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EFFECT

OF

VAPOR-PHASE MASS TRANSFER

ON

AQUIFER

RESTORATION

Cass

T.

Miller Associate Professor

Edward G. Staes Research Assistant

Department of Environmental Sciences and Engineering University of North Carolina at Chapel Hill

Chapel Hill, North Carolina 27599

The research on which this report is based was financed by

the Water Resources Research Institute of The University of North Carolina

Disclaimer Statement

Contents of this publication do not necessarily reflect the views and policies

of

the Water Resources Research Institute nor does men tion

of

trade names or commercial products constitute their endorsement or recommendation for use by the Institute or the State

of

North Carolina.

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Acknowledgements

This work was performed within the Department of Environmental Sciences and Engineer- .

ing of the School of Public Health at the University of North Carolina at Chapel

Hill.

We thank

Mr.

Randall Goodman for his assistance in designing and fabricating both the field

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Abstract

Volatile organic chemicals, or VOC's, are a frequent source of groundwater contamina- tion in North Carolina and throughout the United States

and

other developed countries. Volatile organic chemicals may enter the subsurface in many different ways, but a com- mon phenomenon regardless of the source history, is that these compounds exist in several phases: the gas phase, the aqueous phase, the solid phase, and often in a n immiscible organic fluid phase. Such systems are termed multiphase systems, and the general descrip- tion of fluid %ow and contaminant transport and reaction in these systems is a complex problem. This work considered a subset of the generd multiphase flow and transport problem: fluid %ow and contaminant transport in the gas phase of the unsaturated zone.

Gas-phase %ow and transport phenomena are important for groundwater systems contam- inated with VOC's for several reasons: (1) gas-phase processes can lead to the distribution of a contaminant in all directions from a sourceincluding up-gradient;

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failure to con- sider gas-phase taransport can lead to inappropriate conclusions regarding the source of a contaminant; (3) gas-phase sampling can be used to investigate subsurface contaminant distributions more quickly and economically than groundwater sampling done; and (4)

gas-phase venting is a frequently used technology for aquifer restoration. The purpose of this work was to investigate gas-phase transport phenomena at the field scale to assess the relative importance of operative transport phenomena.

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Table

of

Contents

. . .

Acknowledgements

...

. . .

Abstract

111

. . .

List

of

Figures

vii

. . .

List of Tables

Summary

and

Conclusions

. . .

. . .

Recommendations

. . .

1

Introduction

. . .

1.1 Motivation

1.2 Fundamentals

of

Gas-Phase

Transport

. . .

. . .

1.3 Objectives

. . .

2

Previous Research

. . .

2.1Overview

. . .

2.2 mansport Processes

. . .

2.2.1 Diffusive Transport

2.2.2 Advective Transport

. . .

2.2.3 Dispersive Bansport

. . .

. . .

2.3 Mass Transfer Processes

2.3.1 Aqueous-Gas

Phase

Mass Transfer

. . I . . .

2.3.2 Sorption-Desorption

. . .

3MaterialsandMethods

. . .

. . .

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. . .

3.2

Field Methods

9

3.2.1 Site Description

. . .

9

3.2.2 Contamination History

. . .

10

3.2.3 SubsurfaceCharacteristics

. . .

11

3.2.4 Fbel Description

. . .

12

. . .

3.2.5 Monitoring Wells 12

. . .

3.2.6 Gas-Phase Sampling 14

. . .

3.2.7 Ground-Probe Sampling 16 3.2.8 Groundwater Sampling

. . .

16

3.3 Laboratory

Methods

. . . . . .

17

3.3.1 Materids

. . .

17

3.3.2 Bottle-Point Isotherm

. . .

18

3.3.3 Bottle-Point Rate Study

. . .

19

3.3.4 Sample Extraction Methods

. . .

20

3.3.5 Gas Chromatography Methods

. . .

20

4Results

. . .

22

4.1

Field

Results

. . .

22

4.1.1 Overview

. . .

22

4.1.2 Groundwater Flow

. . .

22

4.1.3 Gas-Phase Solute Distributions

. . .

26

4.1.3.1 Sample Variability

. . .

26

4.1.9.2 Nonaqueow Phase Liquid Zone

. . .

26

4.1.3.3 January 1989 Analyses

. . .

26

4.1.3.4 March 1989 Analyses

. . .

28

4.1.3.5 June 1989 Analyses

. . .

29

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. . .

4.1.3.7 Concentration Trends 32

. . .

4.1.4 Sorption Equilibrium 34

. . .

4.1.5 SorptionRate 35 .

5Discussion

. . .

43

. . .

5.1 Overview 43

. . .

5.2 Fickian Diffusion 43

. . .

5.2.1 FLndamental Physics 43

. . .

