Silica Sources and Water Flowpaths in a Tropical Watershed: A Combined Ge/Si, Oxygen Isotope, and Hydrometric Study

溶解硅浓度和硅浓度-流量关系已被广泛用作集水区地下水流路径的示踪剂。这项研究将采用Ge/Si比率作为一种新的水文示踪剂，并将测试关于陆生植物在控制河流Si通量中的作用的新兴想法。来自波多黎各研究集水区（里约热内卢Icacos）的Ge/Si数据将通过氧同位素分析、水文数据和物理水文模型进行补充，以验证以下假设：a)基流中观察到的高Si浓度反映了腐岩-基岩界面的风化作用。这些硅通过深层地下水排放被输送到溪流中；B)在径流事件中观测到的低硅浓度反映了硅从次生土壤矿物质中快速释放到沿浅层渗透性不连续流动的水中；C)浅层土壤孔隙水中有时存在的高Si浓度反映了植物的循环和植物岩的溶解。在我们的站点，这一Si并不构成整个河流Si通量的重要部分。该项目旨在开发一种新工具，以增加我们对地下水运动、向溪流输送水以及控制溪流化学成分的过程的理解。这些问题是水质的重要决定因素，也是地球化学和水文学基础研究的中心问题。在这里，我们将这个工具（溪流中天然硅和微量元素锗的浓度）应用于波多黎各加勒比国家森林的一个已建立的研究流域。该项目承诺将整合来自水源的其他化学示踪剂信息，以及土壤湿度和地下水位的物理测量，并使用水流的数学模型。从这项研究中收集到的数据和发展出来的想法应该为水文学家和地球化学家在其他环境中广泛应用该工具奠定基础。

Dissolved silicon concentrations and silicon concentration-discharge relationships have been used widely as a tracer of subsurface flowpaths in catchment hydrology. This study will employ Ge/Si ratios as a novel hydrologic tracer, and will test emerging ideas regarding the role of terrestrial plants in controlling streamwater Si fluxes. Ge/Si data from a research catchment in Puerto Rico (Rio Icacos) will be supplemented by oxygen isotope analyses, hydrometric data, and physical hydrologic modeling to test the following hypotheses: A) High Si concentrations observed in streams at base flow reflect weathering at the saprolite-bedrock interface. This silicon is delivered to streams via deep groundwater discharge; B) Low Si concentrations observed during runoff events reflect rapid release of silicon from secondary soil minerals to water flowing along shallow permeability discontinuities; C) High Si concentrations sometimes present in shallow soil porewater reflect plant cycling and dissolution of phytoliths. This Si does not comprise a significant part of the overall streamwater Si flux at our site. This project is aimed at developing a new tool that will increase our understanding of the underground movement of water, delivery of water to streams, and the processes that control the chemistry of stream water. These issues are important determinants of water quality and are central to basic research in geochemistry and hydrology. Here we apply this tool (concentrations of naturally-occurring silicon and the trace element germanium in stream water) to an established research watershed in the Caribbean National Forest in Puerto Rico. The project promises to integrate information from additional chemical tracers of water sources plus physical measurements of soil moisture and water table levels, and use mathematical models of water flow. Data collected and ideas developed from this study should lay the groundwork for widespread application of this tool by hydrologists and geochemists in other settings.