. Recent monitoring activities at MDA G were documented in numerous reports (e.g., Loaiza and Vold, 1995; Vold, 1997) and primarily consist of periodic neutron logs of volumetric water profiles collected over time in cased vertical boreholes.

These historical characterization and monitoring efforts are useful; however, most do not provide any information directly below the solid waste disposal shafts or pits where contaminants are most likely to move. Field experiments and numerical simulation studies (Purtymun et al., 1989; Robinson et al., 2005) have demonstrated that fluid movement is primarily vertically downward in the vadose zone. Hence, it is easy to miss potential contaminant migration using only vertical monitoring wells next to individual pits or shafts. Therefore, we suggest that a combination of both horizontal and vertical boreholes immediately adjacent to individual pits or shafts represent the best available technology for environmental characterization and rapid, periodic surveillance monitoring.


   Hydrogeologic Setting at MDA G
TOP
 ABSTRACT
 INTRODUCTION
 Background and Site Description
 Hydrogeologic Setting at MDA...
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
An interpretive cross section down the axis of Mesita del Buey shows the geologic setting of MDA G (Fig. 5) . Broxton and Vaniman (2005) provided an overview of Pajarito Plateau geology, while Purtymun and Kennedy (1971), Purtymun et al. (1978a), Rogers and Gallaher (1995), Ball et al. (2002), Kleinfelder, Inc. (2003), and Springer (2005) described site-specific geologic and hydrologic conditions at MDA G.



View larger version (86K):
[in this window]
[in a new window]
  		Fig. 5. Geologic cross-section A–A' on Mesita del Buey that passes through Material Disposal Area G (MDA G). Inset shows the location of section A–A' between Wells R-21 and R-22.

 
The waste shafts, pits, and boreholes discussed in this paper occur in subunits of the Tshirege Member of the Bandelier Tuff. These waste shafts and pits are mostly excavated in subunit Qbt 2, the resistant tuffs that form the caprock of the mesa (Fig. 6) . Qbt 2 is a sequence of partly to moderately welded, ash-flow tuffs containing abundant phenocrysts of sanidine and quartz in a matrix of crystalline ash, pumice, and minor lithic fragments. Qbt 2 is about 13 m thick at Pit 3. Vertical cooling joints are abundant in these welded tuffs (Fig. 6) and may provide fast pathways for the movement of both water and air. Purtymun et al. (1978a) characterized the joints in the walls of Pit 3 before the disposal of waste and in the cores collected from horizontal boreholes beneath the pit. They found that 72% of 199 joints in the cores were filled or plated with clay, 9% were filled or plated with pedogenic calcite (or caliche) and minor clay, and 19% were open with slight alteration. The joint spacing averaged 0.9 to 3.3 m in the core samples and 2.2 m in Pit 3.



View larger version (133K):
[in this window]
[in a new window]
  		Fig. 6. (a) Exposures of Bandelier Tuff on the south face of Mesita del Buey at MDA G. Qbt 2, which forms the resistant caprock of the mesa, is made up of moderately welded ash-flow tuffs with numerous high-angle joints. Qbt 1vu is the lighter colored, sloping unit in the lower part of the outcrop. Qbt 1vu is made up of non- to partly welded ash-tuffs with fewer fractures. (b) The sandy surge deposits that occur at the base of Qbt 2. The surge bed is a cross-bedded sandy deposit that is overlain by a nonwelded pumice-rich ash-flow tuff. The 13.5-cm-long pen is for scale.

 
Several pyroclastic surge deposits occur at the contact between Qbt 2 and the underlying Qbt 1v (Reneau and Vaniman, 1998). These pyroclastic surge deposits are well-bedded, ash-poor, sandy deposits that are typically 0.5 to 6 cm thick and form wedge-like deposits (Fig. 6). Purtymun (1973) identified these deposits as preferential pathways for the migration of tritium from solid-waste disposal shafts at MDA G.

Qbt 1v, which crops out on the sides of Mesita del Buey (Fig. 6), is generally similar to Qbt 2, but it is less welded and has higher porosity. Subtle welding variations result in exposures that are characterized by alternating zones of rounded cliffs and sloping benches. The thickness of Qbt 1v decreases eastward, ranging from 27 to 30 m in the vicinity of MDA L (see Fig. 5 map insert) to 5 to 13 m in the vicinity of MDA G.