5.2.2 Interfacial Area 44

. . .

5.2.3 Diffusive Boundary Conditions 44

. . .

5.2.4 MoistureContent\k.riations 44

. . .

5.2.5 Model Simulations 45

. . .

5.3 Non-Fickian Diffusion 50

. . .

5.4 Advective Tkansport 53

5.4.1 Density-Driven Flow

. . .

54

5.4.2 Transient Moisture Contents

. . .

54

. . .

5.4.3 Other Climatic Effects

56

. . .

5.4.4 ThermalMotivatedFlow 57

5.5 Interphase Mass Tkansfer

. . .

57

. . .

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List

of

Figures

. . .

1 Topographic map of the Pope

AFB

Fire Protection Raining Area 4 site 10

2 Monitoring station location map

. . .

13

. . .

3

Schematic of gas-phase monitoring well design 15

. . .

4 GrainsizedistributiondiagramofPopeAFBbulksample 18

. . .

5 January 1989 observed piezometric surface 23

. . .

6

March1989obsen.edpiezometricsurface. 24

. . .

7

June 1989 observed piezometric surface 25

. . .

8

Octane concentrat ion sample rariabili ty 27

. . .

9 Estimated limit of

NAPL

zone 28

. . . .

10 Gas-phase hexane concentration for samples collected in January 1989 29

. . . .

11 Gas-phase octane concentration for samples collected in January 1989 30

. . . .

12 Gas-phase toluene concentration for samples collected in January 1989 31

. . .

13 Gas-phase m-xylene concentration for samples collected in January 1989 32 14 Gas-phase hexane concentration for samples collected in

March

1989

. . . .

33

15

Gas-phase octane concentration for samples collected in March

1989

. . .

34

16 Gas-phase toluene concentration for samples collected in March 1989

. . . .

35

. . . .

17

Gas-phase m-xylene concentration for samples collected

in March

1989

36

. . .

18 Gas-phase octane concentration for samples collected

in June 1989

37

. . . .

19 Gas-phase m-xylene concentration for samples collected in June 1989 38 20 Vertical gas-phase concentration profiles for monitoring well

V1

. . .

39

21 Vertical gas-phase concentration profiles for monitoring well V2

. . .

40

22 Sorption equilibrium in bot tle-point reactors

. . .

41

23 Sorption rate of toluene in bottle-point reactors

. . .

42

24

Two-dimensional domain used for gas-phase diffusion simulations

. . .

48

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25 Log of gas-phase concentration as a percent of source concentration for difi-

sion simulation results after 0.074 days

. . .

49

26 Log of gas-phase concentration as a percent of source concentration for difhx-

sion simulation results after 0.260 days

. . .

.

. .

. . .

.

.

.

50 .

27

Log of gas-phase concentration as a percent of source concentration for difFu-

sion simulation results after

0.720

days

.

. . .

. .

.

. .

.

. . . .

51

28

Log

of gas-phase concentration as a percent of source concentration for difTu-

sion simulation results after

29.5

days

. . .

. .

. . .

.

..

. .

52

29 Log of gas-phase concentration as a percent of source concentration for diffu-

sion simulation results after 73.4 days

.

.

. . . .

.

.

. . . .

. .

53

30 Cross-section location map

.

.

. . .

.

. . . . . . . . .

.

. .

. .

.

. .

58

31 Cross-section distribution of octane concentrat

ions based upon January 1989

and March 1989 sampling

.

.

. .

. . . .

.

.

.

.

.

. . .

. . . .

59

32 Cross-section distribution of m-xylene concentration based upon January 1989

and

March

1989 sampling

. . .

. .

. . .

. .

.

.

.

. .

.

.

. . . . .

.

60

...

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List

of

Tables

.

1 Target Compound Properties

. . .

12

2 Gas-Phase

Diffusion Simulation Input

. . .

49

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Summary

and

Conclusions

1.

A

field and laboratory investigation was performed to investigate the movement of .

volatile organic contaminants in the unsaturated zone of the subsurface environment.

2.

Multi-level gas-phase monitoring wells were designed, fabricated, installed, and Sam- pled to detect spatial and temporal distributions of volatile organic contaminants in

the unsaturated zone. Driveable ground probe sampling was also used to monitor gas-phase concentrat ions.

3.

Gas-phase concentration variability among replicate samples was typically less than

20% at low concentrations and less than 10% at

high

concentrations. Concentration variability aas somewhat less for replicate samples collected &om gas-phase wells when compared to sample variability from driveable ground probe wells.

4.

All

field sampling results showed a steep concentration gradient in the vertical di-

rection compared to the concentration gradient in the horizontal direction. This translates to a dominance in mass transport in the vertical direction compared to the horizontal direction for the field site investigated.

5.

