Nitrate-nitrogen and total phosphorus loads from the major rivers draining Illinois were updated through the 2021 water year using U.S. Geological Survey, USGS, and Illinois Environmental Protection Agency, Illinois EPA, monitoring stations (Figure 3.1). Beginning with the 2023 biennial update to the Illinois Nutrient Loss Reduction Strategy, nutrient loads were estimated using data from the USGS continuous monitoring stations (U.S. Geological Survey, 2023) rather than the original Illinois EPA monitoring stations. To maintain consistency with previous biennial report updates, the loads from the new sites were rescaled to match the original drainage areas (Table 3.1). As before, the statewide nutrient load was estimated by summing the loads of the eight major rivers, adjusting for out-of-state contributions and unmonitored areas.
Prior to water year 2020, nutrient loads were estimated at Illinois EPA sampling sites (Table 3.1) using Weighted Regression on Time, Discharge, and Season, WRTDS, to estimate total phosphorus and linear interpolation to estimate nitrate-nitrogen (Hirsch, et al., 2015; IEPA, IDOA, and University of Illinois Extension, 2015 and 2021). To account for loads coming from out-of-state, the loads in the Illinois and Vermilion rivers were reduced by 16% and 7%, respectively. The loads in the Rock River at Rockton, near the Wisconsin border, were subtracted from the load at Joslin to isolate the Illinois portion of the Rock River. After these adjustments, the total monitored area represents 69.7% of the land area of Illinois. Therefore, to estimate the statewide load, the total load was multiplied by 1.435, which is the ratio of the total area to the monitored area. The resulting loads estimates for nitrate-nitrogen and total phosphorus are shown in the figures and tables that follow.
River
System | Gage Location | Illinois EPA ID | USGS ID | Drainage Area (square mile) | Drainage Area in Illinois (%) | % of Illinois Represented |
| Big Muddy | Murphysboro | N-12 | 05599490 | 2,159 | 100 | 3.8 |
| Embarras | Ste. Marie | BE-07 | 03345500 | 1,516 | 100 | 2.7 |
| Green | Geneseo | PB-04 | 05447500 | 1,003 | 100 | 1.8 |
| Illinois | Valley City | D-32 | 05586100 | 26,743 | 84 | 39.9 |
| Kaskaskia | Venedy Station | O-20 | 05594100 | 4,393 | 100 | 7.8 |
| Little Wabash | Carmi | C-23 | 03381500 | 3,102 | 100 | 5.5 |
| Rock | Joslin | P-04 | 05446500 | 9,549 | 43 | 7.3 |
| Rock | Rockton | P-15 | 05437500 | 6,363 | 13 | 1.4 |
| Vermilon | Danville | BP-01 | 03339000 | 1,290 | 93 | 2.1 |
Table 3.1. Monitoring stations used to estimate the statewide nitrate-nitrogen and total phosphorus loads prior to 2020
Beginning with water year 2020, methods were modified to use data from the USGS continuous monitoring sites located at or near the original sampling sites (Hodson, et al., 2022). Table 3.2 describes the new monitoring stations, including scaling factors representing the change in drainage area from the original sampling site. Loads from the continuous sites were multiplied by their respective scaling factor before applying the total-land-to-monitored-land adjustments.
Loads from the continuous monitoring sites were estimated using covariate-based Bayesian imputation (Hodson, et al. 2021), which fills gaps in the concentration data by using available surrogate data: other parameters that correlate with the parameter of interest, such as the monitored parameters (Table 3.2), streamflow, or season. Daily loads were estimated from the gap-filled concentration data by multiplying the daily loads by streamflow and by a unit conversion factor. Loads from the continuous monitoring sites are available in Hodson et al., 2022. For comparison, estimated annual nutrient loads from long-term Illinois EPA and national sampling sites, including loads to the Gulf of Mexico, are available in Hodson, 2023, and Lee, 2022, respectively.
