LAUSR

Abstract This paper describes the occurrences, mineralogical assemblages and environmental relevance of iron-rich precipitates derived from acidic (pH   4) that dried, cracked and acidified to form deep sulfuric materials (pH   5 that were preserved after crystallization and likely formed by dissolution of pyrite and microbial oxidation of Fe2 + by acidophilic bacteria and (2) fibrous spheres (0.3–3 μm) with filamentous morphology and a high degree of porosity. Speciation calculations (PHREEQC) using the dissolved metal and major ion concentrations in drain waters supported the XRD results as the saturation index (SI) exceeded zero for schwertmannite in many drains. The precipitates contained high concentrations of metals (Al > Cu > As > Zn > Pb > Co) and nutrients (e.g. P) due to co-precipitation/scavenging of these elements during the formation of schwertmannite. There was also spatial variability in concentrations of metal(loids) in precipitates between drains. A conceptual model explains and summarizes the morphological properties, mineralogy, geochemistry and environmental processes influencing the formation and relative stabilities of schwertmannite-rich precipitates from six diverse physical settings. The environmental relevance, which has significant implications for rehabilitation options is shown in three perspectives: (1) the conditions for schwertmannite formation have persisted in irrigation drains for over 7 years, (2) the ability for schwertmannite-rich precipitates to reveal acid sulfate conditions and therefore act as a mineralogical indicator in irrigation systems and (3) the pollution potential of metals and metalloids scavenged by schwertmannite-rich precipitates.