Effects of Vegetation, a Clay Cap and Environmental Variables on 222Rn Fluence Rate from Reclaimed U Mill Tailings

Abstract

We measured 222Rn fluence rate and several environmental variables on two plots with U mill tailings buried beneath 30 cm of overburden and 20 cm of topsoil. An additional 30 cm of clay covered the tailings on one plot and each plot was subdivided into bare soil and vegetated subplots. We used linear correlation, two-way ANOVA and stepwise multiple regression to analyze the effects of the plot characteristics and the environmental variables on 222Rn fluence rate. The most important effect on 222Rn fluence rates from these plots was the combination of a clay cap and a vegetated surface. The mean annual fluence rate from the plot having both of these characteristics (520 +/- 370 mBq m-2 s-1) was over three times that of the vegetated plot without a clay cap (170 +/- 130 mBq m-2 s-1) and 18 times that of the bare plot with a clay cap (29 +/- 13 mBq m-2 s-1). The interaction effect may have been due to the growth of roots in the moist clay and active transport of dissolved 222Rn to the surface in water. This speculation is supported by the observation that on vegetated plots with a clay cap, moisture in the clay enhanced the fluence rate.

Guidance for implementing the long-term surveillance program for UMTRA Project Title I Disposal Sites

“…These studies also show that plant roots have grown into the infiltrationhadon barrier layer a t some of the disposal cells (DOE, 1992;. The impacts of plant root growth into this layer have not been determined although studies on the UMTRA Project and elsewhere indicate that such plant root growth could result in an increase in radon emanation from the disposal cell (DOE, 1995; Morris and Fraley, 1989). Other potential impacts from plant biointrusion into the disposal cell include increased water infiltration into the cell, breakdown of the engineered cover system, and transport of other contaminants out of the cell.…”

UMTRA project disposal cell cover biointrusion sensitivity assessment, Revision 1

“…Consistent with the scope of this paper, no new site-specific data were collected from UMTRA Project sites. Further, this paper does not focus on the issue of plant transport of radon gas or other contaminants out of the disposal cell cover though it is acknowledged that such transport has the potential to be a significant pathway for contaminants t o reach the environment during portions of the design life of a disposal cell where plant growth occurs (Morris a& Fraley, 1989). Rather, this study was performed to evaluate the effects of physical penetration and soil drying caused by plant roots that have and are expected to continue t o grow in UMTRA Project disposal cell covers.…”

“…It is unclear t o what extent transpiration would increase the overall flux from a cell, or compete with the soil as a medium of transport, potentially resulting in a decrease of radon flux from the physical soil barrier itself. Certainly, there is evidence in the literature indicating that transpiration will result in removal of radon from the cell (Morris and Fraley, 1989). However, a comprehensive examination of this phenomenon and the synergism that may exist between transpiration and the parameters discussed herein, is beyond the scope of this assessment.…”

An assessment of plant biointrusion at the Uranium Mill Tailings Remedial Action Project rock-covered disposal cells

“…In the above referenced studies, the roots had penetrated the tailings. In a study by Morris and Fraley (1989), it was determined that radon-222 (Rn-222) emanation from study plots with plants growing In soil over tailings was enhanced over that of unvegetated plots. One study plot had a soil layer and clay cap over the tailings and another was just soil over the tailings.…”