The makers of Coors beer boast of using water from the snowcapped Rocky Mountains. In Virginia, Blue Mountain Brewery draws from deep wells in the Blue Ridge Mountains.

Now there's Baxter's Best, an ale formulated to take advantage of a more urban chemistry: tap water from Philadelphia.

No joke. Beer experts say the stuff from Philly's water mains is ideal for brewing an English-style ale such as Baxter's, made by Saint Benjamin Brewing Company in Kensington.

The exact chemistry is fairly complex, having to do with the interplay of calcium, magnesium, and alkalinity, but those thirsty for knowledge can get a taste Tuesday night at a pop-up beer garden run by the Pennsylvania Horticultural Society at 15th and South Streets.

Brewery owner Tim Patton, who is speaking at 6:30 p.m. about his attraction for the city's water, acknowledged that some people react as if he has quaffed a few too many.

"When we tell people about Philadelphia water, we get very mixed reactions," he said.

But he is on firm ground, said Colin Kaminski, co-author of Water: A Comprehensive Guide for Brewers.

Municipal water is commonly used to make beer, and historically, the mineral profile of each city's water system dictated the kinds of beer made there, he said.

Brewmakers learned early on that the hard waters of Dublin were ideal for stouts, said Kaminski, the brewmaster at Downtown Joe's in Napa, Calif. The softer waters of the Pilsen region, in the Czech Republic, became known for golden pilsners.

Philadelphia's profile makes it ideal for mid-range beers, such as pale ales, without any modification, Kaminski said after looking at the city's water profile.

"That really reminds me of the old-school London waters," he said.

The key is that the water contains moderate levels of calcium and alkalinity, helping to maintain the proper pH during the "mash" phase of brewing, when starches are converted to sugars.

Brewers typically aim for a mash pH in the 5.2 to 5.6 range on that zero-to-14 scale you may recall from chemistry class. (Values below 7 are acid; a number above 7 means the liquid is a base, such as bleach.)

Saint Benjamin's Patton said the malts he uses in Baxter's Best — roasted barley and some chocolate malt — drive the mash pH down to just the right level.

Lighter grains such as those used in pilsner might not drive it down far enough, while the darkest malts could yield a mash that was too acid — in each case possibly compromising the taste unless the brewer modified the water chemistry, Kaminski said.

The only thing Patton does to the city tap water before brewing is to remove the chlorine, by using a charcoal filter.

Baxter's Best is named for the Baxter water-treatment plant in the Northeast section of the city. The brewery owner said he came up with the idea for the beer in the spring after talking with an acquaintance at the Philadelphia Water Department. Saint Benjamin started making it in June.

Speaking along with him at the beer garden Tuesday will be Gary Burlingame, director of the water department's bureau of laboratory services.

It is the second of three events at the garden this week held by the water department, which seeks to publicize its successes with green roofs, rain gardens, and other eco-friendly projects that keep storm water out of the aging city sewers.

The third is Wednesday, complete with a dinner menu and drinks, including Baxter's Best.

The mineral profile of the city water comes from limestone and other sedimentary deposits in the upper reaches of the Delaware and the Schuylkill, Burlingame said.

"These are all the natural minerals that come from the erosion and weathering of the sedimentary rock in the watershed," he said.

In the late 1800s, Philadelphia had dozens of breweries, but toward the latter part of the 20th century the number dwindled to a handful. Now, with the surge in craft breweries, the beer scene is hopping once again.

"It's coming full circle," Burlingame said.

Maybe it's something in the water.