Ramboll developed an innovative, sustainable and economically beneficial approach to addressing the impact of metal contamination on river sediments in the River Nent in Cumbria.
Our experts assessed the chemical and physical composition of heavy metal sediments (predominantly lead and zinc) impacting the river as a result of historical mine workings around Nenthead. After evaluating treatment, re-use and excavation and disposal options, we used a simple but effective bankside sampling method that saved our client approximately £125,000 in hazardous waste landfill tipping fees and a further £100,000 by avoiding the requirement for virgin construction materials.
Contaminated sediments not only affected the condition and habitat quality of the river, but the metals were also carried downstream, impacting the status of other waterbodies within the Tyne catchment. Our team met the objectives of the Coal Authority and Environment Agency to reduce contaminant loading to the river and improve downstream water quality in a sustainable and cost-effective manner, through careful analysis and forward thinking.
Sediment sample analysis
We collected sediment samples from behind existing in-stream check weirs, immediately downgradient of the main pollution source area. The samples were sieved into various particle size fractions and analysed for a range of chemicals. The analysis aimed to establish whether the impact was uniform across all sediment material or concentrated into key fractions, indicating whether a potential cost saving could be made by separating impacted from non-impacted fractions to reduce the overall volume of material requiring disposal. Initial waste classification suggested that the sediments would be classified as hazardous waste and bulk excavation and disposal to a landfill would be costly and unsustainable for the long-term requirements of the UK-wide programme.
Data results for analysis and separation
The data showed that approximately 80% of the contaminant mass contained fine sediment (<10mm), which accounted for around ~25% of the total sediment volume. Ramboll proposed a sustainable reuse option whereby coarse sediment (>10mm) was identified as suitable for use within gabion baskets for ongoing bank stabilisation works, so only the fine sediment needed to be sent to the landfill.
To make this approach cost effective, Ramboll trialled two sediment separation methods. The first involved using a Powerscreen Warrior 600 to trial dry sieving. This approach is effective when large volumes of sediment (>2,000m³) require treating and where there is ample space to sort and locate the equipment. The results indicated that material sent to landfill could be reduced by more than 75% under the right conditions.
Riddle bucket trial method
The second trial method used a riddle bucket attached to an excavator under both wet and dry conditions. This method was highly effective for small volumes of sediment and in confined or constrained conditions, which are the typical conditions encountered in upland areas requiring sediment excavation.
The riddle bucket method was trialled this year and 212m³ of sediment was removed, equating to removing a contaminant loading of 1.9 tonnes of lead and 5.1 tonnes of zinc to the river. This trial represents the first metal removal from the River Nent system, demonstrating a sustainable alternative to bulk excavation and landfilling whilst reducing financial outlays associated with meeting strategically important environmental company objectives.
Benefits of project
- Cut costs by 60% making the removal of sediment a viable remediation option
- Reduced the need to import new material for sister activities such as flood defense and other civils work
- Minimised haulage and transport costs
- Reduced the volume of material going to landfill
- Removed metal loading to River Nent and downstream watercourses
Further potential market benefits resulting from a reduction metal mining include a change in property values due to close proximity to improved water courses and an increase in the availability of water for abstraction, either for drinking or irrigation. Non-market benefits range from informal and in-stream recreational activities through to an increase in the value of natural capital such as biodiversity and habitats.
This simple, sustainable and economic approach demonstrates innovation which has the potential to have a significant impact on the Coal Authority and Environment Agency’s aspiration to address the requirements of the River Basin Management Plans. A national strategy is beginning to emerge for similar areas affected by legacy mining issues.
"Ramboll’s innovative work on the waste classification and reuse assessment has potentially saved a significant sum of money and, more importantly, means that we should be able to meet our programme’s sustainability objectives and avoid sending materials unnecessarily to landfill.
“Both the Coal Authority and the Environment Agency really appreciate the difference that your involvement has made to the project’s overall success.”
Ramboll project director quote:
"Ramboll has always put delivering environmentally sustainable solutions for clients at the heart of its business strategy, but this project has really challenged our team – and motivated us – to find a solution that was practical and economically viable as well as sustainable. It is gratifying in its own right to be part of the team that is striving to make a real difference to improving the water quality of the River Nent, however, it has also been helpful to put a value on our work and the benefits it has brought to clients at a time when public and private bodies are stretched and being able to demonstrate the financial case for environment and sustainability projects is essential.”