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International Groundwater Symposium Valencia, Spain September 22-24, 2010 Greg Ruskauff 1 Nicole DeNovio 2 Edward Kwicklis 3 1 NNES LLC 2 Golder Associates 3 Los Alamos National Laboratory 1

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Page 1: Iahr Ruskauff

International Groundwater Symposium

Valencia, Spain

September 22-24, 2010

Greg Ruskauff1

Nicole DeNovio2

Edward Kwicklis3

1 NNES LLC2 Golder Associates

3 Los Alamos National Laboratory

1

Page 2: Iahr Ruskauff

BackgroundThe Nevada Test Site was used from

1951 to 1992 for nuclear weapons

testing

Great Basin area of western United

States

Facility area is 3,500 km2

(> Luxembourg)

The tests left behind radioactive residue,

which may be migrating in groundwater

NNSA/NSO established the

Underground Test Area (UGTA)

Subproject to ensure the protection of

the public

Site characterization and modeling used

to negotiate compliance boundaries with

Nevada Department of Environmental

Protection2

Page 3: Iahr Ruskauff

Frenchman FlatTen underground shaft tests detonated

between 1965 and 1971

All less than 20 kt

Informally divided into Northern and

Central areas

3

Page 4: Iahr Ruskauff

What happens near an underground nuclear

test?

Rock and water vaporized immediately around device, forming a cavity

Formation of a chimney as pressure subsides

Alteration of rock properties to some distance away from the test

Cavity size related to yield and depth of burial

The exchange volume, where radionuclides can be found, forms out to ~ 2 cavity radii

Preferential distribution of radionuclides

Higher boiling point radionuclides become incorporated in the nuclear melt glass

Lighter radionuclides condense from vapor and remain in the water and sorbed to cavity/chimney rubble

4

Page 5: Iahr Ruskauff

Underground Nuclear Tests in Frenchman Flat

Ten tests

Nine conducted in alluvium

One conducted in vitric tuff

The CAMBRIC event (entirely in alluvium)

has been studied extensively by

Lawrence Livermore National Laboratory,

who developed the following conceptual

model:

In alluvium, shock-compressed zones of lower

permeability, porosity limit groundwater flow

through the cavity

Radionuclides assumed to be evenly distributed

within exchange volume (2Rc), and nuclear melt

glass zone.

Supported by several numerical models

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Page 6: Iahr Ruskauff

The need for other conceptual models

6

Within 2 cavity radii rocks vary,

especially at the water table

IAEA (1998) reviewed French

underground nuclear tests in basalt

lava in the Pacific

“The Radiological Situation at the

Atolls of Mururoa and Fangataufa”

Conceptualized high permeability

zone out to 2.5 cavity radii

Cavity infilling studies broadly support

spherical zone of enhanced

permeability from test effects

Adopt another conceptual model for

tests with the exchange volume

extending to lava and welded tuff

Page 7: Iahr Ruskauff

Consequences of near-test conceptual models

7

Alluvium Concept IAEA Concept

Evaluated the effect of different near-field conceptual models on

radionuclide release

Same inventory uncertainty

Different conceptual models received different parameter assignments

Alluvium/vitric tuff – permeability reduction factor from 1.5 to 100

IAEA – permeability enhancement factor of 5 to 100

Different conceptual model results in higher peak concentrations

Page 8: Iahr Ruskauff

Basin-Scale Conceptual Model

Deep, complicated geology

Initial model didn’t have data on the depth of the alluvium, thickness of confining units

