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

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±0.1μg/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 1.22μg/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.