A substrate classification index for the visual assessment of the impact of acid mine drainage in lotic systems

The impact of acid mine drainage (AMD) on lotic receiving waters is complex and difficult to assess. While low pH, elevated sulphate and metals all characterize AMD these vary both spatially and temporally, with high river dilutions resulting in low absolute concentrations which are difficult to measure. From field observations the best indications of AMD contamination of lotic systems appear to be the brownish-yellow precipitates of iron (III) hydroxide, high densities of chironomids, and the absence of macrophytes. Using these observations a simple index based on the degree of deposition of ochre on the river substrate, and the level of floc formation, has been devised. The index is purely a descriptive one and is calculated rapidly using a visual inspection of individual sites. The derived index values are related to the biological impact that can be expected.