Continental glaciation and nuclear fuel waste disposal: Canada's approach and assessment of the impact on nuclide transport through the biosphere

Ecological Modelling 78 (1995) 249-265

Continental glaciation and nuclear fuel waste disposal: Canada’s approach and assessment of the impact

on nuclide transport through the biosphere

Marsha I. Sheppard *, B.D. Amiro, P.A. Davis, R. Zach Environmental Science Branch, AECL Research, Whiteshell Laboratories, Pinawa, Man., Canada, ROE IL0

Received 1 December 1992; accepted 27 August 1993

Abstract

A concept for disposal of immobilized nuclear fuel waste in a vault mined deep in stable plutonic rock of the Canadian Shield is being investigated in Canada. Far in the future, when man-made and natural protective barriers lose their integrity, radionuclides carried by groundwater may migrate from the vault to the biosphere. During this time, many transitional processes will cause changes to the climate, hydrogeology and surface features of the biosphere. Glaciation is the most severe transitional process and its impacts on the disposal concept must be assessed. Using a discrete-state approach, the effects of glaciation were evaluated by performing separate time-independent radiological dose assessments of interglacial and cold interstadial states assuming that each state persists throughout the entire simulation period of 100000 years. We assume that humans will not inhabit a full glacial environment. One of the major glacial processes is increased runoff during melt. This should decrease nuclide concentrations and doses in the immediate discharge zone of the vault through flushing and dilution. Temperature and moisture fluctuations will have only a minor impact on nuclide transport in soils and surface waters. Our calculations show that it is unlikely that cold interstadial conditions will lead to substantially higher doses to humans resulting from radionuclides in the environment than the current interglacial state.

Keyworak Glaciation; Nuclear fuel wastes; Radioactivity

1. Introduction

12. Geological disposal of nuclear fuel wastes

Several countries are assessing the environmental impact of disposal of nuclear fuel wastes in geological formations (Glasbergen et al., 1988,

* Corresponding author.

p. 179; Hadermann et al., 1988, p. 233; Johnston, 1988, p. 257; Eisenberg et al., 1989, p. 213). The Canadian concept involves deep underground disposal in plutonic rock on the Canadian Shield. We are currently assessing the technical feasibil- ity and the overall acceptability of the disposal concept, which includes natural geological barriers, supplemented by man-made barriers, to en- sure the long-term protection of humans, other biota and the environment (Hancox and Nuttall, 1991, p. 109).

0304-3800/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0304-3800(94)00181-2

250 M. I. Sheppard et al. /Ecological Modelling 78 (1995) 249-265

We recognize that containment of nuclear fuel wastes in an underground vault may not last indefinitely. After a long period of time, dissolu- tion of waste by groundwater, followed by the movement of some radionuclides to the earth’s surface, is likely. Because of the long time peri- ods involved, we use models to evaluate the potential consequences. Radiological effects on humans are of primary importance; however, the chemical toxicity of some elements, and radio- logic and toxic effects on nonhuman biota must also be considered (Amiro and Zach, 1993, p. 341).

The disposal site could be located in the On- tario portion of the Canadian Shield (Fig. 1). Because no site will be selected before the concept has been approved, the assessment of the

potential environmental impacts must be sufficiently generic to evaluate a range of environments. Risk, based on radiation dose, is evaluated for an individual member of a critically exposed group. We assume that the critical group is located at the groundwater discharge zone from the vault where the dose is highest, generally a location where radionuclides enter the biosphere and where dilution and dispersion are minimal.

The complete environmental impact of this proposed concept is assessed using three inte- grated models; the first describes the vault con- taining the wastes, the next describes the host rock or geosphere, and the third describes the biosphere (Hancox and Nuttall, 1991, p. 109). Our biosphere model, BIOTRAC (BIOsphere TRAnsport and Consequence; Davis et al.,

Canadlan Shield

Ontario portion of the Canadian Shield

Fig. 1. The model focusses on a generic site located somewhere in the Ontario portion of the Canadian Shield.

MI. Sheppard et al. /Ecological Modelling 78 (1995) 249-265 251

1993a), integrates four independent submodels representing the surface water (Bird et al., 1993a,b, p. 153), soil (Sheppard, 1992; Davis et al., 1993b, p. 251, atmosphere (Amiro and Davis, 1991, p. 41; Amiro, 1992) and food-chain and dose (Zach and Sheppard, 1991, p. 643, 1992).

Each submodel is based on a set of mass transfer equations with parameters describing the magnitude of the transfers. The model can be used probabilistically and the parameter values may be distributed to account for measurement error, spatial and temporal variability, and uncertainty

Fig. 2. Glaciated regions of the Northern Hemisphere during the last ice age. The arrows indicate the main directions of ice flow. The extent of sea ice covering the Arctic Ocean was much larger than at present and the world sea level was around 100 m below the present level (Eronen and Olander, 1990).

252 M. I. Sheppard et al. /Ecological Modelling 78 (1995) 249-265

in our knowledge of the transfer processes. The biosphere cannot be assumed to remain static because radionuclides released from the vault are unlikely to reach the surface environment for a very long time (Davis et al., 1993a). Canadian regulatory guidelines require predictive calculations of risk to 10000 a (AECB, 1987); however, some assessment of the impact of continental glaciation at longer times must accompany this quantitative treatment.

