Application of U-series disequilibria to evaluate fracture flow and fracture-matrix interactions at Pahute Mesa, Nevada National Security Site.

摘要

The U-Series decay chain was used to better understand groundwater movement through fracture networks on Pahute Mesa at the Nevada National Security Site. Whole rock core samples were taken from five boreholes on Pahute Mesa and were classified as either discrete fracture surfaces, or interior intact or brecciated/rubblized samples. After samples were chemically digested, a Thermo Finnigan TritonRTM mass spectrometer was used to determine 230Th/238U and 234U/238U A.R.s. It was determined that the majority of interior intact and brecciated/rubblized samples had A.R.s that clustered around secular equilibrium, while discrete fracture surfaces consistently had values that were more that 2% away from secular equilibrium. This reveals that groundwater flow on Pahute Mesa is dominated by discrete fracture networks. Rock interior samples at select distances away from discrete fracture surfaces were also collected and analyzed. Modeling attempts to capture the 230Th/238U and 234U/238U A.R.s at selected distances away from the discrete fracture surface were based on diffusion and were unsuccessful, as equation parameters were likely too unconstrained. Modeling samples' A.R.s relationships to the equiline was performed under steady state and transient conditions. This modeling approach was based on a balance between the influx and loss of 234U and 238U. Modeling results suggest that samples plotting on the equiline are at a steady state balance of 234U and 238U sources and sink terms. Samples plotting to the left of the equiline likely show that recent water-rock interaction has occurred and are representative of a U-adsorbing system. Given the uncertainty of the input and loss rates of U nuclides, many different scenarios were generated to explain samples' relationships to the equiline.