ACID-BASE TITRATIONS AS AN INDICATOR OF GEOCHEMICAL HETEROGENEITY IN AQUIFERS

Acid-base titration curves can be used to help distinguish reactive facies, and in turn, to help understand geochemical heterogeneity in aquifers. Acid-base titrations of sediment samples from contaminated aquifers are indicative of the mineral surface properties that control sorption of contaminants. Samples with similar mineralogy will have titration curves with similar shapes and variations in reactive surface area are reflected by shifting locations of titration curve equivalence points. Hence, acid-base titration curves can be a simple cost effective method for distinguishing reactive facies.

This study was conducted at the Savannah River Site (SRS). The focus of this study is the contamination plume associated with the F-Area Seepage Basins. The contaminants of primary concern are uranium, ¹²⁹I, ⁹⁰Sr, and ³H. The aquifer is sandy with minor clay-sized minerals composed primarily of kaolinite and goethite with smectite being observed in the lower part of the aquifer.

Soil core samples were taken from horizons in these basins, at elevations throughout the saturated zone of the core. Titrations were performed using a variation of the back-titration method to account for mineral dissolution and other non-surface effects. The data from this method were used to calculate the soil surface charge.

X-ray fluorescence analysis was conducted on core samples from these same locations performed in the back-titration method. This analysis characterized the core’s chemical composition. Previously collected X-ray diffraction data was used to identify the mineralogy.

Preliminary results of these analyses indicate three groups of similar curves. The mineralogy for these core samples compared to the surface charge suggests a correlation between the titration curves of these reactive facies and their mineralogy. These titration curves aide in the understanding of geochemical heterogeneity in aquifers.