Numerical modeling was performed to simulate the distribution of gas-phase contam- inants as a function of space and time, and the results compared reasonably well with field obsert-ations.

6. Fkom relative time-scale analysis of flow and transport processes, it was determined that steady-state conditions would probably never be achieved for field conditions similar to those investigated.

7.

Mdticomponent, non-Fickian diffusion processes were found to have likely iduenced gas-phase concentration distributions, especially close to the contaminant source.

8. Significant advective transport of the gas phase occurred during the monitoring period

of this study, due to rainfall recharge. Rainfall recharge caused the water table to rise and the mean moisture content to increase, both of which in turn led to displacement of the gas phase and hence advective transport.

9. Gas phase density, barometric pressure, and temperature variations can a l l lead to advective transport. Density and temperature effects were probably slight for the field site investigated, but barometric pressure effects were perhaps a source of the greater horizontal contaminant spreading that was observed compared to model predictions.

10. Interphase mass transfer of contaminants among the phases present was shown to lengthen the time required for the system to approach steady-st ate conditions.

11. Much remains to be learned about gas-phase transport in general and interphase mass

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Recommendat ions

It is important to note that the relative importance of gas-phase transport has been .

widely recognized only within the last few years and that a significant amount of research is currently underway to better understand factors that affect the hierarchy of importance of the relevant processes. Currently, interest in monitoring technologies-such as ground-

probe sampling and surf'ace %w measurements-and remediation methods-such as vapor extraction and in situ steam stripping-has led to a situation in which application has

far exceeded our fundamental understanding. That is, current practice in these areas is governed largely by empirical and professional judgment, often with little theoretical backing. The obvious problem with this approach is that sub-optimal decisions result and little basis exists for making estimates of a system's response to a given action if

fundamental understanding is lacking. - . .

The work summarized in this document suggests several recommendations; some that are immediately applicable, and some that require additional research. These recornmen- dations may be summarized by:

1. Gas-phase monitoring should be used routinely by practitioners to assess sites con- taminated with volatile organic contaminants.

2. The multilevel gas-phase wells designed and used in this study provided excellent results. This design could be used effectively to monitor gas-phase contamination at other sites.

3. Gas-phase venting of volatile organic contaminants is a promising technology for aquifer restoration, which should be considered routinely by consultants and r e p lat ors.

4. Gas-phase monitoring should be used to measure fluxes of non-contaminant gases, such as oxygen, to aid in assessing the rate of biodegradation.

5.

Rates of interphase mass transfer should be investigated as a function of physicochem- ical system properties for homogeneous and heterogeneous media systems. These re- sults should be evaluated with respect to the adequacy of the frequently made linear local-equilibrium assumptions, as a guide in assessing errors introduced by simplifying complex multiphase systems by using such assumptions.

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1

Introduction

1.1 Motivation

The Environmental Protection Agency

(EPA)

estimates that

96%

of the total kesh wa-

ter supply in the United States is contained in groundwater aquifers. Additionally, 50% of our nation's water supply is generated from groundwater sources and this percentage

w i l l

continue to increase. Petroleum hydrocarbons are a major threat to this resource. Petroleum is introduced into aquifers through surface spills, leaking underground storage tanks

and

transport lines. Restoration of a contaminated aquifer is expensive and difficult to a,chieve. Under some conditions, aquifer restoration requires decades or more to achieve using common pump- and- treat met hods.

The potential threat to groundwater quality is demonstrated by the magnitude of petroleum consumption in the United States. Currently, our nation uses nearly 200 billion gallons of petroleum products each year (Moreau, 1985). Most of this petroleum is stored under- ground in greater than two-million storage tanks or transported underground at one or more times until its find use.

EPA

estimates that from

5%

to 35% of these underground tanks may have current or previous problems with leakage. Assuming complete dispersion

and mixing, Marley and Koag

(1984)

calculated that each gallon of gasoline spilled into an

aquifer has the potential of contaminating greater than tw~million gallons of water. Since petroleum storage tanks are often located in densely populated areas, a defective tank or spill may pose an immediate threat to domestic water supplies, amplifying the significance of the hazard.

1.2 Fundamentals of Gas-Phase Transport

When analyzing a hydrocarbon contamination site, the unique properties of hydrocarbon fuels must be considered. Hydrocarbon fuels may contain up to 300 constituents. Lower molecular weight hydrocarbons are highly volatile and once released into the subsurface

will be present in both liquid and gas phases. These chemicals, known as volatile organic chemicals (VOC's), are of increasing environmental interest because of their toxicity and prevalence in the environment.

Recently increased interest in the study of VOC's has led researchers to believe that gas- phase transport is an important process in the subsurface.

