Each bar in figure 2 represents a risk group with the same color on the national map. Data for specific locations give examples of the difference in risk. Additionally, drains and ditches carry water off to streams rather than letting it seep to ground water. Ground-water nitrate data in some areas did not conform to risk patterns shown in figure 1 . The undulating, hilly landscape might be a factor.
Water quickly runs off the hills and collects in low-lying areas, where denitrification can occur.
Other factors not used to create the national map but which can affect nitrate concentration in ground water include land use, aquifer type, and rainfall and irrigation amounts. Where people live and the depth of their ground-water supply determine the quality of the water they drink. Nitrate contamination generally decreases with increasing depth to ground water. The water table in shallow wells is closer to the land surface and to potential sources of contamination, such as fertilizers and septic systems. Areas with the highest risk have high nitrogen input, well-drained soils, and less extensive forested areas relative to cropland.