Application of 234U/238U activity ratios to investigations of subterranean groundwater discharge in the Cádiz coastal area (SW Spain)

•The U concentrations are very similar to those expected for average seawater, though in wells closest to the coast are evident lower.•There is some evidence of the existence of submarine groundwater discharge (SGD) based on the obtained values of U concentrations and U ratios.•This methodology could be applied to others study areas within the region and obtain an overview of the movements of water masses in the zone as a hole.

The activity ratios of 234U/238U were used to investigate processes of subterranean groundwater discharge into coastal marine waters in a study location at Bay of Cádiz (southwest Spain). Marine waters in the bay and surrounding open ocean exhibited U concentrations of 3.4 ± 0.1 μg/L and activity ratios of 1.15 ± 0.01, in agreement with the expected composition of seawater (234U/238U activity ratio = 1.148 ± 0.002). Three water samples obtained from the discharge zone of the Guadalete River exhibited activity ratios of 1.17–1.22 along with slightly lower U concentrations compared to seawater, which is likely due to mixing between seawater and a groundwater end-member. One possible source of groundwater was characterized by sampling and analyzing a well water sample collected in the neighboring village of El Puerto de Santa María; this water sample exhibited an activity ratio of 1.34 ± 0.03 and a U concentration of 1.22 μg/L. Water from the Guadelete River estuarine zone can be explained to result from a two-component mixture of seawater and groundwater from the El Puerto de Santa María well; however, if there are several groundwater reservoirs with different U activity ratios that discharge to the coastal water, then, it may be difficult and more studies are being conducted to address this issue.