THE APPLICATION OF CENTRIFUGE MODELING AND LASER ABLATION ICP-MS TO QUANTIFY RETARDATION IN CLAY-RICH AQUITARDS

The geochemical controls on the long-term transport of reactive solutes in clay-rich aquitards are difficult to quantify. Two novel approaches, combining centrifuge modeling with ICP-MS, were used to characterize and quantify the retardation of lanthanide and trace metals in one undisturbed clay-rich core sample collected from a well-characterized aquitard located in Southern Saskatchewan, Canada. The core sample was 46.0 mm long, had a surface area of 855 mm², a porosity of 0.3, and a bulk density of 1.97 g ml^-1. Influent groundwater was spiked with lanthanide and trace metals, as well as Cl. A UFA Beckman J6-MI centrifuge with installed seal assembly was used to simulate flow through the clay-rich aquitard sample. The sample was run at 3000 rpm (N factor=876). Infusion pumps provided a flow rate of 1.0 ml hr^-1. A total of 144 pore volumes (one pore volume=11.78 ml) were passed through the core sample. The Peclet number for the experiment was calculated to be 187; hence advective-dispersive transport dominated solute transport through the core. Samples of effluent were analyzed for lanthanide and trace metals using ICP-MS and Cl was analyzed using an ion selective electrode. Using the effluent chemical data, Kd values for Cl, V, Cs, Ce, Pr, Nd, Sm, Hf, Tl, and U were calculated to be 0.0, 0.3, 2.1, 3.3, 13.3, 13.7, 14.1, 6.2, 1.0, and <0.1 ml g^-1, respectively. At the termination of the transport experiment, the adsorption sites for the lanthanides and trace metals were identified using a Cetac LSX-200+ UV Laser Ablation System coupled with an ICP-MS. The data collected to date and the experimental method employed will provide much-needed retardation data to quantify the long-term transport of reactive metals (including actinides) in aquitards.