Abstract

Meteoric precipitation samples collected in 2006–09 at Yucca Mountain, Nevada, were analyzed for chemistry and stable isotope composition. Precipitation is the major source of infiltration to the unsaturated zone and of recharge to the saturated zone at Yucca Mountain.

On February 28, 2005, seepage of water was observed about 40 to 80 meters below the ground surface within the Tiva Canyon Tuff in the South Ramp of the Exploratory Studies Facility tunnel within Yucca Mountain. This seepage was preceded by a 5-month period of above-average precipitation. Chemical and isotopic analysis of this seepage could not be used to estimate travel time, extent of water-rock interaction, and (or) evaporative water loss during percolation through the unsaturated zone due to the lack of corresponding chemical and isotopic analyses of precipitation inputs to the infiltration events that produced the seepage.

In February 2006, collection of precipitation samples for chemical (major ions), delta oxygen-18 (δ¹⁸O), and delta deuterium (δD) analyses began at seven meteorological monitoring stations to provide baseline isotopic and chemical analyses. The sampling stations range in elevation from 1,131 to 1,562 meters. Each site has two collectors—one for chemical analysis and the other for isotopic analysis of precipitation. The collectors were sampled and emptied after each precipitation event.

In 2006–09, 36 distinct precipitation events, with an average 3- to 4-day duration and an average 9.9 millimeters of accumulation, have been analyzed. The chemical composition of these samples of Yucca Mountain precipitation is relatively dilute but contains measurable and variable concentrations of Na⁺, Ca²⁺, NH₄⁺, NO₃^-, and SO₄^2-. Dust transported by typical winter storms and generated from soil carbonates is the main contributor to this precipitation chemistry. Elevated nitrate and ammonium concentrations may be linked to agriculture in the nearby Amargosa River valley to the south and west of Yucca Mountain.

Cumulatively, δ¹⁸O values range from 3.0 to –20.4 per mil (‰) and δD values range from 10 to –14‰. Winter-season precipitation commonly has isotopically lighter compositions. The cumulative δ¹⁸O plotted against δD shows that precipitation samples define a line with slope of 6.4, less than the 8 of the Global Meteoric Water Line. This difference in slope, typical of arid environments, is chiefly the result of evaporation of falling raindrops due to warmer air temperatures.

First posted September 15, 2011