Legacy of historic mining and water quality in a heavily mined Scottish river catchment
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Date
28/11/2013Author
Haunch, Simon
Metadata
Abstract
Mine abandonment and the discharge of contaminated mine water is recognised globally as a
major source of surface water and groundwater pollution. Contamination generally arises
from the oxidation of sulphide minerals, principally pyrite, by the mining process, and the
subsequent chemical reactions can lead to the discharge of mineralised, often acidic, iron,
and sulphate rich waters. In many historically mined river catchments, mine water discharge
is the main cause of poor water quality. Within the UK, managing the legacy of abandoned
mines is one of the principal challenges presented by modern environmental legislation,
particularly the EU Water Framework Directive, a challenge that is exacerbated by the
diverse and widespread nature of historical mining.
The impact and hazard associated with abandoned mining in one of the UK’s most
intensively mined regions, the Almond River Catchment, Scotland, was examined via: 1) a
detailed GIS mapping and investigation of historical mining processes in the catchment, 2)
mine site discharge sampling, 3) detailed site investigations, 4) geochemical modelling of
four mine waste sites and 5) analysis of temporal and spatial river water quality in the
catchment. The results are then brought together to produce a catchment scale mine water
hazard map.
Mapping has identified over 300 mine sites in the catchment including coal, oil shale and
ironstone mine wastes and flooded coal and oil shale mines. The historical development of
oil shale retort methods has been shown to have an impact on potential hazard.
Sampling of discharge waters from the different mining activities, in conjunction with
detailed mineralogical analysis and geochemical modelling at the four mine waste sites has
characterised the main hazards. Ironstone and pyrite bearing coal mine wastes discharge
waters with highly elevated Fe and sulphate concentrations, up to 160mgl-1 and 1900mgl-1
respectively, due to extensive pyrite oxidation and acid generating salt dissolution
(principally jarosite). Coal mine wastes show variable mineralogy, due to the diverse nature
of coal bearing strata, and discharge waters with variable chemistry. Oil Shale mine wastes
are generally depleted in pyrite due to historic processing and discharge low sulphate waters
with moderately elevated Fe concentrations, up to 5mgl-1. Flooded coal mines discharge
sulphate dominant alkaline waters, due to the availability of carbonate minerals in the mine
complex, with elevated Fe concentrations, up to 50mgl-1, while flooded oil shale mines
discharge waters with moderately elevated Fe concentrations, up to 4mgl-1, due to lower
pyrite content in mine strata and reduced availability of oxygen related to mine abandonment
age.
Once in the surface water environment iron and sulphate display significant concentration-flow
dependence: iron increases at high flows due to the re-suspension of river bed iron
precipitates (Fe(OH)3); sulphate concentrations decrease with increased flow as a result of
dilution. Further examination of iron and sulphate loading at low flows indicates a close
correlation of iron and sulphate 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 iron,
respectively, to the main Almond tributary. Decreases in iron loading on river sections
demonstrate the deposition and diffuse storage of iron within the river channel. This river
bed iron is re-suspended with increased flow resulting in significant transport of diffuse iron
downstream with load values of up to 50 g/s iron.
Based on this hazard classification, a catchment scale mine water hazard map has been
developed. The map allows the prioritisation of actions for future mine water management.