A groundwater plume beneath a capped landfill in north-central Massachusetts contains dissolved arsenic concentrations exceeding 10,000 ppb at several locations. The landfill closed in the early 1990's and contains minimally documented solid waste materials deposited over the course of nearly a century. The source(s), fate, and transport of arsenic in the landfill aquifer have been studied extensively over the past decade; however, the source and pathways of arsenic are not yet fully defined. The primary source of arsenic likely involves a combination of the landfill waste material, the peat, the underlying overburden sequence, and/or bedrock minerals. Arsenic mobilization is most likely assisted by reducing conditions created by the decomposition of organic materials within the landfill and underlying peat present prior to the initiation of waste disposal. Another possibility is an arsenic-bearing groundwater discharging from the underlying bedrock from the oxidation of naturally occurring sulfides. Aqueous arsenic species, including inorganic arsenite [As(III), As(OH)3] and arsenate [As(V), AsO(OH)3], and organic monomethylarsonic acid [MMA(V), CH3AsO(OH)2] and dimethylarsinic acid [DMA(V), (CH3)2AsO(OH)], provide information as to where the arsenic is primarily originating from and how it is transported through the aquifer. Furthermore, the analysis of major ions, metals, and groundwater parameters from different zones of the landfill with varying arsenic concentrations will aid in the delineation of probable arsenic sources, the mobilization processes, and arsenic transport modes within the aquifer. A more complete conceptual site model with respect to arsenic speciation and groundwater chemistry will lead to a better understanding of geochemical processes within and beneath the landfill waste pile and also assist with future remediation of the aquifer. Using arsenic speciation and groundwater chemistry data, it was determined that although all four potential arsenic sources likely contribute to the total arsenic concentrations, the overall contribution from the landfill material, peat layer, and bedrock is minimal relative to the iron-oxyhydroxides coated on the sands particles throughout the aquifer which acts as the primary arsenic source. Oxidation-reduction potential and dissolved oxygen are the controlling factors in relation to mobilization and transport of arsenic species from aquifer features and an understanding of these processes at the local level can be further applied to global-scale arsenic contamination.