The upper part of Qbt 1v, designated Qbt 1vu, consists of up to 11 m of ash-flow tuffs made up of devitrified pumice supported by a matrix of pinkish-white to light-gray ash. Qbt 1vu has significantly fewer joints than Qbt 2 (Fig. 6), and most of the cooling joints in Qbt 2 terminate at the contact between the two units.

The lower part of Qbt 1v, designated Qbt 1vc, is a 2-m-thick colonnade tuff that is orange brown, resistant to weathering, and has distinctive columnar cooling joints. The colonnade tuff consists of devitrified chocolate-brown to dark purple-gray pumice lapilli (i.e., 4–32 mm diam.) supported by a pink-white to light-gray ashy matrix. The near vertical cooling joints that characterize this unit generally die out at the contact with Qbt 1g below, although some persist across this lithologic boundary before dying out with depth. The joints in Qbt 1vc are commonly free of fracture-lining minerals and may be vertical and lateral pathways for water and air movement.

Qbt 1g is a massive nonwelded vitric ash-flow tuff consisting of gray to tan vitreous pumice supported by a matrix of coarse ash, shards, and pumice fragments. Qbt 1g contains abundant volcanic glass whereas the original volcanic glass in overlying subunits was transformed to microcrystalline assemblages of sanidine, cristobalite, and tridymite by devitrification and vapor phase alteration. Despite their differences in matrix mineralogy, Qbt 1g and Qbt 1v have similar hydrologic properties (Springer, 2005).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 Background and Site Description
 Hydrogeologic Setting at MDA...
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Site 1
At Site 1 (see Fig. 2 and 3), five horizontal boreholes were cored under Pit 3 at MDA G in 1976 (Purtymun et al., 1978a). This coring operation used air as the circulation fluid to avoid sample contamination by drilling mud or air–water–foam mixtures. These holes were oriented to pass 1 to 5 m below Pit 3 following a gentle parabolic trajectory (Fig. 3). The horizontal cores were collected from boreholes about 5 to 20 m below the mesa surface. Individual core sample runs varied between 1.5 and 6.1 m in length. Analytical samples were collected from the core and cuttings at 1.5-m intervals between the hole collar and 5.5 m from the pit. Thereafter, analytical samples were collected at 0.3-m intervals. Samples were analyzed for 90Sr, 137Cs, total U, 238Pu, 239,240Pu, 241Am, gross {alpha}, and gross ß radioactivity. Except for total U, these anthropogenic analytes are known to occur in Pit 3 at activity levels that exceed local background values. Unfortunately, the original study did not incorporate 3H analysis because no liquid 3H was put directly into Pit 3, and the possibility of vapor-phase transport was not recognized. Hence, we cannot compare any potential changes for this analyte since that time.

In April 1992, uncased horizontal Borehole MH-3 was evaluated to see if it was still open and suitable for resampling. A flexible membrane Seamist monitoring system was used for this test. This retrievable device is described in detail elsewhere (Mallon et al., 1992; Keller and Travis, 1993; and Newell et al., 2004). A diagram of the Seamist system (now called FLUTe; Flexible Liner Underground Technologies, LLC, Santa Fe, NM) is shown in Fig. 7 . Current versions of this device are also described elsewhere (see www.flut.com). Seamist is a commercial borehole instrumentation delivery system that consists of a flexible membrane liner tube that can carry a variety of sampling equipment into an open borehole. It also contains a tether and reel for retrieving and storing the liner once sampling has been completed. The liner is inserted into the borehole using an air pressurization system that inflates and tows the ever-expanding liner off the reel and down the open borehole. The waterproof liner is held tightly against the sides of the borehole using internal air pressure, water, sand, or grout to expand the liner and seal the annulus, preventing borehole collapse. The system used here consisted of an 11.4-cm diameter, lightweight (153 g m–2), urethane-coated polyester tube material that was about 70 m long. This membrane tube was constructed with two in situ sampling stations located approximately 32 m (i.e., Station 1) and 66 m (i.e., Station 2) from the borehole surface casing. These sample locations are shown in Fig. 3c for Borehole MH-3 when the Seamist system was fully deployed. Each sampling station actually contained a gas sampling porthole, a permeability test porthole, and an absorbent collector holder that were clustered closely together. Once the membrane tube was fully deployed in the borehole, it located one pair of portholes and its absorbent collector directly under the disposal pit and a second identical pair of portholes and its collector about 20 m from the pit's eastern boundary. For each sampling station, a 1.59-mm-i.d. Teflon tube was connected between the gas sample porthole and the surface, and a second 3.18-mm-i.d. PVC tube was connected between the permeability test porthole and the surface. Gas samples were sequentially drawn from each of the two gas sampling portholes by pulling a metered volume of formation gas through the Teflon tubing and initially venting it to the atmosphere. After a few minutes of continuous purging at about 115 Pa (i.e., gauge pressure relative to atmospheric pressure) at a flow rate of 2.81 L m–1 for Station 1, and about 42 Pa (gauge pressure) at a flow rate of 2.18 L m–1 for Station 2, 1.2 L of formation gases from each porthole were passed through separate charcoal adsorption collector Orbo tubes and sealed. Total sampling time for each station was <5 min. Formation gases were later analyzed for volatile organic compounds by the Laboratory. Air permeability tests were also performed at each permeability test porthole using the high-resolution test procedure described by Lowery and Narbutovskih (1991). Upon test completion, the inflated membrane liner was maintained in the borehole for about 5 h to allow the absorbent collectors to wick formation water from the borehole wall. These collectors were then retrieved and weighed to see how much water was collected. Finally, the original membrane liner was then reinserted into Borehole MH-3 for one remaining experiment. For this test, formation gases were again extracted from the gas sampling porthole located under Pit 3 for the next 48 h, and water vapor was collected in an absorbent silica-gel tube at the surface.



