Calculating water quality
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Calculating Water Quality. Water Quality Index (WQI). The WQI is based on the results of nine chemical/physical tests: Dissolved Oxygen (DO) Fecal Coliform (FC) pH Biochemical Oxygen Demand (BOD) Temperature Change Total Phosphates Nitrates Turbidity Total Dissolved Solids (TDS)

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Calculating Water Quality

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Calculating Water Quality


Water Quality Index (WQI)

  • The WQI is based on the results of nine chemical/physical tests:

    • Dissolved Oxygen (DO)

    • Fecal Coliform (FC)

    • pH

    • Biochemical Oxygen Demand (BOD)

    • Temperature Change

    • Total Phosphates

    • Nitrates

    • Turbidity

    • Total Dissolved Solids (TDS)

  • These tests were chosen because they significantly impact aquatic organisms and are inexpensive to perform.


  • Calculating the WQI

    • After completing the nine tests, the results are recorded and transferred to a weighting curve chart where a numerical value (Q-value) is obtained.

    • For each test, the Q-value is then multiplied by a “weighting factor.”

      • 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).


    Assigning Q-Values

    • Locate the chart for the appropriate test parameter.

    • Locate and mark your test result on the bottom, or horizontal axis, of the chart.

    • Beginning at your mark, draw a vertical line up until it intersects the curve on the chart.

    • From the point where your line intersected with the curve, draw a horizontal line to the left until you reach the vertical axis of the chart.

    • Record the value where this horizontal line intersects the vertical axis of the chart on the form. This would be the Q-value for the test.

    • Repeat each of these steps to find the Q-value for each of the remaining tests results.

    • Make sure you record the correct Q-value in the appropriate column next to each test parameter on the WQI Worksheet before you proceed to the next step.


    What Does the WQI Mean?

    • The WQI uses a scale from 0 to 100 to rate the quality of the water, with 100 being the highest possible score.

    • Once the overall WQI score is known, it can be compared against a scale to determine how healthy the water is on a given day.


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