Chemistry and mineralogy of ochreous sediments in a constructed mine drainage wetland

The objective of this study was to examine the mineralogy and geochemical stability of ochreous sediments accumulated in a compost wetland constructed in 1990 for acid mine drainage treatment. Intact sediment cores were collected in 1996 and 2000 from an area that had accumulated 33 cm of ochre. Solids and pore waters were subsequently separated by centrifugation and analyzed using conventional methods, including X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and wet chemical techniques. The solid phase had an average Fe content of 585 g/kg and was predominantly schwertmannite [Fe8O8(OH)(4.8)(SO4)(1.6)]in the upper portion of the sediment column, but transformed to goethite (alpha-FeOOH) with depth. The rate of transformation was calculated to be 30 mol/m(3)/yr in the initial 6 yr of sedimentation as compared to 10 mol/m(3)/yr for the 4-yr period from 1996 to 2000. Pore water composition was affected by this mineral transformation through production of acidity and the release of Fe and SO,. These results demonstrate that the sediment column was not a static environment. In addition, the transformation of schwertmannite to goethite, which has been observed under laboratory conditions, also occurs in natural systems. Copyright (C) 2004 Elsevier Ltd