間隙水交換(Pore water Exchange, PEX)和海底地下水排放(Submarine Groundwater Discharge, SGD)是河口和近岸區域海底與水體生態系統之間溶解物質(如溶解無機碳、營養鹽和痕量金屬等)交換的兩個主要過程。兩者發生的尺度分別為毫米到米和米到幾十千米，其輸送的物質分別代表再生成分和外源成分。準確定量這兩個過程對水體生態系統的相對重要性將有助于理解河口和近岸區域元素的生物地球化學。但是，目前對這兩個過程的相對貢獻仍沒有準確的評估，PEX甚至經常成為被忽視的一環。

蔡平河教授課題組以九龍江河口為研究對象，采用²²⁴Ra/²²⁸Th不平衡方法結合水體中²²⁴Ra的質量平衡，首次直接、清晰地同時估算了PEX和SGD兩個過程的溶解物質通量的季節變化。結果發現，在整個河口尺度上，PEX輸送的溶解物質通量與SGD的相當，兩者均與河流輸入相當甚至高于河流輸入通量，并且具有明顯的季節變化。此外，我們發現浸灌作用是控制PEX輸送的主要過程，并且其強度主要受沉積物混合速率的影響。

Correlation of F_Ra vs. D_B for the winter survey in 2014

Schematic diagram of the ²²⁴Ra budget in an estuary

該成果展示了PEX對溶解物質輸送的重要性，是河口區溶解物質交換不可忽視的一環。該成果近日發表于Geochimica et Cosmochimica Acta期刊。第一作者為博士生洪清泉，通訊作者為蔡平河教授。

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