Potentially Acid Forming Research Articles

Identification of biotic and abiotic factors coal mine overburden on Warukin rock formation of South Kalimantan

Overburden (the dumping of mine tailings and other reject materials) from the Warukin geological formation is one of the potential acid-forming (PAF) of 33.17%. Mining can cause overburden to be exposed and affect biotic and abiotic conditions. The study aimed to analyze the biotic and abiotic factors of three different age overburdens from the Warukin Formation, South Kalimantan. Biotic factor measurements included vegetation sampling using line transect, soil insects using pitfall traps, and microbial using total plate count. Abiotic factor measurements included air temperature and humidity, light intensity, temperature, moisture, and pH of the substrate (n=3), and chemical overburden. Identification of plants and insect species using information from locals, identification books, and websites. Plant species diversity was analyzed using the Shannon-Wiener Index. Dominance species were analyzed using the Important Value Index (IVI). The relation of overburden age on biotic and abiotic factors was analyzed multivariate with Non-Metric Multidimensional Scaling (NMDS). The results showed that there are 102 species of plants and 11 species of soil insects. The numbers of fungi and bacteria are 10-4 CFU g-1 and 10-3 CFU g-1, respectively. Plant species diversity is classified as medium (H'= 2.34; 2.50 and 2.82). Changes in plant composition occurred as the microclimate and substrate nutrition improved. The organic matter increased over time. Humidity and light intensity influence the presence and composition of organisms, while pH values affect the adaptability of plants, insects, and microbes. Heavy metal concentrations have increased due to the physicochemical properties of the Warukin formation overburden.

Analysis of PAF and NAF Tests Using NAPP and ANC Methods at Hajak Site, North Barito, Central Borneo

Acid mine drainage (AMD) is a significant challenge for the global coal mining industry, necessitating specialized treatment to prevent its occurrence. A crucial step in AMD prevention is identifying rocks that contribute to its formation. These rocks are classified into Potential Acid-Forming (PAF) rocks and Non-Acid-Forming (NAF) rocks. PAF rocks have the potential to produce acid, while NAF rocks do not. Laboratory analyses, including Maximum Potential Acidity (MPA), Acid Neutralizing Capacity (ANC), Net Acid Producing Potential (NAPP), and Net Acid Generating (NAG) tests, identified 3 sample points with potential PAF characteristics out of 35 samples tested (see Table 3.1). Among these, 2 sample points were classified as Potential Acid Forming-Low Capacity, and 1 sample point was categorized as Potential Acid Forming-Medium Capacity (NAG pH 4.5, NAPP 10 kg H2SO4/ton). The acidity level of mine water resulting from landfilling varies significantly based on the mineral content and landfilling techniques used. Mined material with high carbonate mineral content tends to have lower acidity levels in leachate and can even neutralize formed acid. The stockpiling strategy implemented involves layering PAF material followed by a final cover of NAF material and a rooting zone to mitigate acid formation.

Comparative study of long-term laboratory and field kinetic test for potential acid forming (PAF) materials

Acid Mine Drainage (AMD) comes from open pit and underground mining activities, characterized by high level of acidity and high metal solubility. AMD is formed from sulfide minerals that are exposed from mining activities, undergo oxidation by oxygen and contact with rainwater. AMD formation reaction rate must be studied to prevent AMD from being released to the environment. Reaction rate of acid formation can be determined by kinetic test. This study used the Free Draining Column Leach (FDCL) test method for the kinetic test, performed in laboratory scale and field scale. Laboratory scale experiment was done in a cylinder reactor, with 15 cm diameter and 35 cm height. Meanwhile the field scale was carried out in greater amount of rock material and reactor volume. The test was carried out in a weekly cycle for a total of 83 weeks. This study states that field scale experiments tend to produce higher concentrations of ORP, conductivity, sulfate, iron (Fe) and manganese (Mn) rather than the laboratory scale. The pH parameter on field scale experiment is around 1.91 to 3.18, meanwhile on the laboratory scale experiment is around 2.18 to 3.47. It can be interpreted that the field scale experiment tends to produce more acidic pH compared to the laboratory scale experiment. This research is expected to find the differences between laboratory scale and field scale experiments in long-term kinetic tests.

