HR: 0830h
AN: H21D-0890    [PDF]
TI: Stable Isotopes in Snow: Implications for the Design of Precipitation Collectors and Studies of Groundwater Recharge
AU: * Earman, S
EM: earman@nmt.edu
AF: Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 United States
AB: The stable isotopes 18-O and 2-H are widely used as tracers in the hydrologic system. Numerous studies have collected precipitation in order to determine its stable isotopic signature at locations around the world. This signature varies seasonally, theoretically allowing determination of the relative importance of snow and rain to groundwater recharge. Data for the isotopic signature of precipitation collected over large regions are often contoured, and used to make inferences about weather patterns and processes such as evaporation during rainfall. An issue that, until recently, has generally been ignored in hydrologic studies is that the isotopic composition of snow can change significantly prior to, and during melting. Many studies have compared the isotopic signatures of fresh snow and rain to that of groundwater in order to determine the proportion of each responsible for recharge. However, alteration causes the isotopic signature of snow to become heavier, appearing more rain-like as alteration increases. As a result, many mixing models based on fresh snow and fresh rain end members may provide flawed estimates of the proportion of groundwater recharge derived from snowmelt. We have installed precipitation collectors of various designs, similar to those commonly used in other studies. As two to three different designs of collectors are in place at each of several sites, the impact of the samplers on the isotopic composition of snow can be determined. Our data show that one commonly-used type of collector yields values that are significantly enriched compared to values for fresh snow. At one site in New Mexico, the bulk winter-season isotope values for this collector were -13.5 per mill for delta 18-O and -98 per mill for delta D, compared to the values -16.6 and -118 per mill yielded by a collector designed to prevent isotopic alteration of snow. Our results suggest that the duration of snowpack alteration affects the magnitude of isotopic shift. Where melt takes place quickly due to higher temperatures and/or lower amounts of fall, the isotopic shift is smaller than at sites where large amounts of snowfall and low temperatures allow long periods of alteration. Although collectors that allow the isotopic alteration of snow are inappropriate if values for fresh precipitation are desired, they can be useful for determining the relative importance of snow and rain in groundwater recharge. Mixing calculations from one of our field sites show that using a fresh snow signature in a mixing model would predict groundwater recharge was 40 % snow-derived and 60 % rain-derived. Using the isotopic signature of altered snowmelt from one of our collectors, the proportions were reversed, with snow making up 60 % of the recharge.
DE: 1829 Groundwater hydrology
DE: 1854 Precipitation (3354)
DE: 1863 Snow and ice (1827)
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2003 Fall Meeting