Many people are curious about Iowa stream nitrate before modern agriculture became established across the landscape. They want to know what the “natural” level of nitrate was. It turns out we do have some actual data. In 1955, the Iowa Geological Survey published a document titled “Water-Supply Bulletin No. 5, Quality of Surface Waters of Iowa, 1886-1954” (1). Some of these data are shown below (as nitrogen).
Confluence of the Des Moines (right) and Raccoon (left) Rivers
- Iowa River at Iowa City:
- 1906–1907: 0.63 mg/L
- 1944–1951: 2.48 mg/L
- Cedar River at Cedar Rapids:
- 1906–1907: 0.70 mg/L
- 1944–1951: 1.81 mg/L
- Raccoon River at Des Moines:
- 1945–1946: 2.71 mg/L
- Des Moines River at Keosauqua:
- 1906-1907: 0.75 mg/L
- Des Moines River at Des Moines:
- 1944–1945: 3.6 mg/L
Nitrate concentrations in these streams over the past 30 years or so average 5-8 mg/L, with the Raccoon averaging greater than 10 mg/L in the worst years, so roughly a 10-fold increase from a century ago and a 3- to 4-fold increase since 1950. Loads, or the total mass of nitrate transported by these rivers, have increased far more because stream discharge has increased 2-3 times across Iowa over the past century, and loads are the product of discharge and concentration.
So, what exactly happened?
Although commercial fertilizers (inorganic formulations of nitrogen such as anhydrous ammonia, nitrate salts, urea, urea ammonium nitrate (UAN), ammonium phosphates) were not widely used until the 1960s, the process of “breaking the prairie” and “opening up the land” for crop production, along with the first wave of tile drainage, did release nitrogen into […]