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Steady-state seawater–groundwater interactions are simulated for homogeneous, anisotropic unconfined coastal aquifers using the two-dimensional numerical model MARUN. The spatial salinity distributions are approximately independent of the horizontal hydraulic conductivity K x when α L K x K z ⩾ 10 −6 m 2 /s. Here K z is vertical hydraulic conductivity and α L is longitudinal dispersivity. Both the fresh groundwater discharge rate Q f and seawater recirculation rate Q s depend linearly on K x . These conclusions are validated by a semi-analytical method. The increase of anisotropy ratio pushes the saltwater wedge interface seaward and reduces Q s . The increment of the longitudinal/transverse dispersivity ratio with fixed longitudinal dispersivity decreases the slope of isosalines and Q s . When the seabed slope angle θ increases from 0.01 to π /2, the freshwater–seawater interface moves landward. The increment of θ increases Q s slightly when θ < π /4 and reduces Q s when θ > π /4. The inland recharge rate Q f is independent of the seabed slope. [ABSTRACT FROM AUTHOR]

The natural radioisotope radon (222Rn) has been widely used as a tracer for quantifying submarine groundwater discharge (SGD). Applicable equipment for continuous radon-in-water detection using a radon-in-air monitor includes the "RAD AQUA" and the "Water Probe" (both Durridge Inc., USA). Although both devices have been applied to measure 222Rn concentrations in coastal water over time periods from hours to days, the monitoring results for the same water body may differ due to the different physical driving forces for 222Rn degassing from the water, which will result in an inaccurate SGD estimate. Here the two approaches were simultaneously applied for monitoring 222Rn concentrations at the same site in the same surface-water body (Daya Bay, China). The SGD fluxes resulting from both approaches were evaluated and compared, based on the obtained time series and a 222Rn mass balance model. The results demonstrate that the average 222Rn concentration measured by the Water Probe is only ~30% of that detected by means of the RAD AQUA. The mean SGD flux estimated based on 222Rn time series measured with the Water Probe is 69% of that detected by means of the RAD AQUA. The findings of this study suggest that the RAD AQUA can capture more accurately the signals of variation of radon concentration, and is a better approach for continuously monitoring radon and quantifying SGD in coastal zones. Thus, previous studies based on 222Rn data from the Water Probe for estimating SGD and the associated nutrient flux may need re-evaluation. [ABSTRACT FROM AUTHOR]

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 Darcy's 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%. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]