间隙水交换(Pore water Exchange, PEX)和海底地下水排放(Submarine Groundwater Discharge, SGD)是河口和近岸区域海底与水体生态系统之间溶解物质(如溶解无机碳、营养盐和痕量金属等)交换的两个主要过程。两者发生的尺度分别为毫米到米和米到几十千米，其输送的物质分别代表再生成分和外源成分。准确定量这两个过程对水体生态系统的相对重要性将有助于理解河口和近岸区域元素的生物地球化学。但是，目前对这两个过程的相对贡献仍没有准确的评估，PEX甚至经常成为被忽视的一环。

蔡平河教授课题组以九龙江河口为研究对象，采用²²⁴Ra/²²⁸Th不平衡方法结合水体中²²⁴Ra的质量平衡，首次直接、清晰地同时估算了PEX和SGD两个过程的溶解物质通量的季节变化。结果发现，在整个河口尺度上，PEX输送的溶解物质通量与SGD的相当，两者均与河流输入相当甚至高于河流输入通量，并且具有明显的季节变化。此外，我们发现浸灌作用是控制PEX输送的主要过程，并且其强度主要受沉积物混合速率的影响。

Schematic diagram of the ²²⁴Ra budget in an estuary

该成果展示了PEX对溶解物质输送的重要性，是河口区溶解物质交换不可忽视的一环。该成果近日发表于Geochimica et Cosmochimica Acta期刊。第一作者为博士生洪清泉，通讯作者为蔡平河教授。

Abstract

Pore water exchange (PEX) and submarine groundwater discharge (SGD) represent two mechanisms for solute transport from the seabed into the coastal ocean. However, their relative importance remains to be assessed. In this study, we pursued the recently developed ²²⁴Ra/²²⁸Th disequilibrium approach to quantify PEX fluxes of ²²⁴Ra into the Jiulong River estuary, China. By constructing a full mass balance of water column ²²⁴Ra, we were allowed to put various source terms, i.e., SGD, diffusive and advective pore water flow (PEX), and river input in a single context. This led to the first quantitative assessment of the relative importance of PEX vs. SGD in the delivery of solutes into an estuary. We carried out two surveys in the Jiulong River estuary: one in January 2014 (winter survey), the other in August 2014 (summer survey). By virtue of a 1-D mass balance model of 224Ra in the sediment column, we demonstrated that PEX fluxes of ²²⁴Ra were highly variable, both temporally and spatially, and can change by 1-2 orders of magnitude in our study area. Moreover, we identified a strong correlation between ²²⁴Ra-based irrigation rate and ²³⁴Th-based sediment mixing rate. Our results highlighted irrigation as the predominant PEX process for solute transfer across the sediment-water interface.

Total PEX flux of ²²⁴Ra (in 10¹⁰ dpm d^-1) into the Jiulong River estuary was estimated to be 22.3±3.0 and 33.7±5.5 during the winter and summer surveys, respectively. In comparison, total SGD flux of ²²⁴Ra (in 10¹⁰ dpm d^-1) was 11.3±8.6 and 49.5±16.3 in the respective seasons. By multiplying the PEX fluxes of ²²⁴Ra by the ratio of the concentration gradients of component/²²⁴Ra at the sediment-water interface, we quantified the total PEX fluxes of dissolved inorganic carbon (DIC) and nutrients (NH₄⁺, NO₃^-, and H₄SiO₄) into the Jiulong River estuary. In the meantime, net export of DIC and nutrients via SGD were estimated by multiplying the SGD fluxes of ²²⁴Ra by the DIC (nutrients)/²²⁴Ra ratios in the SGD end-members around this area. Our results revealed that PEX-driven fluxes of solutes rival net SGD input and river input in an estuary. An additional new finding is that water column NO₃^- in the surface estuary was effectively sequestered due to SGD, probably as a result of intense denitrification occurring in the anoxic subterranean estuary.

Reference: Hong, Q., P. Cai, X. Shi, Q. Li, and G. Wang (2017). Solute transport into the Jiulong River estuary via pore water exchange and submarine groundwater discharge: New insights from ²²⁴Ra/²²⁸Th disequilibrium. Geochim. Cosmochim. Acta, 198, 338-359.

Link: http://www.sciencedirect.com/science/article/pii/S0016703716306391