Mine Spoil Lasting Impacts of Mining Operations on the Environment

An overview of the environmental hazards imposed by bare mine spoil in our landscapes, and an introduction to the principles and benefits of restoration using 4R Groups’ spoil-to-soil approach

Contents

Introduction ...... 1 Mine Spoil ...... 1 What is it? ...... 1 Mining terminology ...... 2 Colliery spoil or Metalliferous mine spoil ...... 4 Environmental implications of untreated mine spoil ...... 5 Acid Mine Drainage ...... 5 Causes of acidity ...... 6 Enhanced concentrations and bioavailability of potentially toxic elements ...... 7 Physical stability ...... 8 Siltation ...... 9 Fertility ...... 12 Restoration ...... 13 Principles of restoration ...... 13 Liming ...... 15 Phytostabilization ...... 15 Restoration examples ...... 17 Summary...... 20 Detriment of leaving a former mining site bare ...... 20 Benefits of restoration ...... 21 Contact ...... 22 Further reading and reference ...... 22

Introduction

The purpose of this document is to assist land owners, regulators and governing authorities with understanding the environmental risks associated with unconfined mine spoil. The information presented here is designed to provide a concise, but comprehensive guide relating the hazards posed by post-operational mining sites, and options for their remediation. 4R-Group specialises in providing cost-effective, viable reclamation of former mining sites, with proven long-term success. The spoil-to-soil approach is based upon chemical, biological and physical principles that work in nature to create a desirable set of conditions in the substrate. These essential base conditions act as a catalyst for the development and establishment of a natural, healthy, functioning environment that supports a range of vegetation, thus creating new wildlife habitats and green spaces that bring amenity and aesthetic value to an area of formerly polluted and degraded land.

Mine Spoil

What is it?

Where mining is concerned, the term spoil refers to the overburden, or waste material that overlies an area of ore extraction activities. Traditional mining activities targeted the ores by tunnelling underground through the mineral-rich veins. More contemporary methods extract entire bodies of subsurface and surface ore-bearing material. The spoil left behind is the unwanted overburden or processed material that is often still highly concentrated in metallic compounds or other minerals brought up to the surface, however, their lower amounts make them less economically viable to extract, especially given the technology at the time of traditional practices. This results in the visible presence of barren moon-like, acidic, and usually contaminated tracts of land in an otherwise green landscape.

Figure 1. Acidic mine spoil in Devon, at the site of an abandoned lead mine (left). Colliery spoil at a former mining site in Yorkshire (right)

Mining terminology

Figure 2 illustrates the basic process of extracting target minerals/ore from the ground and the associated by-products that are created. Overburden and waste rock from tunnelling and excavating are deposited at the surface in heaps and form the majority of material that makes up the topography associated with former mining sites. Excavated material that is rich in the target ore is further processed to separate the valuable mineral concentrate from the uneconomic residual tailings. In an effort to mitigate against the direct discharge of tailings and other unwanted material into local waterways, sedimentation or settling ponds are constructed to contain much of the by-product and waste. The heavier particulate fraction falls out of suspension in the dammed water and once dried, can be engineered to form large heaps or lagoons (Figure 3). These tailing ponds are easily distinguishable from the counterpart spoil heaps, which are typically unstructured and formless deposits at the ground surface.

Figure 2. Schematic of the general mining process and associated by-products (Figure from Ground Truth Trekking.org)

Figure 3. Settling ponds at a Kaolin mine in Lee Moor, Devon, as shown from google maps satellite (left). The unvegetated perimeter of a former coal mine tailings pond at Fairburn Ings in West Yorkshire (right)

Colliery spoil or Metalliferous mine spoil Differences in the target ore and the subsequent processing techniques are the primary factors that distinguish metalliferous and coal mining. However, the major principles of extraction are the same, and many of the types of environmental hazards that persist following the mines operational life-time are consistent for both colliery and precious metal mines. Mining operations typically follow the path of an ore vein that is rich in a target mineral. However, metal rich deposits will also be concentrated in a range of other minerals that become a source of potentially toxic elements (PTE), such as arsenopyrite (As), chalcopyrite and tetrahedrite (Cu), galena (Pb) and sphalerite (Zn). When brought to the surface, conditions quickly become acidic due to the oxidation of pyritic and other sulphides, which results in the formation of soluble PTE species that can migrate through pore water and into surrounding water courses, or will remain within the spoil in a plant available form, thus further inhibiting biological activity of both vegetation and soil dwelling micro-organisms. An example of lead and zinc concentrations observed in various metal mine spoil heaps is given in Table 1, with reference to soil guideline values for context. Although colliery spoil does not generally contain the extreme concentrations of PTEs that are demonstrated on metal mining sites, acidic discharge, lack of nutrient or substrate, and weak or no vegetation growth are common factors associated with both types of mining spoil.

