An Iterative Approach for Identifying the Causes of Reduced Benthic Macroinvertebrate Diversity in the Willimantic River, Connecticut (Final)

This case study demonstrates that a screening assessment can help to focus sampling for unknown episodic sources of toxic discharges. The removal of the discharge led to the removal of this segment of the Willimantic River from the 303(d) list of impaired waters.

Cover of the Willimantic River Case Study Report The U.S. EPA’s Stressor Identification (SI) guidance (U.S. EPA 2000) was used to identify the cause of a biological impairment in the Middle River and Willimantic River in Stafford Springs, Connecticut. This case study used an iterative approach to identify stressors in the Willimantic River. Impairments were characterized by reduced diversity of benthic invertebrates based on the Connecticut Department of Environmental Protection’s (CT DEP) evaluation system. The case study was initiated as part of total maximum daily load (TMDL) development for a segment of the river below a permitted point source discharge. The SI process was applied as part of that effort with the first iteration conducted in early 2000, using data collected in 1999. We found that there was no convincing evidence for any of the candidate causes. However, the nature and distribution of effects pointed toward an unknown local cause in one section of a tributary of the river. In preparation for a second iteration of causal analysis (2001), samples were collected in the reach that was believed to contain a source. A previously unknown intermittent effluent was identified and corrected. After the presumed source of impairment had been corrected, the diversity of benthic macroinvertebrates increased over a 2 year period. A second iteration of causal analysis in the spring of 2003 confirmed that the illicit effluent had been the cause of impairment. Consequently, this case study shows how SI can be used within a TMDL program to identify localized causes of impairment.

This case study provides an opportunity to illustrate several effective strategies:
  1. Preliminary analysis leading to the collection of additional information (Iteration).
  2. Biological screening methods to bracket an impairment and its sources.
  3. Comparing and evaluating multiple potential causes.
  4. Confirmation of the probable cause by monitoring after manipulating exposure.
  5. Displaying the evidence by annotating causal pathways on conceptual models.
  6. Recognition of different styles for providing information, which are presented for comparison.
  7. Recognition of rates of change following a manipulation or management action.


This is the final report.


U.S. EPA. An Iterative Approach for Identifying the Causes of Reduced Benthic Macroinvertebrate Diversity in the Willimantic River, Connecticut (Final). U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-08/144, 2010.


Sep 2007An external peer review was conducted by an independent contractor.
Jun 2010EPA released the final report.

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