Colloid-facilitated transport of radium and thorium in the Memphis Aquifer, Memphis, Tennessee, U.S.A.

摘要

The significance of groundwater colloidal transport was examined in the context of the Memphis Aquifer (Memphis, Tennessee) in the vicinity of the Sheahan well field. The hydrological and geochemical characteristics of the Memphis Aquifer were characterized using U- and Th-series isotopes in conjunction with environmental parameters. Ground water flows from source areas associated with elevated-recoil rates to sinks where isotopes become highly retarded and are eventually removed via municipal ground water production. Distribution coefficients (K) calculated for the dissolved/stationary (Ra only, according to Luo et al (2000)) and dissolved/colloid systems (Ra and Th) are significantly lower than other reported values in the literature. While colloidal association of thorium and radium varied from <0.1% up to 75%, the degree of association with the colloid phase was hindered by the lack of Mn-oxides. A correlation between radium retardation and increased iron concentrations was found to be counter-intuitive, until the role of iron-reducing microbiology was considered. Retardation of radium is correlated not with the presence of ferrihydrite, but with organic matter instead. Increased ferrous iron concentrations serve as a proxy for microbial activity.