## Learning Outcomes

- Define **hydraulic fracturing** and explain how it differs from wastewater disposal
- Identify the main environmental risks in the hydraulic fracturing water cycle
- Distinguish **flowback** from **produced water** and explain why each matters
- Explain how methane leakage can reduce the climate advantage of natural gas
- Describe how fracking-related earthquakes usually result from deep wastewater injection
- Apply these ideas to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand fracking environmental effects, with a focus on the following syllabus points:

- Know that hydraulic fracturing uses high-pressure water, sand, and chemicals to increase oil or natural gas flow from low-permeability rock
- Identify environmental impacts including water use, groundwater contamination risks, surface spills, air pollution, habitat fragmentation, and noise/light pollution
- Explain the **hydraulic fracturing water cycle**: water acquisition, chemical mixing, well injection, flowback and produced water handling, and wastewater disposal or reuse
- Understand that the EPA concluded in **2016** that hydraulic fracturing activities can impact drinking water resources under some circumstances
- Distinguish **flowback** from **produced water**, and recognise contaminants such as high salinity, **BTEX**, and **NORM**
- Explain why poor **well integrity** and old or poorly sealed **legacy wells** can create contamination pathways
- Know that oil and natural gas operations are the largest industrial source of methane pollution in the United States
- Explain that most larger induced earthquakes are linked to **wastewater disposal wells**, not usually the fracturing step itself

## 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 hydraulic fracturing, and why is it often paired with horizontal drilling?
2. How can fracking activities affect drinking water resources?
3. What is the difference between flowback and produced water?
4. Why can natural gas seem cleaner than coal but still create climate concerns?
5. Why are many fracking-related earthquakes actually linked to wastewater disposal wells?

## Introduction

Fracking is one of the most testable examples of an environmental trade-off in Unit 6. It can increase access to domestic oil and natural gas, often lowering fuel costs and supporting electricity generation, but it also creates risks to water resources, air quality, habitatss, and climate.

For AP Environmental Science, the key is not to memorise a simple “good” or “bad” label. You need to explain **how** the process works, **where** the risks occur, and **which effects come from the fracturing step itself versus the wider oil-and-gas system**. That distinction is especially important in questions about earthquakes and water contamination.

> **Key Term: hydraulic fracturing**
> A method of extracting oil or natural gas by injecting water, sand, and chemicals into low-permeability rock at high pressure to create fractures and increase fuel flow.

### How fracking works and why it is used

Fracking is mainly used in rock formations such as shale, sandstone, and some coal beds. These rocks contain oil or natural gas, but the fuel does not move easily because the rock has **low permeability**. In simple terms, the resource is there, but it cannot flow freely enough to make the well productive.

The process usually begins with drilling vertically down to the target rock layer. The well may then turn sideways through the rock in **horizontal drilling**. This matters because a horizontal well can contact a much larger section of the fuel-bearing rock than a purely vertical well. More contact area means more places to fracture and more pathways for gas or oil to move.

After drilling, operators inject a fluid mixture under high pressure. The water provides the force, the sand helps keep the fractures open, and the chemicals help with operations such as reducing friction or controlling bacterial growth. Once fractures form, oil or gas can move through the rock more easily and into the well.

This is why fracking became more common as technology improved. It allowed producers to access reserves that were previously too difficult or too expensive to extract.

> **Key Term: low-permeability rock**
> Rock through which fluids move very slowly, making extraction difficult unless fractures or other pathways are created.

A common AP mistake is to describe fracking as simply “drilling.” Drilling creates the well, but fracturing is the later step that opens pathways in the rock. These are related but not identical.

> **Test Tip:** If a multiple-choice question asks why fracking expanded in recent decades, look for answers linked to **horizontal drilling** and access to **low-permeability shale formations**, not just “more oil demand.”

### Water impacts: the hydraulic fracturing water cycle

The most exam-relevant way to organise water impacts is by using the **hydraulic fracturing water cycle**. This means looking at every stage where drinking water resources can be affected, not just the underground fracturing event.