Assessing phytoremediation potentials of heavy metals on selected plants species in the restoration environments of Gold Mining Area

Mining activity have impacts on the environment which potentially expose heavy metals. Therefore, it is necessary to design the reclamation in order to improve the restoration. This research aims to assess the phytoremediation of potential acid forming (PAF) reclamation area of Henny Dump. The research was carried out by identifying the plant species, and then measuring the plant vegetation index, and measuring the concentration of heavy metals. The research showed that Falcataria molucana (IVI: 150%) and Gmelina arborea (IVI: 150%) are species with the highest Important Value Index (IVI). The soil analysis result for total concentrations of ferrous (Fe), copper (Cu), zinc (Zn), and mangan (Mn) showed that the most significant decrease in the four-year period i.e., Ferrous. Falcataria molucanna and Gmelina arborea as the fast-growing species for restoring the reclamation area which can survive in low acidity concentrations and high of heavy metals ferrous concentrations. These fast-growing species are tolerant in high concentration of ferrous and low acidity of soil quality, as well have the potential to absorb the essential heavy metals of ferrous, where in 2019 the iron concentration was 39,575 ppm and in 2022 was 7,100 ppm

The Effect of Fly Ash Ratio Addition as Layering Material Using Free Draining Column Leach Test Method on Laboratory Scale

Acid mine drainage (AMD) is formed due to the oxidation of sulfide minerals in the presence of water and air. AMD already entering the environment can damage the aquatic ecosystem due to low pH and high dissolved metal concentrations. Efforts to prevent the formation of AMD are by regulating the stockpiling of materials containing sulfide or potential acid forming (PAF) with non-acid forming (NAF) to avoid oxidation reactions of sulfide minerals. The purpose of this study was to simulate the effect of adding fly ash ratio as a PAF rock coating material to the quality characteristics of leachate using the laboratory-scale free draining column leach test (FDCLT) method. Adding fly ash ratio as a layering material significantly affects the quality of the resulting leachate. This is due to the physical condition of the fly ash material, which cannot prevent the meeting of AMD-forming components.

Dolomite and organic compost application for preventing acid mine drainage formation in a mesocosm system

Acid mine drainage (AMD) is one of most serious environmental problems facing mining industries. It results from oxidation of sulfide minerals contained in mining wastes, such as rock disposals in mine sites. This study was to investigate AMD formation from waste rocks containing potentially-acid forming (PAF) materials treated with dolomite and compost in a mesocosm system. The system comprised a plastic drum of 150-cm height and 50-cm diameter filled with 15-cm sand at the botom layer, over-laid by 50-cm mixture of PAF+10% of either dolomite or compost, and 35-cm top-soil layer on the top. The drum surface of the sand layer was equipped with a valve to drain leached water into a collecting container. The control was a similar constructed mesocosm without addition of either ameliorant. The drums were installed into the ground of the mined land so that their surfaces were as high as the surrounding soil surface and left under field condition. The leachate samples were taken two-weekly in the early two months and monthly until the day 487. The system demonstrated that both dolomite and compost significantly increased pH from around 3 to pH>6 and reduced dissolved Al and Fe concentrations. However, the dissolved Mn concentration was still high (>20 mg L−1) in all treatments even at the pH reached >6. .

Mineralogical and Geochemical Characteristics of the H-Pit of the Chatree Gold Deposit, Central Thailand: A Case Study for Assessment of Acid Rock Drainage and Heavy Metal Sources

H-pit is one of the significant ore lenses of the Chatree mine in Thailand. Au-Ag mineralization mainly occurs as veins, stockworks, and minor breccias hosted by volcanic and volcaniclastic rocks. Disseminated pyrites are commonly present near mineralized veins in the hanging wall zone. This study aims to assess the acid rock drainage (ARD) potential and heavy metal content from the H-pit area. The results indicate that hanging wall rock is a potential acid-forming (PAF) material related to disseminated pyrite formed by hydrothermal alteration. In contrast, the footwall and ore zone materials are classified as non-acid forming (NAF). Because the ore zone has calcite in the veins, it may help buffer the material’s acidity. The results of heavy metal analysis reveal that the ore zone has significantly higher contents of As, Cd, Cu, Pb, and Zn than those in the hanging wall and footwall zones. Moreover, the hanging wall and footwall materials have exceeding values for As, Cd, and Zn compared to those in typical igneous rocks. These heavy metals are interpreted to be sourced from (1) the primary composition in base metal sulfides (e.g., Cu, Pb, and Zn), (2) the substitution of trace elements in sulfides (e.g., As and Cd), and (3) the substitution of trace elements in calcite (e.g., Mn), as evidenced in the EPMA results. In conclusion, the hanging wall rocks in this study containing high sulfur in proximity to the ore zone are a PAF material with heavy metal contaminant sources, whereas the footwall and ore zone materials have a lower potential to be such sources at the Chatree mine.

