Supplemental description of methods and additional tables and figures. Table S1 Number of systems with discrepancies between demographics characterized using SDWIS’s City served andZip code fields. Table S2 Spearman correlation matrix for county-level predictors, all systems. Table S3 Spearman correlation matrix for county-level predictors, Midwest systems. Table S4 Spearman correlation matrix for county-level predictors, Northeast systems. Table S5 Spearman correlation matrix for county-level predictors, South systems. Table S6 Spearman correlation matrix for county-level predictors, West systems. Table S7 Unadjusted univariate regression model results using nationwide data and county-level demographics. Figure S1 Number of community water systems compiled from SDWIS and subsequently used for regression analyses. Figure S2 Percent of CWSs with mean nitrate ≥5 mg/L by quartile for land use and demographic variables. (PDF 13,659 kb)