After the test is completed and the results are recorded, the Water Quality Index (WQI) for the body of water can be computed. To formulate a WQI, you must first compute a Q-value for the results you obtained. The Q-values of all nine tests will be used together to determine the health of the river or lake. See Water Quality Index page. To compute the Q-value for the Phosphate (PO4-P) test, follow these steps:

1. Find the weighting curve chart at the end of this page;
2. Locate your test result on the bottom (horizontal or “x” axis) of the chart;
3. Interpolate the Q-value for your test result using the following steps;
1. From your test result value on the horizontal (“x”) axis of the chart, draw a vertical line up until it intersects the weighting curve line;
2. From this point of intersection, draw a horizontal line to the left hand side (the vertical or “y” axis) of the chart;
3. Where this horizontal line intersects the vertical (“y”) axis of the chart, read off the value. This is the Q-value for this test; it should be recorded in Column B on the WQI chart on the Water Quality Index page.

The Q-value for each test should then be multiplied by the weighting factor listed in the chart on the Water Quality Index page. Record the product of this calculation in Column D of the chart.

Phosphate (PO4-P): mg/L Note: PO4-P>10.0, Q=2.0