With the possible exception of arsenic, nitrate is the most widespread pollutant of groundwater. Unlike arsenic, which is primarily a naturally occurring contaminant, elevated nitrate concentrations are almost always due to human activities, primarily in agricultural settings. Tilling of soil, application of synthetic fertilizers, and livestock manure are the primary sources of nitrate. As all residents of Illinois surely know, there is a lot of agricultural activity in the state, mainly corn and soybeans. And because of this, there are surface water and shallow groundwater resources in Illinois that suffer from excess nitrate (and other forms of nitrogen).
With the possible exception of arsenic, nitrate is the most widespread pollutant of groundwater. Unlike arsenic, which is primarily a naturally occurring contaminant, elevated nitrate concentrations are almost always due to human activities, primarily in agricultural settings. Tilling of soil, application of synthetic fertilizers, and livestock manure are the primary sources of nitrate. As all residents of Illinois surely know, there is a lot of agricultural activity in the state, mainly corn and soybeans. And because of this, there are surface water and shallow groundwater resources in Illinois that suffer from excess nitrate (and other forms of nitrogen).
Widespread nitrogen pollution is a problem in many parts of the developed world, not just Illinois and the U.S. corn belt, and policy makers have proposed many approaches to reducing leaching of nitrate and other nutrients to water bodies. A recent paper suggests that nitrate concentrations have been decreasing in Danish groundwater since around 1980 [Hansen, B. et al. 2011. Trend reversal of nitrate in Danish groundwater - a reflection of agricultural practices and nitrogen surpluses since 1950. Environ. Sci. Technol. 45(1):228-234].
Denmark is a small county that is intensively farmed; about of the land is in agricultural production. For comparison, about 80% of Illinois land is farmland. Denmark has high nitrogen fertilization rates and livestock densities compared with the rest of Europe and the world. The authors of the paper attribute the trend reversal in nitrate concentrations that began in 1980 to a leveling out of the nitrogen surplus in Danish agriculture after a period of large increases, as well as improvements in wastewater handling and storage. If nothing else had changed, there probably would have been a leveling off of nitrate concentrations, not the significant decrease they observed after 1980. They attribute further decreases to national nitrogen reduction policies and activities beginning in 1985, such as limits on application rates and more stringent control of manure application. Development of crop hybrids capable of more efficient utilization of nitrogen has also helped.
Decreases in nitrate concentrations in groundwater have been reported in other parts of the world as well as Denmark, including the Netherlands, Belgium, and Oregon. However, most of the studied regions in the U.S. have increasing trends.
The U.S. corn belt of which Illinois is a major part has the highest nitrogen fertilization rate in the country. Check out this mapusing data from the 1980s showing fertilizer use by county in the U.S. (todays rates may be different, but qualitatively should be the same).
[Map from Battaglin, W.A., and D.A. Goolsby, 1994. U.S. Geological Survey Water-Resources Investigations Report 94-4176.]
Although there are significant differences between Denmark and Illinois, I believe their findings may be pertinent to Illinois. Improved practices, technologies, and policies can reduce nitrate leaching to vulnerable water resources. However, making causal connections between specific activities and resulting water quality improvements is extremely difficult, due to the almost unimaginable complexity of the soil and subsurface environments and the nitrogen cycle. Illinois also does not have a groundwater monitoring network (other than for public supply wells), which the Danes have, making the detection of trends much more difficult.