Introduction
In 2015, the Illinois Nutrient Loss Reduction Strategy (Illinois NLRS or the strategy) provided estimates of nitrate-nitrogen (N) and total phosphorus (TP) loads in the rivers draining the state from 1980 to 2011 and set a long-term goal of reducing those losses by 45% relative to loads that occurred during the baseline period of 1980–96. Interim goals of 25% reduction in TP losses and 15% reduction in nitrate-N losses by 2025 were also adopted. Variation in nutrient losses across the state was quantified with estimates of 2011 point source loads and 1997–2011 annual average non-point source and total nitrate-N and TP losses for 50 eight-digit Hydrological Unit Codes (HUC8) watersheds that cover most of the state. Priority areas for conservation were identified, in part based on nutrient load estimates for HUC8s.
Illinois NLRS also called for biennial progress reports toward meeting the goals. The 2017 biennial report updated nutrient load estimates through the 2015 water year for the state as a whole, but not for HUC8s. This provides a summary of updates to nutrient load estimates through the 2017 water year for the state as a whole and for HUC8s. The full 68-page report titled "Nitrate and Total Phosphorus Loads in Illinois River: Update Through the 2017 Water Year" provides greater detail on methods and results and is available on the strategy page of the Illinois EPA website (go.illinois.edu/NLRS). The primary aim of this effort was to quantify riverine nutrient loads in Illinois.
Identifying and quantifying why changes happened or did not happen in loads is largely beyond the scope of this study. At a few locations, changes in riverine nutrient loads appear to be related to changes in water flow and/or changes in point source inputs—these associations are noted. Quantifying the causes of changes in riverine loads is recommended for future studies and will involve more detailed accounting of nutrient sources and sinks than can be provided in this study.
Methods
Riverine nitrate and TP loads are the product of nutrient concentration and river flow and often expressed in terms of pounds of nutrient per day or per year.
EQ.1 Nutrient Load = Concentration x Flow
Because larger watersheds drain larger areas, they tend to have greater flows and nutrient loads. Comparing nutrient loads across different sized watersheds is problematic because drainage area confounds the comparison. Dividing loads by drainage areas removes the influence of drainage area, and this is referred to as nutrient yield, which is an indicator of the spatial intensity of nutrient losses. Areas with higher nutrient yields typically have higher nutrient concentrations, and these can be compared across watersheds of different sizes. Nitrate and TP yields from watersheds are often expressed as pounds per acre per year, which is the riverine loads divided by the contributing watershed area.
EQ. 2 Nutrient Yield = Nutrient Load / Drainage Area
Load and yield estimates for this report were based on daily stream-flow data from USGS with nitrate and TP concentration data from multiple sources. Most of the concentration data came from the Illinois EPA Ambient Water Quality Monitoring Network, with additional data at a few locations from the U.S. Geological Survey, Fox River Study Group, Metropolitan Water Reclamation District of Greater Chicago (MWRDGC), and the University of Illinois, specifically, Lowell Gentry.
Results
Statewide Water, Nitrate-N, and TP Loads
The statewide load values were derived from monitoring data collected on eight major rivers draining the state (Figure 3.1). Rock River nutrient loads draining from Wisconsin were subtracted from the total by using data collected at Rockton, Illinois. Illinois River nutrient load estimates from Wisconsin and Indiana were also deducted from the load estimated at Valley City, based on 15% of the watershed area that originates in those states. Similarly, Vermilion River nutrient loads at Danville were reduced by 7%, the portion of the HUC8 that is in Indiana. Nutrient losses per acre outside of these eight rivers, but within Illinois, were assumed to equal the average value loss per acre derived from the eight rivers, excluding contributions from neighboring states.
The estimated statewide average annual nitrate-N load in Illinois rivers during 2013–17 was 425 million pounds nitrate-N per year, which was approximately 7% greater than the 1980–96 baseline average of 397 million lb N/yr. The estimated statewide average TP load during 2013–17 was 43 million lb P/yr, a 26% increase over the baseline load of 34 million lb P/yr.
