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

Gamma spectroscopy was used to measure radioisotope (Pb-210, Ra-226, Cs-137) activities in sediment cores from 20 lakes and a wetland in Florida, USA. Nine profiles display relatively low (<5 dpm g(-1)) and constant Ra-226 activities, whereas 12 show high (>5 dpm g(-1)) and variable Ra-226 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(-1)) Ra-226 activities that exceed total Pb-210 activities, clearly illustrating disequlibrium between Ra-226 and supported Pb-210. Supported Pb-210 activity is generally thought to come from in situ, Ra-226-containing detrital mineral particles, and is typically assumed to be in secular equilibrium with Ra-226 activity. Since 1966, Round Lake has been augmented hydrologically with Ra-226-rich (similar to6.2 dpm L-1) groundwater pumped from the local deep aquifer. Adsorption of dissolved Ra-226 to recent Round Lake sediments probably accounts for the high measured Ra-226 activities and the pronounced disequilibrium between Ra-226 and supported Pb-210 in topmost deposits. We suspect that many Florida waterbodies receive some Ra-226-rich runoff and seepage from groundwater pumped for irrigation, residential use, industrial applications, and mining. This may account for up-core increases in Ra-226 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 Pb-210 to come into equilibrium with adsorbed Ra-226 in uppermost deposits. Input of Ra-226-rich groundwater to lakes may occur in any geographic region where local bedrock contains U-238 and its daughters. When dissolved Ra-226 adsorbs to recent sediments, it complicates accurate estimation of supported Pb-210 activity, and confounds calculation of unsupported Pb-210 activity that is used in dating models.