Realistic approaches to modeling transport of reactant solutes in groundwater flow systems must contend with solution and surface complexation reactions that affect solutes as groundwater composition and aquifer surface site chemistry change along flow paths. Consequently, the limitations of applying the linear isotherm approach (i.e., Kd) to modeling advective, dispersive transport of reactive solutes is generally well accepted. To meet these challenges, we have sought to integrate field and laboratory investigations in our studies of the rare earth elements (REE) in well characterized aquifers (Carrizo Sand, Texas, and Upper Floridan, Florida). The REEs are naturally occurring, generally non-radioactive elements that are chemical analogs of radioactive transuranic elements such as Pu(III), Am(III), Cm(III), and Cf(III). Because the REEs occur naturally and are stable in the environment, their study provides a unique way to investigate the geochemical behavior of trivalent transuranics in the environment without the obvious safety concerns and restrictions associated with working with transuranics in the laboratory. Our previous studies of REEs in aquifer systems involved investigations of how REE concentrations and fractionation patterns respond to changing groundwater compositions, including redox conditions, along flow paths. Laboratory adsorption experiments led to a preliminary surface complexation model (SCM), which was linked to an existing solution complexation model, and which allows for quantitative assessments of competition between surface and solution ligands for REEs in groundwater systems. Preliminary observations indicate that adsorption of REEs onto Carrizo sand involves free metal ions (Ln3+) and the dicarbonato complex, Ln(CO3)2-. The fraction of adsorbed dicarbonato complex increased along the flow path as pH and alkalinity increased, explaining the flattening of REE fractionation patterns. Proposed herein is a return to the monomineralic Carrizo Sand, the carbonate Upper Floridan aquifer, and initiation of study of the heterogeneous Aquia aquifer in order to conduct the following work: (1) better characterize REE, Mn, Fe, DOC, sulfide concentrations, and ancillary geochemical parameters along flow paths, to better constrain redox related controls and changing solution composition on REEs in aquifers; (2a) apply nanoscale techniques (XRD, SEM, TEM, synchrotron radiation) to characterize the mineralogy and geochemistry of aquifer sediments with emphasis on mineral surface coatings and the association of REEs with such coatings and (2b) conduct allied batch adsorption experiments of aquifer sediments as a function of pH, REEs, PCO2, and dissolved organic matter concentrations to significantly improve the existing combined solution and SCM for REEs; and (3) develop a 1-D advective, dispersive transport model using PHREEQC, or a more robust computer code, linked to the improved solution and SCM that can reproduce REE breakthrough curves in proposed laboratory column experiments. Emphasis will also be placed on conducting ultrafiltration studies of groundwater REEs in order to better sort out the fraction of the aqueous REE pool that is associated with colloidal materials, large-molecular weight organic ligands/humics, from that which is more truly in solution. Collaborative efforts with colleagues (groundwater flow modelers, molecular geochemists, geomicrobiologists) are planned to further our understanding REE association with aquifer mineral surfaces, colloids, and/or nanoparticles within groundwater flow systems.
