Water-Quality Indices for Specific Water Uses GEOLOGICAL SURVEY CIRCULAR 770 Water-Quality Indices for Specific Water Uses By J. D. Stoner GEOLOGICAL SURVEY CIRCULAR 770 1978 United States Department of the Interior CECIL D. ANDRUS, Secretary Geological Survey H. William Menard, Director Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202 CONTENTS Page Page Abstract_____________________________ 1 Examples of specific indices __ _, _______ _____ 5 Introduction _ ___ _ __ _ _ ______ _ ___ ___ _ 1 Public water supply index _ _ _ _ ___ _ _ _ _ 5 Earlier indices _________________________ 1 Type-I properties_________________ 5 Development of the index_________ _________ 2 Type-II properties __________________ 5 Criteria ____________________________ 2 Application of index _______________ 8 Boundary conditions __________________ 2 Irrigation index ____________________________ 8 Mathematical functions ______ _ __ _____ _ _ 2 Type-I properties_____________ ___ 10 Types of properties ___-______________ 3 Type-II properties ___________________ 10 Type-I properties__________________ 3 Application of index _____________________ 10 Type-II properties ________________ 4 Conclusions ________________________________ 10 Index __________________ ____ 4 References___________________________ 12 ILLUSTRATION FIGURE 1. Plot of the function QF=a+bX2 for constituent A.____________________________________________ 3 TABLES TABLE 1. Type-I properties selected for public water supply index____-___--____-_____-__________________ 5 2. Type-II properties selected for public water supply index ______________________________________ 5 3. Recommended maximum fluoride concentrations _____ ________________________________________________ 7 4. Type-II property effects for the public water supply index ___________________________________ 8 5. Public water supply indices and individual property effects for selected waters _________________________ 9 6. Type-I properties for the irrigation index _________^.-____-___-______-_____________________ 10 7. Type-II properties for the irrigation index ___________________________________________________________ 11 8. Irrigation indices and individual property effects for selected waters ___________________________________ 11 Hi Water-Quality Indices for Specific Water Uses By JERRY D. STONER ABSTRACT used to develop the index. This attribute of an Water-quality indices were developed to assess waters for index allows direct comparison of the overall two specific uses public water supply and irrigation.The as­ quality of different waters even though the con­ sessment for a specific water use is based on the availability of centration ranges of the individual constituents (Da set of limits for each water quality property selected, (2) a may be very different. These two attributes, the rationale for selection, and (3) information that permits one to appraise the relationship of the concentration of the selected quantification of "good" and "bad" and the sum­ property to the suitability of the specific water use. The mation of individual effects, allow the user to ex­ selected properties are divided into two classes: Type-I prop­ amine waters and view them in terms of ranked erties, those normally considered toxic at low concentrations, order for example; bad, poor, good, better, best. and type-II properties, those which affect aesthetic conditions The water-quality index is also a method of pro­ or which at high concentrations can be considered toxic or would otherwise render the water unfit for its intended use. viding water-quality information that can be In the method used, type-I properties affect the index only more readily used by planners, managers, and when their recommended limits are exceeded. The type-II other nontechnical people. In general, managers properties affect the value of the index over the complete and planners will have technical staffs to analyze range, from optimum or ideal concentrations to concentrations the raw data. However, the technical analyses exceeding their respective recommended limits. The index value is the summation of the type-I and type-II effects. The must still be presented to the managers and plan­ range of the index is such that the value 100 represents a ners in a form they can understand and use. The perfect water, zero a water that has the aggregate effect of the water-quality index is a useful tool in bridging properties at their recommended limits, and a negative value this information gap. a water unfit for the use intended without further treatment. The water-quality index can be a good tool for The index is designed to (1) provide numbers so that various waters can be compared directly with one another, (2) allow presenting water-quality data. It can be used in for comparison of water-quality changes with time, (3) indi­ trend analyses, graphical displays, and in tabular cate waters of both "good" and "bad" quality, and (41 provide presentations. It is an excellent format for sum­ values which managers and other nontechnical personnel can marizing overall