Figure 4. Validation of estimated Csw and Cdw (this study). (a) A linear relationship (red line with slope = 0.95 and R2 = 0.94) derived from estimations in this work (colored markers), compared to the linear relationship (black line, slope = 0.83, R2 = 0.70) from measurements (gray markers) compiled in Sudduth et al. (2013). The dotted gray circle at the top left corner is an outliner and was excluded from data fitting. The similar slope and correlation indicate that Cswestimated from stream chemistry approximates measured Csw. The Cstream from this study are flow-weighted concentrations calculated by dividing annual nitrate load by annual discharge. (b) Estimated Cdw from this work vs. measured groundwater concentration (as a proxy for Cdw) from the National Ground Water Monitoring Network. The measured groundwater concentrations were averaged for close by regions (Figure S5). Most symbols fall close to the 1:1 line, indicating estimated Cdw can be used to approximate groundwater concentration. Error bars are for one standard deviation in nearby regions.