Groundwater discharge into the oceans may impair water quality along one-fifth of the coastal United States, a new study reports. While rivers are a visible and clear example of delivery of freshwater to the ocean, a much less obvious form of water transportation occurs underground. Submarine groundwater discharge (SGD) is the flow of underground water, which can contain ions and dissolved chemicals, from continents to oceans, and its spatial distribution affects coastal water quality. Most of the global population lives near and depends on coastal water resources, so it's important to identify coastal waters that may be vulnerable to "hidden" contamination from freshwater SGD. Yet freshwater SGD can be diffuse and diverse across regions, making it costly and difficult to measure. To gain a better understanding of SGD in the U.S., Audrey Sawyer and colleagues analyzed continental-wide hydrography and climate data sets. To validate their analysis, they also assessed 18 sites from various locations along the Pacific, Gulf, and Atlantic oceans. Their results reveal distinct hotspots, or recharge areas, where half of all freshwater SGD is focused along only 14% of the coast. Freshwater SGD rates are substantially higher in the Pacific Northwest, the study reports, likely because of the steep terrain. In terms of the U.S. population as a whole, 3% live along SGD recharge areas. The authors note that regions with above-average freshwater SGD and land use development are particularly vulnerable to groundwater-borne contamination, and these regions represent 12% of the coastline, with the most vulnerable regions being the northern Gulf Coast from Mississippi to the Florida Panhandle, the northern Atlantic Coast, and Pacific Northwest.