Rising Coastal Groundwater as a Result of Sea‐Level Rise Will Influence Contaminated Coastal Sites and Underground Infrastructure.

Sea‐level rise (SLR) will cause coastal groundwater to rise in many coastal urban environments. Inundation of contaminated soils by groundwater rise (GWR) will alter the physical, biological, and geochemical conditions that influence the fate and transport of existing contaminants. These transformed products can be more toxic and/or more mobile under future conditions driven by SLR and GWR. We reviewed the vulnerability of contaminated sites to GWR in a US national database and in a case comparison with the San Francisco Bay region to estimate the risk of rising groundwater to human and ecosystem health. The results show that 326 sites in the US Superfund program may be vulnerable to changes in groundwater depth or flow direction as a result of SLR, representing 18.1 million hectares of contaminated land. In the San Francisco Bay Area, we found that GWR is predicted to impact twice as much land area as inundation from SLR, and 5,282 additional state‐managed sites of contamination may be vulnerable to inundation from GWR in a 1.0 m SLR scenario. Increases of only a few centimeters of elevation can mobilize soil contaminants, alter flow directions in a heterogeneous urban environment with underground pipes and utility trenches, and result in new exposure pathways. Pumping for flood protection will elevate the saltwater interface, changing groundwater salinity and mobilizing metals in soil. Socially vulnerable communities are disproportionately exposed to this risk at both the national scale and in a regional comparison with the San Francisco Bay Area. Plain Language Summary: We estimated the number of sites with known contamination in the US Superfund program at the national scale and found 326 Superfund sites that may be exposed to inundation from below as rising sea levels push groundwater higher along the coast. California, North Carolina, Virginia, and New York have the largest area of federally managed contaminated land that may be exposed. Thousands of additional sites are managed by state agencies. We conducted a comparison in the San Francisco Bay Area that included state‐managed sites. We found that more than 5,000 sites in the San Francisco region may be exposed to rising groundwater with sea‐level rise (SLR) of 1.0 m, including 1,480 open sites, and an additional 3,817 closed sites that may contain residual contaminants. If the ratio of Superfund to state‐managed sites in this region (1:406) holds, the number of at‐risk contaminated sites nationally may be more than 132,000. Low‐income residents and people of color are disproportionately represented near these sites and therefore may face higher risks. Additional sub‐regional research is urgently needed to understand these exposures. Interactions will occur between the salinity of rising coastal groundwater and shallow pumping, affecting infrastructure and building foundations. Adaptation plans must consider rising groundwater to avoid widespread failures. Key Points: Rising sea levels will cause rising groundwater to inundate some coastal contaminated sites, mobilizing pollutants and causing corrosionWe found 326 Superfund sites that may be at risk nationally, and more than 5,000 state managed sites in a San Francisco Bay area comparisonSocially vulnerable communities are disproportionately exposed to this hazard, with potential impacts on indoor air, foundations and infrastructure [ABSTRACT FROM AUTHOR]