Understanding the mobility of radium and plutonium in the environment, especially through soil and into groundwater, is very important. Radium is one of the principal contaminants associated with uranium mining and milling and affects a large number of sites.  It also affects thousands of secondary oil recovery sites, where radium pollution is quite common.  Plutonium is one of the most long-lived and dangerous radionuclides in the nuclear weapons production process.  Plutonium-contaminated wastes have been dumped into unlined disposal areas at several sites around the United States near critically important bodies of water.

Both radium and plutonium mobility vary widely depending on the circumstances.  Estimates of radiation dose to future generations are highly dependent upon the assumptions about how contamination in the soil actually affects groundwater.  IEER undertook a review of the environmental transport of radium and plutonium because of the scientific difficulty of the question as well as the widespread importance of these two radionuclides in cleaning up the messes from decades of uranium and plutonium processing. This piece summarizes that review.[1]

≈Arjun Makhijani  

Given the complex chemical, biological, and physical properties of soil, predicting the mobility of radionuclides is far from simple.  There are a number of examples where models relied upon by the Atomic Energy Commission and later the Department of Energy failed to accurately predict contaminant transport.  After the discovery of radionuclides spreading further and more rapidly into the environment, these models had to be fundamentally revisited. These failures were due in large part to the failure to adequately characterize the systems. 

In real systems there may be chemical or biological processes that affect the mobility of contaminants that may vary over space and time. There may also be more pathways for radionuclides to move than originally expected.  Finally, the transport model itself might be adequate, but information on what parameters to use may not be available.

This article provides a brief review of the transport of two specific radionuclides, radium and plutonium.  Radium is a naturally occurring radionuclide that is part of the uranium and thorium decay series. In the body, radium is a calcium analog, and goes primarily to the bone. Plutonium is a human-made radionuclide produced in nuclear reactors.  Inside the body, plutonium also preferentially goes to the bone.

A number of important site-specific properties can either enhance or retard the mobility of both radium and plutonium. As a result, detailed, site-specific analyses are essential. Performance assessments which are predicated on simplified models or default values should not be accepted for demonstrating compliance with regulatory limits. Transport modelers should seek to learn from past surprises. This is particularly true for radium, plutonium, and many of the other transuranic elements (elements with an atomic number greater than uranium), given the long half lives of many of these radionuclides and their parents.

The Kd approach

Many contaminants, including radium and plutonium, can adsorb onto soil through ion exchange. The strength of this interaction is quantified by the partition coefficient (Kd). The partition coefficient relates the concentration of a contaminant adsorbed onto the soil to that dissolved in the water after the system has reached equilibrium.

Kd =     concentration of contaminant in soil (pCi/kg)

concentration of contaminant in water (pCi/L)

The partition coefficient therefore has the somewhat unusual units of liters per kilogram (L/kg) or, equivalently, milliliters per gram (mL/g).  A large value for the partition coefficient implies that the contaminant is tightly bound to the soil and will therefore migrate slowly. A small value implies the opposite. Due to its relative simplicity, the constant Kd approach is the most widely used transport model today. It is called the ⌠constant■ Kd approach because the model assumes that an entire site can be characterized by one value of Kd that does not change over time.

Limitations of the Kd model

Despite its widespread use, there are a number of important limitations of the constant Kd approach.  Most importantly, the value of Kd is strongly dependent on local chemical and physical conditions and can thus vary greatly between sites and even across a single site.  The values of Kd for plutonium measured at the Hanford site, for example, vary by more than a factor of 1,000. 

This variation is because the adsorption of contaminants, like radium or plutonium, changes depending on various factors.  These factors include the action of plants and bacteria; the oxidation state of the radionuclides, which can change over time (a particular concern for plutonium, which can exist in four different states); and the amount of clay, sand and organic matter in the soil.  Specifically, the adsorption of contaminants decreases as the acidity of the soil increases and as the concentration of chemically similar ions in the system increases.

Even the U.S. Environmental Protection Agency (EPA) explicitly acknowledges that default or generic Kd values can result in ⌠significant error■ when used to predict contaminant migration.[2] 

An additional limitation is that there are five methods for measuring Kd.  As noted by EPA, ⌠it is not only common, but expected that Kd values measured by different methods will produce different values.■[3] For radium, additional uncertainties arise when other alkaline earth metals are present during measurement because co-precipitation can lead to erroneously large estimates of Kd.

More sophisticated models have been developed that overcome some of the limitations of the constant Kd approach. These models have more successfully described migration involving lead contamination and sulfate contamination at uranium mines.  The challenge of these newer models is that they require significantly more information about the site.

Other transport pathways

In some cases, other pathways and natural phenomena can be important.  Sulfate-reducing bacteria can cause radium precipitates to dissolve, thereby enhancing mobility, while the bacteria Bacillus subtilis has been found to impact the mobility of plutonium.  In addition, animals cause soil mixing, which can enhance near-surface contaminant migration.  Finally, the similarity of radium to calcium may cause it to bioconcentrate as it is transferred up the food chain. 

One of the important processes that can affect the transport of plutonium and other transuranic elements is adsorption on colloidal particles.  Colloidal particles are small particles that occur naturally and are easily suspended in ground and surface water.  The potential for colloidal transport to affect the mobility of contaminants like plutonium was recognized more than 50 years ago, but the interest in this phenomenon has grown since the discovery of rapid colloid transport of plutonium at the Nevada Test Site in the late 1990s.  The impact of colloid transport is highly site-specific, and there is evidence both supporting and questioning the importance of colloid-mediated transport at different DOE facilities.

In addition, plutonium adsorbed onto sediment may become mobilized by resuspension. Likewise, contamination bound to surface soil can become mobilized through erosion. The transport of plutonium via surface water and erosion is known to be particularly important at Los Alamos National Laboratory.  The significance of this pathway has increased since the May 2000 Cerro Grande fire that burned approximately 43,000 acres in and around the lab. The loss of vegetation has led to increased erosion, particularly during the flooding that follows storm events.

Finally, at sites contaminated with transuranics, it is important to consider decay and ingrowth of elements which may have significantly different mobility or half lives than their parents.

Conclusions and recommendations

One of the major recommendations by the EPA in a 2004 review of radium▓s mobility was that ⌠for site-specific calculations, partition coefficient values measured at site-specific conditions are absolutely essential.■[4] (Emphasis in the original.)  Similar conclusions are reasonable for plutonium.  Assessments that are predicated on default values of the partition coefficient should not be accepted for regulatory purposes. At a minimum, an effort should be undertaken to determine a well-founded, site-specific value for Kd. 

While in many cases a suitable Kd value can be determined, consideration should be given to the use of more complex transport models in light of the well-known limitations of the Kd approach, especially at sites with highly concentrated and reactive wastes.  In cases where erosion and surface water transport or colloid-mediated transport are potentially important, a model capable of including these pathways should be used, particularly in light of the relatively rapid migration of plutonium already observed at some sites.