Utility of Radium Isotopes for Evaluating Residence Time and Submarine Groundwater Discharge
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摘要: 海底地下水排泄(submarine groundwater discharge, SGD)难以直接测量, 镭同位素和氡-222等天然示踪剂使得间接评价SGD通量成为可能.为了评价五缘湾的水体滞留时间和SGD通量, 实测了湾内海水、湾外海水和地下水中224Ra和226Ra的活度, 利用224Ra和226Ra半衰期的差异, 采用224Ra与226Ra的活度比值计算湾内水团的年龄和平均滞留时间, 利用224Ra和226Ra的质量平衡模型计算SGD通量.五缘湾13个站位的水团年龄在0.6~2.4 d之间, 湾顶水团年龄相对较大, 平均海水滞留时间1.4 d.地下水输入五缘湾的224Ra和226Ra通量分别为5.17×106 Bq/d和5.28×106 Bq/d, 将该通量用地下水端元的活度转换成为SGD通量分别是0.21 m3/m2/d(224Ra平衡模型)和0.23 m3/m2/d(226Ra平衡模型), 两种模型的结果较接近, 其平均值0.22 m3/m2/d可作为五缘湾的海底地下水排泄通量.Abstract: It's very difficult to measure submarine groundwater discharge (SGD) directly, but naturally occurring radium isotopes and radon tracers make it possible to estimate SGD fluxes indirectly. In order to evaluate the residence time of seawater and SGD fluxes in Wuyuan Bay, Xiamen, China, we measured the 224Ra and 226Ra activities in the bay water, open ocean seawater and groundwater, calculated the water parcel ages and average residence time in the Bay based on the differences in half-life between 224Ra and 226Ra, and estimated the SGD fluxes via the mass balance on 224Ra and 226Ra. The water parcel ages range from 0.6 to 2.4 d among thirteen sampling stations, with an average residence time of 1.4 d. The water parcel ages in the bay head relatively get older than those in the bay mouth. The calculated 224Ra and 226Ra fluxes driven by SGD are 5.17×106 Bq/d and 5.28×106 Bq/d, respectively. Both radium fluxes are further converted into SGD fluxes by dividing the activities of groundwater end-member to be 0.21 m3/m2/d (224Ra mass balance model) and 0.23 m3/m2/d (226Ra mass balance model), respectively. The calculated results from two models are so close that their average of 0.22 m3/m2/d can be considered as SGD fluxes in Wuyuan Bay.
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Key words:
- submarine groundwater discharge /
- seawater /
- radium isotopes /
- residence time /
- Wuyuan Bay /
- hydrogeology
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图 2 224Ra和226Ra活度与盐度的线性关系
Fig. 2. The relation of 224Ra and 226Ra activities versus salinity
图 3 湾内海水与湾外海水、地下水的224Ra与226Ra活度比值对比
Fig. 3. 224Ra and 226Ra activities ratio in bay water compared with seawater outside bay and groundwater
表 1 水样的224Ra、226Ra活度和盐度
Table 1. Measured 224Ra, 226Ra activity and salinity in the water samples
水样 编号 224Ra(Bq/m3) 226Ra(Bq/m3) 盐度 湾内海水 S1 4.13 4.71 26.2 S2 3.73 4.06 26.2 S3 3.47 3.77 27.1 S4 3.31 3.65 29.3 S5 4.17 3.76 26.9 S6 2.29 2.77 29.0 S7 3.26 2.95 28.2 S8 2.06 2.37 30.3 S9 2.78 2.54 30.6 S10 2.63 2.29 30.8 S11 2.16 2.53 30.5 S12 2.32 2.05 30.5 S13 2.60 2.25 30.7 湾外海水 O1 1.37 1.48 31.0 O2 1.45 1.68 30.9 地下水 G1 14.27 6.21 0.4 G2 10.82 13.46 0.5 
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