It's a race against growth.

The Twin Falls City Council has to decide within the next few weeks how to find more drinking water for the city and cut down arsenic levels in its existing water by 2011.

But a presentation of the city's remaining options on Monday night showed that even after two years of studies and discussion, council members still don't think they have all the answers they need to decide the future of the city's water supply.

One thing is clear - in one way or another, Twin Falls residents will have a share in the cost of averting a water crisis. Projections from J-U-B Engineers, the firm conducting the studies for the city, show monthly water rates doubling across the board by the time the first phase of any of three options is completed in early 2011.

The city's choices at this point all revolve around the Low Line Canal, J-U-B representative Mark Olsen said, the only water source the city owns enough shares of to be sure of supply in uncertain times. One plan would build a treatment plant for water from the canal and the city's South wells. Another would retrofit as many as 4,000 homes to use pressurized irrigation water while blending Blue Lakes and South well water to dilute the arsenic. The third would combine elements of both, building a smaller treatment plant and only retrofitting half of the homes.

Cost estimates for the three ranged from $54.7 to $66 million in capital costs and $84.8 to $116.2 million in long-term costs. But council members found their own hidden costs. The lowest-cost solution presented to the council, the large-scale retrofitting, was also the most disruptive and would require those homeowners to pay for any part of the retrofit on their property.

Under the estimates, the city would pay for any work up to the curb line. But Bill Block, a senior project manager with J-U-B, said his subdivision had recently gone through a retrofit and he knew of homeowners who paid between $1,000 and $1,500 for the necessary work.

"It's very, very doable," Block said. "People would have to be willing to not only spend the money, but put up the effort."

The council decided to meet at a later date to go over the options in more detail.

Also Monday night, the Historic Downtown Business Improvement District returned home as the council voted unanimously to place it back fully under city supervision and turn its board, which acted in the past as a decision-making body, into a city advisory board. Though a couple of business owners advocated for dissolving the district, council members - after much debate - decided it still had value to the city.

The council voted immediately after to spend $8,500 in district funds on a landscaping study by The Land Group Inc., and arranged to meet with a parking consultant recommended by the Leland Consulting Group, which is also involved in downtown renovations.

Nate Poppino can be reached at 735-3237 or npoppino@magicvalley.com.