The Columbia and Snake River system allows large volumes of freight to be moved with negligible impact on airquality, and replacing this transportation capacity would require use of 120, 000 rail cars or 700, 000 trucks.
"We will be investigating what impact the proposals will have on airquality and the health of local people, " said David Edwell, the north Wales area manager for Environment Agency Wales.
Campaigners against the plans claimed the "car-centric development" would have a detrimental impact on local businesses on nearby Gloucester Road, airquality and traffic congestion.