Soil contaminants pose delayed but pervasive threat to shallow groundwater

Dissolved chemical transport in soils can delay groundwater contamination for years. These lag times, τ, are controlled by location-dependent hydrologic, soil, and chemical properties. Despite thousands of sites with known groundwater contamination globally, numerous sites with unidentified chemical leaks could take decades to reach and degrade groundwater. Here, we quantify the potential for contamination (n = 621 dissolved chemicals) to have reached water tables below 2.7 million chemical sites since their construction and when in the future groundwater contamination could occur. For the most mobile contaminants, groundwater contamination could exceed the maximum contaminant level for about half of these sites (48.1 %, −3.7 %/+18.6 % with source and environmental uncertainties) as of 2021. However, most groundwater contaminants are less mobile and delayed in the vadose zone, where substantially fewer sites could have already contaminated groundwater by 2021 for moderately mobile chemicals (Kd = 1 L/kg, 23.3 % −8.1 %/+21.1 %) and even less mobile chemicals (Kd = 100 L/kg, 0.3 % −0.1/+1.9 %). The modeled urgent (τ ≤ 10 yr) and cross-generational (10 < τ ≤ 100 yr) contamination potential patterns provide the first global sensitivity analysis of groundwater contaminants. These results highlight the need for chemical site monitoring, management, and planning, especially in the developing world.