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.