Concord, New Hampshire, is the first real city on the Merrimack River as it flows south out of the White Mountains.

This plateau area has been rebounding since it was compressed under the immense weight of the Wisconsin glacier 20,000 years ago. Now, it is about 500 feet higher than Portsmouth on the coast only 20 miles to the east.

As the land rebounded, glacial Lake Winnipesaukee, to the north of Concord, broke through a natural dam holding back the lake and its waters carved out Weirs Channel as it roared into the Atlantic Ocean. The ocean, in turn, flooded back into the lake, leaving anomalous marine fossils in its wake.

In Concord, the Merrimack widens as it meanders through a broad valley laced with several meander loops and oxbow lakes.

The tonier parts of the city are built on broad terraces that were once the shores and beaches of glacial Lake Merrimack and many of them are still pitted with sand and gravel quarries.

The Penacook Indians used to fish for salmon, sturgeon and alewives below Concord’s Penacook Falls and are said to have been able to paddle their birch bark canoes from the Merrimack to the Atlantic through Lake Winnipesaukee and Alton Bay.

Concord was originally established as Rumford in 1734, but optimistically renamed Concord after a bitter dispute with the town of Bow.

The new city became known for its Concord granite that graces the Library of Congress and the similarly optimistically named United Nations building in New York City. In the west, however, Concord was more renowned for its elegant Concord stagecoaches, which were the Lincoln Continentals of their time.

Because it is relatively small, Concord always packed several thousand people into its 67 square miles and waste from all those inhabitants ended up in the Merrimack River.

In 1972, Congress created the Environmental Protection Agency, which required that even such small cities had to build plants to treat their sewage. More importantly, it also provided most of the money to build the plants, which it no longer does.

But, the first thing the EPA required Concord to do was to build a facility to treat the wastes being discharged from the Allied Leather Co. that used toxic chemicals like chromium in the tanning process. So Concord built the Penacook plant in 1973.

The conventional sludge treatment plant was converted to a sequencing batch reactor after the tannery closed in 1987. Now, the plant’s former aeration tank is used to store runoff rainwater during severe storms.

This means that plant managers don’t have to release raw sewage into the river to protect their plant from being overwhelmed by the excess water.

The Penacook plant is now supported by the city’s main wastewater plant, which treats about 4 million gallons of wastewater a day and has the capacity to treat an additional 6 million gallons.

The plant also treats 5 million gallons of landfill leachate and 2 million gallons of sewage trucked in from surrounding communities that still use septic tanks. It also produces 7,500 tons of biosolids that are used for fertilizers and manufactured topsoil.

So, Concord is the only city on the Merrimack that has the ability to store wastewater, which prevents the combined sewage overflows, or CSOs, that plague the lower half of the river. Unfortunately, we will have to hear a lot more about these CSOs as we continue our explorations south toward the waiting Atlantic.

Bill Sargent is a North Shore science writer and contributing columnist. His most recent book, “Coastal Discoveries,” is available in local bookstores and through Storm Surge at http://plumislandoutdoors.org and www.ingramcontent.com.

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