Table 1. Mean Pb and Zn concentrations in various soils contaminated by mine spoil

Author Area of Study Pb (mg/kg) Zn (mg/kg) EA Soil Guideline Limit - 450 36 Yanqun et al 2004 Lanping, China 1632 2205 Won Seo et al 2008 Gahak, Korea 4071 3832 Tandy et al 2008 Anglesey, Wales 5041 214 Kucharski et al 2004 Warynski, Poland 9712 11498 Ash 2007 Devon, England 3952 90 Conesa et al 2007 Belleza, Spain 7000 5400 Conesa et al 2007 Brunita, Spain 1800 2000

Environmental implications of untreated mine spoil

Acid Mine Drainage Acid mine drainage (AMD) is formed when certain sulfide minerals in rocks are exposed to oxidizing conditions. The acidification of spoil and the discharge of acidic pore water into the local surroundings results in certain environmental consequences: • Directly excludes the establishment of non-acid tolerant plant species • Increases the total proportion (%) of heavy metals that are bioavailable to vegetation and soil micro/macro fauna • Results in acid mine drainage, leading to contamination of ground water and surface water courses

Figure 4. Top left and top right - Pooling water from a streamlet at Fairburn Ings colliery spoil showing the red ochre colour characteristic of iron oxide. Bottom left and bottom right - The white milky substance is likely caused by precipitation of aluminium oxide

One of the predominant manifestations of AMD is the visible precipitation of iron oxide in water courses on and around the mine spoil. The acidic nature of the spoil causes the solubility of iron, which is leached out during periods of rain into local water bodies. As the distance away from the spoil increases, so does the pH of the water, and so the iron precipitates into its solid oxide form. This is evident without any need for measurement or analysis, due to the characteristic red/orange ochre colour of the water, which is also present as a coating on the surface of any affected stream or pond bed (Figure 4).

Causes of acidity

When spoil is deposited at the land surface, sulphide bearing minerals e.g., pyrite and marcasite (FeS2), galena (PbS), chalcopyrite (CuFeS2), covellite (CuS), arsenopyrite (FeAsS), come into contact with air and water, where they are oxidized, resulting in the formation of sulphuric acid. A series of reactions, as shown below, involving the oxidation of ferrous iron to ferric iron, results in the consecutive release of protons, which further increase acidity.

2+ 2- + Step 1: FeS2 + 7/2 O2 + H2O = Fe + 2 SO4 + 2 H 2+ + 3+ Step 2: Fe + 1/4 O2 + H = Fe + ½ H2O 3+ + Step 3: Fe + 3 H2O = Fe(OH)3 + 3 H 3+ 2+ 2- + Step 4: FeS2 + 14 Fe + 8 H2O = 15 Fe + 2 SO4 + 16 H

If any of the processes represented by the equations above were slowed or altogether stopped, the generation of AMD would also slow or cease. Removal of air and/or water from the system (two of the three principal reactants), would stop pyrite from being oxidized. This is largely achieved by the addition of a deep substrate that is capable of supporting vegetation.

Figure 5. Where there are acid-forming conditions in the environment it is typical to observe a myriad of colours due to solubility and precipitation reactions, and oxidation/reduction of ferrous and other metal bearing compounds. Typical examples are the red-orange hues of iron, greens of copper and chromium and silver-greys of manganese

Enhanced concentrations and bioavailability of potentially toxic elements

Elevated concentrations of potentially toxic elements that are associated with mine spoil become further exacerbated due to the oxidation of sulphides and subsequent acidification of mine spoil leachates. The relationships between metal behaviour and pH can be numerous and complex, with a range of different species and bonding interactions occurring for each element. However, the general association between pH and the bioavailability of most heavy metals can be summarized in a simplified way (Figure 6), shown here as the amount of elemental metal that is bound to solid surfaces under a range of different pH values from pH 3 – pH 8. Generally, as pH decreases, less metal is bound to the surfaces of the solid particles, and therefore more of the metal becomes bioavailable as it is more soluble and freely available for uptake by plants.

