AP Environmental Science

## Learning Outcomes

- Explain the main types of mining waste, including tailings, waste rock, slag, and overburden
- Understand how acid mine drainage forms and why it can continue long after mining stops
- Identify how mining waste pollutes surface water, groundwater, soil, and air
- Distinguish cyanide heap leaching, mercury contamination, and smelting-related pollution
- Describe prevention, treatment, and reclamation methods used to reduce mining damage
- Apply this knowledge to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand mining waste and its environmental impacts, with a focus on the following syllabus points:

- Know that mining produces several waste types, including **tailings**, **waste rock**, **slag**, and contaminated wastewater
- Explain how exposure of sulfide minerals such as pyrite to oxygen and water causes **acid mine drainage**
- Understand that low pH increases metal solubility, leading to heavy-metal contamination of streams, groundwater, and sediments
- Identify environmental risks from cyanide heap leaching, tailings impoundments, and mine waste runoff
- Distinguish chronic pollution such as seepage and dust from sudden events such as tailings dam failure
- Describe ecological effects on aquatic systems, including low pH, metal toxicity, sediment burial, and habitat smothering
- Compare prevention methods with treatment methods, including lime or limestone neutralization, wetlands, and reclamation
- Recognise that mining impacts include not only water pollution but also air pollution from smelting and dust from extraction

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between tailings, waste rock, slag, and overburden?
2. How does pyrite exposure lead to acid mine drainage?
3. Why does a lower pH make heavy-metal contamination worse?
4. What are two ways tailings storage facilities can damage the environments?
5. Why is cyanide associated with gold mining, and how is that different from mercury risk?

## Introduction

Mining provides materials that modern societies depend on, from metals used in wiring and transport to minerals used in construction and manufacturing. For AP Environmental Science, the key issue is not just how resources are extracted, but what happens to the huge amounts of waste left behind and how that waste affects the environments.

Mining waste matters because it often causes long-term pollution. Some waste is chemically reactive, some is physically unstable, and some contains toxic processing chemicals. A mine can stop operating, but polluted drainage, contaminated sediments, and damaged habitats may remain for decades.

> **Key Term: mining waste**
> The unwanted material produced during mining and ore processing, including waste rock, tailings, slag, and contaminated water.

## Introduction to mining waste types

### The main categories of waste and why students mix them up

A common APES error is treating all mining waste as if it were the same. It is not. Different waste types form at different stages and create different environmental problems.

> **Key Term: overburden**
> Soil, sediment, and rock lying above a mineral deposit that must be removed to reach the resource.

**Overburden** is removed before the ore is reached. It may damage the environments mainly through land disturbance, habitat removal, and erosion. It is not the same as tailings.

> **Key Term: waste rock**
> Excavated rock with little or no economic mineral value, often piled near the mine and sometimes capable of producing acid drainage.

**Waste rock** comes out during excavation. If it contains sulfide minerals, it can become a major source of acid mine drainage. This is why “just rock” can still be dangerous.

> **Key Term: tailings**
> Finely ground waste and processing fluids left after valuable minerals are separated from ore.

**Tailings** are produced after ore is crushed and processed. They are often much finer than waste rock and may contain residual metals and chemical reagents. Because they are fine particles, they can be carried by water or wind more easily, and they often need engineered storage.

> **Key Term: slag**
> Waste material left after smelting ore to extract metal.

**Slag** forms during high-temperature processing, not during excavation. Students often confuse slag with tailings, but slag comes from smelting while tailings come from ore separation.

Why do these distinctions matter? Because exam questions may ask for the source of a pollutant or the likely pathway of contamination. Tailings often involve slurry storage and seepage, waste rock often involves sulfide exposure, overburden often relates to erosion and land removal, and slag links more directly to smelting.

> **Test Tip:** If a question mentions “finely ground waste after processing,” think tailings. If it mentions “material above the deposit,” think overburden. If it mentions “waste from smelting,” think slag.

### Why lower-grade ores create more waste

As high-grade ores become depleted, companies increasingly mine **lower-grade ore**, meaning rock with a lower concentration of the valuable resource. This means more rock must be excavated and processed to obtain the same amount of metal.

That creates a cause-and-effect chain:

- lower ore quality means more rock handled
- more rock handled means more waste rock and tailings
- more processing means more water use, energy use, and chemical use
- more waste increases the risk of seepage, runoff, dust, and habitat damage

This is why mining impacts often grow even if the final amount of metal produced stays similar.

## How mining waste pollutes water and habitatss

### Acid mine drainage: the core process

The most tested mining pollution process is **acid mine drainage**, often shortened to AMD. It forms when sulfide minerals, especially pyrite, are exposed to oxygen and water.

