Data_Sheet_1_DSi as a Tracer for Submarine Groundwater Discharge.docx

Submarine groundwater discharge (SGD) is an important source of nutrients and metals to the coastal ocean, affects coastal ecosystems, and is gaining recognition as a relevant water resource. SGD is usually quantified using geochemical tracers such as radon or radium. However, a few studies have also used dissolved silicon (DSi) as a tracer for SGD, as DSi is usually enriched in groundwater when compared to surface waters. In this study, we discuss the potential of DSi as a tracer in SGD studies based on a literature review and two case studies from contrasting environments. In the first case study, DSi is used to calculate SGD fluxes in a tropical volcanic-carbonate karstic region (southern Java, Indonesia), where SGD is dominated by terrestrial groundwater discharge. The second case study discusses DSi as a tracer for marine SGD (i.e., recirculated seawater) in the tidal flat area of Spiekeroog (southern North Sea), where SGD is dominantly driven by tidal pumping through beach sands. Our results indicate that DSi is a useful tracer for SGD in various lithologies (e.g., karstic, volcanic, complex) to quantify terrestrial and marine SGD fluxes. DSi can also be used to trace groundwater transport processes in the sediment and the coastal aquifer. Care has to be taken that all sources and sinks of DSi are known and can be quantified or neglected. One major limitation is that DSi is used by siliceous phytoplankton and therefore limits its applicability to times of the year when primary production of siliceous phytoplankton is low. In general, DSi is a powerful tracer for SGD in many environments. We recommend that DSi should be used to complement other conventionally used tracers, such as radon or radium, to help account for their own shortcomings.

海底地下水排泄（Submarine groundwater discharge, SGD）是近岸海域营养盐与金属物质的重要来源，可对沿岸生态系统产生影响，且作为一类重要水资源正日益受到学界关注。SGD通常采用氡（radon）、镭（radium）等地球化学示踪剂进行定量，但已有少量研究尝试以溶解态硅（Dissolved Silicon, DSi）作为SGD的示踪剂——相较于地表水，地下水中溶解态硅通常更为富集。本研究通过文献综述与两个不同环境背景的案例分析，探讨了DSi作为SGD示踪剂的应用潜力。首个案例以印度尼西亚爪哇岛南部的热带火山-碳酸盐岩喀斯特区域为研究区，该区域海底地下水排泄以陆地地下水输入为主，研究中利用DSi计算了SGD通量；第二个案例则以北海南部施皮克奥格岛的潮间带为研究对象，该区域SGD主要由海滩砂的潮汐泵吸作用驱动，探讨了DSi作为海洋型SGD（即循环海水）示踪剂的可行性。研究结果显示，DSi可作为适用于多种岩性（如喀斯特、火山岩及复杂岩性）环境的有效示踪剂，用于定量陆地与海洋型SGD通量；同时还可用于示踪沉积物与沿岸含水层中的地下水运移过程。但需注意，需明确DSi的所有源与汇，并可对其进行定量或合理忽略。该方法的一项主要局限在于：硅质浮游植物会消耗DSi，因此其应用仅适用于硅质浮游植物初级生产力较低的时段。总体而言，DSi是适用于多种环境的高效SGD示踪剂，建议将其与氡、镭等传统常用示踪剂结合使用，以弥补单一示踪方法的固有缺陷。