It's simple, Just don't give them the water!

Waukesha County is about to get approval to buy water from the City of Milwaukee.  Apparently Racine and Oak Creek are also willing to sell to Waukesha County but they are less desired due to the higher cost and logistics of piping the water.  If Milwaukee does not provide water than it fears that Oak Creek or Racine will and Milwaukee will lose a source of revenue.  Additionally Milwaukee wants conditions for public transit and affordable housing fulfilled.
Link to Daily Reporter article "Housing, Tansit at Center of Water Sale"

A brief history of this topic:
The area of Waukesha that needs water lies in the Mississippi River Watershed (all falling rain water and ground water naturally drains west to the Mississippi River and eventually to the Gulf of Mexico). Developers are having a difficult time developing subdivisions in the remaining farmland of Waukesha County because the ground water (future well water) is contaminated or can not be reached affordably by drilling new wells (excessively deep drilling). Also the public water supply for Waukesha County can not supply any more water for new residents.

Milwaukee water comes from Lake Michigan, the second largest freshwater lake in the world. The supply SEEMS limitless (it isn't). Rain water and ground water from Milwaukee and Racine county flow east to Lake Michigan and eventually to the Atlantic Ocean. Any water that is pulled from Lake Michigan needs to be returned to Lake Michigan in the form of treated waste water, lawn sprinklers, etc. Water sold to the western 2/3 of Waukesha County will not be entirely returned to Lake Michigan. Instead it will go to the Mississippi River. What seems like a small detail or unimportant piece of information is actually a significant imbalance in our ecological system.
The Continental Divide runs through Waukesha County. Many of you may never realize that the big hill you drive over on I-94 between Hwy 100 & Moreland Rd. exits is the Continental Divide. From the top of this divide (which divides the continent from north to south) rain water either drains west or east depending on which side you are on. It has done this for millions of years with minor exceptions where man has interfered. The Mississippi river depends on its supply of water. Lake Michigan does also. Lake Michigan, in the last fifteen years, has been slowly lowering in height. This problem has been already causing significant coastal erosion. It is believed that human over consumption is one of the causes.
A decision to sell Milwaukee, Oak Creek or Racine water to municipalities in Waukesha County is also a violation of the Great Lakes Treaty which says that all water sourced from the Great Lakes must be returned to them.
In addition, the water is primarily needed for residential and industrial development of existing farmlands and forests. Not providing this water may make these unnecessary developments not feasible. Providing the water will open up the "flood gates" of development and turn undeveloped land into housing subdivisions that are not planned properly and should not exist in exo-suburban conditions. STOP SUBURBAN SPRAWL NOW! Besides there may be a large percentage of existing residents that may want their rural life preserved. If Milwaukee won't provide them water maybe Oak Creek