radioactive waste buried beneath runit dome on enewetak atoll, marshall islands

Int. J. Environment and Pollution, Vol. 49, Nos. 3/4, 2012 161

Copyright © 2012 Inderscience Enterprises Ltd.

Radioactive waste buried beneath Runit Dome on Enewetak Atoll, Marshall Islands

M. Lee Davisson*, Terry F. Hamilton, and Andrew F.B. Tompson Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, California 94550, USA E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] *Corresponding author

Abstract: In the early 1970s after extensive characterisation of fallout the US Pacific Proving Grounds located at Enewetak Atoll began rehabilitation in preparation for the return of indigenous people who were relocated during the Cold War. Cleanup entailed removal and collection of ~545 GBq of contaminated topsoil, vegetation, and debris (concrete and metal) that was subsequently entombed within an unlined crater produced by an 18 kT surface test and capped with a concrete dome. The site is now known as the Runit Dome. Currently, the US Department of Energy conducts comprehensive radiological monitoring of people living on Enewetak Atoll, but characterisation of exposure risks posed by Runit Dome have been limited to catastrophic release scenarios and periodic atoll-wide environmental surveys. Furthermore, evidence indicates open hydraulic communication between waste and intruding ocean water, with migration pathways leading to local groundwater and circulating lagoon waters. Radionuclide migration is likely facilitated by colloids and dissolution/complexation reactions under low-pH anoxic conditions.

Keywords: Runit Dome; Enewetak Atoll; radioactive waste; radiological monitoring; radionuclide migration.

Reference to this paper should be made as follows: Davisson, M.L., Hamilton, T.F. and Tompson, A.F.B. (2012) ‘Radioactive waste buried beneath Runit Dome on Enewetak Atoll, Marshall Islands’, Int. J. Environment and Pollution, Vol. 49, Nos. 3/4, pp.161–178.

Biographical notes: M. Lee Davisson is a Research Geochemist in the Chemical and Isotopic Signatures Group at Lawrence Livermore National Laboratory who has developed and applied chemical and isotopic tracing techniques since 1992. His research expertise and experiences include mass spectrometry, multi-media environmental chemistry, age-dating of groundwater, tracing sources of environmental pollutants, characterising and predicting environmental fate of chemical and nuclear weapons material, nuclear countermeasures, and international scientific engagement for proliferation prevention. Previous positions held include Research Chemist in the Nuclear Chemistry Group from 1992–1999 and Group Leader of Environmental Chemistry and Toxicology from 1999–2003.

162 M.L. Davisson et al.

Terry F. Hamilton has over 25 years of international experience solving problems related to the health and ecological impacts of radionuclide releases to the environment. He has been involved in studies in the Russian Arctic, Mediterranean Sea, Adriatic Sea, Sea of Japan, and at test sites in the Aleutian Islands, in French Polynesia and at the Maralinga test site in South Australia. He has multidisciplinary expertise in the fields of marine and terrestrial radioecology, health physics, radiochemistry, low-level radionuclide detection and measurement, and on the fate and transport of environmental contaminants. He is currently serving as the Scientific Director of the Marshall Islands Dose Assessment and Radioecology Program within the Global Security Directorate at the Lawrence Livermore National Laboratory.

Andrew F.B. Tompson is a Hydrologist in the Atmospheric, Earth, and Energy Division at Lawrence Livermore National Laboratory, and recently served as leader of the Hydrologic Sciences Group. His research interests include the physics of multiphase fluid flow, chemical transport, and chemical transformation in porous media and terrestrial environmental systems, and the coupled mathematical modelling of these processes. He has worked on many interdisciplinary teams involving engineers, hydrologists, geologists, geochemists, isotope and radiochemists, applied mathematicians, computer scientists, and climate scientists. He is active in broad-based technical educational activities relating to public and policy awareness of important water supply and pollution issues and serves, or has served on the editorial boards of Water Resources Research, Advances in Water Resources and Computational Geosciences. He received his PhD from Princeton University in 1985 and spent two years at the Massachusetts Institute of Technology as a Post-doctoral Associate before moving to Livermore.

1 Introduction

Following World War II, the US relocated local inhabitants (the ‘dri-Enewetak’) from Enewetak Atoll under a Trust Territory agreement and tested 42 near-surface and air burst nuclear devices from 1948 to 1958 (Figure 1). After intermittent use from 1958 to 1972, a decision was made to return Enewetak Atoll to the Marshallese after establishing and completing cleanup operations. The Atomic Energy Commission (AEC) conducted radiological surveys in 1972 (AEC, 1973) while the Defense Nuclear Agency (DNA) led the development and execution of the cleanup plan (DNA, 1981). Cleanup of affected islands entailed

1 removal of all radioactive and non-radioactive debris (equipment, concrete, scrap metal, etc.)

2 removing soil with > 14,800 Bq/kg (400 pCi/g) of plutonium

3 removal or amending soil between 1,480 and 14,800 Bq/kg (40–400 pCi/g) of plutonium

4 disposing and stabilising the accumulated soil and radioactive debris inside an unlined nuclear test crater on Runit Island and capping it with a concrete dome.

Radioactive waste buried beneath Runit Dome on Enewetak Atoll 163

This burial method evolved at a time when specific management definitions and guidelines for the disposal of radioactive waste were being drafted and submitted to the US Congress, which today form the regulatory guidelines administered by the Nuclear Regulatory Commission (NRC).

