When I visited the new McCook Reservoir, I wasn’t exactly happy to be welcomed by rain pelting down in big summertime drops. I had brought my steel-toed boots but no jacket and no umbrella.

But the rain was appropriate, considering that rain is what this is all about: the 109 miles of deep tunnel, the 10-billion-gallon reservoir this hole in the rock will someday become part of; all so the water that falls from the sky can find its way into a treatment plant without first detouring through your basement, a task that is getting harder for two reasons: the soot we put into the sky and the pavement we slap over the ground.

“Forty percent of Cook County is nonpermeable surface, which means water can’t absorb where it falls,” said Mariyana T. Spyropoulos, president of the Metropolitan Water Reclamation District of Greater Chicago, who accompanied me on a tour of the site tucked between the Stevenson Expressway and the Sanitary and Ship Canal in Bedford Park.

OPINION

Here I interrupted her, incredulous. I’ve heard a lot of stark statistics about Cook County. But 40 percent? How can that be?

“We have concrete,” she said. “We have asphalt. Rainwater cannot absorb into it. Yes, 40 percent. Combine that with the fact that we have climate change, we have more intense rainstorms. In the last 10 years we’ve had three hundred-year rainstorms.”

“You mean the highly controversial theory of climate change?” I quipped.

“Climate change is resulting in more intense rainstorms,” she said. “The fact that we don’t have permeable pavement. The fact that we have aging infrastructure. Smaller pipes. You’re basically trying to push rainwater through a straw into reservoirs.”

When complete in 2029, the McCook Reservoir will be able to hold 10 billion gallons of water.

This first stage of the McCook Reservoir, an old limestone quarry used to hold stormwater, will flood this summer. When complete in 2029, the site be able to hold 10 billion gallons of water. | Neil Steinberg/Sun-Times

No “small pipes” here. Rather, an enormous, 33-foot-diameter outlet of the Deep Tunnel emptying into the reservoir, against blocks of concrete 9 feet tall to disperse the force of the water and keep it from chewing up the bottom of the reservoir, which the district plans to flood by the end of the summer.

“Stage one is going online this year,” said Carmen Scalise, a MWRD engineer, noting the first section will hold 3.5 billion gallons, roughly half the size of the Thornton Reservoir, which opened in 2015. When stage two, holding 6.5 billion gallons, opens in 2029, McCook will be even bigger than Thornton.

Why will completion take a dozen years?

“The reason it’s taking so long is we have to physically mine this rock out, 285 feet deep, and we can only get rid of it as fast as people are buying it,” said Scalise. “With any giant infrastructure projects, it’s not a quick process.”

After the limestone is dug out, a barrier, or “slurry wall,” had to be formed.

“The slurry wall is basically a 3-foot-wide strip of bentonite clay,” said Scalise. “We dug out this trench around the entire perimeter and filled it with clay and it forms an impervious layer. Groundwater from the outside can’t get in. We did the same thing in the rock: two rows of grout curtains: 4-inch-diameter holes drilled . . . down below the bottom of the reservoir and pumped full of grout to keep the ground water out and, when the reservoir is full, keep our CSO in.”

No, not the Chicago Symphony Orchestra. In this world, “CSO” is “combined sewer outflow.”

When I wrote about the Thornton Quarry, I mentioned the MWRD was taking tour groups through, and 3,000 people responded. With McCook, they are again conducting tours before it’s flooded. To schedule a tour, phone (312) 751-6633.

A Hitachi earthmoving truck working in the McCook Reservoir

This Hitachi earthmoving truck, working in the McCook Reservoir, can carry 150 tons of rock. | Neil Steinberg/Sun-Times

GOLF FOR GOOD: During our visit, we paused to remember our mutual friend, Ed McElroy, that grand gentleman of Chicago, a strong advocate of the MWRD, who left us in November at age of 91. I don’t golf, but his associates are holding a 9-hole tournament and luncheon Thursday, July 20, at Palos Country Club. Tee off is 10 a.m., lunch at 1 p.m. Contact Cliff Carlson at (312) 498-1337. All proceeds go to the 100 Club, which supports the families of fallen police officers and firefighters. You can contribute directly at its website, www.100clubchicago.org. I did; it only takes a minute, and Ed would expect you to do it.