Assessing Options for Remediation of Contaminated Mine Site Drainage Entering the River Teign, Southwest England

： The river Teign in Devon has come under scrutiny for failing to meet Environmental 13 Quality Standards for ecotoxic metals due to past mining operations. A disused mine known as 14 Bridford Barytes mine, has been found to contribute a significant source of Zn, Cd and Pb to the 15 river. Recently, studies have been focused on the remediation of such mine sites using low-cost 16 treatment methods to help reduce metal loads to the river downstream. This paper explores the 17 metal removal efficiency of red mud, a waste product from the aluminium industry, which has 18 proven to be an attractive low-cost treatment method for adsorbing toxic metals. Adsorption 19 kinetics and capacity experiments reveal metal removal efficiencies of up to 70% within the first 2 20 hours when red mud is applied in pelletized form. Also, it highlights the potential of biochar, 21 another effective adsorbent observed to remove >90% Zn using agricultural feedstock. Compliance 22 of the Teign has been investigated by analysing dissolved metal concentrations and bioavailable 23 fractions of Zn to assess if levels are of environmental concern. By applying a Real-World 24 Application Model, this study reveals that compressed pellets and agricultural biochar offer an 25 effective, low-cost option to reducing metal concentrations and thus improving the quality of the 26 river Teign.


