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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]

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]