Long-Term Mine Soil Weathering and TDS Release

The Surface Mining Control and Reclamation Act (SMCRA) of 1977 contained a number of contentious provisions including return to original contour (AOC), long-term liability bonding periods, and return to “equal or better” post-mining land use conditions. However, one of the more interesting provisions was SMCRA’s allowance for use of pre-selected overburden materials as topsoil substitutes when (A) the native A+E horizon materials are less than 6 inches thick, and (B) the physical and chemical properties of the proposed substitute spoil materials are deemed suitable for such use. Since native topsoil layers throughout the Appalachian coalfields are usually less than six inches thick, and removing them from steep slopes is difficult and expensive, the vast majority of coal mined lands in the region have employed topsoil substitutes. One of the unintended secondary effects of this practice has been the intentional selection and placement of topsoil substitute materials derived from deeper unweathered strata that are higher in pH and extractable nutrients than near-surface weathered strata. As discussed later, many of these otherwise suitable topsoil substitutes are also generate significant ionic loads to runoff and leaching waters as they weather over time.