Geochemical Transformations in Subterranean Estuaries: Reactions, Rates, and Fluxes to the Coastal Ocean

在这个项目中，伍兹霍尔海洋研究所的研究人员将研究几种关键元素通过沿海含水层地下河口时的地球化学转化和通量。最近的研究表明，海底地下水排放(SGD)可能对海洋中溶解的化学物质有很大的贡献。然而，这种通量的大小受到发生在地下河口的生物地球化学过程的强烈影响，地下河口被定义为沿海含水层中地下水和海水的混合区。越来越多的证据表明，地下河口在地球化学循环中起着重要作用，因此，我们必须了解这些过程，以便确定SGD在海洋元素收支中的作用。为了做出这一决定，我们必须首先了解在地下河口规模上对这些元素进行的主要地球化学反应。这项研究的主要目的是确定控制几种元素循环的过程，这些元素具有截然不同的地球化学行为：(1)氧化还原敏感元素Fe、Mn和U，以及(2)对氧化还原条件(如对锰氧化物的吸附)和通过地下河口的盐运移驱动的离子强度变化都敏感的Sr、Ba和Ra。该项目由地球化学家和水文地质学家进行独特的合作，他们将通过野外研究、受控实验室实验和水文地质流动模拟来实现项目目标。更具体地说，拟议的项目将通过季节性地下水采样和地表水调查，更深入地探讨地下河口的氧化还原和盐度驱动的生物地球化学反应。为了支持现场工作，研究小组将开发现场密度相关流动的模拟模型。最后，将进行一系列实验室柱实验，以跟踪在不同温度、盐度和氧化还原条件下从含水层沉积物中释放和/或去除锶、钡、Ra和U的情况。更广泛的影响：作为地表河口的类似物，地下河口具有很高的显著地球化学变化的潜力，因此，阐明这些机制可能对理解其他地区的沿海地下水地球化学有重大影响。虽然SGD在某些元素的全球循环中的整体重要性目前还不确定，但毫无疑问，SGD在美国和整个世界范围内都是当地规模的重要。营养丰富(磷酸盐、硝酸盐、铵)的地下水进入沿海水域和海湾并不少见，是SGD的一个环境重要组成部分。因此，更好地了解地下河口的地球化学状况对沿海含水层具有重要的水质影响，并有助于深入了解主要淡水/海水界面的基本生物地球化学反应。该项目有一个重要的教育组成部分，并寻求为一名研究生和一名本科生提供资金，他们自高中以来一直致力于该项目(讲座、海报)，与瓦库伊特湾国家河口研究保护区有关。

ABSTRACTOCE-0425061In this project, researchers at the Woods Hole Oceanographic Institution will study e the geochemical transformations and fluxes of several key elements as they pass through the subterranean estuary of a coastal aquifer. Recent studies indicate that submarine groundwater discharge (SGD) may contribute significant fluxes of dissolved chemical species to the oceans. However, the magnitude of such fluxes is strongly influenced by biogeochemical processes occurring in the subterranean estuary, defined as the mixing zone between groundwater and seawater in a coastal aquifer. Evidence is mounting that the subterranean estuary plays an important role in geochemical cycling and it is therefore critical that we understand these processes so that the role of SGD in oceanic elemental budgets can be determined. In order to make this determination, we must first understand the major geochemical reactions operating on such elements on the scale of subterranean estuaries. The main objective of this study is to identify the processes that control the cycling of several elements having contrasting geochemical behavior: (1) the redox sensitive elements Fe, Mn, and U, and (2) Sr, Ba, and Ra, which are sensitive to both redox conditions (e.g. sorption to Mn oxides) and changes in ionic strength driven by salt transport through the subterranean estuary.The project consists of a unique collaborative effort between a geochemist and a hydrogeologist, who will achieve project objectives through a combination of field studies, controlled laboratory experiments, and hydrogeologic flow modeling. More specifically, the proposed project will probe deeper into redox and salinity-driven biogeochemical reactions in a subterranean estuary through seasonal groundwater sampling coupled with surface water surveys. In support of the field work, the research team will develop a simulation model of density-dependent flow at the site. Finally, a series of laboratory column experiments will be conducted to follow the release and/or removal of Sr, Ba, Ra, and U from aquifer sediments under different temperature, salinity, and redox conditions. Broader Impacts: As analogues to surface estuaries, subterranean estuaries have high potential for significant geochemical transformations and so elucidating these mechanisms may have significant influence on understanding coastal groundwater geochemistry in other areas. While the overall importance of SGD on the global cycle of certain elements is uncertain at present, there is little doubt that SGD is important at the local scale both within the United States and throughout the world. Nutrient-rich (phosphate, nitrate, ammonium) groundwater, entering into coastal waters and bays, is not uncommon and represents an environmentally important component of SGD. Hence, improved understanding of the geochemical regime in subterranean estuaries has important water-quality implications for coastal aquifers and allows insights into fundamental biogeochemical reactions at a major freshwater/seawater interface. This project has an important educational component and seeks funding for a graduate student and an undergraduate who has worked on this project since high school outreach programs (lectures, posters) associated with the Waquoit Bay National Estuarine Research Reserve.