Reps. Candice Miller and Debbie Dingell

The Great Lakes Compact, adopted in 2008 by eight states and the federal government to protect our greatest natural heritage, was a prodigious and far-sighted achievement.

By agreeing to legal standards for the use and preservation of Great Lakes water, its signatories demonstrated a commitment to the sustainable health of a surface water system and shared watershed that provides the 40 million people who live within it fresh water, an economic bounty and a source of inspiration.

The central component of the compact is a ban on the diversion of Great Lakes water for use outside the watershed.

And today, the compact and the commitment face a crucial test.

It comes in the form of a request by the city of Waukesha, Wis., which lies outside the Great Lakes watershed, to pump millions of gallons a day from Lake Michigan as a replacement for an underground water source contaminated with radium.

We understand and empathize with the city’s plight. The Waukesha water system is under court order to address its contamination problem, the result of naturally occurring radium in the aquifer, by 2018. And the city has made a good faith effort to minimize environmental concerns associated with the withdrawal (including a pledge to return treated wastewater to a river which flows into Lake Michigan).

Waukesha relies on a provision in the compact to authorize exceptions to the diversion ban for communities within counties which straddle the Great Lakes watershed, and for whom no reasonable alternative supply of clean water is available.

We understand. But we are also convinced that approving the Waukesha diversion would be a grave mistake, a violation of both the spirit and the letter of the law embodied in the Great Lakes Compact.

That is why we, along with nine other members of the Michigan congressional delegation from both political parties, called on Michigan’s Rick Snyder and the other Great Lakes governors last week to exercise their authority under the compact to veto the Waukesha proposal (exceptions to the diversion ban require unanimous approval of the eight compact states). The governors are scheduled to make a decision June 21.

We believe there is a better alternative, for both Waukesha and the Great Lakes. One potential solution — adding robust new radium-removal treatment facilities to Waukesha’s existing water infrastructure — would be both effective and less expensive, according to an independent engineering study commissioned by a Wisconsin-based conservation coalition.

But the most compelling reason to reject an exception to the Great Lakes diversion ban is the precedent it will set.

On a resource hungry planet, demands to access and exploit the fresh waters of the Great Lakes will inevitably recur and intensify. They will come from sympathetic supplicants which straddle the watershed, like Waukesha, and from the distant but politically powerful and predatory.

Our ability to reject the latter may well depend on how exacting we are in responding to the former.

The Great Lakes Compact was a generational pledge that we, the temporary stewards of one of the world’s great natural wonders, would act as its guardians. The first, critical measure of our resolve is at hand.

If we are serious about our pledge, diversion is a door we should not open.

Rep. Candice Miller, R-Harrison Township, represents Michigan’s 10th Congressional District. Rep. Debbie Dingell, D-Dearborn, represents Michigan’s 12th Congressional District.