A Hidden Threat Beneath Our Feet

Imagine living near a peaceful coastal town. The sun shines, waves crash gently on the shore, and everything seems calm. But deep beneath your feet, there might be something dangerous lurking in the soil: arsenic (As). This poisonous element was left behind by factories, chemical plants, and other industries that operated long ago. During calm weather, As stays put, buried and mostly inactive. But when a flood or hurricane strikes, everything changes. Floodwater does not just rise, it swirls, churns, and claws at the Earth. It digs into the ground like a giant muddy spoon, stirring up the sediment and carrying its contents into nearby lakes, rivers, and even drinking water systems. If that sediment is contaminated, the flood can unleash chemicals long thought to be trapped and forgotten. Suddenly, yesterday’s pollution becomes today’s danger. This problem is especially urgent in U.S. Environmental Protection Agency (EPA) Superfund sites, which are locations across the United States that were once polluted so badly that the federal government had to step in and begin cleanup efforts. Many of these sites are in coastal areas or areas prone to flooding—not only shorelines, but also river and lake systems where contaminated sediments can be stirred up during floods.

Building A Storm, One Drop At A Time

We did not wait for a real hurricane to answer our questions. Instead, we brought the storm into our lab. Using a custom-built flood simulator (a device capable of creating turbulent flooding conditions) we mimicked flood conditions on a small scale (Figure 1). The device was filled with sediments collected from a contaminated site in Wilmington, Delaware. Collected sediments were lightly (800 ppm), highly (5,000 ppm), and severely (6,500 ppm) contaminated with As (ppm means “parts per million”, which is a way to say how many tiny pieces of something are mixed into one million total pieces.) Each experiment used one of the mentioned sediment groups and had three phases: pre-flooding, flooding, and post-flooding. The goal was to understand how contaminated sediments behave before, during, and after flooding and hurricanes. At flooding phase, we slowly increased the speed of water over the sediment surface to match what happens during a light rain, a strong storm, and a full-blown flood. During the artificial flood, we gathered water samples at regular intervals. We were curious about what would break loose, and how quickly.

We also analyzed the As form. Arsenic comes in different chemical forms, and some are more dangerous than others. Arsenite, or As³⁺, dissolves easily in water, which increases its mobility and toxicity. Arsenate, or As⁵⁺, is slightly less poisonous and tends to stick to sediment particles. If a flood releases mostly As³⁺, it would mean more serious environmental and human health risks.

How Floods Mobilize Arsenic And Degrade Water Quality

As the water picked up speed, the sediment responded. It began to erode, just like in a real flood event, and tiny particles carrying As broke free. Within minutes of high-speed flow, we observed sharp increases in the As levels in the water column above highly and severely contaminated sediments (Figure 2). It was as if the sediment had been shaken awake and forced to give up its secrets. This process is called sediment resuspension, and it is one of the most important ways that metalloids like As can get into the water column during floods [1]. In the lightly contaminated sediments, arsenic in the water went down, likely because there was not much arsenic available to release. In the more contaminated sediments, faster water movement released more arsenic into the water. This shows that strong flow during floods can make polluted sediment release arsenic, especially when the sediment is heavily contaminated.

The story grew even more concerning after sediment resuspension. The chemical environment in the flooded sediment changed. As water flowed in and removed oxygen, it created conditions that allowed As³⁺ to form [2]. So not only did the water carry As away from sediments, it also helped convert As into a more dangerous form in the water column. This form stays dissolved longer, moves farther, and is harder to clean up.

What Synchrotron X-Rays Revealed

To understand As transformation during flooding at a microscopic level, we used a powerful tool called synchrotron X-ray spectroscopy (Figure 3). At a special government research facility, we blasted tiny bits of sediment with intense X-rays to see exactly what form the As was in and which minerals it was attached to. This technique helped us confirm that flooding conditions can shift As toward the more mobile and toxic As³⁺ form. Even more troubling, the As did not disappear once the flood was over. Instead, it remained in the water column where it could continue traveling through the environment. That means floodwater could carry contamination far from its original source, silently exposing human communities, fish, and plants to toxic pollution long after the storm has passed.

A Widespread And Concerning Risk

After seeing what happened in the lab, we asked a bigger question: how many people might face this hidden danger? Using public maps and environmental data, we studied where EPA Superfund sites overlapped with areas vulnerable to flooding. What we found was eye-opening. Over 40 million people in the United States live in places where polluted soils and sediments could be disturbed by floodwaters. Data show that these flood risks are not shared equally: low-income and minority communities tend to live in areas near contaminated sites and thus face greater risk when floods occur [3]. In many of these areas, families already struggle with aging infrastructure, low incomes, lack of green spaces, and proximity to industrial activities. The danger is not just scientific, it is social. Environmental justice aims to represent and protect all communities equally from environmental risk and harm.

