Accurate estimates for pollution sources are crucial during the permitting process. For permit modeling applications, haul roads are treated as a volume source. The following three model inputs are key for modeling volume sources:
- Release height (hr)
- Initial horizontal plume size (σyo)
- Initial vertical plume size (σzo)
The model results can vary significantly depending upon the assumptions used in these parameters. EPA’s Haul Road Workgroup Final Report recommends the following:
- Initial σyo: width of vehicle + 6 m (for single lane road) divided by 2.15 for multiple volume sources
- Initial σzo: 1.7 times the height of the vehicle divided by 2.15
- Initial release height, hr: 0.5 times the height of the plume (1.7 x vehicle height)
For haul roads very close the property line, accurate σyo, σzo and hr values can significantly affect the estimated concentrations. For better accuracy, CPP uses wind tunnel modeling to determine site-specific σyo, σzo and hr AERMOD inputs. A volume source representing a truck is placed in the wind tunnel (see below) and a tracer gas is released from a series of holes representing the volume source. Vertical and lateral concentration distributions are then measured at the property line and analyzed to determine the key values used as AERMOD inputs.
A recent study showed σyo values about two times higher than the values recommended using the Texas method (which is very similar to the EPA method). This translates to more accurate and lower concentration estimates at the fence line.