HR: 1330h
AN: H32A-0543    [PDF]
TI: Modeling CO$_{2}$ and Carbon-Isotope Dynamics in a Deep Unsaturated Zone Near Beatty, Nevada
AU: * Walvoord, M A
EM: walvoord@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, Box 25046, MS-413, Lakewood, CO 80225 United States
AU: Prudic, D E
EM: deprudic@usgs.gov
AF: U.S. Geological Survey, 333 W. Nye Ln., Suite 203, Carson City, NV 89706 United States
AU: Striegl, R G
EM: rstriegl@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, Box 25046, MS-413, Lakewood, CO 80225 United States
AU: Stonestrom, D A
EM: dastones@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS-421, Menlo Park, CA 94025 United States
AU: Parkhurst, D L
EM: dlpark@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, Box 25046, MS-413, Lakewood, CO 80225 United States
AB: Natural unsaturated-zone gas profiles at the Amargosa Desert Research Site near Beatty, Nevada show an increase in the partial pressure of CO$_{2}$ (PCO$_{2}$) from $\sim$36 Pa at the land surface to $\sim$900 Pa at the 110-m deep water table. The CO$_{2}$ profiles show generally increasing $\delta$$^{13}$C values with depth, from a time-weighted-average -18$\permil$ in the root zone to -14$\permil$ at the water table, and strongly decreasing $^{14}$C activities, from 120 percent modern carbon (PMC) in the root zone to 20 PMC at the water table. Field data suggest the presence of two CO$_{2}$ sources, one shallow and one deep. The shallow source results from seasonally variable respiration in the root zone. The deep source is located near the water table, and is presumed to represent steady production associated with calcite precipitation. We use a geochemical gas-diffusion model to better quantify CO$_{2}$ production and partitioning between shallow and deep sources. The model was developed by coupling the geochemical equilibrium model PHREEQC with a finite-difference gas-diffusion algorithm. The individual isotopic species of CO$_{2}$ are treated as separate chemical components that diffuse and react independently. Because shallow and deep sources are isotopically distinct, the measured $\delta$$^{13}$C and $^{14}$C profiles help constrain model solutions. Shallow CO$_{2}$ production reflects the isotopic influence of biological processes that include root respiration and metabolism of modern soil carbon. Deep CO$_{2}$ production reflects the isotopic composition of old groundwater (20 PMC). Model simulations run to steady state indicate that the shallow CO$_{2}$ source from root and microbial respiration comprises $\sim$97% of the total CO$_{2}$ annual average production at this arid site. Despite the relatively small contribution from groundwater ($\sim$0.1 mol CO$_{2}$ m$^{-2}$ yr$^{-1}$), upward diffusion from this source strongly influences the distribution of CO$_{2}$ and carbon isotopes in the deep unsaturated zone. Transient simulations for seasonally varying soil-respiration rates establish profile-perturbation depths as a function of time for PCO$_{2}$, $\delta$$^{13}$C, and $^{14}$C. The new model of carbon-isotopic profiles provides a quantitative understanding of isotopically distinct fluxes prerequisite for analyzing migration of $^{14}$CO$_{2}$ from radioactive waste in deep unsaturated zones.
DE: 1875 Unsaturated zone
DE: 4820 Gases
DE: 4825 Geochemistry
DE: 4860 Radioactivity and radioisotopes
DE: 4870 Stable isotopes
SC: Hydrology [H]
MN: 2003 Fall Meeting