How many pounds of material should you apply for 90 acre-feet at 3 ppm if one acre-foot weighs 2.7 million pounds?

• A. 72.9 lbs
• B. 7,290 lbs
• C. 72,900 lbs
• D. 729 lbs

Answer: (D)

Ppm is a weight-to-weight ratio: 3 pounds of material per 1,000,000 pounds of water. Start by turning the volume into total pounds of water: 90 acre-feet times 2.7 million pounds per acre-foot equals 243,000,000 pounds of water. Apply the 3 ppm ratio by multiplying by 3 and dividing by 1,000,000: 243,000,000 × (3 / 1,000,000) = 243 × 3 = 729 pounds. So you should apply 729 pounds of material. The other numbers come from treating the million-pound reference incorrectly or misplacing the decimal.