View larger version (36K):
[in this window]
[in a new window]
  		Fig. 7. Diagram showing the operation and deployment of the Seamist flexible membrane liner in horizontal Boreholes MH-2 through MH-5 at Site 1.

 
In August 1992, four of the five boreholes shown in Fig. 3 were evaluated again (the steel cap for Borehole MH-1 was rusted closed so re-entry was unsuccessful). This time the Seamist system was modified so that a Campbell Pacific Nuclear Corporation (Martinez, CA) 503-DR neutron logging tool was towed into each borehole using dedicated liner systems. Volumetric water content readings were recorded every 0.6 m before each membrane liner was removed. This procedure was repeated three times during the month at about weekly intervals. When the first liner system was removed on the last sampling date, a second Seamist membrane system was deployed in Borehole MH-3 with absorbent collectors located every 3.0 m. These collectors were left in place for about 4 h at a liner inflation pressure of 5.2 kPa (gauge pressure) to wick formation water from the unsaturated tuff for 3H analyses. The collectors were then retrieved from the borehole for analyses.

Site 2
At Site 2 (Fig. 2 and 4), three horizontal boreholes were installed underneath Pits 36 and 37 in 1995 by drilling through the sidewall of a depression excavated in the bottom of Pit 38 (Puglisi and Vold, 1995). A fan-like drilling pattern was used so greater spatial coverage under the pits could be obtained without moving the drill rig. Pit 36 was an older filled pit, and Pit 37 was an active pit that had been receiving waste for 5.5 yr before drilling. Pit 38 was a newly opened pit and that had not received any waste. Similar to Site 1, air rotary core drilling was used and analytical samples were collected from the core at 1.5-m intervals. These samples were stored in capped stainless-steel sleeves for organic analyses and Lexan for inorganic analyses. Analyses for volatile and semivolatile organic compounds and metals were conducted later using standard EPA methods. Sample analyses also included 3H, 90Sr, 137Cs, total U, 238Pu, 239,240Pu, 241Am, 210Pb, 226Ra, and 234Th radioactivity. Water contents were determined gravimetrically following Gardner (1986). In addition, a horizontal pore-water chloride profile was determined for Borehole H-2 following the techniques described by Scanlon (1991), Phillips (1994), and Newman et al. (1996). Previous work at MDA G (Newman, 1996) used the chloride mass balance method to examine variations in pore-water chloride concentrations with depth in vertical boreholes and to estimate downward fluxes in the upper vadose zone. Even though the horizontal, pore-water chloride profile from Borehole H-2 could not be used to directly estimate vertical flux (because it is not a vertical profile), chloride data were collected to evaluate whether transient ponding during the months to years that the pits were typically open had any major effect on the hydrologic conditions in the vicinity of the pits.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 Background and Site Description
 Hydrogeologic Setting at MDA...
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
At Site 1, Purtymun et al. (1980) analyzed cores for 90Sr, 137Cs, total U, 238Pu, 239,240Pu, 241Am, gross {alpha}, and gross ß radioactivity. They concluded that no contamination had migrated from Pit 3 in the 13 yr since disposal operations had begun (i.e., 1963–1976) because none of the analyses exceeded established background values for any of the anthropogenic radionuclides. This conclusion is still valid today even if slightly lower modern background values are used (McLin, 2004). However, we cannot preclude the remote possibility that subsequent contaminant migration may have occurred since 1976 because the boreholes have not been resampled. Furthermore, since this earlier study did not monitor for 3H, we cannot compare our results to theirs.

