# Presentation: 018436

## Probabilistic Performance-Assessment Modeling of the Mixed Waste Landfill at Sandia National Laboratories 

**Clifford K. Ho, Timothy J. Goering,****Jerry L. Peace, and Mark L. Miller**
**Sandia National Laboratories****Albuquerque, NM 87185****(505) 844-2384****ckho@sandia.gov**

## Summary

**Probabilistic fate and transport models showed several potential exceedances that merit “triggers” for long-term monitoring**
**Tritium dose via air pathway**
**Surface flux of radon-222 gas**
**PCE concentration in groundwater**
**Key Assumptions**
**Receptor located at MWL (continuous inhalation exposure)**
**Sealed sources of radium-226 (which produces radon-222) allowed to degrade completely**
**Cover allowed to erode completely**
**Waste inventory treated as uncertain**
**Report can be downloaded from **_**[www.sandia.gov/caps](http://www.sandia.gov/caps)**_** **

## Overview

**Background**
**Modeling Approach**
**Modeling Results**
**Recommended Triggers**

## Background

**New Mexico Environment Department request for Corrective Measures Implementation Plan (May 2005)**
**Include comprehensive model to assess the fate and transport of contaminants from the Mixed Waste Landfill**
**Identify monitoring results that will trigger additional testing or remedies**
**Similar recommendations provided in 2003 WERC independent technical peer review**
**Developed probabilistic performance assessment to address these recommendations**
**Conduct comprehensive fate and transport analysis for contaminants of concern; compare to regulatory metrics**
**Quantify uncertainties**
**Perform sensitivity analyses; understand failure modes**
**Identify triggers for long-term monitoring**

**Notes:**
From Final WERC Report:  “Independent Technical Peer Review of the U.S. Department of Energy Sandia National Laboratories’ “Working Draft” Corrective Measures Study dated November 2002 Mixed Waste Landfill,” January 31, 2003, Performed by WERC:
A Consortium for Environmental Education and Technology Development
p. vi Executive Summary:  
“4. The Panel strongly recommends the development of an integral numerical "fate
and transport" model for the simulation of the MWL. It is regrettable that such a
model was not yet developed to take advantage of sophisticated software that are
available in academia, national laboratories, and private industry. This software
should be calibrated with existing MWL data and fine tuned with the results of
future monitoring and fundamental developments. The results of the numerical
modeling should be used as inputs to the risk assessment model, and to probe into
the sensitivity of different MWL options.”
Our recent probabilistic performance-assessment model includes detailed fate-and-transport models for contaminants of concerns and relevant performance metrics.  Simulated results were compared to existing data, when available, to calibrate and build confidence in the models.
p. 29:
“The Risk Assessment lacks modeling of the fate and transport of tritium in the subsurface
in the gas and liquid phases. The migration/release of tritium (and other COCs) needs to
be modeled to failure; i.e., when in time or at what level of source concentration will the
COCs reach the groundwater. This sort of modeling work will allow SNL to establish the
proper trigger levels described above.”
We have simulated the fate and transport of tritium in the subsurface in both the gas and liquid phases.  Results show that tritium can reach the groundwater (when gas-phase transport is included), but the concentrations and groundwater dose remain below regulatory standards.  Simulated dose due to tritium transport upward to the atmosphere resulted in some exceedances relative to the regulatory standard of 10 mrem/year via the air pathway.  For all scenarios, the system was evaluated for possible “failure modes,” and appropriate triggers were recommended.
p. 30: 
“Page I-26 – Section V: Fate and transport assessment does not include a model for
the fate of tritium or organics with time and space. SNL needs to run tritium
diffusion and tritium water migration, calibrated with the historical data from
sampling over time, to the point of contact with the groundwater. In this manner SNL
can determine where the contaminants migrate, how long it takes to reach a given
boundary condition, and what the concentrations/source energies are at that location
and time.”
We have included models of both tritium and organics (PCE), and simulations were compared to historical data to ensure consistency.  

