Advanced Waterworks Mathematics, 2019a

Practice Problems 12.1
1. A well has a nitrate level that exceeds the MCL of 63 mg/L. Over the last 4 sample
results it has averaged 71 mg/L. A nearby well has a nitrate level of 40 mg/L. If both
wells combined pump up to 1,575 gpm, how much flow is required from each well to
achieve a nitrate level of 50 mg/L?
Well A – 71 mg/L Well B – 40 mg/L Desired result “C” – 50 mg/L
To determine the percentage of Source A required for the blend, the desired
result minus the low result is divided by the high result minus the low result.
C - B =
A - B
50 mg/L - 40 mg/L 10 mg/L
= = 0.3225
71 mg/L - 40 mg/L 31 mg/L
This says that 32% of Well A is needed to achieve the desired blended result.
To determine the percentage of Source B required for the blend, the high result
minus the desired result is divided by the high result minus the low result.
A - C =
A - B
71 mg/L - 50 mg/L 21 mg/L
= = 0.6774
71 mg/L - 40 mg/L 31 mg/L
This says that 68% of Well B is needed to achieve the desired blended result.
Well A:
1,575 gpm 0.32 = 504 gpm
Well B:
1,575 gpm 0.68 = 1,071 gpm
2. A well (A) has shown quarterly arsenic levels above the MCL over the last year, of 16
ug/L, 22 ug/L, 20 ug/L and 10 ug/L. A utility wants to blend this well to a level of 6.0 ug/L
with a well (B) that has a level of 2.1 ug/L. The total production needed from both of
these wells is 4,100 gpm. How much can each well produce?
Remember that it is best to take the highest result when calculating blend volumes.
Well A: 22 ug/L Well B: 2.1 ug/L Desired result: 6.0 ug/L
Well A:
C - B =
A - B
6 ug/L - 2.1 ug/L 3.9 ug/L
= = 0.19597
22 ug/L - 2.1 ug/L 19.9 ug/L
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