How many pounds of herbicide are required to treat a 10 acre-foot body of water to reach 0.2 ppm, given that an acre-foot weighs 2.7 million pounds?

• A. 2.7
• B. 3.6
• C. 4.5
• D. 5.4

Answer: (D)

The key idea is converting a concentration in parts per million to an actual mass given the total amount of water. 0.2 ppm means 0.2 pounds of herbicide for every 1,000,000 pounds of water. The body of water weighs 10 acre-feet, and each acre-foot weighs 2.7 million pounds, so total water mass is 10 × 2.7 million = 27 million pounds.

Multiply the water weight by the ppm ratio: (0.2 / 1,000,000) × 27,000,000 = 0.2 × 27 = 5.4 pounds. Therefore, 5.4 pounds of herbicide are needed to reach 0.2 ppm in 10 acre-feet of water.