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.