Submarine groundwater discharge is a process that is often considered negligible in deltaic systems given their low gradient and fine-grained sediment. However, hydrologic budgets and radon surveys indicate that it may be a significant component of the Mississippi River Delta system. To more concretely indicate groundwater's contribution to the local environment, we conducted an analysis of estuarine water chemistry. We focused on the mid-weight alkaline earth metals, which differ significantly in the system's three end-members: river, ocean, and groundwater. We found an anomaly of barium in the estuaries, which could not be completely explained by desorption. Through the construction of a three-end-member mixing model, groundwater was estimated to comprise 14–28% of Terrebonne and Barataria Bay estuarine water, which corresponds to a combined discharge of 160–480 m3/s. This groundwater discharge helps explain the hydrologic budget of the system, and could influence the chemistry of these large deltaic estuaries.

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