Reducing Risk With Science

The good news is that we are not helpless. There are smart, science-based ways to reduce the risk of As escaping during floods. One way is to change the soil environment. By adding materials like iron oxides, we can help As stick more tightly to soil particles. This keeps it in the soil and makes it less likely to be released into floodwater. Still, if flood conditions strongly change the soil chemistry, iron oxides may release some of that As back into the water. Another option is biochar, a special charcoal made from plants, which improves the soil’s structure and helps trap contaminants. Engineers also use physical barriers to keep contaminated sediments and soils from washing away. Special layers of clay or fabric can cover polluted areas like a blanket, stopping erosion and slowing migration. Even plants can help. Grasses and shrubs with strong roots hold the soil in place and retain water. Every site is different, and the best solutions often use a combination of strategies. In flood-prone communities, we can design landscapes that resist erosion, soak up water more slowly, and contain pollution before it spreads. But that takes planning, teamwork, and trust among scientists, city planners, and local residents. To understand why these solutions work, it is helpful to look at the hidden chemistry that controls how arsenic behaves during flooding.

The Secret Chemistry of Floods

Soil is not just dirt. It is a living, breathing mixture of minerals, microbes, and chemical reactions. During a flood, the chemistry can change rapidly. When water flows in, oxygen levels drop. This shifts how electrons move during chemical reactions. Under low-oxygen conditions, As changes from a less mobile form to a mobile, toxic one. Scientists have long known that these reactions help explain why As can become more dangerous in wet or flooded soils [4]. Understanding these transformations helps scientists predict how sediment will behave when storms hit, and it gives us tools to prepare in advance. If we know that a certain area is likely to release more mobile As during a flood, we can better plan monitoring and risk-reduction strategies ahead of time. Although this article focuses on arsenic, similar processes may also influence other sediment-associated contaminants, including metals such as cadmium and lead. The magnitude and direction of these responses depend on each contaminant’s chemical form, sediment-binding mechanisms, and local conditions. That is why combining chemistry, environmental science, and engineering is so powerful. Each part tells a different chapter of the story, and together they help us protect both people and the planet.

Science In Your Hands

You do not need a lab to think like a scientist. Try this: fill a clear jar with soil and water, shake it, and watch what floats and what sinks. You are seeing erosion in action. Add a handful of leaves or a spoonful of charcoal and see if that changes what happens. You are experimenting with natural filters, just like engineers designing solutions for real floods. You can also learn more about where you live. Are there flood zones or EPA Superfund sites nearby? What are your city’s plans for extreme weather? Asking questions and sharing what you learn helps build stronger, safer communities. Every scientist starts with curiosity. Keep asking, exploring, and testing ideas, and you will help write the next chapter in protecting our environment.

Glossary

Arsenic (As): ↑ A naturally occurring element that is poisonous and causes cancer in humans at high levels. It is found in two main forms: arsenate (As⁵⁺) and the more toxic/mobile form arsenite (As³⁺).

Sediment: ↑ Tiny pieces of soil, sand, clay, or rock that settle at the bottom of rivers, lakes, or other water.

Flood Simulator: ↑ A lab setup that mimics the flow of water during floods so researchers can study what happens.

Sediment Resuspension: ↑ When moving water stirs settled sediment back up into the water.

Synchrotron X-ray Absorption Spectroscopy: ↑ An advanced technique that uses powerful X-rays to identify the chemical form and bonding environment of elements like As in sediments and soils.

Environmental Justice: ↑ The idea that all people should be equally represented and protected from environmental risks and hazards.

Acknowledgments

This research was financially supported by the National Science Foundation, United States Grant No. EEC-2127509 and the University of Texas at Dallas Seed Program for Interdisciplinary Research (SPIRe). A part of the research was under the Delaware EPSCoR’s (Established Program to Stimulate Competitive Research) Project WiCCED (Water in the Changing Coastal Environment of Delaware), supported by the National Science Foundation, United States Grant No. 1757353. We thank the Delaware Environmental Institute, our colleagues at the University of Delaware, and the team at Brookhaven National Lab for their support, especially for helping us use powerful tools to study tiny As particles. Parts of this research used XFM beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Big thanks to the people who allowed us to collect soil from the Wilmington site. Science takes teamwork, and we are grateful to all who helped!

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Conflict of Interest

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