River
System
| Gage Location
| USGS ID
| Scaling Factor
| Monitored Parameters as of 2022
|
| Big Muddy | Murphysboro | 05599490 | 1 | Nitrate and turbidity |
| Embarras | Lawrenceville | 03346500 | 0.65 | Nitrate and turbidity |
| Green | Geneseo | 05447500 | 1 | Nitrate and turbidity |
| Illinois | Valley City | 05586100 | 1 | Nitrate, turbidity, orthophosphate, chlorophyl, dissolved oxygen, temperature, conductivity, and pH |
| Kaskaskia | New Athens | 05594100 | 0.85 | Nitrate, turbidity, orthophosphate |
| Little Wabash | Carmi | 03381500 | 1 | Nitrate and turbidity |
| Rock | Joslin | 05446500 | 1 | Nitrate and turbidity |
| Vermilion | Danville | 03339000 | 1 | Nitrate and turbidity |
Table 3.2. Continuous monitoring stations used to estimate the statewide nitrate-nitrogen and total phosphorus loads after 2020
Unlike nitrate, for which there are reliable optical sensors to measure concentration in situ, total phosphorus was estimated from surrogates. Turbidity is often a good surrogate because a portion of phosphorus is bound to particulate matter. In agricultural watersheds, where the particulate-bound portion tends to be greater, turbidity tends to be a better surrogate for total phosphorus. However, where there is a substantial amount of dissolved phosphorus, such as downstream from urban areas, turbidity is a poorer surrogate. Past analysis by USGS assessed which monitoring sites would benefit most from additional data collection (Hodson, et al., 2021), resulting in the installation of orthophosphate analyzers on the Illinois and Kaskaskia rivers in 2022. This additional instrumentation should improve the accuracy of phosphorus loads estimates from those rivers.
Water Measures
The Illinois NLRS assesses progress toward reducing nutrient loss to Illinois waterways by comparing river loads of nitrate-nitrogen and total phosphorus from a five-year period against those of the 1980–96 baseline period. In general, Illinois nitrate-nitrogen and total phosphorus loads are highly correlated with water yield (Figures 3.2 and 3.3). From 2017–21, the nitrate-nitrogen load from Illinois was 416 million pounds per year, 4.8% greater than the baseline, and the total phosphorus load was 46 million pounds per year, 35% greater than the baseline (Table 3.3). Water yield during the same period was 23% greater than the baseline, in part due to high precipitation years in 2019 and 2020 and to higher precipitation years overall. The five-year average water yields have been greater than the baseline since 2008 (Figures 3.2 and 3.3), and that trend may continue into the foreseeable future. Greater runoff and drainage may have caused some of the increase in nutrient loads. However, if runoff and drainage were the only causes, then the changes in nutrient loads would have been more evenly distributed across the watersheds than is indicated by the data. Instead, certain watersheds (Tables 3.4 and 3.5) experienced much larger increases than others, and some have experienced decreased loads, indicating that other factors such as nutrient management, chang- es in population, hydrology, and “legacy” nutrients in the soil or streambed may be as important or more important than climate factors, such as long-term precipitation trends, in driving changes in loads (Figures 3.4–3.6). Distinguishing among these potential sources could be critical for assessing the effectiveness of NLRS-related efforts.
| 1980-96 Baseline | 2013-17
| 2014- 18
| 2015–19
| 2016–20
| 2017–21
|
Average Value
| Average Value
| % Change from Baseline
| Average Value
| % Change from Baseline
| Average Value
| % Change from Baseline
| Average Value
| % Change from Baseline
| Average Value
| % Change from Baseline
|
| Nitrate-N Load (million lb/yr) | 397 | 425 | +7% | 380 | -4.4 | 448 | +13% | 461 | +16.2% | 416 | +4.8% |
| Total Phosphorus Load (million lb/yr) | 34 | 42 | +23% | 41 | +20% | 46 | +35% | 48 | +42% | 46 | +35% |
| Water Yield (in/yr) | 13 | 14.7 | +13% | 14.1 | +9% | 16.3 | +25% | 16.8 | +30% | 15.9 | +23% |
Table 3.3. Statewide estimated water yield, nitrate-nitrogen load, and total phosphorus load for the 1980–96 baseline period and five recent five-year periods
Relative to the 1980–96 baseline, the largest increase in nitrate-nitrogen load occurred in the Illinois portion of the Rock River, between Rockton and Joslin, where load increased 117% and streamflow increased 62% (Figures 3.5 and 3.6). A related watershed study investigating the nitrate trend in the Rock River in more detail is summarized in the next section of this chapter.