In the initial Seamist demonstration study at Site 1, the gas samples that were extracted from Borehole MH-3 showed no volatile organic compounds were present below Pit 3. The inflated membrane was also left in the borehole for 5 h to allow the 7.62-cm2, polypropylene absorbent collectors to fill with liquid formation water. The first absorbent collector had 0.30 g of water, while the second had 0.29 g. This demonstration proved that the borehole was unobstructed and that sufficient liquid water and vapor samples could be collected for 3H analyses. Results from the high-resolution gas permeability tests conducted in each of the two permeability portholes yielded values of 15 and 31 pm2 (15 and 31 darcies), respectively, for Stations 1 and 2. Data analyses relied on a one-dimensional, homogeneous, isotropic, compressible gas flow model described by Lowery and Narbutovskih (1991) for interpretation. These air-permeability values are considered high for undisturbed Bandelier Tuff (Rogers and Gallaher, 1995; Springer, 2005); however, the two measurement stations were intentionally located in the region of previously identified borehole fractures.

The second Seamist demonstration effort at Site 1 measured volumetric water contents in four of five boreholes under Pit 3 using a neutron moisture probe. Volumetric water content profiles along each of the four measured boreholes at Site 1 are shown in Fig. 8 . Note that distances along each borehole are plotted from right to left and correspond to the east–west borehole orientations shown in Fig. 3. The maximum Seamist liner penetration into each borehole is also shown in Fig. 3. These penetration depths were only limited by Seamist tube lengths and probably would have been longer had the tube material been longer. Logging measurements were repeated on five separate occasions in August 1992 during the annual monsoon season (Bowen, 1996). Daily precipitation totals for the month of August and the individual sampling dates are shown in Fig. 9 for comparison. The volumetric water distributions for individual boreholes in the Bandelier Tuff were stable as seen in Fig. 8 because there is almost no variation in water content readings between sampling dates. The volumetric water content is about 3 to 5% near the borehole collar but quickly tapers off to 1 to 3% 10 to 18 m into the borehole (Fig. 8). These volumetric water contents remain relatively low until the eastern margin of Pit 3 is reached and values of 4% or larger are more typical. Once under the pit, volumetric water contents vary between about 2 and 8%. The slightly elevated water content near the borehole entrance collar probably represents an artifact of neutron measurements near the 6-m-long steel surface casing that caps each borehole, or it is related to condensation associated with the surface casing. Increased volumetric water in the vicinity of Pit 3 represents elevated pore water emanating from the pit. These subtle variations in water contents along the borehole length are easily captured by the neutron-logging tool because it samples an idealized, spherically shaped, formation volume with an effective radius of about 10 to 80 cm at these volumetric water contents (Nyhan et al., 1994). This measuring radius varies inversely with water content. Hence, the numerous small peaks along each borehole log are probably related to locations of near-vertical, clay- or pedogenic calcite-filled cooling fractures in the tuff that have higher water contents than the adjacent tuff matrix.



View larger version (20K):
[in this window]
[in a new window]
  		Fig. 8. Volumetric water content vs. distance in horizontal Boreholes (a) MH-2, (b) MH-3, (c) MH-4, and (d) MH-5 recorded on five different dates in August 1992. Note that the distance scale runs from east to west. Land surface footprint of Pit 3 is shown in yellow.

 


View larger version (19K):
[in this window]
[in a new window]
  		Fig. 9. Daily precipitation totals measured at MDA G during August 1992 and compared with the five neutron logging dates at Site 1.

