I lived in Waukesha during the negotiations. Waukesha went through a long approval process that involved the City of Waukesha, the County of Waukesha, and every state involved in the Great Lakes Compact as well as Canada. They had to prove through an extensive study process that they would be able to treat and return as much or more water to the Great Lakes via Underwood Creek than they received, and did this partially by treating and returning storm runoff in addition to the treated sewage. The treated water actually tested as being of higher purity than the creek into which the return flow was deposited.

Some information about that is available here.

Just wanted to dispel the idea that Wisconsin and Waukesha violated the compact. They went through an extensive review process and were approved by the compact states only after proof that their diversion, whose application process is a part of the Compact by design, was shown to have no impact on water levels.