Many transitional processes have shaped the Shield over the last few million years, including glaciations, earthquakes, meteorite impacts and climate changes. Glaciation is the transitional process likely to have the largest effect on the biosphere following the closure of a disposal vault in Canada (Heinrich, 1984; Davis, 1986). This process has also been of concern for waste disposal in other northern countries such as Finland (Eronen and Olander, 1990) and Sweden (Ahlbom et al., 1991). Assessing the impact of glaciation is challenging, partly because there has been little previous concern regarding the effects of glaciation on our society and on the environment. A major workshop on transitional processes (Hein- rich, 1984), the development of an assessment methodology (Davis, 1986) and an analysis of assessment approaches for glaciation (Elson and Webber, 1991) have provided the basis for our assessment approach. Here, we synthesize this information and use BIOTRAC to assess the impact of glaciation on a member of the critical group to see whether glaciation could result in sudden and dramatic increases in doses. Thus, our assessment focuses only on the biosphere and does not consider the potential effects of glaciation on the geosphere and vault. Since the time frame of this assessment is long and the uncertainties in parameter values are large, only order of magnitude differences would indicate appreciable effects.

1.2. The nature of glaciation in Canada

Over the past two to three million years, the Shield has undergone repeated continental glaciation (Hays et al., 1976, p. 1121). The glacial oscillations are hypothesized to be controlled by

DlOXlDE INDUCED

150 125 loo 75 50 25 TODAY -25

THOUSANDS OF YEARS AGO

Fig. 3. The last interglacial-glacial climatic cycle and predicted climatic change during the next 25000 years (Eronen and Olander, 1990).

the flux of solar radiation received at the top of the atmosphere, which varies in response to changes in the earth’s orbital parameters (Milankovitch, 1969). Conditions favourable for glaciation are expected to persist throughout at least the next one million years, and may cause between 10 and 30 glacial advances during this period @hilts, 1984, p. 174). The ice volume record from the past shows a general trend to- ward an increased extent of continental ice with time during a glacial cycle (Shackleton and Opdyke, 1973, p. 39). It is likely that future glacial advances will be similar in extent to those of the last ice age (Fig. 21, and that a disposal vault deep in rock on the Shield will be covered. The exact timing of the next glaciation is uncer- tain, and external forces, such as anthropogenic increases in atmospheric CO, concentrations, may affect the onset. Models relating the earth’s orbital parameters to estimated ice volumes for past glacial cycles predict occurrence of the next glaciation about 20000 a from now (Fig. 3).

At their largest extent, ice sheets several kilo- metres thick may cover up to 30% of the present land mass (Elson and Webber, 1991, p. 31). Thus, the biosphere will undergo catastrophic changes during each advance and retreat of the ice. Tem- perature and precipitation regimes, the volume and pattern of surface water flow and geomor- phology will all vary profoundly during a glacial

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cycle. Old Shield faults may be rejuvenated by the crustal stresses imposed by cyclic glacial loading and unloading (Sanford et al., 1985, p. 52). Soils, vegetation cover and drainage systems will undergo a characteristic pattern of succession following the retreat of the ice (Pielou, 1991). Surface water flow, the nature of glacial deposits, and soil and vegetation types in the vicinity of the discharge zone of the vault may be quite different after each glacial retreat. Obviously, all of these factors would affect human occupation of the areas influenced by glaciation.

1.3. Assessment approaches

We investigated a number of different meth- ods for assessing glaciation (Davis, 1986). Com- puter models have been developed in the United States (Petrie et al., 1981), France (Bureau de Recherches Geologiques et Mini&es, 1985) and the United Kingdom (Frizelle, 1986; Ashton, 1988) to simulate the long-term evolution of the biosphere, taking into account such processes as climatic change, glaciation, erosion and faulting. The validity of this approach has not been estab- lished, and there is a large uncertainty associated with a time-dependent prediction of biosphere evolution.

We decided on a discrete-state approach, in which the continuous range of possible future biospheres is broken down into a small number of distinct steady-state units (Davis, 1986). The effects of glaciation on nuclide transport were evaluated by performing separate, time-invariant assessments of two states, assuming that each state persists throughout the entire simulation period of about 100000 a. Glacially induced transient events in a particular state were also identified and evaluated.

The discrete-state approach provides a practi- cal and credible framework for the evaluation of the role of glaciation in nuclide transport through the biosphere. Rather than attempting to predict the future, we have used historical data from past glaciations, and information from regions of the earth that are currently undergoing glaciation, to define characteristic biosphere states. It is easier and more defensible to define representative pa-

rameter values for a few states than to deduce a time-dependent sequence of values extending through several glacial cycles. The discrete-state approach is also more likely to be conservative. If large consequences are associated with a particular state, the greatest impacts will occur when that state is assumed to persist throughout a simulation, rather than occurring intermittently as one of a sequence of states. The discrete-state approach has been adopted in other countries to address environmental change (Jones, 1986a,b) and it provides reasoned arguments from which to evaluate the impact of glaciation on the dose to humans.

