They are based on the best scientific information available at the time and are subject to periodic re-evaluation as new information becomes available. Water guidelines for several substances commonly derived from agriculture are given below. Some people view Canadian and similar guidelines as too liberal, preferring to operate under the precautionary principle . This principle purports that precautionary measures should be taken when an activity poses a risk to the environment or human health, whether or not sufficient scientific research has been conducted to conclusively support taking these measures. This view leads to more stringent guidelines, such as those found in the European Commission's 1980 Drinking Water Directive.
Chemical indicators are only one way to assess the state of water quality.
For example, the health of the biological community supported by a water body can be an indirect measure of the water's chemistry. Figure 4-1 Holistic approach to assessing water quality Units used in this report General Chemical concentrations in water are expressed in this report in terms of mass per unit volume, such as micrograms or milligrams of the substance in one litre of water. 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.