Figure S5.Validation of estimated deep groundwater nitrate concentration (Cdw) against field measurements from the National Ground-Water Monitoring Network. Within each red circle, estimated and measured Cdw are first averaged and then compared (results see Figure 4b). Red circles are determined by the proximity of location and data availability (i.e., at least one estimated Cgw point for comparison with measurements). Iowa and Florida have more direct comparison because there are more estimated Cgw points (triangle markers) located in these regions and more measured Cgw points (circle markers) in proximity.