Fig. 1: The spatial variation of nitrate concentrations in 4047 groundwater sites in China.

a The upper and right subgraphs show the counts of longitude and latitude distributions of the sample sites, respectively. The red bar chart above the map shows the number of sampling points in different years. b Mean concentrations of nitrate in eight economic regions: North Coast (NC), Middle of Yellow River (MYR), North East (NE), South West (SW), East Coast (EC), North West (NW), Middle of Changjiang River (MCR), and South Coast (SC). c The relative importance of influential drivers for groundwater nitrate concentrations was determined by regression forecasting using random forests. The columns marked * (P < 0.05) and ** (P < 0.01) indicate that drivers have significant impacts on nitrate concentrations, while ns (P > 0.05) indicates that drivers have no significant differences in nitrate dynamics. d, e Nitrate concentrations under different land use types and population densities, respectively (classification methods in “Materials and Methods”). The box plots include three modules: box limits, upper and lower quartiles; small cube, average value; center line, and median.