Figure 1 Map of Enewetak Atoll with names and locations of documented nuclear weapons tests (1948–1958)

A number of scientific studies have been conducted on Runit Dome since it was constructed including assessments on the integrity of the structure (NAS, 1982) and the level of radioactive contamination of the marine environment in the atoll in general (Noshkin and Robison, 1997). The US Department of Energy (DOE) has developed a number of formal task agreements with the local atoll government to address long-term radiological surveillance needs at former test sites. This has included setting up a radiological facility on Enewetak Island to monitor the resident population for internally deposited 137Cs and plutonium based on whole body counting and urinalysis (Bell et al., 2002; Hamilton et al., 2007; Marshall Islands Program, 2007), and to provide additional assurances about unrestricted use of atoll resources. However, it is not uncommon for local inhabitants of Enewetak to collect birds and bird eggs from Runit Island and, over the more recent past, there are obvious signs that people have been digging trenches on Runit Island looking for copper wiring. Moreover, concerns have been raised about elevated levels of plutonium observed in local marine biota, especially in Tridacna clam and other tissues of sessile organisms such as top snails (Trochus niloticus) and sea cucumber (Halodeima) (Hamilton et al., 2008). Any increases in availability of plutonium will have impact on food security reserves for the local population as well as on a growing commercial export market for these types of products. For example, there


has been a recent campaign to harvest sea cucumber at Enewetak for sale in Asia. Sea cucumber is commonly found on the sandy shore zone of the ocean reef on Runit Island. In more general terms, lagoon circulation indicates a net flow from the vicinity of Runit Island to the major passes in the south end of the atoll where the main population resides. The combination of through-reef and cross-lagoon transport has the potential to maximise human exposure, and highlights the need to conduct a more thorough assessment of the relative, long-term risks posed by the Runit Dome waste containment site.

This paper provides the history of decision-making that went into the design and construction of Runit Dome, and highlights possible points of contention over this construction and its long-term stewardship as related to obligations under NRC rules. We further highlight the unique geochemical conditions set up underneath the dome and suggest possible transuranic (TRU) waste migration mechanisms drawing from previous literature. In addition, based on tidal driven flow around the structure, we provide boundary conditions on potential migration of radionuclides in and around Runit Dome by applying a simplified conceptual model of groundwater flow. These estimates suggest a modest characterisation and monitoring approach is needed in order to reduce lingering uncertainty of future impacts of the waste.

2 Methods

Historical reports that document the development and implementation of radiological cleanup and containment of waste now under Runit Dome were reviewed. Most comprehensive of these documents are those produced by AEC (1973) and the DNA (1981), which detail the radiological characterisation phase and the execution of Runit Dome construction, respectively. Complementing these, exploratory drilling reports on Runit prior to and after dome construction were reviewed (Ristvet et al., 1978; Ristvet, 1980). Numerous reports and publications documenting the physical and radiological status of groundwater in and around Runit at the time of dome development were also reviewed. All data and conclusions extracted from these documents were integrated and interpreted with respect to the mechanisms of radionuclide migration in these environments as most currently understood. Further comparison was made between design features of Runit Dome and current NRC regulations for similarly classed disposal sites. Lastly, risk assessments conducted by NAS (1982) and later (Noshkin and Robison, 1997) were evaluated in the context of both near-field and far-field radionuclide migration potential in and around Runit Island. All these reviews and their integration provide a basis for developing a monitoring approach at the Runit Dome site more consistent with NRC guidelines.

3 Results and discussion

3.1 Decision to construct Runit Dome

Two of the 18 surface tests on Runit Island, code named LaCrosse and Cactus, left craters approximately 120 m in diameter with depths exceeding 10 m (Figure 1 inset). Craters like these were focal point of research in subsurface structural deformation for DNA (Ristvet et al., 1978). However, during the initial 1972 radiological survey of Runit


Island, the AEC discovered hazardous levels of disseminated plutonium in the vicinity of nearby craters that resulted in the quarantine of Runit Island (DNA, 1981). This limited further research to borehole drilling only, which ultimately provided important observations for crater geologic structure and in-situ radioactivity (Noshkin et al., 1977; Ristvet, 1987). To this day, Runit Island is still under a quarantined status and has no human inhabitants.

The development of the cleanup plan and the draft environmental impact statement generated extensive debate among the DNA, AEC, and the US EPA (DNA, 1981). Options ranged from total removal of all waste from Enewetak to immobilisation without a cover. Estimates on the projected volume of waste were approximately 80,000 m3, and Cactus crater was deemed adequate for its containment (Figure 2). The final plan of burial and capping in cactus crater was adopted only after many months of delay by AEC to obtain exemption for open-ocean dumping of the waste instead, which had recently been deemed illegal by international treaty. Interestingly, legal counsel for the dri-Enewetak population went on record to make clear their preference for the full restoration of their islands and the complete removal of all radioactive waste from them. However, the US Congress rejected the estimated $200–$300 million cost to meet the dri-Enewetak requirements. Consequently, after three years delay, during which the dri-Enewetak acquiesced and accepted waste burial, a $20 million one-time appropriation for cleanup following the entombment option was passed in Congress (DNA, 1981).

4 Initial conditions before Runit Dome construction

Extensive drilling and drill core characterisation studies clearly showed that the bottom of Cactus crater was highly fractured from the shock of the test detonation (Ristvet et al., 1978). Core sampling often required the use of split-spoon (i.e., continuous) sampling techniques because of borehole instability, contrasting the good core recovery from outside the crater. Evidence for a highly fractured zone beneath the crater was supported by a correlation between the lengths of recoverable cores and distance from Cactus crater (Ristvet et al., 1978). Moderately consolidated material was not encountered until ~23 m below the floor of the crater surface as also evidenced by a distinct reduction in seismic velocities. This suggested that the nuclear blast caused pervasive micro- and macro fracturing of the consolidated rock below the crater (Ristvet et al., 1978), which is consistent with assessments in similar testing areas (IAEA, 1998).

Before Runit Dome was constructed, dye tracer experiments showed that seawater filling Cactus crater had a half-life between 2.5 to 3.5 days (Marsh et al., 1978). This corroborates the existence of hydraulic communication and circulation between the crater floor and the sea. As observed in wells, higher groundwater levels tended to prevent tracer migration into monitoring wells south of the crater (Figure 3), although these levels were subject to temporal fluctuations (Buddemeier and Holladay, 1977). Meteoric groundwater tended to pool around the crater rim with residence times less than one year (Noshkin et al., 1977). Groundwater accrual patterns shifted after construction of the dome which likely concentrated runoff around its perimeter. The salinity of this recharged runoff will depend on season and rate of precipitation, but the encroachment of dense vegetation currently around the rim suggests a sustained freshwater lens beneath the surface.