Nutrient loads tend to be correlated with water flow, which is highly variable over time, largely due to fluctuation in annual precipitation. The estimated statewide average water flow during 2013–17 was about 13% greater than 1980–96, and this likely facilitated the increase in nutrient loads. Additionally, 2012 was an extreme drought year that reduced corn yields, leaving greater than average unused nitrogen fertilizer in cropland soils. Higher than average nitrate-N loads in rivers have frequently been observed following drought years.
Since the baseline period, changes in water flow and nutrient loads were not uniform across the state. Average water flow increased in all eight major river basins during 2013–17 relative to 1980–96. The greatest percent of increase (34%) occurred in the Illinois portion of the Rock River Basin. This location also had the greatest absolute increase in nitrate-N load (18 million lb N/yr), which is more than half of the 28 million lb N/yr increase for the state as a whole. Smaller nitrate-N load increases of 4.1, 2.2, and 1.1 million lb N/yr occurred for the Embarras, Little Wabash and Green river systems, respectively. Small reductions in nitrate-N load were calculated for the Big Muddy, Kaskaskia, Illinois, and Vermilion river systems.
Figure 3.1. Eight major rivers used to estimate statewide nutrient loads.
The greatest increase in TP load (4.12 million lb P /yr) occurred in the Illinois River, followed by the Kaskaskia River (1.64 million lb P/yr). In the Kaskaskia, this is a 68% increase over the average annual load for 1980–96. Increases in other river systems were all less than 1 million lb P/yr, with the exception of the Green River, which had a relatively small reduction in load (0.22 million lb P/yr). However, some of these smaller changes represent large percentage changes for the river system. The 0.22 million lb P/ yr reduction in the Green River was 36% lower than in the baseline period and an 0.98 million lb P/yr increase in the Little Wabash was a 51% increase. Causes for these changes deserve further study.
Statewide Point Source Discharges
Estimated total nitrogen (TN) discharge from point sources to rivers in 2017 totaled 75 million lb N/yr compared to 87.3 million lb N/yr in 2011, a reduction of 14%. (The percent reduction in point source discharge between 2011 and 2017 is not directly comparable to the changes in riverine loads from 1980–96 to 2013–17 discussed above because the changes are evaluated over different time intervals.) The source of the vast majority (93%) of the 2017 TN point source discharge was 213 major publicly-owned treat- ment works (POTWs) that discharge over one million gallons of treated wastewater per day. Point sources appear to be a relatively small contributor to the 425 million lb nitrate-N/yr statewide loads in rivers.
Estimated TP discharges from point sources in 2017 totaled 14.1 million lb P/yr, which is a 22% reduction from the 18 million lb P/yr estimated in 2011. As with TN, the source of the vast majority (81%) of the 2017 estimated TP point source discharge appears to be major POTWs. Approximately half of the 3.9 million lb P/yr statewide reduction in point source TP discharge was due to 1.9 million lb P/yr reductions across six MWRDGC facilities. Several smaller municipalities reported 2017 TP discharges that were 20–60 thousand lb P/yr lower than 2011 values reported in Illinois NLRS (e.g., Springfield, Champaign-Urbana, and Quincy). Increases in TP discharge on the order of 200 thousand lb P/yr were estimated for the Sanitary District of Decatur and the City of Joliet.
The 14.1 million lb P/yr point source discharge in 2017 represents 33% of the 43 million lb P/yr TP load in the state’s rivers in 2013–17. Attributing riverine phosphorus loads to point sources or non-point sources is complicated by phosphorus adsorption to river and lake sediments, which may be retained in a river system for years. Phosphorus measured at a river outlet likely includes considerable amounts of “legacy” phosphorus from both point and non-point sources. Legacy phosphorus deserves further study and may partly explain why riverine TP loads increased, even though point source inputs decreased.