INTRODUCTION
Historic mining in the southwest of England has left a legacy of environmental and socioeconomic impacts. Whilst mining operations have largely ceased throughout Devon and Cornwall, impacts have persisted resulting in localised contamination and elevated metal concentrations in soils, sediment, and waters. In England, pollution from mine waste affects over 1,700km of rivers [1] with the potential to reduce the quality of drinking water and threaten sensitive aquatic ecosystems. This legacy presents a challenge in achieving the requirements set out by the Water Framework Directive (WFD) (Directive 2000/60/EC) which has established Environmental Quality Standards (EQS) for specific pollutants such as arsenic (As), zinc (Zn), copper (Cu), iron (Fe), chromium (Cr) and manganese (Mn), Priority Substances such as lead (Pb) and Priority Hazardous Substances such as cadmium (Cd).
Meeting the standards and protecting the quality of our water bodies is therefore of fundamental importance.
The river Teign, sourced in Dartmoor, Devon, is at risk of not meeting the requirements set out by the WFD and forms the focus of this study. Exploitation of mineral resources at a local disused mine in Bridford, known as Bridford Barytes mine, have contributed to elevated concentrations of potentially toxic metals. Mining for baryte (barium sulphate) took place between 1855 and 1958, however prior to this, Pb-Zn mining occurred within the catchment [2] . Both episodes have been responsible for releasing potentially ecotoxic metals into the river Teign and monitoring data has consistently shown exceedances in metal concentrations, particularly Zn which presents the basis for this investigation.
Metals sourced from mining operations are typically discharged from mine adits, where Acid Mine Drainage (AMD) is generated releasing trace metals into the environment with potentially adverse effects on the ecology. AMD is produced when sulphide-bearing minerals released from mining activities are exposed to atmospheric conditions. The most common sulphide mineral in this process is pyrite (FeS₂). The oxidation of pyrite leads to the generation of sulphate and an increase in proton acidity [3] , this reaction is responsible for considerable increases in acidity within the natural environment. Due to this increase in acidity, pH associated with AMD is typically below 4.0 [4] , in which metals are highly soluble and easily mobilised, commonly these metals include Mn, Cr, Cd, Zn, Pb and As.
Zn is one of the most encountered WFD specific pollutants from mining activities. It is a metal both essential and toxic to organisms, monitoring the concentration of Zn at the catchment scale is therefore critical to help sustain and preserve the environment. Notably, Zn is often present in high concentrations due to the background geology; this presents unique complications in assessing the risk of impacts. However, studies have shown that Zn in sediments of the river Teign and estuary are not entirely naturally occurring and are derived from mining pollution [2] . These elevated levels of Zn have been attributed to the episodes of Ba and Pb-Zn mining throughout the Teign catchment including a major source at Bridford Barytes mine. In catchments affected by AMD, Zn is commonly present in its most ecotoxic form Zn 2+ , as is the case with the river Teign. The release of this hydrated Zn ion into the environment is toxic to aquatic biota at elevated concentrations, with reports of reproductive and developmental responses in fish and other aquatic organisms [5] . With regards to human health, long term excessive exposure has been identified as a contributing factor to chronic diseases, a decrease in immune system function and even infertility [6,7] . Preventing such adverse effects to aquatic life and human health is the driving force behind environmental legislation.
Over the years, growing concern for the environment and human health has led to an increase in legislation governing pollution associated with the mining industry. The WFD has become one of the most influential pieces of EU law concerning water pollution and the quality of our water bodies. The directive is built upon the principles of sustainable development and requires the development of management strategies referred to as River Basin Management Plans (RBMP). It also requires member states to classify the ecological quality of waters as either high, good, or moderate; with pass/fail Environmental Quality Standards (EQS) for chemicals of concern. To achieve good status, all the chemical and ecological parameters have to be 'good' as it is a one out-all out assessment. Currently the lower Teign catchment only achieves 'moderate' status for failing to meet the standards required for good ecological classification and for periodic failures of the Zn EQS [8] . Consequently, understanding the contribution of Zn to this river catchment and undertaking appropriate mitigation is key to meeting the demands of the WFD and is the rationale behind this study.
The EQS established by the WFD are based upon recommendations from the United Kingdom Technical Advisory Group (UKTAG) and are monitored closely by the Environment Agency.
They are derived from present scientific understanding of the conditions needed for a healthy water environment and utilise ecological data from thousands of sites across the UK. The revised standard for Zn in freshwaters is currently 10.9 µg/l bioavailable plus the ambient background concentration 2.9 µg/l [9] . Importantly, the chemical form of zinc is greatly influenced by the hydrological and physiochemical conditions of the water [10] . Metalconcentrations, pH conditions and amount of organic matter all control the bioavailability and toxicity of zinc [11] , not considering the bioavailable fraction of zinc may result in an under or over estimation of the risks posed by the metal. These influences are therefore an important consideration when assessing if a water body is in fact 'failing' due to the presence of the metal and is consequently an environmental concern.
Practical and cost-effective treatment for mine water is topical and extensive research has been undertaken to assess the application of different treatment methods in the UK. The degree of environmental pollution generated by AMD is highly variable, meaning treatment must be flexible and specific to each site. Passive methods to remove heavy metal ions are currently favoured due to their low cost and local availability, with techniques including constructed wetlands, limestone for neutralisation, precipitation, and adsorption [12] .
Red media is an emerging low-cost technology which utilises bauxite residue from the aluminium mining process to form pellets which have the capacity to adsorb heavy metals [13] .
The pellets can be pre-treated to increase the efficiency of adsorption, simultaneously offering flexibility in the design and operation of the method. Importantly, studies have shown that the pre-treatment of pellets is essential in the adsorption process and hence determines the overall effectiveness of removing heavy metals from mine water [14] . This report aims to evaluate the feasibility of each treatment method, weighing up the benefits and costs to see which method will be most applicable for reducing metal loads to the Teign. Principally it focuses on Zn, however the removal efficiencies for the priority substances Cd and Pb have also been considered for comparison.
Ultimately, information collated on these studies of different treatment methods are employed by the competent authorities (Environment Agency and Coal Authority) to develop and build mine water treatment schemes to clean up our waters where the quality has been compromised by pollution from abandoned mine sites [1] . Currently, one of the greatest challenges in treating pollution generated from AMD is finding a method that meets the expectations of efficiency, cost, and sustainability. The objective of this study is to assess the necessity of Zn, Cd and Pb removal in the river Teign and evaluate the efficiency of treatment methods that utilise red media. The results will enable an assessment of the practicalities associated with reducing Zn loads to the catchment, and an overall more comprehensive understanding of adopting low-cost adsorption treatments to mine sites in the UK.