Groundwater that occurs beneath the crater and Runit Island is typical of that found on island atolls (Buddemeier and Holladay, 1977; Oberdorfer et al., 1990; Wheatcraft and Buddemeier, 1981; Buddemeier and Oberdorfer, 1997). Its composition may vary between seawater at deeper locations and areas close to the island shoreline to fresh water accumulated at shallower mid-island locations from infiltrated rainwater. Two prevailing conditions limit meteoric water accumulation. Trade winds tend to force wave action toward the southwest, providing an excess seawater pressure head and formation of a shallow saltwater wedge on the northeastern side of the atoll (Buddemeier and Holladay, 1977). In addition, a Pleistocene-age solution unconformity in the carbonate rock occurs across the atoll at depth (Ristvet et al., 1978), enhancing lateral transport of recharge, and maintaining a shallow freshwater/saltwater interface (Buddemeier and Holladay, 1977). This can be characterised and modelled atoll-wide as a two-layered aquifer system with groundwater flow rate ultimately driven by tidal influence (Oberdorfer et al., 1990). Surface recharge does not penetrate deeply and does not specifically follow well-known Ghyben-Herzberg principles. Ubiquitous occurrence of particulate plutonium in shallow groundwater across affected islands of the atoll is consistent with this observation (Noshkin et al., 1977). The fact that test-induced fracturing beneath the crater was shown to transcend these layers implies that mobilised radionuclides (ions and/or colloids) can be rapidly transported and exchanged with lagoon seawater.

It was further noted from gamma log measurements during drilling of cactus crater, measurements of recovered core, and measurements of surface sediments at the bottom of the crater that there is likely between 30 and 1,000 GBq (0.8–27 Ci) of residual activity that was already residing below the bottom of cactus crater (Ristvet et al., 1978; Davisson and Hamilton, 2008). Most probably the activity is on the lower end of this range, which would be consistent with findings for inventory levels in offshore sediments (Nelson and Noshkin, 1973). The residual activity is most likely associated with the cactus test itself or from fallout of nearby tests, and it should be considered separate from radioactive debris later moved into the crater (below).

5 Construction and waste characteristics of Runit Dome

Contaminated soil and debris were emplaced in three separate sections beneath Runit Dome starting with a slurried mixture comprising three bags of Type II Portland cement to one-half bag of attapulgite clay per cubic yard of contaminated soil tremied up to high tide level from a floating barge (Figure 4) (DNA, 1981). Above the tremie level in a center area or ‘donut’ was placed the largest sized and most contaminated material, including ~2.2 GBq of TRU debris collected in approximately 400 plastic bags and cemented into place. Additional large debris was simply bulldozed into place around the ‘donut’. The remaining was contaminated soil mixed in proportions of one cubic yard for every two bags of cement. The dome was constructed using approximately 370 individual concrete panels averaging approximately 45 cm thick. Records show that TRU inventory lying below Runit Dome totalled 545 GBq, approximately half (278 GBq) is contaminated soils originating from five northern soils, and the remaining material (267 GBq) is surface soil and radioactive debris from Runit Island itself. Only 131 GBq of TRU activity was buried within the tremie portion below sea level, whereas the remaining lies above. As mentioned above, this inventory should be considered in addition to the 30–1,000 GBq estimated for the residual activity measured in the crater


bottom prior to Runit Dome construction. Construction did not include lining the crater bottom to prevent leakage. Consequently, it is likely that both this residual activity and the intentionally buried waste both interact with groundwater. A nine foot high keywall constructed around the crater to buffer wave action and the 10 m high concrete dome significantly increased the overall elevation of the cactus crater area (DNA, 1981).

Figure 2 Map showing the code name and appropriate location of nuclear tests conducted in the vicinity of Runit Island (includes outline of cactus crater) (see online version for colours)

N

0 400 800 feet

ErieOsage

Fig-Quince

Blackfoot

Zebra

Dog

Cactus

LaCrosse

Runit DomeFootprint

Figure 3 Illustrative north-south cross-section through cactus crater before Runit Dome construction showing possible configuration of surface freshwater/sea water interactions (see online version for colours)

N

Cactus

A A’

A

A’

MSL

10m

20m

Pre-shot topography

Post-shot crater

Rubble zone Solution unconformities

Freshwater

Seawater


The Enewetak cleanup was largely focused on cleanup and mitigation of the long-term health risks posed by elevated levels of TRU contamination on the islands. There is no record of the total fission product inventory of soil and debris buried beneath Runit Dome, even though today it is widely accepted that 137Cs and 90Sr can contribute the largest fraction of the dose delivered to inhabitants of the Marshall Islands from exposure to residual fallout concentrated in the local food chain (Robison et al., 1997).

6 Previous risk assessments of Runit Dome

In 1980, the National Academy of Sciences (NAS) was requested to perform a review of Runit Dome integrity (NAS, 1982). They concluded that the structure was structurally sound and secure against disruption by storm surges and typhoons. Based largely on a catastrophic release scenario where all the contents were released instantaneously, NAS also concluded Runit Dome posed no significant exposure risk because the lagoon bottom sediments throughout the Atoll were already significantly higher in TRU activity (i.e., inventory estimated at 67,800 GBq over ~900 km2). Later work (Noshkin and Robison, 1997) concurred with NAS in an extended environmental risk analysis, again based on a catastrophic release, incorporating approximately 15 years of multi-media radiological surveillance data collected in and around Enewetak Atoll. Note however, that the estimated aerial distribution of TRU activity in the lagoon sediments of ~75 GBq/km2 is nearly 1,000 times lower than that for Runit Dome calculated at ~80,000 GBq/km2. Consequently, Runit Dome represents a point-source of higher concentration. Published evidence on Runit Dome indicates open communication of the buried waste with intruding ocean water and local groundwater (Ristvet, 1980). A more probable release scenario would be characterised by localised subsurface leaching and migration into the immediate surroundings, for which no risk assessment has been conducted thus far.

