Methods Used to Facilitate Critical Watershed Identification
Figure 3.11. HUC8 sub-basins in Illinois.
To support the determination of critical watersheds in the state, we evaluated nutrient yields at the HUC8 level (Figure 3.11). There are 50 HUC8s in Illinois that drain into the Mississippi River and one that drains into Lake Michigan. They range in size from 17 to 2,436 sq. mi, with an average size of about 1,100 sq. mi. The HUC8s with a small area in Illinois are actually larger, but they straddle two states. For each HUC8, we looked for available Illinois EPA nutrient data combined with USGS stream flow gauges. In some of the HUC8s, flow and nutrient concentration data were available for a river that drained a large part of the HUC8. In others, the gauged drainage area was much smaller than the overall HUC8. In a few, more than one river gauge was used, and in others, the estimate was made by calculating the difference between upstream and downstream monitoring sites. For the HUC8s without any available data, averages were taken from the surrounding HUC8s with estimated nutrient yields. For the HUC8s with both USGS stream flow gauges and Illinois EPA nitrate-nitrogen and total phosphorus data for 1997-2011, we used these data to estimate annual average nutrient yields in pounds of nitrogen or phosphorus per acre per year. As we did for the large rivers in the state, linear interpolation was used for the nitrate-nitrogen estimates and WRDTS for total phosphorus. For nearly every site, little or no concentration data were available for 2007-2008, and those years were not included when concentration data were not available. This monitoring data allowed us to directly calculate overall nitrate-nitrogen and total phosphorus yields for 39 of the 50 HUC8s, including seven that were calculated using the difference between upstream and downstream sites. We also disaggregated point source nitrate-nitrogen and total phosphorus estimates by HUC8 and subtracted the point source nitrate-nitrogen or total phosphorus yield from the total nutrient yields to obtain estimates of both point and non-point source nutrient yields for each HUC. This worked well for all except three in northeastern Illinois, where the point source yields were quite high and not reflected in the available river data. In those HUC8s, we set the total nutrient yield equal to the point source yield and assumed non-point sources were zero. Data were not available to allow us to determine the urban non-point source contribution in the three northeastern HUC8s, as the point source loads were so much greater than the stream loads estimated from water quality data. In addition, urban density values were not easily available by HUC8.
Nitrate-Nitrogen and Total Phosphorus Yields by HUC8
Figures 3.12-3.15 show total, point source, and non-point source nitrate-nitrogen and total phosphorus yields by HUC8. For nitrate-nitrogen, the greatest yields were found in the tile-drained northern two-thirds of Illinois, where tile drainage is common and land-use is dominantly row crop agriculture. Statewide nitrate-nitrogen yields ranged from 0.7 to 42 lb/acre/yr, with an average of 13 lb/acre/yr. The non-point source yield average was 10 lb/acre/yr, with 27 HUC8s greater than 10 lb/acre/yr. For point sources, the Chicago and Des Plaines HUC8s had very large nitrate-nitrogen yields of 40.9 and 38.3 lb/acre/yr, respectively.
Figure 3.12. Total nitrate-nitrogen yields by HUC8 in Illinois.
Figure 3.13. Point and non-point source nitrate-nitrogen yields by HUC8 in Illinois.
For total phosphorus, the average yield was 1.4 lb/acre/yr and ranged from 0.42 to 9.74 lb/acre/yr across the HUC8s in the state (Figure 3.14). Non-point source total phosphorus yields were typically greater in the southern Illinois HUC8s, with the smallest yields in northern Illinois (Figure 3.15). Point source total phosphorus yields were very large in the Chicago area, the Upper Sangamon HUC8 (due to the sewage treatment plant discharge in Decatur), and along the Mississippi River in some HUC8s. The Chicago and Des Plaines HUC8 point source phosphorus yields were 9.74 and 6.65 lb/acre/yr, respectively.
Figure 3.14. Total phosphorus yields by HUC8 in Illinois.
Figure 3.15. Point and non-point source total phosphorus yields by HUC8 in Illinois.
303(d)/305(b) Impaired Waters in 2012
We conducted an analysis to determine if yields of nitrate-nitrogen or total phosphorus were related to the miles of impaired streams or acres of lakes reported in the 2012 integrated report. We included assessed 2012 303(d) listed streams as well as 305(b) impaired streams and lakes by HUC8 if they were listed for dissolved oxygen, total phosphorus, nitrate-nitrogen, aquatic plants, or aquatic algae (Figure 3.16). There were 4,070 stream miles on the 303(d) list, 4,346 stream miles on the 305(b), and 127,270 lake acres on the 305(b) assessment. We located them within each of the HUC8s and summed the stream miles or acres of lakes by HUC8. Finally, a correlation analysis was conducted to determine if nitrate-nitrogen or total phosphorus yields were related to the miles of impaired streams or acres of lakes. There were no strong relationships. HUC8 non-point source nitrate-nitrogen yields were significantly related to 305(b) stream miles (p=0.002) and 305(b) lake acres (p=0.011). However, this relationship was negative, suggesting that the greater the nitrate-nitrogen yield from non-point sources the fewer impaired stream miles or lake acres. Total phosphorus yield by HUC8 was not statistically correlated with impaired stream miles or lake acres.
Appendix A: Watershed Prioritization Lists
Prioritization Glossary
Name: Generally accepted name for each of the 33 eight-digit Hydrological Unit Code watersheds in Illinois.
HUC8: U.S. Geological Survey defined Hydrologic Unit Code for each of the 33 watersheds.
M lbs/yr: Million pounds of nutrient (phosphorus or nitrates) lost from specific watershed per year by both point and non-point sources.
Non-point source M lbs/yr: Million pounds of nutrient (phosphorus or nitrates) lost from specific watershed per year by non-point sources. This column is only found on non-point source prioritization tables.
Point source M lbs/yr: Million pounds of nutrient (phosphorus or nitrates) lost from specific watershed per year by point sources. This column is only found on point source prioritization tables.
Load rank: Points given to each watershed based on the non-point or point source load by nutrient contributions.
WQ: Water quality ranking based on the number of water bodies identified as not meeting designated uses based on the potential for nutrient impacts.
% meeting: Percent of waters meeting designated uses in the watershed.
*# of WBPs: Number of known watershed-based plans in the watersheds, including active U.S. Department of Agriculture Natural Resource Conservation Service watershed groups.
Total: Total of all points for each of the 33 watersheds. Those with the highest points are the highest priority for each nutrient, non-point and point source.
For additional information on the prioritization process, see chapter 4.