This

process not only affects the movement

and

distribution of volatile contaminants in the unsaturated zone but also affects the amount of contaminant found in the underlying aquifer. The process of gas- phase mass transport

has

been exploited to delineate contaminant plumes with subsurface

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the movement of VOC's in the subsurface accurately, an understanding of the mechanisms that control gas-phase transport is needed.

Gas-phase mass transport is a process by

which

VOC's migrate horizontally or vertically from contaminated spill sites through the unsaturated zone.

In

the unsaturated zone, the void space is partly filled by air and partly by water. Contaminants enter the air-filled portion of the void space by volatilization. Volatilization in the subsurface is controlled by a compound's vapor pressure, mass-transfer resistance, and the rate of movement through the soil (Spencer et al., 1988). Once the contaminant enters the gas phase, movement of the contaminant away from the evaporating surface may be affected by processes that include bulk phase flow, diffusive transport, sorption-desorption, and biotic and abiotic transformation reactions.

The

relative importance of the vruious transport and transfor- mation processes depends on many factors, including physical

and

chemical properties of the media and solute(s), relative heterogeneity of the media, temperature distribution, moisture characteristics, and the fluid pressure distributions.

The

dominant mechanism for transport of VOC's is usually cited as molecular diffusion. However, recent research has shown that other factors may be important as well, such as density and pressure gradients, barometric fluctuations, multicomponent diffusion effects, rising and falling water tables, and thermal gradient effects.

The current appreciation for the potent id importance of gas-phase transport and the qual- i tative underst anding of potentially important factors is an important beginning. However, these potentially important factors must be better understood quantitatively to underst-and more fully gas-phase migration, and to better design remediation schemes for

VOC's.

1.3

Objectives

The overall objective of this project was to gain a better understanding of gas-phase transport processes in the subsurface environment.

This

report describes field, laboratory,

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2

Previous Research

2.1

Overview

The movement of VOC's in the subsurface may be affected by several processes. It fol- lows that the literature that describes each of these processes bears upon the current level of understanding of gas-phase transport. The most important processes are transport processes-such as diffusive processes, advection, and dispersion; mass-transfer processes- such as aqueous-gas phase mass transfer, and sorption-desorption; and reaction processes- such as biological transformation reactions, and chemical reactions. It is beyond the scope of this report to provide a thorough review of the literature in each of these areas. Com- prehensive annual reviews of the groundwater literature are available (Miller, 1987; Miller and Comalander, 1988; Miller and Mayer, 1989, 1990). Recent developments in transport and mass-transfer processes are reviewed in turn in the following sections. The neglect of solute reactions in this review does not imply a lack of importance, rather it suggests the focus of this report.

2.2 Transport Processes

2.2.1 Diffusive Transport

While the theory of multicomponent gas transport in porous media is well developed in the chemistry and chemical engineering literature (e.g., Bird et al., 1960; Cunningham and Williams, 1980; Mason and Malinauskas, 1983), the applicability of these concepts to groundwater systems has received widespread attention only recently (Thorstenson and Pollock, 1989a, 1989b; Baehr and Bruell, 1990). Solute transport in the gas phase of the unsaturated zone is a multicomponent problem that is affected by interactions among solute molecules and the porous media (Thorstenson and Pollock, 1989a, 1989b). Rigorous consideration of multicomponent gas transport in porous media requires consideration of Knudsen transport, molecular and nonequimolar diffusive transport, and viscous transport. Assessing the relative importance of these transport mechanisms as a function of subsurface system conditions is currently an important research issue in the groundwater field.

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where

Jg

is the molar flux of solute in the gas phase, s is a tortuosity factor,

eg

is the volume fraction of the gas phase,

Dg

is the molecular diffusion coefficient for the solute of interest in air, Cg is the gas-phase concentration, and z is a spatial dimension. Equation

(1)

can be readily extended to a governing partial differential equation form to describe the distribution of a gas-phase concentration in time and space

Equations (1) and (2), dong with an appropriate set of boundary and initial conditions, have been used in many instances to derive analytical or numerical solutions to describe gas-phase solute transport in subsurface systems (e.g., Reible and Shair, 1982; Weeks et

. al., 1982; Krearner et al., 1988; Jury et al., 1990). Unknowns in these equations include

T and D,. Appropriate values of

D,

are often available in the literature, or they can be estimated accurately using mailable methods (Reid et al., 1987).

The notion of an effective diffusion coefticient, D,, is also frequently used in the literature (Weeks et

al.,

1982; Kreamer et al., 1988). An effective diffusion coefficient results when a variety of unknown, but presumed constant, variables is grouped with the diffusion coefficient and solved in product form. An effective diffusion coeficient may be a grouping of a mriety of variables, with considerable variation existing in the literature. Typical factors lumped into effective diffusion coefficients include tortuosity, volume fraction of the gas phase, and variable groupings related to mass transfer to the aqueous and solid

phases.

