^(222)Rn指示北部湾北部近海SGD输送的时空特征初探

A preliminary investigation on the spatial and temporal distribution of submarine groundwater discharge in the northern Beibu Gulf as indicated by^(222)Rn activities

放射性同位素氡222(^(222)Rn)是来源于地层中铀衰变生成的惰性元素。由于其性质稳定,容易测量且在地下水和地表水中活度差异显著,近年来被用作示踪剂广泛应用在海底地下水排泄(submarine groundwater discharge,SGD)的研究中。本文选取北部湾北部海水的^(222)Rn活度作为研究对象,通过2021年8月—9月以及12月—次年1月在北部湾北部海域的两个航次采样,分析了北部湾北部海域水体^(222)Rn的空间分布和季节性变化特征,并结合^(222)Rn的质量平衡模型,估算了北部湾北部海域的SGD通量。结果表明,^(222)Rn分布特征受到季节变化和陆源SGD过程的显著影响,冬季^(222)Rn活度的平均值与夏季相比减少了约40%。夏季底层水^(222)Rn活度较高,断面特征显示SGD过程明显且多集中于研究区北部,而冬季底层水^(222)Rn活度较低,断面特征显示SGD过程较弱。通过构建^(222)Rn质量平衡模型,估算出北部湾北部海域夏季与冬季SGD速率分别为4.16 cm·d^(-1)和2.88 cm·d^(-1)。夏季SGD速率明显高于冬季,且夏季以近岸SGD过程为主,冬季以离岸SGD为主。北部湾北部海域存在着明显的SGD过程,而且由于该区域被陆地和岛屿三面环绕,SGD过程很可能是陆源物质向近岸海域输送的重要自然途径。

The radioactive isotope radon-222(222 Rn)is an inert element derived from the decay of uranium in the earth s strata.Due to its stability,ease of measurement and significant difference in activity between groundwater and surface water,it has been widely used as a tracer in the study of submarine groundwater discharge(SGD)in recent years.In this study,the 222 Rn activity of seawater was selected as the object of study,and the spatial and temporal characteristics of 222 Rn were analyzed,and the SGD flux was estimated through a 222 Rn mass balance model,based on two cruises of August-September,2021 and December,2021-January,2022 from the northern Beibu Gulf.The results show a spatial and temporal variation of 222 Rn distribution which was significantly affected by the terrestrial SGD process.The mean 222 Rn activity decreased by about 40%in winter compared with that in summer.In summer,there show high 222 Rn activity in bottom water especially from the northern part of the research area with a strong SGD process,but with low 222 Rn activity in winter with weak SGD process.Based on the 222 Rn mass balance model,the SGD fluxes of 4.16 cm·d^(-1)in summer and 2.88 cm·d^(-1)in winter were calculated from the research area,which shows significantly high SGD flux in summer dominant with nearshore SGD and low SGD flux in winter dominant with offshore SGD.Since the research area is surrounded by land and islands,the SGD processes were likely to be an important natural pathway for the terrestrial sources transporting to the nearshore waters.