Estimation of Unsaturated Zone Traveltimes for Rainier Mesa and Shoshone Mountain, Nevada Test Site, Nevada, Using a Source-Responsive Preferential-Flow Model

ABSTRACTRadionuclide migration experiments in fractured granite at the Climax Stock, Nevada Test Site, are needed to compare field and laboratory measured retardation factors to determine whether laboratory studies accurately reflect in situ conditions. Initial field activities have concentrated on hydrological investigations to determine whether the fractures in Climax granite are suitable for migration experiments. A critical question was whether we could isolate a single vertical fracture between two boreholes and establish flow along that fracture from an upper to a lower borehole. Of the ten fractures tested, one fracture would not take water at pressures up to 200 psig for 24 hours. Several fractures were so permeable they accepted water at a rate which exceeded the pumping capacity of the equipment. Other fractures failed to show a connection between the two boreholes. In two fractures, we were able to establish a circulating system with up to 95 percent of the injected water being recovered. Constant pressure injection tests were conducted. Intrinsic permeabilities of 33 and 75 (μm)2 were estimated using a radial nonsteady flow model. These values correspond to effective fracture apertures of 20 and 30 μm respectively. Concurrent with the hydraulic testing activities is a study of the Climax ground-water chemistry. Our analyses show the natural water to be very different in composition from the granite equilibrated water used in laboratory sorption studies. This paper includes the results of the hydrogeological and geochemical investigations, and describes the overall experimental design plans for the radionuclide migration experiments.

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Phase II Groundwater Flow Model of Corrective Action Unit 98: Frenchman Flat, Nevada Test Site, Nye County, Nevada, Rev. No.: 0

10.2172/891955 ◽

2006 ◽

Author(s):

John McCord

Keyword(s):

Groundwater Flow ◽

Phase Ii ◽

Flow Model ◽

Test Site ◽

Corrective Action ◽

Groundwater Flow Model ◽

Nevada Test Site ◽

Action Unit ◽

Nye County

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Addendum for the Groundwater Flow Model of Corrective Action Units 101 and 102: Central and Western Pahute Mesa, Nevada Test Site, Nye County, Nevada, Revision 0 (page changes)

10.2172/915347 ◽

2007 ◽

Author(s):

John McCord

Keyword(s):

Groundwater Flow ◽

Flow Model ◽

Test Site ◽

Corrective Action ◽

Groundwater Flow Model ◽

Nevada Test Site ◽

Action Units ◽

Nye County

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Simple estimation of fastest preferential contaminant travel times in the unsaturated zone: application to Rainier Mesa and Shoshone Mountain, Nevada

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-7-3879-2010 ◽

2010 ◽

Vol 7 (3) ◽

pp. 3879-3930

Author(s):

B. A. Ebel ◽

J. R. Nimmo

Keyword(s):

Simple Model ◽

Contaminant Transport ◽

Unsaturated Zone ◽

Preferential Flow ◽

Test Site ◽

Water Sources ◽

Travel Times ◽

Saturated Zone ◽

Recharge Rates ◽

Unsaturated Zones

Abstract. Simulating contaminant transport in unsaturated zones with sparse hydraulic property information is a difficult, yet common, problem. When contaminant transport may occur via preferential flow, simple modeling approaches can provide predictions of interest, such as the first arrival of contaminant, with minimal site characterization. The conceptual model for unsaturated zone flow at two areas within the Nevada Test Site, Rainier Mesa and Shoshone Mountain, establishes the possibility of preferential flow through lithologies between potential radionuclide sources and the saturated zone. Lithology, saturated or near-saturated conditions in portions of the rock matrix, and relatively high recharge rates may act in concert at Rainier Mesa to promote preferential flow, despite the semi-arid climate. After identifying preferential flow as a possible contaminant transport process at Rainier Mesa and Shoshone Mountain, we apply a simple model to estimate fastest unsaturated travel times for conservatively-transported radionuclides to initially reach the saturated zone. Preferential flow travel times at Rainier Mesa are tens to hundreds of years for non-ponded water sources and one to two months for continuously-ponded water sources. If preferential flow occurs at Shoshone Mountain, the fastest travel times are approximately twice the Rainier Mesa estimates. A siliceous rock unit is present at Shoshone Mountain that may provide a barrier to preferential flow; if so, estimated transport times increase to more than a thousand years. Our analysis of unsaturated transport of radionuclides via preferential flow, using a relatively simple model, suggests that contaminated locations associated with continuously-supplied water sources, such as effluent ponds and water-filled tunnels, may have significantly shorter radionuclide travel times than locations not associated with such sources. The simple approach demonstrated here for estimating travel times can be useful in situations where predictions are needed by managers for the fastest arrival of contaminants, yet budgetary or time constraints preclude more rigorous analysis, and when additional model estimates are needed for comparison (i.e. model abstraction).

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