In 1996, Illinois added Lake Decatur to its pollution list after farm runoff harmed its water. 28 years later, nitrogen levels fell below the 7.8 mg/L limit

For nearly three decades, Lake Decatur has been at the center of a continuing effort to improve water quality in central Illinois. The lake was added to Illinois' impaired waters list in 1996 because of nitrogen and phosphorus pollution linked to agricultural runoff.

Since then, conservation work across the watershed has focused on slowing runoff, reducing nutrient and sediment loading, and protecting the lake as an important drinking water source.

The latest monitoring results show a significant sign of progress. By the 2024 assessment cycle, nitrogen levels did not exceed the applicable threshold of 7.8 milligrams per liter. Phosphorus, however, remained above Illinois' lake standard, meaning the work is far from finished, as per a report by Illinois EPA water-quality success story.

What caused Lake Decatur’s pollution?

Lake Decatur's watershed covers 592,665 acres across nine central Illinois counties: Macon, DeWitt, Piatt, McLean, Champaign, Ford, Shelby, Christian and Sangamon. The lake flows east into the Sangamon River and serves drinking water, industrial and recreational needs.

Agriculture dominates the watershed, with about 80% of the area devoted to cultivated crops. That makes runoff a major part of the water-quality challenge.

Excess nutrients and sediment from agricultural activities and urban development contributed to poor water quality and impairments involving nitrogen, phosphorus and pesticides. The size of the watershed also makes the problem difficult to address through a single project.

The Illinois State Water Survey estimated that more than 13 million pounds of nitrate loading occurs in the watershed each year. The resulting periodic exceedance of drinking water standards has required approximately $200,000 in annual treatment costs for the water utility serving the City of Decatur and the Village of Mt. Zion.

A total maximum daily load was approved in 2007 for the Lake Decatur and Sangamon River system covering fecal coliform, phosphorus and nitrogen/nitrate.

How did conservation efforts reduce runoff?

The response developed over several years, with agricultural best management practices becoming an important part of the effort.

In 2014, the Macon County Soil and Water Conservation District received a Section 319(h) grant to install conservation practices at 14 sites in the Macon and Piatt county portions of the Lake Decatur watershed.

The projects were selected based on their ability to control sediment, nitrates and phosphorus, as well as their proximity to the river and lake.

The work included streambank and stream channel stabilization, grassed waterways, grade stabilization structures, cover crops, sediment basins, ponds, permanent vegetative cover and water control structures. Other practices included water and sediment control basins, terraces, parallel tile outlets, diversions and heavy use area protection.

The goal was to address runoff and pollutant movement before those materials could reach the lake.

The City of Decatur later expanded the approach through the Lake Decatur Water Quality Initiative. Launched in 2021, the initiative created a broader watershed strategy that included a cost-share program for structural best management practices.

Starting that year, the city received funding through three Section 319(h) grants to develop or update watershed plans and install priority best management practices designed to reduce sediment and nutrient loading.

The strategy focused on identifying parts of the watershed with the greatest potential impact and directing conservation work toward those areas.

What practices were installed?

The conservation effort has included a wide range of best management practices across the watershed.

Reported projects include 378 acres of cover crops, 3,689 feet of diversions, seven grade stabilization structures and 37 acres of grassed waterways. Four ponds and two sediment basins were also installed.

Stream channel stabilization covered 8,900 feet, while streambank and shoreline protection projects covered 575 feet and another 747 feet. The work also included 25,693 feet of terraces and 3,495 feet of water and sediment control basins.

Wetland restoration covered 6.1 acres.

The broader initiative also brought together more than 50 partners. Farmers were offered incentives to voluntarily adopt conservation practices such as cover crops, tillage management and nutrient management.

The updated watershed plans helped identify locations where additional best management practices could be installed. Those efforts were supported by a $9.8 million U.S. Department of Agriculture Regional Conservation Partnership Program grant and $16 million in local funding.

The initiative combines watershed and in-lake treatments as part of the larger effort to reduce the amount of sediment and nutrients reaching Lake Decatur.

What changed by 2024?

Monitoring by the Illinois Environmental Protection Agency's Surface Water Section has shown encouraging trends for phosphorus, dissolved oxygen and nitrogen/nitrates.

The most notable development for nitrogen came during the 2024 assessment cycle. New data showed that nitrogen levels did not exceed the applicable threshold. For total nitrogen to be removed as an observed effect, the most recent applicable nitrogen/nitrite results would need to remain below 7.8 mg/L.

That result provides an important measure of progress after years of conservation work across the watershed.

Phosphorus has also shown improvement. Levels declined between 2013 and 2022, although they remained above Illinois' lake standard of 0.05 mg/L. As a result, phosphorus continues to be listed as an impairment under the Clean Water Act Section 303(d) program.

The monitoring record also included a phosphorus spike in October 2013. That sampling occurred during a wetter-than-usual year in Decatur, and rainfall had occurred during the five days before the sampling event, including a thunderstorm that produced more than one inch of rain. The rainfall likely carried phosphorus into the lake.

Can the progress continue?

The improvement in nitrogen levels does not mean the Lake Decatur water-quality challenge is over.

The watershed is large, and much of it remains agricultural. Pollutants can continue moving toward the lake through runoff, making sustained conservation work necessary.

The long-term strategy calls for developing or updating watershed-based plans, concentrating on areas contributing the most nutrient and sediment runoff, implementing practices such as cover crops, tillage management and nutrient management, and working with partners to secure funding and provide financial incentives for landowners.

Coordination among local governments, environmental organizations, agricultural stakeholders and other partners remains part of the effort.

Lake Decatur's progress has been built through years of projects rather than one single fix. The drop in nitrogen levels below the 7.8 mg/L threshold during the 2024 assessment cycle shows that the work is producing measurable results.

At the same time, continued attention is needed, particularly because phosphorus remains an impairment and the watershed continues to generate runoff. Protecting Lake Decatur therefore depends on maintaining the conservation effort that has helped improve its water quality over time.

FAQs

How much nitrogen was reduced?
An estimated 81,259 pounds annually.

When did the project begin?
The watershed planning effort began in 2012.