The European Water Framework Directive requires the treatment of acid mine water in order to prevent unfavourable effects on surface and groundwaters as well as on aquatic biota.
The treatment can either be accomplished actively or by passive treatment systems (PTS). The latter option decreases metal loads and net acidity by natural chemical and microbial attenuation processes.
One of these passive treatment systems is Bowden Close consisting of two sequential Reducing and Alkalinity Producing Systems (RAPS) which discharge into an aerobic reed wetland.
The PTS is located 12 km southwest of Durham in the Northeast of the United Kingdom and aims to diminish iron and aluminium concentrations and raise the pH by addition of alkalinity in the mine water to allow the discharge into the local Willington burn.
Bowden Close has been commissioned in 2003 after an intensive pilot scale project between 2002 and 2003. Besides an extensive monitoring of the water quality of influents and effluents of Bowden Close since 2003, so far few geochemical analyses have been undertaken to identify its efficiency and leading (bio)geochemical processes involved in the treatment.
However, only by understanding these, remediation can be fully assessed and lessons learnt for future projects of similar nature.
For this reason, the present research study will focus in first instance on the mass balance of major contaminants (Fe, Al, Mn, Zn and SO4) of the PTS to identify their seasonal removal behaviour and therewith asses the treatment efficiency of the passive treatment system. Secondly, an intensive testwork will follow on sediment cores to identify:
- the total concentrations of sulphur, carbon and iron and their main mineral species and content in organic matter as well as their spatial (vertical and horizontally) distribution within the sediment
- the processes causing isotopic fractionation and disproportionation of sulphur (e.g.: redox processes, microbial controlled processes, precipitation/ dissolution, diffusion, adsorption and co-precipitation)
- the processes involved in the generation of alkalinity within the RAP systems and the contribution that the dissolution of limestone and the anaerobic digestion of organic matter have respectively on the increase of pH in the mine water by filtering through the reactive compost-limestone substrate