1.4. Definition of glacial states

Glacial states have been identified for assessing other geological disposal systems using the Astronomical CLimatic INdex (ACLIN) (K&la et al., 1981, p. 298; Stottlemyre et al., 1981). The ACLIN model provides climate severity indices calibrated to three sets of proxy climate indica- tors dated by radiocarbon up to 50000 BP and by U/Th decay series beyond 50000 BP (K&la et al., 1981, p. 295). The ACLIN model is discussed more fully by Findlay et al. (1984, p. 141, Matthews (1984, p. 40) and Elson and Webber (1991, p. 41. We define four basic glacial states, which do not necessarily occur in the sequence presented: interglacial, mild interstadial, cold interstadial, and full glacial (stadial). Presently, we are in an interglacial state. Between now and the next interglacial, the climate should remain cold, except for two temperate interstadials. According to the ACLIN prediction, temperatures reach a first minimum in 5000 a. Given this relatively short time, and the tendency for ice volumes to increase through a glacial cycle, it is unlikely that this initial minimum would actually result in a major glaciation. More likely major glaciations would result from the minimums at about 23000, 60000 and 100000 a from now. Interstadials would likely occur between these three glacia- lions, and the next interglacial would be expected in 125000 a.

A brief summary of the major features distin- guishing climatic conditions for the four glacial

254

Table 1

MI. Sheppard et al. /Ecological Modelling 78 (1995) 249-265

Climatic features of the glacial states

Mean annual conditions Glacial state

Temperature (“C) Precipitation (cm) Wind speed (m . s-l)

Interglacial

1.5 to -0.5 57 to 62 4

Mild interstadial

-1 to -5 42 to 46 4

Cold interstadial

-6 to -10 26 to 30 6.4

Full glacial

- 12 to - 15

states is given in Table 1 for an arbitrary Cana- dian Shield location, which serves as a climatic example. The most important features of the states are that interglacial (our present state) is the warmest and wettest, mild interstadial and cold interstadial are colder and drier, and full glacial corresponds to a fully glaciated, essentially uninhabitable, environment.

mechanisms, and concentrations and doses would remain at or below the levels predicted for other states of the glacial cycle. Transient events will not be considered further in our analysis.

2. Prediction of glaciation effects on nuclide transport

1.5. Transient events 2.1. BIOTRAC

Because glaciation is a continuing process of climate change, there are many transient events. Most of the relatively short transient events (< 2000 a) are associated with either glacial advance or retreat, as the climate changes from one glacial state to another. We have been unable to identify any processes that could increase nuclide transport through the biosphere to humans during a glacial advance. For example, freezing during a full glacial may decrease water flow in the biosphere and in the upper parts of the geosphere, and allow gaseous nuclides, such as i4C and 1291, to accumulate at the ice/ground interface. Dur- ing a glacial retreat, these nuclides may be rapidly released to the biosphere. However, the most striking feature of a glacial retreat is the vast amount of meltwater involved (Lindbom and Boghammar, 19911, which, in past glaciations, has resulted in the formation of huge lakes (Pielou, 1991). The increased dilution would likely more than compensate for any increase in nuclide release to the biosphere by trapped gases or other

Nuclide transport in the biosphere is modelled with four separate but closely linked models representing surface waters (Bird et al., 1993a,b), unsaturated soils (Sheppard, 1992; Davis et al., 1993b, p. 251, the atmosphere (Amiro and Davis, 1991, p. 41; Amiro, 1992) and the food-chain (Zach and Sheppard, 1991, p. 643, 1992) (Fig. 4).

The surface water body is assumed to be a lake (Fig. 4a), and is modelled with two compartments, one representing the water column and the other representing shallow, recently deposited mixed sediments that overlie compacted sediments, which are part of the geosphere model (Davison et al., 1993). Nuclides from the geosphere are directly released into the water column, from which they may be transferred to the mixed sediments. This system is described by coupled mass balance equations that take into account the processes of hydrological flushing, dilution, mixing, sedimentation, gaseous evasion, and radioactive decay and ingrowth. Nuclide inputs as a result of runoff and atmospheric deposi-

Fig. 4. Transport pathways and processes considered in BIOTRAC’s (a) surface water, (b) soil, (c) atmosphere, and (d) food-chain and dose submodels. Delta signs indicate where changes were made to parameter values for the interglacial and cold interstadial comparison.

M.I. Sheppard et al. /Ecological Modelling 78 (1995) 249-265 255

I I=,,. Dlywtrlon ,,

- crop mom/d”. D

t .

I Aquatic Pathways

256 M.I. Sheppard et al. /Ecological Modelling 78 (I 995) 249-265

tion, and the resuspension of nuclides from the sediments to the water column, are treated im- plicitly. The model output includes time-dependent nuclide concentrations in the water column and in the mixed sediments.

