Petroleum-driven groundwater contamination in Azzan city, Yemen: Integrated assessment using hydrochemistry, trace metals, PAHs, and PCBs

In many parts of the world, the ground beneath our feet acts as a vast, hidden reservoir, storing water that has filtered slowly through layers of rock and sand. This groundwater is often the only source of drinking water for communities living in dry, arid regions where rain is scarce and rivers run dry. However, this underground resource is not immune to the activities of human industry. When oil is extracted, transported, or processed, it can leak into the soil and seep down into these aquifers, carrying with it a complex mixture of harmful substances. These contaminants include heavy metals, which are toxic elements that do not break down, and organic chemicals that can linger in the environment for decades. Understanding how these pollutants move, how they mix with the natural minerals already present in the water, and what they mean for human health is a critical task for scientists working to protect public safety.

In the city of Azzan, located in the Shabwah Governorate of Yemen, researchers recently conducted a detailed investigation into just such a scenario. This area is arid and sits near a major corridor for crude oil pipelines, making the local groundwater a potential target for contamination from the petroleum industry. A team of scientists from universities in Yemen and Tunisia set out to map the quality of the water in seven wells situated along this pipeline route. Their goal was not just to measure how salty or dirty the water was, but to perform a comprehensive check that combined the study of water chemistry, toxic metals, and specific organic pollutants known as polycyclic aromatic hydrocarbons and polychlorinated biphenyls. By looking at all these factors together, they aimed to distinguish between pollution caused by natural geological processes and pollution caused by human activity, ultimately determining the risk this water poses to the people who rely on it.

The researchers began by collecting water samples from the seven wells in December 2024. They analyzed the physical properties of the water, such as its temperature and how well it conducts electricity, which serves as a measure of how many dissolved salts and minerals are present. They found that the water varied significantly from one well to another. Some samples were relatively fresh, while others were highly mineralized, containing dissolved solids that ranged from 406 to 3078 milligrams per liter. The chemical makeup of the water was dominated by calcium, magnesium, and sulfate, a pattern that suggests the water is naturally interacting with the rocks it flows through, dissolving minerals like gypsum and limestone. This natural process creates a baseline level of salinity, but the researchers noticed that in certain wells, the levels of these minerals were unusually high, hinting at additional influences.

Beyond the natural minerals, the team looked for toxic trace metals that often accompany petroleum activities. They found that the water in several wells contained dangerous levels of lead, arsenic, and cadmium. In some locations, the concentration of lead was far above the safety limits set for drinking water, and arsenic levels were similarly alarming. The wells named Mahdi and Atash emerged as the most severely affected, showing the highest concentrations of these toxic metals. The scientists calculated various pollution indices to quantify the severity of the problem, and the results were stark: the water in these areas was deemed unsuitable for drinking due to the cumulative burden of heavy metals. This contamination was not uniform; it was concentrated in specific hotspots, suggesting that the pollution was not just a result of the local geology but was driven by localized leaks or spills from the nearby oil infrastructure.

The investigation also turned to organic pollutants, specifically polycyclic aromatic hydrocarbons, or PAHs, and polychlorinated biphenyls, or PCBs. These are complex chemical compounds often found in crude oil and industrial waste. The researchers detected both types of chemicals in every single sample they tested. The PAHs found in the water were primarily low-molecular-weight compounds, with naphthalene making up nearly 90 percent of the total. This specific chemical signature is a strong indicator that the source of the pollution is petroleum itself, rather than something like burning wood or coal. Similarly, the PCBs detected were dominated by tetrachlorinated compounds, which are known to persist in the environment for long periods. The presence of these chemicals in all the wells confirmed that the groundwater system is contaminated with persistent organic pollutants that do not easily degrade.

To understand how these different pollutants relate to one another, the scientists used statistical tools to look for patterns in their data. They discovered a very strong connection between the salinity of the water and the amount of PAHs present; as the salt content went up, so did the concentration of these organic pollutants. This suggests that the same processes moving the salts through the aquifer are also transporting the oil-related chemicals. The analysis also separated the wells into distinct groups based on their contamination levels. The wells in Mahdi and Atash formed a group with the highest pollution, while others like Al-Mashroa showed lower levels. This grouping confirmed that the contamination is not a blanket issue across the entire region but is concentrated in specific areas where the impact of the pipeline corridor is most severe.

Perhaps the most critical part of the study was the assessment of what this contamination means for human health. The researchers calculated the potential risks for people who might drink this water over a long period. They found that the risk of non-cancer health problems, such as organ damage, was extremely high, with a hazard index that was more than sixteen times the safe limit. This danger is driven primarily by exposure to arsenic and lead. Furthermore, the risk of developing cancer from drinking this water was also found to be significantly higher than what is considered acceptable. The presence of both toxic metals and carcinogenic organic compounds creates a double threat, meaning that the water poses a serious, long-term danger to the health of the community.

The study concludes that the groundwater in Azzan City is caught in a complex situation where natural geological processes have created a baseline of mineral-rich water, which is then severely compromised by human activity. The evidence points clearly to petroleum-related activities as the driver of the toxic metal and organic pollutant spikes seen in the most affected wells. The researchers emphasize that this is not a problem that can be ignored or solved by simple dilution, as the arid climate limits the natural ability of the environment to wash away these contaminants. They call for continuous monitoring of the water, stricter controls on the oil infrastructure, and targeted efforts to protect the most vulnerable wells. Without immediate and sustained intervention, the water resources in this region face a future of progressive degradation, leaving the local population with fewer safe options for their most basic need.