Role of the Tree Species Selected in Developing Swampy Forest System for Passive Treatment of Acid Mine Drainage

Coal mining is one of economic sector that requires forestry area in Indonesia. Most of coal mining concession are forestry area loan use from the government and during mining operation facing with acid mine drainage (AMD). AMD passive treatment can be integrated with revegetation process that related with better handling of overburden (OB) with potential acid forming (PAF) during reclamation processed. The most expensive methods and uncertainty process are the use of quick lime in treatment of AMD. During operation and preparing mine closure stage need to create the effective one to comply with the regulation. “Swampy Forest (SF) system” is an alternative method of sustainable and naturally process. Selecting the appropriate tree species as a preliminary process to develop a better SF system by batch experiment in the form of a forest constructed wetland. The tree species selected are Nauclea orientalis (lonkida tree) and Melaleuca cajuputi (kayuputih tree) as non-local species then Nauclea subdita (bangkal tree) and Melaleuca leucadendron (galam tree) as local species have been selected. Local species higher in Fe and Mn adsorption in pH of AMD <4 then nonlocal species higher in Fe and Mn adsorption on pH of AMD >4 condition.

The possibility of reclamation criteria success in Indonesia: soil condition, vegetation structure and species composition

There are two regulations for mine reclamation success in the forestry area in Indonesia, namely Minister of Forestry Regulation No. P.60/Menhut-II/2009 and Minister of Energy and Mineral Resources Decree No. 1827.K/30/MEM/2018. Both regulations rule vegetation and soil success. This study aims to analyse criteria parameters from both regulations in the mine reclamation and compare them to the surrounding secondary natural forest (SNF). This study was conducted in 6 six types of mine reclamation stand structures: 1, 4, 6, 9, 11-year-old plantation and SNF using 1 hectare of the circular plot each (total 6 ha). Soil samples were collected from 40 cm depth to analyse physical, biological and chemical conditions. Mine reclamation areas had almost similar physical, biological and chemical soil conditions with SNF. Nevertheless, due to the potential acid-forming (PAF) material from overburden, the 1-year-old plantation had pH = 3.23-3.27. The highest diversity index and the number of species and families in all reclamation areas were H’ = 1.82 (11-year-old); 14 species (9-year-old); and 11 families (9-year-old), comparing with SNF were H’ = 3.48; 67 species, and 31 families. Conversely, vegetation structure parameters in mine reclamation areas were higher than SNF (diameter at height breast (DBH; 1.3 m) = 28.42 cm; tree density = 469/ha; basal area = 35.04 m2/ha; and total height = 16.85 m). Compared to the SNF, vegetation structure and soil conditions are mostly possible for mine reclamation success. Still, species composition needs to be considered further as a standard interval to meet the criteria.

Evaluation of Non-Acid-Forming material layering for the prevention of acid mine drainage of pyrite and jarosite

Encapsulation is a typical method used to prevent potential acid mine drainage (AMD) in overburden piles. In this method, Potentially Acid-Forming (PAF) material is covered with either Non-Acid-Forming (NAF) material or alkaline material to minimize water infiltration and/or oxygen diffusion through rock pores. The physical and chemical characteristics and thickness of the NAF material layer are critical factors affecting the successful prevention of AMD. Therefore, this study evaluated the method of NAF material layering using laboratory-scale column leaching tests. NAF layers with a ratio of 25 and 50% were used to cover PAF material containing pyrite and jarosite sourced from the Sangatta and Bengalon mining areas, East Kalimantan. The physical and chemical characteristics of leachate collected from samples watered on a weekly wet-dry cycle were analyzed by kinetic tests over a period of 23 weeks. The results showed a trend of increasing pH values and decreasing sulfate and metal concentrations in the leachate. This study shows that NAF layering is an effective method to prevent or minimize the generation of AMD.

Development of a MIP model to maximize NPV and minimize adverse environmental impact-a heuristic approach.

