HR: 14:40h
AN: H43I-04    [Abstracts]
TI: Land-Use Change in Arid Regions Can Mobilize Millennial Accumulations of Unsaturated-Zone Salts
AU: * Stonestrom, D A
EM: dastones@usgs.gov
AF: USGS, MS 421 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Prudic, D E
EM: deprudic@usgs.gov
AF: USGS, Suite 203 333 W. Nye Ln., Carson City, NV 89706 United States
AU: Walvoord, M A
EM: walvoord@usgs.gov
AF: USGS, MS 413 Denver Federal Center, Denver, CO 80225 United States
AU: Jackson, W
EM: Andrew.Jackson@coe.ttu.edu
AF: Texas Tech University, MS 41023, Lubbock, TX 79409 United States
AB: Studies from arid regions around the world have reported elevated pore-water concentrations of chloride just beneath the root zones of xeric vegetation. Recent work in the Amargosa Desert shows that these chloride "bulges" are accompanied by accumulations of other salts, including nitrate, sulfate, and perchlorate. Several lines of evidence indicate that atmospherically deposited salts have been accumulating beneath the root zones of interfluvial desert vegetation in the western United States since at least the end of the Pleistocene, when wetter and cooler conditions gave way to the hot and dry conditions of today. Transport simulations and chloride mass-balance calculations suggest that thousands of years of negligible deep percolation are required to accumulate the observed sub-root salt accumulations. Salt bulges are absent beneath a long-established, incised reach of the ephemeral Amargosa River in southwestern Nevada but are present, though translocated downwards, beneath a non-incised reach that was established during historic flood-induced channel migration in 1969. Desert areas that were converted from native vegetation to irrigated agriculture have salt bulges that have been translocated downward by deeply percolating irrigation water. Downward translocations of bulges indicate that accumulated nitrate and other salts can be rapidly transported to ground water once recharging conditions are established. Travel-velocity estimates of deep water fluxes agree approximately with chloride mass-balance estimates, supporting the idea of quick release of accumulated salts. Estimated fluxes range from 0.02 to 0.5 m/yr. Given modern conversion of large desert areas to agriculture, irrigation-triggered release of sub-root zone accumulations may account for previously unexplained contamination of water-table aquifers with nitrate, perchlorate, and other atmospherically deposited salts.
DE: 1632 Land cover change
DE: 1813 Eco-hydrology
DE: 1831 Groundwater quality
DE: 1834 Human impacts
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: Fall Meeting 2005