 
There is a strong correlation between the average volumetric water content recorded on three separate days along Borehole MH-3 and water uptake collected by the absorbent collectors on the last sampling date (Fig. 10a) . It is also apparent that water contents are higher below the pit than below the undisturbed mesa. But since these collectors were distributed every 3.0 m using a second Seamist membrane, they do not show the same level of detail as the neutron measurements (0.6-m intervals). Furthermore, Fig. 10b shows that vadose zone concentration levels and spatial variability for 3H are significantly higher immediately below the pit than below the undisturbed mesa. These differences are quite striking but not surprising. They clearly imply complex, three-dimensional, multiphase fluid flow along preferential pathways located below the pit. They also suggest the possibility of variable contaminant inputs (i.e., locations of individual concrete blocks in the pit). The USEPA's limit for 3H in drinking water is 740 Bq L–1 (40 CFR Part 141.16, USEPA, 2000; current regulations are available on-line at www.epa.gov/safewater and links to the National Primary Drinking Water Regulations). This limit is shown in Fig. 10b for comparison even though this standard does not apply to water in unsaturated rock. The corresponding USDOE's derived concentration guides for 3H in drinking water is 2960 Bq L–1 (Environmental Surveillance Report, 2004, p. 106–108 and p. 238; DOE Order 5400.5, 1993, Chapter III(2) and III(3), available online at www.directives.doe.gov; USDOE, 1993). This limit is also shown in Fig. 10b and also does not apply to water in unsaturated rock.



View larger version (32K):
[in this window]
[in a new window]
  		Fig. 10. Average volumetric water content (a) at 0.6 m intervals and moisture wicking uptake at 3.0-m intervals vs. distance in Borehole MH-3 in August 1992. Note that the distance scale runs from east to west. (b) Corresponding 3H analyses in MH-3 were taken from the wicked water samples. Error bars represent radioactivity counting uncertainty (±1 SD). The EPA Safe Drinking Water Act (SDWA) and DOE Derived Concentration Guideline (DCG) limits are shown for comparison. Land surface footprint of Pit 3 is shown in yellow.

 
At Site 2, analyses from the three boreholes showed either no detections, or below background concentrations for nearly all organic compounds, metals, and radioactive analytes (Puglisi and Vold, 1995). The lone exception was 3H where above background concentrations were observed in all three boreholes. Approximately 444 GBq of 3H was contained in concrete that was placed in Pit 37 before drilling.

Gravimetric water contents in H-2 samples (data not shown) ranged from approximately 1 to 10% (i.e., about 2–14% volumetric water content), which is typical to slightly elevated compared with undisturbed portions of the MDA G subsurface. Water contents increased with distance along the borehole, which corresponds to an increase in depth because the boreholes were not truly horizontal (Fig. 4). This increase also corresponds to a change in stratigraphy that is consistent with the borehole passing from Qbt 1vu into Qbt 1vc and Qbt 1g. Similar variations in water content are also observed in vertical holes on the mesa (Rogers and Gallaher, 1995; Newman, 1996).

The pore-water chloride profile for Borehole H-2 is shown in Fig. 11 . There was much variability along the profile, some of which is attributed to the stratigraphic changes mentioned earlier. However, many of the samples in the first (northern) part of the profile from about 0 to 15 m have large chloride concentrations exceeding 1000 mg L–1. These sample locations correspond to shallower depths in unit Qbt 1vu that are under Pit 38. These shallow data resemble other typical pore-water chloride profiles from nearby vertical boreholes as seen in Fig. 12 (i.e., at about 0–25 m), and that were not located below any waste pits (Newman, 1996). These shallow H-2 data suggest a low infiltration rate that is comparable to undisturbed tuff and are consistent with a newly opened Pit 38.



View larger version (22K):
[in this window]
[in a new window]
  		Fig. 11. Pore-water chloride concentration profile for Borehole H-2. Land surface footprints of Pits 36, 37, and 38 are shown in yellow.

 


View larger version (24K):
[in this window]
[in a new window]
  		Fig. 12. Pore-water chloride concentration profiles from four vertical boreholes at MDA G (modified from Newman, 1996).