> **Key Term: hydraulic fracturing water cycle**
> The full sequence of fracking-related water use and waste handling: water acquisition, chemical mixing, well injection, flowback and produced water handling, and wastewater disposal or reuse.

The first stage is **water acquisition**. Fracking requires very large water volumes per well. In areas already facing low water availability, these withdrawals can stress streams, reservoirs, or aquifers. This is a quantity problem before it becomes a quality problem. If too much water is removed, less remains for habitatss, agriculture, or human use.

The second stage is **chemical mixing**. At the surface, fracking fluid is prepared before injection. Spills from storage tanks, hoses, pumps, or transfer lines can contaminate soil and nearby water. This is a classic **point-source pollution** risk because the release comes from a specific, identifiable place.

The third stage is **well injection**. Many students assume groundwater contamination mainly happens because fractures travel all the way up into aquifers. That can happen in rare circumstances, but it is not the main pathway you should emphasise. A much more common concern is **well integrity**. If the steel casing or cement around the well fails, fluids or gases may move into surrounding rock layers and potentially into drinking water sources.

> **Key Term: well integrity**
> The ability of a well’s casing and cement barriers to prevent fluids or gases from escaping into surrounding rock or groundwater.

Old, abandoned, or poorly plugged wells can also matter. These **legacy wells** may act as shortcuts that allow fluids or gas to move upward.

The fourth stage is the return of wastewater to the surface.

> **Key Term: flowback**
> Fracturing fluid that returns to the surface soon after injection.
>
> **Key Term: produced water**
> Wastewater that returns during and after oil or gas production, including flowback plus water naturally present in the rock formation.

The distinction matters. **Flowback** is mainly the injected fluid coming back soon after fracturing. **Produced water** is broader and continues during production. Produced water can contain both injected chemicals and substances from deep underground formations.

These wastes may contain high **total dissolved solids (TDS)**, very high salinity, hydrocarbons including **BTEX**, metals, and **NORM**.

> **Key Term: BTEX**
> A group of petroleum-related volatile organic compounds: benzene, toluene, ethylbenzene, and xylene.
>
> **Key Term: NORM**
> Naturally occurring radioactive material brought to the surface from underground rock formations.

These contaminants make wastewater management difficult. If storage pits leak, if tanks spill, or if treatment is inadequate, surface water or groundwater can be polluted.

The EPA’s key conclusion was careful and specific: fracking activities **can impact drinking water resources under some circumstances**. That wording matters. It does not mean contamination always happens, but it also does not support saying fracking is harmless everywhere.

> **Test Tip:** Avoid extreme wording in FRQs. “Fracking always contaminates groundwater” is too strong, but “there is no risk” is also wrong. A high-scoring answer uses the EPA-style wording: **impacts can occur under some circumstances**.

### Air pollution, methane leakage, and climate trade-offs

Natural gas is often presented as cleaner than coal because burning it generally produces less carbon dioxide, less sulfur dioxide, and fewer particulates. That is true at the point of combustion. But AP questions often push one step further: what about emissions before the fuel is burned?

The biggest concern is **methane leakage**. Methane is the main component of natural gas, and oil and natural gas systems are the largest industrial source of methane pollution in the United States.

> **Key Term: methane leakage**
> The release of methane from oil and gas systems during extraction, processing, storage, and transport.

Some methane is released through **venting**, meaning intentional gas release. Some escapes through **fugitive emissions**, which are unintended leaks from valves, compressors, tanks, pipelines, and pneumatic devices.

> **Key Term: fugitive emissions**
> Unintended gas leaks from equipment such as valves, connectors, tanks, compressors, and pipelines.

This matters because the climate effect of natural gas depends on the whole system, not just the power plant or furnace where it is burned. If leakage is high enough, natural gas loses some of its climate advantage over coal.

Fracking operations can also release **volatile organic compounds** and nitrogen oxides. In sunlight, VOCs and NOx react to form **ground-level ozone**, a major component of photochemical smog.

