1999 Aquatic Life Ambient Water Quality Criteria for Ammonia Update
Fact Sheet; December 1999 Update - Technical Version
The U.S. Environmental Protection Agency (EPA) is publishing a 1999 Update of Ambient Water Quality Criteria for Ammonia (1999 Ammonia Update). The 1999 Ammonia Update contains EPA's most recent freshwater aquatic life criteria for ammonia, superseding all previous EPA recommended freshwater criteria for ammonia. The 1999 Ammonia Update pertains only to fresh waters. It does not change or supersede the EPA's aquatic life criterion for ammonia in salt water, published in Ambient Water Quality Criteria for Ammonia (Saltwater) in 1989.
Under the Clean Water Act, EPA is required to publish and periodically update ambient water quality criteria. These criteria reflect the latest scientific knowledge on the effects of water pollutants on public health and welfare, aquatic life, and recreation. These criteria guide states, territories, and authorized tribes in developing water quality standards and ultimately provide a basis for controlling discharges or releases of pollutants into our nation's waterways.
Ambient water quality criteria are based solely on data and scientific judgments on the relationship between pollutant concentrations and the effects on aquatic life, human health, and the environment. These criteria do not reflect consideration of economic impacts or the technological feasibility of reducing chemical concentrations in ambient water.
Prior to today's 1999 Ammonia Update, EPA published the 1984 Ammonia Criteria for the protection of aquatic life in freshwater. In 1992, EPA made minor revisions to the 1984 Ammonia Criteria. In 1998, EPA published significant revisions to the 1984 Ammonia Criteria as part of the 1998 Ammonia Update, and solicited public comment on those revisions made in the 1998 Ammonia Update.
1999 Ammonia Update
The 1999 Update of Water Quality Criteria for Ammonia incorporates comments received from the 1998 Update. The 1999 Update contains EPA's most recent freshwater aquatic life criteria for ammonia and supersedes all previous freshwater aquatic life ammonia criteria. The new criteria reflect recent research and data since 1984, and are a revision of several elements in the 1984 Criteria, including the pH and temperature relationship of the acute and chronic criteria and the averaging period of the chronic criterion. As a result of these revisions, the acute criterion for ammonia is now dependent on pH and fish species, and the chronic criterion is dependent on pH and temperature. At lower temperatures, the dependency of chronic criterion is also dependent on the presence or absence of early life stages of fish (ELS).
The effect of temperature and expected presence of early life stages of fish on the chronic criterion in the 1999 Update is demonstrated graphically in Figure 1. As shown in Figure 1, the temperature dependency in the 1999 Update results in a gradual increase in the criterion as temperature decreases, and a criterion that is more stringent, at temperatures below 15 C, when early life stages of fish (ELS) are expected to be present.
Comparison of the 1999 Update to the 1998 Update and 1984 Criteria
EPA's recommendations in the 1999 Update represent a change from both the 1984 chronic criterion, which was dependent mainly on pH, and from the 1998 Update, in which the chronic criterion was dependent on pH and the presence of early life stages of fish. Figure 2 is a comparison of the criterion values in the 1984 Criteria, and the 1998 and 1999 Updates, when early life stages of fish are not expected to be present (i.e., "ELS protection not needed"). Figure 2 illustrates the non-temperature dependency of ammonia toxicity in the 1984 Criterion, at temperatures below 20 C. The 1984 Criterion recommended that states and tribes develop site-specific criteria for ammonia at temperatures below 20 C. As seen in Figure 2, the temperature dependency of ammonia toxicity at temperatures below 20 C is incorporated directly into the criterion of the 1999 Update.
Averaging Period and Design Flow Recommendations
The other significant revision in the 1999 Update is EPA's recommendation of 30 days as the averaging period for the ammonia chronic criterion. EPA recommends the 30B3 (the lowest thirty-day average flow based on a 3-year return interval when flow records are analyzed using EPA's 1986 DFLOW procedure), the 30Q10 (the lowest thirty-day average flow based on a ten-year return interval when flow records are analyzed using extreme-value statistics), or the 30Q5 as the appropriate design flows associated with the 30-day averaging period of the ammonia chronic criterion. In addition, EPA recommends that within the 30-day averaging period, no 4-day average concentration should exceed 2.5 times the chronic criterion, or Criterion Continuous Concentration (CCC). Consequently, the design flow should also be protective of any 4-day average at 2.5 times the CCC. EPA believes that in the vast majority of cases, the 30Q10 is protective of both the CCC and any 4-day average at 2.5 times the CCC. However, if a State or Tribe specifies the use of the 30Q5, then the State or Tribe should demonstrate that a 7Q10 (the lowest average 7-day once-in-ten-year flow using extreme-value statistics) is protective of 2.5 times the CCC, to ensure that any short term (4-day) flow variability within the 30-day averaging period does not lead to shorter-term chronic toxicity.