 
There is also a pronounced decrease in chloride concentrations at about the 17- to 25-m length in Borehole H-2 and under Pit 37 (Fig. 11). It is possible that this zone has been affected by transient ponding in Pit 37 because the borehole was only about 3 m below the bottom of the pit at this location (Fig. 4). This zone also lies directly under a French drain system that was installed in the bottom of Pit 37 and runs adjacent to the north wall near Pit 38. Another region of relatively low chloride concentrations along the H-2 profile is between about 42- to 47-m length that is also below Pit 37. This zone corresponds to a deeper part of the vadose zone because the borehole was not strictly horizontal. In this zone, water contents increase up to about 10% and the borehole crosses two stratigraphic contacts (Tshirege units Qbt 1vu/1vc, and Qbt 1vc/1g). These lower stratigraphic units have been observed to have higher gravimetric water contents (often in the 10% range) and low chloride concentrations (Newman, 1996). Thus the low chloride concentrations in Borehole H-2 between about 42 to 47 m may not be related to shifts in hydrologic conditions resulting from the period when Pit 37 was open. In fact, if we compare these data with pore-water chloride profiles from nearby undisturbed vertical boreholes (Fig. 12), we see almost identical behavior at comparable depths below ground surface (i.e., at about 30 m in Fig. 12). Finally, the low pore-water chloride concentration profile below Pit 36 (Fig. 11) between about 50 to 73 m along Borehole H-2 resembles similar profiles from nearby undisturbed vertical boreholes (see Fig. 12 below about 35 m). In other words, the chloride profile below Pit 36 suggests a low infiltration rate exists at these depths.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 Background and Site Description
 Hydrogeologic Setting at MDA...
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Our initial demonstration study at Site 1 documents an attempt to reenter one of a series of five uncased, horizontal boreholes that were originally drilled in 1976 (Purtymun et al., 1978a, 1980). This experiment was undertaken because routine and periodic monitoring activities had not previously been possible unless new core holes were installed and samples collected for analyses. Routine vadose zone surveillance sampling had proven extremely difficult because undisturbed volumetric water contents in the shallow, unsaturated Bandelier Tuff typically vary between about 2 and 8%. At these levels in situ water extraction is virtually impossible using traditional methods. We can illustrate this point using a typical water retention curve (Fig. 13) from a core sample that was recovered from the Bandelier Tuff at about 19.8 m below ground surface near MDA G (Rogers and Gallaher, 1995, p. D-15). This water retention curve shows the difficulty of obtaining in situ water samples from cores when the volumetric water content is below about 10%. Here the matrix pressure corresponding to a volumetric water content of 4% exceeds 3000 cm of water. In other words, it takes a suction pressure of about 0.03 MPa (0.3 atmospheres) (gauge pressure) to physically extract water from the tuff. Core samples from the Bandelier Tuff often implode and disintegrate long before these pressures can be applied. Hence, water must be either wicked or distilled from these samples. Distillation techniques require new core to be collected each time new water profiles are needed. This approach is limited by high drilling costs associated with sample collection. The wicked sampling technique is much more cost effective for repetitive in situ surveillance monitoring because it uses the same borehole. The high-absorbency collectors emplaced by the Seamist system are also convenient, and allow for temporal sampling at optimal locations within an open borehole under difficult field conditions. As an alternative, the gas sampling porthole also provides the option of in situ formation gas extraction back to the surface where it can be either condensed in a cold-water bath or collected in a water absorption canister for subsequent analysis. Using the latter procedure, our first demonstration experiment showed that the Bandelier Tuff yields an average of approximately 0.00025 L h–1 (0.25 g h–1) liquid water when the volumetric water content is about 4%. Hence, we were able to collect more than 0.012 L (12 g) of liquid water in 48 h from the gas porthole located in Borehole MH-3 immediately below Pit 3 (i.e., Station 2 in Fig. 3). Thus, gas sample extraction yields larger individual water samples than the collector pad method. These larger volumes might be required for other anthropogentic radionuclide or contaminant analyses. A potential disadvantage of the gas sampling approach, however, is the lack of precise sampling control. Gas extraction for prolonged periods captures air from an ever-expanding formation volume even though the sampling porthole remains at a fixed location and the extraction flow rate is low. Hence, we tend to favor collector pads to gather in situ HTO samples when large sampling volumes are not required for other analyses. In addition, the collector-pad method provides a 3H profile along the entire borehole length in a single deployment. Unfortunately, our demonstration study did not compare 3H concentration levels using these alternative sampling methods. This is important because the collector pads sample liquid matrix water, whereas gas extraction captures humid matrix air. Finally, the first field demonstration showed that the Seamist system could be used to measure air permeability at selected locations along the horizontal borehole. It would be instructive to conduct both air permeability measurements and 3H sampling from the same locations along an open borehole. Do relatively high or low permeability zones correlate with high or low 3H concentration levels from the liquid or vapor phase? Are these phase concentrations in equilibrium and do contaminant levels fluctuate with time? These questions have never been addressed using direct field observations from the vadose zone.



