We’ve known for a long time that too much nitrogen in streams, lakes, and seas can be a bad thing, and many efforts are being made to reduce the amount of nitrogen (primarily nitrate) coming off of agricultural fields. In addition to being smarter about when and how much fertilizer is applied, a number of techniques designed to slow down movement of runoff to streams, such as constructing wetlands or water-table management, are being tested. Decreasing the concentrations and loads of nitrate in streams in agricultural regions has been remarkably difficult, however, and oftentimes do not match expectations based on these new best management practices (BMPs). A recent study by USGS scientists suggests that the long times it takes for groundwater to discharge to streams is the most likely factor.

Groundwater contaminated with nitrate (and other contaminants) can take decades to reach streams. Thus any activities undertaken to reduce nitrate application or runoff may not be reflected in streams for many years. The USGS scientists studied seven watersheds, doing age dating to assess how much “old” nitrate there was in the streams. The closest to Illinois was the Tomorrow River in central Wisconsin, where they found that decades-old water with elevated nitrate levels was currently discharging to the river. These elevated “base flow” nitrate concentrations may be sustained for many years, regardless of what BMPs are currently in place or undertaken in the near future.

There are two important factors that may control things here in most of Illinois. First, a large percentage of agricultural fields in Illinois are tile drained, thus a lot of the surface applied nitrogen never reaches the relatively deep groundwater systems. Tiles essentially skim off the top of the water table, and are the major source of nutrients to streams in Illinois. Second, under suboxic conditions (dissolved oxygen < 0.5 mg/L), denitrification can occur, a biogeochemical reaction that converts nitrate to nitrogen gas. Most groundwater in Illinois, even very shallow groundwater, is suboxic, mainly due to the high organic content of our soils. Thus base-flow nitrate concentrations tends to be low in most Illinois’ streams. In other words, during low flow conditions when groundwater discharge is the dominant source of water to streams, nitrate concentrations tends to be very low.

Full disclosure: I used to work with the lead scientist of the USGS study, Jim Tesoriero, at the Nuclear Regulatory Commission. Cool study, Jim!

A.J. Tesoriero et al., 2013. Vulnerability of Streams to Legacy Nitrate Sources. Environmental Science & Technology: 47(8):3623–3629.