The following article was featured in the 2021 NLRS Biennial Report.
The science assessment was originally published as part of the 2015 Illinois Nutrient Loss Reduction Strategy. To present the most accurate picture of Illinois waters, the science assessment is routinely updated with the latest data and published in chapter 3 of the Illinois NLRS biennial reports. This chapter contains new nitrate-N and total phosphorus river loads conducted by Dr. Gregory McIsaac, supporting data from the U.S. Geological Survey’s continuous loadings network provided by Paul Terrio, and updated implementation scenarios conducted by Dr. Reid Christianson. The final section is the Illinois NLRS Science Team’s proposed practice decisions.
Nitrate-N and total phosphorus loads in the major rivers draining Illinois were updated through the 2019 water year using the same sites (Table 3.1) and methods used in the 2015 NLRS Science Assessment and subsequent biennial reports. These river systems are depicted on the watershed map in Figure 3.1.
Table 3.1. Monitoring stations used to estimate the statewide nitrate-N and total phosphorus loads
River System | Gage Location | Illinois EPA
Station | USGS Station
Number | Drainage
Area (sq. mi) | Drainage Area
in Illinois (%) | % of Illinois
Represented |
Rock | Joslin | P-04 | 05446500 | 9,549 | 43 | 7.3 |
Rock | Rockton | P-15 | 05437500 | 6,362 | 13 | 1.4 |
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 |
Big Muddy | Murphysboro | N-12 | 05599490 | 2,169 | 100 | 3.8 |
Little Wabash | Carmi | C-23 | 03381500 | 3,102 | 100 | 5.5 |
Embarras | Ste. Marie | BE-07 | 03345500 | 1,516 | 100 | 2.7 |
Vermilion | Danville | BP-01 | 03339000 | 1,290 | 93 | 2.1 |
Figure 3.1. Watershed map depicting monitoring stations used to estimate the statewide nitrate-N and total phosphorus loads
Methods
Discrete sample concentrations were measured by Illinois EPA and USGS and flow was measured by USGS. Nitrate-N loads were estimated by linear interpolation and total phosphorus loads were estimated using Weighted Regressions on Time, Discharge, and Seasonality. To account for loads coming from neighboring states, the loads in the Illinois and Vermilion rivers were reduced by the percentage of the drainage areas in neighboring states, 16% and 7%, respectively. Additionally, 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. The sum of the three adjusted loads and the loads in the other five rivers that lie entirely in Illinois represents the loads draining from 69.7% of the land area in Illinois. To account for the remaining 30.3% of the land area that does not drain past any of these monitoring locations, it was assumed that the loads per unit area (or yield) from the unmonitored 30.3% is the same as the estimated yield from the 69.7% that is monitored. Thus, the final step in estimating the statewide loads was to multiply the sum of the three adjusted loads and the loads in the other five rivers by the ratio of the total state land area to the monitored drainage area (1.435).
Water Measures
Statewide nitrate-N and total phosphorus loads have been highly correlated with water yield (Figures 3.2 and 3.3), which in turn are highly correlated with precipitation. The 2015-19 statewide average nitrate-N load estimate was 448 million pounds N per year, 13% greater than the 1980-96 baseline load (Table 3.2). The 2015-19 statewide total phosphorus load estimate was 46 million pounds P per year, 35% greater than the baseline load. Water yield was 25% greater than the baseline period. The 2015-19 averages were influenced by unusually high precipitation and river flows in 2019. Five-year average water yields have been greater than the baseline water yield since 2008. Greater runoff and drainage tend to increase river loads and, thereby, increase the difficulty of meeting NLRS goals. The 2013-17 total phosphorus load (Table 3.2) is slightly lower than the value reported in the 2019 Biennial Report because weighted regressions on time, discharge, and season estimates concentrations using data over a seven-year window. Adding 2018 and 2019 concentration data to the analysis led to slightly lower estimates for 2013-17.