## Mixed Waste Landfill

**Looking Southwest, 1987**

**Received mixed waste from 1959 to 1988**
**100,000 cubic feet**
**6,300 Curies**
**Semi-arid climate**
**Average precip. ~ 9 in/yr**
**Thick vadose zone**
**Nearly 500 feet**
**Proposed 3-foot-thick vegetated soil cover**
**1-foot-thick biointrusion barrier**

**100 ft**

## Trench BLooking South, 1974

**Trench B****Looking South, 1974**

## Trench ELooking South, May 1980

## Classified Waste Pit Disposal1974

**Classified Waste Pit Disposal****1974**

## Contaminants of Concern

**Radionuclides**
**Am-241, Cs-137, Co-60, Pu-238, Pu-239, Ra-226, Rn-222, Sr-90, Th-232, H3, U-238**** **
**Heavy Metals**
**Lead**
**Cadmium**
**Volatile Organic Compounds**
**PCE (proxy for other VOCs; highest potential for vapor transport and exceedance of regulatory standard)**

## Overview

**Background**
**Modeling Approach**
**Modeling Results**
**Recommended Triggers**

## Probabilistic Performance Assessment Process

**Notes:**
First identify regulatory requirements to screen scenarios and various design options to choose from.

When talking about models, mention that they can be integrated into a common framework using FRAMES or GoldSim.

Mention that an example of this method was given at the 2001 Midyear Review, and this year we will focus on how uncertainty, sensitivity, and alternative-design analyses can be performed and how they can benefit the end user.

## Scenarios

**Water percolates through the cover**
**Consideration of wetter future climates**
**Transport of radionuclides**
**Radionuclides leach to groundwater**
**Gas-phase radionuclides (radon and tritium) diffuse to the surface and groundwater**
**Transport of heavy metals**
** ****Lead and cadmium leach to groundwater**
**Transport of volatile organic compounds**
**PCE diffuses/leaches to surface and groundwater**

**Notes:**
Exclusion of bio-intrusion scenario:

The crushed-rock bio-intrusion barrier is intended to mitigate plant and animal intrusion and subsequent uptake of the buried contaminants.
There are an infinite number of scenarios that one can consider in a performance assessment.  We considered what we believed to be the most important pathways and constituents of concern, including transport of radionuclides, heavy metals, and VOCs both to the surface and to the groundwater via gas and liquid-phase transport.  Plant-root uptake and animal intrusion and displacement of these contaminants were not considered in these prioritized scenarios.

## Models

**Water Percolation through Cover**
**UNSAT-H (unsaturated flow, evaporation, transpiration)**
**Leaching and Transport of Radionuclides & Heavy Metals to Groundwater**
**FRAMES/MEPAS (probabilistic modeling of fate and transport of multiple radionuclides (with progeny), heavy metals, and chemicals)**
**Gas and Liquid-Phase Transport of Tritium, Radon, and PCE to Surface and Groundwater**
**Transient tritium and PCE transport: Jury et al. (1983, 1990)**
**Steady radon transport: ****Ho (2005)**
**Probabilistic Monte Carlo analysis in Mathcad****®**

## Probabilistic Modeling

**At least 100 realizations were simulated in a Monte Carlo analysis for each transport model**
**FRAMES/MEPAS used Latin Hypercube Sampling**

**Each realization can be thought of as a different (but equally probable) transport path through the system**

**Distributions were created for uncertain input variables**
**Conservative or bounding values were used when site data were unavailable**

## Uncertain Variables

**Waste Inventory and Size**
**Thickness of Cover and Vadose Zone**
**Transport Parameters**
**Infiltration**
**Adsorption coefficient**
**Saturated conductivity**
**Moisture content**
**Tortuosity coefficients**
**Boundary-layer thickness**

## Uncertainty Analysis

**Multiple computer “realizations” are simulated using a range of input values for uncertain parameters**

**Ensemble of realizations yields probability distribution for “performance metric”**

## Sensitivity Analysis

**Quantifies the most important parameters and processes that impact the simulated performance metric**
**Linear stepwise rank regression**
**Important parameters can be used as triggers for long-term monitoring or to prioritize site characterization**

## Overview

**Background**
**Modeling Approach**
**Modeling Results**
**Recommended Triggers**

## Modeling Results

**Water percolation through cover**
**Fate and Transport**
**Tritium**
**Radon**
**Other non-volatile radionuclides**
**Heavy metals**
**PCE**
**Comparison to field data**
**Comparison to performance metrics**
**Sensitivity analysis**

