Inputs of dissolved and particulate226Ra to lakes and implications for210Pb dating recent sediments

Abstract

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