Jardine P M, Mehlhorn T L, Larsen I L, Bailey W B, Brooks S C, Roh Y, Gwo J P
Environmental Science Division, Oak Ridge National Laboratory, TN 37831-6038, USA.
J Contam Hydrol. 2002 Mar;55(1-2):137-59. doi: 10.1016/s0169-7722(01)00190-5.
Field-scale processes governing the transport of chelated radionuclides in groundwater remain conceptually unclear for highly structured, heterogeneous environments. The objectives of this research were to provide an improved understanding and predictive capability of the hydrological and geochemical mechanisms that control the transport behavior of chelated radionuclides and metals in anoxic subsurface environments that are complicated by fracture flow and matrix diffusion. Our approach involved a long-term, steady-state natural gradient field experiment where nonreactive Br- and reactive 57Co(II)EDTA2- 109CdEDTA2-, and 51Cr(VI) were injected into a fracture zone of a contaminated fractured shale bedrock. The spatial and temporal distribution of the tracer and solutes was monitored for 500 days using an array of groundwater sampling wells instrumented within the fast-flowing fracture regime and a slower flowing matrix regime. The tracers were preferentially transported along strike-parallel fractures coupled with the slow diffusion of significant tracer mass into the bedrock matrix. The chelated radionuclides and metals were significantly retarded by the solid phase with the mechanisms of retardation largely due to redox reactions and sorption coupled with mineral-induced chelate-radionuclide dissociation. The formation of significant Fe(III)EDTA byproduct that accompanied the dissociation of the radionuclide-chelate complexes was believed to be the result of surface interactions with biotite which was the only Fe(III)-bearing mineral phase present in these Fe-reducing environments. These results counter current conceptual models that suggest chelated contaminants move conservatively through Fe-reducing environments since they are devoid of Fe-oxyhydroxides that are known to aggressively compete for chelates in oxic regimes. Modeling results further demonstrated that chelate-radionuclide dissociation reactions were most prevalent along fractures where accelerated weathering processes are expected to expose more primary minerals than the surrounding rock matrix. The findings of this study suggest that physical retardation mechanisms (i.e. diffusion) are dominant within the matrix regime, whereas geochemical retardation mechanisms are dominant within the fracture regime.
对于高度结构化的非均质环境，控制地下水中螯合放射性核素运移的田间尺度过程在概念上仍不明确。本研究的目的是更好地理解和预测水文和地球化学机制，这些机制控制着螯合放射性核素和金属在缺氧地下环境中的运移行为，该环境因裂隙流和基质扩散而变得复杂。我们的方法涉及一项长期的稳态自然梯度场实验，将非反应性的Br-以及反应性的57Co(II)EDTA2-、109CdEDTA2-和51Cr(VI)注入受污染的裂隙页岩基岩的裂隙带中。使用一系列地下水采样井监测示踪剂和溶质的时空分布，这些采样井分布在快速流动的裂隙区域和流动较慢的基质区域内，监测时间长达500天。示踪剂优先沿走向平行的裂隙运移，同时有大量示踪剂质量缓慢扩散到基岩基质中。螯合放射性核素和金属被固相显著阻滞，阻滞机制主要归因于氧化还原反应和吸附，以及矿物诱导的螯合物 - 放射性核素解离。伴随着放射性核素 - 螯合物络合物解离而形成的大量Fe(III)EDTA副产物，被认为是与黑云母表面相互作用的结果，黑云母是这些铁还原环境中唯一含Fe(III)的矿物相。这些结果与当前的概念模型相悖，当前模型认为螯合污染物在铁还原环境中保守运移，因为其中缺乏已知在氧化环境中会强烈竞争螯合物的铁羟基氧化物。模拟结果进一步表明，螯合物 - 放射性核素解离反应在裂隙中最为普遍，在裂隙中加速的风化过程预计会比周围岩石基质暴露更多原生矿物。本研究结果表明，物理阻滞机制（即扩散）在基质区域占主导，而地球化学阻滞机制在裂隙区域占主导。
