Saltwater intrusion can pose a serious threat to groundwater quality in coastal regions. Estimating the extent of saltwater intrusion is vital for groundwater managers to plan appropriate mitigation strategies. The airborne electromagnetic (AEM) method is commonly used to evaluate groundwater resources, but it is challenging to apply in coastal environments because the low resistivity of saltwater-saturated aquifers attenuates the electromagnetic signal quickly and the relationship between electrical resistivity and pore water salinity is complex. However, if successful, the AEM method can supply information to address questions of critical importance in coastal regions. We investigated the extent of, and controls on, saltwater intrusion using the AEM method in the northern Salinas Valley, CA, USA. We collected 635 line-km of AEM data in the study area, the inversion results of which produced estimates of the electrical resistivity of the subsurface, reaching depths of between 50 and approximately 200 m below the ground surface. We have developed a relationship between the AEM electrical resistivity model and groundwater salinity, calibrated from borehole geophysical and water quality measurements, which allowed us to generate images revealing the distribution of saltwater and fresher groundwater in the study area. This fresher groundwater (defined as “a source of drinking water”) was successfully mapped out in the unconfined aquifer (the Dune Sand Aquifer) and the uppermost confined aquifer (the 180-Foot Aquifer) in the study area, illustrating a groundwater recharge process that helps mitigate saltwater intrusion in the 180-Foot Aquifer. Deep, low-resistivity bodies also were mapped, indicating regions where saltwater likely is migrating vertically from the 180-Foot Aquifer into the lower confined aquifer (the 400-Foot Aquifer). The findings from this case study demonstrate the value of acquiring AEM data for investigating the distribution of salinity in coastal aquifers impacted by saltwater intrusion.

中文翻译：

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使用机载电磁方法绘制加利福尼亚北部萨利纳斯山谷的咸水入侵图

咸水入侵会对沿海地区的地下水质量构成严重威胁。估计盐水入侵的程度对于地下水管理者规划适当的缓解策略至关重要。机载电磁法（AEM）通常用于评估地下水资源，但由于沿海地区的盐水电阻率低，它会迅速衰减电磁信号，并且电阻率与孔隙水盐度之间的关系非常复杂，因此在沿海环境中应用存在挑战。 。但是，如果成功，AEM方法可以提供信息以解决沿海地区至关重要的问题。我们在美国加利福尼亚州萨利纳斯山谷北部使用AEM方法调查了盐水入侵的程度和控制措施。我们在研究区域内收集了635线公里的AEM数据，其反演结果产生了地下电阻率的估计值，达到了地下50至200 m之间的深度。我们已经开发了AEM电阻率模型与地下水盐度之间的关系，并通过钻孔地球物理和水质测量进行了校准，这使我们能够生成图像，揭示研究区域内盐水和较新地下水的分布。在研究区域的无限制含水层（Dune Sand含水层）和最上方的受限含水层（180英尺含水层）中成功绘制了这种较新鲜的地下水（定义为“饮用水源”）。这说明了地下水的补给过程有助于减轻180英尺含水层中的盐水入侵。还绘制了深层，低电阻率的物体，表明盐水可能从180英尺含水层垂直迁移到下部承压含水层（400英尺含水层）的区域。该案例研究的结果表明，获取AEM数据对于调查受盐水入侵影响的沿海含水层中盐度的分布具有重要的价值。