The prediction of nuclide concentrations in soil is based on a mechanistic soil model, SCEMRl (Soil Chemical Exchange and Migra- tion of Radionuclides, Version 1). This model provides the detailed treatment of processes and the fine time and space resolutions necessary for simulating nuclide migration through the soil pro- file (Fig. 4b). SCEMRl is a one-dimensional, time-dependent model that uses detailed meteorological data, together with the Darcy equation and the equation of continuity, to calculate water flows between four soil layers on a daily basis. Nuclides introduced from groundwater below, or irrigation water above, may be advected down- ward by leaching or upward by capillary rise. Concentrations in a given soil layer are calculated from a simple mass balance equation involving the flows into and out of the layer, assuming that the nuclides are instantaneously and uniformly mixed within each layer. Nuclides are partitioned between solid and liquid phases using the soil solid/liquid partition coefficient. SCEMRl is driven by the nuclide concentration in the pore water of the soil layer that receives the nuclide input; these concentrations are also calculated using a mass balance approach. The output of SCEMRl is the time-dependent nuclide concentration in the soil root zone for each of three pathways: groundwater discharge, irrigation and atmospheric deposition. The root zone concentrations, as a function of time, are approximated by a simple analytical expression used to write a mass balance equation for the root zone that allows for a time-dependent source term, ingrowth, and losses resulting from gaseous evasion, cropping and radioactive decay.

In each BIOTRAC simulation, we calculate nuclide concentrations in the soils of three distinct fields: a garden, which supplies all the plant food eaten by the critical group; a forage field, which provides the feed required by their live- stock, and a woodlot, which supplies the wood needed to build and heat their home. Non-human

organisms also live on these fields and depend on them for food and shelter. When the soil type is organic, and when the critical group heats its home with peat, we model a fourth field with the characteristics of a peat bog. The transport equations defining the surface water and soil submodels are solved by a response function/convolution approach that is used to treat time-dependent systems (Goodwin et al., 1993).

Nuclides reach the atmosphere as a result of suspension from water bodies, soils and vegetation (Fig. 4c). The atmosphere submodel treats a variety of suspension mechanisms, both natural and anthropogenic, including the suspension of particulate nuclides from terrestrial and aquatic sources, the evasion of gases from terrestrial and aquatic sources, and the release of nuclides when biomass is burnt. Once in the air, the nuclides undergo dispersion, and deposition back to the underlying surface. Additional processes can influence indoor air concentrations differently than outdoor levels. We model the diffusion of volatile nuclides from the soil into buildings, and the release of nuclides from water used inside the home. Our models for simulating suspension mechanisms vary considerably in complexity, depending on the current theoretical understanding of the process, and on the amount and quality of the available data. In some cases, simple mass- loading parameters are used to calculate air concentrations directly from the nuclide concentration in the source compartment, such as soil. This approach allows a number of suspension mechanisms to be modelled collectively, and also accounts for the effects of atmospheric dispersion. In other cases, nuclide fluxes to the atmosphere can be predicted and combined with a dispersion model to calculate air concentrations. In all cases, the models are equilibrium models, in that air concentrations are assumed to adjust instantaneously to changes in the concentration of the source compartment. Total air concentrations are calculated by summing the contributions from the individual suspension mechanisms. Indoor and outdoor concentrations are calculated separately for each nuclide. The rate at which nuclides are deposited from air to soil and vegetation is also predicted by the atmosphere submodel. Deposi-

M.I. Sheppard et al. /Ecological Modelling 78 (1995) 249-265 257

tion velocities are used to model the dry deposition process, and washout ratios to treat wet deposition.

The food-chain and dose submodel, CALDOS (CALculation of DOSe), traces nuclide movement from the physical compartments of the biosphere, i.e., water, soil and air, through the food- chain to humans, and calculates radiological doses from both internal and external exposure pathways (Fig. 4d). Transfer is predicted using simple multiplicative chain equations that assume the nuclide uptake by plants and animals, and doses, are directly proportional to nuclide concentrations in the source compartment. The model is therefore a steady-state, equilibrium model.

The internal exposure pathways considered in CALDOS are the ingestion of contaminated plants, terrestrial animals, water and soil by humans; the ingestion of terrestrial animals and fish that have consumed contaminated plants, water or soil; and the inhalation of air by humans. In treating these pathways, CALDOS accounts for processes such as root uptake, contamination of plant surfaces by irrigation and atmospheric deposition, losses from plant surfaces as a result of environmental processes, transfer to animals and humans, and radioactive decay and ingrowth. The external pathways treated are immersion in contaminated air and water, and exposure to contaminated soil and building materials. The total dose to a member of the critical group and other organisms is found by summing the individual doses from all nuclides and exposure pathways.

Internal doses depend on the amount of contaminated food, water and air taken into the body. For humans, CALDOS calculates these amounts in a self-consistent way from the total energy need, the diet, and the nutritional content of the diet. For modelling purposes, the diet is assumed to consist of five general food types: terrestrial plant, mammalian meat, milk and dairy products, poultry and eggs, and freshwater fish.

2.2. BIOTIC and glaciation

Glaciation is not expected to create any major new nuclide transport pathways within a given

glacial state. This means that BIOTRAC can be used to assess the different glacial states, although some model and parameter value changes may be necessary.