The estimate of tortuosity, r , has received considerable attention in the literature, relating to both field and laboratory studies. Farmer et al. (1980), BrueU and Hoag (1986) and Karirni et al. (1987) have shown that the effective diffusion coefficient and measured tortuosities are in good agreement with

the

values predicted by existing theory, with the Millington model the most often cited (Millington, 1959). These studies were performed .

using similar laboratory methods and the organic compounds hexane, toluene, benzene, hexachlorobenzene, and iso-octane.

.

Techniques have

been

developed to measure tortuosity and effective diffusion coeflicients directly in the field

(Lai

et al., 1976; Weeks et d., 1982; and Kreamer et d., 1988). Such approaches avoid the error that may be introduced by making measurements in

the

laboratory using repacked cores of field-collected materials.

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nitrous oxygen into shallow unsaturated zones with a syringe then remo\ing and analyzing a volume of gas at locations nearby. Jellick and Schnable (1985) found that t.his procedure gave reproducible results and compared well with laboratory studies on intact cores. There are limitations with the studies to date using this method because experiments have been completed only in very shallow unsaturated zones, less than 1

ft.

Weeks et

al.

(1982) determined effective diffusion coefficients and measured tortuosity for thick unsaturated zones in the dry plains of Texas by analyzing the flux of two man-made fluorocarbons from the atmosphere into the subsurface. The procedure consisted of placing nested piezometers at depths from about 30 ft to 144 ft below the surface and monitoring pressure, temperature, and fluorocarbon concentrations over a two-year period. Results fiom the study indicated that the procedures for estimating tortuosity developed from theoretical considerations are useful in predicting transport in the unsaturated zone.

Kreamer et al. (1988) conducted a tracer study within the unsaturated zone at a low-level nuclear waste site in South Carolina. The procedure consisted of continually releasing a small amount of halogenated compound gas into the subsurface. Soil gas was then sampled and analyzed from 15 sampling points located at Mzjous locations fiom the source. Tortuosity calculations based on theoretical relationships developed by Millington (1959) underestimated the measured effective diffusion coefficient.

2.2.2 Advective Transport

As noted in the previous section, advective transport of the gas phase in the unsaturated zone of the subsurface environment has often been ignored-gas-phase transport has been attributed usually to molecular diffusion alone (Kreamer et al., 1988). However, advective transport may result from (1) seasonal changes in the elevation of the water table or local- ized imbibition of a fluid (n7eeks et al., 1982), (2) density-driven flow resulting from large concentration gradients of organic solutes in the gas phase (Sleep and Sykes, 1989; Mendoza and Rind, 1990a, 1990b), or (3) from barometric pressure fluctuations ( M a s s m m , 1990). The magnitude of advective transport is dependent upon system conditions-including solute properties, and media properties and heterogeneity, but the net effect of advective transport can be a significant or even the dominant process affecting gas-phase transport (Mendoza and Rind, 1990b; Massmann, 1990).

A

significant barrier to the investigation of advective gas-phase transport is the small pres- sure gradient needed to cause appreciable flow

in

the gas phase (Thorstenson and Pollock, 1989b). This is a result of the density, viscosity, and relative permeability properties of air

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where q is a superficial face, or Darcy, velocity vector; a is a phase qualifier (9,-gas or a-aqueous phase);

k,

is a relative permeability; k is a permeability tensor; p is a dynamic viscosity; p is a fluid pressure; p is a fluid density; and g is a gravitational acceleration const ant.

The growing popularity of wcuum extraction, or forced venting,

has

also led to an interest in gas-phase transport processes in general and advective transport in particular (Johnson et al., 1988, 1990; Krishnayya et al., 1988; Rainwater et al., 1988, 1990; Regalbuto, et

al., 1988; Stephanatos, 1988; Sleep and Sykes, 1989). Vacuum extraction is a procedure involving the installation of a well(s) that is screened in the unsaturated zone.

A

Tacuum source is connected to the well, resulting in the extraction of gas from the subsurface. Con-

taminants present in the gas stream are thus extracted from the subsurface and removed

&om the gas phase by a treatment process, such as activated carbon adsorption, or are in some instances discharged to the atmosphere.

For volatile contaminants that readily partition into the gas phase, gas extraction is an economical method of subsurface restoration. Economic advantage results from the lower power cost needed to operate a typical Facuum pump, compared to the expense of pumping a similar volume of water. This has led to a widespread use of vacuum extraction meth- ods, while theoretical work to understand the interaction of the operative processes and

yield guidance for economical design of vacuum extraction systems has lagged practicd applications. Recent theoretical work has led to the formulation of numerical models that include mobile gas phase flow and solute species transport (Sleep and Sykes, 1989; Mayer and Miller, 1990a, 1990b; Mendoza and h d , 1990a, 1990b). Analysis performed with such models will lead to new insight into the importance of advective transport in the gas phase.

