A concrete answer to the water-quality definition begins by identifying the quantifiable components of water quality as well as the variables that affect the measurable values for water-quality standards. In Part One of this three-part series, Author Harbans Lal thoroughly explains the process of establishing the Water Quality Index in his full-length feature: “Introduction to the Water Quality Index: Expressing water quality information in a format that is simple and easily understood by common people”
Water Quality Definition and Importance (Part 1) By Harbans Lal
Water quality is the condition of the water body or water resource in relation to its designated uses. It can be defined in qualitative and/or quantitative terms. Parameters in defining water quality can be grouped into three broad categories: physical, chemical, and biological. Physical factors include temperature, sediment and bed material, suspended sediments, turbidity, color, and odor. Chemical factors consist of the major and minor elements, and other chemical parameters such as pH, Dissolved Oxygen (DO), Biological Oxygen Demand (BOD), and Chemical Oxygen Demand (COD).
The major elements include agro-nutrients such as Nitrogen and Phosphorus; and minor elements include elements such as arsenic (As), lead (Pb), and mercury (Hg), etc. Biological Constituents include Fecal Coli-form and E. coli. Conventionally water quality is expressed in terms of the measured value(s) of one or more of these parameters in relation to their accepted or implied limits. They are expressed in different units, and their magnitudes can vary significantly from one location to another and over time. For example, the temperature is expressed in degrees Celsius or degrees Fahrenheit, and coliforms in numbers, and most chemicals and nutrients in milligrams per liter (mg/L) or in parts per million (ppm).
The conventional approach of expressing different parameters of water quality in varying units is well accepted by water resource experts. However, it is not readily understood by the general public and policymakers who have profound impact on water resource policies. Thus, the need for expressing water quality in a format that is simple and easily understood by common people has been recognized for a long time. Experts have worked internationally—including in the United States—for the past several years and have designed the term Water Quality Index (WQI). The WQI takes the complex scientific information and synthesizes into a single number between zero and 100, by normalizing the observed values to subjective rating curves. It summarizes the relative changes in the underlying group of the water-quality variable.
WQI is easily comprehended and appreciated by common citizens and policy makers. It can also help in meeting regulations and/or making personal lifestyle adaptations for the benefit of the environment. Several organizations in the United States and around the world including United Nations have adopted the WQI concept for expressing the water quality (OR-DEQ 2008, OR-DEQ 2008a, Hallock 2002, CCME 2001, and UNEP 2007) for their water resources.
This paper elaborates on the WQI concepts and reviews different WQI models from the literature. It also presents a case scenario of calculating WQI using different models with an example dataset. All these WQI models have been developed for flowing or standing water resources such as lakes, rivers, streams, and such. There is no reference in the literature for WQI for the runoff water from agricultural fields. The paper also emphasizes the need for developing such a WQI model which could be used for evaluating the effects of agricultural management and conservation practices on private lands supported and cost-shared by the US Department of Agriculture/Natural Resources Conservation Service (USDA/NRCS).
What Is WQI?
WQI is a dimensionless number that combines multiple water-quality factors into a single number by normalizing values to subjective rating curves (Miller et al. 1986). Factors to be included in WQI model could vary depending upon the designated water uses and local preferences. Some of these factors include DO, pH, BOD, COD, total coliform bacteria, temperature, and nutrients (nitrogen and phosphorus), etc. These parameters occur in different ranges and expressed in different units. The WQI takes the complex scientific information of these variables and synthesizes into a single number. Several authors have worked on these concepts and presented examples with case scenarios (Bolton et al. 1978, Bhargava 1983, House 1989, Mitchell and Stapp 1996, Pesce and Wunderlin 2000, Cude 2001, Liou et al. 2004, Said et al. 2004, Nasiri et al. 2007, NSF 2007).
WQI Development Process
The process of developing a WQI involves the following steps:
- Identify water quality parameters of interest and their ranges of acceptability for the intended uses of the water body.
- Compare the measured value with the subjective rating curve and arriving at a dimensionless sub-index value (0–1) for each parameter.
- Define the weighing factor and/or heuristics for each parameter to be considered while building an overall WQI.
- Select an algorithm and computing the WQI with the available data and assumptions.
Part 2 of this series, will revisit the WQI Development Process and will explain various arithmetic models utilized in calculating WQI. Inserting defined parameter values and solving the equations will provide further clarity to the question, “How is water quality measured?”