Water, Air and Climate Change Branch

WATER QUALITY

BC Water Quality Index

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What is This Brochure About? This brochure describes, in general terms, how a water quality index is derived and used. It shows how the index ranks the suitability of water for drinking, swimming, for fish and for other uses. The brochure is based on a more detailed document, The British Columbia Water Quality Index, available from the ministry.

What is a Water Quality Index?

A water quality index is a system of ranking the quality of water in the environment by numerical means. The index assigns a number to a body of water to indicate its quality.

In keeping with other environmental indices, the water quality index gets bigger as conditions worsen. Thus the index scale starts at zero for the best water and increases as the quality deteriorates.

Why Do We Need a Water Quality Index for BC?

There is a considerable amount of technical information collected by government and others to describe the state of water quality. This information is based on sampling water, sediments and aquatic life and measuring their many chemical and physical properties.

The index reduces these large quantities of technical information, in a systematic way, to a simpler category or rank describing the state of water quality. This enables the public and non-scientific community to share and understand the monitoring data. It also helps managers of the resource to grade water quality problems before solving them.

What is the Basis for the BC Water Quality Index?

The BC index is based on the attainment of water quality objectives. The objectives are safe limits, set by the ministry, to protect all the uses of a body of water. Objectives are set for contaminants or substances of concern as well as for key properties of water, sediments and aquatic life.

If monitoring shows that all objectives are usually met everywhere in a water body, all the time, then the index will be close to zero indicating excellent water quality. The more that monitoring shows objectives not being met, the more the index increases indicating a worsening of water quality.

How are Water Quality Objectives Set and Expressed?

The objectives are based on values called water quality criteria. These criteria are safe levels of a substance, or safe conditions, which scientific studies have shown will protect various water uses-such as those by aquatic life and wildlife as well as use by humans for drinking, swimming, etc.

The objectives are simply the criteria applied to a specific body of water. They take into account the natural quality of the water body and the types of threat from wastes and other sources that the water body may be exposed to.

For substances that can be toxic, the objective is expressed as a maximum and average value which must not be exceeded. For other properties, such as the dissolved oxygen needed to sustain aquatic life, the objective is given as a minimum value that must be met. In all cases, objective values are conservative with a safety margin built in.

Which Uses of Water are Considered for Protection?

We recognize six uses of water for protection. Uses by aquatic life and wildlife are almost always considered for protection because fresh and marine waters of the province generally support such life naturally.

The other four uses are for humans and include drinking, recreation (such as swimming), irrigation and livestock watering. We consider these uses for protection only if the water is suitable for them in its natural state.

How is the Index Calculated?

For a given body of water, the index is based on three factors called F₁, F₂ and F₃. These factors measure the following attributes:

• F₁: the number of objectives not met.
• F₂: how often objectives are not met.
• F₃: the amount by which objectives are not met.

Data collected from monitoring programs to check objectives are used to work out the factors. 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.

What are the Index Categories and What do they Mean?

The index ranks water quality into five categories ranging from excellent to poor. The categories and their meaning are as follows:

Excellent:
	the index rank is 0 to 3 and all uses are protected without any being threatened or impaired. Conditions are similar to natural levels.
Good:
	the index rank is 4 to 17 and all uses are protected with only a minor degree of threat or impairment. Conditions are close to natural levels.
Fair:
	the index rank is 18 to 43 and most uses are protected with only a few being threatened or impaired. Conditions are sometimes different from natural levels.
Borderline:
	the index rank is 44 to 59 with several uses threatened or impaired. Conditions are often different from natural levels.
Poor:
	the index rank is 60 to 100 and most uses are threatened or impaired. Conditions are usually different from natural levels.

Why do the Ranges for the Index Differ Between Categories?

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.

How do You Calculate an Index for a Specific Water Use?

The index founded on three factors ranks water quality generally, taking into account all possible uses of the water. However, we can also generate an index which ranks the water for a particular use.

This is done by calculating the F₃ factor, the one measuring the amount by which objectives are not met, for the use in question. The index is then based on this factor alone. This F₃ factor is a sensitive measure of non-attainment of water quality objectives and can be readily calculated for those objectives pertinent to a given use.

How Can the Index be Applied and Used?

The index is applicable to any body of water where objectives have been set and tested. These include rivers and streams, marine areas such as bays and inlets and ground water aquifers. The index ranks the water in a general way with a breakdown showing its suitability for various uses.

We have used the index to prepare a Report Card on water quality for the province. In that document, the index translates a large amount of monitoring data into a user-friendly analysis of the state of water quality for a number of water bodies. This answers people's basic questions and will help in making decisions on correcting specific water quality problems.

What are the Limitations of the Index?

The index index provides a general statement about the state of water quality and water's many uses over time. While it has the advantage of reducing technical complexity, it cannot always show the effect of a random short-term event, such as a spill, unless it occurs more frequently or for a longer time. Indeed, the index is designed to describe the ongoing water quality that would be at risk from such a random or extreme event.

The index also looks at the waterbody as a whole. This means that variations in water quality which are highly localized may not be immediately evident. Another change not necessarily reflected in the index is that to the habitat of fish. This is because water quality, on which the index is based, does not always account for habitat problems such as low water levels, high stream velocities, or disruption of gravels. These factors would be incorporated into an ecosystem index, the development of which remains a challenge for the future.

updated: August 3, 2001