Role of Reef-Flat Plate on the Hydrogeology of an Atoll Island: Example of Rangiroa
Role of Reef-Flat Plate on the Hydrogeology of an Atoll Island: Example of Rangiroa
复制标题
礁滩板块对环礁岛水文地质的作用：Rangiroa 岛为例
作者：
J. Maréchal;V. Hakoun;Pauline Corbier
在环礁岛上，淡水透镜（FWL）对岛上居民来说是一种非常重要的淡水资源，可用于各种用途。它的形状和厚度高度取决于构成该岛地下的几种岩层的地下特征：上部全新世沉积物和下部更新世石灰岩，由全新世更新世不整合面隔开。在这项研究中，几个非常简单的调查方法应用于太平洋环礁的Rangiroa，以表征含水层的水动力学及其对FWL的影响。提出了含水层的水量平衡，包括FWL的深层入渗。抽水试验和潮汐分析表明了含水层的承压特性及其主要的水动力特性（储水量和导水率）。强调了珊瑚礁平板对环礁水文地质的作用。其不透水性质有助于减少对含水层的深层渗透。它还有助于含水层的承压流动状态，引起潮汐力引起的水位高而快的波动，从而有助于增加盐水混合区的厚度。这两种现象都有助于减少FWL的厚度，该环礁的FWL只有2米厚。
On atoll islands, the fresh water lens (FWL) constitutes, for the island population, a very important fresh water resource for various usages. Its shape and thickness highly depend on the underground characteristics of several rock formations which constitute the underground of the island: upper Holocene sediments and lower Pleistocene limestone rocks separated by the Holocene Pleistocene Unconformity. In this study, several very simple investigation methods were applied on the Pacific atoll of Rangiroa in order to characterize the aquifer hydrodynamics and their impact on the FWL. A water budget of the aquifer was proposed, including the deep infiltration to the FWL. Pumping tests and tidal analysis demonstrated the confined character of the aquifer and its main hydrodynamic properties (storage and hydraulic conductivity). The role of the reef-flat plate on the hydrogeology of the atoll was highlighted. Its impermeable nature contributes to reduce the deep infiltration to the aquifer. It also contributes to the confined flow regime of the aquifer, inducing high and fast water level fluctuations due to tide forces, and consequently contributes to increase the thickness of the saline mixing zone. Both phenomena contribute to reduce the thickness of the FWL, which is only 2 m-thick in that atoll.