Week 16 - Remediation Flashcards
(10 cards)
What is the historical context of metal mining in the UK, and why is remediation needed today?
Metal mining in the UK dates back to the Bronze Age and was especially active during Roman and Industrial periods. Mines were largely abandoned by the 20th century. These sites were often left untreated, leading to widespread acid mine drainage and metal pollution (notably Zn, Pb, Cd). Over 2,500 km of UK rivers are now impacted, making abandoned metal mines a major source of water pollution.
What are the main pollutants from abandoned metal mines in the UK and their environmental impacts?
Zinc (Zn), Cadmium (Cd), Lead (Pb) are the primary pollutants.
They exceed Environmental Quality Standards (EQS) in many rivers.
These metals are toxic to aquatic life, reduce biodiversity, and impair river ecosystems (e.g., Danio fish growth is stunted in Pb-contaminated water).
Pollution is persistent and can be mobilised during rainfall or groundwater discharge.
What approaches are used to prioritise metal mine remediation in the UK?
Extent of pollution (e.g. how far EQS are exceeded)
Ecological impact
Catchment importance (e.g. downstream water supplies)
Cost-effectiveness of remediation
Supported by data from the Environment Agency and Welsh monitoring systems (e.g. NRW archives, river basin plans)
What is the difference between passive and active mine water treatment systems?
Passive systems:
Rely on natural processes (e.g. microbial sulphate reduction, vegetation uptake)
Examples: reed beds, vertical flow ponds
Low cost, but limited control and efficiency
Active systems:
Use engineered approaches (e.g. lime dosing, electrocoagulation, pumping)
Higher cost and maintenance
Suitable for high-flow or high-metal sites (e.g. Wheal Jane)
Describe the Wheal Jane mine pollution incident and treatment strategy.
Mine decommissioned in 1991; adit collapse in 1992 caused 50 million L of metal-rich acidic water to flood into Carnon River and Falmouth Bay.
Contaminants: Cd, As, Cu, Zn, Fe
Treatment:
Pumping and lime dosing
Treated ~5 million m³/year
Annual operational cost: ~£1.5 million
Residual sludge stored in Clemows Valley tailings dam
What are key features of the Force Crag metal mine treatment system?
Located in the Lake District
Uses passive treatment: vertical flow ponds and wetlands
Vertical flow ponds:
Vertical flow wetlands are a type of engineered passive treatment system designed to remediate acid mine drainage (AMD) by combining microbial sulfate reduction, alkaline neutralization, and metal precipitation in a controlled, gravity-fed, layered environment. They are especially useful at sites with moderate AMD flow and acidity, where active treatment would be too costly or impractical.
The typical structure of a vertical flow wetland consists of a shallow pond or basin constructed with multiple layers of reactive materials:
1. An upper organic-rich layer, composed of compost, manure, or woodchips,
2. A lower layer of crushed limestone, and
3. A drainage system at the base to collect treated water.
AMD is introduced at the surface and flows vertically downward through these layers. In the organic layer, the environment is anaerobic, promoting the activity of sulfate-reducing bacteria (SRB). These microbes utilize sulfate (SO₄²⁻) as an electron acceptor and organic carbon as an energy source, producing hydrogen sulfide (H₂S).
This H₂S reacts with dissolved metals in the water (e.g., Fe²⁺, Zn²⁺, Cu²⁺) to form insoluble metal sulfides (e.g., FeS), which precipitate out and accumulate harmlessly in the organic substrate. This process removes a significant portion of the toxic metals before the water reaches the limestone.
The partially treated water then enters the limestone layer, where the remaining acidity is neutralized. Calcium carbonate (CaCO₃) in the limestone reacts with hydrogen ions (H⁺) in the acidic water, consuming it which raises pH and releasing bicarbonate, which adds alkalinity to water as it acts as a buffering system. Bicarbonate ions act as proton acceptors by binding to H⁺ and forming carbonic acid, which then decomposes into CO₂ and water. They neutralize additional hydrogen ions and thus increase the water’s alkalinity, helping to stabilize pH and prevent acid swings — which is crucial in acid mine drainage treatment.
If any metals remain in solution, the elevated pH causes them to precipitate as metal hydroxides (e.g., Fe(OH)₃, Al(OH)₃).
This additional removal step improves effluent quality but carries the risk of armoring if hydroxides form directly on the limestone surface. Armoring is the coating of limestone grains with insoluble metal precipitates, which inhibits further dissolution and reduces treatment efficiency. The organic layer mitigates this risk by removing most metals before they reach the limestone.
(Although metal hydroxide precipitation is a desired outcome of AMD treatment, premature precipitation at the limestone surface — triggered by a sharp local pH increase — causes armoring, which reduces further neutralization efficiency. To avoid this, vertical flow ponds use an overlying organic layer to gently raise pH and promote metal removal before the water contacts the limestone, maintaining system performance over time)
Finally, the clean, neutralized water is collected through a bottom drain and discharged to receiving streams or further treatment cells. Vertical flow wetlands require minimal mechanical input and can operate for extended periods with little maintenance, though periodic replenishment of organic material or unclogging may be necessary.
In summary, vertical flow wetlands treat AMD by combining anaerobic microbial processes and limestone buffering in a stratified system. Metals are precipitated as sulfides in the organic layer and as hydroxides in the limestone layer, while acidity is neutralized through carbonate dissolution. Their design protects the reactive media from armoring and ensures long-term, low-cost treatment of mine-influenced waters.
Removes metals like Zn, Pb, Cd
Operated in partnership with the National Trust and Environment Agency
One of the earliest scaled-up passive systems in the UK
What is significant about Saltburn Gill and its treatment?
Former ironstone mining area
Polluted streams stained orange from iron oxide (ochre)
Treatment included flow diversion and wetland construction
Successfully reduced Fe loads and improved visual and ecological quality
How does pollution from the River Nent affect the South Tyne, and what are the remediation challenges?
The River Nent contributes >50% of the metal load (Pb, Zn) to the South Tyne
Exceeds EQS by up to 57× for Zn
Impairs biodiversity across 60 km downstream
Treatment requires a catchment-scale approach, with multiple point and diffuse sources:
A catchment-scale approach to AMD addresses the entire drainage area affected by mining, integrating management of both point and diffuse sources of contamination. Point sources are easily located and treated, while diffuse sources require broader landscape interventions. This holistic approach is essential for sustainable AMD control and long-term ecological recovery of impacted waterways.
Newcastle University research supports targeted interventions
What novel approaches are being developed for mine water treatment?
Electrocoagulation: high efficiency but needs power, generates sludge, and requires anode replacement
Phytoremediation: using plants to accumulate or stabilise metals
Sediment capping and channel diversion (e.g. Fron Goch Mine)
Hydropower integration to generate energy at treatment sites (e.g. Nenthead turbines, 405 kW total)
What are the future considerations for metal mine remediation in the UK?
Integration of new technologies with traditional methods
Addressing legacy pollution in low-flow systems
Managing treatment residues (e.g. sludge disposal)
Sustainable funding models and catchment-based management
Research into biomaterials (e.g. alginate beads at Cwmystwyth) and adaptive systems
