Average concentrations are often given a flow-weighted mean, which is the total mass of a substance over a period divided by the total amount of water that flowed during that time. The amount of a substance leaving a farm field or a watershed may be expressed in terms of mass per unit area, such as grams, kilograms, or tonnes per hectare. The term loading refers to the amount of a substance that is discharged or lost from a farm or watershed over a period. It is expressed in units such as grams per year, or kilograms per hectare per year. Biological indicators can reflect the combined effects of pollutants, changes in habitat, and other environmental impacts that monitoring chemical and physical properties alone cannot reveal.
Human health is another angle in assessing water quality.
For example, large population studies may shed light on possible synergistic or additive effects of contaminated water on human health. Age studies may show that people at certain life stages are more susceptible than others to the effects of contaminated water or are more likely to manifest these effects. However, such studies often involve complex analyses, and the effects of many chemical interactions are still poorly understood.