What does the Great Lakes-St. Lawrence River Basin Sustainable Water Resources Agreement have to do with the Upper York Sewage Solutions? 
Quite a lot, according to some Canadian environmental experts.

With the proposed sewage plant in Queensville funneling treated water back into the Lake Simcoe watershed via the East Holland River, Environmental Law Association researcher Sarah Miller said the region is following provincial direction by keep wastewater within the Simcoe watershed.

The agreement Ontario signed with Quebec and eight American states in 2005 aims to avoid bulk water transfers out of the great lakes, Ms Miller said. 
“Ontario is the only jurisdiction to have not implemented the agreement,” she said. 
“They directed regions and municipalities to act as if the agreement was in full force.”

While many East Gwillimbury residents would prefer to see their sewage sent down a pipe to Lake Ontario, this would raise complications because it would be considered an intra-basin water transfer, Ms Miller said.
An intra-basin water transfer involves moving water from one river basin to another so it can be put to better use.

A bulletin, issued in 2009 to all municipalities, stated water and wastewater projects must follow the 2005 agreement.

more: YorkRegion Article: 2005 act impacts East Gwillimbury sewage project.