This study assesses the physical and chemical characteristics of hyperalkaline steel slag leachate from a former steelworks on two streams in England and their impacts on 24 benthic invertebrate communities. Using multivariate methods (CCA), we related 25 invertebrate richness and diversity with chemical parameters along the environmental 26 gradient from point sources to less impacted sites downstream. Point discharges are characterised by high pH (10.6-11.5), high ionic strength (dominated by Ca-CO3-OH 28 waters), elevated trace elements (notably Li, Sr and V) and high rates of calcium 29 carbonate precipitation. This combination of stressors gives rise to an impoverished 30 benthic invertebrate community in source areas. The total abundance, taxonomic 31 richness and densities of most observed organisms were strongly negatively correlated 32 with water pH. Analysis using biological pollution monitoring indices (e.g. BMWP and Functional Feeding Groups) shows the system to be highly impacted at source but when pH approaches values close to aquatic life standards, some 500m downstream, complex biological communities become established. In addition to showing the rapid recovery of invertebrate communities downstream of the discharges, this study also provides a baseline characterisation of invertebrate communities at the extreme alkaline range of the pH spectrum.