HR: 0800h
AN: H41A-0396    [Abstracts]
TI: Evolution of Oxygen and Hydrogen Stable Isotopic Composition in the Edwards-Trinity Aquifer, South-Central Texas
AU: * Thomas, S M
EM: sheeba.thomas@utsa,edu
AF: Earth and Environmental Science Department, University of Texas at San Antonio, 6900 N Loop 1604 West, San Antonio, TX 78249 United States
AU: Paul, D
EM: debajyoti.paul@utsa.edu
AF: Earth and Environmental Science Department, University of Texas at San Antonio, 6900 N Loop 1604 West, San Antonio, TX 78249 United States
AU: Murray, K E
EM: kyle.murray@utsa.edu
AF: Earth and Environmental Science Department, University of Texas at San Antonio, 6900 N Loop 1604 West, San Antonio, TX 78249 United States
AB: The evolution of stable isotope composition of oxygen and hydrogen(δ¹⁸O and δD) in the Edwards-Trinity aquifer, in South Central Texas is modeled in both closed and open aquifer system to explain possible evaporation effects and mixing of various sources of water. Our modeling uses δ¹⁸O and δD data reported by United States Geological Survey (USGS) from 1988 to 2001, in the Edwards unconfined, Edwards confined and the Trinity aquifers, as well as the Medina lake that overlies parts of these aquifers. The δ¹⁸O isotopic compositions of the Edwards confined (av.=-4.20±0.31‰) and Trinity (av.=-4.45±0.37‰) are generally more homogeneous than the Edwards unconfined (av.=-4.05±0.87‰). This is consistent with the higher groundwater velocity (3,000-12,000 ft/day) for the Edwards aquifer which may facilitate faster mixing resulting in homogenous isotopic composition. However, a closer look at data for each aquifer shows periodic variations with time that can be attributed mainly to seasonal effects (i.e.,temperature and precipitation). Groundwater compositions plot to the right of the Local Meteoric Water Line (LMWL) while δ¹⁸O and δD for each aquifer has a unique slope. To account for these trends, isotopic evolution both in open and closed system was modeled assuming Rayleigh fractionation effects during evaporation (both kinetic and equilibrium effects) and binary mixing. Results indicate that an average of 6%-10% evaporation can explain deviations (and the slope of the respective trends) of the groundwater data from LMWL. The isotopic composition of Edwards confined and Edwards unconfined water suggests derivation from a similar meteoric source. Simple binary mixing (10-60%) between end-members(average of Edwards/Trinity aquifer and Medina lake)accounts for water samples plotting along the mixing trend otherwise cannot be explained by evaporation model alone. One interesting result of this modeling is that in selective wells in the Trinity aquifer, the δ¹⁸O shows negative correlation (r²= 0.95) with altitude, and decrease by 0.04‰ per 100 ft increase in altitude.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0400 BIOGEOSCIENCES
DE: 1640 Remote sensing (1855)
DE: 1818 Evapotranspiration
DE: 3322 Land/atmosphere interactions (1218, 1631, 1843)
SC: Hydrology [H]
MN: Fall Meeting 2005