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We assessed the effect of redox conditions on the mobility of lead (Pb), copper (Cu), and iron (Fe) from sediments affected by acid mine drainage (AMD). This was accomplished by integrating laboratory microcosm experiments, aqueous chemistry, diffraction, and electron microscopy. Microcosm experiments underwent 3 consecutive 5 day redox phases: oxic-anoxicoxic. The sediments contained Fe (51,000 mg/kg), Pb (307 mg/kg), and Cu (30 mg/kg), and minerals such as Illite, albite, and goethite. Microscopy analyses revealed that Pb and Cu are associated with Al-silicates and jarosite. Iron release peaked under anoxic conditions (∼250 mg/L), then decreased in the second oxic phase (<70 mg/L). Extraction experiments confirmed that Pb and Cu are water-labile at pH 3.4 (Pb: 27 μg/L exceeding the United States Environmental Protection Agency drinking water action level of 15 μg/L, Cu: 75 μg/L), but less labile at pH 6.4 (Pb: 7 μg/L, Cu: 3 μg/L). DNA sequencing detected metal-tolerant fungal genera (Trichoderma, Fusarium, Penicillium, and Aspergillus) in the sediments. This study provides insights into the biogeochemical processes influencing the lability of metals in AMD-affected sites, which have relevant implications for risk assessment, remediation strategies, and recovery of critical minerals. 

Sealing of oil and gas production wells is done to protect overlying aquifers and the land surface from vertical migration of drilling fluids, produced water (PW), and natural gas. The integrity of the contact between the well casing and cement seal is especially important as both the steel and the cement are subject to reactions caused by exposure to very high salinity PW. The objective of this study was to identify corrosion and precipitation products that form at this contact and determine their effect on the microannular space between the two materials. Steel cylinders were embedded in Type G Portland cement to simulate a sealed wellbore. They were then exposed to simulated and PW sampled from the Permian Basin in the southwestern United States. Solid phases in the cement were identified by X-ray diffraction and electron microprobe analyses and included portlandite, a calcium silicate, and brownmillerite. Gas flow measurements were used to estimate the aperture of the microannulus between the steel surface and the cement. A decrease in the aperture with increasing reaction time was detected for all experiments. The findings suggest that exposure to PW has the potential to reduce the microannular space between the casing and the cement seal as a result of precipitation of calcium- and magnesium-carbonate as dominant phases, with the co-occurrence of sulfate and silicate minerals. These results have implications related to the long-term integrity of annular seals used to seal oil wells exposed to very high salinity PW.