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 x 10^7 m3/d to 6.36 x 10^7 m3/d, while SGD ranges from 5.32 x 10^8 m3/d to 6.20 x 10^8 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.