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Water Quality The British Columbia Water Quality Index November
1995 NB: The British Columbia Water Quality Index has been replaced by the new federal Water Quality Index, developed by the Canadian Council of Environment Ministers (CCME). The CCME index is based on the British Columbia Water Quality Index, which is maintained here for historical purposes. The CCME Water Quality Index can be found at this site: http://www.ccme.ca/initiatives/water.html?category_id=42 Setting of Water Quality Objectives The BC water quality index reduces technical water quality information to a simple description of the state of water quality. This allows the Ministry to share its wealth of water quality monitoring information with the public, and to assist managers of the resource to grade and remedy water quality problems. This short document is a guide to those who wish to gain a basic understanding of how the water quality index works. We recommend that readers who wish to calculate the index on a routine basis become familiar with provincial water quality objectives and the data collected to measure their attainment. The index is based on the attainment of water quality objectives. The objectives are safe limits, set by the Ministry in areas of human activity, to protect the most sensitive water uses of a body of water. The index is thus a systematic way of interpreting measurements of ambient water quality that have been checked against natural or desirable conditions. The index ranks water quality into one of five categories: excellent, good, fair, borderline, and poor. Each category describes the state of water quality compared to objectives that usually represent the natural state. The index thus indicates the degree to which the natural water quality is affected by human activity. The index was tested using historical data collected over several years to check the attainment of water quality objectives in the Province. The model relies on known factors that affect water quality and empirical relationships derived from data testing. The index can be used to describe the state of water quality as a whole in a body of water. It also shows the suitability of water for various uses. These include drinking, recreation, irrigation, and livestock watering where such uses are naturally sustainable, or the suitability for fish and wildlife which is usually always naturally sustainable. One of the first applications was to prepare a report card, the Status Report on water quality for the Province. This project used data collected between 1987 and 1993 on streams and their tributaries, lakes, a few ground water aquifers and certain marine inlets and bays.
We wish to thank all attendees to our workshop held in Kamloops, November 30 1994, whose ideas were invaluable in developing the index. We are also grateful to regional staff of the Ministry of Environment, Lands and Parks (now called Ministry of Water, Land and Air Protection) who provided much needed comments on our first test of the index model with historical monitoring data. Thanks are also due to the Ministry of Health, the federal Department of Fisheries and Oceans, Environment Canada, members of the Water Caucus of the BC Environmental Network, the Coquitlam Centennial School, the Council of Forests Industries, and the Union of BC Municipalities for their useful comments on revising and improving the index. The concept of a water quality index for British Columbia arose from two needs. First was the need to share and communicate with the public, in a consistent manner, the technical results from many years of monitoring ambient water. Second was the need to provide a general means of comparing and ranking various bodies of water throughout the Province. The Ministry monitors ambient water by collecting data to check the attainment of water quality objectives in the Province. The water quality objectives are safe levels, set by the Ministry in areas of human activity, to protect the most sensitive use of a body of water. The procedures for setting water quality objectives and measuring their attainment are described in the next chapter. The results of water quality objectives attainment are used as the basis for the water quality index. The index:
The index is intended to reflect the overall and ongoing condition of the water. It will not usually show the effect of spills, and other such random and transient events, unless these are relatively frequent or long-lasting. Indeed, the index is designed to show the type of water quality that would be at risk from a random accident such as a spill. One of the benefits of the index is elimination of jargon and technical complexity in describing water quality. The index strives to reduce an analysis of many factors into a simple statement. A problem may arise because readers are not informed about uncertainties and limitations in our knowledge. Without caution, this can lead to an incorrect conclusion about the state of water quality. This problem is minimized by setting limits on how the index is calculated and ensuring that measurements used for water quality objectives attainment are complete and accurate. A November 1994 workshop on the index brought together personnel from Ministry's regional offices and federal agencies, as well as other experts on the subject. The model for a BC index presented at the workshop was later modified as a result of input from the workshop, and results from model-testing with historical data. The modified index is presented in this report.
