Seasonal Groundwater Quality, WAWQI Weighting Sensitivity and Untreated-Use

Water is the most essential ingredient for life, yet in many rapidly growing cities, the water flowing from a tap or drawn from a well is not always safe to drink without treatment. In places where central plumbing systems cannot keep up with population growth, people rely on shallow wells dug by hand or deeper holes drilled by machines to access groundwater. This hidden water is vulnerable. Rain can wash waste from the surface down into the soil, carrying invisible contaminants like bacteria from human waste or chemicals from fertilizers and industrial processes. To understand if this water is safe, scientists often use a scoring system called a water quality index. This system takes many different measurements—such as how clear the water is, how salty it feels, and what chemicals are dissolved in it—and combines them into a single number. The idea is to make complex data easy to understand: a low score means the water is excellent, while a high score means it is poor. However, this method has a hidden flaw. If the scoring system gives too much importance to one specific chemical and too little to another, the final number can tell a story that is dangerously different from the reality of the water's safety.

A team of researchers in Nigeria set out to test how well this scoring system works when the seasons change. They focused on a neighborhood called Lamurde, in the Jalingo area, where residents depend heavily on shallow groundwater. The researchers monitored thirty fixed water sources, a mix of hand-dug wells and boreholes, over two distinct periods. They took samples during the wet season, from July to September 2025, and again during the dry season, from November 2025 to January 2026. For each source, they measured eight different things: how acidic or basic the water was, how cloudy it was, how much dissolved solid material it contained, its ability to conduct electricity, and the levels of nitrate, iron, zinc, and total chromium. They also checked for the presence of Escherichia coli, a common bacterium that signals fecal contamination. The goal was not just to see if the water changed, but to see if the standard scoring system would accurately reflect those changes, especially when some things got worse while others got better.

The results revealed a striking contradiction that challenges how we interpret water safety. When the researchers calculated the standard water quality score for the dry season, the average was 69.79, which falls into a "fair" category. When they calculated the score for the wet season, the average dropped to 45.00, which falls into a "good" category. By the rules of the scoring system, the water appeared to have improved significantly during the rainy months. Yet, the actual safety of the water for drinking without treatment had gotten much worse. During the wet season, the level of nitrate in the water jumped from an average of 24.79 milligrams per liter to 82.14 milligrams per liter. Every single one of the thirty water sources exceeded the safe limit of 50 milligrams per liter. Furthermore, the bacteria that indicates fecal contamination was found in every single source during both seasons, and its levels actually increased slightly during the wet months. The water was chemically "better" according to the score, but biologically and chemically more dangerous for anyone trying to drink it untreated.

The reason for this misleading improvement lies in how the scoring system weights the different chemicals. In this specific system, the importance of a chemical is determined by how strict its safety limit is. Because the safe limit for total chromium is extremely low, the system assigns it a massive amount of importance in the final score. In fact, total chromium and iron together accounted for nearly 97 percent of the weight in the calculation. Nitrate, which is a major health concern, was assigned a tiny fraction of the weight. During the wet season, the levels of chromium and iron in the water went down, which caused the overall score to drop and look better. At the same time, the nitrate levels skyrocketed, but because nitrate carried so little weight in the calculation, its dangerous rise was almost invisible to the final number. It is like a report card where a student's grade is 90 percent based on their handwriting and only 10 percent based on their math test; if the handwriting improves but the math score fails completely, the final grade might still look good, even though the student has failed the most important subject.

To prove that the scoring system was the problem and not the water itself, the researchers ran a series of tests. They recalculated the scores using a different method where every chemical counted equally. When they did this, the story flipped: the water looked worse in the wet season, not better. They also tried removing nitrate from the calculation entirely, and the classification of the water sources did not change at all, proving that nitrate was not driving the score. Conversely, when they removed chromium, the classification of most sources changed, showing that the score was being dominated by that single chemical. The study concluded that while the scoring system is mathematically consistent, it is too dependent on its own rules to be trusted as a standalone measure of safety. The researchers found that a water source can be labeled "good" by the index while simultaneously being unsafe to drink due to high nitrate and bacteria.

This discovery has important implications for how communities and regulators manage water. The study suggests that a single number can never tell the whole story of water safety. If a community relies only on the index score, they might believe their water is getting safer when it is actually becoming more dangerous. The researchers argue that any report on water quality must show the index score alongside specific warnings for critical issues like nitrate levels and bacteria. They also emphasized the need to monitor water during the rainy season, as the dry season alone would have missed the universal spike in nitrate contamination seen in this study. Ultimately, the work shows that while tools like water quality indices are useful for summarizing data, they must be used with caution. They should never hide the fact that a small change in one chemical can mask a large, life-threatening change in another. For the people in Lamurde, the lesson is clear: a "good" score does not mean the water is safe to drink without treatment, and the real danger often lies in the details that the score ignores.