Multivariate Pollution Risk Assessment of Natural Radionuclides and Heavy Metals in Topsoils of Nonwa, Rivers State, Nigeria

Soil is more than just dirt underfoot; it is a living archive that holds the history of a place, storing nutrients for plants while also acting as a sink for whatever humans have poured onto the land. In regions where industry and nature collide, this archive can become a record of pollution, holding traces of heavy metals and natural radioactive elements that seep into the food chain and the air we breathe. When people live on this land, they interact with it constantly, breathing in dust, touching the ground, and eating crops grown in it. This interaction creates a direct pathway for contaminants to enter the human body, posing risks that range from organ damage to an increased chance of developing cancer over a lifetime. Scientists study these hidden dangers by measuring the specific amounts of radioactive materials and toxic metals in the soil, calculating how much radiation a person might absorb, and determining whether the pollution comes from natural rock formations or from human activities like oil spills and farming.

In the Niger Delta of Nigeria, a region known for its vast oil reserves and complex industrial history, a team of researchers turned their attention to a community called Nonwa. This area is not only a hub for oil and gas infrastructure but also a place where residents rely heavily on subsistence farming and where the soil has been subjected to decades of crude oil spills, gas flaring, and illegal artisanal refining. The researchers wanted to understand the full picture of the health risks facing the people there. They did not just look for one type of danger; instead, they examined the soil for both natural radionuclides—naturally occurring radioactive elements found in the earth—and heavy metals, which are toxic substances often released by industrial processes. By collecting soil samples from the top layer of the ground in three different sub-communities within Nonwa, they aimed to measure the concentration of these substances, calculate the potential health risks to residents, and use statistical tools to figure out exactly where these pollutants were coming from.

The team collected soil from a depth of twenty centimeters, a standard depth that captures the layer most likely to be disturbed by farming or construction. They analyzed these samples for three specific natural radioactive elements: uranium-238, thorium-232, and potassium-40. They also tested for eight different heavy metals, including lead, arsenic, zinc, copper, cadmium, chromium, manganese, and nickel. The results showed that the levels of uranium and thorium in the soil were lower than the global average found in soils around the world. However, the levels of potassium-40 were higher in one specific area, Nonwa-Tai, likely due to the natural mineral composition of the local rocks or the use of phosphate fertilizers in nearby farms. When the researchers calculated the radiation risks, they found that most safety indicators were within acceptable international limits. Yet, there was a concerning exception: the risk of developing cancer over a lifetime for people living indoors was about fourteen percent higher than the safety standard. This is because people spend most of their time inside their homes, where radiation from the ground and building materials can accumulate. In the Nonwa-Tai community, the radiation dose to reproductive organs was also twenty-two percent higher than the safety limit, suggesting a potential threat to the health of the residents' bone marrow and reproductive systems.

Beyond radiation, the study looked at the heavy metals to see if the soil was contaminated by human activity. The researchers found that while the metal levels were generally low compared to global averages, they were significantly higher than the natural background levels for the Niger Delta region. Specifically, manganese, zinc, chromium, and lead were found in concentrations that were higher than what is naturally expected in the area's soil. This indicated that human activities had indeed altered the chemical makeup of the ground. The researchers used advanced statistical methods to group these metals together based on how they behaved, which helped them trace the sources of the pollution. They discovered that the metals did not come from a single source but rather from a mix of activities. Some metals were linked to the burning of crude oil and the waste from illegal refining, while others were associated with the use of agricultural chemicals on nearby farms. The analysis revealed that the heavy metals were not responsible for increasing the natural radioactivity levels in the soil; instead, the radioactivity was driven by the natural geology of the area, specifically the types of minerals like feldspar and clay present in the ground.

The study also explored how the soil's acidity, or pH, influenced the behavior of these pollutants. They found that the soil in the polluted areas was more acidic, which can make certain toxic metals more soluble and easier for plants to absorb. This is a critical finding because it means that crops grown in these soils could accumulate dangerous levels of metals, which would then be passed on to the people who eat them. The researchers noted that the heavy metals were not just sitting passively in the dirt; their movement and toxicity were closely tied to the chemical environment of the soil. For instance, the presence of acidic conditions from gas flaring and oil spills appeared to increase the mobility of lead and chromium, making them more likely to leach into groundwater or be taken up by crops. The statistical grouping of the metals confirmed that the pollution was a result of petrogenic sources—meaning it came from oil and gas activities—as well as the use of agrochemicals. The study explicitly ruled out the idea that the heavy metals were causing the natural radioactivity to rise, clarifying that the two types of hazards have different origins in this region.

To address these risks, the researchers proposed practical steps to protect the community. They suggested applying lime to the soil in areas where crude oil has spilled. Lime is a common material that can raise the pH of the soil, making it less acidic. By doing this, the toxic metals would become less soluble and less likely to be absorbed by plants or to wash into the groundwater. They also recommended stopping the gas flaring and the illegal artisanal refining activities, as these are the primary sources of the pollution. The study concluded that while the soil in Nonwa is not universally toxic, specific pockets of contamination pose real health risks, particularly regarding cancer and organ damage. The findings provide a clear, data-driven guide for local authorities and residents, highlighting the need for remediation strategies that focus on neutralizing the soil's acidity and removing the sources of pollution to safeguard the health of the community and the safety of the food they grow.