Thesis advisor: Rudolph Hon / Groundwater in the Shepley's Hill Landfill (SHL) area had a high arsenic concentration for at least 20 years. This study is aimed at understanding the migration pathways of arsenic in the SHL area and postulating the possible sources and mechanisms for the mobilization of arsenic. A direct-push sampling technique (DPT) was applied in the summer of 2010 within the SHL and its downgradient area, which provided groundwater samples from various depths and locations. A long-term monitoring dataset from 1998 to 2009 was utilized as a subsidiary source for temporal analysis. Spatial distributions of arsenic and other major dissolved compositions were analyzed. Extremely high arsenic concentrations (up to ~ 15000 µg/L) were detected in the deep glacial sand deposits close to a peat layer within the SHL. Arsenic concentrations decrease dramatically in the downgradient area north of the SHL. The transport of arsenic in the SHL area is similar to that of iron. The source of arsenic is likely to been within the boundary of the SHL. The glacial sand overburden within the SHL provides enough source for the arsenic mobilization. A possible mechanism of arsenic mobilization in the SHL area is that the reductive dissolution/desorption of arsenic from iron bearing minerals under a lasting reducing environment created by decompositions of organic matter in waste and peat. / Thesis (MS) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.