The factors are then combined in a formula to produce the water quality index. As any one factor increases, so does the index, indicating a decrease in water quality. The index ranks water quality into five categories ranging from excellent to poor. Conditions are similar to natural levels. Conditions are close to natural levels.
Conditions are sometimes different from natural levels.
Conditions are often different from natural levels. Conditions are usually different from natural levels. Index values do not increase regularly. For example, the ranges for excellent and borderline are much narrower than those for good and fair. This is because the formula used to calculate the index is really a model for water quality change which is partly based on empirical factors.