Temperature and Oxygen Response to Groundwater Discharge in the Ice-Covered Razdolnaya River Estuary (Sea of Japan)

Abstract The study demonstrates a dynamic connection between the saltwater wedge region and its underlying aquifer. In winter, this estuary is covered with ice and the river flow is at its lowest; that is why its hydrological and oxygen-specific response to groundwater discharge is best marked in this season. Groundwater discharge was detected in the saltwater wedge region by highly active radium isotopes: 224 Ra — 66.32 ± 0.60 dpm 100L −1 , 223 Ra — 2.85 ± 0.17 dpm 100L −1 , 228 Ra — 159.15 ± 0.13 dpm 100L − 1 . The temperature of ground and river waters was +4.3 °С and +0.09 °С, respectively, that of sea water −1.2 °С, and the near-bottom layer temperature increased up to +1.2 °С in the water discharge region. According to the data provided by an anchored autonomous station installed in the area of saltwater discharge influence during the freeze-up period, the temperature reaches 2.5 °С in the saltwater wedge. Groundwater discharge is accompanied by a lower level of oxygen saturation – 67%, at that time oxygen saturation in sea water was 151%. Thus, we present evidence documenting the natural process that may result in a lower level of oxygen saturation in an estuary irrespective of man-induced effects, supply of nutrients and further intensification of production-destruction processes.