Is 36Cl from weapons' test fallout still cycling in the atmosphere?

³⁶Cl concentrations measured in rivers and lakes surface waters in Eastern North America are higher than those expected from natural production and fallout from the atmosphere. We tested the hypothesis that ³⁶Cl concentrations in these systems are elevated because ³⁶Cl from weapons test fallout is still cycling through the biosphere. Four sets of observations support this interpretation of the elevated concentrations of ³⁶Cl. First, ³⁶Cl:Cl atom ratios in small lakes and streams are lower than the ratios measured in precipitation. Secondly, ³⁶Cl:Cl atom ratio in shallow groundwater (that does not contain ³H) is lower than those measured in surface waters or rain. Thirdly, ³⁶Cl and Cl concentrations in biota are higher than groundwater and similar to precipitation. Fourthly, the inventory of Cl in the vegetation, litter and soil is large relative to the annual inputs from the atmosphere. These observations are consistent with the decadal residence time of ³⁶Cl in the biosphere calculated using a mass balance model of stable Cl^- and ³⁶Cl distribution.