The cause-and-effect chain is very testable:

equipment leaks or venting → methane and VOC emissions → climate warming plus ozone formation

Beyond that, truck traffic, diesel engines, and flaring can worsen local air quality. Nearby communities may experience respiratory irritation, while wildlifes can be affected by ozone and particulates.

The broader lesson is that “cleaner-burning” does not automatically mean “low impact.” AP Environmental Science rewards answers that compare full-system impacts.

### Earthquakes, wastewater disposal, and broader ecological effects

Fracking has been linked in news reports to earthquakes, but exam questions often test whether you know the more precise explanation. The most important distinction is that **most larger induced earthquakes associated with oil and gas are linked to deep wastewater disposal wells, not to the short-term fracturing step itself**.

> **Key Term: induced earthquake-relatedity**
> Earthquakes caused by human activities, such as fluid injection into deep rock formations.

The mechanism is straightforward. When wastewater is injected deep underground, the added fluid raises **pore pressure** in rock. Higher pore pressure reduces friction on preexisting faults. If the fault is already near the point of slipping, this pressure increase can trigger movement.

wastewater injection → pore pressure rises → friction on fault drops → fault slips → earthquake

This is why disposal wells are often more strongly linked to felt earthquakes than fracking treatments themselves. Disposal wells usually inject much larger fluid volumes over longer periods.

> **Key Term: Class II injection well**
> A deep well used to inject fluids associated with oil and gas operations, including wastewater disposal.

USGS evidence is especially useful here. Only a small fraction of the many Class II injection wells in the United States have caused earthquakes large enough to concern the public. In Oklahoma, only about **1–2%** of induced earthquakes were linked directly to hydraulic fracturing, with most linked to wastewater disposal.

This is a classic AP distinction. Students often lose marks by saying “fracking causes most earthquakes.” A better answer separates the direct fracturing step from associated waste management.

Fracking also causes broader land-use effects. Well pads, roads, pipelines, and compressor stations can fragment habitat. Noise pollution may interfere with animal behaviour. Night lighting changes local conditions. Heavy truck traffic increases dust, road damage, and noise. Water withdrawals can stress aquatic systems. Surface spills can damage vegetation and streams.

These are not always as dramatic as earthquakes or drinking-water contamination, but they matter because AP questions may ask for the **full range** of environmental impacts, not only the most famous ones.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Hydraulic fracturing uses high-pressure water, sand, and chemicals to increase oil or gas flow from low-permeability rock
- Fracking commonly works with horizontal drilling to reach more of the shale or other target rock
- The hydraulic fracturing water cycle includes water acquisition, chemical mixing, well injection, wastewater return, and disposal or reuse
- The EPA concluded in **2016** that fracking activities can impact drinking water resources under some circumstances
- Major water risks include large withdrawals, surface spills, poor well integrity, legacy wells, and inadequate wastewater treatment or disposal
- **Flowback** is fluid returning soon after fracturing, while **produced water** includes longer-term wastewater from production
- Produced water may contain high salinity, high TDS, **BTEX**, metals, and **NORM**
- Natural gas burns more cleanly than coal at combustion, but methane leakage reduces that climate advantage
- Oil and natural gas systems are the largest industrial source of methane pollution in the United States
- Most larger induced earthquakes related to oil and gas are linked to **wastewater disposal wells**, not usually the fracturing step itself
- Fracking and associated infrastructure can also cause habitat fragmentation, noise pollution, light pollution, and surface water contamination
- Strong AP answers avoid extremes and clearly distinguish direct fracking effects from broader oil-and-gas system effects

## Key Terms and Concepts

- hydraulic fracturing
- low-permeability rock
- hydraulic fracturing water cycle
- well integrity
- flowback
- produced water
- BTEX
- NORM
- methane leakage
- fugitive emissions
- induced earthquake-relatedity
- Class II injection well


Fossil Fuels - Fracking environmental effects