Figure 6. A general interpretation of selected metal solubility as a function of pH, shown here as amount of elemental cation bound (sorbed) to the surface of ferrihydrite (Figure from Appelo and Postma 2005)

Physical stability

The root cause of all environmental damage associated with mining spoil is the lack of substrate; this is the core principle that underlies ‘spoil to soil’ restoration. Fundamentally, creating an ecologically sound and habitable substrate is the foundation for eliminating the range of problems caused by bare spoil. One of which is the physical stability of the spoil. Spoil is coarse, structureless and unstable when left uncovered, which is associated with various issues:

• Land slip and subsidence • Aeolian transport of particles (wind deposition) • Fluvial transport of particles (water deposition) Due to the inhospitable conditions of spoil that inhibits growth of vegetation, the spoil is constantly exposed to the elements. Some pioneer plants such as mosses and grasses may temporarily grow on the surface of the spoil, but small disturbances such as bird or animal movement, and extreme weather can easily dislodge the plants, thus creating a persistently uncovered surface with little to no chance of a natural succession of plant life. Without the possibility for pioneer plants to establish themselves, there is no ‘paving the way’ for intermediate plants that could build up a soil naturally, and there is no chance for a climax community, which we recognise as a woodland with a diverse species composition.

Figure 7. Aeolian processes that can transport contaminated particles from bare spoil (left), (Huggett 2011). Example of a highly eroded bare spoil bank (right)

Figure 8. Erosion on the slope of a spoil bank in West Yorkshire due to inability of vegetation to establish. The right image is the same site as seen from above

Siltation

One of the greatest threats to the wider environment by untreated spoil is the transport of sediment into local water courses. Once rain water falls onto the spoil surface, its movement is dependent on three main processes, surface runoff, percolation, and evapotranspiration. Although spoil is quite free-draining, due to its grit and sand-like particles, there is also a rapid over-land runoff as surface water. This surface flow is enhanced due to a lack of vegetation, which would otherwise intercept and remove rainwater partially as evapotranspiration. The increased flow of surface water accelerates the erosional process, whereby the energy of flowing water concentrates to take the path of least resistance, forming stream like channels that dissect the spoil at numerous points. Figure 9 illustrates this movement of surface water 9

on a coal spoil heap in West Yorkshire. In steep parts of the slope, the water moves with high velocity, carrying the contaminated spoil particles until it reaches a flat area, at which point the waters energy decreases, and so deposition of finer particles occurs with the ponding water. Sulphide deposition in such areas, followed by their oxidation due to contact with the air will result in the formation of sulphuric acid, and so areas of ponding and siltation in water courses are also likely to be extremely acidic. Some weathering experiments on spoil have shown a drop of pH in the leachate from pH 7 to pH 3 within one year (Wiggering 1993).

Figure 9. Examples of water erosion and particle transport on colliery spoil. The upper two pictures show streams and channel-like features that have cut their way into the spoil. The bottom image shows an area where multiple stream inlets meet, the pH here is very acidic and plant life is scarce

Streams, tributaries or major rivers and estuaries become the discharge point for leachate and sediments being carried from the spoil heap, depending on the spoil deposits location. Accordingly, over time the finer particles that were brought up to the surface with the mineral rich ores when the ground was mined will be transported into these waters. The effects of this have been long evident. In the mineral rich regions of Devonshire and Cornwall where large-scale mining has taken place for centuries, river beds such as those of the Tamar River and Plym Estuary have become so heavily silted, that salt marshes and flood plains now exist where shipping ports once stood (Figure 10).

Figure 10. Siltation of the Plym estuary due to historic metaliferous mining as seen from google earth (left), and looking across the estuary at low tide (right)

Siltation of watercourses from spoil heaps has a marked effect on the water quality and the diversity of species. Besides the aesthetic degradation of a river affected by heavy siltation, the environmental consequences are numerous;

• Silt clogs the gills of fish causing suffocation • Siltation destroys fishes spawning sites • Silt and sediment destroys invertebrate habitats on the river bed, removing a vital food source at the bottom of the food chain • Silt coats the leaves of aquatic plants, hindering their growth and development • Build-up of silt and sediments can cause the blockage of water ways resulting in localized flooding

In addition to the direct effects of siltation, the acidification of water and coating of fauna and flora with iron hydroxide diminishes the abundance of aquatic organisms, and depending on the extent of pollution, only highly tolerant species will persist in such places.

Figure 11. View from Fairburn Ings spoil tip in Yorkshire, which drains directly into the River Aire (left). Erosion channels on a spoil heap created by rain water runoff at Birch Coppice industrial park (right)

Fertility

Spoil contains various minerals that can potentially weather into nutrient bearing compounds or elements, which we recognise in plant fertilization, i.e Fe, Mg, S, Zn, Mn. However, bare spoil is largely inert and lacking in fertility. There is a common misconception that soil fertility is a measure of the amount of nutrients in the ground, however, in reality, the fertility of a soil is a reflection of its ability to bind, release, and cycle nutrients within a balanced and sustained system.

