Groundwater level observations in 250,000 coastal US wells reveal scope of potential seawater intrusion

Scott Jasechko,Ying Fan,Hansjörg Seybold,James W Kirchner,Debra Perrone

doi:10.1038/s41467-020-17038-2

Abstract

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.

Highlights

Seawater intrusion into coastal aquifers can increase groundwater salinity beyond potable levels, endangering access to freshwater for millions of people
Our objective is to identify coastal areas where most well water elevations lie below sea level, rendering these areas potentially vulnerable to seawater intrusion
Reported well water depths were converted to elevation above sea level using a ~10 m-gridded digital elevation dataset

Summary

Introduction

Seawater intrusion into coastal aquifers can increase groundwater salinity beyond potable levels, endangering access to freshwater for millions of people. The 108 local-scale studies compiled in Supplementary Table 5 demonstrate important features that motivate our study: (a) areas with high topographic relief and high recharge rates can maintain water tables above sea level, inducing groundwater discharge into the sea and limiting seawater intrusion (e.g., Puget Sound[12,13,14]; Fig. 1a); (b) extensive groundwater pumping can lead to seawater intrusion (e.g., Salinas Valley[15,16,17]; Fig. 1b), making seawater intrusion relatively common in low-lying alluvial valleys on the West Coast (e.g., Los Angeles[18,19]; Fig. 1c) and along the Gulf[20,21,22,23] and East[24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40] Coasts where extensive coastal plain aquifers are tapped for irrigation and public water supplies (Fig. 1d–g); and (c) seawater intrusion occurs in both shallower unconfined and deeper confined aquifers, and, in many places, the deeper aquifers are more vulnerable due to their isolation from local recharge and low storativity, making hydraulic heads sensitive to pumping These insights provide a conceptual framework that demonstrates how seawater intrusion depends on climate, terrain and groundwater use, and imply that seawater intrusion assessments should include both shallow and deep aquifers (Fig. 1). Some continental-scale coastal hydrogeologic models embed assumptions that cannot be straightforwardly evaluated, such as an assumed relationship between population density and groundwater pumping

Objectives

Methods

Results