If the salinity of ocean water at the entrance to an estuary is 30 percent and the average salinity of the estuary is 20 percent, what fraction of the volume of water in the estuary is fresh water?

Question

Quizplus · Accepted Answer

Suppose the volume of ocean water entering the estuary is 100 units. Then, the volume of salt in that water is 30 units. Let the volume of fresh water entering the estuary be x units. The volume of salt in that water is 0 units. The total volume of water in the estuary is (100 + x) units, and the total volume of salt in that water is (30 + 0) = 30 units. The average salinity of the estuary is 20 percent, so the total volume of salt in the estuary is (20/100)(100 + x) units. Setting these two expressions for the total volume of salt equal to each other and solving for x, we get x = 50. Therefore, the total volume of water in the estuary is 150 units, and the fraction of that which is fresh water is 50/150 = 1/3.

If the Salinity of Ocean Water at the Entrance to an Estuary