Elson and Webber (1991) derived meteorological parameter values for different glacial states for an arbitrary location on the Shield. They listed representative values for the four glacial states, identified the pathways by which nuclides could reach humans, and discussed the associated parameter values. We have used this information to evaluate the impact of glaciation on the dose to humans. The lifestyle of the critical group may change with glacial state. The present interglacial environment supports agriculture, and a self-suf- ficient farming culture is assumed to thrive. The mild interstadial state should also permit such activity, with summer temperatures being close to those of an interglacial, but with colder winters and less precipitation (Elson and Webber, 1991, p. 35). However, the cold interstadial state may not support agriculture as it is practised today; temperatures are too low, permafrost may be common, and there is much less precipitation. The critical group living in a cold interstadial environment may have to rely more on a hunter/gatherer existence typical of northern Canadian natives. Alternatively, there may be extensive import of food from warmer climates (a common practice now>, or development of advanced technology for food production in cold climates. We assume the import of food would decrease the dose to the critical group. It is not likely that technological advances in food production will increase nuclide transport to humans appreciably.

In the full glacial state, an extensive ice sheet would cover the potential disposal site. Humans and most other biota cannot survive in such an environment, so the nearest inhabited areas would lie far from the vault, at the ice margin or in ice-free corridors and similar refugia (Pielou, 1991). These areas would likely correspond to the cold interstadial state. Nuclide concentrations and doses would be very low in these areas because of the great distances from the vault, the dispersive power of the glacier and the reduced rate of water flow in the geosphere and biosphere.

2.58 M.I. Sheppard et al. /Ecological Modelling 78 (1995) 249-265

We evaluated the effects of glaciation on nu- elide transport by comparing total dose to humans for the interglacial and cold interstadial states. Since mild interstadial conditions are just somewhat colder and drier than interglacial, they are intermediate between those for interglacial and cold interstadial and were not calculated. A full glacial state is assumed to be uninhabited and was not assessed.

BIOTRAC is generally used in a probabilistic mode (Davis et al., 1993a); however, since there is no evidence that parameter variability changes from state to state, the glaciation assessment is based on median parameter values. The analysis was simplified further by including 1291, 14C, and 99Tc only. These nuclides are expected to be among the greatest contributors to dose (Wuschke et al., 1985; Goodwin et al., 1993). Technetium-99 is handled by BIOTRAC in the same way as the majority of nuclides, whereas both 12’1 and 14C

exhibit special pathways and modelling approaches (Davis et al., 1993a). The results from these three nuclides are expected to be representative of all nuclides and to provide a reasonable estimation of the total dose.

Our interglacial climate is similar to one expe- rienced at Geraldton, Ontario (49”N, 86”W) today. For the assessment of the cold interstadial state, we assume that the climate corresponds to the present climate near Ennadai Lake in the Northwest Territories (61”N, lOO’W> (Elson and Webber, 1991, p. 40). It is difficult to find a true analog of a cold interstadial state today, because higher latitudes have different daylight conditions than lower-latitude sites experiencing continental glaciation. We also do not have an appropriate analog for precipitation, and we assume that the Ennadai Lake location is suitable. We model our cold interstadial critical group on present-day native northern cultures. The cold climate limits

Table 2 Comparison of parameter values for interglacial and cold interstadial states

Sedimentation rate (kg. rn-‘. a-‘) 14C evasion rate lake to air (a- ‘1 Mean annual air temperature CC) Precipitation (m . a-‘) Runoff (m . a-‘) Effective precipitation (rn. a-‘) Probability of peat fuel use (unitless) Domestic beating needs (MJ . d-l) 14C evasion rate soil to air (a-‘) 129I evasion rate soil to air (a-‘) Wind speed (rn. s-l) Yield for wood (kg. rnp2) Plant interception fraction for wood (unitless) Plant yield (kg. mm21

for PLANT for MEAT for MILK for POULTRY

Above-ground exposure time (d) Plant environmental half-time (d) Man’s total energy need (MJ . d-l) Food type energy weighting factor (unitless)

for PLANT for MEAT for MILK for POULTRY for FISH

Interglacial Cold interstadial

0.16 0.044 0.92 0.138 5 -8 0.78 0.29 0.31 0.12 0.47 0.17 0.01 1.0

302 484 8.8 4.4 0.032 0.016 2.36 3.78

10.5 0.89 1.0 0.1

0.8 0.068 1.0 0.085 0.8 0.068 1.2 0.102

50 18250 12 18250 14.6 18.6

0.32 0.05 0.26 0.49 0.36 0.01 0.05 0.20 0.01 0.25

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agriculture, and it is possible that the critical group will belong to a hunter/gatherer society; hence we account for an increased reliance on wild game and fish. However, marginal agriculture may be possible, and we consider agriculture because it results in conservatively high doses, ensuring that we do not underestimate the consequences. We make no allowance for food impor- tation or advanced food production technologies,

3. Parameter values for the glacial assessment

For the interglacial state, we used BIOTRAC median parameter values (Davis et al., 1993a) with few exceptions. In the following sections, we define and justify all the changes to the parameter values required for the glaciation assessment of the cold interstadial state. The parameter values that were adjusted are listed in Table 2 for both states. Pathways where parameters are changed are also flagged using a delta symbol throughout Fig. 4. Parameters that were not changed are either insensitive to different climates, or are sufficiently conservative or uncer- tain that there is no basis for change.