7 Runit Dome in the context of US regulatory requirements

Under the current US regulatory framework, Runit Dome and its contents can easily be construed as a low-level radioactive waste (LLRW) disposal site. An LLRW site is classified by the type of radionuclides present and their individual concentrations (US NRC, 2011). As mentioned, records show that Runit Dome contains 80,000 m3 of excised soil with a total TRU inventory of 545 GBq (DNA, 1981). Using a dry weight soil density of 1.29 g/cm3 (AEC, 1973), the average soil TRU concentration in the undiluted soil fill is estimated to be around 5 Bq/g. Consequently, the excised soil would easily satisfy the alpha-emitting TRU criteria for near-surface disposal as Class A waste (i.e., TRU < 370 GBq/m3) (US NRC, 2011). Note, residual radioactivity below the buried waste and left by the surface test has not been included in this calculation.

Between 1976, when cleanup began, and 1986, when Marshall Islands was granted independent nation status, LLRW disposal guidelines on non-federal sites in the USA were regulated by the NRC and specific guidelines for disposal operations were established as early as 1980. These guidelines dictate methods of disposal, site preparation, waste packaging, and post-operational environmental surveillance needs. Even though the US DOE has since developed a comprehensive individual radiological protection monitoring program at Enewetak, and concluded Runit Dome does not pose a


significant excess radiological risk to the people living on Enewetak Atoll as long as Runit remains quarantined, current monitoring and management of Runit Dome falls short of specific US NRC requirements (US NRC, 2011).

8 Potential for radionuclide release in groundwater from beneath Runit Dome

For more than 40 years, attention has been given to the environmental fate of radionuclides released from nuclear power generation, nuclear fuel processing, and nuclear weapons testing activities. Significant events such as the Chernobyl disaster (Bugai et al., 1996), radionuclide releases at Chelyabinsk-65 in Russia (Cochran et al., 1993), or the extensive characterisation and cleanup efforts at former nuclear enrichment or nuclear weapons test sites (Zorpette, 1966; IAEA, 1998; Tompson et al., 2002) provide a wealth of laboratory and field observations to benchmark radionuclide behaviour. Like most of these other sites, the key long-lived radionuclides of potential health and ecological concern in the Marshall Islands are fission products, such as 137Cs and 90Sr, and residual fuel products such as plutonium and americium (Robison and Noshkin, 1999). In general, these radionuclides constitute a significant amount of residual environmental radioactivity in many locations and are variously distributed in different compartments throughout the ecosystem at Enewetak Atoll (Robison and Noshkin, 1999). Residual 137Cs is readily mobile in carbonate soil of the Marshall Islands, but through addition of potassium amendments its bioconcentration in locally grown fruit crops can be avoided (Robison et al., 2006). 90Sr readily substitutes for calcium in soil and is highly retarded. The low solubility of plutonium and americium in Marshall Island soils preclude their soluble migration through the food-chain, however, particulate forms are of ongoing monitoring concern because of their heterogeneous distribution (Hamilton et al., 2009) and because of previous measurements in mobile colloids associated with groundwater (Noshkin et al., 1977).

The bulk waste material beneath Runit Dome comprises an intimate contaminated mixture of cement, ferrous metal debris, calcareous soil, and decomposing organic matter, the latter inferred from rotten egg and ammonia smell noted in drilling operations during the NAS study (Ristvet, 1980). This implies that dissolved oxygen is consumed faster than it can be replenished. This bulk chemistry represents a unique contaminant condition not observed nor investigated in any other environmental compartment of the Marshall Islands. Consequently, in the absence of direct measurements of the buried waste, our current investigations requires us to estimate radionuclide migration potential by drawing upon similar observations in published experimental and field studies.

137Cs is known to be readily mobile in dilute or saline water, and retards by ion exchange, with partitioning coefficients ranging from < 40 ml/g in marine carbonate sediments to >105 ml/g in fresh water lake sediments dominated by montmorillonite clay (NCRP, 1984). A minor adsorption to Fe-oxyhydroxides has been noted in some studies (Ferris et al., 2000; Xiangke et al., 2000), however 137Cs appears to have a stronger affinity towards organic matter (Ferris et al., 2000; Xiangke et al., 2000; Stauton et al., 2002; Kruyts et al., 2004; Chibowski and Zygmunt, 2002; Caron and Mankarios, 2004). Although frayed edge sites on hydrous layered silicates have been shown to predominate adsorption in soil, high ratios of organic matter to clay can equally retard 137Cs mobility and enhance its biological uptake (Rigol et al., 2002).


Measurements of 137Cs in and around cactus crater before Runit Dome construction ranged from 0.5 to 2.4 Bq/kg (Ristvet et al., 1978), and similar concentrations are anticipated in the buried waste. The 137Cs is likely readily soluble in the associated pore water. Adsorption leading to retardation would be strongest on attapulgite, although it has a cation exchange capacity nearly five times lower than montmorillonite (Arnould et al., 1991). Non-specific adsorption to insoluble organic matter or to any amorphous silica formed on cement surfaces could also be significant. However, the majority of the buried waste comprises carbonate soil and given the absence of abundant layered silicate clay minerals, it is probable that 137Cs is relatively mobile.

90Sr is retarded in carbonate soils compared to 137Cs (Robison et al., 2000) due to substitution for calcium in the carbonate matrix. Beneath Runit Dome this retardation may be enhanced during dissolution/re-precipitation reactions of the carbonate soil or on cement surfaces in the presence CO2 reacting with Ca(OH)2.The extent of adsorption onto other mineral phases depends on presence of competing ions such as Na, Ca, or Mg. In clay loam soil, 90Sr adsorption is strongest and reversible at near-neutral pH and at low oxidation potential (Wang and Stauton, 2005). Fe-oxyhydroxides have a weak affinity for Sr but adsorption increases in the presence of humic or bacterially-derived organic matter (Ferris et al., 2000). Adsorption is also enhanced with increases in silica gel (Hakem et al., 2004). It is anticipated that 90Sr will not mobilise significantly from the buried waste.

