Unsaturated Zone Waters from the Nopal I Natural Analog, Chihuahua, Mexico - Implications for Radionuclide Mobility at Y
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waste at Yucca Mountain (YM), Nevada'-'. Key features that are analogous to those at YM include the Basin and Range tectonic setting; the fractured, silicic, volcanic tuff host rocks; the location in the hydrologic unsaturated zone (UZ); oxidizing, bicarbonate-rich geochemical conditions; desert climate; and the occurrence of uraninite (UO 2+j) as an analog to spent nuclear fuel. Direct observation of aqueous transport of U and other natural radionuclides is challenging clue to the arid and unsaturated
distance in meters
Figure 1 - Map of the Level +00 and Level +10 exposures at Nopal I, showing water-sampling locations. Water was collected at Borehole 12 (BH-12) and in the Level +00 adit at projected locations marked by X's. Shading indicates the zone of visible uranium mineralization, or uranium deposit. Contour interval is 2 m, referenced to zero at Level +00; axes (in meters) correspond to a NS-EW field grid. Site is located approximately at 29"07'N, 106'02'W.
zone setting. However, past excavation at the site has provided opportunities for sampling UZ waters. Perched and seep water samples yield information on both radionuclide content and chemical characteristics that allow modeling of interactions with waste analogs. This report presents chemical and isotopic data on these waters, describes geochemical 809
Mat. Res. Soc. Symp. Proc. Vol. 556 1 1999 Materials Research Society
modeling of uraninite dissolution in these groundwaters, and interprets isotopic data in the context of previously reported data4' 6. SAMPLES AND METHODS UZ waters were collected as both perched and seep samples. In August 1995, water was bailed from a 10.7 m-deep uncased borehole on Level +10 (BH-12 on Figure 1). The hole, which was drilled but not used for blast excavation, contained water for several years and was capped three months prior to sampling. In the field, measurements were made of (i) temperature, pH, and oxidation potential using an Accumet 1002 pHImV meter with a combination pH electrodetemperature probe and a Pt redox combination electrode, (ii) specific conductance with a YSI Model 33 Meter with a model LN3422 probe, and (iii) alkalinity, dissolved 02, and hardness using a Hach Ecology Combination Test Kit (Model AL-36DT). Aliquots of selected samples
were filtered (0.2 pm PVDF syringe filters) and acidified the day of sampling. Saturated-zone samples were collected and similarly measured and treated from a well in the regional carbonate aquifer located 1.3 km southeast of Nopal I (water depth 94 in). A seep-water collection system was constructed in an adit on Level +00, where previous evidence of seepage (dampness and dripping) had been observed. Plastic sheets approximately 2 m square were suspended from the adit ceiling and walls, leading to funnel filters feeding plastic collection bottles. Water entering the adit traversed at least 8 m of rock from the surface. During September 1995, three rain storms resulted in rapid infiltration into the adit, causing the collection bottles to overflow. The bottles were
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