|Wall Street Journal chart, from USDA and USGS data|
Both produce nitrates, which rose significantly in 21 percent of the regions where the U.S. Geological Survey "tested groundwater from 2002 through 2012, compared with the 13 prior years," Jesse Newman and Patrick McGroarty report. "The greatest increases were in agricultural areas. More recent sampling shows the pattern is continuing, at a potentially greater rate."
The reporters add, "More than 16 percent of groundwater from wells sampled between 2002 and 2012 topped the federal nitrate limit of 10 parts per million, versus 12 percent in the 1990s. The percentage above the limit fell slightly in the wells sampled after 2013 but remained elevated."