HR: 0800h
AN: H11E-0821    [Abstracts]
TI: Natural Arsenic Mobilization by Counterion Effect, Ogallala Aquifer, Southern High Plains, Texas, USA
AU: * Nicot, J
EM: jp.nicot@beg.utexas.edu
AF: The Univeristy of Texas at Austin, Jackson School of Geosciences Bureau of Economic Geology University Station, Box X, Austin, TX 78713, United States
AU: Scanlon, B R
EM: bridget.scanlon@beg.utexas.edu
AF: The Univeristy of Texas at Austin, Jackson School of Geosciences Bureau of Economic Geology University Station, Box X, Austin, TX 78713, United States
AU: Reedy, R C
EM: bob.reedy@beg.utexas.edu
AF: The Univeristy of Texas at Austin, Jackson School of Geosciences Bureau of Economic Geology University Station, Box X, Austin, TX 78713, United States
AB: A sharp contrast in arsenic levels between the northern part (median 4.3 ppb) and southern part (median 12 ppb – more than half of the water wells exceed the MCL of 10 ppb) of the Ogallala Aquifer in the Southern High Plains (SHP) coincides with a change in several aquifer characteristics (from north to south: decrease in saturated thickness, decrease in water table depth, and increase in total dissolved solids). Earlier analyses ruled out cotton crops and associated historical application of arsenic defoliants as the source of groundwater arsenic contamination. The most likely source of arsenic is adsorption onto Fe-Mn (oxyhydr)oxides, similar to arsenic sources in most semiarid, oxidizing systems and mobilization through a change in environmental conditions. Changes along flow lines in the southern Ogallala aquifer of the SHP include, in addition to increasing levels of arsenic and other oxyanions, increasing TDS (from <500 to >2000 ppm) and evolution from a calcium- bicarbonate water type to a sodium chloride/sulfate type. There is, however, no change in pH, which remains around neutral. Increase in pH is often described as an important factor in arsenic mobilization. It is hypothesized that waters of the southern Ogallala aquifer of the SHP mix with arsenic-poor waters from the underlying Triassic Dockum Formation releasing sorbed arsenic in the process. Change from Ca to Na type water has been documented in laboratory experiments as a mechanism for mobilizing sorbed arsenic. Several other studies, including those on phosphate, an ion with chemical properties very similar to arsenate, in allied fields (soil science, water treatment) strongly support this counterion effect as a mechanism for mobilizing arsenic.
DE: 1800 HYDROLOGY
SC: Hydrology [H]
MN: 2007 Fall Meeting