Nitrapyrin, a nitrification inhibitor, 6-CPA, a nitrapyrin degradate, and three co-applied herbicides (acetochlor, atrazine, and metolachlor) were measured in water samples from seven streams across Iowa and Illinois. Iowa water samples were collected from March 2016 to June 2017, and Illinois water samples were collected from September 2016 to June 2017. The compounds studied are applied to corn, and Iowa and Illinois are the two largest corn producing states in the U.S. The seven stream sites are all adjacent to agricultural fields in corn production. Water samples (1-L) were collected in amber glass bottles using a grab or depth weighted approach. With the exception of one site, Iowa River at Wapello (USGS Site ID 05465500), all water samples were collected using a hydrologic based sampling approach. This means that samples were collected near peak flow during storm events and periodically during base flow conditions. Water samples were filtered (0.7 micrometers), extracted via solid phase extraction (Oasis® MAX cartridge), and analyzed (nitrapyrin and herbicides by gas chromatography with mass-spectrometry; 6-CPA by liquid chromatography tandem mass-spectrometry).