Tony Gaudry is a miner who works 300 feet underground – and a mile from Homewood. Since 2011, he
has shored up a 30-foot-high tunnel with steel, rebar and concrete, preparing four slots for steel doors
that will slide open one morning in August, letting billions of gallons of storm water and sewage flood
the north lobe of the Thornton Quarry.

What was under water 350 million years ago will be under water again this summer when the Thornton
Composite Reservoir opens. A 17-year project by the Metropolitan Water Reclamation District, it will be
big enough to prevent flooding in an area ranging from the south side of Chicago to Homewood and
Flossmoor.

As a miner – that’s his title in the construction industry – Gaudry works with concrete, steel and
carpentry and prides himself on “being able to solve problems and build things” in a below-ground
world of ever-expanding tunnels.

Gaudry’s below-ground world, soon to be filled with water, presents a dramatic sight to the casual
visitor. However, viewing the Thornton Quarry is equally dramatic.

All around the quarry’s north end, workers are putting the finishing touches on the new reservoir, which
will act as a storage basin for up to 7.9 billion gallons of combined stormwater and sewage.

From above, the heavy machinery looks like a collection of toys so tiny, you could sweep them all into
one hand. But 350 feet below, on the quarry floor, the cranes, trucks, bulldozers and other earthmoving
equipment appear gigantic; they are machines being used to build a one of the world’s biggest
reservoirs. Looking up the limestone walls of the eons-old quarry, you have a good idea of your own
positively miniature size.

John Lemon, principal civil engineer for the Metropolitan Water Reclamation District of Greater Chicago,
pointed to the north lobe of the quarry and rattled off statistics. The reservoir area is one-half mile long
by one-quarter mile wide by 350 feet deep.

“There is no place like it in the world,” he said.

In part, that’s because the reservoir will be so enormous. But the rest of the story is hidden beneath the
ground. The quarry reservoir will be linked to a system of 30-foot tunnels – also carved into the rock and
more than 300 feet – that will be connected to an MWRD treatment plant at 130th Street in Chicago.

The Thornton Quarry is literally in the H-F area’s backyard It is located just east of big box stores along
Halsted Street in Homewood. A second water storage facility on quarry property — the Thornton
Transitional Reservoir, completed in 2003 — is only a few blocks away from the Homewood border.

Homewood and Flossmoor are both served by MWRD.

Lemon said the Thornton Quarry oldest limestone mining operations in the country, with quarrying
starting in the 1830s. But the quarry site is, of course, much older than that. Many millions of years ago,

it was probably a reef in an ancient tropical ocean. Fossils found in the quarry show that the long-ago
existence marine animals with shells, sponges and some trilobites. Lemon said geologists have been
studying the quarry area for generations.

Area residents may feel closest to the quarry when they drive over the Interstate 80/94 bridge that
separates the north lobe from the main pit area.

When you are at the bottom of the quarry, you can see cars on the interstate. But you also notice that,
just below the bridge construction crews built a concrete dam that will separate the two sections of the
quarry and prevent water from the new reservoir from flooding into the main pit. The dam is 120 feet
high and was built on rock that is 200 feet above the quarry floor. It is, Lemon says, one of the biggest
dams in Illinois.

The new Thornton reservoir is part of MWRD’s Tunnel and Reservoir Plan, or TARP, which was first
initiated in 1975. TARP, well known as the “Deep Tunnel” system, provides a series of large diameter
tunnels and large reservoirs designed to reduce flooding and improve water quality in rivers and
streams.

Lemon said the new reservoir is expected to be operational this August but that MWRD engineers won’t
know how well it works until the first major rainstorm hits the area. He speculated that a rainfall of
three or four inches in a short period of time may nearly fill the quarry reservoir to the top.

At the bottom of the quarry Lemon pointed to a 33-foot- high opening on the quarry’s east wall. That’s
where water will enter the reservoir from the tunnel system. Underground, and less than a quarter-mile
away, two huge gates will regulate the flow of water from the tunnels to the reservoir. The iron gates
are nearly 25 feet high two feet thick.

During heavy rains, stormwater and sewage will be pumped to the new reservoir. Following the storm,
water will be sent through the tunnel to the 130 th Street treatment plant, where it will be processed and
returned to waterways flowing toward the Mississippi River.

Lemon said local residents were concerned that the water flowing into the reservoir would produce
unpleasant odors, so MWRD has installed seven solar-powered aerators to freshen the air in the quarry.
Lemon said MWRD started working on engineering plans for the Thornton tunnel system in the 1980s.

Work on the Thornton Composite Reservoir began in earnest in 1998, when MWRD reached an
agreement with Hanson Material Service, the quarry operator, to use the north lobe for a reservoir.

Construction began after Hanson Material Service completed mining in the north lobe; he said about 58
million tons of stone were removed after the company signed its agreement with MWRD. Lemon said
Hanson Material Service still reserves the right to mine for limestone underneath the reservoir.