Use of chlorine-36 and other geochemical data to test a groundwater flow model for Yucca Mountain, Nevada

Defining the spatial distribution and timing of subsurface fluid percolation is one of the most important factors determining long term performance of the potential high-level radioactive waste repository at Yucca Mountain, Nevada. The nonwelded interval of the Paintbrush Group (PTn), which overlies most of the potential repository, has high matrix porosities and permeabilities and is mostly unfractured. The Exploratory Studies Facility (ESF) is a 8-km long, 7.6-m diameter, tunnel excavated beneath Yucca Mountain to the level of the potential repository horizon in order to provide access for characterization of these rocks. Several samples collected within the ESF have measured {sup 36}Cl/Cl ratios that record anthropogenic {sup 36}Cl (bomb-pulse {sup 36}Cl), indicating that at least some fraction of the water has traversed the overlying PTn in 40 years or less and that flow is not confined to the matrix of that unit. The presence of a fast path transmitting bomb-pulse {sup 36}Cl to depth appears to require the simultaneous presence of a structure (such as a fault) cutting the PTn and sufficiently high magnitude to surface infiltration to initiate and sustain at least a small component of fracture flow along the connected fracture path associated with the structure. The {sup 36}Cl data have been simulated using the flow and transport model FEHM in order to establish bounds on infiltration rates at the site and to provide greater confidence in the understanding of unsaturated flow processes at the site by showing consistency between the observed and simulated data sets. An analogous effort simulating the distribution of porewater chloride concentrations is providing an independent means for confirming the conceptual model

Using chloride to trace water movement in the unsaturated zone at Yucca Mountain

The nonwelded Paintbrush Tuff (PTn) hydrogeologic unit is postulated as playing a critical role in the redistribution of moisture in the unsaturated zone at Yucca Mountain, Nevada. Fracture-dominated flow in the overlying low-permeability, highly fractured Tiva Canyon welded (TCw) unit is expected to transition to matrix-dominated flow in the high-permeability, comparatively unfractured PTn. The transition process from fracture to matrix flow in the PTn, as well as the transition from low to high matrix storage capacity, is expected to damp out most of the seasonal, decadal, and secular variability in surface infiltration. This process should also result in the homogenization of the variable geochemical and isotopic characteristics of pore water entering the top of the PTn. In contrast, fault zones that provide continuous fracture pathways through the PTn may damp climatic and geochemical variability only slightly and may provide fast paths from the surface to the sampled depths, whether within the PTn or in underlying welded tuffs. Chloride (Cl) content and other geochemical data obtained from PTn pore-water samples can be used to independently derive infiltration rates for comparison with surface infiltration estimates, to evaluate the role of structural features as fast paths, and to assess the prevalence and extent to which water may be laterally diverted in the PTn due to contrasting hydrologic properties of its subunits

Distribution of fast hydrologic paths in the unsaturated zone at Yucca Mountain

Development and testing of conceptual flow and transport models for hydrologic systems are strengthened when natural environmental tracers are incorporated into the process. One such tracer is chlorine-36 ({sup 36}Cl, half-life, 301,000 years), a radioactive isotope produced in the atmosphere and carried underground with percolating groundwater. High concentrations of this isotope were also added to meteoric water during a period of global fallout from atmospheric testing of nuclear devices, primarily in the 1950s. This bomb-pulse signal has been used to test for the presence of fast transport paths in the unsaturated zone at Yucca Mountain and to provide the basis for a conceptual model for their distribution. Yucca Mountain is under investigation by the US Department of Energy as a potential site at which to host an underground high-level radioactive waste repository. Under wetter climatic conditions, fast-flow pathways will respond quickly to increases in infiltration and have the potential to become seeps in the tunnel drifts. The {sup 36}Cl data are also being used in numerical flow and transport models to establish lower bounds on infiltration rates, estimate ground water ages, and establish bounding values for hydrologic flow parameters governing fracture transport

