Characteristics Of C Isotopes And Its Biogeochemistry Significance In High As Groundwater System In The Hetao Basin, Inner Mongolia

Biogeochemical cycle of groundwater arsenic is one of the hot topics in onenvironmental study. Microbially-mediated arsenic metabolism plays a key role in arsenicenrichment in high arsenic groundwater. The source of organic carbon is still controversial.With improved technology for stable isotope analysis, stable carbon isotope has extensivelybeen used in geochemical field. In this study, pretreating conditions of samples forinorganic and organic isotope were optimized. Groundwater major constituent, minorcomponent and isotopes were analyzed. Carbon isotopes of dissolved organic carbon (DIC)and dissolved inorganic carbon (DOC) were characterized. The source, migration andevolution of carbon isotope and its effect on arsenic enrichment were evaluated. Results notonly provide theoretical basis for formation mechanism of high arsenic groundwater, butalso help in ensuring drinking water safety in high arsenic area.New findings were obtained as follows:(1) The stability and accuracy of carbon isotope in DIC was optimized with pretreatingsamples in water bath at60℃for60min. The stability and accuracy of carbon isotope inDOC was optimized with pretreating samples in water bath at60℃for60min for sufficientremoval of inorganic carbon and then oxidizing organic carbon using sodium persulfide asoxidant under chromatographic column temperature of25℃.(2) δ13CDICvalues in groundwater DIC ranged between-5‰and-15‰, while δ13CDOCvalues in groundwater DOC between-22‰and-30‰. In comparison with dissolvedinorganic carbon, dissolved organic carbon had a depleted δ13C value.(3) Dissolved inorganic carbon was mainly derived from dissolution of carbonatiteduring water-rock interaction in the pediment region. δ13CDICvalues were gradually depletedin alluvial-proluvial plain area, which showed that contribution of exogenous carbonatedissolution gradually decreased and contribution of endogenous organic microbialdegradation gradually increased. (4)There was an increasing trend in δ13CDOCalong the groundwater flow path from thepediment to the alluvial-proluvial plain. δ13CDICwas significantly negatively correlated withδ13CDOC,which showed that effect of organic microbial degradation gradually increased alongthe flow path, with the increase in arsenic concentration.(5) Shallow groundwater δ13CDICand δ13CDOCwere depleted, which would be the resultof surface microbial or recharge of surface water. Groundwater δ13CDICand δ13CDOCshoweddecreasing trends with sampling depth. Contribution of microbial degradation to carbonisotope increased with the samping depth. In comparison with δ13CDOC, less variation inδ13CDICwith groundwater depth was observed.