All You Need to Know about Water Quality Index
Clean water is as important to humans as it is important to any other lifeforms on earth. Although there isn’t an ultimate tool that can tell us whether the water we use and consume everyday is 100% clean, there are certain ways for us to determine the chemicals that occur in water. Just like how the Body Mass Index (BMI) equation helps in determining if you weigh according to your height, there’s also what we call as the Water Quality Index, which provides a single number (like a grade) that expresses the overall water quality at a certain location and time, based on several water quality parameters.
Some Facts You Need to Know About Water Quality Index
The purpose of Water Quality Index is to convert complex water quality data into information that is understandable and useable by the public. While a single number cannot tell the whole story of water quality; a water index based on some very important parameters can provide a simple indicator of water quality. It gives the public a general idea the possible problems with the water in the region.
In the state of Oregon, the Oregon Water Quality Index (OWQI) is a single number that integrates 8 water quality parameters, such as temperature, dissolved oxygen, biochemical oxygen demand, pH, ammonia + nitrate nitrogen, total phosphates, total suspended solids, and faecal coliform – used to make powerful assessments of their rivers and streams
In Mexico, they developed a Water Quality Index (ICA by its Spanish acronym) – a weighted average of 18 quality indexes – to classify bodies of water according to their degree of pollution.
Performing the Water Quality Index Test
When performing the Water Quality Index test, one must exercise caution in handling the collected water samples. A collected sample should be representative of the river or lake being tested. Near-shore samples may not be representative of the river at that location. If possible, water samples should be collected from a bridge spanning the river, from a boat, or off the end of a dock. A rule of thumb for sampling is to sample midway across the river and below the surface.
The water sample undergoes nine different tests, which consist of the following: dissolved oxygen, fecal coliform, pH, biochemical oxygen demand, temperature, total phosphates, nitrates, turbidity, and total solids. This standard is designed and created by the National Sanitation Foundation (NSF) in an attempt to devise a system to compare rivers and lakes in various parts of the country. For each water test, the numerical value or Q-value is multiplied by a “weighting factor.” The resulting values of the nine tests are added to arrive at an overall water quality index (WQI), with 100 as the highest score a body of water can receive.
Understanding the Index Parameters
Dissolve Oxygen Level: Dissolved oxygen level is a test for the ability of the body of water to support life. For underwater habitats like fish, the oxygen that they get from the water is dissolved in the water in much smaller quantities than what is in the air. There are two ways that dissolved oxygen that enters water, either from photosynthesis from aquatic plants or through diffusion with the surrounding air.
When more oxygen is consumed than produced, dissolved oxygen levels in the water will decline. When water has high, relatively stable levels of DO, it is usually considered a healthy ecosystem, capable of supporting many different kinds of aquatic organisms. However, low DO (usually called hypoxic) levels usually indicate pollution or some type of human-caused change.
According to the Cary Institute of Ecosystem Studies, when measuring dissolve oxygen, concentrations range from 0 to 14 ppm or mg/L (they measure the same thing, but sometimes your test kit will use only one of the measurements), and 0-125+ percent saturation. In addition, you need to know the temperature of the water as that can change the result.
0-2 mg/L: not enough oxygen to support most animals
2-4 mg/L: only a few kinds of fish and insects can survive
4-7 mg/L: good for most kinds of pond animals
7-11 mg/L: very good for most stream fish
For percent saturation:
Below 60%: poor quality, bacteria may be using up the DO
60-79%: acceptable for most stream animals
80-125%: excellent for most stream animals
125% or more: too high
Fecal Coliform Bacteria: These are normally found in the feces of humans and animals. It is a test that aims to find out whether or not the water will harbor disease-causing pathogens that can affect humans.
Water pH: pH is a test whether or not the water is acidic or alkaline. A pH value of 7 means a substance is neutral. The lower value indicates acidity, and a higher value is a sign of alkalinity.
Biochemical Oxygen Demand: BOD primarily measures the amount of oxygen consumed by microorganisms when they decompose organic matter in stream water. It also measures how much oxygen is used up by chemical reactions in the water. The rate of oxygen consumption in a stream is affected by many of the same variables as were described above: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water.
Total Phosphorus: Although phosphorus is usually present in natural water as phosphates, an excess of this can cause extensive algal growth called “blooms,” which are a classic symptom of cultural eutrophication and lead to decreased oxygen levels in creek water. The recommended level of total phosphorus in estuaries and coastal ecosystems to avoid algal blooms is 0.01 to .1 mg/l and 0.1 to 1 mg/l of nitrogen (a 10:1 ratio of N:P). The higher concentrations support less diversity
Nitrates: Nitrate is a colorless, odorless, and tasteless compound that is present in some groundwater. The formation of nitrates is an integral part of the nitrogen cycle in our environment. In moderate amounts, nitrate is a harmless constituent of food and water. However, high nitrate levels in water can cause several hazards both in humans and animals. Nitrate values are commonly reported as either nitrate (NO3) or as nitrate-nitrogen (NO3-N). The maximum contaminant level (MCL) in drinking water as nitrate (NO3) is 45 mg/l, whereas the MCL as NO3-N is 10 mg/l.
Temperature: Temperature affects almost every aspect of water quality. It affects the amount of dissolved oxygen, the rate of photosynthesis, the metabolic rates of the aquatic animals, and the aquatic organisms’ sensitivity to known toxics in the water.
Turbidity: This element refers to how clear the water is. Typical sources of turbidity in drinking water include water discharges, runoff from watersheds, algae or aquatic weeds, humic acids, high iron concentrations, and air bubbles from treatment process. Clarity is important when producing drinking water for human consumption and in many manufacturing uses.
The table below shows the Water Quality Index Ranges. The highest score that water can receive is 100 and the lowest is 0.
Water Quality Index Ranges and Interpretation
90 – 100
70 – 90
50 – 70
25 – 50
0 – 25
In all, water is a very essential component on earth. Humans and other living organisms use and consume water to function well. Having access to clean water assures us of a healthier existence.
All You Need to Know about Water Quality Index
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