What is the Saturation Index of a 330,000-gallon outdoor pool with total alkalinity 50 ppm, pH 7.2, calcium hardness 150 ppm, and total dissolved solids 500 ppm?

To understand why D is the correct answer for the Saturation Index calculation, it's important to know how to compute the Saturation Index using the formula:

Saturation Index (SI) = pH + TF + CF - AF

Where:

• pH is the pH of the pool water.

• TF (temp factor) is derived from the water temperature; typically at 77°F it is around +0.4.

• CF (calcium factor) is calculated as (calcium hardness / 10) for values in ppm.

• AF (alkalinity factor) is (total alkalinity / 10) for values in ppm.

Given the parameters:

• pH = 7.2

• Total Alkalinity = 50 ppm

• Calcium Hardness = 150 ppm

• Total Dissolved Solids = 500 ppm

Here’s how each factor breaks down:

1. TF is approximately +0.4 (assuming the pool temperature is around 77°F).

2. For CF, divide calcium hardness by 10: 150 ppm / 10 = 15.

3. For AF, divide total alkalinity by 10: 50 ppm / 10 = 5.

Now, substit