## Water Percolation

** ****Peace and Goering (2005)**
**Simulated net annual percolation through the cover was less than the regulatory metric of 10****-7**** cm/s for alternative scenarios and conditions**
**Predicted average infiltration rates through the MWL cover range from 1.18 X 10****-9**** cm/s for present conditions to 6.12 X 10****-9**** cm/s for wetter future conditions **

## Tritium Fate and TransportGas and Liquid-Phase Transport to Groundwater and Surface

## Tritium Surface FluxComparison to Field Data

## Tritium Surface ConcentrationsComparison to Field Data

**Concentrations also compared at depths of 15 and 115 feet**

## Tritium Aquifer Concentrations

## Tritium Dose via Groundwater

## Tritium Dose via Air Pathway

## Tritium Sensitivity Analysis

## Tritium Key Results

**Simulations showed no exceedances in groundwater concentration or dose**
**A small percentage (2%) of the simulated peak dose due to tritium via the air pathway exceeded the regulatory metric of 10 mrem/year**
**However, the average of the peak doses (1.7 mrem/yr) is less than the regulatory metric (as prescribed in NUREG-1573)**
**Key assumptions**
**Continuous receptor inhalation and exposure above landfill**
**Maximum inventory set equal to twice estimated value**
**Allowance of complete erosion of cover**
**Use of bounding tortuosity factors that maximized tritium diffusion**

## Radon Fate and Transport Gas and Liquid-Phase Transport to Groundwater and Surface

## Radon Calibration

**Calibration based on “Emanation Factor” of sealed radium sources**
**Governs how much radon-222 gas emanates from radium-226 source**
**Ranges from 0 (complete containment) to 1 (no containment)**
**Minimum value (10****-6****) calibrated to yield values of measured surface radon fluxes in 1997**
**Maximum value assumed to range between 0.01 to 1 to accommodate container degradation**

## Radon Surface Flux

- 40 CFR 192 states that the *average* Rn-222 surface flux shall not exceed 20 pCi/m2/s

## Radon Sensitivity Analysis

## Radon Key Results

**Average simulated surface flux is greater than**** regulatory metric of 20 pCi/m****2****/s if maximum emanation factor is 1 (up to 100% of radium-226 containers fail)**
**Releases of radon to groundwater were negligible**
**Key Assumptions**
**Up to 100% of radium-226 sealed sources allowed to fail in 1000 years**
**1-D model: maximizes gas transport to surface**

## Fate and Transport of Other Radionuclides  (Leaching to Groundwater)Am-241, Cs-137, Co-60, Pu-238, Pu-239, Ra-226, Rn-222, Sr-90, Th-232, H3, U-238

## Key ResultsLeaching of Radionuclides to Groundwater

**None of the simulated radionuclides reached the groundwater within 1,000 years for all realizations.**
**Only uranium-238 (and some of its decay products) were predicted to reach the water table for extended periods (>10,000 years).   All peak aquifer concentrations were still less than the EPA regulatory metric of 30 μg/L.  **
**Infiltration rate was found to be the most significant parameter impacting the variability in the simulated groundwater concentrations and dose via groundwater**
**Simulated uranium groundwater concentrations exceeded the regulatory metric of 30 ********g/L if the Darcy infiltration increased two orders of magnitude above the maximum stochastic value to 6.12x10****-9**** m/s. **

## Heavy Metal Fate and Transport(Leaching to Groundwater)Lead and Cadmium

## Key ResultsLeaching of Lead and Cadmium to Groundwater

**Simulations showed that neither lead nor cadmium reached the groundwater in 1,000 years (or extended periods past 10,000 years)**
**Additional increases in infiltration (3-4 orders of magnitude over expected maximum infiltration rates) allowed cadmium and lead to reach the groundwater in 1,000 years **

**Notes:**
With a 3 order of magnitude increase in infiltration, cadmium was found to exceed regulatory metric of 5 micrograms/L.  With a 4 order of magnitude increase in infiltration, lead reached the groundwater but did not exceed metric of 15 micrograms/L.

