National Water-Quality Assessment (NAWQA) Program
Changes of nitrate concentrations were evaluated in groundwater samples collected from 1,225 wells in 56 well networks across the United States (Lindsey and Rupert, 2012). Of the 56 well networks that were analyzed for changes in nitrate concentrations using the Wilcoxon-Pratt signed-rank test (1959), 13 had statistically significant increases in concentrations of nitrate. Five well networks had statistically significant decreases in concentrations of nitrate.
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