Solute transport into the Jiulong River estuary via pore water exchange and submarine groundwater discharge: New insights from Ra-224/Th-228 disequilibrium

摘要

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 Ra-224/Th-228 disequilibrium approach to quantify PEX fluxes of Ra-224 into the Jiulong River estuary, China. By constructing a full mass balance of water column Ra-224, 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 Ra-224 in the sediment column, we demonstrated that PEX fluxes of Ra-224 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 Ra-224-based irrigation rate and 234 Th-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 Ra-224 (in 10(10) 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 Ra-224 (in 10(10) dpm d(-1)) was 11.3 +/- 8.6 and 49.5 +/- 16.3 in the respective seasons. By multiplying the PEX fluxes of Ra-224 by the ratio of the concentration gradients of component/Ra-224 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 Ra-224 by the DIC (nutrients)/Ra-224 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. (C) 2016 Elsevier Ltd. All rights reserved.