Study on water-rock interactions of trace elements in groundwater with leaching experiments.

Measurements of trace elements leached from host rocks can indicate the source of flowing groundwater and provide the host rock's fingerprint information that may be useful in understanding the mixing patterns of different groundwaters and flowpaths. In this study, the leaching characteristics of three widely-distributed aquifer host rocks in Southern Nevada were investigated through leaching experiments of 13 trace elements under different experimental conditions (leaching time, pH, grain size, and rock mineralogy). The fraction of leached trace elements from different aquifer host rocks was calculated for examining the water-rock interaction in different conditions. The effect of host rock mineralogy on leachate composition was examined, which could provide useful information about groundwater source. Solute speciation and saturation indices were simulated using geochemical modeling to further explore the redistribution and leaching process of trace elements.

Modeling ground water flow and radioactive transport in a fractured aquifer.

Three-dimensional numerical modeling is used to characterize ground water flow and contaminant transport at the Shoal nuclear test site in north-central Nevada. The fractured rock aquifer at the site is modeled using an equivalent porous medium approach. Field data are used to characterize the fracture system into classes: large, medium, and no/small fracture zones. Hydraulic conductivities are assigned based on discrete interval measurements. Contaminants from the Shoal test are assumed to all be located within the cavity. Several challenging issues are addressed in this study. Radionuclides are apportioned between surface deposits and volume deposits in nuclear melt glass, based on their volatility and previous observations. Surface-deposited radionuclides are released hydraulically after equilibration of the cavity with the surrounding ground water system, and as a function of ground water flow through the higher-porosity cavity into the low-porosity surrounding aquifer. Processes that are modeled include the release functions, retardation, radioactive decay, prompt injection, and ingrowth of daughter products. Prompt injection of radionuclides away from the cavity is found to increase the arrival of mass at the control plane but is not found to significantly impact calculated concentrations due to increased spreading. Behavior of the other radionuclides is affected by the slow chemical release and retardation behavior. The transport calculations are sensitive to many flow and transport parameters. Most important are the heterogeneity of the flow field and effective porosity. The effect of porosity in radioactive decay is crucial and has not been adequately addressed in the literature. For reactive solutes, retardation and the glass dissolution rate are also critical.