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Vadose Zone Clays and Water Content beneath Wet and Dry Canyons of the Pajarito Plateau, New Mexico

^a MS D462, Los Alamos National Lab., Los Alamos, NM 87545
^b MS T003, Los Alamos National Lab., Los Alamos, NM 87545
^c MS D469, Los Alamos National Lab., Los Alamos, NM 87545

View larger version (24K): [in a new window]   Fig. 1. The Pajarito Plateau, showing Los Alamos and Mortandad canyons, the boundaries of Los Alamos National Laboratory (LANL), and the boreholes discussed in this paper (labeled dots).

View larger version (37K): [in a new window]   Fig. 2. Raw and corrected conductivity logs from borehole R-8. The instrument was calibrated before the logging run. Steady decrease in signal with time is seen in comparison of data from downward and upward instrument paths (2a). The rate of decrease in signal is systematic and can be fit by a second-order function (d = depth in feet) with R2 of 0.84 (2b). Correction functions can be applied to the signal values at each depth in both downward and upward paths (2c) to generate corrected curves that account for instrument drift (Fig. 2d).

View larger version (59K): [in a new window]   Fig. 3. Gravimetric water abundance profiles and clay abundances in drill holes LAOI(A)-1.1 and R-8 within Los Alamos Canyon. Note that the lower horizontal scales, for weight % water and clay, differ for the two drill holes (R-8 is wetter), and clay data for R-8 cover a more limited depth range than the moisture data. The figure for LAOI(A)-1.1 also shows hydraulic transmissivity data (Ksat, cm/s; Stephens et al., 1995). Hachures in the figure for LAOI(A)-1.1 indicate near-vertical clay-filled fracture zones; other markings indicate a pumice swarm and a horizontal fracture zone as marked.

View larger version (35K): [in a new window]   Fig. 4. Gravimetric water abundance profiles for drill holes MCOBT-4.4 and MCOBT-8.5 in Mortandad Canyon. Clay mineralogy of particulates and salt mineralogy concentrated from leached samples is indicated in italics (mineralogic data from Table 4).

View larger version (28K): [in a new window]   Fig. 5. Comparison of water contents determined from core samples (Washington Group International, 2003) and conductivity data determined by borehole induction geophysics in drill hole R-8. Samples for gravimetric water measurement and conductivity data were collected in the same season (fall). Points on the water abundance curve indicate locations of core samples analyzed. Blue circles highlight water abundance data that deviate most from locally increasing or decreasing trends in conductivity data.

View larger version (32K): [in a new window]   Fig. 6. Plot of water content vs. conductivity for drill hole R-8, using corresponding induction data for each water abundance data point shown in Fig. 5. Blue circles highlight the same data points circled in Fig. 5. Gravimetric water abundance in the Puye dacitic pumice bed is greater than the water abundance in any of the pumice-poor Puye fanglomerates, although the pumice-poor fanglomerates include many outliers of high conductivity. The basal sample from the Guaje Pumice Bed has high conductivity, similar to the other Guaje samples, but lower water content.