USGS1993
Radium isotope activities ( 226 Ra, 228 Ra, and 224 Ra), chemical compositions, and sulfur isotope ratios in sulfate were determined for water samples from thermal areas in Yellowstone National Park, Wyoming. Activities of 226 Ra in these waters range from <0.2 to 37.9 dpm/kg. Activity ratios of 228 Ra 226 Ra "> 228Ra226Ra range from 0.26 to 14.2, and those of 224 Ra 228 Ra "> 224Ra228Ra range from 0.73 to 3.1. Radium concentrations are inversely correlated with aquifer equilibration temperatures (estimated from dissolved silica concentrations), while[ Ra Ba "> RaBa ] aq and 228 Ra 226 Ra "> 228Ra226Ra activity ratios depend upon U Ba "> UBa and Th U "> ThU ratios in aquifer rocks. Major controls on Ra concentration in Yellowstone thermal waters are inferred to be 1. (1) barite saturation (at Morris Geyser Basin, Mammoth Hot Springs, and other northern areas) and 2. (2) zeolitewater ion exchange (at Upper Geyser Basin). The data are consistent with a model in which 3. (1) radium and barium are supplied to water by bulk dissolution of aquifer rock, and 4. (2) chemical equilibration of water with rock is rapid relative to the 1602 year half-life of 226 Ra. The 228 Ra 226 Ra "> 228Ra226Ra activity ratios of the waters may in some cases reflect surface enrichments of 232 Th and/or may indicate that α-recoil input of 228 Ra is rapid relative to water-rock chemical equilibration. Activity ratios of 224 Ra 228 Ra "> 224Ra228Ra indicate a nearly ubiquitous 224 Ra excess that generally increases with decreasing pH. Near-surface (≤100 m) thermal water flow velocities at Mammoth Hot Springs are estimated from 224 Ra 228 Ra "> 224Ra228Ra variation to be ≥ 1 m h −1 .