Nitrate-N Yields at River Monitoring Locations and HUC8s
Estimated non-point source nitrate-N loads (Figure 3.2) were calculated by identifying point source discharging facilities upstream of monitoring locations and subtracting 90% of the sum of TN discharges from the nitrate load estimated at monitoring location. Based on point source data from Illinois NLRS, it was assumed that point source TN discharge was 90% nitrate. It was further assumed that the area upstream of the monitoring location was representative of the HUC8 as a whole. The HUC8 load was estimated by multiplying the monitored non-point source load by the ratio of the HUC8 area to the monitored area. For HUC8s with no data or inadequate monitoring data, load estimates were based on yield averages measured in neighboring HUC8s.
The greatest increase in TP load (4.12 million lb P /yr) occurred in the Illinois River, followed by the Kaskaskia River (1.64 million lb P/yr). In the Kaskaskia, this is a 68% increase over the average annual load for 1980–96. Increases in other river systems were all less than 1 million lb P/yr, with the exception of the Green River, which had a relatively small reduction in load (0.22 million lb P/yr). However, some of these smaller changes represent large percentage changes for the river system. The 0.22 million lb P/ yr reduction in the Green River was 36% lower than in the baseline period and an 0.98 million lb P/yr increase in the Little Wabash was a 51% increase. Causes for these changes deserve further study.
Statewide Point Source Discharges
Estimated total nitrogen (TN) discharge from point sources to rivers in 2017 totaled 75 million lb N/yr compared to 87.3 million lb N/yr in 2011, a reduction of 14%. (The percent reduction in point source discharge between 2011 and 2017 is not directly comparable to the changes in riverine loads from 1980–96 to 2013–17 discussed above because the changes are evaluated over different time intervals.) The source of the vast majority (93%) of the 2017 TN point source discharge was 213 major publicly-owned treat- ment works (POTWs) that discharge over one million gallons of treated wastewater per day. Point sources appear to be a relatively small contributor to the 425 million lb nitrate-N/yr statewide loads in rivers.
Estimated TP discharges from point sources in 2017 totaled 14.1 million lb P/yr, which is a 22% reduction from the 18 million lb P/yr estimated in 2011. As with TN, the source of the vast majority (81%) of the 2017 estimated TP point source discharge appears to be major POTWs. Approximately half of the 3.9 million lb P/yr statewide reduction in point source TP discharge was due to 1.9 million lb P/yr reductions across six MWRDGC facilities. Several smaller municipalities reported 2017 TP discharges that were 20–60 thousand lb P/yr lower than 2011 values reported in Illinois NLRS (e.g., Springfield, Champaign-Urbana, and Quincy). Increases in TP discharge on the order of 200 thousand lb P/yr were estimated for the Sanitary District of Decatur and the City of Joliet.
The 14.1 million lb P/yr point source discharge in 2017 represents 33% of the 43 million lb P/yr TP load in the state’s rivers in 2013–17. Attributing riverine phosphorus loads to point sources or non-point sources is complicated by phosphorus adsorption to river and lake sediments, which may be retained in a river system for years. Phosphorus measured at a river outlet likely includes considerable amounts of “legacy” phosphorus from both point and non-point sources. Legacy phosphorus deserves further study and may partly explain why riverine TP loads increased, even though point source inputs decreased.
Nitrate-N Yields at River Monitoring Locations and HUC8s
Estimated non-point source nitrate-N loads (Figure 3.2) were calculated by identifying point source discharging facilities upstream of monitoring locations and subtracting 90% of the sum of TN discharges from the nitrate load estimated at monitoring location. Based on point source data from Illinois NLRS, it was assumed that point source TN discharge was 90% nitrate. It was further assumed that the area upstream of the monitoring location was representative of the HUC8 as a whole. The HUC8 load was estimated by multiplying the monitored non-point source load by the ratio of the HUC8 area to the monitored area. For HUC8s with no data or inadequate monitoring data, load estimates were based on yield averages measured in neighboring HUC8s.
