Iron and Manganese in Groundwater: Using Kriging and GIS to Locate High Concentrations in Buncombe County, North Carolina by Crystal D. Johnson 1 , Arpita Nandi 2 , T. Andrew Joyner 1 , and Ingrid Luffman 1 Abstract For health, economic, and aesthetic reasons, allowable concentrations (as suggested by the United States Environmental Protection Agency) of the secondary contaminants iron (Fe) and manganese (Mn) found present in drinking water are 0.3 and 0.05 mg/L, respectively. Water samples taken from private drinking wells in rural communities within Buncombe County, North Carolina contain concentrations of these metals that exceed secondary water quality criteria. This study predicted the spatial distribution of Fe and Mn in the county, and evaluated the effect of site environmental factors (bedrock geology, ground elevation, saprolite thickness, and drinking water well depth) in controlling the variability of Fe and Mn in groundwater. A statistically significant correlation between Fe and Mn concentrations, attributable to bedrock geology, was identified. Prediction models were created using ordinary kriging and cokriging interpolation techniques to estimate the presence of Fe and Mn in groundwater where direct measurements are not possible. This same procedure can be used to estimate the trend of other contaminants in the groundwater in different areas with similar hydrogeological settings. Introduction About 14% (42 million people) of the United States (U.S.) population relies on unregulated private domestic water wells (Kenny et al. 2009). An estimated 2.3 million individuals in North Carolina (N.C.) use private wells for drinking water, the fourth largest state population in the U.S. that relies on private well water for drinking (Kenny et al. 2009; Sanders et al. 2012). In N.C., wells are predominately situated in rural areas where residences are located outside municipal water supply infrastructure. The large number of private wells in N.C. that contain elevated amounts of Fe and Mn raises concerns about the factors responsible for the contamination in the wells (Herman 1996). Both Fe and Mn are listed under the United States Environmental Protection Agency (USEPA) National Secondary Drinking Water Regulations as secondary contaminants in drinking 1 Department of Geosciences, College of Arts and Sciences, East Tennessee State University, Johnson City, TN 37614. 2 Corresponding author: Department of Geosciences, College of Arts and Sciences, East Tennessee State University, Johnson City, TN 37614; nandi@etsu.edu Article impact statement : Kriging interpolation to predict iron and manganese in groundwater in rural United States, can be used by agencies for groundwater well planning. Received December 2016, accepted June 2017.  2017, National Ground Water Association. doi: 10.1111/gwat.12560 water at an allowable concentration of 0.3 mg/L for Fe and 0.05 mg/L for Mn (USEPA 2017). Neurotoxic effects from greater exposure to Fe and Mn via drinking water were studied by several researchers (Woolf et al. 2002; Bouchard et al. 2007; Farina et al. 2013). Spangler and Spangler (2009) and Spangler and Reid (2010) examined groundwater Mn concentrations in well water in N.C. and found associations with increased infant and cancer-related mortality rates. Polizzotto et al. (2013) noted high levels of Mn concentrations in the N.C. Piedmont area associated with bedrock weathering and groundwater redox conditions; however, they realized the need for geospatial analyses to better understand the causes of spatial variability in well water concentrations. Giese et al. (1986) related elevated concentrations of Fe and Mn in groundwater with crystalline aquifers of North Carolina. Specifically, evidence of high concentrations of Fe and Mn in groundwater in the crystalline aquifers of mountainous regions of western N.C. was documented by Campbell (2013). Campbell (2013) analyzed 63 private wells and four springhouses in Haywood County, N.C. for water quality; Fe and Mn concentrations were found to be elevated, although the sources of the elevated concentrations were not identified conclusively. Several private wells in North Carolina indicated elevated Fe and Mn concentrations, consequently the N.C. Department of Health and Human Services (NCD- HHS) in conjunction with the N.C. Division of Public NGWA.org Groundwater 1