> **Key Term: acid mine drainage (AMD)**
> Acidic, metal-rich water formed when sulfide minerals exposed by mining react with oxygen and water.

Pyrite is iron sulfide, **FeS2**. When mining breaks open rock and leaves it exposed, rainwater and air reach minerals that were previously buried. This starts oxidation reactions that produce acidity.

A commonly shown reaction is:

4FeS2 + 15O2 + 14H2O -> 4Fe(OH)3 + 8H2SO4

A useful stepwise version is:

2FeS2 + 7O2 + 2H2O -> 2Fe2+ + 4SO4^2- + 4H+

The key APES takeaway is simpler than the chemistry looks:

- exposed pyrite reacts with oxygen and water
- sulfuric acid forms
- pH drops
- acidic water dissolves metals more easily
- dissolved metals enter streams, groundwater, and sediments

That is why AMD is not just “acidic runoff.” It is a reaction sequence that creates both acidity and metal mobilization.

> **Key Term: pyrite oxidation**
> The chemical reaction in which iron sulfide minerals react with oxygen and water, producing acidity and helping create acid mine drainage.

### Why low pH makes metal pollution worse

Acidic water has a low pH, meaning it contains more hydrogen ions. In that acidic environment, many metals become more soluble. In simple terms, they dissolve more easily and move more freely through water.

That means lower pH does two kinds of damage at once:

- direct stress to aquatic organisms that cannot tolerate acidic conditions
- indirect increase in metal toxicity because more iron, manganese, and other metals enter solution

Streams affected by AMD may therefore look polluted even when the visible waste source is upstream and old. This is also why abandoned mines can remain dangerous long after operations cease.

> **Key Term: heavy-metal contamination**
> Pollution caused when metals released from rock or mine waste enter soil, water, or sediments at harmful levels.

### Visible and biological effects in streams

One clear sign of AMD is the orange or yellow coating often seen on streambeds.

> **Key Term: yellow boy**
> The orange-yellow iron precipitate, mainly ferric hydroxide, that forms in streams affected by acid mine drainage.

This forms when dissolved iron precipitates out:

Fe3+ + 3H2O -> Fe(OH)3(s) + 3H+

The result is not just discoloration. The precipitate can coat rocks, gravel, and streambeds, physically smothering habitat used by aquatic insects, fish eggs, and bottom-dwelling organisms. So mining pollution harms aquatic lifes chemically and physically at the same time.

A typical ecological chain is:

- acidic drainage enters stream
- fish and invertebrates experience stress
- metals dissolved in water increase toxicity
- iron precipitates coat substrate
- spawning areas and benthic habitat are buried or smothered
- biodiversity declines

### Tailings impoundments, seepage, and failure

Tailings are often stored as slurry in large engineered storage areas.

> **Key Term: tailings impoundment**
> An engineered storage area, often behind a dam, used to hold tailings slurry.

These facilities create at least three major risks.

First, **chronic seepage**. Water can leak through liners, walls, or foundations and enter groundwater or nearby streams over long periods.

Second, **runoff and dust**. During rainfall, pollutants can wash off tailings surfaces into freshwater. When tailings dry, wind can carry fine contaminated particles into surrounding areas.

Third, **catastrophic failure**. If a tailings dam fails, a huge volume of slurry and sediment can move downstream, burying habitat and spreading pollution rapidly.

Two widely cited examples are:

- **Mount Polley, Canada (2014)** — a tailings storage facility breach in British Columbia
- **Samarco/Fundão, Brazil (2015)** — a major failure near Mariana that caused severe downstream river damage and loss of life

These examples matter because they show tailings pollution as point-source pollution with immediate and long-term effects: turbidity, sediment burial, contaminated floodplains, fish mortality, and slow recovery.

> **Test Tip:** If an FRQ asks for a mining-related water pollution example, AMD is often the strongest process answer. If it asks for a disaster example, tailings dam failure is usually the clearest event answer.

## Mining chemicals, air pollution, and food-web effects

### Cyanide heap leaching and mercury risk

Gold mining is often linked with **heap leaching**, where chemical solution is run through crushed ore to dissolve the target metal.

> **Key Term: heap leaching**
> A mining method in which a chemical solution is passed through crushed ore to extract valuable metals.

In gold mining, the solution is often cyanide-based. Cyanide can leak from heaps, pipes, ponds, or storage areas and contaminate soil, surface water, or groundwater. APES questions often connect cyanide with gold extraction because it is intentionally used in the process.

But students often miss an important distinction: cyanide and mercury do not behave the same way in food webs.

Mercury, especially when converted in aquatic systems to methylmercury, is the stronger concern for **bioaccumulation** and **biomagnification**.

