Arsenic-contaminated groundwater poses a major environmental hazard and adversely affects the health of over 100 million people worldwide. Prolonged consumption and exposure leads to arsenicosis, which can result in skin lesions, impaired cognitive development, cancer and death. One current groundwater remediation technique involves injecting microbial nutrients (i.e. sulfate and carbon) into the subsurface of contamination sites to promote the formation of minerals (through microbial-induced reducing conditions) that either integrate arsenic into their matrix or adsorb arsenic onto their surface. However, studies have shown that post-treatment arsenic levels can increase beyond pre-treatment levels if the predominant removal mechanism is adsorption, and if sufficiently high levels of organic carbon are present to further reduce the adsorbing minerals, resulting in the re-mobilization (release) of arsenic. Therefore, it is important to determine the dominant removal process in order to develop effective long-term remediation techniques. Presently, we are working to determine the effectiveness of techniques involving induced microbial sulfate reduction through nutrient injections, which are currently being used for arsenic removal near Tacoma, WA. To simulate field conditions I pumped contaminated groundwater, extracted from the site, through sediment test columns to determine the arsenic retention capacity of the system. Once effluent and influent arsenic concentrations matched, we used the method of sequential chemicals extractions, along with μXRF and μXRS (microscale x-ray fluorescence and absorption, respectively) to determine the speciation and prevalence of the various arsenic-utilizing and -adsorbing minerals created. Next, we will pump uncontaminated groundwater through the columns to determine the post-treatment concentrations expected for this site due to re-mobilization. If our results show that the dominate removal mechanism is the incorporation arsenic into precipitates, preliminary treatment will be sufficient. However, if adsorption is the primary removal mechanism, a long-term treatment plan will need to be developed to minimize the remobilization of arsenic.