--- title: "Geochemistry – Part I: What is metal leaching and acid rock drainage (ML/ARD), and how does it occur?" date: 2024-04-15T11:19:00-04:00 author: jessica napert canonical_url: "https://www.bbaconsultants.com/es-cl/publications/geochemistry-part-i-what-is-metal-leaching-and-acid-rock-drainage-ml-ard-and-how-does-it-occur" section: Publications --- 1. ## Introduction In the mining process, substantial amounts of sulphide-bearing rock are often excavated to extract the economic metals essential to modern day living. If left untreated, the excavation of sulphidic rocks can lead to metal leaching and acid rock drainage (ML/ARD), which may negatively impact the environment, human health and surrounding infrastructures. ML/ARD is considered to be the most important environmental challenge for the mining industry, given the high cost and technically difficult approach required for prevention1. While a modern-day approach to ML/ARD treatment seeks early prediction and prevention, previous mining operations in Canada were considered “reactive” to acid generation issues, and remedial actions were initiated only after significant environmental damage had occurred. In 1995, it was estimated that the cost for environmental liabilities in Canada due to ML/ARD was somewhere between $2 billion and $5 billion2, highlighting the importance of early prevention in the design phase of a mining operation. 2. ## The chemistry behind acid generation How exactly does ML/ARD occur? If not managed or neutralized, the process of acid generation begins with a relatively slow reaction process. After the series of reactions gains momentum, the latter stages of acid generation can be especially devastating to the environment and difficult to remediate (Figure 1). The following three generalized reactions are detailed below to help clarify the sequence of reactions, which determines acid generation and the potential for metal leaching through the initial oxidation of pyrite: 3. [ ![REACTION 1](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-1_2024-04-15-132446_idpw.png) ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-1_2024-04-15-132446_idpw.png) 4. In this initial reaction, the sulphide mineral (most commonly pyrite), is broken down through fresh exposure to oxygen and water from the atmosphere, which often occurs through the crushing or blasting of waste rock material. As a result, the reduced iron (ferrous iron) is separated from sulphur to produce sulphate and acidity. Relatively minor amounts of acidity are produced during this first step. 5. Reaction 2 [ ![REACTION 2](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-2_2024-04-15-132542_hxem.png) ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-2_2024-04-15-132542_hxem.png) 6. The separated ferrous iron from dissolved pyrite begins to react with oxygen and acidity to oxidize the ferrous iron to ferric iron (Fe2+ to Fe3+). On its own, this rate-limiting reaction typically takes a long time to oxidize and generate ferric iron. However, this oxidation reaction is often significantly accelerated from the presence of naturally occurring bacteria (i.e., thiobacillus ferrooxidans), which serve to catalyze the rate of reaction by approximately 100,000 times. It is important to prevent this reaction because once Fe2+is oxidized to Fe3+, acid generation can become rapid and very difficult to manage. 7. Reaction 3 [ ![REACTION 3](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-3_2024-04-15-133046_lapn.png) ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-3_2024-04-15-133046_lapn.png) 8. In this late-stage reaction, oxygen is no longer required, and pyrite can be simply oxidized in the presence of Fe3+ and water. This stage can be devastating as the acid generation process becomes cyclic and self propagating, leading to very rapid rates of acid generation, and continues until either ferric iron or pyrite is fully consumed. At this stage, affected waters are strongly acidic with pH values lower than 3, which has significant implications for metal leaching transport as described below. 9. [ ![Figure 1](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/Figure-1.jpg) ML/ARD migrating from the entrance of an abandoned mine shaft (Photo: Christopher Miller - csmphotos.com). ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/Figure-1.jpg) 10. ## Metal leaching in acidic waters Acid rock drainage is particularly dangerous because low pH (highly acidic) waters can dissolve and transport high concentrations of heavy metals, such as copper (Cu), lead (Pb), zinc (Zn) and several others (Figure 2). This dissolution and transport process is called “metal leaching”. When metals are present in their dissolved form, they are considered toxic, as they can be readily absorbed by plant and animal life. Metal accumulation can be passed along the food chain through biomagnification, leading to further toxicity throughout these ecosystems. While acid rock drainage is the most common mechanism for metal leaching, it is important to note that other potentially toxic compounds, such as arsenic (As), selenium (Se) and sulphate (SO4 (2-)), can be effectively transported at near-neutral and alkaline pH conditions. Similarly, if the drainage chemistry is not properly managed, this can lead to neutral mine drainage (NMD) and saline drainage (SD). 11. [ ![Figure 2](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/Figure-2.jpg) The solubility of base metals in water (µg/L) versus pH with regions highlighting ARD, NMD and SD (modified from INAP – GARD Guide, 2009). ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/Figure-2.jpg) 12. ## Conclusions ML/ARD starts with the generation of acidic waters, often caused by improperly managed sulphidic mine material. Acidic waters at low pH levels can dissolve heavy metals from the mine site and transport them into the surrounding environment. In high concentrations, these metals become toxic to the environment and ecosystems. In the following article **“**[Geochemistry Part II: Predicting metal leaching and acid rock drainage (ML/ARD)](https://www.bbaconsultants.com/en-ca/publications/geochemistry-part-ii-predicting-metal-leaching-and-acid-rock-drainage-ml-ard-chemistry)**”**, we discuss a scientific and multidisciplinary approach used to predict ML/ARD. With effective predictions established prior to mine development, solutions can be engineered to manage and prevent ML/ARD from occurring in the first place, thereby protecting the environment and potentially saving mining companies several millions of dollars in environmental liabilities. 13. References 1 - Price (2009) Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials. MEND Report 1.20.1. 2 - Geocon – SNC Lavalin Environment (1995) Economic Evaluation of Acid Mine Drainage Technologies. MEND Report 5.8.1. *This blog article was written in collaboration with Neal Sullivan.* This content is for general information purposes only. All rights reserved ©BBA 1. ![Mm employee men todd mccracken retouch web SQ LR BBA](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/avatars/_size_avatar/204324/mm_employee_men_todd-mccracken_retouch_web_SQ_LR_BBA.webp) Todd McCracken, P.Geo Director de Departamento, Minería y Geología Leer bio [ ](https://www.linkedin.com/in/todd-mccracken-921b6927/) **Todd McCraken cuenta con más de 30 años de experiencia en estimación de recursos y estudios económicos,** reduciendo riesgos y maximizando el valor de los proyectos mineros. **Áreas de especialización y certificaciones** - Estudios económicos avanzados (PEA, PFS, FS) y optimización de proyectos - Persona calificada según la norma NI 43-101 y el estándar S-K 1300 - Gestión de proyectos y programas de QA/QC - Asociaciones: PGO, QOG, PEGNL **Principales logros:** Más de 55 proyectos multisectoriales en más de 10 países de América, Europa, Asia y África, integrando modelado avanzado y cumplimiento normativo. **Enfoque y valor añadido:** Todd emplea una estrategia metódica y cooperativa, combinando el cumplimiento normativo con un análisis estratégico exhaustivo para resguardar las inversiones, mitigar los riesgos y maximizar el valor de los proyectos mineros. - [ 17 ago 2026 Structural engineering early phase: unlocking value through early decisions - ![Nicolas Samson](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/avatars/_size_avatar/197852/Nicolas-Samson.webp) Nicolas Samson, P.Eng. Senior Engineer, Structural ](https://www.bbaconsultants.com/es-cl/perspectivas/structural-engineering-early-phase-unlocking-value-through-early-decisions "Structural engineering early phase: unlocking value through early decisions") - [ 12 ago 2026 Four mistakes that can harm the future performance of industrial assets - ![Caroline provost](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/avatars/_size_avatar/208557/Caroline-provost.webp) Caroline Provost P.Eng. Director, Operational Strategy Implementation ](https://www.bbaconsultants.com/es-cl/perspectivas/four-mistakes-that-can-harm-the-future-performance-of-industrial-assets "Four mistakes that can harm the future performance of industrial assets") - [ 27 jul 2026 Minería y metalurgia Por qué las decisiones tempranas sobre relaves definen el desempeño de la mina a largo plazo - ![Geraldine Cosset](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/avatars/_size_avatar/206707/Geraldine-Cosset.webp) - ![Jared Purdy](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/avatars/_size_avatar/212424/Jared-Purdy.webp) ](https://www.bbaconsultants.com/es-cl/perspectivas/las-decisiones-sobre-relaves-definen-desempeno-mina-a-largo-plazo "Por qué las decisiones tempranas sobre relaves definen el desempeño de la mina a largo plazo") - [ 02 jul 2026 Minería y metalurgia Python: Una herramienta clave para el análisis de datos en el procesamiento de minerales - ![Charles Blais](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/avatars/_size_avatar/211197/Charles-Blais.webp) - ![Juan Pablo Vergara](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/avatars/_size_avatar/211266/Juan-Pablo-Vergara.webp) ](https://www.bbaconsultants.com/es-cl/perspectivas/python-una-herramienta-clave-para-analisis-datos-en-procesamiento-minerales "Python: Una herramienta clave para el análisis de datos en el procesamiento de minerales") [ Ver todo ](https://www.bbaconsultants.com/es-cl/perspectivas "Ir a la página del listado de Publications") - [ 01 may 2024 Minería y metalurgia Geochemistry – Part II: Predicting metal leaching and acid rock drainage (ML/ARD) chemistry - ![Mm employee men todd mccracken retouch web SQ LR BBA](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/avatars/_size_avatar/204324/mm_employee_men_todd-mccracken_retouch_web_SQ_LR_BBA.webp) Todd McCracken, P.Geo Director de Departamento, Minería y Geología ](https://www.bbaconsultants.com/en-ca/publications/geochemistry-part-ii-predicting-metal-leaching-and-acid-rock-drainage-ml-ard-chemistry "Geochemistry – Part II: Predicting metal leaching and acid rock drainage (ML/ARD) chemistry") [ Ver todo ](https://www.bbaconsultants.com/es-cl/perspectivas) Un mañana sostenible que empieza hoy [ Descubra nuestro Informe ESG 2022 ](https://www.bbaconsultants.com/es-cl/sobre-nosotros/social-engagement-2) 1. ## Introduction In the mining process, substantial amounts of sulphide-bearing rock are often excavated to extract the economic metals essential to modern day living. If left untreated, the excavation of sulphidic rocks can lead to metal leaching and acid rock drainage (ML/ARD), which may negatively impact the environment, human health and surrounding infrastructures. ML/ARD is considered to be the most important environmental challenge for the mining industry, given the high cost and technically difficult approach required for prevention1. While a modern-day approach to ML/ARD treatment seeks early prediction and prevention, previous mining operations in Canada were considered “reactive” to acid generation issues, and remedial actions were initiated only after significant environmental damage had occurred. In 1995, it was estimated that the cost for environmental liabilities in Canada due to ML/ARD was somewhere between $2 billion and $5 billion2, highlighting the importance of early prevention in the design phase of a mining operation. 2. ## The chemistry behind acid generation How exactly does ML/ARD occur? If not managed or neutralized, the process of acid generation begins with a relatively slow reaction process. After the series of reactions gains momentum, the latter stages of acid generation can be especially devastating to the environment and difficult to remediate (Figure 1). The following three generalized reactions are detailed below to help clarify the sequence of reactions, which determines acid generation and the potential for metal leaching through the initial oxidation of pyrite: 3. [ ![REACTION 1](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-1_2024-04-15-132446_idpw.png) ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-1_2024-04-15-132446_idpw.png) 4. In this initial reaction, the sulphide mineral (most commonly pyrite), is broken down through fresh exposure to oxygen and water from the atmosphere, which often occurs through the crushing or blasting of waste rock material. As a result, the reduced iron (ferrous iron) is separated from sulphur to produce sulphate and acidity. Relatively minor amounts of acidity are