View larger version (18K):
[in this window]
[in a new window]
  		Fig. 13. Water retention curve for a core sample collected from Technical Area 54 Well LLC-86-22 at 19.8 m below ground surface (modified from Rogers and Gallaher, 1995). This curve is typical of Bandelier Tuff samples collected on Pajarito Plateau mesas.

 
The second demonstration study at Site 1 measured volumetric water contents using a neutron logging device that was towed into each borehole and safely retrieved. Water content profiles along individual boreholes showed almost no changes between data collection runs conducted on five separate days. These demonstration tests verified the operational ease of the Seamist apparatus and the repeatability of the individual neutron moisture measurements. The neutron moisture data also showed that either direct rainfall infiltration effects did not rapidly penetrate down through the near-vertical surface fractures in the Bandelier Tuff to the horizontal boreholes between sampling dates, or that potential infiltration quickly pulsed downward and water content levels rapidly returned to previously measured levels faster than the sampling rate at about once per week. Purtymun et al. (1978a) reported that about 81% of the fractures in Boreholes MH-1 through MH-5 were filled with either clay or pedogenic calcite, and that the remaining 19% were open but were slightly altered. Hence, rapid liquid recharge through open fractures followed by an equally rapid return to previous water content levels seems unlikely.

The second demonstration study at Site 1 also collected sufficient water samples for 3H analyses along Borehole MH-3. As expected, the liquid volumes of water that were collected were strongly correlated to the average water content profiles obtained with the neutron logging tool even though the respective sampling intervals were different (Fig. 10a). Furthermore, the analyses showed elevated and erratic 3H peaks below Pit 3, and significantly less 3H below the undisturbed mesa adjacent to the pit (Fig. 10b). These data clearly identify an elevated HTO moisture envelope that emanates from Pit 3. They also suggest that 3H movement may be concentrated in several preferential pathways below the pit and that these pathways may be related to the locations of open fractures. However, further study is required to verify these simple correlations. A flexible liner system towing a borehole video camera could be used for this purpose.

The elevated 3H envelope below Pit 3 at MDA G resulted from the diffusion of tritiated water (HTO) from solid cement–HTO mixtures, followed by nonuniform dispersive mixing into undisturbed Bandelier Tuff that is driven by atmospheric pressure fluctuations (Auer et al., 1996; Newman, 1996; Neeper, 2001, 2002, 2003). In all probability, this 3H migration has been in the vapor phase because liquid water infiltration rates and volumetric water contents are simply too low to support significant liquid-phase transport. Furthermore, the HTO diffusion rate has apparently remained relatively constant over the years. However, since this rate is so much smaller than the decay rate of 3H, the elevated volumetric water contents under Pit 3 appear to contain relatively high 3H levels even though more than two half-lives have elapsed (i.e., the 26 yr from 1966 to 1992). Thus, the elevated HTO envelope has probably remained relatively constant in both volumetric water content and 3H concentration levels despite the relatively short half-life of 3H. Hence, there is probably only a very small mass of exponentially decaying 3H available for vapor-phase transport in the vadose zone. The Seamist system provides us with the capability of routine and periodic monitoring at extremely low water contents. It also provides us with an experimental means of measuring the effective diffusion–dispersion coefficient of 3H from concrete and then into unsaturated tuff under field conditions using a conceptually similar technique to that described by Szántó et al. (2002) for laboratory samples. Such an experiment would require that we periodically collect 3H concentration distributions along the borehole so we could construct concentration breakthrough curves with time. We would also need to know something about the number and sizes of concrete blocks in Pit 3. Alternatively, we might estimate the amount of 3H originally contained in the concrete blocks if we assumed a laboratory diffusion value of HTO from concrete.

As at Site 1, the analyses of core samples from the horizontal boreholes at Site 2 were useful in determining the kinds of contaminants and their concentrations that were moving (or not moving) beneath the waste pits. Another similarity with Site 1 was that 3H was the primary contaminant detected and the likely mode of transport was also in the vapor phase. Many of the pore water chloride concentrations from Borehole H-2 beneath Pits 37 and 38 in the 0- to 40-m zone are >1000 mg L–1 (Fig. 11). Such accumulations of chloride suggest low downward fluxes. Chloride profiles from nearby vertical boreholes that were not drilled below waste pits show similar pore-water chloride concentrations (Fig. 12), and chloride mass balance flux estimates for high concentration zones in the vertical boreholes are low, ranging from 0.03 to 0.10 mm yr–1 (Newman, 1996). The similar chloride concentrations between horizontal Borehole H-2, and these vertical profiles suggest that the limited time that the pits remain open for disposal causes minimal impact on the hydrological conditions beneath the pits. This conclusion is also supported by the <5% gravimetric water contents observed below Pits 37 and 38.

