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              Section 7
Methodology Development for the Cahokia Canal Watershed
FINAL 7-7
from a slightly less urbanized drainage area of 37 square miles. The Harding Ditch
watershed encompasses 33 square miles up to sampling site JMAC02 and the Canteen
Creek watershed drains 27 square miles at sampling site JNA01.
Data were downloaded through the USGS for the Indian Creek gage and minor
corrections were made to account for point source flow influence. First, daily average
flows (DAF) from the Bunker Hill STP (located upstream of the Indian Creek
streamgage) were subtracted from the Indian Creek data to account for natural flows
resulting from precipitation and overland contributions. These values were then
multiplied by the area ratio discussed above to estimate natural flows for each
watershed. The Canteen Creek flows were further adjusted to account for the
Collinsville STP which contributes 4.4 mgd to Canteen Creek upstream of the
sampling location. Likewise, the Harding Ditch flows were adjusted to account for the
Caseyville Township West STP which contributes 0.786 mgd to the watershed
upstream of the sampling location.
7.2.2.2 Total Manganese Analysis for Canteen Creek Segment JNA01
A flow duration curve for segment JNA01 of Canteen Creek was generated by ranking
the estimated daily flow data generated through the area ratio method discussed above,
determining the percent of days these flows were exceeded, and then graphically
plotting the results. The flows in the duration curve were then multiplied by the water
quality standard of 1,000 μg/L for total manganese to generate a load duration curve.
Total manganese data collected from USEPA STORET and Illinois EPA databases
during Stage 1 of TMDL development were paired with the corresponding flow for the
sampling date and plotted against the load duration curve. Figure 7-4 shows the load
duration curve as a solid line and the observed pollutant load as points on the graph.
Appendix G contains the spreadsheet used for this analysis.
The load duration curve shows that only two exceedences of the standard have
occurred since 1990. One occurred under high-flow conditions and one occurred
during low-flow conditions. Under average conditions, the standard is not being
exceeded.
7.2.2.3 Fecal Coliform Analysis for Harding Ditch Segment JMAC02
A flow duration curve for segment JMAC02 of Harding Ditch was also generated by
ranking the estimated daily flow data generated through the area ratio method
discussed above, determining the percent of days these flows were exceeded, and then
graphically plotting the results. Because the fecal coliform standard is seasonal and is
only applicable between the months of May and October, only flows during this time
period were used in the analysis. The flows in the duration curve were then multiplied
by the geometric mean water quality standard of 200 cfu/100mL to generate a load
duration curve. Fecal coliform data collected between May and October were compiled
from USEPA STORET and Illinois EPA databases during Stage 1 of TMDL
development and were paired with the corresponding flow for the sampling date and
plotted against the load duration curve. Figure 7-5 shows the load duration curve as a