UPWr Experts – protecting our soil after the flood

We're all focusing on the damage caused by flooding, but little is said about how destructive this natural disaster is to the soil.

Exactly. Unfortunately, a flood not only destroys homes, bridges, streets, and takes away people's belongings and livelihoods. The enormous mass of water flowing at great speed also causes significant disintegration of the top layer of the soil, leading to its rapid degradation. Additionally, the soil we cultivate gets washed away to random places, and often torrential rains or a flood wave can undo many years of a farmer's work. But that’s not the only problem soil scientists are highlighting.

There’s another one?

Yes, during the movement of the tidal wave, bottom sediments from the riverbed get stirred up. The water transports them to floodplains, creating serious issues.

I was taught in school that bottom sediments are a good thing, citing the floods in the Nile Valley as an example, as they contain many organic substances beneficial to soil fertility.

Ancient Egypt is a very distant past. Today we live in the Anthropocene, where human activity—industrial, agricultural, and urban—plays a key role. This activity also impacts bottom sediments, which record the history of pollution emissions into water and the atmosphere over the last few hundred years. During a flood, the water in the riverbed mixes and disturbs these sediments, spreading them. Precisely speaking, bottom sediments are a ticking time bomb.

Why?

Because these sediments accumulate pollutants. At the bottom of the river, under anaerobic conditions, they remain dormant. But when they are disturbed, as happens during a flood, they become aerated, releasing various pollutants into the atmosphere, such as mercury compounds, toxic elements that can become volatile, and aromatic hydrocarbons. Since water is an excellent carrier of many pollutants, they are easily absorbed from the bottom sediments onto flooded lands. We are also dealing with a new type of pollution in rivers and bottom sediments—microplastic contamination, residues from pharmaceuticals, antibiotics, and hormonal agents, which easily bind to organic matter.

In the past, there were fewer pollutants, but we also didn’t have the means to identify them. Now, with advancements in science, we have sophisticated techniques that allow us to detect many substances resulting from industrial activities and domestic sewage, which are present in water, soil, and bottom sediments, even in trace amounts. Identifying these substances is one issue, but many are classified as emerging contaminants, which require urgent attention from scientists and relevant authorities because we don’t have developed technologies to remove or prevent their spread into the environment.

Like a cleaning emergency?

You could call it that, but the key concern is how these disturbed bottom sediments—or rather, what’s in them—will interact with plants and animals as they transition from anaerobic to aerobic conditions, and eventually affect humans.

In addition to cleaning, the process of soil reclamation is also critical and will need to be carried out in these areas. People are asking us, soil scientists, what to do with the sediment that the flood brought to their fields, ponds, and gardens—whether to use it, leave it, or lime it.

Do you treat these sediments like typical sewage sludge?

Not exactly. Bottom sediments have a different chemical composition. They carry both organic matter and mineral particles, but also many chemical and microbiological contaminants, making them similar to sewage sludge in some respects. At the moment, we don’t know precisely what types of pollutants have been deposited by floodwaters onto the soil, which is why it should be carefully examined, especially for contaminants that could pose a threat to the quality of crops or animals grazing in these areas.

What if the contamination levels in post-flood soil exceed acceptable limits?

The soil needs to be reclaimed. The problem, however, is that bottom sediments, which should be removed from the area, are not subject to the same disposal and management procedures as sewage sludge. First of all, there are no legal regulations in Poland defining the quality of bottom sediments or how to manage them if they accumulate outside the riverbed, as can happen during a flood. In Poland, we don’t have facilities dedicated to cleaning soil or sediments after a flood. Such installations exist in many European countries, where thorough analyses of collected samples are conducted to determine how to clean them. For now, in Poland, contaminated soils and sediments are often placed in open heaps in designated areas within the municipality to be composted. In situ remediation methods—cleaning the soil where the contamination occurred—are also commonly used. This approach is often better, less destructive to the soil environment, and promotes faster regeneration of the soil’s biological life, which is essential.

Can various compounds and pollutants from bottom sediments deposited by floodwaters seep into the soil? Is this a serious threat?

Let’s not panic. At this point, we don’t know what we should be afraid of, or whether, for example, the organic matter in these sediments will undergo faster biological oxidation. Underwater conditions are anaerobic, but on the soil’s surface, oxygen is present, and microorganisms multiply, initiating biodegradation processes. In that case, the pollutants stored in the sediment may start to be released into the soil. But this is a slow process… There’s another issue, though—some people will try to minimize their losses by harvesting crops or what’s left of them from flooded areas. The fields currently have corn, but that which was flooded should be tested, and if contamination levels exceed acceptable limits, it should be destroyed. Right now, we also have a lot of cucurbit vegetables in the fields, and people might think they are safe because they have thick skins, but everything that is water-soluble can easily enter the plant. The potato harvest is coming up, as well as other root crops like beets and carrots, which are also at risk of direct contamination. And we’re not sure that during their processing—such as peeling and cooking—the health risks will be reduced.

There is a discussion about how to protect ourselves from extreme weather events. This discussion is not just about reservoirs and hydrological protection but also about changing forestry management practices in mountainous areas, reducing deforestation in favor of creating rain-protective forests.

We certainly need to implement systemic solutions because, of course, we can’t guarantee that a flood won’t happen again in one, two, or ten years. However, we should not only discuss protecting people and their homes but also the soil environment, which is being polluted by atmospheric deposits, sewage, and during floods by river waters and bottom sediments. If we actively stop these pollutants, they will be unavailable to plants, won’t leach into groundwater, and will be permanently bound. But we must also be aware that any intervention causing erosion processes, even sunlight exposure on the soil surface or leaving fields bare in the autumn-winter season, will accelerate the degradation of organic matter in the soil. This leads to carbon dioxide emissions into the atmosphere, while accumulated contaminants, such as heavy metals, are absorbed by plants.

I know that during the flood, you went to key locations and collected soil samples.

Yes, because from my perspective as a soil scientist, being in the field is crucial. Satellite observations and models are important, but personal observation is much more significant. You have to see for yourself what’s happening with the soil, whether it’s properly cultivated and supplied. And most importantly, protected.

Interview by Katarzyna Kaczorowska.

The cultivation of food plants like vegetables, herbs, or fruits on post-flood lands carries the risk of accumulating substances harmful to health. Soil retains contaminants for a long time as it does not have self-cleaning abilities. The most at-risk soils are organic soils or hortisols, which are enriched with organic matter, such as compost or peat—typical soils in our gardens. If your gardens and fields have been flooded, seek help from UPWr experts and bring soil samples to the Environmental Quality Analysis Center, where specialists will conduct a comprehensive analysis to determine if the soil is safe for growing food plants. Contact: Environmental Quality Analysis Center, Wrocław University of Environmental and Life Sciences, pl. Grunwaldzki 24a (room 010a), 50-363 Wrocław, tel./fax 71 320 1547, e-mail: cajs@upwr.edu.pl.

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  UPWr Experts – flooding in Poland

  Prof. Mirosław Wiatkowski, head of the Department of Hydrology and Water Management, discusses how to fight floods, the effectiveness of reservoirs and floodplains, and water management planning, while Prof. Tomasz Kowalczyk from the Department of Environmental Shaping and Protection discusses the lessons we should learn from recent difficult days. Meanwhile employees of the Institute of Environmental Engineering have been conducting unique research on the Oder River since it all began.