AP Environmental Science

## Learning Outcomes

- Understand why **heavy metals** in groundwater are serious persistent pollutants
- Identify major sources of groundwater metal contamination, including natural and human causes
- Explain how **leaching**, pH, redox conditions, and speciation control metal mobility
- Distinguish clearly between arsenic, lead, and mercury rather than treating them as one problem
- Recall key U.S. drinking-water standards for arsenic, mercury, and lead
- Apply this knowledge to environmental impacts, human health risks, and likely AP Environmental Science FRQ prompts

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand heavy metals in groundwater, with a focus on the following syllabus points:

- Know that groundwater contamination can come from both **geogenic** sources in aquifer materials and **anthropogenic** sources such as mining, industry, landfills, and corrosion
- Identify how **leaching** moves dissolved contaminants downward through soil into aquifers
- Explain why heavy metals are **persistent pollutants** that do not biodegrade
- Understand how **pH**, **redox conditions**, and **speciation** affect metal mobility, toxicity, and treatment
- Distinguish groundwater contamination in an aquifer from contamination that happens in pipes, fixtures, or the distribution system
- Know the EPA public drinking-water limit for **arsenic** as **0.010 mg/L** and for **inorganic mercury** as **0.002 mg/L**
- Know that **lead** is commonly regulated through an **action level** rather than a traditional MCL, and that older AP-style teaching often uses **0.015 mg/L**
- Understand that mercury is especially significant in the environments because **methylmercury** bioaccumulates and biomagnifies in food webs

## 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 a geogenic source of groundwater contamination and an anthropogenic one?
2. Why do heavy metals in groundwater remain a problem for long periods?
3. How can low pH increase the mobility of some metals such as lead?
4. Why is lead at the tap not always evidence that the aquifer itself is contaminated?
5. Which heavy metal is most strongly linked to biomagnification in aquatic food webs?

## Introduction

Heavy metals in groundwater are a high-yield AP Environmental Science topic because they connect **pollution**, **water resources**, **human health**, and **ecological impacts**. Groundwater is widely used for drinking water and agriculture, and once an aquifer is contaminated, cleanup is slow, expensive, and often incomplete. Water in aquifers may remain underground for very long periods, so pollutants can stay in place for decades or longer.

This topic also tests careful distinctions. Students often lump arsenic, lead, and mercury together, but they behave differently. **Arsenic** is often naturally released from aquifer materials. **Lead** in drinking water often comes from plumbing corrosion rather than the groundwater itself. **Mercury** matters most in AP Environmental Science because it can be converted to methylmercury and then move through food webs.

> **Key Term: groundwater**
> Water stored underground in pores and cracks within soil, sediment, or rock.

### Why groundwater metal pollution is especially serious

Groundwater pollution is harder to fix than many surface-water problems. Rivers can flush, dilute, and sometimes recover from degradable waste. Aquifers usually cannot do this quickly. Groundwater moves slowly, often has low dissolved oxygen, and is surrounded by porous rock and sediment that can absorb pollutants.

> **Key Term: aquifer**
> A permeable underground layer of rock or sediment that stores and transmits groundwater.

Heavy metals are especially dangerous because they do not biodegrade. Bacteria can break down some organic pollutants, but they cannot destroy an element like arsenic, lead, or mercury. That means management usually focuses on **prevention, immobilization, removal, or treatment**, not natural breakdown.

> **Key Term: persistent pollutant**
> A pollutant that remains in the environment for long periods because it does not break down easily.

This is why even a small contamination event can matter. The source material may stop entering the aquifer, but the metal already present can remain in sediments, dissolved in water, or attached to mineral surfaces.

A key APES link here is with mining. Mining exposes buried sulfides and metals to air and water. Rainwater can then leach metals out of exposed rock or tailings. This process may continue long after the mine closes.

> **Key Term: leaching**
> The downward movement of dissolved substances through soil into groundwater.

Cause-and-effect chain students should be able to state clearly:

- mining or waste releases metals
- rain or infiltrating water dissolves them
- leaching carries them downward
- contaminants enter groundwater
- slow water movement limits flushing
- contamination persists for long periods
  > **Test Tip:** In FRQs, do not stop at “metals pollute groundwater.” Show the pathway: source -> leaching/infiltration -> aquifer contamination -> ecological or health effect.

