Summary from the 2023 Biennial Report
Of the eight major rivers in Illinois used to estimate Illinois statewide nutrient losses, the Illinois River is by far the largest and carries the largest total phosphorus load that leaves the state. Much of the total phosphorus load comes from the Chicago region, where more than half of the state’s population resides. At the furthest downstream monitoring locations of Valley City and Florence, total phosphorus loads increased 39% from the 1989–96 period, to the 2015–19 period, despite reductions in point source total phosphorus discharges in the Chicago region and extensive efforts to reduce total phosphorus loads from non-point sources throughout the basin.
To identify where and why the total phosphorus load increases occurred, concentration and streamflow data from 41 locations (Figure 3.9) across the Illinois River Basin were compiled, and annual loads of total phosphorus, dissolved and particulate phosphorus, total and volatile suspended solids, chloride, sulfate, and nitrate were calculated (McIsaac et al., 2022). All loads were calculated using Weighted Regressions on Time, Discharge, and Season with Kalman filtering. Annual loads were also calculated for sites outside the Illinois River Basin and made publicly available as a USGS data release (Hodson, 2021). At many monitoring sites, routine measurement of total phosphorus concentrations began between 1982 and 1985. Additionally, the combination of regular concentration and flow measurements for the Mackinaw River were only available after the 1988 water year. Consequently, loads could not be consistently calculated across the Illinois River Basin for the full 1980–96 baseline period. Instead, average annual loads for the 1989–96 period were calculated and compared to the 2015–19 period; the latter period was the last five years for which data were available at the start of the project. The yields, or the load per unit area, and incremental yields, or the yield from the drainage area between two or more monitoring locations, were also calculated from the average annual loads.
For most Illinois River Basin subwatersheds, changes in incremental total phosphorus yields from the 1989–96 to 2015–19 periods were relatively small. Exceptions included Thorn Creek at Thornton (Cook County) and Addison Creek at Bellwood (western Cook and eastern DuPage counties) where there were large reductions (Figure 3.10). Large increases in total phosphorus yield were observed for the Lower Mainstem subwatershed (which is between Marseilles and Valley City and excludes monitored tributaries); the Sangamon River between Monticello and Riverton; and Kickapoo Creek at Waynesville in McLean County. For watersheds primarily composed of agricultural land cover and less than 9% developed or urban land, the change of total phosphorus yield was weakly correlated with the change in water yield.
Most (78%) of the increased total phosphorus load at Valley City and Florence came from the Lower Mainstem subwatershed. During 1989–96, more total phosphorus entered the Lower Mainstem than exited at Valley City, suggesting an accumulation of phosphorus in the Lower Mainstem subwatershed. The Lower Illinois River and its floodplain are very flat and accumulate sediment and associated phosphorus. From the 1989–96 period to the 2015–19 period, this section had shifted from being a net sink to a net source of phosphorus. The reasons for this shift are unknown, but correlations suggest several possible causative mechanisms, including increased dissolved phosphorus and chloride loads; reduced sulfate and nitrate concentrations influencing ionic strength and redox potential in the sediments; and increased vol- atile suspended solids loads at Valley City, which may indicate greater algal production and contribute to hypoxia in lower river sediments. Additional research is needed to determine the relative influence of these mechanisms. The remaining 22% of the increased total phosphorus load at Valley City and Florence came from the Sangamon River Basin, where the increase was similar in magnitude to the increased phosphorus discharge from the Sanitary District of Decatur.
In many watersheds across the basin, dissolved phosphorus loads increased and particulate phosphorus and total suspended solids loads decreased from the 1989–96 to 2015–19 periods. A similar pattern has been observed in northwest Ohio (Jarvie et al., 2017) and may be a consequence of the adoption of conservation tillage and/or expansion of tile drainage. Conservation tillage reduces soil loss from cropland and can also reduce the incorporation of phosphorus fertilizer and manure into the soil. Higher concentrations of phosphorus at the soil surface contribute to increased loss of dissolved phosphorus in surface runoff. Tile drainage tends to reduce surface runoff, erosion, and particulate phosphorus loss from cropland. However, dissolved phosphorus can move through soil macropores to tiles and can be discharged from the tiles into drainage ditches and streams.
Dissolved phosphorus may be less likely to be deposited in the Lower Illinois River than particulate phosphorus. While some research indicates that increased chloride concentrations can promote phosphorus release from some sediments, the mechanism is unclear. High chloride concentrations have been shown to reduce zooplankton (Hintz et al., 2022), which may allow greater accumulation of phytoplankton and more extensive hypoxic and anoxic conditions in which phosphorus may be released from sediment.
References
Hintz, W. D., Arnott, S. E., Symons, C. C., and Weyhenmeyer, G. A., 2022. Current water quality guidelines across North America and Europe do not protect lakes from salinization. Proceedings of the National Academy of Sciences of the United States of America, 119(9). doi.org/10.1073/pnas.2115033119
Hodson, T.O. 2021. WRTDS-K Nutrient and Sediment Loads at Illinois EPA Ambient Water Quality Monitoring Network Sites through Water Year 2020. U.S. Geological Survey Data Release. doi.org/10.5066/ P9JX8YVL
Jarvie, H.P., L.T. Johnson, A.N. Sharpley, D.R. Smith, D.B. Baker, T.W. Bruulsema, and R. Confesor, 2017. “Increased Soluble Phosphorus Loads to Lake Erie: Unintended Consequences of Conservation Practices?” Journal of Environmental Quality 46 (1): 123–32. doi.org/10.2134/jeq2016.07.0248
McIsaac, G.F., T.O. Hodson, M. Markus, R. Bhattarai and D. C. Kim, 2022. Spatial and Temporal Variations in Phosphorus Loads in the Illinois River Basin, Illinois USA. Journal of the American Water Resources Association. doi.org/10.1111/1752-1688.13054