Setting of Water Quality Objectives The Ministry sets objectives in streams, lakes and marine areas and measures their attainment to check on how well it is protecting water quality. The process begins with the development of water quality guidelines. In simple terms, a water quality guideline is a safe condition or level of a substance that, according to scientific studies, will protect a particular water use. Safe, in this context, means a level or condition at which no deleterious effect occurs to the user, be it a human, aquatic life or wildlife. The guidelines developed by the Ministry are summarized in a publication available from the Water Quality Branch, British Columbia Water Quality Guidelines (Criteria): 1998 Edition. They cover a variety of characteristics for the water column, sediments, and aquatic life and apply province-wide. The objectives are the guidelines adapted to a particular body of water. We set objectives in bodies of water where the water quality is affected by human activity. The first step is to identify water uses which need to be protected. These uses must be naturally sustainable and include those for drinking, recreation, irrigation, livestock watering, and use by aquatic life and by wildlife. The next step is to assess the sources of all waste streams entering the body of water, be they from point sources (industry or municipalities) or diffuse sources (agriculture or forestry). This process identifies characteristics that threaten water quality and for which objectives will therefore be needed. Using concentrations and loadings plus data on flow regime, we calculate the potential worst-case effects for each water quality characteristic. These effects are checked against results from monitoring ambient water, sediments, and aquatic life. We use this information, together with the water quality guidelines, to derive the objective for each water quality characteristic that will protect all water uses. The objectives are expressed in several ways. They can be a single maximum or minimum value, a thirty-day arithmetic or geometric mean, a thirty-day ninetieth percentile, or a range. In the case of protection of aquatic life, they can be a maximum and a thirty-day average. In such cases, the maximum is the threshold of acute toxicity and the average is the threshold of chronic toxicity. We monitor to check the attainment of objectives and report the data annually. The objectives are viewed as the goal and the reference point against which water quality can be checked. Results thus measure performance in protecting water quality and help to assess possible corrective measures. For more information on objectives, please refer to: Principles for Preparing Water Quality Objectives in BC, 1986, available from the Water Management Branch.
Few water quality index systems have been developed and none are in widespread use. Available indices are either highly specialized (e.g., those applicable only to lakes), or are very simple in terms of the number of variables considered. None seem to be geared to the protection of multiple water uses or to encompass the variety of measurements of water quality we now gauge, including physical, chemical and biological parameters. For these reasons, a water quality index was developed that would be based on the attainment of water quality objectives for the water column, sediment and aquatic life. The main advantages to an objectives-based system are as follows:
The index is founded on three factors involving the measurement of the attainment of water quality objectives. The factors measure the following:
These three factors are combined to form the index which can fall into one of the following five rankings: excellent, good, fair, borderline or poor. These rankings describe the state of water quality compared to its desirable or natural state. The following brief descriptions related to water use and natural conditions are helpful in interpreting the meaning of each water quality ranking. We recognize six uses of water: drinking, recreation, irrigation, livestock watering, use by aquatic life, and use by wildlife. The first four uses are related to human use and are only considered in the rankings when they are naturally sustainable. The uses by aquatic life and wildlife are usually always naturally sustainable in BC waters. Note that drinking water in this context always refers to the quality of the raw water source, as it exists in the environment before it is delivered to a consumer's tap. Such raw water, even if ranked as excellent, always needs, at the least, disinfection before drinking. Natural Natural water quality conditions refer to conditions that exist in the absence of any human interference. Desirable conditions are those which will sustain the most sensitive water uses. Natural and desirable are usually synonymous, although they may differ when human activity has wrought permanent change. The rankings are described as follows:
Conditions Followed in Calculating the Index There are six steps to follow in calculating the index. The first is to define the body of water to which the index will apply. The second is to choose the period of time over which the index will apply. The last four steps are to work out the three factors that make up the index and calculate the index itself. In working out the three factors, we recommend the following condition regarding the data on water quality objectives attainment:
The body of water to which the index will apply can be a stream or river in its entirety, or in just certain reaches, or a ground water aquifer. Tributaries, lakes, river arms, estuaries, inlets and bays are usually considered separately or they can be combined, if desired, to calculate an index for a whole watershed. We recommend caution in the watershed approach since extreme results from one reach can unduly influence the index for a much wider area. The more that bodies of water are combined, the more the index or ranking will average variable conditions; the more that bodies of water are separated, the fewer data available to work with and the higher the likelihood of getting an index or ranking which fluctuates unduly with time.