### Sources of heavy metals: natural vs human-caused

A frequent exam trap is assuming all heavy metals in groundwater come from industry. That is not true.

**Geogenic sources** are natural sources from the earth itself. Groundwater moves through mineral-bearing rocks and sediments, and this water-rock interaction can release metals and trace elements. Arsenic is the best example. In some aquifers, especially under certain chemical conditions, arsenic is released from sediments into groundwater without any factory spill or mine nearby.

**Anthropogenic sources** are caused by human activity. These include:

- mining and mine tailings
- industrial waste disposal
- landfill leachate
- deep-well injection of hazardous waste
- urban runoff
- some agricultural inputs
- corrosion of pipes, solder, brass fixtures, pumps, and well components
  > **Key Term: geogenic**
  > Originating from natural geological materials or processes rather than direct human activity.

This distinction matters because it changes the solution. If arsenic is naturally present in aquifer sediments, the answer is not simply “stop the factory.” The likely response is testing, alternate water sources, or treatment. If lead comes from plumbing corrosion, replacing pipe materials or improving corrosion control may be more useful than treating the aquifer.

A strong example of natural contamination is the **Bangladesh arsenic crisis**. Millions of people were exposed to arsenic in groundwater from tube wells. The wells reduced microbial disease compared with surface water, but many tapped naturally arsenic-rich sediments. This is a classic APES lesson: one environmental problem can be reduced while another is created.

A strong example of contamination after water leaves the source is the **Flint water crisis**. In **2014**, Flint changed water source, and improper treatment increased corrosivity. Lead leached from old pipes into the drinking water. That means the lead problem was mainly in the **distribution system**, not that the aquifer itself suddenly became lead-rich.

### Why chemistry matters: pH, redox, and speciation

Heavy metals do not behave the same way in every aquifer. Their mobility depends on groundwater chemistry.

**pH** affects how strongly metals stay attached to soil and mineral surfaces. In general, low pH means more acidic water, and acidic water often increases the mobility of many positively charged metal ions such as lead. That is why corrosive water can dissolve lead from pipes or plumbing fixtures more easily.

Arsenic is more complicated. It often behaves as an oxyanion rather than a simple metal cation, so higher pH can sometimes increase its mobility by promoting desorption from mineral surfaces.

**Redox conditions** also matter. Redox means whether conditions are oxidizing or reducing. In reducing, low-oxygen groundwater, iron oxides that once held arsenic can dissolve, releasing arsenic into the water. This is one reason arsenic contamination can be severe in certain aquifers even when there is no nearby pollution source.

> **Key Term: redox conditions**
> The oxidation-reduction state of an environment, which affects the chemical form and mobility of substances.

Another key idea is **speciation**. This means the chemical form of an element, not just the element name.

> **Key Term: speciation**
> The specific chemical form of an element in water, which affects its toxicity, mobility, and treatment.

For arsenic, the two major forms are:

- **arsenite, As(III)**, more common in reducing groundwater
- **arsenate, As(V)**, more common in oxidizing groundwater

This matters because As(III) is generally more toxic and often harder to remove than As(V).

Mercury also has important forms. Inorganic mercury in water is dangerous, but the biggest environmental concern is **methylmercury**, which forms under low-oxygen microbial conditions in aquatic sediments and wetlands.

> **Key Term: methylmercury**
> A highly toxic organic form of mercury produced by microbial processes in low-oxygen aquatic environments.

Students often miss an important link here: groundwater mercury is one issue, but mercury’s greatest APES importance is often what happens after it enters aquatic systems and is transformed into methylmercury.

> **Test Tip:** If an exam question asks how chemistry changes contamination, use precise language such as “acidic water increases lead mobility” or “reducing conditions can release arsenic from iron oxides.”

### Arsenic, lead, and mercury: do not treat them as identical

A high-scoring response compares these metals rather than listing them as a group.

#### Arsenic

Arsenic is often a **groundwater** problem in the aquifer itself. It is commonly geogenic and released by water-rock interaction or changing redox conditions. Chronic arsenic exposure is linked to skin damage, circulatory problems, nervous system effects, and increased cancer risk.

For public drinking water in the United States, the EPA **Maximum Contaminant Level (MCL)** for arsenic is **0.010 mg/L**, which is also **10 µg/L** or about **10 ppb**.

