Like most big questions, the answer is complicated.  As highlighted in the State of the River Report, nitrates are one of the big contaminants affecting water quality here in the Twin Cities, and further downstream, creating a huge dead zone in the Gulf of Mexico the size of Connecticut and Rhode Island.

Even if we had perfect drainage tile and nutrient management, we would only see a 30% drop in nitrate levels.

Great explanation of how Crop Tile and Nitrates affect our environment.