Study Area
The study is based on a former baryte mine in Bridford, situated south west of Exeter (SX83148643). The mine is located within the Teign valley on the north eastern edge of Dartmoor ( Figure 1) where metalliferous mineral deposits have been extracted since the bronze age due to the presence of a large granite batholith. Mineral deposits in the area consist mainly of shales, mudstones, cherts and tuffs, also known as the Culm measures; these deposits contain the Ba-Pb-Zn loads [15] . Initially, Pb mining took place at the site dated at around 1804, however, low profits moved production to Ba in 1855, with final abandonment of the mine in 1958 [2,16] . water courses currently exceed the Zn EQS [14,16] . However, these tributaries comprise a small area of the catchment (approximately 6km 2 out of 540km 2 for the Teign catchment [17] ) and at first instance, seem unlikely to contribute greatly to the elevated Zn concentrations of the Teign.

Red Media Technology
Red Media Technology has been trialling the capability of 'Red Mud' (RM) for adsorbing heavy metals from discharged mine waters. Millions of tonnes of hazardous RM waste is produced each year as a by-product of the aluminium industry; the utilization of this material therefore supports the concept of waste-recycling. Importantly, it is highly alkaline with a pH of 10-13, the red colour comes from the presence of oxidised iron which comprises up to 60% of the mass of the product [18] . The RM is in pellet form, pre-treatment of the pellets via heat and acid treatment has been found to increase adsorption and the removal efficiency of heavy metals [19] . Previous laboratory studies have investigated the capabilities of four different types of pellet which have undergone treatment: Compressed (CP), fired (FP), fired-acid-etched (FAE) and a new powdered pellet (PP). These treatment methods have been described and their adsorption efficiencies compared and outlined in this methodology. They have also been compared with a field scale trial using the RM and another treatment method utilising biochar, a carbon-rich material produced from plant biomass.

Compressed Pellet
Compressed pellets have been tested in the laboratory and during a field scale trial by Hill (2016) [14] and Comber (2015) [16] respectively. CP have been compacted under high pressure, forming small and crumbly pellets of varying sizes ( Figure 2a) [14] . They have lost porosity during compaction and have a high surface area; however, the field trial shows that the pellets lack structural integrity and suffered degradation during the experiment [16] .

Fired Pellet
Lab based experiments using fired pellets have been conducted by Hill (2016) [14] and Turner (2017) [13] . Production of the FP involves heating in a kiln at 1050°C for 2 hours and allowed to cool for a further 2 days [14] . The pellets are more uniform in size, with a coarse texture and an overall lower surface area compared with the compressed pellets ( Figure 2b) [13] .

Fired Acid Etched Pellet
The adsorption efficiency of the fired acid etched pellets have been tested by Turner (2017) [13] , where they are described as small, bright orange pellets with a powdery texture and a smoother surface produced from etching ( Figure 2c).

Powdered Pellet
The powdered pellets described by Turner (2017) [13] , are similar in appearance to the compressed pellets, with a cylindrical shape, powdery texture, and a dark orange colour, fired at 800°C.

