IMPROVING OUR UNDERSTANDING OF THE CONTROLS OF RADON-222 ACTIVITIES IN GROUNDWATER WITH A GOAL OF ENHANCING ITS VALUE AS A TRACER OF GROUNDWATER DISCHARGE TO THE COASTAL OCEAN

Groundwater discharge has in recent years emerged as an important source of both nutrients and certain contaminants to the coastal ocean. Quantifying this discharge is challenging, because discharge is heterogeneous and often occurs below the water surface, where direct observation and measurement are difficult. One means of quantifying this discharge is via the use of radon-222. Radon-222 is an excellent tracer of groundwater discharge to the ocean for several reasons: 1) it is strongly enriched in groundwater relative to surface water; 2) it is non-reactive; 3) it is continuously supplied by long-lived parent isotopes; and 4) its activity varies little over distance scales of tens of meter, such that radon-based discharge estimates integrate over a comparable distance. One of the largest uncertainties in quantitative estimates of groundwater discharge to the coastal ocean using radon as a tracer stems from spatial variability in the radon content of the groundwater. Improved understanding of the causes of this variability would increase the value of radon as a groundwater discharge tracer. With this in mind we carried out a study in the groundwaters of West Falmouth Harbor to try to better understand the causes of radon variability. A reasonably consistent two-fold increase in radon activity was observed between surficial groundwaters and groundwaters from a depth of ~9 m in five different locations adjacent to the harbor. Possible controls on these radon activities include redox conditions, gas solubility control, and solid-phase concentrations of the parent isotope, Ra-226. The controls on these radon concentrations will be examined in this talk in order to increase the utility of radon-222 as a quantitative tracer of groundwater discharge to the coastal ocean.