Total Maximum Daily Load(TMDL)

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Total Maximum Daily Load

The TMDL was written by the Ohio EPA.  The goal of the TMDL is full attainment of the Aquatic Life and Recreational Uses.  According to the biological and chemical stream surveys conducted by the Ohio EPA, the primary causes of impairment are nutrient enrichment, siltation, organic enrichment, pathogens, low dissolved oxygen, and habitat alterations.

The TMDL report was on public notice in the summer of 2005 and approved by the director of the United State Environmental Proctection Agency(USEPA) in March 2006.  The TMDL  is available on Ohio EPA's website.

What exactly is the TMDL?

TMDL is the amount of a particular pollutant that a particular watercourse can 'handle' without violating state water quality standards.  In healthy lake and streams, nutrients are needed for the growth of algae which forms the base for the complex food web which supports the entire aquatic ecosystem.  

The TMDL was written to address three pollutant concerns: nitrogen, phosphorous and sediment.  Pollutants can come from either natural or man-made sources.  The natural source of nitrogen is from leaves and organic debris from riparian vegetation and wildlife.  Phosphorous comes from soil and rocks and sediment is from soil both from bank erosion and overland runoff.  

Potential sources of pollutants from man-made sources for nitrogen and phosphorous is from fertilizers and manure from golf courses, urban or agricultural areas.  Although bank erosion and overland runoff sediment erosion is natural, the amount can be varied based on user practices.

Watercourses are in dynamic equilibrium and have the ability to mitigate certain quantities of pollutants.  Dynamic equilibrium means that the amount of sediment delivered to the watercourse from its watershed is a long-term balance with the capacity of the watercourse to transport and discharge sediment.  The amount of nutrients put into the system also must be equal to the amount that the stream system can mitigate. 

Under certain conditions, including abundant nutrients, algae and aquatic plants will continue to grow and multiply well beyond the amount needed to support the food web.  The excess growth then dies and microorganisms break it down, consuming dissolved oxygen from the water in the process.  The fish and other organisms need Dissolved oxygen in order to breathe.  If this happens, aquatic organisms may die from lack of oxygen and a fish kill may result.

Need additional information?

For more information on the Darby Watershed, check out the  other watershed documents or contact  the Darby Watershed Coordinator or use the comments section.


Other Watershed Documents:

    1. 2001-2002 Biological and Water Quality Study  

    2. Scioto Basin Water Quality Standards Use Designations

    3. Darby Watershed Inventory

    4. Big Darby Creek Subwatershed plans


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Last updated: April 7, 2009.