When people talk about rising prices these days, the discussion usually is about food and fuel.

But local officials are also reeling from the sudden spike in prices for another staple product - potassium hydroxide, a chemical compound used to treat water for corrosion after it runs through copper pipes.

The inflation rate for potassium hydroxide makes gas look cheap. In just one year, the price for a gallon of the chemical has jumped an astronomical 178 percent, from $1.75 to $4.87.

That $3-plus increase was the lowest bid received by the Southeastern Regional Services Group, a bulk-buying consortium of local communities that includes Foxboro, Mansfield and Norton.

Other communities have seen an even larger increase. Dartmouth, which isn't part of a purchasing group, is now paying $5.82 a gallon. In Mansfield, the water system uses about 65,000 gallons of potassium hydroxide a year, which means the town's annual bill for the chemical will jump from $113,750 in 2008 to $316,550 in 2009 - more than double the amount officials had budgeted.

The unexpected increase may force Mansfield to adjust the water department's budget later this year, said Lee Azinheira, the town's public works director.

"Just to put it in perspective," Azinheira said, "that increase alone represents 5 percent of our budget - just that one thing that we did not plan on."

"Everyone I've talked to is facing the same problem," he added. "We've just never seen any escalation like this."

Some communities are paying as much as $8 a gallon, he said.

Strangely enough, farmers in Iowa are the ones making it more costly to treat the water in Mansfield, said Steven Monica, sales and marketing manager at Univar, an industrial chemical distributor in Providence.

"Chemicals follow the energy markets," he explained.

Demand for fertilizer, which is made from potassium hydroxide, is growing quickly because of farmers planting more corn crops to cash in on the ethanol boom, as well as developing countries buying more food, he said.

In addition, the weak dollar has increased foreign demand for American-made chemical products, while a decade-long consolidation in the industry has left the country with fewer chemical manufacturing plants.

Three of the four major producers of potassium hydroxide have stopped shipping to New England because it costs too much, Monica said. Mansfield used to gets its supply of potassium hydroxide from Delaware, for example, but since that plant shut down, it has to come from Louisiana - and the rising cost of fuel means it costs more to transport it from Louisiana to Massachusetts. Dow Chemical recently added a $600 fuel surcharge to its shipments.

"Across the chemical industry, we're seeing the highest prices we've ever seen," Monica said. "We created a perfect storm and it all came together this year."

Monica said prices could continue to rise over the next few years until more production plants come online, and he did not rule out the possibility of shortages in the interim.

"We're watching it closely," he said.

TED NESI can be reached at tnesi@thesunchronicle.com or 508-236-0333.