HR: 0830h
AN: H11G-0955    [PDF]
TI: Rare Earth Element Speciation Along Groundwater Flow Paths in Two Different Aquifer Types (i.e., Sand vs. Carbonate)
AU: * Tang, J
EM: jtang@odu.edu
AF: Old Dominion University, Dept. of Ocean, Earth, and Atmospheric Sciences, Norfolk, VA 23529-0276 United States
AU: Johannesson, K H
EM: kjohanne@uta.edu
AF: The University of Texas at Arlington, Dept. of Geology, Arlington, TX 76019-0049 United States
AB: Groundwater samples were collected in two different types of aquifer (i.e., Carrizo sand aquifer, Texas and Upper Floridan carbonate aquifer, west-central Florida) to study the concentrations, fractionation, and speciation of rare earth elements (REE) along groundwater flow paths in each aquifer. The solution complexation of REEs in these two aquifers was modeled using Humic Ion-Binding Model V. The results of the REE speciation modeling indicate that the solution complexation of REEs is controlled by pH, alkalinity, and DOC concentration. Because DOC is low (less than 0.65 mg/L) in Carrizo aquifer, organic complexation of REEs is not significant in these groundwaters. For LREEs, LnCO3+ and/or Ln(CO3)2- are the dominant species when pH is above 7.0, but when pH is below 7.0, besides LnCO3+, Ln3+ and LnSO4+ are also significant and their importance increases with decreasing pH. For HREEs, LnCO3+ and/or Ln(CO3)2- are always the dominant species, however, LnPO4o is important in some groundwaters. The similar general patterns of inorganic solution complexation of the REEs are also observed in groundwaters from the Upper Floridan aquifer. However, because of the relatively high DOC concentrations measured in the Floridan groundwaters, organic complexation of REEs, especially LREEs, is also predicted to be significant in some groundwaters. Linear correlation coefficients for total REE, Nd, Gd, and Yb concentrations, and (Yb/Nd)SN vs. pH, CO3, DOC, SO4, Fe, and Mn indicate that for the Carrizo groundwaters, REE concentrations are not correlatd to any of these factors. However, (Yb/Nd)SN is inversely related to pH and the [CO32-] concentration of these groundwaters. Because REE speciation is strongly related to pH and the [CO32-] concentration, it is reasonable to expect that REE solution complexation plays a greater role in fractionating REE than controlling absolute concentrations in the Carrizo Sand aquifer. In Upper Floridan aquifer, LREE concentrations are positively correlated to DOC and inversely related to [CO32-] and pH. MREEs do not correlate with DOC but show a weakly inverse relationship to [CO32-] and pH. The HREEs are inversely related to DOC and positively correlated with the [CO32-] concentration and pH. (Yb/Nd)SN is positively related to pH and [CO32-] and negatively related to DOC in the Floridan groundwaters. Thus, in contrast to our observations for the Carrizo Sand aquifer, REE solution complexation appears to affect both REE fractionation patterns and their absolute concentrations for groundwaters of the Upper Floridan aquifer.
DE: 1045 Low-temperature geochemistry
DE: 1065 Trace elements (3670)
DE: 1806 Chemistry of fresh water
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2003 Fall Meeting