Hydrogeologic Heterogeneity Impacts on Fresh-Saltwater Interaction in Jeju Volcanic Island, Korea.

Ground water Pub Date : 2025-03-10 DOI:10.1111/gwat.13472
Chang-Seong Koh, Eun-Hee Koh, Won-Bae Park, Min-Choel Kim
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Abstract

Jeju volcanic island of South Korea is characterized by hydrogeological heterogeneity, which has resulted in complex environments in a coastal aquifer system. The shape of the fresh-saltwater transition zone (FSTZ) and depth-dependent tidal influences on fresh-saltwater interaction in the eastern part of Jeju Island were examined by assessing geological logs from drilling cores, vertical profiles of specific conductance (SC) and temperature from geophysical logging, and performing time series analysis of groundwater level and multi-depth SC (collected from multiple sensors installed at various borehole depths). A sharp interface and step-like FSTZ were developed in the hyaloclastite and lava layers, respectively. The tidal influences on groundwater levels were highly associated with the distance from the coastline; however, SC data revealed different responses to tidal changes according to depth. Based on these data, we propose a conceptual hydrogeological model that incorporates different volcanic structures, including hyaloclastite and lava layers. Conduit flow through the highly permeable hyaloclastite layers led to the development of a sharp interface of FSTZ and disturbed the tidal signals on SC by acting as a preferential pathway for fast and abundant fresh groundwater discharge. Conversely, in the lava layers characterized by the successive formation of high- and low-permeability layers, boundary flows in the geological boundaries created a step-like FSTZ and showed a relatively high association between the tide and SC. This study highlights the crucial role of hydrogeological heterogeneity in determining the complex behaviors of fresh-saltwater interactions in the coastal aquifers of volcanic regions.

韩国济州火山岛水文地质非均质性对淡水-盐水相互作用的影响
韩国济州岛火山岛具有水文地质非均质性的特点,导致其沿海含水层系统环境复杂。通过评估钻探岩心的地质记录、地球物理测井的比电导(SC)垂直剖面和温度,以及地下水水位和多深度SC(由安装在不同钻孔深度的多个传感器收集)的时间序列分析,研究了济州岛东部淡水-盐水过渡带(FSTZ)的形状和深度依赖潮汐对淡水-盐水相互作用的影响。透明晶岩层发育尖界面,熔岩层发育阶梯状FSTZ。潮汐对地下水位的影响与离海岸线的距离密切相关;然而,SC数据显示了不同深度对潮汐变化的不同响应。基于这些数据,我们提出了一个概念水文地质模型,其中包括不同的火山结构,包括透明碎屑岩和熔岩层。通过高渗透性透明晶岩层的导管流导致FSTZ形成一个尖锐的界面,并作为快速和丰富的新鲜地下水排放的优先通道,干扰了SC上的潮汐信号。相反,在以高、低渗层连续形成为特征的熔岩层中,地质边界的边界流动形成了阶梯状的FSTZ,显示出潮汐与SC之间相对较高的相关性。该研究强调了水文地质非均质性在决定火山地区沿海含水层淡水-盐水相互作用的复杂行为中的关键作用。
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