COLLABORATIVE RESEARCH: Selenium Stable Isotopes as Indicators of Selenium Transport and Cycling

9725799约翰逊拟议的工作发展了地表水和地下水系统中硒(Se)稳定同位素比率的测量，用于研究Se的运输和化学循环。以前的工作非常有限，因为我们最近克服了技术限制。Se和S同位素系统学的相似性结合前人的Se同位素工作表明，Se同位素分馏的主要机制是将可溶的氧阴离子(Se_6或Se_4)还原为不可溶的SEO。由于这种转化可以决定Se的迁移性和生物可利用性，Se同位素测量在Se的运输和循环研究中可能是一个有价值的工具。例如，地下水中的同位素比率随时间或流动距离的变化可以为主动还原硒提供证据。相反，硒浓度的降低可能是由于还原、可逆吸附或改变输入浓度而引起的。因此，硒同位素研究为标准技术提供了补充信息。此外，如果给定系统中的硒来源在同位素上是不同的，则硒同位素测量可用于确定主要来源或划定受点源输入影响的区域。研究前半部分的实验将在以前工作的基础上，测量由各种过程引起的同位素分馏，并填补对Se同位素变化的基本理解的主要空白。实验侧重于化学还原(细菌和非生物)作为主要的同位素分馏机制，但也研究植物生长、烷基硒的挥发、吸附和沉淀。在确定了同位素分馏“规则”之后，计划在地下水、地表水和河口环境中进行野外实验，以展示这项新技术的三个不同的应用。组建的小组包括独特的专业知识组合：1)它是目前唯一在环境样品S上测量硒同位素比率的小组。2)一名合作者在细菌还原硒方面拥有丰富的专业知识。3)该小组在圣华金河谷和其他地方受Se影响的地点有经验，并与该领域的许多其他研究人员有良好的联系。

9725799 Johnson The proposed work develops measurements of selenium (Se) stable isotope ratios in surface water and ground water systems for use in studies of Se transport and chemical cycling. Previous work is very limited because of technical limitations that we have recently overcome. Parallels between selemium and sulfur isotope systematics combined with previous Se isotope work, suggest that the dominant mechanism for Se isotope fractionation is reduction of soluable oxyanions (Se6 or Se4) to insoluble SeO. Because this transformation can determine the mobility and bioavailablity of Se, Se isotope measurements could be a valuable tool in Se transport and cycling studies. For example, isotope ratio shifts in groundwater over time or flow distance could provide evidence for active reduction of selenium. In contrast, Se concentration decreases could be caused by reduction, reversible sorption or changing input concentration. Se isotope studies thus provide complementary information to standard techniques. Also, if sources of selenium in a given system are isotopically distinct, Se isotope measurements may serve to determine the dominant source or delineate areas impacted by point source inputs. Experiments in the first half of the study will measure isotope fractionation caused by various processes, build on previous work, and fill in major gaps in the basic understanding of Se isotope variation. The experiments focus on chemical reduction (bacterial and abiotic) as the dominant isotope fractionation mechanism, but alos investigate plant growth, volatilization of alkyl selenides, sorption and precipitation. After the isotpic fractionation "rules" have been ascertained, field experiments in groundwater, surface water, and estuarine settings are planned to demonstrate three distinct applications of the new technique. The group assembled includes a unique combination of expertise: 1) It is currently the only group measuring selenium isotope ratios on environmental sample s. 2) One collaborator has extensive expertise in bacterial reduction of selenium. 3) The group has experience with Se-impacted sites in the San Joaquin Valley and elsewhere, and has good connections with many other researchers in the field.