Pu and Am are relatively insoluble elements when introduced into the natural environment. PuO2 particles have been shown to be highly stable (Dahlman et al., 1976). In oxygenated aqueous environments soluble Pu forms a +5 oxidation state, however, the presence of humic material has a tendency to reduce Pu to the +4 state (Tan et al., 1993). The latter valence has high adsorption affinity to numerous mineral and organic substrates (Bondietti et al., 1976; Nash et al., 1981), rendering it essentially insoluble. Water of low oxidation potential would likewise convert Pu to reduced states and induce adsorption. Chemically reduced water beneath Runit Dome would prevent oxidation and mobility of Pu and Am as soluble ions in the buried soil and debris. They would likely form or maintain adsorption complexes with the carbonate matrix or organic matter (Zavarin et al., 2005). Although actinides should be stable in this state, some evidence suggests that the higher ionic strength of seawater could lead to desorption from carbonate-based materials (Fried et al., 1976). While it is highly unlikely that Pu or Am would be significantly mobile in an ionic state because of reducing conditions under the dome, actinides are effectively transported on colloids under both oxidative and reducing conditions (Noshkin et al., 1977; Fried et al., 1976; Nelson et al., 1985; Kersting et al., 1999). Colloidal organic matter is common in the environment in which Pu and Am will strongly adsorb. Consequently, colloids may form the most important vector for transport of radionuclides buried beneath Runit Dome. Other potential colloidal phases include minerals associated with the local rock matrix, organic-mineral complexes, and dispersed clays as well as amorphous material such as silica. In all cases, typical colloidal concentrations can range from < 1 mg/L to > 100 mg/L (Ryan and Elimelech, 1996). The abundance of colloids will be strongly dependent on the local rock mineral aqueous geochemistry and water flow (Ryan and Elimelech, 1996). Colloidal transport is recognised as the main mechanism for mobilisation of actinides contained in Enewetak groundwater (Noshkin et al., 1977). It was further noted that Pu activity in water sampled from boreholes drilled during the NAS study was 1,000 times higher in the particulate


phase than in a dissolved state (Robison and Noshkin, 1981). Both 137Cs and 90Sr have also been observed in the particulate phase at Enewetak (Noshkin and Robison, 1997).

The crater beneath Runit Dome is unlined and was highly fractured by detonation effects from the Cactus test (Ristvet et al., 1978). Contaminated waste material located within the crater but below the historical topographic surface under the dome, as well as any residual radionuclides below the crater floor derived from test effects, will be routinely exposed to both fresh, brackish, and saline groundwater that circulate and mix as a result of transient tidal forces (along the sea perimeter), competing freshwater mounds (that cyclically accrue and dissipate on the land side), and variable hydraulic permeability (Figure 3). The potential for such circulation was indicated by the aforementioned tracer tests conducted before the dome construction (Marsh et al., 1978). The local topography of the Dome structure itself enhances meteoric water infiltration around its rim, as inferred by encroaching surface vegetation along the crater edge (Figure 4). The potential for radionuclide migration in groundwater will be highly dependent on solubility, dissolution and sorption effects associated with individual radionuclides and their intrinsic physical forms (e.g., aqueous, sorbed, soluble precipitate), prevailing redox conditions and secondary mineralisation, both of which are largely unknown, as well as highly transient hydraulic forcing mechanisms. Plutonium migration, for example, will be enhanced by the subsurface anoxia and the presence of complexing humic material.

Figure 4 Diagrammatic cross-section of Runit Dome showing the placement and contents of buried soil and debris and location of boreholes drilled during the NAS study (see online version for colours)

10 m

20 m

Concrete Dome

Tremie Layer

Soil CementSoil Cement "Donut Hole"

Rubble Zone

CD-1 CD-12

CD-17

South North

N

Back Reef

Remnant Cactus Crater Lip

CD-1

CD-12

CD-17

OversizeDebris

OversizeDebris

"DonutHole"

Lagoon

MSL

Keywall


These factors potentially contribute to the flushing and mixing of both fresh water, seawater, and their mixtures through the waste pile at a rate proportional to both its local permeability and the cycling pressure and pore water density gradients associated with groundwater conditions around and below the dome’s perimeter. Under one idealised mixing scenario (A), radionuclide migration in a shallow groundwater mound away from the dome would occur primarily through transient flushing episodes that will serve either to dilute and disperse existing aqueous species or promote dissolution and release of other non-aqueous species, all as a function of the flushing cycle and groundwater residence times in each cycle. A different scenario (B) may be more likely due to the underlying steady trend in the hydraulic forces (due to asymmetry flow towards the lagoon driven by tides and prevailing winds (Oberdorfer et al., 1990; Buddemeier and Oberdorfer, 1997), which will promote, on the average, a mean, unidirectional groundwater flow through the saturated waste material, and a subsequent, more steady opportunity for radionuclide release into the lagoon.

9 A simple example

Because prevailing evidence indicates layered groundwater flow toward the lagoon at Enewetak (Buddemeier and Holladay, 1977; Oberdorfer et al., 1990), we chose to model the simplified scenario (B). Under steady and constant density flow conditions, as a bounding condition for radionuclide transport we use an idealised flow configuration shown in Figure 5 for an idealised (prismatic) portion of the crater below the water table of dimensions L × L × D, where L ~ 93 m (mean diameter of crater; L2 × D = waste volume). Steady horizontal groundwater flow in the positive x-direction through this volume will be described by a simple one-dimensional flow equation of the form

Q A v AK h= = − ∇φ (1)

where Q is the volumetric flow rate (L3 / T), A = L • D is the area of an idealised control plane located at the end of the crater (L2), φ is the mean porosity of the rubble, v is the mean groundwater flow velocity in the x-direction (L / T), K is the hydraulic conductivity (L / T), and ∇h is the hydraulic gradient directing flow in the positive x-direction. If the buried waste is dominated by fracture porosity that is interconnected, as suggested by tidal-driven water responses observed in boreholes during the NAS study (Ristvet, 1980), then hydraulic conductivity represents the greatest uncertainty in the calculation. All other parameters can easily be estimated from topography and construction of the buried waste.

Consider a radionuclide contaminant confined initially in the rubble behind the control plane with a total inventory of M Bq (decay corrected to an appropriate date). Its physical form and chemical reactivity will govern its initial concentration in groundwater, while its elution in groundwater past the control plane will be affected by these same factors as well as the rate of groundwater flow through the crater:

• Case 1: Transport of inert colloidal species. If the radionuclide is non-reactive and completely dissolved in groundwater, or if it is completely adsorbed onto a mobile colloidal suspension, then an initial aqueous concentration c = M / (φLA) will exist in the pore spaces and will result in a radionuclide flux


J Qc= (2)

passing the control plane for a period of time equal to the water residence time,

/γT L v= (3)

(This ignores effects produced by hydrodynamic dispersion and colloidal filtration). According to the data in Table 1, idealised residence times were calculated in the range from several days to hundreds of thousands of years. However, the previous observations during the NAS study that water level in boreholes responded to tidal fluctuations would suggest that residence times are likely days to months rather than millennium.

