Nemerow Pollution Index and Inter-Variable Relationships in Esuk Oro River, Southern Nigeria

Rivers are living systems that carry a complex story in every drop of water, a story written in the language of chemistry. To understand if a river is healthy or struggling, scientists often look at specific ingredients dissolved or suspended within it, such as nutrients, salts, and oxygen. However, a river contains so many different measurements that it is difficult to see the big picture at a glance. To solve this, researchers use a tool called a pollution index, which acts like a translator. It takes all those different numbers and condenses them into a single score that indicates whether the water is safe or if it has been contaminated. The challenge lies in doing the math correctly; if the rules for comparing the measurements to safety limits are applied in the wrong way, the final score can tell a completely false story, making clean water look dirty or dirty water look clean.

In the Esuk Oro River in Southern Nigeria, a team of researchers from Akwa Ibom State University recently took a closer look at the water quality to ensure the story being told was accurate. They focused on three specific spots along the river: the upstream section near the source, the middle section, and the downstream section closer to where the river flows out. Over a five-month period from late 2022 to early 2023, they collected water samples and measured nine different indicators, ranging from how salty the water was to how much oxygen it held. Their goal was to recalculate the pollution scores for these locations because they suspected that a previous assessment had made a fundamental error in how it handled the data.

The researchers found that the earlier assessment had treated a vital ingredient, dissolved oxygen, incorrectly. In a healthy river, oxygen is something the water needs to have in high amounts to support life. If the oxygen level drops too low, it is a sign of pollution. However, the previous calculation had treated high oxygen levels as a problem, as if too much of a good thing were bad. By fixing this logic, the new study revealed a very different picture. The water in the Esuk Oro River was not suffering from a lack of oxygen; in fact, the oxygen levels were surprisingly high, ranging from 20.50 to 29.91 milligrams per liter, though the researchers noted these numbers were unusually high for warm river water and would need to be double-checked against the original field notes.

Once the oxygen calculation was corrected, the true source of the river's pollution became clear. The study identified phosphate, a nutrient often found in fertilizers and detergents, as the main culprit. At every station along the river, the amount of phosphate in the water was far higher than the safe screening limit. The researchers calculated a pollution score for each location by comparing the measured amount of phosphate to the safety benchmark. The scores for phosphate were between 4.172 and 4.438, meaning the river contained more than four times the acceptable amount of this nutrient. This high level of phosphate was the primary driver behind the overall pollution score for the entire river.

When the researchers combined the scores for all nine indicators into a single composite number, the results showed a river under stress from nutrient enrichment. The overall pollution score started at 3.015 at the upstream station and rose slightly to 3.200 at the downstream station. While phosphate was the overwhelming problem, the study also found that alkalinity, a measure of the water's ability to neutralize acid, was slightly above its limit at all three locations. Additionally, the upstream section had a small amount of suspended solids, or tiny particles floating in the water, that just barely exceeded the safety threshold. In contrast, other common pollutants like nitrates and salts were well within safe limits, and the water was not as polluted as the earlier, flawed report had suggested.

Beyond just measuring pollution, the team looked at how these different chemical ingredients moved together over the five months of observation. They discovered strong connections between certain variables. For instance, when the levels of alkalinity went up, the levels of nitrate and chloride also tended to go up. Conversely, when the total amount of dissolved solids in the water increased, the levels of nitrate and chloride tended to drop. These patterns suggest that the different chemicals in the river are responding to the same seasonal changes or inputs, such as rainfall or runoff from nearby land. However, because the study only looked at five months of data, the researchers are careful to say these are hints rather than proof of a specific cause. They cannot yet say exactly where the phosphate is coming from, whether it is from sewage, agricultural runoff, or household detergents.

The study concludes that the Esuk Oro River is currently facing a significant challenge from phosphate pollution, which is the most pressing issue for water managers to address. The researchers emphasize that their work serves as a screening tool to flag problems, not as a final verdict on whether the water is safe to drink. They recommend that local authorities verify the raw data for phosphate and oxygen to ensure accuracy. Furthermore, they suggest mapping the local area to find the sources of the phosphate, such as sewage outlets or agricultural fields, and establishing a full year of monitoring to see how the river changes with the seasons. By correcting the mathematical errors of the past, this study provides a clearer, more honest foundation for protecting the health of the Esuk Oro River.