Uranium and strontium isotopic evidence for strong submarine groundwater discharge in an estuary of a mountainous island: A case study in the Gaoping River Estuary, Southwestern Taiwan

To further define the hydro-geochemical circulation in estuary from a mountainous island, over 50 river water samples were collected in three sampling schemes from the Gaoping River Estuary (GRE), the largest river catchment in southwestern Taiwan and were analyzed for major/trace elements, uranium (U) and strontium (Sr) isotopic compositions using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Dissolved U and Sr concentrations (0.46–13.8 nM and 2.27–86.2 μM), as well as U activity ratio (denoted as (234U/238U)) and 87Sr/86Sr (1.145–2.427 and 0.709155–0.712918), vary significantly and show strong seasonal variability. Three characteristic source waters were identified in the study region, seawater (SW), river water (RW), and groundwater (GW or evolved RW). SW has the highest U and Sr concentrations among the three end-members with low and constant (234U/238U) and 87Sr/86Sr. In contrast, RW shows the lowest U and an intermediate Sr concentration, but with medium (234U/238U) and the most radiogenic 87Sr/86Sr. GW has the medium U and lowest Sr content with rather radiogenic (234U/238U) and 87Sr/86Sr, possibly evolved from RW. The GW is most clearly identified in the vertical profile of the near estuary stations. These high (234U/238U) values in RW and GW reflect the outcome of intense weathering in the upper catchment, efficient leaching of α-recoil products in soils. Dissolved Sr and 87Sr/86Sr data confirm similar U mixing scenarios, but the index is more sensitive to water source variations. Estimated fluxes of submarine groundwater discharge (SGD) in the GRE reach a maximum value of 456–2107 × 1011 L yr− 1, implying important contribution to the chemical budget in coastal regions. This study reveals that U and Sr isotopes are useful tracers for monitoring SGD in estuaries.