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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.
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