Current studies using Red Media Technology products
Laboratory studies have been undertaken to assess the removal efficiency of pre-treated pellets [13,14] . Samples were collected from several different locations of the water course at Bridford along with in situ measurements of pH, temperature, dissolved oxygen content and redox potential. For the purpose of this study, only data from samples taken at the mine adit have been utilised. CP, FP, FAE and PP were supplied by Red Media Technologies to determine their metal removal efficiency and suitability to a mine environment [20] . An Pellets [14] . (B) Fired Pellets [14] . (C) Fired acid etched pellets [13] . Recoveries for Zn, Pb and Cd were 100% +/-10% and precision for the 3 replicate analyses for each sample were typically less than 5% relative standard deviation [13,14] . The pH was also tested at the start and end of the experiment to reveal any neutralising capabilities of the pellets [13] .
As well as a kinetics experiment, an adsorption capacity column experiment was undertaken to determine the adsorption behaviour and optimum capacity of the pellets in mg of metal sorbed/kg of media used. The adsorption capacity was calculated using the starting concentrations of the elements, the amount of solution which had flowed through them and the weight of pellets within the column (ESI, S2).The highest capacity achieved for each metal has been recorded [14] .
A field scale trial of the removal efficiency of toxic metals using the pellets was undertaken by Comber (2015) [16] in conjunction with Red Media Technology at Bridford Barytes mine, Bridford. The trial period was a duration of 3 months to assess the performance of the pellets on a realistic timescale. The experiment consisted of a 1m 3 tank containing compressed pellets; mine water was delivered to the tank and samples were taken throughout the operation, including pH readings. Metal concentrations were determined by ICP-MS.
Analyte concentration data collected from the laboratory tests and field scale trial have been used to calculate the metal removal efficiency of each pellet form, as well as the adsorption capacity and pH neutralising capability; the results will allow an evaluation of which pellet is most suitable for reducing the Zn load from Bridford to the river Teign.

Alternative Treatment Method using Biochar
Biochar is a black, carbon rich solid produced by thermal decomposition of biomass, similarly to charcoal. Typically, it has a wide range of characteristics which depend upon the feedstock used; this affects the chemical and physical properties of the biochar and consequently how it acts as an adsorbent. A lab study carried out by Roberts (2018) [21] , tests the sorption capabilities of pelletized biochar supplied by the United Kingdom Biochar Research Centre (UKBRC). Varying forms of feedstock were tested at different pyrolysis temperatures (550°C and 700°C) including forestry waste, municipal waste, and agricultural waste. Following a similar methodology to the experiments using red media, samples of mine water were taken from the adit and used in an adsorption kinetics experiment and a titration capacity experiment.
Metal concentrations were analysed by ICP-MS and ICP-OES as described above.

River Teign Metal Concentrations
The Environment Agency (EA) act as the competent authority to implement the requirements set out by the WFD and closely monitor the quality of water courses within England. Data provided by the EA's water quality archive has been extracted to determine the mean concentrations of the river Teign for dissolved Zn from 2000 to 2020. Analysing total dissolved metal concentrations forms the first stage of a tiered approach to assessing the classification of a water body in the UK [10] , if the Teign exceeds the standard EQS value of 13.8, then it will progress to the next tier. Bioavailability data is accessible after 2015 from EA monitoring data, these results therefore have a high uncertainty.

Real-World Application Model
Using mean bioavailable metal concentration data and flow data from Chudleigh, the average load of Zn, Cd and Pb into and within the river Teign at Chudleigh has been calculated using the "Real-World Application Model" -(Hill 2016) [14] (ESI, S10). Acquiring the load data has enabled the determination of the tonnes of pellets/biochar feedstock required to reduce Zn levels below the EQS at Chudleigh.

Removal Efficiency Results
Zn concentration data at Bridford mine adit is presented in figure 3.    [13] and Hill (2016) [14] .  [13,14] The high removal efficiency of the CP is supported by the field scale trial conducted by Comber (2015) [16] . Figure 4 shows the removal efficiency of the pellets over a 3-month period, the results reveal >80% of the Zn is removed within the first 10 days of the experiment. After this period, the removal efficiency gradually falls until it remains at below 40% after 40 days. Cd and Pb follow a similar trend but with a marked increase in removal after 70 days. These results suggest that the RM pellets require at least 2 hours to be efficient and achieve >70% removal, uptake is reduced greatly up to 24 hours and beyond (ESI, S6).  [16] .

