The water quality index at Higher Level: This statement is Higher Level only. It combines several water tests into one weighted score, using Vernier's index as the example.
Practise this as you read
- Calculate a WQI from Q-values and weights.
- Evaluate the use of a WQI to judge water quality.
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Many tests, one score: Oxygen, pH, bacteria, nutrients: each test tells part of the story. A water quality index puts them together into one score, giving the most important tests the most weight.
What a WQI is
- A water quality index (WQI) combines several water tests into one number.
- It is a weighted average: tests that matter more for life count more.
- It shows the degree of contamination of a water sample, from 0 to 100.
- Vernier's WQI uses nine tests, from dissolved oxygen (0.17) to total solids (0.07).
- The weights add up to 1.00.
Remember it as: Test it, weight it, add it: one number for the whole sample.
Faecal coliform bacteria show the risk of disease. Turbidity and total solids show how much soil and other particles it carries.
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Worked example: the same class tested Millbrook below its sewage works for all nine tests.
Working out the WQI
- Measure each test, e.g. dissolved oxygen with a probe.
- Turn each result into a Q-value (0-100) using that test's chart.
- Multiply each Q-value by its weight.
- Add the answers: that total is the WQI.
- Fewer tests? Divide the total by the sum of the weights used.
- Read the rating: 90-100 excellent, 70-90 good, 50-70 medium, 25-50 bad, 0-25 very bad.
Remember it as: Q times weight, then add: divide by the weights if tests are missing.
Formula
WQI = sum of (Q-value × weight).
Work it out
14.45 + 6.40 + 9.90 + 6.60 + 8.00 + 5.50 + 7.00 + 6.00 + 5.60 = 69.45.
Rate it
69 is in the 50-70 band: medium quality, just short of good.
Explain it
The low Q-values for bacteria (40) and phosphate (55) pull it down: sewage.
Canada and India publish water quality indices for their rivers. One number is powerful, but it can also hide things.
Strengths and limits of a WQI
- Strength: one simple number to compare sites and years, and to explain to the public.
- Strength: it combines many tests, so it is broader than any one test.
- Limit: averaging hides one very bad result, e.g. high bacteria.
- Limit: the weights are a judgement, and the charts were made for rivers in the USA.
- Limit: it leaves out toxins such as metals and pesticides, and one sample is only a snapshot.
Remember it as: Simple to share, easy to compare, but an average can hide a danger.
Strengths
- One number: easy to compare.
- Easy for the public to understand.
- Combines many tests.
Limits
- Hides one dangerous result.
- Weights are a judgement.
- Leaves out metals and pesticides.
An average can mislead: At Millbrook, the bacteria Q-value was only 40, a real health risk for swimmers, yet the overall WQI of 69 sounds almost good. Always look at the individual tests too.
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How this comes up: Paper 2 data questions: a table of Q-values and weights, with 'calculate the WQI' [1-2], then 'evaluate the use of a WQI' or 'suggest why one test has a larger weight'.
A school could run only five of the nine tests on a stream. The Q-values were: dissolved oxygen 80 (weight 0.17), BOD 50 (0.11), pH 90 (0.11), nitrate 60 (0.10) and total phosphate 40 (0.10).
Calculate the water quality index of the stream and state its rating.
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