2.2.3 Dispersive Transport

It is well known that mass transport in flowing systems is affected by processes other than

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2.3

Mass

Transfer Processes

2.3.1

Aqueous-Gas Phase Mass Transfer

The unsaturated zone of the subsurface environment contains a gas phase, an aqueous phase, a solid phase, and, in some contamination instances, a nonaqueous phase liquid

(NAPL).

The mass of a volatile organic solute is distributed among the phases present. Accurate description of contaminant transport requires knowledge about the factors that influence the rate and equilibrium of interphase mass transfer among the phases present.

A

convenient assumption is that all phases are

in

equilibrium at each spatial location-the local equilibrium assumption. Local equilibrium has been a common assumption for both laboratory (Peterson et al., 1988) and field (Weeks et al., 1982; Kreamer et d., 1988) studies of gas-phase mass transport, as well as a common assumption used in modeling studies

(Baehr

and Corapcioglu; 1984, 1987; .4briola and Pinder, 1985a, 1985b; Lindstrom

and Piver, 1985; Piver and Lindstrom, 1985;

Baehr,

1987; Corapcioglu and

Baehr,

1987; Mendoza and F'rind, 1990a, 1990b; Shoemaker, et al., 1990).

Recently, the propriety of assuming local equilibrium between the gas and aqueous phases has been questioned (Corn et al., 1989; Sleep and Sykes, 1989; Szatkowski and Miller, 1989a, 1989b; Miller et al.,

1990;

Rainwater et d., 1990; Cho and

J&6,

1990). However, some recent evidence suggests that local equilibrium between the gas and aqueous phases may be an appropriate assumption for some systems (Gierke et al., 1990). Clearly, more work is needed to resolve this issue. Research is underway in this regard.

Sorption processes in the unsaturated zone are complex and as a result are not well under- stood.

A

compljcat,ing factor is the competition for sorption sites by the diffusing solute gas and infiltrating water. Chiou and Shoup (1985) have shown that sorption of five non-polar organic gases on soils with low moisture content is a highly nonlinear process. Equilibrium sorption concentrations of organic gas on dry soils were found to be two orders of mag- nitude higher than in saturated soils. Sorption capacity of dry soils is increased because of strong binding to mineral surfaces, which occurs for dry conditions. Upon wetting the soils, water displaced the contaminants and decreased the soil sorption capacity.

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In

addition to the above experimental work, English and Loehr (1989) and Shoemaker et

al.

(1

990)

have recently suggested t, he potential significance of gas- phase sorption processes. In

addition to the lack of inf~rmat~ion concerning single solute equilibrium sorption processes, little is known about desorption processes, multicomponent sorption processes, or the rates

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3

Materials and

Methods

3.1 Overview

Research was undertaken to gain a better understanding of gas-phase transport processes in the subsurface environment. In support of this objective, field and laboratory data were collected for gas-phase mass transport in porous media Field data were collected at a site located on Pope Air Force Base (AFB), which is located about 10 mi northwest of Fayetteville, North Carolina, in the Piedmont sandhills region. Laboratory analyses were performed using gas, water, and solid samples collected at the Pope AFB field site. Field samples were also collected and used to perform a series of laboratory experiments to inves- tigate gas-phase transport properties of the porous media. These laboratory experiments were performed to gain a fundamental understanding of gas-transport processes and to aid in the interpretation of field results.

3.2

Field

M e t h o d s

3.2.1 Site Description

An agreement was negotiated between the University of North Carolina-Chapel

Hill

and Pope AFB, which allowed access to and use of Fire Protection Raining Area 4 (FPTA4) for research purposes. FPTA4 is one of four FPTA's located on Pope AFB and it is the only active site. The purpose of the site, as the name implies, is to provide a location and facility for the ignition and extinguishment of fies designed to simulate the conditions of a burning aircraft. Such training has been common at Pope

AFB

since the mid 1940's and at the current site since 1955.