Decide the period of time for which the index applies. This will be the period from which all objectives attainment data are drawn. A minimum period of one year is usually chosen because attainment data are collected on an annual basis. Data from several years can also be consolidated to obtain an index over a longer time frame. Combining data from several years masks year-to-year variations, but it has the advantage of filling in data gaps that frequently occur due to incomplete monitoring.
The first factor, called F1, measures the number of objectives not met. It is expressed as a percentage of the number of objectives checked. Calculate F1 for one year by summing the number of objectives not met in that year, dividing by the total number of objectives measured that year and multiplying by 100.
To calculate F1 over several years, sum all the objectives not met over the period in question and divide by all the objectives checked in that period. For example, if 5 objectives were measured in 1990, 8 in 1991 and 7 in 1992 for a total of 20 and the number of objectives not met were 1, 3, and 2 in each of these years for a total of 6, then F1 is 6 in 20 or 30%. This approach is used in preference to averaging F1 from each year because the averaging procedure can be unduly affected by extreme values in years with incomplete data when only a few objectives were checked.
The second factor, called F2 is the number of times objectives were not met, at all sites and dates in a given time period, expressed as a percentage of all instances of objectives being checked. It is calculated for one year by summing all events of objectives not met in that year dividing by the total number of instances objectives were checked that year, and multiplying by 100. The factor can range from 0, indicating objectives are met at all sites, to 100% indicating that none of the objectives are met at any of the sites. Note that for an objective value such as an average, which is based on 5 measurements in 30 days, the instance of an objective being checked is counted as 1, not 5. To calculate F2 over several years, perform the same summations for 1 period under consideration as for one year.
The third factor, called F3, is a measure of the maximum amount by which objectives are not being met in a given year. For the common case of an objective expressed as a maximum, this deviation is calculated by subtracting the objective value from the maximum measurement exceeding the objective, dividing by this maximum measurement, and multiplying by 100. The highest deviation obtained in a year from all objectives checked is the value of F3 used in the index.
This factor is readily influenced by extreme values in the data and can thus affect the index unduly. It is mainly for this reason that it is recommended that outliers be censored due to suspected contamination in the field or laboratory, as indicated by quality assurance. To calculate F3 over several years, average the F3 factors obtained from each year. Using the average instead of the maximum from all the years avoids characterizing the period by an extreme event that occurred in only one year. For objectives expressed as a minimum instead of the more common maximum (e.g., dissolved oxygen or water clarity), the deviation is calculated slightly differently. Subtract the minimum measurement from the objective, divide by the objective, and multiply by 100. These deviations are then treated in the same way as the others with the highest value among all becoming F3.
