Groundwater comes from precipitation that falls on land. Some of this water evaporates into the atmosphere, gets taken up by plants, or flows into streams, but some infiltrates into the ground and recharges aquifers. Groundwater can flow from inland aquifers to lakes, streams, or coastal waters. On the seaward side, denser saltwater flows into sediments and establishes equilibrium with fresh groundwater. Tides and mixing along the freshwater-saltwater interface results in seawater circulation through the sediments. Scientists have estimated that groundwater provides only 5 percent of the water flowing into the ocean, but groundwater can easily contain more than 20 times the amount of chemicals in it as other freshwater sources, so groundwater can have significant impacts on coastal ocean chemistry and ecology. (Illustration by Jack Cook, Woods Hole Oceanographic Institution)
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