3.1. Geosphere /biosphere inteflace

For the glaciation assessment, we consider the biosphere in isolation, without incorporating glacially induced changes in the flow of nuclides from the geosphere (Goodwin et al., 1993). The input to BIOTRAC consisted of median time series of nuclide discharges, as calculated by a geosphere model (Davison et al., 1993). We assume that the processes modelled at the geosphere/biosphere interface (Davis et al., 1993a) apply equally well in the interglacial and cold interstadial states. The occurrence of transient events during glacial advance and retreat could affect the geosphere/biosphere interface. A glacial advance could change the hydraulic head conditions, perhaps altering the location of the discharge zone (Elson and Webber, 1991). We assume that the critical group will occupy the area around the new discharge zone, which is assumed to be identical to the original discharge

zone, from a modelling point of view. During a glacial retreat, additional meltwater in the vicinity of the interface would likely dilute nuclides reaching the surface. Since this will decrease concentrations and doses, and since this dilution is difficult to quantify, we have not explicitly considered it for the cold interstadial state.

3.2. Surface water

Most of the parameter values describing processes and transport in surface waters (Bird et al., 1993a,b) were not changed for the cold interstadial state. However, the sedimentation rate is generally lower in more northern lakes because of lower production rates of organic matter at lower temperatures (Elson and Webber, 1991, p. 54). Accordingly, we decreased our interglacial value of 0.16 kg dry sediment * m-’ - a-l to 0.044 kg * rnp2 9 a-‘, on the basis of paleolimnological post-glacial data from 17 Shield lakes (Bird et al., 1993a,b). The gaseous evasion rate for 14C from lake water was decreased to 15% of its interglacial value (Table 2). This lower rate reflects a shorter ice-free season and slower diffusion rates (Thurber and Broecker, 1970). Runoff was also decreased. The remaining parameter values for surface water transport are representative of both the interglacial and cold interstadial states.

3.3. Soil

Since permafrost was not considered for the interglacial state and will likely be present in the cold interstadial state (Elson and Webber, 1991), our soil model (Sheppard, 1992) needed to be modified. We assumed that permafrost will limit the upward movement of nuclides in solution, essentially blocking soil contamination from groundwater discharge.

The existence of permafrost is to a large extent inconsistent with agriculture, but vegetation can thrive over permafrost. To support agriculture, irrigation would have to be practised to compensate for low precipitation in the cold interstadial state (Table l>, and this could result in soil contamination. The soil types chosen for this glaciation assessment were organic and sand. Organic

260 M.I. Sheppard et al. /Ecological Modelling 78 (1995) 249-265

soils may be formed at the edge of the ice sheet as small lakes evolve into wetlands and bogs characterized by peat soils. Sand soils might predominate in outwash areas after glacial melts.

Soil transport processes are driven by fluxes of water at the atmosphere-soil interface. The meteorological parameter values required to drive the moisture fluxes in the soil are precipitation, air temperature, solar radiation, vapour pressure and wind speed. Of these, precipitation and moisture are most important. The effective precipitation, the precipitation that actually infiltrates the soil, was reduced to 0.17 m water * a-i for the cold interstadial state. New temperatures, representative of cold interstadial conditions, were defined by calculating the difference between the monthly mean temperatures used for interglacial conditions at Geraldton, Ontario (Environment Canada, 1982a,b) and those at Ennadai Lake, Northwest Territories (Environment Canada, 1982~). By subtracting this difference from the generic interglacial data each day during a given month, a new data set for the cold interstadial state was produced. These data give a mean annual air temperature of -8”C, corresponding to a much shorter growing season in cold interstadial than in interglacial conditions. Appropriate ambient vapour pressures were also determined for cold interstadial conditions (Sheppard, 1992, p. 125).

The data used for interglacial conditions were selected to represent a typical year in terms of precipitation rate, because it is the most important meteorological parameter (Sheppard, 1992, p. 38). Although the year was typical for precipitation, it was not for temperature, which was warmer. The resulting 13°C mean temperature difference between interglacial and cold interstadial conditions (Table 2) reflects a large temperature change, so that our assessment includes a correspondingly large difference in climate.

We assumed that chemical exchange in the soil is temperature-independent, and used median solid/liquid partition coefficients to describe soil nuclide retention for both glacial states. Gaseous emissions from soil are partly mediated by biolog- ical activity, which is temperature-dependent. Given a 13°C temperature difference between

interglacial and cold interstadial states, and not- ing that many physiological functions are approxi- mately halved with a 10°C temperature decrease (Lehninger, 1975) we reduced the gaseous evasion rates from soil to air to 0.016 a-l for 1291 and to 4.4 a-i for 14C (Table 2).

The remaining parameters describing nuclide transport in the soil are representative of both the interglacial and cold interstadial states.

