A new look at remediation

Two project applications have recently been approved by the CL:AIRE Technology and Research Group (TRG). The first involves the use of tree species as an indicator of contaminated soil, while the second involves the use of a reactive material which is placed in the subsurface to treat contaminated groundwater.

In many instances, clean up costs exceed the value of the land, although low cost action such as the planting of landscape flora can create a vegetative cap to reduce the amount of wind-blown and waterborne contamination, and at the same time dramatically improve the site’s aesthetics and socio-economic value.

Planting of landscape flora is a low cost option compared to full scale remedial works but can nevertheless be expensive, and the plantings may be sensitive to residual contamination. It is therefore useful for landscapers to know the contaminant history at the site, which species may be impacted by any residual contamination, and which species may be more suitable for planting.

This project, which is being carried out by Forest Research in collaboration with Arup, aims to use quick growing common tree species as indicators of potential impacts of contamination to common landscape flora. The uptake of heavy metals into tree tissue is a measure of the bioavailability of the heavy metals. The bioavailability of contaminants is the mobile fraction of a contaminant which poses a potential risk to living organisms.

Nursery experiments have been conducted to investigate the correlation between the uptake of select contaminants from soil by common tree species and the uptake of the same contaminant by common landscape flora. This correlation has been developed into a model which predicts contaminant uptake in the flora based on the measured uptake in tree species. The research involved with this project will field-test and refine the model, and extend the range of contaminants and landscape flora.

The results allow landscapers to predict which flora will survive, given the levels of residual heavy metal concentrations that exist at a site. The method will also be refined to indicate the ability of soil amendments at reducing the mobility and toxic impacts of contaminants.

A site would be planted with low cost fast growing tree species. Once established, the leaves and stems would be sampled and analysed for heavy metals, and the tree species would be harvested and removed from the site. The results of the tissue samples would be modelled to predict the optimum species for landscape planting.

Reactive remediation

Shallow groundwater discharging from spoil heaps at abandoned coal and mineral mines is often highly acidic and contaminated with iron, manganese and aluminium. This water is often termed acid mine drainage (AMD) and is a significant problem both aesthetically and environmentally to surface water bodies and aquatic life both in the UK and world-wide.

AMD occurs when pyrite, which is commonly associated with coal and certain types of metal deposits, is discarded in spoil heaps where it reacts with infiltrating rainfall and oxygen in the air to produce an acidic discharge containing metals and sulphate. Iron, manganese and aluminium precipitate to form hydroxide minerals which discolour water courses and smother aquatic life.

One such spoil heap is located at the former Shilbottle Colliery in Northumberland. AMD from the spoil discharges to the Tyelaw Burn, a tributary of the River Coquet. A system of reedbeds, a type of constructed wetland, was installed in an attempt to treat the mine water. This is shown clearly in Figure 1, where a bright red colouration is seen in the lower portion of the photograph, where ochre is being precipitated within the lowermost reedbed.

The reedbed precipitates iron oxide in the presence of oxygen (aerobic), with the pH being raised by decay of plant leaf and stem debris. However, not long after installation of this reedbed, it was discovered that the visible discharges of AMD intercepted by the reedbeds was actually only about a third of the total, the remainder flowing unseen into the adjoining stream by subsurface pathways, thus bypassing the treatment.

The proximity of the discharge area to the lower reedbed and the Tyelaw Burn required a change in the orientation of the treatment system. There was insufficient room to allow another wetland, so a permeable reactive barrier (PRB) system was considered instead.

A PRB treatment system consists of a reactive material which is placed to intersect the flow of contaminated water. The reactive material is permeable to allow contaminated water to flow through it and react with the material to remove or decrease the toxic effect of the contaminants.

At Shilbottle, the University of Newcastle, in conjunction with Northumberland County Council, has developed a design for a PRB system. The reactive material was designed to enhance the activity of a special type of naturally occurring sulphate-reducing bacteria which use carbon within the PRB and sulphate in the groundwater to reduce the acidity of the groundwater, and precipitate the iron and other metals from solution as metal sulphide minerals. The process is anaerobic.

Chips, waste and horses

Experiments have been carried out in the laboratory to determine the appropriate reactive material and its permeability, and to develop the full scale design. The reactive material finally selected is a mixture of 50 per cent limestone chips, 25 per cent green waste compost and 25 per cent horse manure.

This material has been placed in a two metre wide trench over a length of 180 metres and to a depth of up to three metres, making it the largest system of its type in the world, and also required re-routing of the Tyelaw Burn.

As the partly-treated groundwater exits the PRB, it flows into a new settling pond (pond (which incorporates original riparian trees, now on islands) and into the pre-existing reedbeds. The pond and reedbeds serve to ‘polish’ and aerate the treated groundwater before final discharge to Tyelaw Burn.