A modeling study of geochemical interactions at the Sheffield, Illinois low-level radioactive waste disposal site

This report summarizes an attempt to model the geochemical behavior of leachate migration from the Sheffield, Illinois low-level radioactive waste disposal site. The reaction path code PHREEQE was used to simulate mixing between groundwater altered by trench leachate and ambient groundwater. The thermodynamic speciation code WATEQF was used to calculate the aqueous speciation and mineral saturation indices of the pertinent waters. The mass balance code BALANCE was used to predict the probable chemical reactions occurring as groundwater migrates. Results suggest that the most important factor influencing water quality is dilution. Chemical reactions that may also be affecting water chemistry are ion exchange, calcite precipitation, iron sulfide mineral dissolution, and exsolution of CO₂ Limitations of the modeling include lack of consideration of reaction kinetics under surficial conditions, uncertainties in flowpaths, and limited reliability of thermodynamic data. The study suggests that anionic and gaseous radionuclides have the greatest potential for migration away from the site.