Influence of Physical Variability of Highly Weathered Sedimentary Rock on Nitrate in Area 3 of the ENIGMA Field Research Site at Y-12

Uranium processing and waste storage in unlined waste ponds leached contaminants into the groundwater at Y-12, Oak Ridge, Tennessee, from the 1950s to 1980s. Groundwater wells near the S-3 ponds have had the highest nitrate concentrations of groundwater anywhere in the world (>10,000 mg/L). For reference, the maximum contaminant level for nitrate in drinking water set by the U.S. Environmental Protection Agency is 10 mg/L. Since 2012, the ENIGMA (Ecosystems and Networks Integrated with Genes and Molecular Assemblies) group has been characterizing, monitoring, and conducting field experiments to understand the interactions between contaminants, microbes, and the subsurface. The goals of this project are to measure the variability of physical and hydrogeological properties in the weathered sedimentary rock (i.e., saprolite), (ii) determine how physical properties control contaminant distribution, and (iii) assess nitrate and geochemical correlations, which could provide insight into nitrate transformation processes. Physical characteristics of the shallow subsurface materials were evaluated using cone penetrometer testing (CPT) in 131 locations. Profiles of material type and hydraulic conductivity were generated from the CPT data. Colloidal borescope measurements from ENIGMA wells confirmed groundwater flow is to the south, away from the S3 ponds (now a parking lot), to Bear Creek. Slug tests were performed in six wells to calculate hydraulic conductivity, which ranged from 0.06 (at well FW115-32) to 3.5 m/day (at well FW127). Nitrate concentrations and other geochemical parameters were measured from ten wells. Hydraulic conductivity values estimated by the CPT corresponded to hydraulic conductivity values measured by slug tests in nearby wells. The study concludes that there is lateral connectivity amongst neighboring soil types and that the distribution of soil types across Area 3 is highly variable. Saprolite above the interface with intact rock generally has higher hydraulic conductivity values compared to the shallower saprolite and nitrate concentrations do not appear to have a relationship with hydraulic conductivity. Regions of low nitrate concentrations may be due to flushing by flow through fractures and high concentrations of nitrate may be attributed to low hydraulic conductivity and groundwater being in a state of sulfate reduction or methanogenesis.