PNNL-11176

PNNL-11176
UC-630

The Multimedia Environmental Pollutant
Assessment System (MEPAS)^®:
Riverine Pathway Formulations

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 radioactive and hazardous carcinogens and hazard quotients for noncarcinogens. This system has wide applicability to environmental problems using air, groundwater, surface-water, overland, and exposure models. MEPAS enables users to simulate release of contaminants from a source; transport of contaminants through the air, groundwater, surface-water, or overland pathways; and 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 platform 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 surface-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

This report describes the mathematical formulations used for contaminant fate and transport in the riverine pathway of the Multimedia Environmental Pollutant Assessment System (MEPAS). Of the many types of surface-water bodies (e.g., nontidal rivers, estuaries, lakes, open coasts, reservoirs, impoundments, etc.) in which contaminant fate and transport could be simulated, only a nontidal river model is currently incorporated into MEPAS. Nontidal rivers refer to freshwater bodies with unidirectional flow in definable channels. Because the MEPAS methodology is compositely coupled, other surface-water models can be added when deemed necessary.

The surface-water component of MEPAS provides estimates of contaminant concentrations in a river at locations downstream from a release point. The computed contaminant concentrations are used by the exposure assessment component of MEPAS to calculate dose and the resulting health effects to the surrounding population. Potential exposure of humans to contaminants via rivers can be associated with ingestion (e.g., drinking contaminated water), inhalation of volatile pollutants (e.g., showering), dermal contact to chemicals (e.g., swimming), or external dose from radionuclides (e.g., swimming).

Because annual-average contaminant releases to a river in the MEPAS methodology are relatively long term compared to typical contaminant travel times in a river, the migration and fate of contaminants through the riverine pathway are described by the steady-state, two-dimensional advective-dispersive equation for solute transport. The results are based on an analytical solution that is well established in the scientific literature. The surface-water equation accounts for the major mechanisms of constituent persistence (i.e., degradation/decay), advection, and hydrodynamic dispersion. 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 longitudinal direction. Hydrodynamic dispersion is accounted for in the lateral direction. The processes associated with adsorption/desorption between the water column and suspended and bed sediments are not addressed. Neglecting these processes should, in most cases, represent a conservative assumption with regard to water column contaminant concentrations.

Contamination can enter the riverine environment in one of three ways. The groundwater pathway can supply transient contaminant fluxes along the stream bank adjacent to the aquifer. Overland runoff can supply nonpoint-source contaminant fluxes from the land adjacent to the stream. Finally, the surface-water component of MEPAS allows direct discharges to the stream.

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.

Flow is steady and uniform in the longitudinal direction.
The effects of contaminant adsorption to or desorption from sediment particles suspended in the water column or in the river bed are negligible; therefore, all contaminants travel at the same speed as the river flow.
Contaminant releases to the riverine pathway are assumed to be long term relative to the travel time in the river. Therefore, a steady-state solution to the advective-dispersive equation is applicable for describing contaminant transport (Section 2.0).
The river geometry can be represented by a rectangular cross-section.
A line source along the stream bank can be approximated as a point source located at the center of the line source; therefore, only the point source equation is used in MEPAS (Section 2.1).
Advection dominates dispersion in the longitudinal direction, and the contaminant plume is assumed to be fully mixed over the depth of the river; therefore, dispersion is only considered in the lateral direction.
An appropriate lateral dispersion coefficient can be estimated based on the flow velocity and depth of the river (Section 2.5).
Degradation/decay for all contaminants is first-order.
The single concentration value specified by the user for the measured concentrations option is assumed to be temporally constant.

Acknowledgments

The authors thank Keith Shields for his technical review of this document, Robert Buchanan for editorial review, and Vickie Atkinson for helping to prepare the manuscript. Thanks are also extended to Larry Bagaasen, John Buck, Karl Castleton, Jim Droppo, Gariann Gelston, Andre dé Hamer, Bonnie Hoopes, Chikashi Sato, Dennis Strenge, and Monique Van der Aa, 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.

