What is a TMDL (Total Maximum Daily Loading)?

Under section 303(d) of the 1972 Clean Water Act, states, territories, and authorized tribes are required to develop lists of impaired waters that do not meet water quality standards. In Colorado, this list is called the 303(d) List. Law requires that states, territories, and tribes establish priority rankings for waters on the lists and develop total maximum daily loads for these waters. Barr Lake and Milton Reservoir were included on Colorado’s 2004 303(d) List with a “medium” priority for elevated pH and then later in 2012 for dissolved oxygen (DO) and ammonia.

Once on the 303(d) list, a TMDL needs to be developed. A TMDL is the calculation or estimate on the amount of pollutant that the body of water can handle without violating the standards. This amount is usually expressed in pounds per year for lake (annual load) but then converted to a daily value. This total amount is divided up among all the point sources and non-point sources. Each source gets a piece of the pie. These slices of the pie are called allocations – wasteload allocations are for point sources like wastewater treatment plants and load allocations are for non-point sources like stormwater and internal loading.

pH, DO, and ammonia are all related to the eutrophication process and phosphorus is the driving factor for Barr and Milton. So the pH and DO TMDL for Barr and Milton calculates the annual load of phosphorus that can go into each reservoir so that pH (between 6.5 and 9.0, 85% of the time) and DO (above 5.0 mg/L) standards are achieved.

In mathematical terms:


where LA is the nonpoint source load allocation, WLA is the point source waste load allocation, and MOS is a margin of safety.

Barr Lake Annual P load and TMDL Allocations

Milton Reservoir Annual P load and TMDL Allocations

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