江汉平原高砷地下水稳定碳同位素特征及其指示意义

Characteristics of stable carbon isotopes and its implications on arsenic enrichment in shallow groundwater of the Jianghan Plain

溶解性有机碳(DOC)是地下水中砷释放过程的关键因素,为查明江汉平原高砷地下水稳定碳同位素特征,识别有机质的降解过程对砷富集的影响,采用稳定碳同位素分析测试技术并结合地下水化学特征,对江汉平原典型砷中毒病区的浅层地下水进行了区域采样分析。结果表明:浅层承压水的砷质量浓度为0.23~2621μg/L。地表水较地下水具有更负的δ^13 C DOC、δ^13 C DIC值。地下水中溶解性无机碳(DIC)的δ^13 C DIC值在-11.9‰^-3.99‰之间,溶解性有机碳的δ13 C DOC值在-28.5‰^-19.6‰之间。地下水的δ^13 C DIC-δ^13 C DOC差值与ρ(As)呈一定负相关关系,表明微生物作用下有机质的降解促进了As的富集。δ^13 C DIC-δ^13 C DOC差值与δ^13 C DIC和ρ(DOC)均具有较显著的正相关关系,表明地下水中有机质的氧化分解是导致δ13 C DIC贫化的重要过程,微生物作用下溶解性有机质的降解是地下水中无机碳的重要来源。此外,江汉平原少数高砷地下水呈现较大的δ^13 C DIC值,推断江汉平原高砷含水层强还原环境下可能存在的产甲烷过程导致了明显的碳同位素分馏。

Dissolved organic matter of groundwater is one of the most important factors controlling arsenic release.In order to elaborate the characteristics of stable carbon isotopes and the effects of degradation of organic matter on arsenic enrichment in groundwater,shallow groundwater samples were collected using hydrochemistry and stable carbon isotope analysis in typical arsenicosis areas of the Jianghan Plain.Results indicate that the concentration of As in the shallow groundwater range in 0.23-2621μg/L.Surface water has lower values ofδ^13 C DOC andδ^13 C DIC compared with groundwater.The value ofδ^13 C DIC ranges from-11.9‰to 3.99‰and the values ofδ^13 C DOC ranges from-28.5‰to-19.6‰in groundwater.There is a negative correlation betweenδ^13 C DIC-δ^13 C DOC and As concentration.It indicates that the degradation of organic matter promotes the enrichment of As.There is a positive correlation betweenδ^13 C DIC-δ^13 C DOC andδ^13 C DIC also betweenδ^13 C DIC-δ^13 C DOC and DOC concentration.It indicates that the microbially involving oxidative decomposition of organic carbon in groundwater leads to the fractionation of carbon isotopes,and the degradation of dissolved organic matter is the main source of inorganic carbon in groundwater.Moreover,some groundwater samples have highδ^13 C DIC values in the Jianghan Plain,which is significantly higher than other typical arsenic affected areas(Hetao Plain and Datong Basin).It is suggested that methanogenic process occurred in arsenic affected aquifer of the Jianghan Plain,which results in significant carbon isotope fractionation.