• Case 2: Transport of a chemically retarded species. If the radionuclide is soluble in groundwater but partakes in surface complexation or ion exchange (adsorption) reactions with solids in the crater, and if these reactions can be described with an equilibrium exchange model associated with a retardation coefficient R, then an initial aqueous concentration c = M/(RφLA) will exist in the pore spaces (reduced by a factor of R) and will result in a radionuclide flux [equation (2)] past the control plane for an extended period of time equal to

γT RT= (4)

• Case 3: Transport of a solidified species. If the radionuclide is initially bound within a soluble solid (e.g., such as a melt glass) or otherwise associated with a solid precipitate that can dissolve, then its presence in the aqueous regime will be limited by dissolution rate of the solid. Depending on these rates, this kind of behaviour will serve to lower the observable concentrations passing the control plane, but otherwise lengthen the time over which elution occurs. Aqueous species derived from dissolution may also partake in adsorption reactions.

Figure 5 Idealised conceptual model of steady flow (from the left to right) and radionuclide release from the contaminated waste materials in the tremie layer, Runit Dome, in the crater below the water table

Note: Residual radioactivity beneath the crater floor, not part of the deposited waste debris, is not considered.

The data in Table 1 are derived for a series of representative variables based upon Case 1 and an initial concentration assumed to be similar to those observed in groundwater in earlier studies (i.e., ~15 Bq/L as TRU; Robison and Noshkin, 1981). Unidirectional flow beneath Runit Dome is likely dominant and only an exception during transient, multidirectional flow driven by high surface recharge, storm surges, and local density


effects. Either way, the potential exists for localised migration of radionuclides in groundwater away from the dome’s debris field. Table 1 Simple case hydraulic parameters and radioactivity release predictions

φ K m/s ∇h Q m3/yr v m/yr Residence time (yr) GBq/yr leached

0.1 1.E-03 0.1 184,500 3,154 0.030 2.77 0.05 1.E-05 0.05 18 0.615 152 2.70E-04 0.01 1E-09 0.01 1.80E-03 3.08E-04 304,267 2.70E-08

Although radionuclides measured in groundwater to date would appear to reflect the operation of these processes at some level (Noshkin et al., 1977; Robison and Noshkin, 1981), the argument is in favour of enhancing the current level of characterisation and monitoring of groundwater and radionuclide concentrations at Runit Dome. Such efforts will provide a means to better understand the competing hydraulic flow and mixing mechanisms at play under the dome and the chemical processes associated with releasing and mobilising radionuclides in groundwater toward the lagoon.

10 Summary

Ultimately, the buried waste below Runit Dome has a significantly higher radionuclide burden per unit mass than water, sediment and vegetation in the surrounding islands and lagoon. What is not understood is the propensity of radionuclides from this waste to migrate at a rate above and beyond of that known and observed for surrounding areas. Evidence gathered in this study and integrated with current knowledge of radionuclide migration supports migration and interconnectivity of groundwater below Runit Dome with surrounding areas. The subsurface redox conditions and the mixed mineralogy below the dome suggests that a greater variety of mechanisms for radionuclide transport likely exist, in particular colloidal movement from both physical breakdown of cement and chemical generation of organic/inorganic colloidal matter with a high degree of affinity for actinide and fission product adsorption. The construction of Runit dome served the purpose of confining and securing radioactive waste distributed around Enewetak atoll to facilitate repopulation. We show in this work that Runit dome construction was not consistent with NRC regulations, lacks specific monitoring required by the regulations, and lacks any detailed investigations for the potential migration into near-field lagoon setting.

A full and comprehensive assessment of the migration potential of radionuclides contained in the aged waste pile buried beneath Runit Dome will require:

1 measurements on the levels and distribution of fallout radionuclides in the underlying groundwater

2 solid-phase characterisation of the waste pile

3 a fundamental understanding of groundwater hydrology on the northern end of Runit Island

4 supplemental information on physiochemical conditions beneath the dome (e.g., pH, redox conditions, temperature, and groundwater chemistry) enabled


through a network of drill holes established in and around the concrete façade, and sampled at regular intervals.

This would provide data for an improved risk assessment based on projected release of contaminated groundwater locally into the surface environment of the lagoon. Furthermore, regular monitoring of radionuclide uptake in the local marine food chain near Runit Dome could be established.

These measures would directly address concerns of the local population and help alleviate perceived safety concerns about Runit Dome.

Acknowledgements

We thank our partners at the Office of International Health Studies at the DOE for funding support. Dr. William Robison provided helpful comments on the original manuscript. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

References Arnould, M., Clement, C., Gouvenot, D. and Struillou, R. (1991) ‘New sorbing grouts for

radioactive and toxic heavy metals’, Engineer. Geol., Vol. 30, No. 1, pp.127–139. Atomic Energy Commission (AEC) (1973) Enewetak Radiological Survey, Atomic Energy

Commission NTIS NVO-140, Springfield, VA. Bell, R.T., Hickman, D., Yamaguchi, L., Jackson, W. and Hamilton, T.F. (2002) ‘A whole body

counting facility in a remote Enewetak Island setting’, The Radiation Safety Journal, Health Phys., Vol. 83, Suppl. 1, pp.S22–S26.

Bondietti, E.A., Reynolds, S.A. and Shanks, M.H. (1976) ‘Interaction of plutonium with complexing substances in soils and natural waters’, Paper presented at Proceedings of a Symposium on Transuranium Nuclides in the Environment, 17–21 November 1975, San Francisco, California, International Atomic Energy Agency: Vienna, Austria, pp.273–287.

Buddemeier, R.W. and Holladay, G. (1977) ‘Atoll hydrology: Island groundwater characteristics and their relationship in diagenesis’, Paper presented at Proceedings Third International Coral Reef Symposium,. Miami, Florida, Vol. 2.

