AN ESTIMATE OF EFFECTIVE SURFACE AREA FROM AN IN SITU AQUIFER TITRATION

Effective surface area of aquifer material was estimated to be between 20 and 30% of the total surface area of the aquifer minerals. The estimate was derived from an in situ aquifer titration done to evaluate remediation of an acidic plume by injection of alkaline solutions. The plume is associated with the F-Area Seepage Basins on the Savannah River Site in South Carolina and contains elevated concentrations of Sr-90, I-129, Tc-99, uranium isotopes, and several contaminant metals. The aquifer is sandy with less than 5% clay-sized material.

For the aquifer test, a solution of sodium hydroxide and sodium bicarbonate with a pH of 10 and an alkalinity of 5.2 x 10^-3 eq/L was injected into the aquifer. An extraction well located approximately 30 meters down gradient from the injection well was pumped at a rate of 60 liters/minute during the test. Regular pH measurements of the extracted water yielded a titration curve for the aquifer that was compared to modeled titration curves to estimate effective surface area.

The modeled titration curves were constructed using the U.S.G.S. equilibrium geochemistry code PHREEQC. X-ray diffraction analysis of 5 core samples showed aquifer mineralogy to be dominated by quartz, kaolinite, and hematite. Bulk chemistry of these samples was obtained using X-ray fluorescence spectroscopy. A normative mineralogy of 21 mg/g kaolinite, 12 mg/g hematite, and 970 mg/g quartz was then calculated from these data. The hypothetical surface used in the model was based on these data, literature data on mineral surface area, and literature values for acidity constants.

To estimate effective surface area of the aquifer, the titration test data were compared to a family of titration curves in which effective surface area was varied. This provides a method to estimate a fundamental aquifer property that is difficult to measure. The approach also demonstrates that important information about an aquifer can often be obtained from field tests that were not specifically designed for this purpose.