Use of 222Rn and 226Ra to trace groundwater discharge into the Chesapeake Bay

Activities of 222Rn and 226Ra were measured in water samples from the Chesapeake Bay during two summers and one winter in 1995 and 1997 in order to develop a potential groundwater tracer. A major estuary to the Bay, the Potomac, was also sampled for these nuclides during 1997 in order to assess their inputs to the Bay. The 222Rn activities ranged between about 20 and 140 dpm/100 l within the Chesapeake Bay. Higher activities were encountered at the northern end of the Bay. In the upper Potomac, activities as high as 200–300 dpm/100 l were observed. These activities are twice to as much as two orders of magnitude higher than coastal ocean 222Rn. The activities generally decrease with salinity and depth to the south and show little change between summer and winter. The 222Rn activities are much in excess of its parent 226Ra which suggests an active disequilibrated source of 222Rn to these waters. Considering the short half-life of 222Rn (3.8 days) and its limited exchange with the atmosphere, another important source is suggested to be the exchange of groundwater in lower salinity regions of the Bay or its tributaries. The inverse trend of 222Rn against salinity is consistent with a fresh groundwater end member from the Chesapeake Bay watershed. The 222Rn concentrations of the Potomac tributary are even greater than in the Bay, increasing from 47 dpm/100 l at the Bay entrance to about 280 dpm/100 l upstream and suggest an even more significant groundwater discharge. Calculations suggest as much as 10% of freshwater inputs to the Chesapeake Bay could be from groundwater.