Low cation exchange capacity The reason that spoil cannot bind nutrients is because the particles are made up largely of dense, non-porous silicaceous gravel-like material, which has little to no exchangeable surfaces.

The main sources of cation exchange surfaces in soil are organic matter, clays, and amorphous oxides of Fe/Al/Mn. Without these, the substrate has no potential for ‘holding onto’ nutrients.

K+

Mn2+ Cu2+

Figure 12. Organic matter has an abundance of cation exchange surfaces available for nutrient bonding due to hydroxyl and carboxyl groups in fluvic, humic, and simple organic acids (Image adapted from Brady and Weil 2004)

Restoration

Principles of restoration

Metalliferous and colliery spoil deposits are highly unfavourable environments for plants. Vegetation growth is excluded due to a myriad of growth limiting factors. High levels of residual heavy metals, soil acidity, a lack of organic matter and associated nutrients, and poor substrate structure with only skeletal materials are the principal reasons that mine spoil remains bare, decades after its deposition.

The aim of revegetation is to stabilize the former mining site, by providing a cover crop that will: A) prevent dispersion of metal-contaminated and acid causing particles by water and wind erosion B) Stop the acid forming conditions by blocking the pathway between air and rain, and the pyritic sulphides in spoil C) Create an ecologically balanced system, where a succession of plant life can flourish, providing a visually pleasing and stable environment that has both ecological and aesthetic value The focus of reclamation efforts is therefore on creating a substrate that will remove the primary inhibiting factors that exclude plant growth. The principles of beneficial substrate formation can be summarized by a simple analogy that is already visible on untreated spoil, involving the addition of nutrient and organic matter to spoil in the form of animal droppings. Figure 13 illustrates how the addition of nutrient rich soil amendments can help to build a microenvironment suitable for the establishment of plant life. In the left image, rabbit droppings and/or other animal faeces become concentrated in an area favoured by the animal. The addition of nitrogen, phosphate, plus other trace nutrients and organic matter becomes a life source for simple pioneer plants such as grasses and mosses. The new grass has become a popular grazing spot on otherwise bare spoil, which accumulates more droppings and so gives the vegetation a chance to establish itself due to sufficient inputs of material vital to substrate formation. This process is analogous to the spoil to soil approach, which is based on a substantial incorporation of organic biodegradable residues (e.g. sewage sludge, manure, compost) that give an instant substrate, ready for seeding and/or planting according to the desired end use. These materials play three major roles when applied to mine soils: (i) improvement of the physical nature of the rooting medium, especially improving water and nutrient holding capacity; (ii) supply of plant nutrients in a slow release form, facilitating plant establishment; (iii) Create a suitable environment for soil micro and macro fauna that will sustain a healthy and functioning ecosystem

Figure 13. The natural process of soil formation triggered by concentrated animal droppings

Liming

In addition to supplying nutrient and a viable substrate, a key factor that remains to be addressed is the spoils acidity. Although the presence of a cover will contribute to a decrease in sulphide oxidation, a crucial part of the spoil to soil reclamation is to tackle the soil acidity directly using lime or other acid-neutralizing amendments. The combined provision of lime and organics to spoil addresses the major constraints to vegetation development, bringing together a multitude of beneficial effects that characterize a successfully reclaimed spoil heap. Phytostabilization

Phytostabilization is the end goal of all of the restoration practices. Once established, vegetation will continuously improve the chemical and biological characteristics of the contaminated spoil by increasing the organic matter content, nutrient levels, cation exchange capacity and biological activity, without further need for intervention. The main principles of phytostabilization are summarized in Figure 14. Further to the mitigating effects on the immediate environment, another significant benefit to revegetating quite vast areas such as former mining sites is the carbon sequestering capabilities of green spaces and woodlands. A review of carbon uptake in British forests and woodlands by Milne and Brown (2002) suggested that conifer woodlands in the UK accrue carbon in forest floor litter at an average rate of 0.25-0.32 t/ha per year, whilst broadleaf forests store carbon in litter at an approximate rate of 0.24 to 0.28 t/ha per year. Other models predict carbon storage in the whole biomass of various forest types to be in the range of 2.4 – 3.6 tonnes of carbon per hectare per year.

