PNNL-10907: MEPAS Ground Water Pathway Formultations

PNNL-10907
UC-630

Multimedia Environmental Pollutant
Assessment System (MEPAS^®):
Ground Water Pathway Formulations

^(a)

Preface

The Multimedia Environmental Pollutant Assessment System (MEPAS) is a physics-based environmental analysis code integrating source-term, transport, and exposure models for concentration, dose, or risk endpoints. Developed by Pacific Northwest National Laboratory^(a) for the U.S. Department of Energy, MEPAS is designed for site-specific assessments using readily available information. Endpoints are computed for chemical and radioactive pollutants. For human health impacts, risks are computed for radionuclide and hazardous carcinogens and hazard quotients for noncarcinogens. This system has wide applicability to environmental problems using air, ground water, surface-water, overland, and exposure models. MEPAS enables users to simulate release of contaminants from a source; the transport of contaminants through the air, ground water, surface water, or overland pathways; and the transfer of contaminants through food chains and exposure pathways to the exposed individual or population. Whenever available and appropriate, guidance and/or models from the U.S. Environmental Protection Agency, International Commission on Radiological Protection, and National Council on Radiation Protection and Measurements were used to facilitate compatibility and acceptance.

Although based on relatively standard transport and exposure computation approaches, MEPAS uniquely integrates these approaches into a single system providing a consistent basis for evaluating health impacts for a large number of problems and sites. Implemented on a desktop computer, a user-friendly shell allows the user to define the problem, input the required data, and execute the appropriate models. This document describes the mathematical formulations used in the ground water component of MEPAS.

^(a)Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.

Summary

    The ground water component of the MEPAS methodology models solute transport through the ground water environment (i.e., partially saturated and saturated zones). Specifically, this component provides estimates of ground water contaminant fluxes at various transporting medium interfaces (e.g., water table or aquifer/river interface) and contaminant concentrations at withdrawal wells. Contaminant fluxes at transporting medium interfaces represent boundary conditions for the next medium in which contaminant migration and fate is to be simulated (e.g., ground water contamination entering a surface-water environment). Contaminant concentrations at withdrawal wells provide contaminant levels for the exposure assessment component of MEPAS. This document describes the mathematical formulations for the ground water pathway.

    The migration and fate of contaminants through the ground water environment are described by the three-dimensional, advective-dispersive equation for solute transport. The results are based on semianalytical solutions (i.e., solutions that require numerical integration) that are well established in the scientific literature. The ground water model accounts for the major mechanisms of constituent mobility (i.e., adsorption/desorption), persistence (i.e., degradation or decay), advection, and hydrodynamic dispersion. Mobility is described by an equilibrium coefficient that assumes instantaneous adsorption/ desorption between the soil matrix and the pore water. Persistence is described by a first-order degradation/decay coefficient. Radionuclide decay products are also accounted for. Advection is described by constant, unidirectional flow in the vertical direction in the partially saturated (vadose) zone and in the longitudinal direction in the saturated zone. Hydrodynamic dispersion is described in one dimension for the partially saturated zone and three dimensions for the saturated zone.

    The assumptions listed and/or discussed in this document are itemized below for easy reference. Section numbers are provided where a particular assumption is discussed in more detail.

The ground water environment is initially free of contamination.
All transport media properties are homogeneous and isotropic.
Flow in both the partially saturated and saturated zones is uniform.
The saturated zone is of finite, constant thickness and of infinite lateral extent.
The flow system is at steady state. Drawdown effects of withdrawal wells and other transient stresses on the aquifer are not considered by the semianalytical solutions.
Density differences between a contaminant plume and the natural ground water are negligible.
The contaminant sorption process can be described by a constant (K_d) epresenting the ratio between the contaminant adsorbed to the soil matrix and the contaminant dissolved in solution (Section 2.0).
A unit hydraulic gradient exists in the partially saturated soil beneath a waste site (Section 2.0).
The moisture content in the partially saturated zone fluctuates between field capacity and saturation (Section 2.0).
The soil transmission rate is the maximum possible infiltration rate for a leachate migrating through a partially saturated zone (Section 2.0).
A unit hydraulic gradient and saturated conditions, as described by a saturated hydraulic conductivity, occurs beneath ponded sites (Section 2.0).
If a surface-water body is part of the transport scenario, then it will be the final transporting medium, and all contaminants entering the subsurface environment will eventually enter the water body, except contaminants lost through degradation/decay.
Contaminant diffusion through the water table does not occur once a contaminant has entered the aquifer. Thus, contaminant spreading in the vertical direction is only downward (Section 2.4).
The leachate volume infiltrating from a waste site is insignificant compared to the total flux of contaminated water at the receptor well (Section 3.3).
Degradation/decay is first-order for all contaminants.
Decay products have the same equilibrium coefficient as the parent, and therefore travel at the same speed as the parent (Section 3.5).

The following assumptions are applicable to the option of using measured concentrations for evaluating health impacts (Chapter 4.0):

The monitoring and receptor wells occur along the same ground water flow streamline.
The user-supplied concentration is temporally constant.
Differences in concentrations at a monitoring well and a receptor well are due solely to vertical and lateral dispersion.

Acknowledgments

The authors thank Mark Rockhold and Keith Shields for their technical review of this document, Robert Buchanan for editorial review, and Vickie Atkinson and Text Processing staff for preparing the final manuscript. Thanks are also extended to Larry Bagaasen, John Buck, Karl Castleton, Jim Droppo, Gariann Gelston, Andre dé Hamer, Bonnie Hoopes, and Dennis Strenge, all of whom have, in some way, influenced the development of the waterborne codes with their technical guidance and suggestions. Appreciation also goes out to all the people who use MEPAS and have alerted us to potential problems in the code and offered suggestions for its improvement.