River System
| Monitoring Locations
| 1980-96
| 2013-17
| 2014-18
| 2015-19
| 2016-20
| 2017-21
|
Pounds of nitrate-N per acre per year
|
| Big Muddy | Murphysboro | 1.5 | 1.3 | 1.2 | 1.5 | 1.8 | 2.0 |
| Embarras | Ste. Marie | 15.3 | 19.5 | 17.6 | 19.0 | 18.6 | 16.5 |
| Green | Geneseo | 12.2 | 14.0 | 13.1 | 16.5 | 17.1 | 15.2 |
| Illinois | Valley City | 13.9 | 13.6 | 12.1 | 14.3 | 14.5 | 13.2 |
| Kaskaskia | Venedy Station | 3.6 | 3.1 | 2.4 | 3.2 | 3.7 | 3.4 |
| Little Wabash | Carmi | 3.1 | 4.3 | 3.6 | 4.1 | 3.6 | 3.7 |
| Rock (Illinois portion) | Joslin-Rockton | 8.6 | 17.3 | 16.6 | 20.3 | 22.1 | 18.7 |
| Vermilion | Danville | 24.3 | 23.3 | 20.7 | 22.2 | 20.9 | 19.6 |
Table 3.4. Monitoring stations used to estimate and evaluate the statewide nitrate-N yields
Relative to the baseline period, the largest increase in total phosphorus load occurred in the Illinois River, where load increased 23%, and streamflow increased 17% (Figure 3.6). Compared to the Kaskaskia and Little Wabash rivers, the percentage increase of the Illinois River load was relatively small, but because the Illinois River is by far the largest watershed, it was the largest contributor to the statewide increase in phosphorus load (Figures 3.7 and 3.8). Results from a study investigating the sources of phosphorus in the Illinois River are summarized later in this chapter in the section Summary of Spatial and Temporal Variation in Phosphorus Loads in the Illinois River. The greatest percentage increases in phosphorus loads occurred in the Kaskaskia River, 102%, and the Little Wabash River, 86%, while streamflow from both rivers increased by 28% and 30%, respectively (Figure 3.6). The cause of the load increase has not yet been investigated.
River System
| Monitoring Locations
| 1980-96
| 2013-17
| 2014-18
| 2015-19
| 2016-20
| 2017-21
|
Pounds of total phosphorus per acre per five-year average
|
| Big Muddy | Murphysboro | 0.69 | 0.86 | 0.87 | 1.02 | 1.06 | 1.00 |
| Embarras | Ste. Marie | 1.23 | 1.18 | 1.14 | 1.38 | 1.59 | 1.56 |
| Green | Geneseo | 0.97 | 0.60 | 0.59 | 0.93 | 0.98 | 0.89 |
| Illinois | Valley City | 0.95 | 1.19 | 1.17 | 1.24 | 1.25 | 1.17 |
| Kaskaskia | Venedy Station | 0.86 | 1.41 | 1.24 | 1.59 | 1.73 | 1.74 |
| Little Wabash | Carmi | 0.96 | 1.43 | 1.40 | 1.71 | 1.80 | 1.79 |
| Rock (Illinois portion) | Joslin-Rockton | 0.86 | 0.95 | 0.91 | 1.15 | 1.24 | 1.10 |
| Vermilion | Danville | 1.19 | 1.06 | 1.14 | 1.31 | 1.30 | 1.16 |
Table 3.5. Monitoring stations used to estimate and evaluate the statewide phosphorus yields
Overall, the pattern in Illinois is typical of the Mississippi River Basin, where both nutrient loads and flow to the Gulf of Mexico have remained above 1980–96 baseline levels. USGS and others are researching why loads in some watersheds have increased or not decreased despite substantial management efforts. In all likelihood, a variety of factors, including climate change, “legacy” nutrients which cause lag times, and watershed management, contribute to varying extents across the state. Further research could help explain which of these factors are most important to riverine nutrient loads and inform more targeted and cost-effective reductions. Moreover, this information could affect how the Illinois NLRS assesses progress toward reducing nutrient loads from non-point sources; some nutrient reductions might be masked by legacy or climate effects and could be accounted for by other means.
See figures 8.1 and 8.2 in chapter 8 of the 2023 Illinois NLRS Biennial Report for nutrient loads information in context of the strategy goals.
