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Overview Turbidity is the presence of suspended materials (silt, clay, organic and inorganic matter) in the water column that reduce light transmission through scattering or absorption, causing water to turn brown and muddy after heavy rain falls. It is one of the most pervasive sources of non-point source (NPS) pollution throughout the country, and this is especially true in the Catskill Mountains where the glacial geology, steep mountain slopes and high energy runoff are but some of the factors that contribute to erosion and entrainment of sediments in the water system.
In the Catskills, this problem is most pronounced in the Schoharie Reservoir system, a basin that is characterized by a small reservoir which is supplied by a rather large watershed – 314 square miles. The watershed is for the most part steep, and deep lacusterine clays underlie much of the watershed surface. Whether it is deep clays laid down in former glacial lakes, or the more compacted lodgement till characteristic of glacial action on the valley walls, even the most minor disturbances result in significant impacts to water clarity.
In recent years, the significance of turbidity from the Schoharie basin has become better known as the result of legal action taken by a coalition of watershed groups which resulted in a federal court ruling that the Shandaken Tunnel, an aqueduct built in 1928, requires a State Pollution Discharge Elimination System permit under the Clean Water Act. While there has been much debate on the solution to this problem, there is consensus amongst most parties that a holistic, or watershed-based approach must be taken to reduce turbidity at the source.
Problems for NYC When NYC expanded its water supply source up into the Catskills in the early 1990’s, turbidity was a known problem at that time. Local stream names, such as Muddy Creek and Red Falls, are indicative of the prominence of turbidity in the watershed.
In order to control turbidity, NYC is compelled to a series of regulatory, monitoring and operational actions. These actions include in-reservoir applications of turbidity reducing agents, typically Alum, which is an aluminum-based coagulant. Alum treatment of the Catskill supply is generally a rare occurrence, in that between 1985 and 2005, treatment was necessary only four times. Significant storm events in 2005 and 2006, however, required alum treatment four times in the two year period, and the amount of turbidity in the Schoharie Reservoir has increased as a consequence of the major storms in the past ten years, notably 1996, 1999 and 2005.
The limit for turbidity in an unfiltered surface water source, such as New York City’s, is set forth in the federal Safe Water Treatment Rule and the NYS Sanitary Code, which states turbidity cannot exceed 5 NTU (the standard used to measure turbidity). The frequency and magnitude of storms, such as those in January 1996 and April 2005, resulted in the largest increases in turbidity in the Catskill System Reservoirs where annual turbidities were 2.5 to 15 times higher than historic annual medians.
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Erosion & sedimentation related to development can be a significant source of turbidity. |
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Unstable drainage systems can also contribute sediment to the Schoharie increasing turbidity. |
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The transfer of turbid water from Schoharie Reservoir to the Esopus Creek via the Shandaken Tunnel is an important resource management issue. |
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Watershed Assistance Program
PO Box 996, Main Street Tannersville, NY 12485 Phone 518.589.6871 Fax 518.589.6874 |
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Michelle Yost WAP Coordinator |