Stable Carbon Isotopes In Dissolved Organic Matter With Different Molecular Weights In High Arsenic Groundwater From The Hetao Basin, Inner Mongolia

Formation and mechanisms of high arsenic groundwater are the hot topics in the field of high arsenic groundwater study. The colloid and stable carbon isotope in high arsenic groundwater are studied, but the combination of organic matter and stable carbon isotope analysis with different molecular weight is lacking. Based on ultrafiltration of groundwater and the advanced technology of carbon isotope analysis, the role of organic matter in arsenic enrichment and migration process was investigated. The following conclusions have been obtained.(1) Groundwaters were of Na-HCO3 and Na-Cl types, with the ranges of pH from 7.35 to 8.80,and ORP from-234 to 45.4mV. High As groundwater had high concentrations of Fe, and Mn. As(III)was the major form of As species in groundwater samples.(2) Dissolved inorganic carbon was mainly derived from carbonate dissolution and degradation of dissolved organic carbon, while dissolved organic carbon was mainly derived from endogenous organic matter. The δ13CDIC values of dissolved inorganic carbon lay in the range of-1.65‰ to-23.61‰. The δ13CDOC values of dissolved organic carbon were in the range of-14.28‰ to-25.4‰.(3) Inorganic colloids containing Fe and Mn and organic colloids were the major colloids in groundwaters. Inorganic colloids mostly had the molecular size >30 kDa, while the organic colloids had the molecular sizes between 30 kDa and 5kDa. Groundwater As and organic matter mostly existed as truely dissolved forms.(4) Arsenic-containing colloids were categorized into four types.(1)Fe-inorganic colloids above30 kDa, accounting for 13% samples;(2)Organic colloids between 30kDa~5kDa, accounting for13% samples;(3) Fe-inorganic–organic colloids, accounting for 20%, and forming ternary complexes.(4) Both Fe-inorganic colloids above 30 kDa and Organic colloids between 30kDa~5kDa,accounting for 27% samples; In the remaining water samples, As was completely dissolved,accounting for 27%.(5) During the sequential ultrafiltration of groundwater, DOC concentration gradually decreased with the increase of δ13CDOC values. It indicated that microorganisms preferentially degraded the low molecular weight organic colloids and used the lighter 12 C as the carbon sources. Organic matter degradation causesd groundwater environment tend to reduce, then Fe, Mn oxide/hydroxide minerals reductive dissolution, adsorped arsenic release.