Modeling unsaturated flow and transport processes at the Busted Butte Field Test Site, Nevada.

A numerical model was used to simulate the flow and transport processes at the Busted Butte Field Test Site for the purpose of quantifying the effects of hydrogeologic conditions beneath the potential Yucca Mountain repository horizon. In situ experiments were conducted on a 10 x 10 x 7 m block comprising a layered Topopah Springs/Calico Hills formation with two imbedded faults. Tracer solution was continuously injected in eight parallel boreholes arranged on two horizontal planes. Twelve collection boreholes were emplaced perpendicular to the injection holes and were both horizontal and inclined. Solution samples were collected regularly using a sampling assembly consisting of an inverted membrane and sorbing-paper sampling pads. Comparisons between measurements and predictions show that, except for the occasional drops of concentrations observed in the field, the current model is able to capture the general characteristics of the system with varying levels of agreement using laboratory-measured mean hydraulic properties. Simulation results and field observations revealed a capillary-driven flow in the system. Good quantitative agreement is generally observed for near-field boreholes, however, this agreement deteriorates and the simulated solute concentration is underestimated at boreholes farther away from the injection points. Increasing the spatial resolution of the simulation improves the model predictions only to a limited extent. Scaling issues may need to be considered to describe flow and transport events, as the travel distance becomes large.

Testing and parameterizing a conceptual solute transport model in saturated fractured tuff using sorbing and nonsorbing tracers in cross-hole tracer tests.

Two cross-hole tracer tests involving the simultaneous injection of two nonsorbing solute tracers with different diffusion coefficients (bromide and pentafluorobenzoate) and one weakly sorbing solute tracer (lithium ion) were conducted in two different intervals at the C-wells complex near the site of a potential high-level nuclear waste repository at Yucca Mountain, NV. The tests were conducted to (1) test a conceptual radionuclide transport model for saturated, fractured tuffs near Yucca Mountain and (2) obtain transport parameter estimates for predictive modeling of radionuclide transport. The differences between the responses of the two nonsorbing tracers and the sorbing tracer (when normalized to injection masses) were consistent with a dual-porosity transport system in which matrix diffusion was occurring. The concentration attenuation of the sorbing tracer relative to the nonsorbing tracers suggested that diffusion occurred primarily into matrix pores, not simply into stagnant water within the fractures. The K(d) values deduced from the lithium responses were generally larger than K(d) values measured in laboratory batch sorption tests using crushed C-wells cores. This result supports the use of laboratory-derived K(d) values for predicting sorbing species transport at the site, as the laboratory K(d) values would result in underprediction of sorption and hence conservative transport predictions. The tracer tests also provided estimates of effective flow porosity and longitudinal dispersivity at the site. The tests clearly demonstrated the advantages of using multiple tracers of different physical and chemical characteristics to distinguish between alternative conceptual transport models and to obtain transport parameter estimates that are better constrained than can be obtained using only a single tracer or using multiple nonsorbing tracers without a sorbing tracer.

Comparison of the pollutant loads in dry and wet weather runoff in a southern California urban watershed.

This research compares the relative contributions of potential contaminants discharged in dry weather flow (DWF) and wet weather flow (WWF) from the highly urbanized Ballona Creek watershed (BCW) in southern California using empirical and deterministic models. These models were used to compare the loading of the following pollutants: total suspended solids, biochemical oxygen demand, total nitrogen, total inorganic nitrogen, total Kjeldahl nitrogen, total phosphorus, copper, lead, arsenic, nickel, cadmium, and chromium. The results indicate DWF contributes approximately 10-30% of the total annual flow discharged from Ballona Creek. The annual DWF volume was fairly consistent; the variation in DWF percentage contribution was dependent on the highly variable volume of WWF. The relative contribution to the annual pollutant load varied considerably between each pollutant. In general, the DWF load was found to be significant, especially in years with lower precipitation totals. The results from this investigation have identified the relative relationship between DWF and WWF loads in the BCW and will aid in the decision-making process during the development of an integrated DWF-WWF management plan and allocation of water pollution control funds between DWF and WWF management.

Chlorine-36 in fossil rat urine: an archive of cosmogenic nuclide deposition during the past 40,000 years.

Knowledge of the production history of cosmogenic nuclides, which is needed for geological and archaeological dating, has been uncertain. Measurements of chlorine-36/chlorine (36Cl/Cl) ratios in fossil packrat middens from Nevada that are radiocarbon-dated between about 38 thousand years ago (ka) and the present showed that 36Cl/Cl ratios were higher by a factor of about 2 before approximately 11 ka. This raises the possibility that cosmogenic production rates just before the close of the Pleistocene were up to 50% higher than is suggested by carbon-14 calibration data. The discrepancy could be explained by addition of low-carbon-14 carbon dioxide to the atmosphere during that period, which would have depressed atmospheric radiocarbon activity. Alternatively, climatic effects on 36Cl deposition may have enhanced the 36Cl/Cl ratios.