Submarine groundwater discharge (SGD) is widely acknowledged as a key driver of environmental change in tropical island coral reefs. Previous work has addressed SGD and groundwater-reef interactions at isolated submarine springs; however, there are still many outstanding questions about the mechanisms and distribution of groundwater discharge to reefs. To understand how groundwater migrates to reefs, a series of offshore ²²²Rn (radon) and submarine electrical resistivity (ER) surveys were performed on the tropical volcanic island of Mo’orea, French Polynesia. These surveys suggest that fresher water underlies the fringing reef, apparently confined by a <1-m-thick low-permeability layer referred to as a reef flat plate. Reef flat plates have been documented elsewhere in tropical reefs as thin, laterally continuous limestone units that form through the super-saturation of calcium carbonate in the overlying marine waters. In other tropical reefs, the reef flat plate is underlain by a highly permeable karstic limestone formation, but the submarine reef geology on Mo’orea is still uncertain. Numerical modeling of two-dimensional reef transects and SGD quantifications, based on water budget and radon/salinity mass balance, support the confining nature of the reef flat plates and indicate important implications for SGD impacts to tropical reefs. Except where incised by streams or local springs, reef flat plates may route SGD to lagoons or to the reef crest 100s of meters offshore. Because groundwater can transport pollutants, nutrients, and low pH waters, the reef flat plate may play an important role in the spatial patterns of reef ecology and coastal acidification.

海底地下水排放（SGD）被公认为热带岛屿珊瑚礁环境变化的主要驱动力。先前的工作已经解决了孤立海底温泉中的SGD和地下水-礁石相互作用；但是，关于地下水向礁石排放的机理和分布仍然存在许多悬而未决的问题。为了解地下水如何向礁石迁移，一系列近海²²²Rn（rad）和海底电阻率（ER）调查是在法属波利尼西亚的莫奥雷阿热带火山岛上进行的。这些调查表明，淡水是边缘礁的基础，显然是由厚度小于1米的低渗透层（称为礁平板）限制的。在热带珊瑚礁的其他地方，珊瑚礁平板被记录为薄的，横向连续的石灰岩单元，它们是通过上覆海水中碳酸钙的过饱和而形成的。在其他热带珊瑚礁中，珊瑚礁平板位于高渗透性的岩溶石灰岩地层之下，但莫奥雷阿岛的海底珊瑚礁地质仍然不确定。基于水量预算和ra /盐度质量平衡的二维礁样面和SGD量化的数值模型，支持珊瑚礁平板的封闭性，并指出SGD对热带珊瑚礁的影响具有重要意义。除非受到溪流或当地温泉的影响，否则礁石平板可将SGD导向泻湖或离岸100米的礁顶。由于地下水可以输送污染物，养分和低pH值的水，因此礁石平板可能在礁石生态系统和沿海酸化的空间格局中发挥重要作用。