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Figure 1. Topographic map of the Pope

AFB

Fire Protection Training Area 4 site

POPE

A I R

FORCE

BASE

\ \

AI,M R,

NORTH CAROLINA

\ \

FIRE T W M M U 3 I S

\ \

FTS

0

v1

SS Hnttaing Well FT 1

0 PVC Honftoring Well

TOPOGRAPHIC MAP

C o n t a r I n t s r v s l 2

3.2.2 Contamination Histow

The Air Force has conducted fire training exercises at the FPTA4 site since 1955. A typical exercise consists of applying 300 gal to 600 gal of aviation waste fuels, primarily consisting of contaminated

JP-4

jet fuel, to a water-filled f i e pit. Fbels are then ignited and the resulting fire is extinguished during the exercise. On occasion, extremely large fires consisting of greater than 2000

gal

of fuel were noted in base records. Records also indicate that other fuel products were burned at the site in the 1950's and 1960's. These products included other jet fuels, diesel fuel, thinners, paints, waste oils, and transmission and hydraulic fluids. Seepage of these hydrocarbon products into the subsurface

has

occurred, contaminating the underlying soils

and

infiltrating into the groundwater system. The burn

(33)

The frequency of fires at the training area has varied. During the 1950's and 1960's, installation documents indicate that from one to seven training exercises were conducted per week. In the 1970's, the number of exercises was reduced to about 15 per year. Currently, there are between 12 and 20 training exercises per year.

Since individual records of training exercises are not kept by the base, only an estimate can be made on the total amount of fuel that has been used since 1955. About 1.5 million gal of fuel have been applied to the fire pit based on average amounts of fuel used and average number exercises year. limit on the total amount of fuel applied to the burn pit is about 3.5 million gal. The amount that has seeped into the subsurface is unknown.

In May 1987, the Department of Defense completed an Installation Restoration Program

(IRP)

site assessment at the Base. It was determined in this program that

FPTA4

posed an environmental hazard after pet,roleurn-type leachate was discovered at two locations to the west of the burn'pit in the upper tributaries to

Tank

Creek.

3.2.3 Subsurfa.ce Charac t,eristics

The subsurface at the site can be characterized as a combination of various sized sands and silt layers interbedded with lenticular clay lenses. This type of stratigraphy is typical of the Tbscaloosa formation in the Piedmont sandhills region. Geophysical logs obtained fiom water wells completed in the area indicates that this thin veneer of unconsolidated material, with a thickness of less than 100 ft, overlies a metamorphic basement rock of schists and gneiss.

Grain size analyses were completed on selected samples collected during the installation of monitoring wells. Representative samples were taken to quantify the range of soil particle sizes found at the site. Particle sizes found in the subsurface vary dramatically, ranging from silty clay size particles (0.005 mm) to a medium-grained sand size particles

(0.4

mrn).

Various authors have indicated that sorption of organic compounds is related to the

to-

tal organic carbon (TOC) content of the soil (Karickhoff et al., 1979). Therefore, TOC analyses were completed on samples from an uncontaminated up-gradient well.

A

total of 30 replicate samples was analyzed over a composite sample collected fiom a single 4 f t i n t e n d of the well. These data indicate that the soil has a carbon concentration at this locat ion of 0.01 %&an extremely low value.

(34)

3.2.4 h e 1 Description

JP-4

jet fuel is the primary fuel used by United States Air Force military aircraft. JP-4 is a complex blend of up to 300 different hydrocarbon compounds (Bishop, 1983). On average, JP-4 is composed of 86% aliphatic hydrocarbons, while aromatic compounds account for an additional 10% to 11%.

I .

comparison to gasoline, JP-4 has a higher distillation temperature, higher average molecular weight, and lower vapor pressure. Since the vapor pressure of

JP-4

is less than

half

that of gasoline, the amount of JP-4 expected in the gas phase would be significantly less than that associated with gasoline contamination.

Since JP-4-like

all

petroleum fuel products--contains many individual compounds, it 0c7as not practical to track

all

components of JP-4 in the subsurface samples collected. As a

compromise, six representative target compounds were tracked. The target compounds were all significant components of

JP-4

and possess a wide range of chemical characteris- tics. The compounds selected were: n-hexane, n-heptane, toluene, n-octane, o-xylene, and

m-xylene. These compounds were chosen because of their variability in sorption charac- teristics, volatility, diffusion coefficients, and water solubility. Physical properties of these targeted compounds are listed in Table 1.

Table 1. Target Compound Properties

Compound Toluene n-Hexane n-Hept ane n-Oct ane eXylene m-Xvlene Molecular Weight (g) - - -Vapor Pressure (in Hg) Henry's Const ant Diffusion Coefficient

(ft2 /day

Aqueous

Solubility

(mg/l)

et al., 1966; Hansch and Leo, 1979; Verschueren, 1983.

.

3.2.5

Monitoring Wells

(35)

Figure

2.

Monitoring station location map

POPE

A I R

FORCE

BASE

NORTH CAROLINA

FIRE TMININC AREA W4

V I

0 SS b l t o r l n g W e l l FT 1

0 P K tknftorlng W e l l

PHI

Probe Hole

WORTH

8 lW

of contamination was used as a basis for locating

11

combined gas and groundwater mon- itoring wells. These additional monitoring wells were installed during December of 1988 and January of 1989 at the locations depicted on Figure

2

(V1 to V11).

