Inputs of dissolved and particulate super(226)Ra to lakes and implications for super(210)Pb dating recent sediments

Gamma spectroscopy was used to measure radioisotope ( super(210)Pb, super(226)Ra, super(137)Cs) activities in sediment cores from 20 lakes and a wetland in Florida, USA. Nine profiles display relatively low (5 dpm g super(-1)) and variable super(226)Ra activities. In the latter group, most display up-core increases in activity. Upper sediments from two lakes (Round and Rowell) possess very high (>20 dpm g super(-1)) super(226)Ra activities that exceed total super(210)Pb activities, clearly illustrating disequlibrium between super(226)Ra and supported super(210)Pb. Supported super(210)Pb activity is generally thought to come from in situ, super(226)Ra-containing detrital mineral particles, and is typically assumed to be in secular equilibrium with super(226)Ra activity. Since 1966, Round Lake has been augmented hydrologically with super(226)Ra-rich ( similar to 6.2 dpm L super(-1)) groundwater pumped from the local deep aquifer. Adsorption of dissolved super(226)Ra to recent Round Lake sediments probably accounts for the high measured super(226)Ra activities and the pronounced disequilibrium between super(226)Ra and supported super(210)Pb in topmost deposits. We suspect that many Florida waterbodies receive some super(226)Ra-rich runoff and seepage from groundwater pumped for irrigation, residential use, industrial applications, and mining. This may account for up-core increases in super(226)Ra activity measured in sediment cores from some Florida lakes. Significant groundwater pumping began within the last century, and there has been insufficient time for supported super(210)Pb to come into equilibrium with adsorbed super(226)Ra in uppermost deposits. Input of super(226)Ra-rich groundwater to lakes may occur in any geographic region where local bedrock contains super(238)U and its daughters. When dissolved super(226)Ra adsorbs to recent sediments, it complicates accurate estimation of supported super(210)Pb activity, and confounds calculation of unsupported super(210)Pb activity that is used in dating models.