produced during this first step. 5. Reaction 2 [ ![REACTION 2](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-2_2024-04-15-132542_hxem.png) ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-2_2024-04-15-132542_hxem.png) 6. The separated ferrous iron from dissolved pyrite begins to react with oxygen and acidity to oxidize the ferrous iron to ferric iron (Fe2+ to Fe3+). On its own, this rate-limiting reaction typically takes a long time to oxidize and generate ferric iron. However, this oxidation reaction is often significantly accelerated from the presence of naturally occurring bacteria (i.e., thiobacillus ferrooxidans), which serve to catalyze the rate of reaction by approximately 100,000 times. It is important to prevent this reaction because once Fe2+is oxidized to Fe3+, acid generation can become rapid and very difficult to manage. 7. Reaction 3 [ ![REACTION 3](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-3_2024-04-15-133046_lapn.png) ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/REACTION-3_2024-04-15-133046_lapn.png) 8. In this late-stage reaction, oxygen is no longer required, and pyrite can be simply oxidized in the presence of Fe3+ and water. This stage can be devastating as the acid generation process becomes cyclic and self propagating, leading to very rapid rates of acid generation, and continues until either ferric iron or pyrite is fully consumed. At this stage, affected waters are strongly acidic with pH values lower than 3, which has significant implications for metal leaching transport as described below. 9. [ ![Figure 1](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/Figure-1.jpg) ML/ARD migrating from the entrance of an abandoned mine shaft (Photo: Christopher Miller - csmphotos.com). ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/Figure-1.jpg) 10. ## Metal leaching in acidic waters Acid rock drainage is particularly dangerous because low pH (highly acidic) waters can dissolve and transport high concentrations of heavy metals, such as copper (Cu), lead (Pb), zinc (Zn) and several others (Figure 2). This dissolution and transport process is called “metal leaching”. When metals are present in their dissolved form, they are considered toxic, as they can be readily absorbed by plant and animal life. Metal accumulation can be passed along the food chain through biomagnification, leading to further toxicity throughout these ecosystems. While acid rock drainage is the most common mechanism for metal leaching, it is important to note that other potentially toxic compounds, such as arsenic (As), selenium (Se) and sulphate (SO4 (2-)), can be effectively transported at near-neutral and alkaline pH conditions. Similarly, if the drainage chemistry is not properly managed, this can lead to neutral mine drainage (NMD) and saline drainage (SD). 11. [ ![Figure 2](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/Figure-2.jpg) The solubility of base metals in water (µg/L) versus pH with regions highlighting ARD, NMD and SD (modified from INAP – GARD Guide, 2009). ](https://www.assets.bbaconsultants.com/bba-production-assets/production/images/Figure-2.jpg) 12. ## Conclusions ML/ARD starts with the generation of acidic waters, often caused by improperly managed sulphidic mine material. Acidic waters at low pH levels can dissolve heavy metals from the mine site and transport them into the surrounding environment. In high concentrations, these metals become toxic to the environment and ecosystems. In the following article **“**[Geochemistry Part II: Predicting metal leaching and acid rock drainage (ML/ARD)](https://www.bbaconsultants.com/en-ca/publications/geochemistry-part-ii-predicting-metal-leaching-and-acid-rock-drainage-ml-ard-chemistry)**”**, we discuss a scientific and multidisciplinary approach used to predict ML/ARD. With effective predictions established prior to mine development, solutions can be engineered to manage and prevent ML/ARD from occurring in the first place, thereby protecting the environment and potentially saving mining companies several millions of dollars in environmental liabilities. 13. References 1 - Price (2009) Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials. MEND Report 1.20.1. 2 - Geocon – SNC Lavalin Environment (1995) Economic Evaluation of Acid Mine Drainage Technologies. MEND Report 5.8.1. *This blog article was written in collaboration with Neal Sullivan.* This content is for general information purposes only. All rights reserved ©BBA