What is a TMDL?

A TMDL specifies the maximum amount of a pollutant a waterbody can receive and still meet water quality standards. Think of a TMDL as a pollution budget.

Virginia Strives for all streams to attain appropriate beneficial uses including:

  • drinking water use
  • primary contact/swimming use
  • fishing use
  • shell fishing use
  • aquatic life use

When waters become impaired and beneficial uses are not met the state must take steps to restore water quality. This is where the TMDL comes in…

TMDLs serve as a watershed planning guide by creating a report of pollution sources and needed reductions. They become a one-stop-shop for all watershed data. TMDLs include point sources and non-point sources of pollution. Point sources of pollution include residential municipal, or industrial discharges. Non-point sources of pollution include residential, urban, or agricultural runoff.

TMDL Improvement Process


  • Gather data
  • Reassess waterbody
  • Hold a stakeholder meeting
  • Produce draft TMDL report
  • Hold a stakeholder meeting
  • 30-day comment period
  • Final TMDL
  • EPA approval

For more TMDL information please go here.


 What Indicates “Good” Water Quality?

Dissolved oxygen – Dissolved oxygen tells scientists how much oxygen is available in the water for fish and other aquatic organisms to breathe. Healthy waters typically have high levels of DO (some areas, like swamps, naturally have low levels of DO). Aquatic organisms need oxygen to survive just as humans do. Several factors can affect how much DO is in the water. These include temperature, the amount and speed of flowing water, the plants and algae that produce oxygen during the day and take it back in at night, pollution in the water, and the composition of the stream bottom. (Gravelly or rocky bottoms stir up the water more than muddy ones do, creating bubbles that put more oxygen into the water.)

Biological sampling – Scientists determine the health of waters by taking samples of fish, plants and smaller organisms called macroinvertebrates. Macroinvertebrates include things like snails, worms, fly larvae, and crayfish. You find them under rocks and tree roots in the water. Macroinvertebrates are indicators of water quality. Some macroinvertebrates thrive in water that’s dirty, so if scientists find a lot of those in a sample, they know there’s a problem. Other organisms can survive only in water that’s very clean, so finding those means the water is probably healthy

Temperature – Scientists measure water temperature for several reasons. First, it determines the kinds of animals that can survive in a stream. If the temperature gets too hot or too cold for some organisms, they die. Temperature also can affect the chemistry of the water. For example, warm water holds less oxygen than cold water. A healthy cluster of trees and vegetation next to a stream or river helps keep temperatures cool for trout and other fish.

Nutrients – The two major nutrients of concern to water quality are nitrogen and phosphorus. An excess of nutrients can harm aquatic life by allowing increased algal growth. Nutrients can also affect pH, water clarity and temperature, and cause water to smell and look bad.

Bacteria – Fecal coliforms are found in the intestinal tract of humans and animals. While by themselves they are not inherently harmful, they are usually associated with viruses and pathogens which can cause illness. The major sources of fecal coliforms are faulty septic systems, discharges from wastewater treatment plants, and animal waste.

Turbidity – Turbidity is a measure of water clarity; the number of particulates suspended in water. Low turbidity is when you can see the bottom of a stream. A body of water experiencing high turbidity may appear murky.

pH – The concentration of hydrogen in water is referred to as pH (The p stands for “potential of” and the H is hydrogen.) pH ranges from 0 (very acidic) to 14 (very basic), with 7 being neutral. Most waters range from 6.5 to 8.5. Changes in pH can affect how chemicals dissolve in the water and whether organisms are affected by them. High acidity can be deadly to fish and other aquatic organisms.

Source: http://water.epa.gov/learn/resources/measure.cfm