Modeling the Potential for MNA of a Uranium Plume at a Fractured Rock Site

A pump and treat system to remove uranium-impacted groundwater has operated on the site for two years, reducing concentrations to less than or near action levels. An evaluation of potential uranium mobility after system shutdown was conducted using geochemical and contaminant transport modeling to evaluate MNA as a component of the long-term remedy at the site. Geochemical testing for a suite of parameters was conducted for wells upgradient, downgradient, and within the plume at a former manufacturing facility in the Mid-Atlantic region that used uranium in its operations.  The suite of laboratory-tested parameters include alkalinity, phosphate, chloride, sulfate, iron (total, ferric, and ferrous), calcium, potassium, magnesium, manganese, sodium, and uranium. Measured field parameters include pH, temperature, conductivity, dissolved oxygen, oxidation-reduction potential, and turbidity. Field and analytical data were input into PHREEQC to evaluate the speciation of uranium in groundwater.  Subsequent PHREEQC modeling then evaluated changes in speciation with plume movement, and with upgradient, groundwater flowing through the core of the plume. Results of PHREEQC modeling were applied to the construction and calibration of an MT3D model to evaluate long-term plume behavior, adequacy of the existing monitoring network, and potential time frames involved in plume migration. The modeling suggests that post-shutdown, the plume remains relative immobile, due to plume dependence on sorption.  Sensitivity analyses indicate that drastic changes in groundwater chemistry would be the only probable reason for equilibrium to change at the site.  Such changes are unlikely given the lack of use of groundwater for water supply or industrial purposes current site use and that monitored attenuation coupled with groundwater use restrictions over a limited area would be protective of human health and the environment.  The groundwater monitoring network is generally adequate for monitoring changes in plume dynamics.  Monitored natural attenuation is a potentially viable option for this site.