DETECTION LIMIT OF SEQUENTIAL LEACHING METHOD FOR ASSESSING SMALL AMOUNTS OF CHROMIUM RETARDATION IN SUBSURFACE

Retardation is an important process that can slow down the aqueous transport of hexavalent chromium, Cr(VI), either by adsorption of Cr(VI) to sediments, or by reduction to immobile trivalent chromium, Cr(III). In previous (Musa et al., 2015) and ongoing studies, sequential leaching tests were performed on chromium contaminated sediments and non-contaminated sediments from the regional aquifer beneath Los Alamos National Laboratory to look for evidence of retardation of chromium on the contaminated sediments. In this study, the detection limit of the sequential leaching method was determined using bentonite and biotite, representing two types of minerals present in the aquifer that could potentially reduce or adsorb the chromium. Prior to the sequential leaching tests, the bentonite and biotite samples were exposed to potassium dichromate solutions with concentrations of 5, 2, 1, 0.5, and 0.2 ppm, respectively. The last three concentrations corresponding to those of the Cr(VI) concentrations measured in groundwater within the plume. The samples were then leached sequentially for three days with 0.01 M nitric acid followed by three days of 0.1 M nitric acid, to remove the chromium. By carefully measuring the amount of chromium that became associated with the bentonite and biotite samples during the exposures to dichromate solutions, we could assess how much of the non-natural chromium was leached during each sequential leaching step. A leaching of blank samples, not exposed to chromium, yielded a correction for background levels to determine how much and in which steps the natural chromium might be leached. Quantifying the retardation of chromium in the regional aquifer will help develop an integrated remediation strategy for the chromium plume.