Submarine Groundwater Discharge in a Coastal Bay: Evidence from Radon Investigations

Abstract

Jiaozhou Bay, an urbanized coastal bay located in the southern part of Shandong Peninsula, China, has been deeply affected by anthropogenic activities. Here, the naturally occurring 222Rn isotope was used as a tracer to assess the submarine groundwater discharge (SGD) in this bay. The time series of 222Rn concentrations in nearshore seawater were monitored continuously over several tidal cycles at two fixed sites (Tuandao (TD) and Hongdao (HD)) during the dry season in spring and the wet season in autumn of 2016. 222Rn concentrations in seawater were negatively related to the water depth, indicating the influence of tidal pumping. A 222Rn mass balance model revealed that the mean SGD rates were 21.9 cm/d at TD and 17.8 cm/d at HD in the dry season, and were 19.5 cm/d at TD and 26.9 cm/d at HD in the wet season. These rates were about 8–14 times the discharge rates of the local rivers. Enhanced groundwater inputs occurred at HD in the wet season, likely due to the large tidal amplitudes and the rapid response to local precipitation. Large inputs of SGD may have important influences on nutrients levels and structure, as well as the water eutrophication occurring in coastal waters.

In-situ radon-in-water detection for high resolution submarine groundwater discharge assessment

“…The long-time series measurements revealed that the variation of radon activities in seawater (~2.5 m water depth) is inversely correlated with the local tides, with radon peaks generally at low tide (Figure 5). The radon activities at 31.0 ± 11.4 Bq/m 3 during a tidal cycle near the same place of the bay were previously documented by Luo et al (2020), which match well with our results. We measured the 226 Ra activities (6.8 ± 0.6 Bq/ m 3 ) of three seawater samples at different tidal times including high and low peaks, indicating that the radon contributed from the ingrowth of 226 Ra were limited.…”

“…The SGD fluxes in this bay were previously summarized in a range of 12.3-26.9 cm/d by using the radon mass balance models (Luo et al, 2020). Our results were basically in the scope of former documents.…”

“…where l Rn is the decay constant of 222 Rn; j is the porosity of sediment, 0.33 in this study refer to Luo et al (2020); C eq is the 222 Rn activity in pore water that is equilibrium with that in sediment, 4745 Bq/m 3 in this case according to Luo et al (2020); C 0 is the 222 Rn activities in overlying seawater; and D m is the molecular diffusivity coefficient of 222 Rn, as a function of temperature (T ) (Peng et al, 1974):…”

“…Based on our high temporal resolution observations, the radon fluxes contributed from SGD were evaluated by applying a radon mass balance model, which varied from 0-143 Bq/m 2 /h (mean: 48 ± 31 Bq/m 2 /h). Previously reported 222 Rn activities in groundwater samples (2 870-7 960 Bq/m 3 , mean: 4 560 Bq/m 3 ) in same season were used as the end-member to calculate SGD fluxes (Luo et al, 2020). Using these values, the SGD fluxes in the Jiaozhou Bay were estimated to be 0-3.1 cm/h (mean: 1.1 ± 0.60 cm/h), i.e., 0-75.1 cm/d (mean: 25.4 ± 14.5 cm/d).…”

Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment

“…A number of contributions to this Special Issue add specifically to the growing body of literature on submarine groundwater discharge. Luo et al [1] show, based on radon time-series measurements, that submarine groundwater discharge rates to the Jiaozhou Bay (China) are 8-14 times higher than those of the local rivers. By combining radium isotopes, radon, and stable isotopes of nitrate, Knapp et al [2] reveal the importance of submarine groundwater discharge for the primary productivity of coastal waters off Saipan (Northern Mariana Island, US).…”