If you remember from Lesson 1, fish are the favorite food of Bald Eagles. Since fish live in the water and can be affected by what is in the water, it is important to know if a lake or river is “healthy.”

In an attempt to devise a system to compare rivers and lakes in various parts of the country, the National Sanitation Foundation (NSF) created and designed a standard index called the Water Quality Index (WQI). The WQI is one of the most widely used of all existing water quality procedures. The overall results of nine separate tests can be used to determine if a particular stretch of river is healthy.

The WQI consists of nine tests:

Dissolved Oxygen
Fecal Coliform
pH
BOD (Biochemical Oxygen Demand)
Temperature
Total Phosphate
Nitrates
Turbidity
Total Solids

After completing the nine tests, the results are recorded and transferred to a weighting curve chart where a numerical value is obtained. For each test, the numerical value or Q-value is multiplied by a “weighting factor.” (See individual tests for more information on Q-value.) For example, dissolved oxygen has a relatively high weighting factor (.17); because it is more significant in determining water quality than the other tests. The nine resulting values are then added to arrive at an overall water quality index (WQI). The highest score a body of water can receive is 100.

Water Quality Index Ranges

90-100	Excellent
70-90	Good
50-70	Medium
25-50	Bad
0-25	Very Bad

 

 

 

 

If you are unable to run all nine tests and you want to estimate the “Overall Water Quality Index,” students could determine the Q value of missing data by examining known data. For example , if the “fecal coliform” test were not run, the results or Q-value of three related tests (dissolved oxygen, nitrates, total phosphate) could be averaged to get a score for that test. Using the table below as an example, the average Q-value of the three tests is 62 - or a fecal coliform count of 20 colonies per 100 ml of water.

So is this water excellent, good, medium, bad or very bad?

---

the following table to enter your own data.

Note on Sampling

It is important to exercise care in the way samples are collected for analysis. 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.

A simple device can be constructed from a series of metal rods that can be extended and rubber tubing attached that holds the sample bottle. This device might be extended out from shore if no bridges are available and particularly if the river is narrow or shallow. A golf ball retriever can also be adapted very easily for this purpose.