A basin-wide water quality survey for the radionuclide tritium during 2017 and 2019 provides an overview of levels in Great Lakes surface waters. All data, together with those from similar basin-wide surveys since the early 1990s, are included in the Supplemental Material. Values of tritium are lowest in Lake Superior and are highest within a region of northwestern Lake Ontario, as well as locally near a known source in Lake Huron. Twenty-year trends show declines in all of the lakes, and this is consistent with the decline in fallout from past nuclear weapons testing, the major source of tritium to the lakes. Longer-term trends, developed using values from the literature, demonstrate a marked overall reduction in tritium values since maxima in the late 1960s, with a slowing rate of decline in the most recent decade. As atmospheric fallout is reduced, the relative importance of other sources is increasing. Known releases, primarily from nuclear generating stations using heavy water, could therefore drive any future changes in Great Lakes tritium levels.

2017 年和 2019 年期间对放射性核素氚进行的全流域水质调查概述了五大湖地表水的水平。所有数据以及自 1990 年代初期以来类似的全流域调查的数据都包含在补充材料中。氚的价值在苏必利尔湖最低，在安大略湖西北部地区以及休伦湖已知来源附近的当地最高。20 年的趋势表明所有湖泊都呈下降趋势，这与过去核武器试验产生的放射性尘埃的下降是一致的，后者是湖泊氚的主要来源。使用文献值得出的长期趋势表明，自 1960 年代后期达到最大值以来，氚值整体显着下降，最近十年下降速度放缓。随着大气沉降物的减少，其他来源的相对重要性正在增加。已知的排放，主要来自使用重水的核电站，因此可能会推动五大湖氚水平的任何未来变化。