3.4. Atmosphere

Few modifications to the atmosphere submodel (Amiro, 1992) were required for the glaciation assessment; many of the model parameter values are appropriate for both the interglacial and cold interstadial states. For example, the building parameters and those related to indoor air are similar for both states, either because they are independent of climate or because the values selected for interglacial conditions are conservative. However, for some parameters, specific cold interstadial values are needed.

We assume that peat-burning for heating purposes is more common during a cold interstadial, and the probability of doing so is assigned a value of 1.0 rather than 0.01 (Table 2) when organic soil is considered. Increased home heating is required during cold interstadial conditions, and we assumed that the amount of energy required to heat a single family dwelling increases linearly with heating-degree days below 18°C. Long-term averages in present conditions (represented by Geraldton, Ontario) and at Ennadai Lake are 6278 and 9961 heating-degree days, respectively (Environment Canada, 1982a,b,c). Hence the amount of energy required to heat a single family dwelling is 484 MJ * d-’ for a cold interstadial, 1.6 times higher than that for the interglacial state.

Elson and Webber (1991, p. 39) indicate that wind speeds will generally increase in the cold interstadial state by about a factor of 1.6. In- creased wind speeds will enhance atmospheric dilution, thereby decreasing local air concentrations. For dispersion from a chimney, we increased the annual average wind speed by a factor of 1.6 to 3.78 m - s-l for cold interstadial

M. I. Sheppard et al. /Ecological Modelling 78 (1995) 249-265 261

conditions. For dispersion from terrestrial and aquatic area sources, increased wind speeds were also modelled.

Particle suspension, as defined by the terrestrial and aquatic dust loads, was left unchanged. We believe that the values chosen for interglacial conditions are appropriate for both interglacial and cold interstadial states, even though cold interstadial conditions are drier and windier.

A cold interstadial climate may have an increased frequency of stable atmospheric conditions over terrestrial regions. However, increased wind speeds may counterbalance this, resulting in no appreciable change in the average stability. To be conservative, we assumed that stable conditions, which maximize air concentration, prevail during a cold interstadial and new atmospheric dispersion factors for these conditions were calculated. We assumed that stability conditions will not change appreciably over aquatic surfaces, with neutral conditions continuing to predominate; thus the calculation applies to both states.

All other parameter values describing nuclide transfers in the atmosphere can be applied to both interglacial and cold interstadial states.

3.5. Food-chain and dose

Human anatomy and physiology are assumed to remain constant in the various glacial states, so many of the food-chain and dose parameters will not change. For example, the dose conversion factors will not vary with climate. However, some parameters will vary, such as those related to diet and ecosystem type.

The dominant native plant community in a cold interstadial environment may be transitional between boreal forest and tundra, typical of the vegetation found near the tree line in northern Canada today. From the data on plant yields given by Elson and Webber (19911, we estab- lished a yield for wood of 0.89 kg wet wood . rnF2 forest and a plant interception fraction for wood of 0.1 for cold interstadial conditions to reflect a sparsely forested open woodland (Table 2).

We assumed that the critical group can sustain some agriculture, or will rely on wild native plants for food and animal fodder. In either case, the

colder climate will decrease yields in comparison with an interglacial climate. Following the recom- mendations of Elson and Webber (1991, p. 59), we decreased plant yields to 0.068, 0.085, 0.068 and 0.102 kg wet biomass * me2 soil for the food types terrestrial plant, mammalian meat, milk and dairy products, poultry and eggs, and freshwater fish, respectively (Table 2).

The critical group may rely more heavily on wild game, such as caribou, during a cold interstadial. Native animals may obtain a larger portion of their diet from perennial vegetation that could accumulate and retain nuclides over a longer period than an annual agricultural crop. For instance, lichens are known to efficiently retain nuclides, which can subsequently be transferred via caribou to humans (Whicker and Schultz, 1982). To include this pathway, we increased the time of above-ground exposure for terrestrial plants and mammalian meat, and the plant environmental half-time to 50 a or 18250 d for cold interstadial conditions. This reflects the long potential times over which some wild plants could accumulate and retain nuclides. This large value for plant half-time will result in radioactive decay becoming a more important depletion term.

A colder environment would increase man’s total energy need, so we increased the energy need from 14.6 MJ - d-’ to 18.6 MJ . d-’ for cold interstadial conditions. Also, northern diets, typi- fied by today’s inhabitants, generally include a large proportion of venison and fish. Hence, we modified the food type energy weighting factors to reflect this (Table 2). The remainder of the food-chain and dose parameters were assumed to vary little between the two glacial states and were not altered.

3.6. Water balance parameters

Elson and Webber (1991) indicate that both precipitation and runoff should decrease in a cold interstadial environment. Following their recom- mendations, we set precipitation to 0.29 m * a-r and runoff to 0.12 m * a-‘, so that the effective precipitation, the amount of water that infiltrated the soil, was 0.17 m . a-l for cold interstadial conditions rather than 0.47 m - a-* for inter-


glacial conditions. This change assumes that glaciers are not actively retreating, which would increase runoff substantially. This assumption is conservative because melting would increase dilution.

