Measurement of unsaturated hydraulic conductivity and chemical transport in Yucca Mountain Tuff: Milestone Report 3044-WBS1.2.3.4.1.4.1

Hydraulic conductivities, K, were experimentally determined as a function of volumetric water content, {theta}, in tuff from the Yucca Mountain site. In addition, the retardation factor, R{sub f}, in Yucca Mountain tuff with respect to selenium, as the selenite species, was measured under unsaturated conditions. These data were used to determine the feasibility of applying a new unsaturated flow technology (UFA) to further hydrologic studies at Yucca Mountain. The UFA directly measures K({theta}) rapidly in Yucca Mountain tuff and is shown to agree well with traditional methods. Hysteresis does not appear important during this testing. Hydraulic steady-state is achieved fastest during desaturation from a saturated state. Imbibition into dry tuff requires a long time for steady-state to occur because of slow filling of the diffusion porosity which can take a few weeks. The existing UFA is a prototype, and a new design of the next generation UFA is completed that eliminates some of the earlier problems. These preliminary investigations demonstrates that the UFA is a useful investigate technique that should be used to compliment existing techniques for hydrogeochemical characterization at Yucca Mountain and other arid sites

Validity of batch sorption data to describe selenium transport through unsaturated tuff

As part of project for characterizing Yucca Mt. as a potential site for high-level nuclear waste respository, we used UFA {trademark} technology (centifuge-induced flow) to directly measure selenite retardation coefficients and hydraulic conductivity under unsaturated conditions on two tuff samples from Yucca Mt. Retardation factor for the selenite species was 2.5 in both Yucca Mt. vitric tuff at 62.6% saturation and zeolitic nonwelded tuff from G-tunnel at 52.8% saturation. For these column experiments, we prepared a solution, using J-13 well water from NTS, with a Se conc. of 1.31 mg/L(ppM). The retardation factor of 2.5 measured for both tuffs translates into a sorption distribution coefficient K{sub d} of 0.9 mL/g for the vitric tuff and 0.8 mL/g for the zeolitic tuff. For batch sorption experiments, using the same zeolitic tuff as for the column experiments and solutions of J-13 well water with a Se conc. of 1.1 mg/L(ppM), the average K{sub d} was determined to be 0.1{+-}0.2 mL/g. Given the small K{sub d} values for Se sorption, general agreement between batch and column measurements (obtained under unsaturated conditions) was observed. Unsaturated hydraulic conductivities during the experiments were 2.5x10{sup -8} cm/s for the Yucca Mt. vitric tuff and 1.2x10{sup -8} cm/s for the zeolitic nonwelded tuff from G- tunnel

Selenite transport in unsaturated tuff from Yucca Mountain

Direct measurements of unsaturated selenite retardation coefficients and unsaturated hydraulic conductivity were obtained on two tuff samples from Yucca Mountain using the UFA{trademark} technology. The retardation factor for the selenite species was only 2.5 in both Yucca Mountain vitric member at 62.6% saturation and zeolitized nonwelded tuff from G-tunnel at 52.8% saturation with respect to J-13 well water from the Nevada Test Site contaminated with selenium at 1.31 mg/l (ppm). In batch tests on the same material using 1.2 mg/l (ppm), the average K{sub d} was determined to be 13, giving retardation factors higher than the UFA column breakthrough tests by an order of magnitude. The difference could result from preferential flow paths in the UFA column as might occur in the field or differences in residence times between the two types of test. The unsaturated hydraulic conductivities during the experiments were 2.49 {times} 10{sup {minus}8} cm/s for the Yucca Mountain vitric member and 1.16 {times} 10{sup {minus}8} cm/s for the zeolitized nonwelded tuff

Solid waste leach characteristics and contaminant-sediment interactions Volume 2: Contaminant transport under unsaturated moisture contents

The objectives of this report and subsequent volumes include describing progress on (1) development and optimization of experimental methods to quantify the release of contaminants from solid wastes and their subsequent interactions with unsaturated sediments and (2) the creation of empirical data that become input parameters to performance assessment (PA) analyses for future Hanford Site disposal units and baseline risk assessments for inactive and existing solid waste disposal units. For this report, efforts focused on developing methodologies to evaluate contaminant transport in Trench 8 (W-5 Burial Ground) sediments under unsaturated (vadose zone) conditions. To accomplish this task, a series of flow-through column tests were run using standard saturated column systems, Wierenga unsaturated column systems (both commercial and modified), and the Unsaturated Flow Apparatus (UFA). The reactants investigated were {sup 85}Sr, {sup 236}U, and {sup 238}U as reactive tracers, and tritium as a non-reactive tracer. Results indicate that for moderately unsaturated conditions (volumetric water contents >50 % of saturation), the Wierenga system performed reasonably well such that long water residence times (50-147 h) were achieved, and reasonably good steady-state flow conditions were maintained. The major drawbacks in using this system for reactive tracer work included (1) the inability to achieve reproducible and constant moisture content below 50% of saturation, (2) the four to six month time required to complete a single test, and (3) the propensity for mechanical failure resulting from laboratory power outages during the prolonged testing period