Characterization, Modeling, Monitoring and Remediation of Radionuclides in the Subsurface

Thomas J. Nicholson, Mark Fuhrmann and Ralph CadyOffice of Nuclear Regulatory Research

September 15 - 16, 2009EPRI Ground-Water Protection Workshop

Charleston, South Carolina

Overview

• Dose Assessment to Determine Need for Remediation

• Characterization of Sources and Subsurface Processes

• Field Tests and Monitoring to Test Conceptual Site Model (CSM)

• Range of Remediation Methods Available

• Selection of Remediation Method Based upon Confirmed CSM and Monitoring Baseline

• Confirmatory Performance Monitoring to Evaluate Remediation Effectiveness

• Analysis of Monitoring Data to Determine Compliance with Dose Criteria

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Dose Assessment to Determine Need for Remediation

Decision to Remediate Based upon Source and Site Characterization

Identify contaminant sources Determine if releases are continuous or

episodic, their radiochemistry and mass flux Collect soil and water samples to estimate

contaminant concentrations and properties Identify subsurface pathways to receptors Estimate attenuation and sorption characteristics of subsurface Calculate dose to receptors and compare to dose criteria

If Dose exceeds regulatory criteria consider the need forremediation

3

secs

min

sho

urs

days months years

surface water

groundwater

Transport Processes – Hydraulic Residence Times (trans )

Attenuation Processes – Reaction Times (rxn )

Radioactive Decay

solid phase transformationsolid-state diffusionprecipitation

co-precipitationexchange-adsorption

222Rn3.82 d

3H12.3 y

131I8 d

1

10

100

1,000

10,000

100,000

226Ra1600 y

99Tc210000 y

241Am432 y

220Rn55.6 s

secs

min

sho

urs

days months years

surface water

groundwater

Transport Processes – Hydraulic Residence Times (trans )

Attenuation Processes – Reaction Times (rxn )

Radioactive Decay

solid phase transformationsolid-state diffusionprecipitation

co-precipitationexchange-adsorption

222Rn3.82 d

3H12.3 y

131I8 d

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

226Ra1600 y

99Tc210000 y

241Am432 y

220Rn55.6 s

ImmobilizationReactions}

from Ford, EPA at 2009 RIC

Relative Process TimescalesRelative Process Timescales

4

5

Does radioactive material move from slag to ground water?

Precipitation percolates through pile

Unsaturated Zone

Water SaturatedZone

Groundwater moves slowly

Soil Samples Water Table

Is radioactive materialgetting to the groundwater?

Weathering Processes of Slag• Leaching of U, Ra, Pb, Ca, Si• Formation of secondary minerals• Adsorption

Retardation Processes in Unsaturated Zone

• Adsorption (Kd)• Solid solution (Ra in calcite)

Processes in Saturated Zone• Adsorption (Kd)• Dispersion• Transport

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Does radioactive material move from slag to ground water ?

Three approaches are recommended to determine if slags are releasing radioactive material into water:

• Laboratory leach tests of multiple samples of slag • Soil samples from under the piles with analysis for

radioactive material • Ground-water monitoring for radioactive material

Performance Indicators are pH and recharge rate

The Conceptual Site Model

• Hypothetical representation of the site

• Select, organize and communicate information in 3-dimensional visualizations

• Subject to testing with new characterization and monitoring data

Conceptual cross-section for the Rocky Flats Facility (Modified from K-H, 2004)

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Conceptual Site Hydrologic Model

Important to characterize the following:

• Natural and engineered features, structures, backfills and soil-rock interfaces, boundary conditions and time-dependent processes

• Potential sources of accidental, incidental and regulatory releases• Regional and site hydrologic setting (aquifers, surface- water bodies,

springs, wetlands and drainage systems) and relationships• Local drinking water sources (ground- and surface-water sources)• Existing ground-water wells and monitoring points onsite and offsite• Depth to the water table and surface-water body elevations• Historical details on contaminant releases• Ground-water gradients, flow directions and velocities

Once surface and/or subsurface contamination is detected,evaluate its significance and develop an appropriate response

8

from Ward et al. (1997) after Caggiano et al. (1996)

Conceptual Model of Complex Site with Multiple Sources

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---137 Cs c::::::J 90Sr Il!I:II Backfill c::::::J Mobile Constituents in Tank Waste, 3H, Na+, Tc04-' U02(C0:V2=' Cr04=

High Level Waste Supernate or Liquid Phase ..........- Natural Precipitation and Movement Along

Solid Phases of High Level Waste (FeOH3 and Other Precipitated Phases)

Culturally Disturbed and Natural Pathways

Surface Runoff and Artificial Sources of Water

Fig. 4Composites of Geophysical Logs, Transmissivity and Hydraulic-Head Difference Estimates and Measurements & Tracer-Test Results for Test Wells (Williams, 2008)

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- - - ,- -- .- - -~ - --, , • , , , , ,

" • " • III -<., ~~ ... - " 0.'" ~ •• " .. n~ • , -- "T .... .- ._-, , , • 0 - ~~ ~. "'. -' ., .• = ~ .. •

Site-Specific FEP’s for DevelopingAlternative Conceptual Site Models

• Pathways for rapid spread of leaking contaminants – pipe or cable trenches – gravel backfill

• May drive contaminants in directions not predicted by contouring a few data points on a water-table map

• Local precipitation drainage (roof and storm drains)

