The role of aquifer microorganisms in controlling arsenic contamination of Bangladesh aquifers via oxidation of organic carbon coupled with reduction of sedimentary iron oxyhydroxides and concomitant arsenic dissolution is generally accepted. What remains to be ascertained is the in situ biogeochemical mechanisms of cycling different carbon sources and directly relating indigenous microbiota to arsenic release. Using biomarker fingerprint approaches, this dissertation expanded the presently growing research in the biogeochemical carbon cycling controlling arsenic contamination in Bangladesh aquifers. Comprehensive profiles of microbial cell membrane components (PLFA and sterols) at three different aquifers tested the regional distribution of aquifer microbial community abundance, structure, and organic input potential across Araihazar. The highly variable bulk viable microbial biomass observed across these three sites confer both regional-scale and localized heterogeneous distributions of in-aquifer microbial communities which control carbon cycling in the aquifer. The lack of correlation between PLFA biomarkers and dissolved arsenic challenges the assumption that greater extent of microbial community metabolism results in an increase in arsenic in groundwater. Natural abundance radiocarbon isotope Δ14C analysis of cell membrane PLFA and available carbon pools (SOC, DOC, DIC) confirmed that young organic carbon substrates are being cycled at two of the three sites investigated here. This corroborates previous reports at nearby sites (Site B and F) thereby contributing to a well-constrained carbon source which actively support microbial metabolism over a regional scale. Sterol biomarker distributions were characterized to determine potential sources of organic input into the aquifer. In particular, the importance of raw human and/or animal sewage waste as a source of labile carbon was assessed by measuring the faecal biomarker Coprostanol and comparing its abundance to other sources of biogenic sterols using sewage input proxies (Sewage Contamination Index, Coprostanol/Cholesterol ratio). This was motivated by previous findings which correlated sewage contamination with dissolved arsenic at depth at nearby sites. While sewage contamination was low in the shallow aquifers at these sites, it is more likely that plant organic matter supported the elevated microbial abundance at shallow depths. On the other hand, evidence presented in this project suggests that sewage contamination intrudes into deeper aquifers (e.g. buried Pleistocene) and contributes to the vulnerability of previous pristine aquifers to future arsenic contamination. / Thesis / Master of Science (MSc)