Water-Resources Investigations Report 99-4275
A retrospective analysis of selected data on nutrients and suspended solids in surface waters of the upper Illinois River Basin was done as part of the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program. Approximately 91 percent of the upper Illinois River Basin is drained by three principal rivers: the Kankakee (and its major tributary, the Iroquois), the Des Plaines, and the Fox. The data analyzed were collected by the Illinois Environmental Protection Agency (IEPA), which operates 39 monitoring sites in the study area as part of its Ambient Water-Quality Monitoring Network, and included analyses for total ammonia nitrogen, total nitrite-plus-nitrate nitrogen, total ammonia-plus-organic (total Kjeldahl) nitrogen, dissolved and total phosphorus, and total suspended solids and volatile solids. Nutrient and suspended-sediment data collected by the USGS as part of the upper Illinois River Basin NAWQA pilot study from 1987–90 were compared to IEPA data.
For the 1978-97 period, in general, nutrient concentrations, with the exception of nitrate, were highest at streams in the urban areas of the Des Plaines River Basin. Streams in the Kankakee and Fox River Basins generally had lower concentrations, although the data indicate that concentrations increased in a downstream direction in these basins. These spatial patterns in nutrient concentrations correspond closely with land use in the respective basins. The elevated concentrations of ammonia and phosphorus in the urbanized Des Plaines River Basin, with respect to other sites in the study area, indicate that municipal- and industrial-waste discharges into streams of the basin increase concentrations of these nutrients in the receiving streams. In contrast, nitrate concentrations were highest in agricultural areas. Relatively large ratios of nitrogen to phosphorus and nitrate to ammonia are characteristic of agricultural drainage. On the other hand, urban tributaries were characterized by smaller ratios of nitrogen to phosphorus and nitrate to ammonia. The apparent, but nonuniform, correspondence of nutrient concentrations to urban and agricultural land use in the upper Illinois River Basin was generally consistent with findings in other river basins. A seasonal pattern of nutrient concentrations characterized by high concentrations in the winter months, depletion during the spring and summer, and minimum levels in the late summer or early fall was observed in some of the data from the upper Illinois River Basin. Monthly median concentrations of total ammonia nitrogen and nitrite plus nitrate nitrogen were at minimum levels from July through October, whereas phosphorus concentrations did not display a strong seasonal trend.
The net result of nutrient inputs and transport through the river system were elevated nutrient concentrations at the most-downstream site in the study area on the Illinois River. At this site, the median concentrations of nitrate, total phosphorus, and orthophosphate were among the highest in the Mississippi River Basin, and the concentration of ammonia was the highest.
Suspended-solids concentrations do not indicate any particularly strong spatial patterns among major river basins in the study area. Instead, higher suspended-solids concentrations are observed at sites draining areas of low-permeability, easily eroded soils in agricultural and urban areas alike. Seasonal variation of suspended solids were consistent at sites across the study area. In general, suspended-solids concentrations were highest in the summer and lowest in the winter. The increase during the summer can be attributed to higher streamflow and the associated increase in runoff and transport, as well as increased phytoplankton growth.
Because of the high nutrient concentrations in the upper Illinois River Basin, annual loads and yields also were relatively large; however, yields of phosphorus from the Fox and Kankakee River Basins were not unusually high. The major contributor of total ammonia nitrogen, total Kjeldahl nitrogen, and phosphorus loads to the total study-area output was the Des Plaines River Basin, the Chicago Sanitary and Ship Canal in particular. The high concentrations in this waterway, coupled with the relatively high volume of streamflow, contribute to the large load output. The high loads in the Ship Canal reflect the input from the three large Metropolitan Water Reclamation District of Greater Chicago treatment plants. In contrast, nitrate loads were higher from the agricultural Kankakee River Basin. Total suspended-solids loads were also greatest from agricultural areas, in particular the Iroquois River Basin and tributaries to the lower Fox River. These are areas of intensive row-crop agriculture and fine, easily erodable soils.
The total nitrogen export from the upper Illinois River Basin for 1978-97 was 91,800 ton/yr (tons per year). This figure corresponds well with estimates of loads from urban, agricultural, and other sources, and is about 30 percent of the estimated total nitrogen input to the basin of about 300,000 ton/yr. The total phosphorus export from the study area during 1978-97 was about 5,400 ton/yr, or about 6 percent of estimated phosphorus inputs of 94,000 ton/yr. Loads and yields of nutrients from the upper Illinois River Basin are among the very highest in the entire Mississippi River drainage system.
Significant downward trends in total-ammonia concentrations were observed at many sites during the period of analysis, along with correlative upward trends in nitrate. This opposite relation is consistent with the reversible capacity for transformation between the reduced form (ammonia) and the oxidized form (nitrate), and may be related to nitrification of wastewater effluents. Significant downward trends in total ammonia plus organic nitrogen were related to downward trends in ammonia concentrations. Few trends in phosphorus concentrations were observed, but upward trends were observed at 2 sites downstream of major wastewater-treatment plants.
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