Seasonal trends in uranium distributions of two Florida rivers: Influence of geology and hydrology

Uranium (U) is commonly associated with carbonate bedrock and phosphate deposits, both of which can be found within Florida. Much of central Florida's land is mined for phosphate to produce fertilizer, resulting in elevated U levels in local water bodies, although causes for any temporal trends are not well understood. Seasonal U distributions in the Alafia and Hillsborough Rivers, which contribute to the Tampa Bay estuary, were evaluated by collecting water samples in the wet season (May to July 2022), the hurricane season (October to November 2022), and the dry season (January to March 2023). These rivers had U concentrations (0.8 – 12.5 nM) up to nine times greater than the global average concentration for rivers and estuaries. Located closer to the phosphate deposits, the Alafia River had higher U and phosphorus (P) concentrations than the Hillsborough River during all seasons, while both rivers had similar calcium (Ca) concentrations. On the Alafia River, U was strongly correlated with both P (r = 0.80, p < 0.001) and Ca (r = 0.85, p < 0.001) due to the phosphate deposits and carbonate bedrock. On the Hillsborough River, U was more strongly correlated with Ca (r = 0.86, p < 0.001) than P (r = 0.43, p < 0.001) likely due to the influence during the dry season of groundwater flow from Sulphur Springs. These correlations were skewed by data from the hurricane season when U and Ca concentrations were low, but P concentrations were high as a result of increased fertilizer runoff. Overall, this study reveals that U concentrations in Tampa Bay are primarily controlled by the underlying geology, but the local rainfall has a strong influence as well.