CADDIS Volume 3: Examples & Applications
Causal Assessment of Biological Impairment in the Bogue Homo River, Mississippi Using the U.S. EPA’s Stressor Identification Methodology (Final)
This assessment of biological impairment in the Bogue Homo River, Mississippi is taken from more than 700 court ordered assessments of the causes of impairments requiring development of a total maximum daily load (TMDL). Therefore, it is a good example of several strategic techniques for expediting causal analyses and TMDLs. A TMDL is the calculation of the maximum amount of a pollutant that a body of water can receive and still meet water quality standards, and an allocation of that amount to the pollutant's sources. The calculation of a TMDL is required for all waters that are listed as impaired, in accordance with §303(d) of the Clean Water Act.
The U.S. Environmental Protection Agency (U.S. EPA)’s Stressor Identification (SI) procedures were used to identify causes of biological impairment in the Bogue Homo River in southeast Mississippi prior to the development of a TMDL. The Bogue Homo was initially listed as impaired based upon an evaluation of geographic information with no field measurements. Follow up biological monitoring of benthic macroinvertebrates by the Mississippi Department of Environmental Quality (MDEQ), confirmed that the stream was impaired. Candidate causes included decreased dissolved oxygen (DO) and altered food resources (organic matter), unsuitable habitat, increased temperature, increased ionic strength, and/or increased toxicity.
A standard list of candidate causes and measurements routinely used by MDEQ was developed by the State to expedite the process. Field data collected during the 303d listing verification visits were used to develop tables for the bioregion (similar biological communities) and site classes (similar physical and chemical characteristics). Once made, these tables were used in this case and all subsequent cases again saving time while strengthening the assessments.
Data for these cases were limited. And yet, for the Bogue Homo, MDEQ developed evidence to show that some causes co-occurred with the biological impairment, were a part of a larger causal chain of events, occurred at sufficient levels known to cause the observed effects, and were coherent with general ecological and scientific theory. Although available evidence was not equivalent in amount or quality for all candidate causes, it was enough to identify some probable causes and to suggest what additional, targeted data might improve the confidence in the determination.
|Sep 2007||An external peer review was conducted by an independent contractor.|
|June 2010||EPA released the final report.|
- (81 pp, 4 MB, about PDF)