Pat Mulroy, the Las Vegas-based general manager of SNWA, told her board in August that it should sue Utah for balking.
The agency wants to move forward with a $15.5 billion pipeline proposal to pump water from desert aquifers for transfer to Las Vegas.
Earlier this week, an independent panel of water attorneys appointed by Herbert reported the Snake Valley Water agreement with Nevada would be an "equitable" deal for Utah.
"The Agreement allocates this shared ground water resource on an equal 50-50 basis," their report states. "It allows for additional water development in both states, but that is conditioned upon protecting existing rights from interference."
The Millard County representatives disagree.
"A 50-50 split is non-sensical," the commissioners state, "because it contradicts the geographical reality that the vast majority of groundwater consumption and historic usage, have occurred and continue to occur on the Utah side."
Utah's historic allocation is about 55,000 acre feet per year to Nevada's 12,000, according to the commissioners. The agreement also would give Nevada 36,000 acre feet per year of un-allocated groundwater to 5,000 for Utah.
"Once the split takes effect," the commissioners' letter says, "the groundwater that is rightfully and fairly Utah's property and birthright is forever forfeited."
Wednesday, Salt Lake County Mayor Peter Corroon urged the governor to listen to the Millard County Commission. Juab and Tooele counties also have asked Herbert not to sign the agreement.
Herbert spokeswoman Ally Isom said the governor "is determined to protect every drop of Utah's water."
As such, the Millard County Commission's letter will be considered by the state Water Development Commission along with the legal analysis by the governor's panel of water attorneys.
The Water Development Commission will take up the matter Nov. 13 at 3:30 p.m. in Room 30 in the House Building, northwest of the Capitol.