Block sequencing is of great importance in an open-pit mining operation. Sequencing is usually performed to maximize the net present value (NPV). Also, from the environmental viewpoint, the sequence of dumping mined materials is of significant value in the sulfide mines. The potential acid-forming (PAF) waste rocks in these mines can seriously damage the environment due to the formation of acid mine drainage (AMD). To prevent the exposition of the PAF materials, it is essential to design suitable block sequencing. For this purpose, encapsulation of the PAF rocks by non-acid forming (NAF) rocks should be considered during waste dumping. However, this method can impose unnecessary re-handling costs. This issue is due to the determination of the waste-dump sequence based on improper block sequencing obtained from the previous models with the NPV maximization strategy. In the present study, a mixed-integer programming (MIP) model is proposed for generating proper block sequencing taking into account the composition of waste rocks. The main objective of the proposed MIP model is to maximize NPV and minimize the destructive environmental effects of PAF materials dumping. The CPLEX solver was applied to solve the proposed model in small datasets. Then, an artificial bee colony (ABC) is implemented to find out optimum block sequencing and waste dumping (BSWD) on a large scale. The proposed approach was examined employing several sets of data. The obtained results were compared with those of the CPLEX solver as a benchmark. An approximate gap of 2% demonstrates the efficiency of the proposed approach.

Potensi Pembentukan Air Asam Tambang Pada PIT 3000, Blok Toraja, PT Trubaindo Coal Mining Berdasarkan Studi Karakteristik Geokimia dan Mineralogi Batuan Overburden dan Underburden

Coal mining in Indonesia is mostly applied by an open pit system which causes the rocks in the mining location to be exposed to the surface, so that it will be easy to react with air and water directly. The high content of sulphide minerals such as pyrite and marcasite in coal mining site, has the potential to produce mine acid water. Mine acid water causes environmental pollution and a threat to aquatic and soil ecosystems by increasing the concentration of other heavy metal ions. This study aims to identify the mineral content of the overburden rock layers to determine the type of sulfide minerals forming acid mine and neutralizing minerals in rocks. The methods used in this study are field investigations, mineralogical content analysis, geochemical characteristics and measured stratigraphic to evaluate mineralogical content, geochemical characteristics and depositional environmental facies of overburden and underburden rocks from the Pit 3000 coal seam. The results showed that overburden rocks were composed by claystone with the thickness ranges 1.60 m – 5.15 m, whereas underburden rocks are characterized by diverse lithology such as claystone, siltstone and sandstone with the thickness ranges from 0.20 m – 4.50 m. Based on geochemical analysis on 18 overburden rock samples, it is known that overburden layers have characteristics that are dominated by Non Acid Forming (NAF) rocks with 83% NAF, 11% Potential Acid Forming (PAF) and 6% uncertain. The characteristics of the underburden layer are dominated by PAF rocks with 45% PAF, 19% NAF and 36% uncertain.

Characteristic of pro delta environment on coal seam PAF and NAF, warukin formation, South Kalimantan

Coal seam of the Kusan Bawah block, Warukin Formation, Asam-Asam Basin, South Kalimantan has age middle Miosen to Late Miosen. The purpose of this study is to investigate the characteristic of the coal, PAF, and NAF, the depositional environment. The method used in this research is taking rock samples from core drilling and then testing the quality of rock geochemistry by using Net Acid Generating test. Rock geochemical analysis results are entered into modeling software to determine its distribution based on stratigraphic principle. Results of rock geochemical quality analysis based on SNI 6597- 2011 to Non-Acid Forming and Potential Acid Forming. The advantages of identification PAF or NAF value are about preventive activity to decrease acidic pollutant material in the environment. Potential Acid Forming material is above seam G and is between EL and seam D. The percentage of Potential Acid Forming material is 33.17% and NAF 66.83%. The depositional environment of the Kusan Bawah block is interpreted in pro delta and transgressive conditional.

