See the Lil' Miss Atrazine project's Facebook page - they coordinated "citizen science" groups across the Mississippi River basin to take water samples on a single day, measuring levels of the widely-used herbicide atrazine, and reporting results via social media.
Blue pins mark "negative" tests, red pins "positive" - with "positive" meaning that it exceeds the USEPA threshold of 3 parts per billion atrazine.
There were 15 reported positive tests, 36 negative, out of 51 valid-looking tweets.
Sorry I don't have city names associated with the dots.