Figure 3.2. Statewide estimated annual water yields, annual nitrate-N loads, five-year moving averages, and average load for the 1980-96 baseline period
Figure 3.3. Statewide estimated annual water yields, annual total phosphorus loads, five-year moving averages, and average load for the 1980-96 baseline period
Table 3.2. Statewide estimated water yield, nitrate-N load, and total phosphorus load for the 1980-96 baseline period and three recent five-year periods
|
1980-96 Baseline | 2013-17 | 2014-18 | 2015-19 |
| Average Value | Average
Value | % Change
from Baseline | Average
Value | % Change
from Baseline | Average
Value | % Change
from Baseline |
Water Yield | 13.0 | 14.7 | +13% | 14.1 | +9% | 16.3 | +25% |
Nitrate-N Load (million lb/yr) | 397 | 425 | +7% | 380 | -4.4% | 448 | +13% |
Total Phosphorus Load
(million lb/yr) | 34 | 42 | +23% | 41 | +20% | 46 | +35% |
Of the major rivers, the largest nitrate-N loads occurred in the Illinois River, partly because it drains the largest area of all these rivers, including much of the tiled cropland of central Illinois, in addition to receiving considerable discharge of effluent from wastewater treatment in Chicago and Decatur (Figure 3.4). The 2015-19 average nitrate-N loads in the Illinois River were only 3% greater than the baseline period and the 2014-18 loads were 13% less than the baseline.
The largest increase in nitrate-N load from the baseline period occurred in the Illinois portion of the Rock River (between Rockton and Joslin) where the 2015-19 nitrate-N load was 135% greater and water yield was 60% greater (Figures 3.5 and 3.6). Increased precipitation is likely one causal factor. Additionally, flow through large groundwater aquifers may have played a role in delaying the appearance of increased nitrate-N concentrations in the lower Rock River. In the baseline period, nitrate-N yield from the portion of the Rock River watershed downstream of Rockton and Perryville was unusually low (4.2 pounds N/acre/year) despite intensive row crop production, irrigation, and tile drainage. In comparison, baseline average nitrate-N yield from the Kishwaukee River was 17 pounds N/acre/ year and the neighboring Green River at Geneseo was 12.2 pounds N/acre/year (Table 3.3). In 2015-19, the nitrate-N yield from the Rock River, excluding the Kishwaukee River, was 21.5 pounds N/ acre/year, slightly larger than yields from the Kishwaukee River (17.9 pounds N/acre/year) and the Green River (16.5 pounds N/acre/ year). Understanding why nitrate-N yields from the lower section of the Rock River were unusually low during the baseline period may shed light on the reasons for the large increase since the baseline period. There is evidence from groundwater wells in the region that nitrate-N has been accumulating in the extensive aquifers below the lower Rock River watershed. Some portion of the increased nitrate-N load in the Rock River may be due to nitrate-N contamination of groundwater in the 1970s, 1980s, and early 1990s that increasingly emerged in the river in the late 1990s and early 2000s.
Figure 3.4. Nitrate-N loads in eight major rivers draining Illinois during the 1980-96 baseline period and three recent five-year periods. The percentage change from the baseline load to 2015-19 for each river system is also indicated.
Figure 3.5. Changes in nitrate-N loads in eight major rivers draining the state for three recent five-year periods relative to the load during the 1980-96 baseline period. The percentage change from the baseline load to 2015-19 for each river system is also indicated.
