Xuejing Wang

Submarine fresh groundwater discharge into Laizhou Bay comparable to the Yellow River flux

Near- and off-shore fresh groundwater resources become increasingly important with the social and economic development in coastal areas. Although large scale (hundreds of km) submarine groundwater discharge (SGD) to the ocean has been shown to be of the same magnitude order as river discharge, submarine fresh groundwater discharge (SFGD) with magnitude comparable to large river discharge is never reported. Here, we proposed a method coupling mass-balance models of water, salt and radium isotopes based on field data of 223Ra, 226Ra and salinity to estimate the SFGD, SGD. By applying the method in Laizhou Bay (a water area of ~6000 km2), we showed that the SFGD and SGD are 0.57 ~ 0.88 times and 7.35 ~ 8.57 times the annual Yellow River flux in August 2012, respectively. The estimate of SFGD ranges from 4.12 × 107 m3/d to 6.36 × 107 m3/d, while SGD ranges from 5.32 × 108 m3/d to 6.20 × 108 m3/d. The proportion of the Yellow River input into Laizhou Bay was less than 14% of the total in August 2012. Our method can be used to estimate SFGD in various coastal waters.

Seawater-groundwater Exchange in a Silty Tidal Flat in the South Coast of Laizhou Bay, China

ABSTRACT Hou, L. J.; Li, H. L.; Zheng, C. M.; Ma, Q.; Wang, C. Y.; Wang, X. J., and Qu, W. J., 2016. Seawater-groundwater exchange in a silty tidal flat in the south coast of Laizhou Bay, China. There were few studies about seawater-groundwater exchange in silty, low-permeability tidal flats with very gentle slopes. This paper reports the monitoring data and preliminary analytical results on a typical transect in a silty tidal flat with large-scale seepage faces at the south coast of Laizhou Bay, China. The “pair-wells method”, which was an improvement of the single-well method used by Ma et al. (2015) was used to estimate the seawater-groundwater exchange rate. We selected 14 locations along a typical transect in the intertidal zone, with a slope of 0.4‰ and cross-shore length of 3514 m, to install pair-wells for monitoring the groundwater head, salinity and temperature at two different depths simultaneously once per hour from 20:00 August 10th to 23:00 September 12th, 2014. The vertical hydraulic conductivity measured in situ ranges from 5.4 ×10−7 to 1.1×10−5 m s−1. The salinity of the underground brine ranges from 25 to 56 g l−1. Based on the observed data and the generalized Darcys law, the submarine groundwater discharge (SGD) and inflow along the entire transect were estimated to be 164.0 and 6.5 m2 d−1, respectively. It is found that 80.4% of the total SGD occurred between W7 and W10, where the hydraulic conductivity is one or two orders of magnitude greater than that at other wells; and 64.6% of the total inflow occurred between W2 and W4. The single-well method, on the other hand, yields a SGD value of 223.0 m2 d−1 and inflow value of 13.3 m2 d−1. Neglecting the density effect may lead to an overestimation of the SGD by 25.6% and underestimation of the inflow by 27.4%.

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.

Coastal water quality assessment and groundwater transport in a subtropical mangrove swamp in Daya Bay, China

Coastal water quality assessment is challenging due to the complex hydrological environment in mangrove swamps. Such assessment requires a good understanding of swamp hydrology and potential solute reactions. In this study, we investigated the concentration variations of a suite of major elements (Na+, K+, Ca2+, Mg2+, SO42-, HCO3- and Cl-), nutrients such as dissolved inorganic phosphorous (DIP) and silicate (DSi), and heavy metals (Cu, Zn, As, Hg, Cd, Pb and Cr) along a typical mangrove transect in Daya Bay, China. The transect comprises a tidal creek, a mudflat and two mangrove zones. The major elements exhibited various degrees of dilution and loss, such as losses of K+ due to uptake by mangrove trees and of SO42- due to microbial sulfate reduction. Numerical simulations of groundwater flow showed that the inland fresh groundwater can continuously discharge through the landward mangrove zone, where high concentrations of DIP and DSi occurred. However, the middle mangrove zone served as a sink of DIP and DSi due to the weak hydrodynamic environment. The spatial distribution of heavy metal concentrations showed that only Cu pollution occurred in the creek zone. The Metal Pollution Index (MPI) was developed to compare the enrichment capability of heavy metals at the different sampling sites. Results showed that the intertidal groundwater has the strongest heavy metal enrichment capability, followed by sea water, inland groundwater and river water. Within the intertidal zone, the creek zone has the highest MPI, followed by the mangrove zone and the mudflat zone. The local water circulation occurred around the creek should contribute the high MPI in the creek zone.

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.

Groundwater–surface water exchange associated metals at two intertidal transects, Dan’ao Estuary, Daya Bay, China

In the evaluation of aquatic environments in estuarine waters, the groundwater–surface water exchanges and associated metal fluxes are difficult to quantify and often ignored. This study made such an attempt based on field data at two intertidal transects in the estuarine wetland of Dan’ao River, the largest river flowing into Daya Bay, China. The results of groundwater–surface water exchange associated six common metals (Cu, Zn, As, Cd, Pb, and Cr) indicate the following: the cumulative concentrations of the six metals are ranked as seawater > groundwater > river water. Among all estuarine groundwaters, the sum of metal concentrations in rhizospheric groundwater at the upstream transect is the highest, owing to the enriching effects of mangrove rhizospheres on metals. The net fluxes of Cu, Zn, As, Cd, and Cr at the upstream transect are 4.8–13.8, 11.9–16.7, 147.8–190.2, 4.1–4.9, and 10.0–12.6 times higher than those at the downstream one. The much higher groundwater discharge rates at the upstream mangrove transect plays a vital role in increasing metal fluxes from groundwater to surface water in this estuary.