Vertical migration of 134Cs bearing soil particles in arid soils: implications for plutonium redistribution

doi:10.1016/j.jenvrad.2006.01.010

Journal of Environmental Radioactivity

Volume 88, Issue 2, 2006, Pages 171-188

https://doi.org/10.1016/j.jenvrad.2006.01.010 Get rights and content

Abstract

Vertical migration of plutonium in soils at the Waste Isolation Pilot Plant (WIPP) and the Rocky Flats Environmental Technology Site (RFETS) was evaluated based on observed ¹³⁴Cs migration in soil column experiments. After applying ¹³⁴Cs-labeled soil particles to the surfaces of large, undisturbed soil cores collected from each site, resulting soil columns were subjected to experimental cycles of irrigation plus drying (treatment columns) or to cycles of irrigation only (control columns). Mean losses of ¹³⁴Cs inventory from soil surfaces were 3.1 ± 0.6% cycle⁻¹ and 0.7 ± 0.6% cycle⁻¹ respectively for RFETS treatment and control columns. WIPP columns had mean respective losses of 1.3 ± 1.2% cycle⁻¹ and 0.5 ± 0.2% cycle⁻¹. Bulk transport of labeled soil particles through soil cracks was an important process in RFETS soils, accounting for 64–86% of total ¹³⁴Cs migration. Colloidal transport processes governed migration in WIPP soils.

Introduction

The mobility of plutonium in near-surface soils is an important issue with respect to environmental management policies at US Department of Energy facilities. Plutonium transport models require knowledge of site-specific environmental factors and related processes. Under most environmental conditions, transport of Pu is governed by the physical transport of contaminated soil particles (Higley, 1994, Hakonson and Bostick, 1975). Of the basic physical mechanisms for transporting Pu-contaminated soil particles, wind and water erosion acting on the soil surface are of greatest concern in terms of spreading activity beyond initial contamination areas and creating health risks to the public. However, the rate of vertical Pu migration into soil profiles is important because it affects erosional loss rates. On time scales of a few decades or less, plutonium residing deeper than a few centimeters into the soil profile is generally not subject to erosional processes. The purpose of this study was to evaluate the potential vertical migration of Pu-contaminated soil particles (using irreversibly bound ¹³⁴Cs tracer as a predictor for Pu migration) in soils from the Waste Isolation Pilot Plant (WIPP), New Mexico, USA, and the Rocky Flats Environmental Technology Site (RFETS), Colorado, USA. This information is necessary for future development of credible, site-specific inventory loss rate expressions for environmental Pu transport models.

This study focused on precipitation and drying sequences as a driving force for the vertical migration of ¹³⁴Cs-contaminated soil particles. Other potential driving forces such as bioturbation (Bunzl, 2002) or freeze–thaw action (Higley, 1994) were not considered due to practical constraints on the scope and duration of the study. In the arid climatic regimes at both WIPP and RFETS, it is fairly common for drought-like conditions to be punctuated by occasional large precipitation events. How such climatic sequences affect the vertical migration of soil particles or colloids is not well known. Potential mechanisms for vertical ¹³⁴Cs migration under simulated cycles of drought and precipitation, as depicted in Fig. 1(A), include “colloidal migration” (transport of colloid-sized particles through interstitial micropores in the soil matrix) and “bypass migration” (bulk transport of many characteristic soil particle sizes through soil cracks).

Specific study objectives were to (1) quantify the total amount of vertical migration of ¹³⁴Cs tracer in soils from each site due to the combined affects of bypass migration and colloidal migration processes, (2) partition and quantify bypass migration and colloidal migration processes at each site, and (3) evaluate spatial and temporal variability in vertical migration.

Section snippets

Conceptual framework

Although Pu (an actinide) and Cs (an alkali metal) are chemically different elements, there is substantial evidence in the literature that under a wide range of soil and weather conditions, both Cs and Pu bind strongly to soil particles and that Cs can be used to estimate both horizontal and vertical Pu distribution in soil (EPA, 1999, Hulse et al., 1999, Hodge et al., 1996, Lee et al., 1997, White et al., 1981). The characteristic adsorption of both Cs and Pu as a function of particle size is

Total vertical migration

Experimental evidence relevant to the primary objective of this study (estimating total vertical migration) was obtained directly from treatment column results. This was because respective cycles of wetting and drying served to approximate actual periodic field conditions under which both colloidal and bypass migration processes could potentially occur. The initial unit bases for our estimates were % cycle⁻¹ for loss of inventory from the original ¹³⁴Cs-labeled layer at the surface, and cm cycle

Conclusions

The data generated from this study support a general conclusion that in arid or semi-arid climates, soils with high clay contents such as those found at the RFETS site (about 50% clay) have significantly enhanced potential for vertical transport of Cs-bearing soil particles due to soil cracking and bypass migration versus sandy soils with low clay contents such as those found at the WIPP site (about 6% clay). In this study, bypass migration through drying-induced cracks in RFETS soils was

Acknowledgements

Funding for this effort was provided by the US Department of Energy Environmental Management Science Program, and by the Office of Nuclear Energy, Science and Technology through the Radiochemistry Education Award Program (REAP). Special thanks are extended to Dr John Pinder, III, Dr Ward Whicker, and Chuck Sampier for technical assistance and to Michelle Whicker for assistance with data collection.

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Vertical migration of 134Cs bearing soil particles in arid soils: implications for plutonium redistribution

Abstract

Introduction

Section snippets

Conceptual framework

Total vertical migration

Conclusions

Acknowledgements

Geoderma

The Science of the Total Environment

Chemosphere

Soil and Tillage Research

Journal of Environmental Radioactivity

Journal of Environmental Radioactivity

Geoderma

The migration of 137Cs and 90Sr in multilayered soils: results from batch, column, and fallout investigations

Nuclear Technology

Vermiculites

Water in Environmental Planning

Understanding Variation in Partition Coefficient, Kd, Values. Volumes I and II

Cesium-137 and plutonium in liquid waste discharge areas at Los Alamos

An overview of Los Alamos research on soil and water processes in semi-arid ecosystems

Vertical migration of ¹³⁴Cs bearing soil particles in arid soils: implications for plutonium redistribution

The migration of ¹³⁷Cs and ⁹⁰Sr in multilayered soils: results from batch, column, and fallout investigations

Understanding Variation in Partition Coefficient, K_d, Values. Volumes I and II