Black Lagoon: The monster's cage weakening

PHOTO GALLERY: The concrete, carved-stone and earthen dam off Walker Street in Marlborough, Mass., has been poorly maintained for the past 20 years. Several dams in Massachusetts are in similar disrepair; but none are like the bulwark at The Black Lagoon, which contains untold tonnage of contaminated silt. The man-made pond is contaminating the nearby Sudbury Reservoir, part of the MWRA water system's emergency water supply for 2.2 million Greater Boston residents and 5,500 businesses.

THE MONSTER'S CAGE: The primary features of the dam at The Black Lagoon include a rusty steel-and-plank walkway, a main spillway constructed of concrete several feet thick, and a 4-foot-wide flood gate. /Christopher Cheney photo

INFILTRATED: Many trees, which compromise the structural integrity of packed-earth dams, have taken root in the earthen structure at The Black Lagoon. /Christopher Cheney photo

ISLAND OF THE BLACK LAGOON: The largest silt deposit in The Black Lagoon has a tree-dotted section about 30 feet wide. The silt is mixed with chemicals from more than a century of gas and oil spills at commercial properties along nearby Maple Street. /Christopher Cheney photo

WORN TO THE BONE: A washout on the back side of The Black Lagoon's dam is at least 5-feet-wide and 10-feet long. The washout runs along the concrete portion of the structure's western abutment, which is constructed with carved stone and mortar. /Christopher Cheney

EROSIVE EFFECT: Water is the most erosive natural force on Earth. When I first saw floodgate spillway at The Black Lagoon in April 1996, there were boards between these concrete frame posts that created a de facto settling and holding basin for oil, heavy contaminants such as lead and silt that came over the floodgate, primarily during storm events. One of the missing boards (photo below) is snagged in brush along the spillway about 6 feet from the damaged concrete frame posts. /Christopher Cheney photos

SNOWFLAKES AND SLUDGE: In April 1996, the floodgate's spillway floor was slathered with an orange, iron-rich slime, primarily from construction sites upstream disturbing iron-rich soil, which is not a serious environmental threat. With fresh flakes of snow lining a hole in the ice this week, a thick black slime from The Black Lagoon is mixed into a coating that appears more noxious now. /Christopher Cheney photo

STAINED ICE: A 15-foot-wide patch of ice covers part of the main spillway at The Black Lagoon's dam. The main spillway and the floodgate spillway feed the final half-mile run of the Sudbury Reservoir tributary. Upstream, the creek flows past several Maple Street commercial properties that have undergone toxic waste cleanups. /Christopher Cheney photo

The rounded crown of The Black Lagoon dam's main spillway is chipped and cracked. In addition, the dam's footbridge steel is heavily corroded at some junction points of the structure. /Christopher Cheney photo

WEIGHTY PRESSURE: Silt in The Black Lagoon rests uneasily, particularly during storm events, when new silt flows into the man-made pond and old silt is eroded and flows over the dam toward the Sudbury Reservoir. The channel of open water in The Black Lagoon is bigger than an Olympic Hockey rink, stretching from the dam to the man-made pond's inlet. /Christopher Cheney photo