Studying Submarine Groundwater Discharge at the Cape Ayia: a Multi-Tracer Approach

Pupose. The study of submarine groundwater discharge is one of the most pressing issues of modern hydrogeology and oceanography. The purpose of the paper is to represent a comprehensive study of the submarine groundwater discharge focus at the Cape Ayia using the hydrological, hydrochemical and radiochemical methods, as well as to determine the flux of submarine groundwater including the nutrients. Methods and Results. The scientific investigations were carried out during the coastal expeditions nearby the Cape Ayia on March 24, 2019, and also on April 22–24, 2019 during the 106th cruise of the R/V “Professor Vodyanitsky” (April 18–May 13, 2019). The isotopes 226Ra, 228Ra were extracted using the proprietary MnO2-based fibers. After concentration, the sorbent was squeezed to remove excess water, dried and ashed. Then the ash was placed in the Petri dishes and poured with epoxy resin. Activity of the radionuclides was measured on a low-background semiconductor γ-spectrometer with a detector of high-purity germanium (GC3020) in 3 weeks after the resin casting. The activity of 226Ra was determined by the daughter radionuclide 214Pb with the energy 351.9 keV (qγ = 37.2%), and that of 228Ra – by the daughter 228Ac (T1/2 = 6.1 h, qγ = 27.7%) with the energy 911.6 keV. The basic elements of the main biogenic cycle were determined using the photo-colorimetric means: mineral phosphorus – by molybdenum blue, silicon – by silicon-molybdenum complex. Nitrates (reduced to nitrites) and nitrites were determined by azo dye; ammonium – by indophenol blue also by the photo-colorimetric method. Conclusions. Distribution of hydrophysical, hydrochemical and radiochemical parameters in the water area connected with the known focuses of the groundwater submarine discharge was studied. For the first time, the distribution of the 226Ra, 228Ra isotopes was studied. The data on salinity, volumetric activity of the 226Ra, 228Ra isotopes and the nutrients concentration permitted to determine the flows of submarine underground waters in the Cape Ayia area, which amounted to 8220 ± 1200 m3/day. The anthropogenic contribution to pollution of the groundwater forming the submarine sources is shown.