Farm in Southern Wisconsin (1 of 2) (image) US Geological Survey Share Print E-Mail Caption An irrigation system waters crops in southern Wisconsin. A new USGS report shows that drought periods followed by rainfall caused nitrate levels to increase to the highest ever measured in some Midwest streams during a 2013 study. Credit USGS Usage Restrictions Credit: USGS Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.