The index is obtained by summing the three factors as if they were vectors. Thus, the square of the index is equal to the sum of the squares of each factor. This approach is used because the index is envisaged as a three-dimensional space defined by each factor along one axis. With this model, the index, or space defined by the factors, changes in direct proportion to the changes in all three factors regardless of the type of water body involved. When we tested the index with historical attainment data, we found that the third factor, F3 which measures the extent of non-attainment, tended to dominate the index. We brought the effect of this factor into balance by applying a weighting factor and dividing F3 by three. The index is therefore given by the following formula:
(Index)2 =
(F1)2 + (F2)2 +
(F3/3)2
or Index = [(F1)2 + (F2)2 + (F3/3)2]1/2
The index values are rounded to the nearest integer to produce an index rank, on a scale of 0 to 100, by dividing each index value by 1.45. This gives an index rank that ranges from 0 for the best water, to 100 for the poorest water. The index scale gives values which increase numerically as water quality worsens. This type of scale is in keeping with other environmental indices now in use. such as the air quality index or the index for UV radiation, where values increase as conditions deteriorate. The model for the index is a mixture of: a) factors that are known to affect water quality, and b) empirical relationships established by testing with historical data from monitoring. Users will note that the index values do not increase regularly. For example, the range for excellent water quality is narrower than for good water quality, etc. This result is due to empirical factors which were introduced when we tested the model with actual water quality data. In the case when only one or two objectives have been set in a body water, calculate the index by summing the two factors F2 and F3 in the same manner as for three factors. Thus the index for the case of less than three objectives is given by: (Index)2 =
(F2)2 + (F3/3)2
or Index = [(F2)2 + (F3/3)2]1/2
The index calculated ranks a body of water for all water uses considered together. It is therefore a statement about the quality of the water in general. To describe how the water rates for a particular use, the F3 factor is calculated for each of the uses. The use is then ranked according to the range in which F3 falls in the index table. The reasons for using the F3 factor are that it can be targeted to specific objectives, and therefore uses, and that it is a sensitive measure of non-attainment. It is also a relatively simple calculation to perform, although it can produce some unexpected results as explained in the last paragraph of this section. The procedure for calculating F3 and relating the index to water use is given below.
Example In most cases, the rank of the overall index for a body of water will equal or be close to the lowest rank obtained for the uses. For example, if water were excellent for irrigation, good for aquatic life and wildlife, and fair for drinking, then the overall rank is usually fair. This result is caused by the F3 factor outweighing the other two factors in the overall index. In about 15 percent of cases, however, we found that the overall rank was below the lowest rank obtained for the individual uses. In the above example, this might produce an overall rank of borderline. Such a result is due either to the other two factors, F1 and F2, having more influence on the overall index than the F3 factor, or to the impairment of several uses which lowers the grade overall.
The purpose of the index is twofold. The first is to inform the public in an easy-to-understand and non-technical way of the state of water quality relative to its natural state. The second is to assist those who manage the resource rank bodies of water in a general way and consequently take action to remedy unacceptable water quality conditions. The immediate application of the index was to prepare a report card, the Status Report on water quality for the Province. This is a user-friendly document for the public that will rank the state of water quality in streams, lakes, marine areas and ground water aquifers. The rankings are not absolute but relative to the natural or desirable state of water quality, thus measuring the effects of human activity. In applying the index to obtain a report card, we plan to rank water quality according to its suitability for all uses in general as well as suitability for specific water uses. For example, the index may show that water in a particular stream is generally good with a breakdown showing it is excellent for recreation, good for aquatic life and fair for drinking. The analysis could also show, for example, that there is poor water quality for aquatic life in certain areas of a lake although it is good elsewhere. Rankings obtained using the index can also help in decisions on correcting water quality problems. Highlighting a specific stream or reach of stream, and a particular use that is affected, can assist managers in grading and targeting the work needed to rectify a problem. The index has the flexibility to incorporate future kinds of water quality objectives that will be developed. For example, objectives for the aquatic habitat or for the aquatic ecosystem as a whole would apply. Such objectives will need to be expressed numerically and their attainment will need to be measured in order to incorporate them into the index model. Whenever the index is applied, follow the conditions outlined in the section on calculation of the index. To use the index in an area for which there are water quality objectives, we recommend use of the water quality guidelines given in the report: British Columbia Water Quality Guidelines (Criteria): 1998 Edition, or subsequent editions. This report is available from the Water Management Branch and is updated regularly.
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