Radon and radium isotope assessment of submarine groundwater discharge in the Yellow River delta, China

Abstract

Naturally occurring chemical tracers were used to assess the magnitude of submarine groundwater discharge (SGD) during two different sampling periods at a coastal site south of the Yellow River delta, China. We used salinity and pH as indicators of the terrestrial and recirculated seawater components of discharging groundwater and radium isotopes to quantify offshore transport rates. We then used an hourly time series of multiple radium isotopes (224Ra, 223Ra, and 226Ra) to quantify SGD rates and also used 222Rn and seepage meters to independently quantify SGD rates as a comparison to the radium results. Offshore transport rates were found to range from 3.3 to 4.7 cm s−1. Modeled time series radium activities indicated average SGD rates ranging from 4.5 to 13.9 cm d−1 in September 2006 and from 5.2 to 11.8 cm d−1 in July 2007. Temporal trends associated with the radium approach agree with SGD patterns revealed by automated seepage meters deployed nearby, but the absolute fluxes are about 70% lower than those determined by the seepage meters. Modeled SGD rates based on 222Rn (mean = 13.8 cm d−1 in 2006 and 8.4 cm d−1 in 2007) agree with those determined by the radium analysis. Differences in derived SGD rates between the different radium isotopes (226Ra highest; 224Ra lowest) are likely results of uncertainties in the background activities and our limited selection of appropriate groundwater/pore water end‐member values. Scaling our results to the entire Yellow River delta, we find SGD fluxes (and corresponding nitrate fluxes) 2–3 times that of the Yellow River.