Source of radium in a well-water-augmented Florida lake

A study of the lake waters of Saddleback Lake, Florida was undertaken with the goal of determining the source of elevated radium activities in the lake. Four radium isotopes, 226Ra, 228Ra, 223Ra and 224Ra, were measured and activities of all the four radium isotopes were substantially greater in the well water used to augment the lake as compared to the lake waters. In the surface water, radium activities were highest close to the well used for augmentation in the initial sampling. Activities initially decreased with time after augmentation from the well ceased. The 223Ra/226Ra activity ratio decreased during the first month of sampling and closely followed an exponential decay curve based on the 223Ra decay constant. Trends in the activities and the 223Ra/226Ra activity ratios support the conclusion that the well used to augment the lake was the dominant source of 223Ra and 226Ra to Saddleback Lake during this study. The 224Ra/226Ra activity ratio did not follow the expected trend of exponential decay based on the 224Ra decay constant. While the augmentation well supplied some 224Ra, these results suggest that there must be an additional source of 224Ra to the lake. The most likely additional source of 224Ra appears to be the ingrowth of 224Ra on the sediment within the lake from 228Ra (via 228Th).