1.0 Introduction

This report describes the mathematical formulations used for contaminant fate and transport in the riverine pathway of the Multimedia Environmental Pollutant Assessment System (MEPAS). It is one in a series of reports that collectively describe the components of MEPAS. Other volumes address the following topics:

Source-Term Release (Streile et al. in press) - presents the mathematical formulations for simulating the release of contaminants from a source term to the atmospheric and waterborne transport pathways.
Ground Water Pathway (Whelan et al. 1996) - presents the mathematical formulations for contaminant fate and transport in the groundwater pathway of MEPAS.
Atmospheric Pathway (Droppo and Buck 1996) - presents the mathematical formulations for contaminant fate and transport in the atmospheric pathway of MEPAS.
Exposure Pathway and Human Health Impact Assessment Models (Strenge and Chamberlain 1995) - describes the methods used by MEPAS to compute dose and human health impacts to selected individuals and populations caused by exposure to pollutants.

The surface-water component of MEPAS provides estimates of contaminant concentrations in a river at locations downstream from a release point. The computed contaminant concentrations are used by the exposure assessment component of MEPAS to calculate dose and the resulting health effects to the exposed population. Potential exposure of humans to contaminants via rivers can be associated with ingestion (e.g., drinking contaminated water), inhalation of volatile pollutants (e.g., showering), dermal contact to chemicals (e.g., swimming), or external dose from radionuclides (e.g., swimming). A schematic diagram illustrating the riverine pathway is presented in Figure 1.1.

Because contaminant releases to a river in the MEPAS methodology are generally of long duration relative to the travel time from the point of release to a receptor, the migration and fate of contaminants through the riverine pathway are described by the steady-state, two-dimensional advective-dispersive equation for solute transport. The results are based on an analytical solution that is well established in the scientific literature. The surface-water equation accounts for the major mechanisms of constituent persistence (i.e., degradation/decay), advection, and hydrodynamic dispersion. 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 longitudinal direction. Hydrodynamic dispersion is accounted for in the lateral direction. The processes associated with adsorption/desorption between the water column and suspended and bed sediments are not addressed. Neglecting these processes should, in most cases, represent a conservative assumption with regard to water column contaminant concentrations.

Contamination can enter the riverine pathway in one of three ways. The groundwater pathway can supply transient contaminant fluxes along the stream bank adjacent to the aquifer. Overland runoff can supply nonpoint-source contaminant fluxes from the land adjacent to the stream. Finally, contaminants can be discharged directly to the river.

The specific topics addressed in Chapter 2.0 of this report are as follows:

Advective-Dispersive Equation - The advective-dispersive equation describes solute migration in the riverine environment. The form of the equation used is briefly discussed.
Contaminant Concentration Equation - The solution to the advective-dispersive equation used in the riverine component is presented. The solution describes contaminant concentrations.
Multiple River Receptors - The technique employed to estimate concentrations for more than one river receptor is presented.
Mixing Length - A lateral mixing length is defined, which describes the lateral distance in which the migrating solute plume can be considered to be fully mixed, and is used in the multiple river receptor estimation technique. The time for the contaminant to travel from the waste site through the river to a receptor of concern as well as the lateral dispersion coefficient are also described. Both are used in determining mixing lengths.

The specific topics addressed in Chapter 3.0 of this report are as follows:

Contaminant Degradation/Decay - The technique for computing the degradation of chemicals and/or the decay of radionuclides is described, as well as the algorithms used to assess degradation/decay at the source of contamination only, in the environment only, or both.
Surface Water Mass Balance at the Source - This section briefly describes the features included in the MEPAS methodology to ensure that mass is conserved in the analysis.
Measured Concentrations in the Riverine Environment - This section describes the option within the methodology of using measured environmental contaminant levels in the assessment of health impacts to downstream sensitive receptors, as opposed to performing transport calculations to estimate these environmental concentrations.

Chapter 4.0 provides a listing of the equation notations found throughout the report.