Prediksi Laju Pembentukan Air Asam Tambang dengan Metode Column Leaching Test

Prediction of Acid Mine Drainage Forming using Method of Column Leaching Test. One of the environmental problems in coal mining activities is the formation of acid mine drainage. Prediction of the formation of acid mine drainage is important as an effort to control environmental impacts. Acid mine water occurs with interactions between potentially acid-forming material with oxygen, bacteria and water. Objective of study is to analyze the potential for acid mine drainage forming based on material characteristics. The research method was carried out using static and kinetic tests. The static test method classifies materials according to the ability to produce clean acids with observed parameters such as paste pH, total sulfur, Acid Neutralizing Capacity (ANC), Net Acid Generation (NAG), Maximum Potential Acid (MPA), and Net Acid Producing Potential (NAPP). The Kinetic test method predicts the rate of acid-forming of a material. The kinetic test uses the Column Leaching Test Method by using material with composition designed to represent field condition. The kinetic method parameters are pH, electrical conductivity, acidity, alkalinity, sulfate content, and dissolved metal content (Fe, Mn, and Cd). Results of the static test classified the material into NAF Non-Acid Forming (NAF), Potential Acid Forming (PAF) and Uncertain (UC) material categories. The results of the Column Leaching Method classified the material into categories of potential and no potential to form acid mine water. The columns that have the potential to form acid mine drainage occur in columns with large amounts of tonnage of PAF material or those in the upper layer so that it reacts with oxygen. The columns that have no potential to produce acid mine drainage in columns with PAF material are in the middle layer or mixed with NAF material.

Prediction Potential Acid Mine Drainage of Epithermal High Sulfidation Deposits using Static Test

Acid mine drainage can be predicted by testing the geochemical characteristics of rocks, to be knowing acid-forming rocks and acid neutralizing. Prevention and mitigation efforts from acid mine drainage can be predicted by knowing the characteristics of rocks and looking for the direct contact of acid rocks with air and water. The static test aims to determine the potential for acid formation that occurs in rocks as a first step to characterize rocks. Tests were carried out on argillic silica alteration rock samples from the Lanut site, PT J-Resources Bolaang Mongondow. The calculation of total sulfur shows a smaller percentage of the value of the advance argillic than the silisic sample. The results of advance argillic samples, NAG pH> 4.5 were obtained and the value of Net Acid Producing Potential (NAPP) in the range of 110-153 kg H2SO4 / ton rocks which means that the potential nature of rocks cannot be ascertained PAF or NAF. Whereas for silisic alteration rock samples the pH value of NAG <4.5 and NAPP values in the range of 146 - 355 kg H2SO4/ton rocks means silisic alteration has the potential acid forming.

Identifikasi Potensi Pembentukan Air Asam Tambang Batubara (PAF /NAF) dengan Menggunakan Metode Uji NAPP (Net Acid Producing Potential)

&lt;p&gt;&lt;em&gt;Several laboratories tests have been developed in Indonesia and another countries that carry out mining activities, especially coal mining.&lt;/em&gt; &lt;em&gt;The purpose of this analysis is to find out the presence and absence of soil layers which have the potential to form acidity.&lt;/em&gt; &lt;em&gt;The most commonly known analytical methods are static and kinetic methods. The most commonly known method of analysis is static and kinetic methods. Our test this time are uses the NAPP method (Net Acid Producing Potential)&lt;/em&gt;&lt;em&gt;.A&lt;/em&gt;&lt;em&gt;and then, from the results of this calculation, was known that the mine area had acidic or not.&lt;/em&gt; &lt;em&gt;This time, an investigation was conducted to determine the acidity of PAF&lt;/em&gt;&lt;em&gt; (Potential Acid Forming)&lt;/em&gt;&lt;em&gt; and NAF&lt;/em&gt;&lt;em&gt; (Non Acid Forming)&lt;/em&gt;&lt;em&gt;, which is owned by the mining company PT GIE (Globalindo Inti Energy) at the Handil, Muarajawa, Balikpapan, East Kalimantan regions.&lt;/em&gt; &lt;em&gt;From the results of tested and mapped , it was found that this mining area had NAF (Non Acid Forming) soil content, with a PH range between 4.65 - 5.75 and had a negative NAPP price.&lt;/em&gt;&lt;/p&gt;

Potential Formation of Acid Mine Drainage In Putra Perkasa Abadi Coal Mining Company - Girimulya Site (BIB), Tanah Bumbu Regency, South Kalimantan

Coal is a combustible fossil fuel, formed from plants that have been consolidated between rock strata and can be used by geothermal. World Energy Council said Indonesia is the fifth world coal producer after China, United States, India and Australia. Acid Mine Drainage is become one of environmental damage from coal massive exploitation in Indonesia. Precautions are required to reduce risks and minimize the formation of acid mine drainage at overburden disposal in order to meet the quality standards set by the Government. The action is carried out by isolating Potential Acid Forming (PAF) materials with non-acidic (NAF / Non Acid Forming) materials to avoid exposure to air and water with sulphide minerals. Good PAF and NAF material management in the disposal area will minimize the potential of acid mine drainage.