Figure 3.6. Percent changes in river flow or water yield from the 1980-96 baseline period to three recent five-year periods in eight major rivers draining the state
Table 3.3. Monitoring stations used to estimate and evaluate the statewide nitrate-N loads
River System | Monitoring Locations | 1980-96 | 2013-17 | 2014-18 | 2015-19 |
pounds of nitrate-N per acre per year |
Big Muddy | Murphysboro | 1.5 | 1.3 | 1.2 | 1.5 |
Embarras | Ste. Marie | 15.3 | 19.5 | 17.6 | 19.0 |
Green | Geneseo | 12.2 | 14.0 | 13.1 | 16.5 |
Illinois | Valley City | 13.9 | 13.6 | 12.1 | 14.3 |
Kaskaskia | Venedy Station | 3.6 | 3.1 | 2.4 | 3.2 |
Little Wabash | Carmi | 3.1 | 4.3 | 3.6 | 4.1 |
Vermilion | Danville | 24.3 | 23.3 | 20.7 | 22.2 |
Rock (Illinois portion) | Joslin-Rockton | 8.6 | 17.3 | 16.6 | 20.3 |
Rock (Illinois portion, excluding the Kishwaukee) | Joslin-Rockton-Kishwaukee | 4.2 | 18.4 | 17.8 | 21.4 |
Kishwaukee | Perryville | 17.1 | 15.4 | 15.2 | 17.9 |
Sangamon | Oakford | 15.5 | 13.4 | 10.2 | 13.6 |
In contrast to the Rock River, nitrate-N loads have declined about 10% in the Vermilion (Wabash Basin) and Kaskaskia rivers, despite increases in water flow of 17% and 28%, respectively (Figure 3.5). A similar reduction also occurred in the Sangamon River at Oakford (Table 3.3). This may be due to increased efficiency of nitrogen fertilizer use and the absence of significant groundwater retention of nitrate-N. Corn yields have increased substantially since 1996, largely due to improved varieties, while nitrogen fertilizer use has increased only modestly. Increased precipitation and drainage tend to promote increased nitrate-N loss from cropland, but crop management factors can also have a counteracting influence. Another factor to consider is the role of reservoirs, where significant denitrification can occur. Changes in the seasonality of flow and warmer temperatures may be promoting more denitrification in lakes and reservoirs, thereby reducing nitrate-N concentrations and loads downstream.
Total phosphorus loads have also been variable across the state (Figures 3.7 and 3.8). The greatest load occurred in the Illinois River because of the size of the watershed and wastewater discharges from Chicago and Decatur. There was a small (4%) reduction in total phosphorus loads in the Green River, despite a 40% increase in water flow (Figure 3.6). The greatest percentage increases in total phosphorus loads occurred in the Kaskaskia (86%) and Little Wabash (77%) rivers, which were associated with increased river flows of 28% and 37%, respectively. These two watersheds also had the greatest total phosphorus yields of the major rivers during 2015-19 (Table 3.4). In the Illinois section of the Rock River, total phosphorus loads increased 34%, while water flow increased 60%. Total phosphorus loads are influenced by factors other than precipitation and river flow. In the Kaskaskia, legacy phosphorus released from reservoir sediments may be playing a significant role. In the Little Wabash, land slope and greater propensity for surface runoff may be important. Additional research is needed to quantify these and other influences on both phosphorus and nitrate-N loads in rivers.
Figure 3.7. Total phosphorus loads in eight major rivers draining Illinois during the 1980-96 baseline period and three recent five-year periods. The percentage change from the baseline load to 2015-19 for each river system is also indicated. The estimated loads for the Illinois and Vermilion rivers reflect loads attributed to the portion of watershed drainage areas in Illinois.
Figure 3.8. Changes in total phosphorus loads in eight major rivers draining the state for three five-year periods relative to the load during the 1980-96 baseline. The percentage change from the baseline load to 2015-19 for each river system is also indicated.
Table 3.4. Total phosphorus yields from major river systems draining Illinois for the 1980-96 baseline period and three recent five-year periods
River System | Monitoring Locations | 1980-96 | 2013-17 | 2014-18 | 2015-19 |
annual average pounds of total phosphorus per acre per year |
Big Muddy | Murphysboro | 0.69 | 0.86 | 0.87 | 1.02 |
Embarras | Ste. Marie | 1.23 | 1.18 | 1.14 | 1.38 |
Green | Geneseo | 0.97 | 0.60 | 0.59 | 0.93 |
Illinois | Valley City | 0.95 | 1.19 | 1.17 | 1.24 |
Kaskaskia | Venedy Station | 0.86 | 1.41 | 1.24 | 1.59 |
Little Wabash | Carmi | 0.96 | 1.43 | 1.40 | 1.71 |
Rock (Illinois portion) | Joslin-Rockton | 0.86 | 0.95 | 0.91 | 1.15 |
Vermilion | Danville | 1.19 | 1.06 | 1.14 | 1.31 |
