High Arsenic (As) in the shallow groundwater (GW) of the Southern Western Bangladesh has been recognized as the natural catastrophe since 1990s. This paper aims to unveil the likely geochemical phenomena that may dominate in both release and transport of As in that shallow alluvial aquifer. Observed linear relationships between As and Iron (Fe) contents both in sediment (R2 = 0.42) and in GW (R = 0.46) reflect their probable co-existence and co-precipitation characteristics. Speciation analysis with geochemical code PHREEQC shows that some Fe minerals have already been reached at the super saturation state, indicating further the subsequent co-precipitation of the accompanying As. Linear Equilibrium partition coefficient Kd (As) was noticed to vary inversely with dissolved Fe in a relatively high pH, reflecting the leaching potentiality of As from sediment to groundwater, due to the loss of promising sorption sites. HCO3 was reportedly found as the efficient leaching (24.5 μg/l) agent in releasing adsorbed As from the aquifer sediment.