303(d) List Prioritization Changes
Using the 1998 Illinois Section 303(d) list, the West Branch of the DuPage River was initially
listed as impaired for the following parameters: phosphorus, nitrogen, nitrate, salinity/total
dissolved solids (TDS)/chlorides, total suspended solids (TSS), ammonia, pathogens,
siltation, flow alterations and other habitat alterations.
In Developing the 2002 Illinois Section 303(d) List, the Illinois EPA has revised its method of
prioritization that accounts for severity of pollution and the uses to be made of such waters.
This prioritization was done on a watershed basis. A brief explanation of the process will be
given here. For a detailed explanation, please refer to the Illinois 2002 Section 303(d) list,
available at http://www.epa.state.il.us/water/watershed/reports/303d-report/
The prioritization process involves three steps:
• Step 1: The first step in the prioritization process is based on use designations,
establishing a High, Medium, and Low Priority for specific uses.
− High Priority – Watersheds containing one or more water bodies in which potential
causes of impairments pose a threat to a drinking water use.
− Medium Priority – Watersheds containing one or more water bodies in which
potential causes of impairments pose a threat to aquatic life use, fishing use, or
− Low Priority – Watersheds containing one or more water bodies in which potential
causes of impairment pose a threat to secondary contact recreation.
• Step 2: The second step in the prioritization process is based on the degree of confidence
assigned to potential causes in the water bodies within a watershed. A Confidence Level
of “3” indicates Illinois EPA has relatively high confidence that the identified potential
cause is contributing to impairment of the water body. A Confidence Level of “2”
indicates moderate confidence, and a confidence level of “1” indicates low confidence.
The watersheds in Step 1 were prioritized by taking into consideration watersheds
which contain one or more water bodies that have potential causes with a Confidence
Level of “3”. The numbers of Confidence Level 3’s were summed using the major cause
categories (i.e., 900—nutrients, 1200—organic enrichment) for each watershed.
Watersheds with a greater number of Confidence Level 3’s were identified as a higher
priority for TMDL development within each of the priority groups identified in Step 1.
Step 2 was used only when a Confidence Level 3 was assigned to potential causes of
impairment in a water body within a watershed.
• Step 3: Those watersheds with potential causes of impairment without an assigned
Confidence Level 3 were prioritized based on the overall severity of pollution. For the
purposes of this process, “severity of pollution” is determined by summing the major
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