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

9725799 约翰逊 拟议的工作开发测量硒（Se）稳定同位素比值在地表水和地下水系统中使用的研究硒运输和化学循环。 由于我们最近克服了技术限制，以前的工作非常有限。 硒和硫同位素系统学与以前的硒同位素工作相结合之间的平行关系，表明硒同位素分馏的主要机制是可溶性含氧阴离子（Se6或Se4）不溶性SeO的减少。 由于这种转化可以确定硒的流动性和生物有效性，硒同位素测量可能是一个有价值的工具，在硒的运输和循环研究。 例如，地下水中的同位素比值随时间或流动距离的变化可以为硒的主动还原提供证据。 相反，Se浓度的降低可能是由于还原、可逆吸附或改变输入浓度引起的。 因此，硒同位素研究提供了补充信息的标准技术。 此外，如果在一个给定的系统中硒的来源是同位素不同的，硒同位素测量可以用来确定主要来源或划定受点源输入影响的区域。 在研究的前半部分实验将测量各种过程引起的同位素分馏，建立在以前的工作，并填补硒同位素变化的基本认识的主要空白。 实验重点是化学还原（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，烷基硒化物的挥发，吸附和沉淀。 在确定同位素分馏“规则”后，计划在地下水、地表水和河口环境中进行现场实验，以展示新技术的三种不同应用。 该小组包括一个独特的专业组合：1）它是目前唯一的小组测量环境样品的硒同位素比率。2)一位合作者在细菌减少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.