#### Lead

Lead is a strong neurotoxin, especially harmful to children and developing fetuses. It can reduce IQ, damage the nervous system, and contribute to kidney damage and high blood pressure.

But in drinking water, lead often enters **after** the groundwater is pumped. The source is frequently corrosion of lead service lines, solder, brass fixtures, or well parts. That is why boiling contaminated water is not a solution: boiling removes water, not lead, so concentration can even increase.

Lead is unusual in regulation. For public systems, it has often been regulated through the **Lead and Copper Rule** with an **action level** of **0.015 mg/L** rather than a classic MCL.

> **Key Term: action level**
> A regulatory threshold that triggers required treatment or corrective action, rather than a standard “safe level.”

#### Mercury

Mercury can come from mining, industry, and atmospheric deposition. Coal combustion is a major mercury source to the environment. Mercury deposited onto land or water can eventually enter aquatic systems.

The EPA public drinking-water MCL for **inorganic mercury** is **0.002 mg/L**. But for APES, the bigger food-web story is methylmercury. Once formed, it builds up in organisms and becomes more concentrated at higher trophic levels.

> **Key Term: biomagnification**
> The increasing concentration of a substance at successively higher trophic levels in a food chain.

This is why fish consumption advisories often focus on mercury. Top predators accumulate the greatest concentrations.

The classic case study is **Minamata** in Japan, where industrial mercury discharge led to methylmercury contamination of fish and shellfish, causing severe neurological disease in people who consumed them.

### Human health, the environments, and regulation

Heavy metals affect both people and natural systemss, but AP students should distinguish **drinking-water standards** from broader ecological effects. A drinking-water MCL is designed for human exposure through public water systems. It is not the same as an aquatic natural environments threshold.

Groundwater contamination can also create environmental justice issues. Private wells are a good example. Public systems are regulated under federal drinking-water rules, but **private wells are not regulated by the EPA** in the same way. That means homeowners are often responsible for testing and treatment. So a family using a private well may face serious contamination that goes undetected for years.

This creates a pattern examiners like: hazard exists -> regulation gap -> exposure risk remains.

Important laws to connect accurately:

- **Clean Water Act (1972)** regulates many point-source discharges into surface waters
- **Safe Drinking Water Act (1974)** focuses on safe public drinking water, especially groundwater and other drinking-water sources

Be careful, though: the Safe Drinking Water Act does **not** generally cover private wells.

### Treatment and prevention strategies

Because metals persist, prevention is usually better than cleanup.

Possible strategies include:

- preventing releases from mines, landfills, and industry
- testing groundwater and private wells
- corrosion control in public water systems
- replacing lead-containing plumbing
- treating contaminated water with reverse osmosis, distillation, or adsorptive media
- pump-and-treat or containment at contaminated sites

Treatment depends on the contaminant and its chemical form. Arsenic treatment is often more effective for As(V) than As(III), so students should avoid saying “one filter fixes all metal pollution.”

A final APES comparison:

- **prevention** works best because contamination is hard to reverse
- **treatment** protects drinking water but may not restore the aquifer
- **containment** may limit spread without removing all pollutants

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Heavy metals in groundwater are serious because they are persistent and do not biodegrade
- Groundwater contamination can come from geogenic sources as well as anthropogenic sources
- Leaching moves dissolved contaminants downward through soil into aquifers
- Aquifers recover slowly because groundwater moves slowly and pollutants are not flushed out quickly
- Arsenic is often naturally released from aquifer materials under certain pH and redox conditions
- Lead in drinking water often comes from corrosion of pipes or fixtures rather than the aquifer itself
- Mercury is especially significant because it can be converted to methylmercury and biomagnify in food webs
- Low pH often increases the mobility of metals such as lead
- Reducing conditions can release arsenic from iron oxides into groundwater
- Speciation affects toxicity, mobility, and treatment, especially for arsenic and mercury
- The EPA arsenic MCL is 0.010 mg/L and the inorganic mercury MCL is 0.002 mg/L
- Lead is commonly regulated through an action level, and private wells may still be contaminated even though EPA public-system rules do not usually apply to them

## Key Terms and Concepts

- groundwater
- aquifer
- persistent pollutant
- leaching
- geogenic
- redox conditions
- speciation
- methylmercury
- action level
- biomagnification


Human Impacts on Ecosystems - Heavy metals in groundwater