In the Illinois Nutrient Loss Reduction Strategy (Illinois NLRS) Science Assessment, annual nitrate-nitrogen and total phosphorus loads from 1980 through 2011 water years were estimated in the eight major rivers that carry water and nutrients out of Illinois: Rock, Green, Illinois, Kaskaskia, Big Muddy, Little Wabash, Embarras, and Vermilion. In 2016, these load estimates were updated through 2015 using the same methods described in Illinois NLRS.
Total nitrate-nitrogen losses leaving Illinois from the eight major rivers in 2011–2015 were 10 percent less than losses during the 1980 to 1996 baseline period (Figure 3.1). This may be due to improved nitrogen balances in the agricultural areas of the state, particularly the tile-drained region of central and northern Illinois that are in the Illinois and Vermilion (Wabash) river watersheds. Fertilizer sales have had little change since 1980, and harvest removal of nitrogen in grain has greatly increased.
This has led to a large decrease in residual nitrogen, and likely has led to a decline in both tile and riverine nitrogen losses. The 10 percent reduction in nitrate load at near average flow conditions for the state may suggest that the load reduction was not primarily due to below average water flows during 2011 to 2015, but that some progress has been made in decreasing nitrate-nitrogen losses from Illinois due to improved management and conservation efforts.
Figure 3.1. Comparison of nitrate-nitrogen flux in eight major Illinois rivers from 1980–1996 to 2011–2015
Almost all the nitrate load reduction for the state occurred in the Illinois and Vermilion (Wabash) rivers, where the 2011–2015 average nitrate-nitrogen loads were 15 and 22 percent less, respectively, than the 1980–1996 loads, while the river water flows were only 4 and 16 percent less, respectively. In contrast to these promising trends, the 2011–2015 average nitrate-nitrogen load in the Rock River was 72 percent greater than during the baseline period, while river discharge was only 8 percent greater.
The combined total phosphorus load leaving the state from the eight rivers in 2011–2015 was 17 percent greater than the baseline period (Figure 3.2)—the likely large drivers are increased point source contributions as well as increased river flows in the Kaskaskia and Little Wabash Rivers, where total phosphorus loads per acre tend to be higher than the rest of the state. The Metropolitan Water Reclamation District of Greater Chicago, the state’s largest point source, has only recently adopted water treatment processes to address total phosphorus.
Figure 3.2. Comparison of total phosphorus flux in 8 major Illinois rivers between 1980-1996 and 2011-2015
The average total phosphorus loads in 2011–2015 in the Illinois, Kaskaskia, Big Muddy, and Little Wabash rivers ranged from 16 to 58 percent greater than loads during the baseline period. All these rivers, except the Illinois, had greater discharge during this time compared to the baseline period. Total phosphorus loads in the Green, Vermilion (Wabash) and Embarras rivers were 17–56 percent lower. The average river flows for the Green and Vermilion (Wabash) rivers from 2011 to 2015 were 12 and 16 percent less, respectively, with no difference for the Embarras River. There was little difference in the total phosphorus loads in the Rock River between the baseline period and 2011–2015, even though river flows were 8 percent greater during this time.
Five-year averages of nutrient loads seem appropriate for evaluating the loss of these nutrients through time. Future analyses should include evaluating five-year average loads in conjunction with five-year average river flows.
The method used to estimate total phosphorus loads is most uncertain at the beginning and end of the data record. Consequently, future estimates of the 2011–2015 average total phosphorus loads may differ from these estimates when data from 2016 and beyond are included in the analysis.
