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We present a new method to characterize and quantify groundwater discharge to estuaries and the coastal ocean.

The Eocene‐Oligocene Upper Castle Hayne Aquifer (UCH), a well‐indurated limestone with a very high percentage of secondary moldic porosity, is one of the most productive and extensively developed aquifers in the North Carolina Coastal Plain. Ground water from western wells in the UCH is Ca‐ and HCO3‐rich and ground water from easternmost wells is alkali‐ and Cl‐rich. In general, from west to east across the study area, Sr concentrations [Sr] and isotopic ratios of ground water from the UCH and other aquifers evolve toward those of the host aquifer. At the same well site, water from older aquifers usually has a lower 87Sr/86Sr ratio than water from younger aquifers, due to interaction between ground water and sedimentary material in the host aquifers. Comparison of 87Sr/86Sr ratios and [Sr] suggests that most UCH water represents mixing of strontium‐poor recharge water from the Surficial Aquifer with varying amounts of strontium from the aquifer rock. For samples that deviate from the calculated mixing line, strontium ratios can often be used to indicate the source of strontium that did not come from UCH rock. Surface waters are characterized by high 87Sr/86Sr ratios and variable [Sr] that depend on the proportion of intermixed sea water. Water from the overlying Yorktown and Pungo River aquifers can be recognized by higher 87Sr/86Sr ratios than those of the UCH and water from the underlying LCH, Beaufort, and Peedee aquifers can be recognized by lower 87Sr/86Sr ratios and higher [Sr].

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