高砷地下水中溶解性有机碳和无机碳稳定同位素特征

随着稳定同位素分析技术的逐步完善，碳稳定同位素被广泛应用于地球化学领域。高砷地下水砷的生物地球化学循环是目前环境化学研究热点之一。分析概括了碳稳定同位素应用于地下水领域的研究现状，介绍了地下水中有机碳和无机碳稳定同位素的前处理方法以及测试技术。在此基础上，选取了内蒙古河套平原具有代表性的高砷地下水进行氧化还原敏感组分、碳稳定同位素的测定与分析。结果表明，As分布极不均匀，其含量为1.24~387 μg/L。地下水溶解性有机碳(DOC)含量相对较高，与溶解性无机碳(DIC)浓度基本呈正相关。δ¹³C_DIC相对δ¹³C_DOC较富集¹³C；δ¹³C_DIC-δ¹³C_DOC与δ¹³C_DIC之间具有显著的正相关关系；表明δ¹³C_DIC值越贫化，δ¹³C_DIC-δ¹³C_DOC越小，地下水中来源于有机物氧化分解的无机碳越多，进一步说明有机碳的氧化分解在无机碳稳定同位素贫化过程中起主要作用。此外，δ¹³C_DIC-δ¹³C_DOC与As浓度呈一定的负相关关系；表明有机物的微生物降解对砷的富集具有明显的促进作用。微生物可利用的碳源增加，促进异养微生物的代谢，并消耗氧气，最终形成有利于地下水As富集的还原环境。

Stable Isotope Characteristics of Dissolved Organic Carbon and Inorganic Carbon in High Arsenic Groundwater

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.