Buddemeier, R.W. and Oberdorfer, J.A. (1997) ‘Hydrogeology of Enewetak Atoll’, in Vacher, H.L. and Quinn, T.M. (Eds.): Geology and Hydrogeology of Carbonate Islands, pp.671–693, Elsevier, Amsterdam.

Bugai, D.A., Waters, R.D., Dzhepo, S.P. and Skal’skij, A.S. (1996) ‘Risks from radionuclide migration to groundwater in the chernobyl 30-km zone’, Health Phys., Vol. 7, No. 1, pp.19–18.

Caron, F. and Mankarios, G. (2004) ‘Pre-assessment of the speciation of 60Co, 125Sb, 137Cs, and 241Am in a contaminated aquifer’, J. Environ. Rad., Vol. 77, No. 1, pp.29–46.

Chibowski, S. and Zygmunt, J. (2002) ‘The influence of the sorptive properties of organic soils on the migration rate of 137Cs’, J. Environ. Rad., Vol. 61, No. 2, pp.213–223.

Cochran, T.B., Norris, R.S. and Suoko, K.L. (1993) ‘Radioactive contamination at Chelyabinsk-65, Russia’, Annual Reviews of Energy and Environment, Vol. 18, pp.507–28.

Dahlman, R.C., Bondietti, E.A. and Eyman, L.D. (1976) ‘Biological pathways and chemical behavior of plutonium and other actinides in the environment’, in Friedman, A.M. (Ed.): Actinides in the Environment, ACS Symposium Series 35, American Chemical Society, Washington, DC, pp.47–80.


Davisson, M.L. and Hamilton, T.F. (2008) An Historical Perspective on Technical and Scientific Issues Related to the Runit Island Low-Level Radioactive Waste Disposal Site (Runit Dome) on Enewetak Atoll, Lawrence Livermore National Laboratory UCRL-TR-403015, Livermore, California.

Defense Nuclear Agency (DNA) (1981) The Radiological Cleanup of Enewetak Atoll, Defense Nuclear Agency, Washington, DC.

Ferris, F.G., Hallberg, R.O., Lyven, B. and Pedersen, K. (2000) ‘Retention of strontium, cesium, lead, and uranium by bacterial iron oxides from a subterranean environment’, Appl. Geochem., Vol. 15, No. 7, pp.1035–1042.

Fried, S., Friedman, A.M., Hines, J.J., Atcher, R.W., Quarterman, L.A. and Volesky, A. (1976) ‘The migration of plutonium and americium in the lithosphere’, in Friedman, A.M. (Ed.): Actinides in the Environment, ACS Symposium Series 35, American Chemical Society, Washington, DC, pp.19–46.

Hakem, N., Apps, J.A., Moridis, G.J. and Al Mahamid, I. (2004) ‘Sorption of fission product radionuclides, Cs-137 and Sr-90, by Savannah River site sediments impregnated with colloidal silica’, Radiochim. Acta, Vol. 92, No. 7, pp.419–432.

Hamilton, T.F., Jernströem, J., Martinelli, R.E., Kehl, S.R., Eriksson, M., Williams, R.W., Bielewski, M., Rivers, A.N., Brown, T.A., Tumey, S.J. and Betti, M. (2009) ‘Frequency distribution, isotopic composition and physical characterization of plutonium-bearing particles from the fig-Quince zone on Runit Island, Enewetak Atoll’, J. Radioanal. Nucl. Chem., Vol. 282, No. 3, pp.1019–1026.

Hamilton, T.F., Kehl, S.R., Hickman, D.P., Brown, T.A., Martinelli, R.E., Tumey, S.J., Jue, T.M., Buchholz, B.A., Langston, R.G., Johannes, K. and Henry, D. (2007) Individual Radiation Protection Monitoring in the Marshall Islands: Enewetak Atoll (2005–2006), Lawrence Livermore National Laboratory, UCRL-TR-231397, Livermore, CA.

Hamilton, T.F., Martinelli, R.E., Kehl, S.R. and McAninch, J.E. (2008) ‘The plutonium isotopic composition of marine biota on Enewetak Atoll: a preliminary assessment’, J. Environ. Monit., Vol. 10, No. 10, pp.1134–1138.

International Atomic Energy Agency (IAEA) (1998) The Radiological Situation at the Atolls of Mururoa and Fangataufa, Reports by an International Advisory Committee, International Atomic Energy Agency, Vienna, Austria.

Kersting, A.B., Efurd, D.W., Finnegan, D.L., Rokop, D.J., Smith, D.K. and Thompson, J.L. (1999) ‘Migration of plutonium in groundwater at the Nevada Test Site’, Nature, Vol. 397, No. 6714, pp.56–59.

Kruyts, N., Titeux, H. and Delvaux, B. (2004) ‘Mobility of radiocaesium in three distinct forest floors’, Sci. Total Environ., Vol. 319, Nos. 1–3, pp.241–252.

Marsh, K.V., Jokela, T.A., Eagle, R.J. and Noshkin, V.E. (1978) Radiological and Chemical Studies of Groundwater at Enewetak Atoll, 2. Residence Time of Water in Cactus Crater, Lawrence Livermore National Laboratory Report UCRL-51913, Part 2, Livermore, California.

Marshall Islands Program, Marshall Islands Dose Assessment and Radioecology Program, available at https://marshallislands.llnl.gov (accessed in June 2007).

Nash, K., Fried, S., Friedman, A.M. and Sullivan, J.C. (1981) ‘Redox behavior, complexing, and adsorption of hexavalent actinides by humic acid and selected clays’, Environ. Sci. Technol., Vol. 15, No. 7, pp.834–837.

National Academy of Sciences (NAS) (1982) Evaluation of Enewetak Radioactivity Containment, National Academy of Science, National Research Council, National Academy Press, Washington, DC.

National Council on Radiation Protection (NCRP) (1984) Predicting the Transport, Bioaccumulation and Uptake by Man of Radionuclides Released to the Environment, Radiological Assessment, National Council on Radiation Protection and Measurements, Report No. 76, Bethesda, Maryland.