Hours
Removal Efficiency for zinc (%) When compared with the priority substances Cd and Pb ( Figure 5), Zn appears to have the most similar adsorption rate to Cd, which is highest when influenced by the CP and lowest with the FP and FAE pellets. The results show Pb to have the greatest removal compared to Cd and Zn with all the pellets, especially the CP which exhibit nearly 100% removal efficiency.
Notably, the data only shows results for a 2-hour duration; the FP are recognized to have the slowest removal efficiency during this time despite the greatest overall removal efficiency as demonstrated by figure 3 [13] .

River Teign Metal Concentration Results
Selected sample locations downstream of Bridford mine are presented in figure 7 with average dissolved Zn concentrations at each locality calculated from 2000-2020. Estimates of Zn concentrations at the adit have been made: 11,170µg/l [22] , 8,911µg/l [14] , 11, 200µg/l [13] and 11,400µg/l [16] . These values show that the adit acts as a point source of consistently high Zn values of around 11,000µg/l. Downstream of the adit, mine waters enter the Rookery brook where average Zn concentrations are 471.8µg/l, this is considerably higher than upstream values of 49µg/l documented by Hill (2016) [14] , owing to the mine discharge from Bridford.  times the EQS. Cd follows a similar trend but appears to be steadily declining in recent years to 0.19µg/l, 2 times the EQS. Pb levels, however, continue to stay below the EQS at 2.65µg/l. Pb EQS mean Pb from 24.2 to 37.3mg/l. Together, these results show that the bioavailable Zn fraction is high and exceeds the EQS at both Chudleigh bridge and Beadon brook ( Figure 8). Although, the data suggests that these levels are beginning to decline. For the river Teign at Preston, bioavailable Zn has stayed closely below the EQS, suggesting that the bioavailable proportion of Zn decreases downstream where it is less impacted by discharges from Bridford Barytes mine and Wheal Exmouth.

Figure 8. Annual mean bioavailable Zn levels for the river Teign at Chudleigh bridge (A), Beadon Brook (B) and Preston (C), compared to the bioavailable EQS for Zn.
Data calculated from EA water quality archive [8] .

Real-World Application Model
The mean bioavailable concentration of Zn at Chudleigh Bridge was 18.1µg/l in 2019. This was combined with flow data at Chudleigh (5.32m 3 /s) to estimate the annual load from Bridford downstream to the river Teign; the load was calculated to be 1210kg/yr. As well as this, the capacity of the pellets was retrieved from adsorption capacity experiments (ESI, S10). The CP were found to have a capacity of 8743mg/kg for Zn, and 35.40mg/kg for Cd. The duration of the experiment for the FP was limited, therefore maximum adsorption capacities were not reached, however the highest recorded capacities were >150mg/kg for Zn and >1.56mg/kg for Cd [14] . The most efficient biochar from the experiment was agricultural biochar, with a capacity of 11000mg/kg [23] . Compared to the capacity of the wood biochar which has been observed at 395.8mg/kg [24] . The results from the model are shown in table 3; estimates of the costs of the pellets have been calculated on the basis that 1 tonne of pellets costs £88.95 to dispose of at landfill [13] . The results from the model reveal that the agricultural biochar costs the least amount to reduce Zn levels in the Teign. However, the removal efficiency was only tested up to 2 hours, therefore this is not a realistic value as the removal efficiency is expected to drop over time.

