Geochemical and physical sources of radon variation in a subterranean estuary — Implications for groundwater radon activities in submarine groundwater discharge studies
Section snippets
Introduction and study site
Submarine groundwater discharge (SGD) facilitates transport of dissolved components from coastal aquifers to the sea (Valiela et al., 1990, Moore, 1996, Burnett et al., 2001, Slomp and van Cappellan, 2004). Groundwater advection rates to surface waters are often difficult to quantify but can be assessed indirectly via geochemical tracers such as radium isotopes and radon (Moore, 1996, Moore, 2000, Charette et al., 2001, Krest and Harvey, 2003, Burnett et al., 1996, Cable et al., 1996, Burnett
Methods
Groundwater samples were collected at the head of Waquoit Bay, MA at a location just above the mean sea level of the water line. In order to exclude variability in the STE due to tides, monthly sampling was always performed during the same phase of the tidal cycle (a few days before low spring tide) between the period of October 2004 and September 2007. A multi-level well constructed from nylon tubing and stainless-steel well points was set up at 8 discreet levels covering the full depth of the
Results
The time-series groundwater salinity profiles plotted in Fig. 2C show strong seasonality at almost all depths. In the usually fresh (salinity 0–1) top layer, salinity increases to 10–20 in some fall and winter months perhaps due to overtopping of seawater during storm events. The salinity gradient is the steepest between 3.5 and 4.5 m within which salinity ranges from 1 to 25. This region shows seasonal fluctuations, with low salinity in the spring and high salinity in the summer and fall. The
The mechanism of radon enrichment in the STE and the sources of its variability
The sole source of 222Rn (t1/2 = 3.8 d) in groundwater is the radioactive decay of 226Ra (t1/2 = 1600 y), where 50% of the equilibrium radon activity builds up in 3.8 days and 98% is reached within 21 days. Radium, the parent of radon, can be bound in the mineral lattice in aquifer solids, retained on grain surface coatings, and dissolved in pore water. The former two fractions are collectively referred to as the labile 226Ra pool. The fraction of radon produced from lattice bound radium and
Conclusions
Complex redox and mixing processes in the subterranean estuary influence the distributions of Mn and 226Ra between sediment particle surfaces and pore water. We postulate that radium accumulation on Mn (hydr)oxides ultimately drives 222Rn pore water activities. Seasonal monitoring of the STE or sampling across the full salinity gradient, may be necessary to derive the best representative groundwater end-member radon activities. In designing a groundwater radon sampling strategy for a
Acknowledgements
The authors thank Gillian Smith, Paul J. Morris, and DeAnna McCadney for assistance in the field and laboratory. David Schneider of the WHOI ICP-MS Facility performed the trace metal analyses. We thank Billy Moore, Alex Rao, and two anonymous reviewers for their constructive comments on the manuscript. We extend our continued appreciation to the director and staff of the Waquoit Bay National Estuarine Research Reserve for their assistance with logistics during field sampling. This work is a
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