Flow dependent water quality impacts of historic coal and oil shale mining in the Almond River catchment, Scotland

摘要

The Almond River catchment in Central Scotland has experienced extensive coal mining during the last 300 years and also provides an example of enduring pollution associated with historic unconventional hydrocarbon exploitation from oil shale. Detailed spatial analysis of the catchment has identified over 300 abandoned mine and mine waste sites, comprising a significant potential source of mine related contamination. River water quality data, collected over a 15 year period from 1994 to 2008, indicates that both the coal and oil shale mining areas detrimentally impact surface water quality long after mine abandonment, due to the continued release of Fe and SO_4~(2-) associated with pyrite oxidation at abandoned mine sites. Once in the surface water environment Fe and SO_4~(2-) display significant concentration-flow dependence: Fe increases at high flows due to the re-suspension of river bed Fe precipitates (Fe(OH)_3); SO_4~(2-) concentrations decrease with higher flow as a result of dilution. Further examination of Fe and SO_4 loading at low flows indicates a close correlation of Fe and SO_4~(2-) with mined areas; cumulative low flow load calculations indicate that coal and oil shale mining regions contribute 0.21 and 0.31 g/s of Fe, respectively, to the main Almond tributary. Decreases in Fe loading along some river sections demonstrate the deposition and storage of Fe within the river channel. This river bed Fe is re-suspended with increased flow resulting in significant transport of Fe downstream with load values of up to 50 g/s Fe. Interpretation of major ion chemistry data for 2005-2006 indicates significant increases in Ca~(2+), Mg~(2+) and HCO_3~- in coal mined areas probably as a result of the buffering of proton acidity in mine waters; in the oil shale areas Na~- and Cl~- become increasing dominant possibly associated with increased urbanisation and saline pore water discharge from unprocessed oil shale waste. The study demonstrates the importance of considering the cumulative impact of point and diffuse contamination sourced from numerous small coal and oil shale mine sites on surface water quality.