Pellet Removal Efficiency
The data from this study suggests the most suited pellet for removing Zn loads from Bridford are the CP. Fast adsorption rates allow >70% of the metal to be absorbed within the first 2 hours of experimentation. This efficiency is supported by the field-scale trial, with rapid adsorption of Zn within the first 20 days (>80%). At the end of the 3-month trial, the removal efficiency drops to <40%, suggesting that the capacity of the pellets had not been exhausted.
However, precipitation becomes the dominant process blocking sorption sites and consequently lowering the adsorption efficiency [22] . The higher removal efficiency of the CP can be attributed to its higher surface area of 27.9m 2 /g compared with other pellets (35 times greater than the FP) [14] . Notably, the FP have a slower initial removal efficiency, yet remove 99.9% of Zn at 53 hours. Both pellets cause an immediate pH increase when added to solution, although the FP result in the highest pH increase of 4.68, enabling the formation of precipitates such as iron hydroxide to further drive metal removal. Whilst the adsorption kinetics experiments have identified the CP and FP as having the greatest removal efficiency, the faster removal rate of the CP means a lower phase-contact time is needed between the pellets and water; this makes it more suited to a real-world application. Pb is observed to adsorb more strongly than Zn and Cd, this is possibly due to its greater partition coefficient [25] .
The results from the experiment are supported by other studies using RM pellets [26] . Crushed pellets with a greater surface area (like the CP) have been found to be most efficient, with enhanced metal adsorption taking place at an optimum pH of 5/6 for Zn. Significant uptake of Zn has been documented within the first few hours of experimentation, with a less pronounced uptake after 24 hours [27] , in line with the results from this study. Interestingly, the FAE pellets had a lower removal efficiency compared to the CP and PP; however other studies have proved acid treatment to be highly effective in aiding adsorption [18] .

Biochar Removal Efficiency
Previous studies have demonstrated the remediation potential of Biochar, particularly as a soil modification where application has been seen to reduce bioavailability of toxic metals and simultaneously promote plant growth. Maximum removal efficiencies (>95%) have been observed at high pyrolysis temperatures (650°C) which greatly influence the success of the treatment method [28] . Other parameters such as contact time, particle size and the type of biochar feedstock used have also been considered as important factors.
Results from this study have shown the type of feedstock to be an important influence on the removal efficiency of Zn, Cd and Pb, rather than pyrolysis temperature. Forestry feedstock had the lowest removal efficiency whilst agricultural waste had the overall highest. This can be explained by the pyrolysis temperature at which the biochar is produced at. Higher temperatures produce a higher ash content which raises the pH and consequently aids metal adsorption, with maximum adsorption recorded at pH 5 [29] , similarly to the RM pellets. This ash component is accountable for significant Pb immobilisation, explaining why Pb had the greatest adsorption rate in the experiment. Forestry waste has a low ash content and hence low adsorption rates. Other studies support this concept where lower pH (7.9) has been observed in wood biochars, compared to other feedstock which significantly increases the pH to 9 and above [24] .  [31] . Therefore, the effects of Zn may not be as damaging to ecosystems as studies suggest.

River Teign Compliance
Cd levels in the Teign also exceed the EQS and are rising at some of the sample locations.
Independently, the impacts of Cd and resulting effects on ecosystems are beyond the scope of this project. However, the synergistic effects of metals such as Zn, Cd and Pb together have been documented and observed to increase fish mortality [5] . It is therefore important to investigate the effects of combinations of metals to assess the threats posed to the environment.

