Kai Xiao

Nitrogen fate in a subtropical mangrove swamp: Potential association with seawater-groundwater exchange

Coastal mangrove swamps play an important role in nutrient cycling at the land-ocean boundary. However, little is known about the role of periodic seawater-groundwater exchange in the nitrogen cycling processes. Seawater-groundwater exchange rates and inorganic nitrogen concentrations were investigated along a shore-perpendicular intertidal transect in Daya Bay, China. The intertidal transect comprises three hydrologic subzones (tidal creek, mangrove and bare mudflat zones), each with different physicochemical characteristics. Salinity and hydraulic head measurements taken along the transect were used to estimate the exchange rates between seawater and groundwater over a spring-neap tidal cycle. Results showed that the maximum seawater-groundwater exchange occurred within the tidal creek zone, which facilitated high-oxygen seawater infiltration and subsequent nitrification. In contrast, the lowest exchange rate found in the mangrove zone caused over-loading of organic matter and longer groundwater residence times. This created an anoxic environment conducive to nitrogen loss through the anammox and denitrification processes. Potential oxidation rates of ammonia and nitrite were measured by the rapid and high-throughput method and rates of denitrification and anammox were measured by the modified membrane inlet mass spectrometry (MIMS) with isotope pairing, respectively. In the whole transect, denitrification accounted for 90% of the total nitrogen loss, and anammox accounted for the remaining 10%. The average nitrogen removal rate was about 2.07g per day per cubic meter of mangrove sediments.

Improving Estimation of Submarine Groundwater Discharge Using Radium and Radon Tracers: Application in Jiaozhou Bay, China

Radium and radon mass balance models have been widely used to quantify submarine groundwater discharge (SGD) in the coastal areas. However, the losses of radium or radon in seawater caused by re-circulated saline groundwater discharge (RSGD) are ignored in most of the previous studies for tracer-based models and this can lead to an underestimation of SGD. Here, we present an improved method which considers the losses of tracers caused by RSGD to enhance accuracy in estimating SGD and SGD-associated material loadings. Theoretical analysis indicates that neglecting the losses of tracers induced by RSGD would underestimate the SGD by a percentage approximately equaling the tracer activity ratio of nearshore seawater to groundwater. The data analysis of previous typical case studies shows that the existing old models underestimated the SGD by 1.9 ∼ 93%, with an average of 32.2%. The method is applied in Jiaozhou Bay (JZB), North China, which is experiencing significant environmental pollution. The SGD flux into JZB estimated by the improved method is ∼1.44 and 1.34 times of that estimated by the old method for 226Ra mass balance model and 228Ra mass balance model, respectively. Both SGD and RSGD fluxes are significantly higher than the discharge rate of Dagu River (the largest one running into JZB). The fluxes of nutrients and metals through SGD are comparable to or even higher than those from local rivers, which indicates that SGD is an important source of chemicals into JZB and has important impact on marine ecological system.

Submarine groundwater discharge and chemical behavior of tracers in Laizhou Bay, China

Naturally occurring radon (222Rn) and radium isotopes are widely used to trace water mixing and submarine groundwater discharge (SGD) in the coastal zones. However, their activities in groundwater are variable both spatially and temporally. Here, time series sampling of 222Rn and radium was conducted to investigate their behavior in intertidal groundwater of Laizhou Bay, China. The result shows that groundwater redox conditions have an important impact on the behavior of tracers. The activities of tracers will decrease under oxidizing conditions and increase under reducing conditions. Radon and radium mass balance models were used to evaluate the flushing time and SGD based on spatial surveys in Laizhou Bay. The flushing time is estimated to be 32.9-55.3 d with coupled models, which agrees well with the result of tidal prism model. The trace-derived SGD in the whole bay ranges from 6.1 × 108 to 9.0 × 108 m3/d and the re-circulated seawater (RSGD) ranges from 5.5 × 108 to 8.5 × 108 m3/d. The average SGD and RSGD fluxes are 22.8 and 21.1 times greater than the Yellow River discharge in April 2014, respectively. The study provides a better understanding of the dynamics of coastal groundwater and behavior of tracers in a well-studied bay system.