MODELING FLOW AND TRANSPORT PATHWAYS TO THE POTENTIAL REPOSITORY HORIZON AT YUCCA MOUNTAIN

The isotopic ratios of {sup 36}Cl/Cl are used in conjunction with geologic interpretation and numerical modeling to evaluate flow and transport pathways, processes, and model parameters in the unsaturated zone at Yucca Mountain. By synthesizing geochemical and geologic data, the numerical model results provide insight into the validity of alternative hydrologic parameter sets, flow and transport processes in and away from fault zones, and the applicability of {sup 36}Cl/Cl. ratios for evaluating alternative conceptual models

Chlorine-36 investigations of groundwater infiltration in the Exploratory Studies Facility at Yucca Mountain, Nevada

Chlorine-36, including the natural cosmogenic component and the component produced during atmospheric nuclear testing in the 1950`s and 1960`s (bomb pulse), is being used as an isotopic tracer for groundwater infiltration studies at Yucca Mountain, a potential nuclear waste repository. Rock samples have been collected systematically in the Exploratory Studies Facility (ESF), and samples were also collected from fractures, faults, and breccia zones. Isotopic ratios indicative of bomb-pulse components in the water ({sup 36}Cl/Cl values > 1,250 x 10{sup {minus}15}), signifying less than 40-yr travel times from the surface, have been detected at a few locations within the Topopah Spring Tuff, the candidate host rock for the repository. The specific features associated with the high {sup 36}Cl/Cl values are predominantly cooling joints and syngenetic breccias, but most of the sites are in the general vicinity of faults. The non-bomb pulse samples have {sup 36}Cl/Cl values interpreted to indicate groundwater travel times of at least a few thousand to possibly several hundred thousand years. Preliminary numerical solute-travel experiments using the FEHM (Finite Element Heat and Mass transfer) code demonstrate consistency between these interpreted ages and the observed {sup 36}Cl/Cl values but do not validate the interpretations

Flow focusing in unsaturated fracture networks: A numerical investigation

A numerical modeling study is presented to investigate flow-focusing phenomena in a large-scale fracture network, constructed using field data collected from the unsaturated zone of Yucca Mountain, Nevada, the proposed repository site for high-level nuclear waste. The two-dimensional fracture network for an area of 100 m x 150 m contains more than 20,000 fractures. Steady-state unsaturated flow in the fracture network is investigated for different boundary conditions and rock properties. Simulation results indicate that flow paths are generally vertical, and that horizontal fractures mainly provide pathways between neighboring vertical paths. In addition to fracture properties, flow-focusing phenomena are also affected by rock-matrix permeability, with lower matrix permeability leading to a high degree of flow focusing. The simulation results further indicate that the average spacing between flow paths in a layered system tends to increase and flow tends to becomes more focused, with depth

The in situ production of radioisotopes in rock matrices with particular reference to the Stripa granite.

The in situ production of 3H, 14C, 36Cl, 129I, 37Ar, 39Ar, 81Kr and 85Kr by neutron-induced reactions and spontaneous nuclear decay processes has been estimated within the Stripa granite, its associated fracture minerals and surrounding leptite. Experimental measurements of the neutron flux within the Stripa granite are within 15% of theoretical values and have been used to calculate neutron-induced production rates. Activities of radionuclides that may enter groundwaters from the rock matrix are discussed in relation to their initial distribution in the host rock and their subsequent release into solution. The measured radioactivities in groundwaters from the high radioélément-content granites at Stripa are compared with estimates of their in situ production within the granite and of their solution by fracture-borne fluids. The activities of 3H and 14C which enter fracture fluids from the rock matrix are close to present detection limits. For 3H, the most significant production is within the rock matrix whilst for 14C it is within the circulating fracture fluids. Chlorine-36 production by neutron-capture within the granite is much more significant than inputs from cosmogenic and nuclear fallout sources. The 36Cl Cl ratio may be used to evaluate Cl- sources in saline groundwaters. In situ production of 129I by spontaneous fission of 238U is an important source of 129I in the high radioactivity Stripa granite. Argon-37,39Ar and 85Kr are produced by in situ reactions within the rock matrix, fracture mineralisation and at the sites of U-mineralisation, respectively. The use of some cosmogenic radionuclides for the estimation of groundwater residence times may be limited because of in situ production