Dissolved organic matter (DOM) plays critical roles in biogeochemistry of arsenic in groundwater systems. To characterize the spatial–temporal variations of groundwater DOM and its impacts on arsenic dynamics, surface water and shallow groundwater from two different depths were collected during monsoon and non-monsoon periods in Jianghan Plain, central Yangtze River Basin. Parallel factor analysis (PARAFAC) of the three-dimensional fluorescence excitation–emission matrix (EEM) reveals that DOM comprises three major components, one microbial humic-like component (C1) and two terrestrial humic-like components (C2 and C3). Abundant terrestrially-derived humic-like components were observed during monsoon while more microbially-derived DOM occurred in groundwater during non-monsoon season. These seasonal DOM variations are found to be related to groundwater-surface water (GW-SW) interactions, which is evidenced by stable H and O isotopes and Cl/Br mass ratios. During monsoon season, groundwater is recharged from dilute rainwater, evaporated surface water, and polluted surface water. More terrestrial humic-like components are introduced by precipitation and local agricultural activities (e.g. irrigation, paddy cultivation) during monsoon. The fluorescence spectroscopy and stable carbon isotopes (δ13CDIC and δ13CDOC) results indicate that seasonal As variations are affected by seasonal DOM dynamics. Smaller Δδ13C values (defined as δ13CDIC – δ13CDOC) with higher As concentrations are found during non-monsoon while larger Δδ13C values with relatively lower As concentrations are observed during monsoon season. The strong correlation between DOC and Fe(II) concentrations (r = 0.64, p < 0.01) and that between C2 maximum fluorescence intensity and As concentrations (r = 0.57, p < 0.01) clearly reflect the critical role of DOC in As mobilization. The seasonal variations of As concentrations are therefore correlated with biodegradation of labile organic matter, and terrestrially-derived high molecular weight humic-like components promote seasonal As mobilization via complexation reactions. These findings reveal the effects of seasonal DOM variations on As dynamics in shallow Quaternary alluvial aquifers and are applicable to similar geogenic As-affected Quaternary aquifers in floodplains and river delta regions of monsoon climate.