用氡-222评价五缘湾的地下水输入

Assessment of submarine groundwater discharge into the Wuyuan bay via continuous Radon-222 measurements

海底地下水排泄(SGD)近年来成为陆-海相互作用的研究热点,地球化学示踪方法是其主要研究手段,尝试用天然示踪剂氡-222评价厦门五缘湾的SGD。为了评价五缘湾SGD的入海通量及其变化,对五缘湾海水中222Rn和226Ra活度、大气中222Rn活度、风速、水温和水深进行了连续2 d的测量,对沉积物进行了培养实验用以获得其222Rn扩散通量和孔隙水中222Rn活度。基于海水中222Rn通量的质量平衡,对实测的海水中222Rn活度实施了母体支持、涨落潮影响、大气逃逸损失、沉积物扩散输入、混合损失的校正,保守估计SGD输入的222Rn通量在0～126.7 Bq/(m2·h)范围内变化,对海水中222Rn的平均贡献达54%。以井水和孔隙水中222Rn的加权平均值作为SGD端元的代表,获得SGD的输入速率为0～29.3 cm/d,平均输入速率9.3 cm/d。SGD输入速率的动态变化基本围绕12 h的周期波动,是对本海域正规半日潮的具体响应。假设SGD以平均速率在五缘湾海底输入,则五缘湾海底的SGD输入量为1.86×105m3/d。以陆源地下淡水占SGD输入量的10%考虑,五缘湾的陆源地下淡水输入量约为1.86×104 m3/d。

In recent years,submarine groundwater discharge（SGD） has been recognized as an important process in land-ocean interactions in the coastal zone（LOICZ）.The subject has thus become the focus of intensive research.Geochemical tracers can be effective tools for estimating SGD in LOICZ.The objective of this study is to estimate SGD in the Wuyuan Bay using Radon-222（222Rn） as naturally occurring tracer.The dynamic variation of SGD can thus be subsequently assessed.We continuously measure the seawater 222Rn and 226Ra activities,the near-sea surface air 222Rn,wind speed,sea water temperature and depth for two consecutive days.We also deploy an incubation device to measure the diffusive flux of 222Rn from sediments and the pore-water 222Rn activities.Based on the mass balance principle for the 222Rn flux,the measured seawater 222Rn are corrected for the decay product of parent 226Ra,the effects of tides,the losses to the atmosphere,the diffusive influxes from the sediments,and the mixing with offshore seawaters.The result shows a conservative estimate of 222Rn flux attributed to SGD is between 0 and 126.7 Bq/（m2 h）,which can account for 54% 222Rn in seawater.Taking a SGD end-member as example,the calculated SGD input rates range from 0 to 29.3 cm/d with an average value of 9.3 cm/d.The end-member is made of the weighted average of groundwater and pore-water 222Rn activities.The fluctuation in the SGD input rate has a 12-h period,which matches well with the semi-diurnal tides in this region.The SGD input to the Wuyuan Bay could be as much as 1.86×105 m3/d if the estimation were made under the assumption that the average SGD input rate is applicable to the entire bay area.The input of terrestrially derived fresh groundwater in the Wuyuan Bay could be approximately 1.86×104 m3/d if the fresh groundwater were 10% of the total SGD input.