water-quality conditions over use more easily to characterize water quality. The method space and time. developed can be applied to water-quality indices for specific uses that are very broad or very narrow in scope. This report presents a new concept in the devel­ opment of water-quality indices. Application of INTRODUCTION the method to a wide range of use categories and waters should show its utility and test the va­ At the present time an increased emphasis has lidity of the concept. The method should not be been placed upon the development of water- construed to be an official U.S. Geological Survey quality indices. Much of the effort in developing technique. quality indices is directed toward quantifying such terms as "good" and "bad," and the values EARLIER INDICES between these extremes. In this context, a water-quality index is a grading system for com­ A general water-quality index not directed to­ parison of various waters. ward any specific water use was developed and A water-quality index is also the summation of reported by Brown, McClleland, Deininges, and the individual effects of the several properties Tozer (1970). They concluded that a single numer- ical expression reflecting the composite influence CRITERIA of significant properties of water quality is feasi­ Two criteria were adopted to develop a base ble.Various investigators have developed water- from which a specific-use index could be gen­ quality indices for specific uses. Amongst the erated. The first criterion was that the number oldest is the classification scheme for irrigation generated as the index value from one water must waters by Wilcox (1955). More recently, Harkins be directly comparable to the index number gen­ (1974) developed an index specifically for use in erated from a different water. The second criter­ trend analysis, and Walski and Parker (1974) de­ ion was that the number generated should repre­ veloped an index to be applied to recreational use. sent the "fitness" of the water for the specific-use category under consideration. These criteria dif­ DEVELOPMENT OF THE INDEX fer from the criteria of other indices in that most Classification of waters according to specific indices developed to date have been concerned uses has become increasingly important. Apply­ with judging waters in terms of general overall ing a general water-quality index to specific-use quality that is, how "good" the waters are waters may lead to conclusions that are not en­ irrespective of their intended use. tirely valid, primarily because the importance and influence of water-quality properties vary for BOUNDARY CONDITIONS different uses. As an example, water temperature In order to meet the established criteria, the is relatively unimportant in water used for irriga­ QF's (quality function), which are the mathemat­ tion but is of vital importance in waters used for ical functions representing individual water- the maintenance of aquatic life. With the method quality properties making up the WQI (water- to be described, a water-quality index for any quality index), and the WQI itself assign to an water use, broad or narrow in scope, can be devel­ "ideal" water the arbitrary value of 100. Because oped if certain information can be provided. The of the method of computation, the boundary con­ minimum information needs are (1) a set of limits ditions for the individual properties were applied for each water-quality property to be considered, to the respective QF's. The QF's and WQI for a (2) a rationale for establishment of the limits, and water at the recommended concentration limits (3) some information on, or appraisal of, the rela­ were arbitrarily set at zero. In this way, when the tion of various concentrations of each property to QF's or WQI (which is the sum of the individual the specific water use for which the index is being effects), are somewhere in the range of 0 to 100, developed. the "goodness" of the water for a specific use can Two broad water-use categories, public water be judged. The QF of an individual property supply and irrigation, were analyzed to develop whose value has exceeded the limit becomes the method. The National Academy of Sciences negative. The more the limit is exceeded, the and National Academy of Engineering report, larger the negative number becomes. No limit is "Water Quality Criteria 1972" (1972), provided placed upon the value that a negative number can the necessary information for the development of become. If the sum of the individual effects is the water-use indices. negative, then the WQI becomes negative. Thus, The water-quality properties, rank order, if the value of a property normally not considered weighting factors, and mathematical expressions toxic at commonly found concentrations reaches a used in this report are the author's subjective toxic concentration, or if a concentration renders choices based upon his experience and the Na­ the water unfit for use, the value will make the tional Academy of Sciences report, and as such, WQI negative. they will probably not be accepted by all readers.