基于223Ra和224Ra的桑沟湾海底地下水排放通量

海底地下水排放（SGD）是陆地向海洋输送水量和营养物质的重要通道之一,对沿海物质通量及其生物地球化学循环有重要的影响,对生态环境起着不可忽视的作用。本文运用天然放射性同位素223Ra和224Ra示踪估算了我国北方典型养殖基地桑沟湾的海底地下水排放通量。结果表明,海底地下水样尤其是间隙水中Ra活度224Ra=（968±31）dpm/（100 L）,223Ra=（31.4±4.9）dpm/（100 L）,n=9]远高于表层海水224Ra=（38.7±2.0）dpm/（100 L）,223Ra=（1.70±0.50）dpm/（100 L）, n=21]。假设稳态条件下,考虑Ra的各源、汇项,利用Ra平衡模型,估算出桑沟湾SGD排放通量为（0.23~1.03）×107 m3/d。潮周期内的观测结果显示,涨潮时,水力梯度较小,SGD排放变弱,落潮时,水力梯度较大,导致了相对较多的SGD排放。在一个潮周期间,基于223Ra和224Ra得到的SGD排放通量平均为0.39×107 m3/d。潮汐动力下的SGD排放平均占总SGD排放的61%,因此桑沟湾沿岸的地下水排放主要受潮汐动力的影响,并对海水组成及海陆间物质交换有显著贡献。

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=(1.70±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) ×10⁷ m³/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 0.39×10⁷ m³/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.