Pesticides in groundwater of the United States: decadal-scale changes, 1993-2011.

The national occurrence of 83 pesticide compounds in groundwater of the United States and decadal-scale changes in concentrations for 35 compounds were assessed for the 20-year period from 1993-2011. Samples were collected from 1271 wells in 58 nationally distributed well networks. Networks consisted of shallow (mostly monitoring) wells in agricultural and urban land-use areas and deeper (mostly domestic and public supply) wells in major aquifers in mixed land-use areas.

Nitrate in groundwater of the United States, 1991-2003.

An assessment of nitrate concentrations in groundwater in the United States indicates that concentrations are highest in shallow, oxic groundwater beneath areas with high N inputs. During 1991-2003, 5101 wells were sampled in 51 study areas throughout the U.S.

Decadal-scale changes of nitrate in ground water of the United States, 1988-2004.

This study evaluated decadal-scale changes of nitrate concentrations in ground water samples collected by the USGS National Water-Quality Assessment Program from 495 wells in 24 well networks across the USA in predominantly agricultural areas. Each well network was sampled once during 1988-1995 and resampled once during 2000-2004. Statistical tests of decadal-scale changes of nitrate concentrations in water from all 495 wells combined indicate there is a significant increase in nitrate concentrations in the data set as a whole.