High As groundwater was found in Chaobai River aquifers of North China Plain (NCP) which poses risk to drinking water resource. Arsenic shows patchy distribution in both horizontal and vertical profiles with concentration of <0.1–304 μg L–1. The enrichment of As is related to reductive dissolution, supported by contradictory distribution of As and NO 3 − https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781351046633/8978195a-ffaa-498f-8e06-bee4769ddb72/content/eq19.tif"/> , negative correlation between As and SO4/Cl, and positive and then negative trend between As and Fe. DOM decomposition was evidenced by positive link between As and P. Four florescence components were identified in groundwater DOM including humic acid-like, fulvic acid-like and two kinds of protein-like substances. Arsenic concentration shows negative correlation with humic substances (HS) except for the sample with the highest As. It seems that HS may enhance As mobilization possibly via electron shuttle or complexation effect only when it is in high content. The river water which is characterized with high humic-like substances would infiltrate to supply more HS to groundwater and further promotes As mobilization.