228Ra, 226Ra, 224Ra and 223Ra in potential sources and sinks of land-derived material in the German Bight of the North Sea: implications for the use of radium as a tracer

Abstract

Activities of the naturally occurring radium nuclides 228Ra, 226Ra, 224Ra and 223Ra were determined in waters of the open German Bight and adjacent nearshore areas in the North Sea, in order to explore the potential use of radium isotopes as natural tracers of land–ocean interaction in an environment characterised by extensive tidal flats, as well as riverine and groundwater influx. Data collected at various tidal phases from the Weser Estuary (228Ra: 46.3±4.6; 226Ra: 17.1±1.1; 224Ra: 26.1±8.2 to 36.5±6.1; 223Ra: 1.8±0.1 to 4.0±0.4), tidal flats near Sahlenburg (228Ra: 39.3±3.8 to 46.0±4.5; 226Ra: 15.5± 1.5 to 16.5±1.7; 224Ra: 34.3±2.2 to 85.3±6.3; 223Ra: 3.6± 0.5 to 8.0±1.2), freshwater seeps on tidal flats near Sahlenburg (228Ra: 42.1±4.1; 226Ra: 21.3±2.2; 224Ra: 5.1±0.9; 223Ra: 2.6±1.3) and also in permanently inundated parts of the North Sea (228Ra: 23.0±2.3 to 28.2±2.8; 226Ra: 8.2±0.8 to 11.8±1.2; 224Ra: 3.1±1.0 to 10.1±0.9; 223Ra: 0.1±0.02 to 0.9±0.05; units: disintegrations per minute per 100 kg water sample) reveal that, except for the fresh groundwater, the potential end-members of nearshore water mass mixing have quite similar radium signatures, excluding a simple discrimination between the sources. However, the decreasing activities of the short-lived 224Ra and 223Ra isotopes recorded towards the island of Helgoland in the central German Bight show a potential to constrain fluxes of land-derived material to the open North Sea. The largest source for all radium isotopes is generally found on the vast tidal flats and in the Weser Estuary. Future work could meaningfully combine this so-called radium quartet approach with investigations of radon activity. Indeed, preliminary data from a tidal flat site with fresh groundwater seepage reveal a 222Rn signal that is clearly lower in seawater.