Amman – Flash floods and runoff triggered by climate change pose environmental risks that are not immediately visible in surface water. They can remobilize pollutants and toxic metals that have remained trapped in soil and valley sediments for years or even decades, potentially transporting them to new areas and threatening soil quality, agricultural crops and living organisms while creating health risks whose effects may not become apparent immediately.
The phenomenon, scientifically known as the remobilization of legacy pollutants, has raised concerns among experts over the pollutants stored in Jordan’s land, particularly in areas affected by industrial, mining and agricultural activities.
It also raises the possibility that flash floods could become more than a flood-related hazard, serving instead as a mechanism for redistributing pollution, increasing the bioavailability of some toxic substances and facilitating their movement through the food chain.
To reduce the potential environmental risks associated with the remobilization of legacy pollutants by floods, experts have called for a preventive approach based on establishing an integrated program to monitor soil and valley-sediment pollution, particularly in industrial, mining and agricultural areas.
Sources of Pollution
Dr. Maan Al-Ghazawi, an assistant professor and researcher in environmental chemistry and human health at the Department of Natural Resources and Environment at Jordan University of Science and Technology, said contaminated soil and valley sediments affected by various pollutants could be among the most important sources of pollution, particularly toxic and heavy elements.
Al-Ghazawi cited arsenic (As), one of the most toxic elements, as well as lead (Pb), cadmium (Cd), zinc (Zn) and others.
He told Al-Ghad that these elements can remain stored in soil for long periods, while flash floods and surface runoff resulting from heavy rainfall over short periods can redistribute them across different areas in varying concentrations and quantities.
He added that floods can increase the dissolution of some of these elements and the minerals containing them, as well as their mobility and bioavailability.
Land use, he said, plays a major role in determining the nature and type of these pollutants, as well as their quantities and distribution beyond the natural geological background.
A number of factors also determine the level of risk posed by the interaction between these pollutants and floodwaters, including the source of pollution, soil mineral composition, chemical and climatic conditions, and the intensity of flooding.
In Jordan, mining areas, quarries and regions affected by agricultural and industrial activities could play a significant role in influencing floodwater and surface runoff in valleys and redistributing heavy metals and toxic elements to different locations, he said.
Al-Ghazawi based his assessment on a study he conducted in the southwestern United States examining chemical changes in the forms of toxic elements and their biological availability at several abandoned mining sites, as well as the impact of different climatic conditions on their distribution in contaminated soil.
The study showed that these elements could reach human communities and enter the human body through consumption of contaminated food and water. Some, such as arsenic, have been linked to several diseases, including diabetes and cancer.
He warned that soil and valley sediments could serve as long-term reservoirs for pollutants.
Oxidative Processes
Al-Ghazawi noted that risk is not determined solely by total concentration. It is also affected by the chemical form of an element, its solubility and absorption potential, the duration of exposure and the nature of the organisms affected.
He explained that erosion and exposure of pollutants buried underground to water and oxygen, known as oxidative processes, can occur through flood movement. This is one of the key mechanisms that can release pollutants into the environment and alter their chemical composition in ways that make them more biologically available in many cases, as demonstrated by numerous studies.
He stressed that the toxicity of a metal can change without any change in its total concentration in soil.
Floods can alter the conditions controlling these characteristics, including pH, redox potential (Eh), oxygen concentration, dissolved organic matter and the composition of minerals carrying the pollutants, he said.
However, Al-Ghazawi stressed that not every flood necessarily increases bioavailability. The opposite can also occur, such as metals being deposited in less soluble forms or becoming reattached to iron oxides. Therefore, actual changes must be measured rather than assumed.
In Jordan, where large areas are arid or semi-arid, soil may have geological and chemical conditions that allow heavy metals to remain trapped for years before being suddenly released, he said.
However, identifying the areas most vulnerable to such risks requires field data on rocks, soil, sediments and potential pollution sources.
He cited his study of five sites in the United States, all located in arid or semi-arid areas, which showed that the risk was associated with surface soil layers containing higher concentrations of toxic elements and relatively high bioavailability near the soil surface.
