Submarine groundwater discharge into Sanggou Bay traced by ²²³Ra and ²²⁴Ra

Submarine groundwater discharge (SGD) with inputs of nutrients and water into the ocean in certain regions plays a significant role in coastal material fluxes and their biogeochemical cycle, which cannot be ignored for its impact on the eco-system. In this paper, SGD into the typical aquaculture base of northern China, Sanggou Bay, was estimated using naturally occurring radium isotopes,²²³Ra and ²²⁴Ra. Results showed that the radium activities of submarine groundwater, especially in the pore water [²²⁴Ra = (968 ± 31) dpm/ (100 L),²²³Ra = (31.4 ± 4.9) dpm/ (100 L), n = 9], were obviously greater than those in the surface water [²²⁴Ra = (38. 7±2.0) dpm/ (100 L),²²³Ra = (l.70 ± 0.50) dpm/(100 L), n = 21]. Assuming steady state and using a radium mass balance model with sources and sinks, the SGD rate was estimated to be among (0.23~1.03) × 10⁷m³/d. SGD flux varied from spring tide to ebb tide. During spring tide, SGD flux was smaller because of weak hydraulic gradient while it was much larger during ebb tide because of strong hydraulic gradient. During a tidal cycle, the SGD flux was averaged to be 0. 39×10⁷m³/d based on ²²³Ra and ²²⁴Ra. Tidal-driven SGD can account for 61% of the total SGD. Therefore, the main driving force of SGD into Sanggou Bay was tidal pumping and SGD of Sanggou Bay in coastal zone contributes a lot to seawater composition and material exchange between land and sea.