Irrigation is a potentially important source of contamination for both soil and plants, given the dry conditions during a cold interstadial. We consider irrigation of a garden with lake water for both the interglacial and cold interstadial states. Because of the drier conditions during a cold interstadial, more irrigation water would be needed to maintain soil moisture at field capacity.

4. Assessment results

BIOTRAC results were used to compare the doses predicted for the interglacial and cold interstadial states, using the selected parameter values (Table 2). For both states, BIOTRAC was driven by realistic, median geosphere inputs. These inputs increase with time (Goodwin et al., 1993) so that the times of peak dose for each of the three nuclides considered were partially dependent on the release rates from the geosphere. The ratios of the cold interstadial to interglacial maximum doses, irrespective of their time of occurrence within 100000 a for 1291, 14C, and *Tc are given in Table 3 for both soils.

The cold interstadial environment produces slightly lower predicted doses than the interglacial for organic soil and slightly higher doses for sand soil for 1291 (Table 3). The effect is opposite for 99Tc and somewhat more extreme for organic soil, though 99Tc doses were relatively insignificant. Doses from 14C increase by up to a

factor of six for the cold interstadial compared with the interglacial, depending on soil type.

For sandy soils, concentrations at steady state were slightly lower in the colder state, because evapotranspiration rates are lower and less contaminated irrigation water is required, despite the lower effective precipitation. Comparisons of the soil concentrations scale correctly with the amount of contaminated irrigation water applied. For organic soil, however, concentrations at steady state in the colder state were higher even though total evapotranspiration was lower. This occurs because of the very large value chosen for the field capacity moisture of the peat, which is used to calculate the amount of irrigation water applied. Also, the evapotranspiration and field capacity moisture are not linked mechanistically in the model, and it cannot reduce the amount of irrigation water added when evapotranspiration is low. This results in large estimates of the amount of irrigation water needed, and consequently high soil concentrations.

For 129I and 14C, the ingestion of plants growing in contaminated soil is the pathway that con- tributes the greatest portion of the dose during an interglacial (Reid and Corbett, 1993). During a cold interstadial, this pathway is more important because more irrigation water is used in the drier cold interstadial state.

If we assume that the geosphere is unaffected by glaciation, BIOTRAC predicts that doses to humans in a cold interstadial environment would differ by less than an order of magnitude from those for present-day interglacial conditions. Only differences as large as an order of magnitude are generally thought to be significant, considering the complexity and uncertainties in such a challenging modelling problem. Doses from individ-

Table 3 Dose ratios for cold interstadial and interglacial states for selected radionuclides

Ratio of maximum doses for lo5 a 129 I

14 C 99Tc

Cold interstadial/interglacial (organic soil) 0.45 1.6 6.5 Cold interstadial/interglacial (sand soil) 1.62 5.1 0.70 Time of peak dose (a) 105 6 x lo4 105 Radiological half-life (a) 1.6 x lo7 5730 2.1 x 10s

MI. Sheppard et al. /Ecological Modelling 78 (I 99s) 249-265 263

ual nuclides will vary slightly. However, the total dose to man, even though it is low, is dominated by I291 for times up to 100000 a (Goodwin et al., 19931, and a critical group living in an interglacial or cold interstadial environment would be exposed to similar doses.

5. Conclusions

Glaciation is a major trauma to the biosphere. The environmental consequences, in the form of ecosystem change, are immense, and would greatly exceed perturbations caused by small amounts of nuclides originating from a disposal vault. The cultural impact of glaciation on human development and habits would also be large, effectively dwarfing potential radiological consequences.

Regardless of the large physical impact of glaciation, we have quantitatively assessed the potential implications of glaciation on the dose to humans. Our comparative analysis indicates that the interglacial and cold interstadial states will likely result in similar total doses. The reason for this similarity is that glacially induced biosphere changes do not affect any of the highly sensitive model parameters (Reid and Corbett, 1993), such as the use of lake versus well water. The intermediate mild interstadial state should result in doses that lie between those predicted for the interglacial and cold interstadial states. In the full glacial state, humans are assumed not to inhabit the vault region. The closest possible critical group would be at the edge of the ice sheet, effectively experiencing a cold interstadial environment, but with lower doses because of the large distance from the vault location. Transient events associated with the evolution of the glacial cycle are not expected to introduce processes that could increase the dose to humans. The major transient process during a glacial melt is increased runoff, which should decrease local nu- elide concentrations through flushing and dilution.

The central question addressed is whether glaciation could lead to substantially increased doses to humans. Our analysis shows that this is

unlikely, because doses for various glacial states tend to be similar.

Acknowledgments

We wish to thank Dr. Michael Stephens and Laveme Wojciechowski for our comparison simu- lations using BIOTRAC, and Drs. Sid Whitaker and Steve Sheppard for helpful comments on the manuscript. The work was carried out for the Canadian Nuclear Fuel Waste Management Pro- gram, which is jointly funded by AECL Research and Ontario Hydro under the auspices of the CANDU Owners’ Group.

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Documents

Continental glaciation and nuclear fuel waste disposal: Canada's approach and assessment of the impact on nuclide transport through the biosphere