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