Elsevier

Marine Chemistry

Volume 110, Issues 1–2, 16 May 2008, Pages 120-127
Marine Chemistry

Geochemical and physical sources of radon variation in a subterranean estuary — Implications for groundwater radon activities in submarine groundwater discharge studies

https://doi.org/10.1016/j.marchem.2008.02.011Get rights and content

Abstract

Submarine groundwater discharge (SGD), from springs and diffuse seepage, has long been recognized as a source of chemical constituents to the coastal ocean. Because groundwater is two to four orders of magnitude enriched in radon compared to surface water, it has been used as both a qualitative and a quantitative tracer of groundwater discharge. Besides this large activity gradient, the other advantage of radon stems from its classification as noble gas; that is, its chemical behavior is expected not to be influenced by salinity, redox, and diagenetic conditions present in aquatic environments.

During our three-year monthly sampling of the subterranean estuary (STE) in Waquoit Bay, MA, we found highly variable radon activities (50–1600 dpm L 1) across the fresh–saline interface of the aquifer. We monitored pore water chemistry and radon activity at 8 fixed depths spanning from 2 to 5.6 m across the STE, and found seasonal fluctuations in activity at depths where elevated radon was observed. We postulate that most of the pore water 222Rn is produced from particle-surface-bound 226Ra, and that the accumulation of this radium is likely regulated by the presence of manganese (hydr)oxides. Layers of manganese (hydr)oxides form at the salinity transition zone (STZ), where water with high salinity, high manganese, and low redox potential mixes with fresh water. Responding to the seasonality of aquifer recharge, the location of the STZ and the layers with radium enriched manganese (hydr)oxide follows the seasonal land- or bayward movement of the freshwater lens. This results in seasonal changes in the depth where elevated radon activities are observed.

The conclusion of our study is that the freshwater part of the STE has a radon signature that is completely different from the STZ or recirculated sea water. Therefore, the radon activity in SGD will depend on the ratio of fresh and recirculated seawater in the discharging groundwater.

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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