Meteoric precipitation at Yucca Mountain, Nevada: Chemical and stable isotope analyses, 2006-09

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 (&delta¹⁸O), and delta deuterium (&deltaD) 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⁺, NH