WATER QUALITY

Sedimentation and the Charles River

In the Lower Basin of the Charles River, below the Watertown Dam, sediment and attached chemicals have the potential to cause serious, persistent environmental and public health problems. During the past century, sediment that accumulated in the Lower Basin has contained many inorganic elements (such as chromium, lead, copper, and mercury) as well as organic compounds (such as DDT and fossil-fuel combustion products like polyaromatic hydrocarbons). While some of the inorganic elements could have resulted from the natural breakdown of rocks, the presence of organic compounds can only be from human activities.

Suspended sediments are particles of soil and vegetative matter that are transported by a river. If the river is slow moving or blocked by a dam, like the Lower Basin, suspended sediment carried from upper portions of the river can settle to the river bottom in a process called sedimentation. In very slow-moving waters like the Lower Basin, the bottom sediment is rarely subject to river scour or erosion so it remains trapped on the river bottom.

Sewer and industrial wastewater discharges, accidental spills, and urban runoff are sources of pollutants which still affect the health of the river today. For example, DDT is a pesticide that was banned in the early 1970's and classified by the USEPA as a probable carcinogen. Because DDT adheres well to fat cells, it accumulates up the food chain.  Its presence in river sediment demonstrates the ability of some compounds to resist breakdown in the environment and to be a persistent problem long after their use has stopped.

The potential harmful effects that these pollutants could have on human and animal life are extensive. Inorganic elements are essential to life in very small doses and will therefore only be toxic at high concentrations. A high, and thus toxic, level of inorganic elements was found at less than 25% of sample sites in the Lower Basin (USGS, 2000). Organic compounds are much more foreign to humans and animal organisms; so only a small dosage is needed for these compounds to be harmful. Many sites proved to have toxic levels of organic compounds particularly public use areas such as Daly Field and Herter Park.

Most sediment in the Lower Basin contains a high accumulation of inorganic elements and organic compounds, both of which pose a serious threat to plants and animals living in the river. Despite the fact that some of these compounds have been banned and that extensive effort has been expended into cleaning up the water column in the Charles, the continued presence of these pollutants in the sediments will most likely affect the health of the Charles River for many years to come.

This information is based on the U.S. Geological Society (USGS) Water Resources Investigations Report 00-4180  entitled Distribution and Potential for Adverse Biological Effects of Inorganic Elements and Organic Compounds in Bottom Sediment, Lower Charles River, Massachusetts, released in 2000.