Cross Section through Center of Model Domain

Alluvial Aquifers

Regional Carbonate

Aquifer

VolcanicAquifers

Aquitards

Top of the Model Domain

8

HLCA < HAA

Page 9: Iahr Ruskauff

Hydrogeologic Conceptual Model

Easterly component from

leakage across Cane Spring fault

from CP basin

Based on age and head

relationships groundwater flows

from the perimeter of the basin

toward the basin center and out to

the south-south east

For the regional carbonate flow

system, Rock Valley fault system

provides regional drainage that

will focus flow to the southwest

out of Frenchman Flat

Groundwater velocity in

alluvium from 0.1 to 1 m/yr

9

1.10.85

0.20

0.43

0.25

Page 10: Iahr Ruskauff

Approach to Uncertainty AnalysisExternal project review during 1999 suggested

the need to consider conceptual uncertainty

reflected in basin conceptual model

Revised approach incorporated 5 interpretations

of geology

Tried to focus alternative interpretation in portions

of the model likely to have transport

Several sets of boundary conditions

One different hydrologic conceptual model

Three permeability parameterization

approaches

Several discrete flow model cases of different

geologic models, parameterization approaches

Performed Monte Carlo transport analysis on

each case

Flow model calibration constrained Monte Carlo

analysis

Monte Carlo transport analysis on ensemble

HFM Alternative Description

DETA – detachment

fault alternative

This alternative is a no detachment fault

model.

DISP –

Displacement fault

alternative

This alternative is concerned with the

locations and displacement of basin-

forming faults.

CPBA – CP basin

alternative

The CP basin alternative extends the

UCCU beneath all of CP basin.

BLFA – Basalt Lava-

Flow Aquifer

The BLFA HSU is modeled as a single

continuous flow, rather than three

separate zones

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Page 11: Iahr Ruskauff

Pooled Uncertainty Analysis

All discrete cases considered

“Null-space Monte Carlo” of Tonkin and Doherty (2009) (100 realizations)

Calibration constrained uncertainty of flow model parameters

Specified plausible parameter ranges

Heads and flows always calibrate acceptably

11

BASE Hydrologic

Conceptual Model

Alternative Hydrologic

Conceptual Model

Page 12: Iahr Ruskauff

Which models matter?Try to identify similar behaving

models that can be used as

surrogates for others

Used cavity flow rate as

transport surrogate

Evaluate range to see if relevant

uncertainty is being sampled

Cannot discern that alternative

geologic models have any

importance

Scatter among models using

variable parameterization similar

to alternative geology scatter

Two (NHA and BASE-USGSD)

really are different12

BASE Hydrologic

Conceptual Model

Alternative Hydrologic

Conceptual Model

Alternate geology

interpretations

BASE geology

variable

parameterization

Page 13: Iahr Ruskauff

Remaining Geological Uncertainty

Welded tuff under PIN STRIPE modeled as continuous

North-south normal faulting appears to die out, not certain

Is the welded tuff really continuous?

Missed conceptual uncertainty

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Page 14: Iahr Ruskauff

Things We LearnedIn one instance, exchange volume altered properties, conceptual model uncertainty was clearly important

We need to be alert to conceptual uncertainty when we are extrapolating outside the information we have

CAMBRIC conceptual model vs. other tests

Refsgaard, J.C., J.P. van der Sluijs, J. Brown, and P. van der Keur. 2006. “A Framework for Dealing with Uncertainty due to Model Structure Error.” In Advances in Water Resources, Vol. 29, 1586-1597.

Can exist at all scales

Having an initial idea of what might be uncertain is useful, but it can’t override the ability to make adjustments

Two of the alternative geologic models were not insightful

It may take some time to understand the base interpretation enough to identify sensitive assumptions (e.g. the geologic uncertainty at PIN STRIPE)

“If the confidence intervals associated with the models predictions are greater than the difference between the predictions, this difference cannot be considered meaningful, at least in a statistical sense.” National Research Council, Ground Water Models Scientific and

Regulatory Applications, 1990.

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Page 15: Iahr Ruskauff

International Groundwater Symposium

Valencia, Spain

September 22-24, 2010

Greg Ruskauff1

Nicole DeNovio2

Edward Kwicklis3

1 NNES LLC2 Golder Associates

3 Los Alamos National Laboratory

15