Federal ELG requires that there be no discharge from a CAFO except for the overflow from a system designed and operated to retain the 25-year, 24-hour storm event except if the precipitation event exceeds the 25-year, 24-hour storm?

To demonstrate the ability of VTS to achieve the runoff control equivalent to the ELG, the results are compared using a computer model for predicting performance for a traditional system based on the ELG of the CAFO regulations and using the weather files generated at the site in question.

It is incorrect to assume that traditional systems designed and managed according to the ELG of the CAFO regulations overflow only during storm events in excess of the 25-year, 24-hour event. Chronic wet periods are the primary reason for discharges in many wetter regions. Several published modeling studies for traditional system design suggest discharge is common for storm events less than the ELG. No discharge except for storm events greater than the ELG is achieved only in the western regions of the High Plains (e.g., western Kansas and Nebraska). Chronic rainfall events over a several day period (none exceeding the design storm event) are a common cause for discharges from traditional well-designed and managed systems in many Corn Belt states.

Author: Lara Moody, Iowa State University