Abstract:

With improved technology for stable isotope analysis, stable carbon isotope has extensively been used in the field of geochemical study. Biogeochemical cycle of groundwater arsenic is one of the hot topics in environmental studies. This paper summarizes research status of stable carbon isotope in groundwater, and introduces pretreating methods and measuring technology of organic and inorganic carbon isotope. Representative groundwater samples with high As concentrations were collected from the Hetao basin, Inner Mongolia, and analyzed for redoxsensitive components, stable carbon isotope in dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). Results indicate that As distribution is patchy with concentrations between 1.24 and 387 μg/L, and that DOC concentration is relatively high and positively correlated with DIC. In comparison with DOC, DIC is enriched in ¹³C. There is a positive correlation between δ¹³C_DIC-δ¹³C_DOC and δ¹³C_DIC. It indicates that groundwater with low  δ¹³C_DIC and low  δ¹³C_DIC-δ¹³C_DOC values has high contribution of DOC to DIC, which suggests that degradation of organic carbon results in depletion of ¹³C in DIC of groundwater. In addition, there is a negative correlation between  δ¹³C_DIC-δ¹³C_DOC and As. In groundwater with low  δ¹³C_DICand low  δ¹³C_DIC-δ¹³C_DOC values, high As concentrations are observed. High As groundwater experiences intensive biodegradation of organic carbon, and microbial degradation of organic matters plays a significant role in promoting the enrichment of As in groundwater. The increase in bioavailable organic carbon promotes microbial dissimilation, consumes dissolved oxygen, and leads to formation of reducing conditions favorable for As enrichment in groundwater.

Key words: dissolved organic carbon, dissolved inorganic carbon, stable carbon isotope, groundwater, arsenic