Illinois EPA and Indiana DEM Kankakee/Iroquois Watershed TMDL Report
Final 87
6.0 LINKAGE ANALYSIS
An essential component of developing a TMDL is establishing a relationship between the source loadings
and the resulting water quality. Water quality data within the Kankakee/Iroquois watershed are discussed
in Section 3.2 and potential point and nonpoint sources are inventoried in Section 4.0. The purpose of this
section of the report is to evaluate which of the various potential sources is most likely to be contributing
to the observed water quality impairments.
Establishing a linkage analysis for bacteria is challenging because there are so many potential sources and
because bacteria counts have a high degree of variability. While it is difficult to perform a site-specific
assessment of the causes of high bacteria for each location in the Kankakee/Iroquois watershed, it is
reasonable to expect that general patterns and trends can be used to provide some perspective on the most
significant sources.
Table 59 summarizes several of the potential bacteria sources in each subwatershed along with the E. coli
data collected by IDEM in 2008. E. coli counts are highest in the Yellow River, Upper Iroquois, and
Upper Kankakee subwatersheds which are all characterized by relatively high animal unit densities. It is
therefore possible that waste generated by livestock in these subwatersheds is contributing to the elevated
bacteria counts. In fact, the animal unit density of each subwatershed is strongly correlated with the
geomean of E. coli counts in each subwatershed (Figure 25). Similar trends are not apparent with the
other sources listed in Table 59. However, it is also possible that some other factor could explain the
higher counts. For example, the Yellow River, Upper Iroquois, and Upper Kankakee are also headwater
subwatersheds and many of the sampled tributaries therefore have a relatively small drainage area.
Streams with smaller drainage areas generally have relatively higher E. coli counts because there is less
opportunity for dilution compared to larger streams. Bacteria patterns associated with drainage areas, as
well as flow conditions, are further discussed in the next several sections.
Table 59. Potential sources of pathogens in the Kankakee/Iroquois Watershed.
Upper
Kankakee
Middle
Kankakee
Yellow
River
Upper
Iroquois
Lower
Iroquois
Lower
Kankakee
Mean of All 2008 E. coli data
(#/100mL) 760 435 1014 767 473 514
Geomean of All 2008 E. coli data
(#/100 mL) 308 165 545 375 156 139
Source Type or Concern
Total Average Design Flow of
Wastewater Treatment Plants
(MGD)
10.8 10.4 8.6 2.6 3.1 31
Total Number of Combined
Sewer Overflows 0 1 23 9 16 2
Point
Square Miles of MS4 Storm
Water 18.3 31.9 7.0 0 0.2 184.2
Rural Population Density
(persons/square mile) 214 315 141 29 22 310
Animal Unit Density
(units/square mile) 146 65 329 185 53 37
Non-
Point
Deer Density (animals/square
mile) 3 4 5 2 3 6