> **Key Term: bioaccumulation**
> The buildup of a pollutant in an organism over time.
>
> **Key Term: biomagnification**
> The increase in pollutant concentration at higher trophic levels of a food chain.

Mercury can move up food chains and become more concentrated in predatory fish, birds, and humans. Cyanide is dangerous and toxic, but it does not generally biomagnify in the same way. That distinction is a high-value exam point.

### Smelting and why mining pollution is not only water pollution

Mining impacts also include major air pollution during ore processing.

> **Key Term: smelting**
> High-temperature extraction of metal from ore.

Smelting can release sulfur dioxide, which contributes to air pollution and can also contribute to acid deposition. This matters because some students describe all mining harm as runoff, when the processing stage can pollute air as well as water and soil.

Dust from excavation, blasting, transport, and crushing is another major impact. It can harm workers and nearby communities, and it can settle on vegetation and soil. So the environmental effects of mining stretch beyond streams:

- water pollution from AMD, seepage, and chemicals
- soil pollution from deposition and waste storage
- air pollution from dust and sulfur dioxide
- habitat loss from excavation and waste disposal

## Prevention, treatment, and reclamation

### Preventing pollution before it forms

The best control method is often prevention rather than cleanup. Once AMD begins, it can continue for a long time and may need ongoing treatment.

Prevention focuses on stopping sulfide-bearing waste from contacting oxygen and water. Strategies include:

- covering waste piles
- using liners
- diverting clean surface water away from reactive waste
- storing waste underwater when suitable
- limiting exposure of tailings and waste rock

This is usually more effective than waiting for acidic drainage to form and then trying to neutralize it.

### Active and passive treatment

When acidic drainage already exists, treatment aims to raise pH and remove dissolved metals.

> **Key Term: active treatment**
> Pollution treatment that requires regular management, such as chemical addition or operation of treatment systems.
>
> **Key Term: passive treatment**
> Pollution treatment that relies more on natural or low-maintenance systems, such as wetlands or limestone drains.

Active treatment often uses lime or limestone to neutralize acidity. A simple neutralization reaction is:

CaCO3 + 2H+ -> Ca2+ + CO2 + H2O

This raises pH and can help dissolved metals precipitate out.

Passive systems include constructed wetlands and anoxic limestone drains. These can be useful, but they are not automatic solutions for every site. Their success depends on water chemistry, oxygen levels, flow conditions, and whether clogging occurs.

That is an important exam judgement point: passive treatment can work, but not everywhere and not equally well.

### Reclamation and its limits

> **Key Term: reclamation**
> The process of restoring land after mining to reduce environmental damage and improve stability or ecological function.

Typical reclamation steps include:

- recontouring the land
- replacing topsoil or adding soil nutrients
- planting fast-growing native vegetation
- using limestone or lime to reduce acidity
- constructing wetlands for bioremediation

Reclamation helps reduce erosion, lower exposure of reactive wastes, and improve habitat conditions. But students should avoid overstating it. Reclaimed mines are often safer and more stable, yet they usually do not fully return to the original natural state condition. Soil structure, species composition, hydrology, and food webs may all remain altered.

That balanced judgement is strong APES thinking: reclamation reduces damage, but full restoration is rarely immediate or complete.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Mining waste includes overburden, waste rock, tailings, slag, and contaminated wastewater
- Tailings are finely ground waste left after ore processing, while overburden is material removed above a deposit
- Acid mine drainage forms when sulfide minerals such as pyrite are exposed to oxygen and water
- AMD lowers pH and increases the solubility and movement of dissolved metals
- “Yellow boy” is an iron precipitate that can coat streambeds and smother aquatic habitat
- Tailings impoundments can cause chronic seepage, contaminated runoff, dust, and catastrophic dam failure
- Cyanide heap leaching is linked to gold extraction and leak risk to soil and water
- Mercury contamination is especially important because it can bioaccumulate and biomagnify through food webs
- Smelting causes air pollution, especially sulfur dioxide, so mining impacts are not limited to runoff
- Prevention focuses on limiting contact between sulfide waste, oxygen, and water
- Treatment may be active, such as lime addition, or passive, such as wetlands and limestone drains
- Reclamation reduces damage and erosion but usually does not fully restore the original habitat

## Key Terms and Concepts

- mining waste
- overburden
- waste rock
- tailings
- slag
- acid mine drainage (AMD)
- pyrite oxidation
- heavy-metal contamination
- yellow boy
- tailings impoundment
- heap leaching
- bioaccumulation
- biomagnification
- smelting
- active treatment
- passive treatment
- reclamation


Impacts of Mining - Mining waste and impacts