Yucca Mountain Project

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Saturated-Zone Hydrochemistry

Assessment of the overall performance of the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, requires a thorough understanding of the factors influencing ground-water flow and transport in the saturated zone. Some of the critical factors are the length and direction of the flow paths, flow velocity, and mixing. Because these factors cannot be readily measured, the chemical and isotopic composition of ground water throughout the flow system must be used to assess the saturated-zone flow.

Since 1997, approximately 250-ground water samples have been collected from wells throughout the Amargosa Farms and Yucca Mountain area, including new wells drilled by Nye County as part of their Early Warning Drilling Program (EWDP). The cation, anion, trace element, and isotope analyses are entered into the USGS hydrochemical and isotopic database, which also includes more than 600 historical analyses of ground-water samples from the Yucca Mountain region. These chemical and isotope analyses have been interpreted using a variety of methods to help determine source areas, flow directions, mixing relations, ages, and travel times.

illustration showing hydrochemical facies at Yucca Mountain

Water samples from the Western Yucca Mountain facies contains elevated Na, HCO₃, F, U, and B as compared to water from either the Eastern Yucca Mountain or Fortymile Wash facies
Water samples from the Eastern Yucca Mountain and Fortymile Wash facies are similar except that water from the Eastern Yucca Mountain facies contains slightly higher Na and HCO₄ and water from the Fortymile Wash facies contains higher Mg and K. Also, water from the Eastern Yucca Mountain facies contains higher ²³⁴U/²³⁸U ratios than that of any other facies
Water samples from the Bare Mountain and Amargosa River facies are distinguished by higher concentrations of SO₄, HCO₃, and U
Water from the Amargosa River facies contains higher concentrations of B, Na, and Li than water from the Bare Mountain facies
The Eastern Amargosa River facies is the least distinct because of mixing of water from Fortymile Wash, the Spring Mountains, and Jackass Flats which are the suspected source areas

Water chemistry in ground-water samples from both the Western and Eastern Yucca Mountain facies is distinctly different than water from the Timber Mountain area to the north
Water from the Eastern Yucca Mountain facies contains elevated F concentrations and elevated ²³⁴U/²³⁸U ratios that have been interpreted as evidence of local recharge
Estimates of net infiltration from both the mean glacial transition and mean modern climates indicate that the largest infiltration rates occur along northwest-trending fault-controlled washes on the north end of Yucca Mountain
Water from the Eastern Yucca Mountain facies appears to be a mixture of water from the Timber Mountain area to the north and local recharge from the northwest-trending washes on the north end of Yucca Mountain
Water from the Timber Mountain area does not appear to flow beneath the crest of Yucca Mountain and mix with water from the Western Yucca Mountain facies

From Patterson, G.L., and Oliver, T.A., 2004, Trace and Minor elements in saturated-zone water near Yucca Mountain, Nevada. (abst.): Geological Society of America Abstracts with Programs-Annual Meeting, Nov. 7-10, 2004, and Vol.36, No. 5, and poster.

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