Leaching and geochemical evaluation of oxyanion partitioning within an active coal ash management unit

•Low liquid-to-solid test (0.3–1 L/kg) described As, B, Mo, and V release to porewater.•Field leaching mechanisms are inferred using geochemical speciation modeling.•Carbonates and ettringite in weathered impoundment ash immobilized B, Mo, and V.•Ca-bearing minerals controlled concentrations of As and V in porewater (0.01–1 mg/L)•Field suboxic (pe 0–4) and alkaline conditions (pH 9–11) inhibited Cr and Se leaching.