The United States Geological Survey monitors the quality of ground water in the United States as part of their National Water Quality Assessment Program. They have prepared the map above to represent the risk of nitrate contamination in shallow ground water. Areas show in red have the highest risk of nitrate contamination. Three risk factors were used to make this map 1) the level of nitrogen input in the area (typically from fertilizer, livestock waste or high human population density), 2) how well drained the soils are, and, 3) how much land area is devoted to cropland.
A report that includes this map can be seen at the USGS National Water Quality Assessment Website.