Spence Michael J, Bottrell Simon H, Thornton Steven F, Richnow Hans H, Spence Keith H
Groundwater Protection and Restoration Group, Department of Civil and Structural Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK.
J Contam Hydrol. 2005 Sep;79(1-2):67-88. doi: 10.1016/j.jconhyd.2005.06.003.
Hydrochemical data, compound specific carbon isotope analysis and isotopic enrichment trends in dissolved hydrocarbons and residual electron acceptors have been used to deduce BTEX and MTBE degradation pathways in a fractured chalk aquifer. BTEX compounds are mineralised sequentially within specific redox environments, with changes in electron acceptor utilisation being defined by the exhaustion of specific BTEX components. A zone of oxygen and nitrate exhaustion extends approximately 100 m downstream from the plume source, with residual sulphate, toluene, ethylbenzene and xylene. Within this zone complete removal of the TEX components occurs by bacterial sulphate reduction, with sulphur and oxygen isotopic enrichment of residual sulphate (epsilon(s) = -14.4 per thousand to -16.0 per thousand). Towards the plume margins and at greater distance along the plume flow path nitrate concentrations increase with delta15N values of up to +40 per thousand indicating extensive denitrification. Benzene and MTBE persist into the denitrification zone, with carbon isotope enrichment of benzene indicating biodegradation along the flow path. A Rayleigh kinetic isotope enrichment model for 13C-enrichment of residual benzene gives an apparent epsilon value of -0.66 per thousand. MTBE shows no significant isotopic enrichment (delta13C = -29.3 per thousand to -30.7 per thousand) and is isotopically similar to a refinery sample (delta13C = -30.1 per thousand). No significant isotopic variation in dissolved MTBE implies that either the magnitude of any biodegradation-induced isotopic fractionation is small, or that relatively little degradation has taken place in the presence of BTEX hydrocarbons. It is possible, however, that MTBE degradation occurs under aerobic conditions in the absence of BTEX since no groundwater samples were taken with co-existing MTBE and oxygen. Low benzene delta13C values are correlated with high sulphate delta34S, indicating that little benzene degradation has occurred in the sulphate reduction zone. Benzene degradation may be associated with denitrification since increased benzene delta13C is associated with increased delta15N in residual nitrate. Re-supply of electron acceptors by diffusion from the matrix into fractures and dispersive mixing is an important constraint on degradation rates and natural attenuation capacity in this dual-porosity aquifer.
利用水化学数据、化合物特定碳同位素分析以及溶解烃和残留电子受体中的同位素富集趋势，推断了裂隙白垩含水层中BTEX和MTBE的降解途径。BTEX化合物在特定的氧化还原环境中依次矿化，电子受体利用的变化由特定BTEX组分的耗尽来定义。氧气和硝酸盐耗尽区从羽流源向下游延伸约100米，伴有残留的硫酸盐、甲苯、乙苯和二甲苯。在该区域内，TEX组分通过细菌硫酸盐还原被完全去除，残留硫酸盐的硫和氧同位素富集（ε(s)=-14.4‰至-16.0‰）。在羽流边缘以及沿羽流流动路径更远的距离处，硝酸盐浓度随着δ15N值高达+40‰而增加，表明发生了广泛的反硝化作用。苯和MTBE持续存在于反硝化区，苯的碳同位素富集表明沿流动路径发生了生物降解。残留苯的13C富集的瑞利动力学同位素富集模型给出的表观ε值为-0.66‰。MTBE没有明显的同位素富集（δ13C=-29.3‰至-30.7‰），并且在同位素上与炼油厂样品相似（δ13C=-30.1‰）。溶解MTBE中没有明显的同位素变化意味着，要么任何生物降解引起的同位素分馏幅度很小，要么在存在BTEX烃的情况下发生的降解相对较少。然而，由于没有采集到同时存在MTBE和氧气的地下水样品，MTBE有可能在没有BTEX的有氧条件下发生降解。低苯δ13C值与高硫酸盐δ34S相关，表明在硫酸盐还原区几乎没有苯降解发生。苯降解可能与反硝化作用有关，因为苯δ13C的增加与残留硝酸盐中δ15N的增加相关。通过从基质扩散到裂隙以及分散混合来重新供应电子受体，是该双孔隙含水层中降解速率和自然衰减能力的一个重要限制因素。
