Seawater intrusion into coastal aquifers can increase groundwater salinity beyond potable levels, endangering access to freshwater for millions of people. Seawater intrusion is particularly likely where water tables lie below sea level, but can also arise from groundwater pumping in some coastal aquifers with water tables above sea level. Nevertheless, no nation-wide, observation-based assessment of the scope of potential seawater intrusion exists. Here we compile and analyze ~250,000 coastal groundwater-level observations made since the year 2000 in the contiguous United States. We show that the majority of observed groundwater levels lie below sea level along more than 15% of the contiguous coastline. We conclude that landward hydraulic gradients characterize a substantial fraction of the East Coast (>18%) and Gulf Coast (>17%), and also parts of the West Coast where groundwater pumping is high. Sea level rise, coastal land subsidence, and increasing water demands will exacerbate the threat of seawater intrusion.

海水侵入沿海含水层会使地下水盐度超过可饮用的水平，危及数百万人获取淡水的危险。如果地下水位低于海平面，则很可能会造成海水入侵，但也可能是由于地下水位高于海平面的某些沿海含水层中的地下水泵入引起的。但是，尚不存在基于全国范围的基于观测的潜在海水入侵范围评估。在这里，我们汇总并分析了自2000年以来在美国连续进行的约25万个沿海地下水位观测结果。我们表明，观察到的大多数地下水位都沿着连续海岸线的15％以上位于海平面以下。我们得出的结论是，陆上水力梯度是东海岸（> 18％）和墨西哥湾沿岸（> 17％）的很大一部分，还有西海岸的地下水抽水量很高的地区。海平面上升，沿海土地沉降和水需求增加将加剧海水入侵的威胁。