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Watersheds, TMDLs and Water Quality Standards

What are watersheds and why are they important?

A watershed is known as the area of land where all of the water that is running under it, drains off or flows into the same end point. All watersheds vary in shape and size and can flow through several county, state, and even national boundaries. In the United States, there are over 2,267 total watersheds (That includes Hawaii, Alaska, and Puerto Rico).

What Exactly is a TMDL?

Watershed and TMDL (Total Daily Maximum Load)Total Maximum Daily Load (TMDL) is the sum of the individual wasteload allocations (WLAs) for point sources and load allocations (LAs) for nonpoint sources and natural background. If receiving water has only one (1) point source discharger, the TMDL is the sum of that point source WLA plus the LAs for any nonpoint sources of pollution and natural background sources, tributaries, or adjacent segments. TMDLs can be expressed in terms of either mass per time, toxicity, or other measures.

Since 1995, over 34,000 approved TMDLs have been developed by the EPA, states and/or other governing bodies. Both the EPA and non-delegated states anticipate an additional 70,000 new TMDLs within the next eight (8) to thirteen (13) years. The undiscounted cost to develop 36, 225 TMDL’s is approximately equal to $1 billion. The anticipated 70,000 TMDLs are estimated to cost in the range of $2.04 billion.

The formula to calculate TMDL is as follows: TMDL = WLA + LA + MOS

WLA (Wasteload Allocation) - is the portion of TMDL which takes existing or future point sources into consideration (ex: sediment, concentrated flow channels).

“The undiscounted cost to develop 36,225 TMDL’s is approximately equal to $1 billion. The anticipated 70,000 TMDL’s are estimated to cost in the range of $2.04 billion”

LA (Load Allocation) - is the portion of TMDL which takes nonpoint sources and natural background into consideration when calculating (rainwater flowing through vegetation while transporting nutrients).

MOS (Margin of Safety) - is taking into account the uncertainty of relationships between pollutant loads and receiving water quality when calculating a Total Maximum Daily Load (TMDL).

Is “Turbidity” Directly Related to Water Quality Standards (WQS)?

Turbidity is the cloudiness or haziness of a fluid caused by individual particles (total suspended solids) which are mostly invisible to the naked eye – The cloudier the water, the worse the water quality.

How Bad Can Turbid Waters Really Be?

Turbid Waters from StormwaterSampling and testing for turbidity levels is a key component to discovering water quality. Turbidity’s unit of measurement (NTU) measures the “cloudiness” of water through the penetration of sunlight through water. Increased levels of turbidity are often caused by sediment and other suspended solids in the water sample.

As of today, scientific research has found that an excess of turbidity will reduce growth of aquatic plants and interfere with the ability for aquatic species to sustain life. Increased turbidity levels can also hinder recreational uses for water. This hindrance can cripple shoreline communities whose economies are largely dependent on tourism and water recreation. Finally, high levels of suspended sediment, which is typically measured as turbidity for regulatory purposes, can cause public drinking water treatment systems to increase operational costs.

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