Nelson, D.M., Penrose, W.R., Karttunen, J.O. and Mahlhaff, P. (1985) ‘Effects of dissolved organic carbon on the adsorption properties of plutonium in natural waters’, Environ. Sci. Technol., Vol. 19, No. 2, pp.127–131.

Nelson, V. and Noshkin, V.E. (1973) ‘Marine program’, Enewetak Survey Report, Vol. 1, pp.131–224, United States Atomic Energy Commission, Nevada Operations Office NVO-140, Las Vegas, Nevada.

Noshkin, V.E. and Robison, W.L. (1997) ‘Assessment of a radioactive waste disposal site at Enewetak Atoll’, Health Phys., Vol. 73, No. 1, pp.234–247.

Noshkin, V.E., Wong, K.M., Eagle, R.J., Holladay, G. and Buddemeier, R.W. (1977) ‘Plutonium radionuclides in groundwater at Enewetak Atoll’, Paper presented at Proceedings IAEA Symposium on Transuranium Nuclides in the Environment, Vienna, Austria.

Oberdorfer, J.A., Hogan, P.J. and Buddemeier, R.W. (1990) ‘Atoll island hydrogeology: flow and freshwater occurrence in a tidally dominated system’, J. Hydrol., Vol. 120, Nos. 1–4, pp.327–340.

Rigol, A., Vidal, M. and Rauret, G. (2002) ‘An overview of the effect of organic matter on soil-radiocaesium interaction: implications in root uptake’, J. Environ. Rad., Vol. 58, Nos. 2–3, pp.191–216.

Ristvet, B.L. (1980) Summary of Drilling Operations Conducted at Cactus Dome, Runit Island, Enewetak Atoll, in Support of the National Academy of Sciences investigations, Defense Nuclear Agency, Kirtland Air Force Base, New Mexico.

Ristvet, B.L. (1987) ‘Geology and geohydrology of Enewetak Atoll’, The Natural History of Enewetak Atoll, Vol. 1, pp.37–56, United States Department of Energy, Washington, DC.

Ristvet, B.L., Tremba, E.L., Couch, R.F., Fetzer, J.A., Goter, E.R., Walter, D.R. and Wendland, V.P. (1978) Geological and Geophysical Investigations of the Enewetak Nuclear Craters, Air Force Weapons Laboratory Report AFWL-TR-77-242, Kirtland Air Force Base, New Mexico.

Robison, W.L. and Noshkin, V.E. (1981) Analysis of Core ‘Soil’ and ‘Water’ Samples from the Cactus Crater Disposal Site at Enewetak Atoll, Lawrence Livermore National Laboratory Report UCID-18935, Livermore, California.

Robison, W.L. and Noshkin, V.E. (1999) ‘Radionuclide characterization and associated dose from long-lived radionuclides in close-in fallout delivered to the marine environment at Bikini and Enewetak Atolls’, Sci. Total Environ., Vols. 237/238, pp.311–327.

Robison, W.L., Conrado, C.L. and Hamilton, T.F. (1997) A Comparative Study on 137Cs Transfer from Soil to Vegetation in the Marshall Islands, Lawrence Livermore National Laboratory UCRL-JC-128490, Livermore, California.

Robison, W.L., Conrado, C.L., Hamilton, T.F. and Stoker, A.C. (2000) ‘The effect of carbomate soil on transport and dose estimates for long-lived radioniuclides at U.S. Pacific test sites’, J. Radioanal. Nucl. Chem., Vol. 243, No. 2, pp.459–465.

Robison, W.L., Stone, E.L., Hamilton, T.F. and Conrado, C.L. (2006) ‘Long-term reduction in cesium-137 concentration in foodcrops on coral atolls resulting from potassium treatment’, J. Environ. Radioactivity, Vol. 88, No. 3, pp.251–266.

Ryan, J.N., and Elimelech, M. (1996) ‘Colloid mobilization and transport in groundwater’, Colloids Surf. A. Physioch. Engineer. Aspects, Vol. 107, pp.1–56.

Stauton, S., Dumat, C. and Zsolnay, A. (2002) ‘Possible role of organic matter in radiocaesium adsorption in soils’, J. Environ. Rad., Vol. 58, Nos. 2–3, pp.163–173.

Tan, J.X., Chen, Y.Z. and Lin, Z.J. (1993) ‘A kinetic study of the reduction of plutonium with humic acid’, Radiochim. Acta, Vol. 61, No. 2, pp.73–75.

Tompson, A.F.B., Bruton, C.J., Pawloski, G.A., Smith, D.K., Bourcier, W.L., Shumaker, D.E., Kersting, A.B., Carle, S.F. and Maxwell, R.M. (2002) ‘On the evaluation of groundwater contamination from underground nuclear tests’, Environmental Geology, Vol. 42, Nos. 2–3, pp.235–247.


US Nuclear Regulatory Commission (NRC) (2011) NRC Regulations, Part 61, available at http://www.nrc.gov/reading-rm/doc-collections/cfr/part061/full-text.html (accessed in June 2007).

Wang, G. and Stauton, S. (2005) ‘Evolution of Sr distribution coefficient as a function of time, incubation conditions and measurement technique’, J. Environ. Rad., Vol. 81, Nos. 2–3, pp.173–185.

Wheatcraft, S.W. and Buddemeier, R.W. (1981) ‘Atoll island hydrology’, Groundwater, Vol. 19, No. 3, pp.311–320.

Wang, X.K., Dong, W.M., Li, Z., Du, J.Z. and Tau, Z.Y. (2000) ‘Sorption and desorption of radiocesium on red earth and its solid components: relative contribution and hysteresis’, Appl. Rad. Isotop., Vol. 52, No. 4, pp.813–819.

Zavarin, M., Roberts, S.K., Hakem, N., Sawvel, A.M. and Kersting, A.B. (2005) ‘Eu(III), Sm(III), Np(V), Pu(V), and Pu(IV) sorption to calcite’, Radiochim. Acta, Vol. 93, No. 2, pp.93–102.

Zorpette, G. (1996) ‘Hanford’s nuclear Wasteland’, Scientific American, Vol. 274, No. 5, pp.88–97.

radioactive waste buried beneath runit dome on enewetak atoll, marshall islands

Documents