10 Dewatering Days

By: John George, P.E. Agricultural Engineering Associates

Q: How Did EPA Come Up with the 10 Day Time Limit for Dewatering Runoff Detention Ponds? This requirement can be difficult for my yard to meet.

A: When the Clean Water Act (CWA) was passed, EPA was to define pollution control performance metrics for each class of discharging facilities. For CAFOs, the prescribed performance level reflected runoff detention ponds capturing all manure and wastewater runoff resulting from a 25 year – 24 hour duration precipitation event; the largest local rainfall that occurs once every 25 years.

While the sizing of detention structures was addressed by the CAFO technology assessment, the timing for pumping captured stormwater out of the detention pond(s) was not addressed. Permit conditions, however, must provide for timely dewatering of runoff detention ponds to re-establish holding capacity for ensuing runoff events. There was no data available at the time to define what the pump-out requirements should be? As EPA’s primary focus was the burgeoning development of large open feedlots throughout the arid high plains, the permit writers were tasked to assume that every feedlot would have a number of nearby center pivot irrigation fields to utilize in accomplishing runoff removal. Permit conditions were written requiring detention pond pump out within 10 “dewatering days.” A “dewatering day” was defined as one where the temperature is above freezing, the ground is not frozen, and there has not been more than 0.05” of precipitation on any of the last three days prior.

The CWA focus and impact has progressively broadened to encompass thousands of CAFOs, many of which are located in wetter climates where irrigation often is neither needed nor practically achieved. For such CAFOs the burden of dewatering a detention pond in 10 days can be really daunting. As the early permit writer tasked with conceiving the dewatering requirement, I had anticipated from it’s inception that the requirement would morph within the bounds of climatic/environmental practicality as more knowledge and experience evolved. Such has never happened nor apparently even been broached within the environmental bureaucracy.