The following is an example of the need for delivery ratios in pollutant trading and demonstrates why delivery ratios must be understood before any trades can be made. The example examines a potential trade between a nonpoint source (located at River Mile 15) and a point source at River Mile 3. The receptor point for this trade would be at River Mile 0. This is the point at which the phosphorus from each source will be compared.

The Nonpoint Source has 100 kg of TP load in its runoff. Much of this phosphorus is lost to the river, however, so much less than 100 kg can be traded.

  • 10% of the runoff load is removed through uptake or settling between River Mile 15 and River Mile 7 (approximately 90 kg is left at River mile 7)
  • At River Mile 7, half of the remaining phosphorus is removed at an agricultural diversion (approximately 45 kg left).
  • Another 10% of the load is lost through uptake or settling as the remaining phosphorus moves from River Mile 7 to the Receptor point at River Mile 0 (approximately 40.5 kg left)

Therefore only 40.5 kg of the original 100 kg from the nonpoint source located at River Mile 15 can be traded. It’s delivery ratio is 40.5/100 = 0.405.

In contrast, a point source located at River Mile 3 loses only 10% from its point of discharge at River mile 3 to River mile 0 (approximately 90 kg left). Its delivery ration is 90/100 = 0.9.