• Water-sources of leaks – can inject large amounts of water into the

vadose zone, sometimes creating perching – drive ground water and contaminants in

directions not predicted based on water levels from scattered monitoring wells

GPR Images

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Uncertainties in Conceptual Models

Sources of Hydrogeologic Uncertainty:

Incomplete knowledge of the subsurface system

Measurement error in characterizing the system’s features, events and processes (FEPs)

Natural variability in the system’s spatial properties, temporal events and transient external stresses

Disparity in scales of sampling, monitoring and simulation relative to actual dimensions of the FEPs

Parameter estimation Scenario definition

612

1

ANALYSIS:

Site and Facility characterization

& monitoring data

Conceptual Site Model (CSM)

Flow, transport

Site modeling (PA)

HOW:Monitoring Devices (MD)

WHERE & WHEN:Monitoring Points (MP)

WHAT:Performance Indicators

(PI)

PERFORMANCE CONFIRMATION MONITORING:

Data Collection & Analysis

FEEDBACK to update:

• CSM

• PA

• Choice of PIs, MDs, and MPs

• Stopping Rules

NUREG/CR-6948

13[ from Price and others, 2007 in NUREG/CR-6948 ]

Integrate Modeling with Monitoring

Why monitor and model?Couple sources to site characteristics and receptors:

Develop and evaluate site conceptual models Guide data collection including monitoring, sampling and

geophysical surveys Formulate scenarios involving human and natural conditions Identify Performance Indicators as measureable quantities

for both monitoring and modeling Analyze Monitoring Data to Determine Compliance to Dose

Collect information to identify significant Features, Events and Processes

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Integrate Modeling with Monitoring

Site-Specific Modeling Benefits:

Integrates disparate characterization and monitoring data into a logical framework

Reduces uncertainties and help to identify location of monitoring to confirm hydrogeologic system behavior

Forecasts impacts (doses due to exposure and uptake) Provides bases for decision-making on the need to interdict,

mitigate and remediation abnormal releases Assists in designing and monitoring remediation program

(e.g., monitored natural attenuation thru pump-and-treat) Communicates understanding of the system to the public

and facilitates technical interactions15

Range of Remediation Techniques

Increasing Costs

Increasing Technical Uncertainty

Passive RemediationNo ActionActive Remediation

Excavation&Removal

Pump &Treat

Chemical Remediation, AbioticTreatment Zones or Walls

Bioremediation:Forced Gradient

Bioremediation:Natural Gradient

MonitoredNaturalAttenuation

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(after Long et al, 2008, NUREG/CR-6973)

Case Study Case Study -- Immobilization Immobilization Hanford 300Hanford 300--Area Area -- UraniumUranium

daily fluctuation

ColumbiaRiver

ContaminatedVadose

Zone

Regional GW Flow

TransientFlow

Reversal

GroundwaterPlume

A

SmearZone

Hanford 300 Area

Historical Disposal Unit 11

2233

44 55

Contaminated surface soils (source removal)Dispersed residual contamination in vadose solidsZone impacted by water table fluctuations (GW-SW interactions)Plume in continuously saturated zoneTransition zone between GW & SW (includes sediments)

1

2

3

4

5from Ford, EPA at 2009 RIC 17

Monitoring to Determine Efficacy of In Situ Bioremediation

Photo of wells and sampling apparatus including flow cell for measurement of pH, Eh, DO, T,and conductivity during slow purge sampling. Foreground shows injection manifold and

stainless steel injection lines to injection wells. [ From Long, Yabusaki et al, 2008 (PNNL-17295) ] 18

Case Histories - Brookhaven

Regional Geologic Setting

Regional Ground-Water Flow

Tritium Plume Cross-SectionSite Ground-Water Flow

Tritium Plume Plan Map

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Synthesis of CSM & Flow and Transport Models

• AES developed a CSM that incorporated influence of nearby pumping wells and 3-D visualization of data

• Used flow and transport modeling to validate that our CSM reflected observed monitoring data. (Modeling was done in Excel, MODFLOW under the GMS umbrella)

• Once validated, used flow and transport modeling to evaluate remediation alternatives

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On-Site Monitoring Needed atNuclear Facility Sites

Minimization of contamination required at new nuclear facilities [10 CFR Part 20.1406 (a) (b)] • How facility design and operating procedure will minimize to the

extend practicable contamination of the facility and the environment• Facilitate eventual decommissioning and minimize generation of

radioactive waste

Existing radionuclide contamination at operating facilities

Facilities being decommissioned

Modeling and Monitoring establish the Technical Basis for Decision-Making such as the need for and selection of Remediation Methods

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Information Source – NUREG/CR-6948

• Technical bases for developing guidance on ground-water monitoring for NRC-licensed sites

• Systematic methodology to integrate monitoring with modeling

• http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6948/v1/index.html

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Information Source – NUREG/CR-6948

• Lessons-Learned for developing guidance on ground-water monitoring for NRC-licensed sites

• Case Studies which includes Brookhaven radionuclide plume remediation and monitoring

• http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6948/v2/index.html

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Information Source – NUREG/CR-6973

• Overview of in situ uranium bioremediation which identifies and prioritizes field performance indicators for evaluating its effectiveness.

• Performance indicators to be monitored and modeled are based on current biogeochemical understanding of uranium.

• Confirmation monitoring is vital to demonstrating long-term success of U-bioremediation and provides a significant assurance that regulatory goals will be met.

• http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6973/index.html

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