Exploratory Data Analysis of the Groundwater Contaminants at the Former Panna Maria Uranium Recovery Facility

The former Panna Maria mill was a uranium recovery facility that operated from 1979 to December 1992. Sulfuric acid leach was used to process several tons of ore daily. Therefore, the groundwater was sampled at the site to monitor the concentration of background chemical pollutants. The purpose of this study was to perform exploratory data analysis to evaluate whether the arsenic, nickel, selenium, sulfate, and uranium concentrations were in accord with state-regulated groundwater drinking standards at the site. To accomplish this, a substitution method was used to replace concentration values that were below measurable ranges of well monitoring units. Additionally, random forest regression was employed to account for missing not-at-random values in the dataset. Groundwater samples collected in wells before the facility began operations were used to determine an upper bound on background pollutants consistent with US Environmental Protection Agency Standards. The upper tolerance limit comprising 90% of the groundwater sample at a confidence coefficient of 95% was used to establish alternate concentration limits for each chemical based on well data from the National Uranium Resource Hydrogeochemical and Stream Sediment Reconnaissance program. Results obtained in this analysis establish a baseline on the chemical concentrations in the background groundwater at the former Panna Maria mill.