Application to Bridford Mine
According to the real -world application model, the CP and agricultural biochar are the most promising treatment methods for adsorbing Zn at Bridford mine. The removal efficiency of the RM pellets is a result of pre-treatment which affects the porosity, surface area and adsorption capacity of the pellets. The CP have the highest adsorption capacity due to their larger surface area and would ultimately require less production and lower disposal costs. The FP have a much lower adsorption capacity, resulting in the need for 58 times more tonnes of pellets a year compared to the CP. Hill (2016) [14] and Turner (2017) [13] similarly found that you would need 44 times more FP than CP to efficiently remove Zn at the mine site. However, despite the slower adsorption rate of the FP, its ability to significantly raise pH may prove useful for increasing precipitation reactions and consequently removing metals via the formation of hydroxides. The PP have potential for effectively removing metals at Bridford, however adsorption capacity data and a field scale trial would be necessary.
Despite the success of the CP, the field scale trial by Comber (2015) [16] highlighted a few issues that may affect the pellets ability to act as an adsorbent. Firstly, the pellets lacked rigidity, resulting in a loss of structural integrity during the trial; this is problematic for a realistic application of the treatment method. Also, the precipitation of ochre (iron hydroxide) resulted in a build-up of iron on the pellet surface, blocking adsorption sites. Although, it also leads to an increase in co-precipitation of other metals, thus limiting the mobility of dissolved metals in the mine water [32] .
Field scale trials of biochar treatment have shown that the effectiveness decreases over time (biochar ageing effect) [33] . However, unlike the CP, biochar is persistent in the environment and its application may be prolonged. This is particularly the case with high temperature biochars which have a greater carbon stability [34] , making it a more effective adsorbent.
Biochar therefore offers an attractive remediation alternative to the RM pellets. Although, the effects of potentially hazardous substances in biochars because of the feedstock used and the pyrolysis process are still largely unknown [35] .
Notably, RM is a toxic by-product of the aluminium production process. Each year, 90 million tonnes of RM are produced globally, making it widely available as an adsorbent [18] . Due to its hazardous nature, disposal is costly, valued at £88.95 per tonne as from April 2018 [13] .
Currently, the pellets can be disposed of in mine tailings in agreement with the EA, however, where this is not possible, they are sent to an inert landfill. One viable solution to reduce disposal costs would be to drain the pellets after use to achieve a greater % of dry weight [14] .
Also, to further increase the efficiency of the pellets, a cell-based system could be used where pellets are placed successively next to each other. This design would ensure that pellet capacity is not all exhausted at once, prolonging their effect of metal removal.
Adsorption is an economical remediation technique, owing to the abundance of waste materials, their low cost, and high capacities. It is a much more practical option for mine sites than the current most widely used treatment method activated carbon (AC). AC is inaccessible for most remediation projects due to its high cost, which is typically more than 1000 Euros/tonne, equivalent to £914.281/tonne [36] . The metal-removing capabilities shown by the RM pellets and biochar are therefore more suited to application at Bridford mine than limited methods like activated carbon.
Realistically, for the river Teign to comply with water quality standards, other inputs need to be addressed. Whilst Bridford mine is a significant source of Zn, it cannot solely be accounted and 1874), outputs of Pb and Zn are estimated to be 11,759 tonnes and 1589 tonnes respectively [2] . Treatment of mine water at Wheal Exmouth is necessary to reduce metal concentrations below the EQS, particularly in the case of Cd which would require 898 tonnes of pellets a year applied at Bridford alone to reduce levels below the EQS. Moreover, mine adits only represent point sources of pollution, diffuse sources such as runoff from tailings and road surfaces should also be investigated for their contribution to Zn, Cd and Pb levels.

CONCLUSIONS
This study has highlighted the long-term impact of historical mining on our local water resources, demonstrating the need for protection and assurance of water quality, implemented by key legislation like the WFD.
The consistent exceedance of Zn and Cd environmental quality standards in the river Teign has formed the rationale for evaluating potential treatment methods. Adsorption techniques for mine remediation are topical due to their low cost and abundance; this study has proven the potential for pelletized RM and biochar as effective adsorbents. Pellets with a greater surface area and higher adsorption capacity such as the CP demonstrate high removal efficiencies for Zn, Cd and Pb. Agricultural biochar formed at high pyrolysis temperatures has also been observed as a promising material for removing ecotoxic metals. Limited data on the FAE pellets and PP meant that their application to a mine site could not be determined, however, they do exhibit neutralizing capabilities as well as effective adsorption.
Treatment methods need to follow the principle of sustainable development by improving the status of a water body whilst considering the costs and benefits of their application. Reusing the hazardous RM as an adsorptive material supports this concept of sustainability, especially the CP which can be disposed of at only £34,067; this is much more economically viable than other treatment methods like activated carbon.