He warned, however, that the same conditions that trap metals may also allow them to be released when the chemical environment changes.
For example, sediments rich in iron oxides can effectively retain arsenic under certain conditions, but may release it if those oxides dissolve under reducing conditions, such as water saturation resulting from heavy rainfall.
Similarly, areas rich in sulfides may experience the release of certain metals when exposed to oxygen and water and acidic conditions develop. By contrast, carbonates can neutralize acidity and reduce the dissolution of some metals, he said.
Therefore, risk assessments in Jordan should integrate geology, hydrology and soil chemistry rather than relying solely on rainfall or flood maps, he said.
Chemical Associations
Al-Ghazawi did not rule out the possibility that some metals could accumulate in plant roots, leaves or edible parts without producing visible symptoms such as yellowing or wilting.
The absence of visible symptoms, he stressed, does not necessarily mean that a crop is safe for consumption.
He summarized the potential consequences by noting that each toxic element is associated with a range of health effects and diseases. For example, exposure to lead is associated with neurological effects, with risks particularly significant for children.
He also cited arsenic as an example, saying chronic exposure to the element is associated with serious health risks, including an increased risk of certain cancers.
He expected groundwater in Jordan could also be affected if such flooding occurs, given the presence of groundwater resources in several areas across the country.
Applying Scientific Reasoning
Dr. Naeem Mazahreh, an expert in irrigation water management, said the phenomenon of legacy pollutant remobilization should be addressed through scientific and diplomatic approaches.
Mazahreh described the phenomenon as a natural interaction between the force of surrounding water, the terrestrial environment and rainfall, which can contribute to cleansing the land. With sound management, pollution can be prevented, he said.
He did not rule out the possibility that valleys could become geological pathways that redistribute pollution accumulated over decades. However, he said they should also be viewed as natural channels for distributing water and fertile soil on which agriculture depends.
Valleys function as natural drainage networks, and the high kinetic energy of heavy rainfall can wash and rearrange soil, he explained.
Some areas are more vulnerable to pollutant redistribution because of characteristics including steep topography, which increases surface runoff speed; limited vegetation cover, which reduces soil and sediment stabilization; and clay or sandy sedimentary formations that are easily broken down and transported.
The benefit, he said, is that this movement of water can renew agricultural soil in low-lying basins by transporting silt and nutrient-rich sediments.
Under certain conditions, however, these channels can transport sediments containing certain elements or historically accumulated compounds, particularly in areas with steep terrain, fragile or inadequately vegetated soils and high surface runoff rates.
“This erosion can release pollutants that had been buried. But scientifically and diplomatically, this same erosion is the natural mechanism that creates mineral- and nutrient-rich agricultural soils in floodplains,” he said.
He explained that water reaching crops through modern irrigation techniques such as sprinklers or drip irrigation is generally clean, whereas floodwater is loaded with old sediments and reaches natural or artificial sedimentation basins where it can be filtered before being pumped into irrigation networks.
Changes in rainfall patterns also alter sediment movement and transport capacity, he said.
He illustrated this by noting that a severe and sudden storm can move within hours what accumulated over decades, while at the same time contributing to the replenishment of dams and springs in Jordan.
In this regard, he proposed using precise spatial planning to direct floodwater pathways toward agricultural areas in need of water, while establishing sedimentation basins at convergence points to remove any legacy pollutants before the water is pumped into irrigation networks.
As for harmful materials, he said their geological accumulation sites can be identified using digital elevation models (DEMs) to determine slope contours and flow accumulation points.
This step, he explained, would help establish sedimentation reservoirs at the entrances to these basins to filter the water from legacy pollutants before using the treated water to irrigate surrounding crops.
Other techniques he cited include remote sensing and spatial environmental-risk distribution models, such as those developed by Marghany and Hashim in 2019, to map potential flow scenarios before they occur.
He stressed that the vast majority of flood-risk studies in Jordan focus primarily on flood hydrology, peak-flow calculations, water volumes and infrastructure protection.
In his view, this focus is justified given Jordan’s severe water scarcity.
Source: Al-Ghad
