Socially vulnerable populations in the United States are bearing the highest costs of water scarcity, which is likely to worsen with climate change, population growth, and growing disparities between areas with high demand and the location of available supplies. Prior research showing that socially vulnerable groups are inequitably exposed to some of the worst shortages has focused on singular dimensions of social vulnerability, typically in relatively localized geographies, leaving us with an incomplete understanding of the national scope of the problem. This study combines data on surface water shortages lasting 12 months and with a 50-year return period with the U.S. Center for Disease Control's Social Vulnerability Index (SVI) to spatially identify clusters of high-shortage, high-vulnerability hotspots from 71,195 census tracts across the conterminous United States. We estimate that 5 percent of the population of the lower 48 states -- nearly 15 million people -- lives in high-SVI, high-shortage hotspot areas. We examine the relationship between water shortage volumes and i) SVI, (ii) SVI themes, and (iii) 15 indicators used to construct SVI for the U.S. and within hotspots. We find evidence that water shortages constitute an environmental injustice, as multiple dimensions of social vulnerability are disproportionately exposed to the worst shortages. However, distinct dimensions of vulnerability that are correlated with increasing shortage volumes vary across regions and within hotspots, indicating that water shortage adaptation strategies will have to be tailored to their specific contexts.