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.

We describe a method for measuring 224Ra and 228Th activities in coastal sediments based on a delayed-coincidence counting system (the RaDeCC system). Milli-Q water is added to bulk sediment to form a slurry. 224Ra in interstitial water is co-precipitated by MnO2 suspension. The MnO2 suspension and the sediment with absorbed 224Ra and 228Th are filtered onto a 142-mm 0.7 μm (nominal pore size) GFF filter. The filter is placed onto a sample holder specified for sediment samples and counted in a RaDeCC system. 224Ra and 228Th activities are calculated from two measurements that are conducted within 6–12 h and in 8–10 days after sample collection. The RaDeCC system is calibrated with a 232U-228Th standard using the method of standard addition. The reproducibility and the overall accuracy of 224Ra and 228Th measurements based on this method are estimated to be ± 5% and ± 5–7%, respectively. We have applied this method to a coastal sediment and observed a significant deficit of 224Ra with respect to 228Th in the upper 3–4 cm.Highlights►224Ra and 228Th activities in coastal sediments were measured using a RadeCC system. ► Counting efficiency doesn't change with water content in the range of ~ 0.4–0.5. ► Counting efficiency doesn't change with sample load in the range of 2–25 g. ► The reproducibility and the overall accuracy are ± 5% and ± 5–7%, respectively. ► A deficit of 224Ra relative to 228Th was observed in a coastal sediment.

We utilized 224Ra/228Th disequilibrium in the sediment to investigate processes that regulate solute transfer across the sediment–water interface. Depth profiles of dissolved and surface-bound 224Ra and 228Th in the upper 0–20 cm sediment column were measured using a delayed coincidence counter during a cruise to the Yangtze estuary from 15 to 24 August 2011. Along with 224Ra and 228Th, depth profiles of 234Th were collected to determine the bioturbation rate in the sediment. At most study sites, a significant deficit of 224Ra relative to 228Th was observed in the upper 0–10 cm. In contrast, 224Ra was in excess with respect to 228Th in the upper 0–5 cm at the river mouth, possibly due to redistribution of 224Ra from the mid-salinity region. By modeling the 224Ra depth profiles in the sediment using the general diagenetic equation, we demonstrated that in most cases molecular diffusion and bioturbation together can account for only ∼20–30% of the measured flux of 224Ra. We concluded that other mechanisms, especially irrigation, must be invoked to explain the remnant 70% of the observed deviation of 224Ra relative to 228Th. On the basis of the 224Ra/228Th disequilibrium in the sediment and a concept of increased surface area for exchange by irrigation as developed by early investigators, we proposed a new approach – the 224Ra/228Th disequilibrium approach to quantify the transfer rate of other dissolved species across the sediment–water interface. We have utilized this new approach to determine the benthic consumption rate of dissolved O2. The result reveals that benthic consumption is an important loss term of dissolved O2 in the Yangtze estuary and must be considered as one of the mechanisms that lead to hypoxia in this area.