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. These factors were introduced by testing the model extensively with water quality data collected over many years. The index founded on three factors ranks water quality generally, taking into account all possible uses of the water.