In summary, the approach of horizontal boreholes and flexible membrane liners below waste sites can be extremely effective for both characterization and monitoring. Advantages include the collection of hydrogeological and contaminant information directly below wastes. These observations greatly aid in early detection of problems and minimize the possibility of not observing vertical contaminant transport using vertical holes drilled adjacent to buried waste. But there are also limitations. Horizontal boreholes may not be suitable for all sites because boreholes may not remain open without special casing support, or this technique may be prohibitively expensive if not installed before the onset of waste disposal operations. Nevertheless, using systems such as Seamist, horizontal boreholes can be incorporated into periodic sampling and monitoring programs at many sites. Despite these advantages, the specific mechanisms of water and 3H transport below MDA-G remain unclear. For example, do water and 3H predominately move in the liquid or vapor phases, or as complex multiphase fluid flow? These questions are important because recent detections of low 3H levels (<5 Bq L–1) at Well R-22 (Longmire, 2002) suggest that a relatively rapid pathway to the regional aquifer may be present in the vadose zone below MDA-G.


   ACKNOWLEDGMENTS
 
Portions of this work were supported by the Water Quality and Hydrology Group, the MDA G Performance Assessment Project, and the Groundwater Protection Program at Los Alamos National Laboratory (LANL). LANL is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36. Mention of trademark propriety names are for the benefit of readers and do not constitute an endorsement by the Department of Energy or the University of California to the exclusion of alternative products that may also be suitable. Special thanks are extended to William D. Purtymun, who developed many of the vadose zone characterization techniques used at the Laboratory today, and to William Lowery and Carl Keller for providing the Seamist demonstration systems. This manuscript also benefited greatly from comments and suggestions provided by Bruce Robinson, Guest Editor, and by two anonymous reviewers. The LANL document number for this paper is LA-UR-04-4475.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Background and Site Description
 Hydrogeologic Setting at MDA...
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




This article has been cited by other articles:


Home page
 Vadose Zone JHome page
O. Dahan, R. Talby, Y. Yechieli, E. Adar, N. Lazarovitch, and Y. Enzel
In Situ Monitoring of Water Percolation and Solute Transport Using a Vadose Zone Monitoring System
Vadose Zone J., November 17, 2009; 8(4): 916 - 925.
[Abstract] [Full Text] [PDF]

Home page
 Vadose Zone JHome page
J. A. Vrugt, P. H. Stauffer, Th. Wohling, B. A. Robinson, and V. V. Vesselinov
Inverse Modeling of Subsurface Flow and Transport Properties: A Review with New Developments
Vadose Zone J., May 27, 2008; 7(2): 843 - 864.
[Abstract] [Full Text] [PDF]

Home page
 Vadose Zone JHome page
B. D. Newman and B. A. Robinson
The Hydrogeology of Los Alamos National Laboratory: Site History and Overview of Vadose Zone and Groundwater Issues
Vadose Zone J., August 16, 2005; 4(3): 614 - 619.
[Abstract] [Full Text] [PDF]

Home page
 Vadose Zone JHome page
B. A. Robinson, G. Cole, J. W. Carey, M. Witkowski, C. W. Gable, Z. Lu, and R. Gray
A Vadose Zone Flow and Transport Model for Los Alamos Canyon, Los Alamos, New Mexico
Vadose Zone J., August 16, 2005; 4(3): 729 - 743.
[Abstract] [Full Text] [PDF]

Home page
 Vadose Zone JHome page
P. H. Stauffer, K. H. Birdsell, M. S. Witkowski, and J. K. Hopkins
Vadose Zone Transport of 1,1,1-Trichloroethane: Conceptual Model Validation through Numerical Simulation
Vadose Zone J., August 16, 2005; 4(3): 760 - 773.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (2)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by McLin, S. G.
Right arrow Articles by Broxton, D. E.
Right arrow Search for Related Content
PubMed
Right arrow Articles by McLin, S. G.
Right arrow Articles by Broxton, D. E.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by McLin, S. G.
Right arrow Articles by Broxton, D. E.
Related Collections
Right arrow Radionuclides
Right arrow Downhole/Borehole Methods
Right arrow Vadose Zone Processes and Chemical Transport

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome