Abstract: Dissolved organic matter (DOM) controls the mobilization of arsenic (As) in several ways. Although higher As concentrations were observed in confined vs. unconfined aquifers in the Guide Basin, the roles of DOM in As mobilization are unclear. In order to evaluate DOM influence on As mobilization in confined aquifer under reducing environment in the Guide Basin, we conducted excitation-emission-matrix spectroscopic (EEMs) analysis on surface water and groundwater samples and used parallel factor analysis (PARAFAC) to determine DOM properties. The PARAFAC model identified four components in the Guide Basin and they are all humics: C1 is terrestrial humic-like; C2 is humics and affected by agriculture; C3 is semiquinone-like; and C4 is microbial humic. C1 is highly enriched in groundwater at about 40%55%. Compared to groundwater, surface water has higher C2 and C3 contents. Meanwhile, high As confined aquifer is characterized by higher percentage of C2 and C3. The occurrence of high As groundwater in confined aquifer shows that C1 can form complex with As to increase aqueous As concentration; and C3 serves as an electron shuttle to enhance microbial reduction of iron oxides and As mobilization. The HCO-3 anion generated from organic matter degradation causes Fe(II) precipitation to form FeCO3 and adsorbs some of arsenic. These results demonstrate that DOM can lead to As accumulation in groundwater by complexation or by serving as electron shuttle to promote reductive dissolution of iron oxides. This finding provides an theoretical explanation for the formation of high arsenic groundwater in the upper reaches of the Yellow River.

Key words: Guide Basin, dissolved organic matter, high arsenic groundwater, reducing environment, excitation-emission-matrix spectra, parallel factor analysis