Investigation of Accelerated Casing Corrosion in Two Wells at Waste Management Area A-AX

Abstract

The sidewall core samples from well 299-E24-19, which were comprised of a mixture of bentonite and silt lens material, had an average porewater chloride concentration of 376 mg/L. The sidewall core samples collected from well 299-E25-46 had calculated porewater chloride concentrations ranging from 1,200 to more than 10,000 mg/L. Clearly, the sidewall core samples tested were capable of generating porewaters with sufficient chloride concentrations to cause corrosion of the stainless steel well casing. Furthermore, analysis of the sidewall core samples yielded a clear relationship between chloride concentration and well casing corrosion. The sidewall core samples containing the greatest amount of chloride, 3000 {micro}g/g of sediment, came from the well that experienced the longest length of casing failure (4.2 feet in well 299-E25-46). All of the sidewall core samples tested from both decommissioned wells contained more chloride than the Wyoming bentonite test material. However, since chloride was present as a trace constituent in all of the sidewall core samples (less than 0.4 weight percent), it is possible that it could have been introduced to the system as a ''contaminant'' contained in the bentonite backfill material. Therefore, it is likely that chloride leached from the bentonite material and/or chloride carried by/as a constituent of the liquid waste stream caused the advanced well casing corrosion found at wells 299-E24-19 and 299-E25-46 via crevice corrosion and stress corrosion cracking. The sample of Enviroplug{trademark} No.8 high swelling Wyoming bentonite was characterized for its potential to generate porewaters of sufficient chlorinity to lead to accelerated corrosion of type 304L stainless steel. Overall, the bentonite sample had considerably high water extractable concentrations of sodium, chloride, fluoride, sulfate, and alkalinity (measured as calcium carbonate). Interpretation of the laboratory data indicated that the Wyoming bentonite test sample was capable of generating localized vadose zone porewater with chloride concentrations in excess of 700 mg/L. However, the vadose zone at Hanford is primarily composed of coarse-grained sands with an in-situ moisture content ranging from 5-12%. Therefore, it is doubtful enough moisture will be available throughout the majority of the vadose zone to sufficiently ''wet'' the bentonite and leach chloride from the material. Consequently, Wyoming bentonite material should be suitable as an annulus filling agent in all low-moisture zones and those regions that lack the potential to accumulate perched water. This report contains all the geochemical and selected physical characterization data collected on archived vadose zone sediment recovered during the early 1990s installation of four RCRA monitoring wells: 299-E24-19, 299-E24-20, 299-E24-22, and 299-E25-46, sidewall core samples collected during the decommissioning (in 2004) of wells 299-E24-19 and 299-E25-46, splitspoon core samples collected during the installation (in 2004) of two RCRA monitoring wells: 299-E24-33 and 299-E25-95, a sample of Wyoming bentonite, as well as a perched water sample collected during the installation of well 299-E24-33. Laboratory tests were conducted to characterize the sediment and to identify water-leachable constituents. Testing consisted primarily of 1:1 sediment:water extractions, which were used to calculate the elemental concentrations of water soluble constituents in the solid and to estimate in-situ porewater chloride concentrations. Additionally, 8M nitric acid extractions and X-ray diffraction analysis of the solids were used to provide a measure of the total leachable sediment content of constituents and to search for the formation of new crystalline phases that may have formed during the corrosion process, respectively.