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

^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

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

View larger version (99K): [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 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 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.

View larger version (86K): [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.

View larger version (133K): [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.

View larger version (36K): [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.

View larger version (20K): [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 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.

View larger version (32K): [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.

View larger version (22K): [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 a new window]   Fig. 12. Pore-water chloride concentration profiles from four vertical boreholes at MDA G (modified from Newman, 1996).

View larger version (18K): [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.