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

建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，用于硒运输和化学循环的研究。由于我们最近克服了技术上的限制，以前的工作是非常有限的。硒和硫同位素系统的相似性结合以往的硒同位素工作表明，硒同位素分馏的主要机制是可溶氧离子（Se6或Se4）还原为不溶性的硫代氧化物。由于这种转变可以确定硒的迁移率和生物利用度，因此硒同位素测量可以成为硒迁移和循环研究的有价值的工具。例如，地下水中同位素比例随时间或流动距离的变化可以为硒的活性还原提供证据。相反，硒浓度的降低可能是由还原、可逆吸附或改变输入浓度引起的。因此，硒同位素研究为标准技术提供了补充信息。此外，如果给定系统中硒的来源在同位素上是不同的，则硒同位素测量可以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测量各种过程引起的同位素分异，在以往工作的基础上，填补对硒同位素变化基本认识的主要空白。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，但也研究了植物生长、烷基硒化物的挥发、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实地实验，以演示新技术的三种不同应用。该小组汇集了独特的专业知识：1)它是目前唯一一个测量环境样品中硒同位素比率的小组。2)一位合作者在细菌还原硒方面具有广泛的专业知识。3)该小组在圣华金河谷和其他地方的硒影响地点有经验，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.