HR: 1340h
AN: B23B-1051    [Abstracts]
TI: ²²⁶Ra, ²²⁸Ra, ²²³Ra and ²²⁴Ra in a groundwater-augmented Florida lake
AU: * Smoak, J M
EM: smoak@stpt.usf.edu
AF: University of South Florida, Environmental Science, Davis Hall 258 140 7th Ave. South, St. Petersburg, FL 33701 United States
AU: Krest, J M
EM: krest@stpt.usf.edu
AF: University of South Florida, Environmental Science, Davis Hall 258 140 7th Ave. South, St. Petersburg, FL 33701 United States
AB: A study of the lake waters of Saddleback Lake in Hillsborough County, Florida was undertaken with the goal of determining the source of elevated radium activities in the lake. The groundwater-augmentation well was sampled on three occasions and four stations within the lake were sampled numerous times over a 664 day period. Four radium isotopes, ²²⁶Ra, ²²⁸Ra, ²²³Ra and ²²⁴Ra, were measured and activities of all four radium isotopes were substantially greater in the groundwater-augmentation well water as compared to the lake waters. In the surface water, radium activities were highest close to the groundwater augmentation well in the initial sampling. Specific activities initially decreased with time after groundwater augmentation ceased. The ²²³Ra/²²⁶Ra activity ratios decreased during the first month of sampling and closely followed an exponential decay curve based on the ²²³Ra decay constant. Trends in the specific activities and the ²²³Ra/²²⁶Ra ratios support the conclusion that the groundwater-augmentation well was the dominant source of ²²³Ra and ²²⁶Ra to Saddleback Lake during this study. Radium-224/²²⁶Ra ratios during the study did not follow the expected trend of exponential decay based on the ²²⁴Ra decay constant. While ²²⁴Ra decreased after the initial sampling, the activity remained greater than expected after input from the groundwater-augmentation well ceased. While groundwater augmentation supplied some ²²⁴Ra , these results suggest that there must be an additional source of ²²⁴Ra to the lake. Because meteoric precipitation did not cause activities or ratios of ²²⁴Ra to increase, the most likely additional source of ²²⁴Ra appears to be the ingrowth of ²²⁴Ra on the sediment within the lake from ²²⁸Ra. Radium-224 is a member of the ²³²Th decay series and is unique from the other isotopes in that it has another radium isotope as one of its parents in the decay series. Groundwater augmentation apparently supplied ²²⁴Ra directly to the lake, as well as ²²⁸Ra that subsequently generated ²²⁴Ra.
DE: 0458 Limnology (1845, 4239, 4942)
DE: 1040 Radiogenic isotope geochemistry
DE: 1830 Groundwater/surface water interaction
DE: 1872 Time series analysis (3270, 4277, 4475)
DE: 1880 Water management (6334)
SC: Biogeosciences [B]
MN: Fall Meeting 2005