ISOTOPIC INVESTIGATION OF PORE WATER INTERACTION WITH BURIED CARBONATE LAYERS AT THE HANFORD SITE, PASCO BASIN, WA

Clastic sediments of the Hanford and Ringold formations contain laterally extensive carbonate layers at the Hanford Site, located in the Columbia Plateau of south-central Washington. Pu for nuclear weapons was produced and chemically separated from other reactor products at the Hanford Site throughout the Cold War, which has resulted in some of the most serious and complex groundwater contamination problems in the country. The carbonate layers, known as “caliche” are associated with high moisture contents and high cation concentrations in vadose zone pore waters. These layers likely impede infiltrating fluids due to their low permeability relative to the surrounding sands and gravels, and may interact with contaminants migrating through the unsaturated zone. The morphology of the carbonates has been attributed to soil forming processes during depositional hiatuses and with later modification by groundwaters. This study uses Sr, C, and O isotopes as tracers to infer the depositional environment and test for later modification of buried carbonate layers at Hanford.

In hand sample, some carbonate layers display laminae that are a few millimeters thick, separated by centimeters thick micritic or sandy zones. A thin rind of carbonate commonly coats clastic pebbles included in the carbonate layers. Crushed “whole rock” samples and subsamples of the laminae, groundmass, and pebble rinds have d¹³C_PDB values of –8.7 to –4.4 per mil and d¹⁸O_SMOW values of 15.2 to 18.6 per mil. Carbon isotopes from these samples show a generally positive, linear correlation with oxygen isotope values. Disseminated carbonates from cored sediments have higher d¹³C_PDB values, and lower d¹⁸O_SMOW values than the “caliche” layers. Unsaturated zone pore waters at Hanford have d¹⁸O_SMOW values up to –7.5 per mil near the surface and are –17 to –14 per mil at depth, while groundwater values are generally –18 to –16 per mil. The “caliche” layers at Hanford retain an isotopic signature consistent with deposition from strongly evaporated, near surface waters, whereas disseminated carbonates are generally near isotopic equilibrium with less evaporated pore waters. Preliminary Sr isotope results suggest little modern interaction between the carbonate layers, which have ⁸⁷Sr/⁸⁶Sr ratios around 0.710 and their pore waters, which are generally 0.715.