The creation of a sufficient organic substrate is the key to longevity and lasting success of restoration on forming mining sites, because it provides the fundamental foundation for vegetation to establish itself. Addressing the acidity, poor nutrient status and a lack of substrate enables the plants to overcome the initial barriers of establishment on spoil tips, and once present, nature takes over to create a self-sufficient ecosystem in which all spheres of the environment are balanced.

Figure 14. Schematic showing the processes and associated benefits of a phyto-stabilised substrate (Image from WordPress.com)

Figure 15. Failed vegetation on a spoil heap at a West Midlands industrial park where no organic amendment was used to create a substrate

Restoration examples

Unaided vegetation of mine spoil can take decades or centuries, and effective plant establishment will never be as prominent on a non-remediated site than on a remediated one. Large-scale restoration depends on the import of significant quantities of inorganic and organic wastes such as gypsum, biosolids, compost-like organics from anaerobic digestion facilities, wood chips and dredgings, among others, and the type and mix of wastes used are tailored to the conditions of each individual site. The following figures demonstrate successful cultivation of former mining sites, which have been restored using the approaches outlined in this document.

Figure 16. Dubbers restoration site in Cornwall before seeding in July (Top) and after seeding in the following January (Bottom)

Figure 17. Views from Seven Sisters colliery spoil in Wales, before and after restoration

Figure 18. Hatfield former colliery mine in South Yorkshire, at different stages of the restoration process

Figure 19. Langton spoil tip, before and after restoration – Residual coal was recovered from the spoil tip by a washing process, the remaining colliery discard was restored with lime and organics to create three habitats

Summary

Detriment of leaving a former mining site bare

• Acid forming conditions: sulphide oxidation on bare spoil creates extreme acidity, which excludes plant growth, increases the toxicity of heavy metals, and leads to contamination of local waters by way of acid mine drainage • Potentially toxic elements: Heavy metals and metalloids that are often present in spoil in high concentrations are exposed to air and surface waters, causing them to accumulate in the surrounding environment • Erosion of spoil: Without a sufficient cover, spoil is exposed to the elements, resulting in subsidence, wind erosion and subsequent deposition of potentially contaminated particles, and massive erosion by water which leads to contamination of water courses • Siltation of water: Movement of loose particles from spoil into local streams and rivers results in significant build-up of sediment over time. This has numerous consequences including, damage to wildlife and aquatic habitats, and localised flooding • Low biodiversity: Inability of plant establishment, and exposure of biota to toxic and acidic conditions leads to a relatively sterile environment with poor biodiversity at all levels of the food chain • Visual eyesore: Spoil heaps disrupt the aesthetic appearance of the landscape. Standing testament to the anthropogenic activities of the past, the unvegetated gravelly spoil tips do not fit into the surrounding environment

Benefits of restoration

• Ameliorates acidity: Incorporation of lime neutralises the present acidity and will help buffer against any potential further acid formation. The substrate and vegetation that it supports forms a physical barrier that removes the pathway between air/water and pyritic sulphides present in spoil, thus rendering them unreactive • Blocks transport of contaminants: A physical barrier between potentially toxic metals in spoil and the natural elements that transport them is created by the thick organic substrate and vegetation, therefore, the potential for these contaminants to migrate into the surrounding environment is removed • Stops erosion of spoil: The newly established vegetation prevents contact of spoil particles with strong winds, and evapotranspiration of rain reduces both the percolation and overland flow of water, which protects the spoil heap from erosion • Newly formed habitat: Depending on the desired end use, the organic substrate can support grass and/or trees, providing a number of key habitats for wildlife, creating a whole new ecosystem • Carbon sink: Spoil restoration creates a vegetated area on land that was previously bare. Whether grass or woodland, this newly created habitat provides a valuable ecosystem service in the form of carbon sequestration and long-term storage of carbon in the biomass and in the soil organic matter • Recreational value: Grass and woodland that a restoration site supports can offer various amenity and recreational options, such as hiking, dog walking, bird watching, or leisure and sporting activities • Aesthetically pleasing site: The green, vegetated appearance of a restored site is in stark contrast to the bleak, grey bare spoil. A restored site blends seamlessly into the surrounding countryside, raising the aesthetic value of an area

Contact

For more information about the restoration work that 4R Group does, call 0800 0121 769 for an informal discussion, or contact by email ([email protected]) with any questions or queries

Visit our website for more information about 4R-Group and our operations: www.4r-group.co.uk

4R Group Technical director: Paul Whyatt, email [email protected]

4R Group Restoration operations manager: Graham Roberts, email graham.roberts@4r- group.co.uk

This document was prepared by Dr Christopher Ash Environmental consultant at 4R Group, email [email protected]