Improved mine waste characterisation through static blended test work

Prediction of the geoenvironmental characteristics of mine waste materials typically relies on the static and kinetic testing of individual waste units. In reality, waste rock piles are heterogeneous landforms containing potentially acid forming (PAF) units juxtaposed with non-acid forming (NAF) units. Despite this, predictive geoenvironmental test work programs rarely characterise blends of waste materials. Through undertaking laboratory-based blended static testing and mineralogical assessments, a first insight into the leachate chemistry of a waste landform can be ascertained. This was tested using three waste units (Type A – alkaline, B – neutral and D – PAF; n=31) obtained from the Savage River mine, Western Tasmania. Seven daughter blend types (1–7; n=42) were prepared using different ratios of the individual waste types, and characterised by acid base accounting and mineralogical techniques. Blend types were NAF when a maximum of 20% of Type D was used, with daughter blend 4 (90% Type A+10% Type D) the most inert when screened against ANZECC (2000) aquatic protection trigger values. This study demonstrates that through first-pass static blended testing, opportunities exist to improve waste-rock handling practices and design NAF waste piles once field trials, involving a range of particle sizes, have been conducted.

Control of Acid Generation from Pyrite Oxidation in a Highly Reactive Natural Waste: A Laboratory Case Study

Laboratory kinetic leach column (KLC) tests were carried out to define the conditions required to control acid generation from a highly reactive, potentially acid-forming (PAF) iron ore waste rock. It was found that lime addition (0.1 wt % blended) plus either blending of silicates (25 wt % K-feldspar and 25 wt % chlorite), or addition of a non-acid forming (NAF) top cover containing about 10% dolomite (PAF:NAF = 5:1 wt %), when watered/flushed with lime-saturated water, greatly reduced acid generation as compared to the control KLC (PAF alone, watered/flushed with Milli-Q water), but did not result in circum-neutral pH as required for pyrite surface passivation and effective acid and metalliferous drainage (AMD) mitigation. In contrast, the combined use of these treatments—blended lime and silicates with an NAF cover and watering/flushing with lime-saturated water—resulted in leachate pH of 12 (up to 24 weeks). Mass balance calculations for Ca2+ and scanning electron microscopy (SEM) analyses suggest that calcite or gypsum may have formed in the NAF-amended KLCs and lime with added silicate KLC. Although the combined approach in the form trialled here may not be practical or cost-effective, control of a highly reactive natural PAF waste by pyrite surface passivation appears to be possible, and an improved treatment methodology (e.g., slightly increased lime blending without the need for further lime watering/flushing) could usefully be examined in the future.

Strategies for Reduced Acid and Metalliferous Drainage by Pyrite Surface Passivation

Acid and metalliferous drainage (AMD) is broadly accepted to be a major global environmental problem facing the mining industry, requiring expensive management and mitigation. A series of laboratory-scale kinetic leach column (KLC) experiments, using both synthetic and natural mine wastes, were carried out to test the efficacy of our pyrite passivation strategy (developed from previous research) for robust and sustainable AMD management. For the synthetic waste KLC tests, initial treatment with lime-saturated water was found to be of paramount importance for maintaining long-term circum-neutral pH, favourable for the formation and preservation of the pyrite surface passivating layer and reduced acid generation rate. Following the initial lime-saturated water treatment, minimal additional alkalinity (calcite-saturated water) was required to maintain circum-neutral pH for the maintenance of pyrite surface passivation. KLC tests examining natural potentially acid forming (PAF) waste, with much greater peak acidity than that of the synthetic waste, blended with lime (≈2 wt %) with and without natural non-acid-forming (NAF) waste covers, were carried out. The addition of lime and use of NAF covers maintained circum-neutral leachate pH up to 24 weeks. During this time, the net acidity generated was found to be significantly reduced by the overlying NAF cover. If the reduced rate of acidity production from the natural PAF waste is sustained, the addition of smaller (more economically-feasible) amounts of lime, together with application of NAF wastes as covers, could be trialled as a potential cost-effective AMD mitigation strategy.