## VOC Fate and TransportTetrachloroethylene (PCE) Gas and Liquid-Phase Transport to Groundwater and Surface

## PCE Soil Gas ConcentrationsComparison to Field Data

**Notes:**
Similar results to comparisons with data at 10 feet as well.

## PCE Groundwater Concentrations100 Realizations

## PCE Peak Groundwater Concentrations

## PCE Sensitivity Analysis

## PCE Key Results

**1% of the realizations yielded peak PCE concentrations in the groundwater that exceeded the regulatory metric of 5 ********g/L**
**The majority of the realizations showed that the peak PCE groundwater concentration occurred within 100 years**
**Key Assumptions:**
**1-D model: maximizes transport to groundwater**

## Overview

**Background**
**Modeling Approach**
**Modeling Results**
**Recommended Triggers**

## Recommended Triggers

**Surface emissions of tritium and radon**
**Water percolation through the vadose zone**
**Concentrations of uranium in groundwater**
**Concentrations of VOCs in groundwater**

## Surface Emissions of Tritiumand Radon

**Trigger**
**20,000 pCi/L of tritium in soil moisture at  environmental monitoring locations along MWL perimeter **
**4 pCi/L of Rn-222 (measured by Track-Etch radon detectors) along MWL perimeter**
**Performance Objective**
**Dose to the public via the air pathway shall be less than 10 mrem/yr (excludes radon)**
**Average flux of radon-222 gas shall be less than 20 pCi/m****2****/s**

**Notes:**
Tritium soil concentration was on the order of 10^9 to 10^10 pCi/L for the few realizations that yielded doses via the air pathway that exceeded 10 mrem/yr.

Radon soil gas concentrations were greater than 10,000 pCi/L for realizations that yielded surface fluxes greater than 20 pCi/m^2/s

## Water Percolation Throughthe Vadose Zone

**Trigger**
**Increases in moisture content above 25% as measured by neutron probes 10-100 ft beneath MWL**
**Performance Objective**
**Percolation through the cover shall be less than the EPA-prescribed technical equivalence criterion of 31.5 mm/yr  [10****-7**** cm/s]**
**Large increases in percolation were shown to pose increased risks for groundwater contamination**

**Notes:**
Characteristic curves for soils at site show that 25% volumetric moisture content corresponds to a percolation flux of 10^(-7) cm/s

## Concentrations of Uraniumin Groundwater

**Trigger**
**15 ********g/L in groundwater (half the EPA MCL)**
**Performance Objective**
**Uranium concentrations in groundwater shall  not exceed the EPA MCL of 30 ********g/L.**

## Concentrations of VOCsin Groundwater

**Trigger**
**10 VOCs will be monitored in the groundwater and the trigger will be half the MCL for each constituent**
**1,1,1-Trichloroethane(1,1,1-TCA); 1,1-Dichloroethene; Benzene; Ethyl benzene; Methylene chloride; Styrene; PCE; Toluene; TCE; Xylenes (total)**
**Performance Objective**
**VOC concentrations in groundwater shall be less than EPA MCLs**

## Trigger Evaluation Process

**Trigger Evaluation Process**

## Summary

**Probabilistic fate and transport models showed several potential exceedances that merit “triggers” for long-term monitoring**
**Tritium dose via air pathway**
**Surface flux of radon-222 gas**
**PCE concentration in groundwater**
**Recommended Triggers**
**Surface emissions of tritium and radon**
**Water percolation through the vadose zone**
**Concentrations of uranium in groundwater**
**Concentrations of VOCs in groundwater**

## Summary (continued)

**Key Assumptions**
**Receptor located at MWL (continuous inhalation exposure)**
**Diffusion of tritium and PCE were maximized**
**Sealed sources of radium-226 allowed to degrade in 1000 years**
**Cover allowed to erode completely**
**Maximum waste inventory set equal to twice estimated value**
**Use of 1-D models (maximizes transport to surface and groundwater)**
**Report can be downloaded from **_**[www.sandia.gov](http://www.sandia.gov/caps)**__**[/caps](http://www.sandia.gov/caps)**_** **