A good water quality index (WQI) score falls between 70 and 89 on the most widely used 0-to-100 scale. Scores of 90 to 100 are considered excellent, while anything below 70 starts to signal increasing levels of contamination. The index condenses multiple lab measurements into a single number, giving you a quick snapshot of whether a body of water is safe for drinking, swimming, or supporting aquatic life.
How the Scoring Scale Works
The National Sanitation Foundation Water Quality Index (NSFWQI) is the standard most often referenced in the United States and in international research. It divides water quality into five categories:
- Excellent (90–100): Pristine or near-pristine water with virtually no contamination concerns.
- Good (70–89): Minor deviations from ideal conditions, but safe for most uses including drinking after standard treatment.
- Medium (50–69): Noticeable pollution. Water may still be usable but often requires significant treatment.
- Bad (25–49): Substantial contamination that poses risks to human health and aquatic ecosystems.
- Very bad (0–24): Severely polluted water unsuitable for nearly all purposes.
If you’re looking at a local water report or an environmental study, any score in the “good” range or above generally means the water meets the thresholds needed to protect both human health and wildlife. A score in the “medium” band is a yellow flag, not an emergency, but worth investigating which specific parameter is dragging the number down.
What the Index Actually Measures
A single WQI score is built from several individual measurements, each weighted by how important it is to overall water safety. The most common version tracks nine parameters: dissolved oxygen, fecal coliform bacteria, pH, biochemical oxygen demand, temperature change, total phosphates, nitrates, turbidity (cloudiness), and total dissolved solids. Each parameter gets its own sub-score based on how far the measured value strays from what you’d find in perfectly clean water, and those sub-scores are combined into the final number using a weighted average.
Not every parameter carries equal importance. Dissolved oxygen and fecal coliform levels, for example, are weighted more heavily because they have the most direct impact on whether water is safe to drink or can support fish and other organisms. A water body could have a nearly perfect pH but still score poorly overall if bacteria levels are high.
Key Parameters That Drive the Score
Dissolved Oxygen
Dissolved oxygen is one of the strongest indicators of water health, especially for rivers, lakes, and estuaries. Healthy aquatic ecosystems generally need concentrations of 5 mg/L or higher. Fish, crabs, and oysters that feed along the bottom need at least 3 mg/L, while spawning fish and their eggs require up to 6 mg/L. When levels drop below 2 mg/L, scientists classify the water as hypoxic, a condition that can suffocate most aquatic life. High dissolved oxygen levels push the WQI score upward; low levels pull it down significantly because of the heavy weighting this parameter receives.
pH
The EPA’s secondary drinking water standard sets an acceptable pH range of 6.5 to 8.5. Water within this band is neutral enough to avoid corroding pipes (which can leach metals like lead) and won’t cause the bitter or soda-like taste that comes with extreme acidity or alkalinity. For the WQI calculation, a pH near 7.0 earns the highest sub-score. Values that drift toward either end of the acceptable range reduce the score modestly, while readings outside 6.5 to 8.5 reduce it more sharply.
Turbidity
Turbidity measures how cloudy the water is, typically reported in Nephelometric Turbidity Units (NTU). Cloudiness matters because suspended particles can shield bacteria and parasites from disinfection, making treatment less effective. High turbidity is also a sign of runoff carrying sediment, nutrients, or pollutants into the water. Clear water scores well on this parameter; murky water does not.
Bacteria and Nitrates
Fecal coliform bacteria indicate contamination from human sewage or animal waste. Their presence signals that disease-causing microorganisms, including viruses and parasites, may also be in the water. Consuming contaminated water can cause gastrointestinal illness and more serious infections. Nitrates, commonly introduced by agricultural fertilizers and sewage, reduce the blood’s ability to carry oxygen. This is particularly dangerous for infants under six months, who can develop a life-threatening condition within days of exposure.
What a Low Score Means for Health
Water that scores in the “bad” or “very bad” range often contains elevated levels of contaminants that carry real health consequences. Heavy metals like arsenic, lead, and cadmium can cause liver and kidney damage, anemia, and increased cancer risk with long-term exposure. The EPA sets a maximum contaminant level goal of zero for both arsenic and lead, meaning no amount is considered truly safe. The enforceable limit for arsenic in drinking water is 0.010 mg/L, and lead triggers action at the same concentration.
Organic chemicals from household products, agriculture, and industrial processes can damage the kidneys, liver, nervous system, and reproductive system. Radioactive elements like uranium and radium, released through mining and energy production, add another layer of risk in certain regions. A low WQI score doesn’t tell you which specific contaminant is the problem, but it tells you something is wrong and warrants a closer look at the individual parameters.
WQI for Drinking Water vs. Natural Water Bodies
The context of the score matters. A WQI of 75 for a river means the ecosystem is in reasonably good shape, supporting healthy fish populations and recreational use. That same score applied to treated tap water would be less reassuring, because drinking water typically needs to meet stricter standards across all parameters before it reaches your faucet.
The World Health Organization publishes international guidelines for drinking water quality that many countries use as the basis for their own regulations. These guidelines emphasize preventive risk management from the water source all the way to the tap, rather than relying on a single index number. In practice, your local water utility tests for dozens of individual contaminants and must meet specific legal limits for each one. The WQI is a useful summary tool, but it doesn’t replace the detailed contaminant reports that utilities are required to publish annually.
How to Use the Index Practically
If you’re evaluating a local waterway, a community well, or a report from your municipality, the WQI gives you a fast read on overall conditions. Scores of 70 and above are where you want to be. If the number falls into the 50–69 range, look at which individual parameters are underperforming. A slightly elevated turbidity reading after heavy rain is a different concern than chronically high bacteria counts.
For private well owners, the WQI can be especially useful as a tracking tool. Testing your well annually and calculating the index over time lets you spot trends before they become health problems. A score that drops from 80 to 65 over two years is a clear signal to investigate the source of the change, whether it’s nearby agricultural runoff, a failing septic system, or natural geological shifts.
Keep in mind that newer contaminants like PFAS (sometimes called “forever chemicals”) are not part of the traditional nine-parameter WQI. The EPA recently set an enforceable limit for PFOA at 0.000004 mg/L, an extraordinarily low threshold that reflects how potent these substances are. A water source can score well on the classic index and still carry PFAS contamination, so the WQI is best used as one tool among several rather than the final word on safety.