Effects of bioremediation on uranium and arsenic geochemistry at a former uranium mill site

Abstract

Identifying effective strategies of groundwater uranium remediation has been a focus of research at the former uranium and vanadium mill site in Rifle, Colorado. Monitored natural attenuation, bicarbonate flushing and biostimulation techniques have been investigated for aqueous uranium removal and impacts to other groundwater contaminants. Biostimulation, which is based on adding a carbon substrate to promote microbial growth and the reduction of soluble U(VI) to immobile U(IV), has been the primary focus at this site over the past decade. Passive flux meters (PFMs) were developed using anion exchange resin to sorb uranium from the groundwater as it flows through the PFM. From the fraction of tracer removed and the amount of uranium sorbed, these PFMs can be used to determine uranium and groundwater fluxes using the same sample. Accurate flux measurements were made using this improved technique, and results showed lower uranium fluxes in reduced zones. While reducing conditions are favorable for uranium immobilization, this biostimulation has been shown to increase arsenic concentrations substantially. The strongest arsenic release has been observed during sulfate reduction with and concomitant sulfide generation. Speciation studies, using ion chromatography coupled to inductively coupled plasma mass spectrometry (IC-ICP-MS), were conducted to understand this arsenic release. Highly mobile thioarsenates, specifically trithioarsenate, comprised the majority of the increased arsenic concentration. However, post sampling conversion from thioarsenites is possible. Laboratory mechanistic experiments revealed that arsenite and sulfide react in the groundwater to form thioarsenite, which can then oxidize to thioarsenate. Although accurate identification of the oxidation state remains a challenge using the current IC-ICP-MS methods, improvements have been made. Bicarbonate flushing was shown to remove uranium without the large increase in arsenic concentrations. If biostimulation is chosen for uranium remediation, care will need to be taken to minimize the arsenic release during sulfate reduction.