All

wells were installed using a Mobile

Drill

Minuteman rig, which had a

7

H P

gasoline engine and

30

ft of 3-in continuous-flight, solid-stem auger. Each boring was drilled about

4

(36)

of a well into a boring the holes were backf3led to within 2 ft of the surface with native sand materials, the upper 2 ft of the hole were widened to a diameter of about 1 ft and backfilled with about 1 ft of 1/8 in bentonite pellets, the remaining 1 ft was filled with concrete and terminated in a steel cover to protect the well casing. Once installed, all wells were surveyed with a theodolite and electronic distance meter.

The six initial groundwater monitoring wells were constructed of 2-in diameter PVC casing that terminated in 24in long, 2-in diameter, 6-slot PVC screens and a drive point. The wells were developed to remove fine materials fiom the screen section and the well by bailing.

Each of the stainless-steel, multilevel monitoring wells was constructed as detailed in Figure

3. Well casings were 2-in diameter stainless-steel, with stainless-steel drive points and 26-in long, 2-in diameter wound stainless-steel, 6-slot well screens. All connecting joints were flush t headed.

Sampling ports were constructed by drilling a 112-in diameter hole through the exterior wall of the well casing.

A

recessed stainless-steel filter plug was then mounted inside the well casing and welded in place.

A

1/8-in stainless-steel tube, silver soldered to one end of the f3t.er plug, was extended from the sampling port to the top of the well for sampling access as shown in Figure 3. To ensure that silt and clay particles did not inhibit entry of gas to the sampling port, a 100-mesh, stainless-steel screen was placed over the filter. Sampling ports were mounted every 12 in within the well casing, an exception being the first port, located 20 in below the top of the well casing. This additional length was needed to allow adequate sealing with the ground surface. The number of sampling ports varied in each well depending on the depth to the water table.

In

the interior of each well, a 1.0-inch diameter

PVC

pipe was installed to allow for water level measurements and sampling.

3.2.6 Gas-Phase Sampling

Soil gas samples were collected at the

FPTA4

site on a bimonthly schedule starting in late January 1989 and ending in early June 1989. Three sampling trips were completed during the monitoring period. The number of samples collected was 54, 53, and 39 respectively. Gas-phase wells V9 and V11 were installed after the initial sampling trip.

A

rising water table precluded sampling from the lower gas sampling ports during the latter two sampling trips.

(37)

Figure 3. Schemat,ic of gas-phase monitoring well design

1

S t a l n l e s v

Steel

Tublng

1' PVC

Tublng

-

Stainless

Steel

26'

Well

Scre

6-slot

\

I

-

M l e

Cover

2'

Stalnless

Vapor-Phase

Stalnless

d~

Well Pofnt

Steel

At each multilevel gas-sampling site, the procedure for sampling was a s follows. The sampled port was connected to a Thomas dual-diaphragm vacuum pump with Swagelock fittings and then pumped at a rate of approximately 300 to 350 mI/min for

2

min.

This

evacuation method ensured that the sample represented the gas-phase conditions in the formation at the port depth. After purging, the flow rate

A s

adjusted to a value between 100 to 200 ml/min; activated carbon sampling tubes were then connected to the outflow of the vacuum pump; gas was extracted from the sample location and trapped onto an

(38)

losses to the atmosphere were minimized by immediately sealing the carbon sample tube

and storing on ice. One system blank and one travel blank were taken for every 10 to 15 samples.

The activated carbon sampling tubes used for adsorbing contaminants contained two sec- tions of carbon, separated with a polyethylene plug, and retained with polyethylene at both ends. Both ends of the tube were flame sealed and at the time of analysis the ends could be easily broken. The front section of the tube, labeled

A,

contained 100 mg of carbon while the lat ter section, labeled

B,

contained

50

mg of carbon. Each section of the sampling tube was analyzed separately to determine if solute breakthrough had occurred.

In

an effort to reduce the potential of cross-contaminating sampling equipment, locations upgradient or transverse to the burn pit were sampled h t . These monitoring stations contained the lowest levels of contaminants. Sampling then proceeded down gradient of the bum pit to the more highly contaminated areas.

3.2.7 Ground-Probe Sampling

As part of the agreement between

UNC

and Pope

AFB,

permanent monitoring wells were located on the perimeter of the active training area. Gas-phase concentration sample data were collected in the restricted area using a driveable ground probe

(DGP)

method developed by M7allingford et al. (1988). Fourteen temporary probe holes were located as

shown in Figure 2. To maintain continuity, DGP holes were located by compass and tape in approximately the same location during each sampling trip.

The

DGP

method involved the boring of a small diameter bore hole to approximately 1 ft above the desired sampling elevation. The DGP was then inserted and hammered to the desired depth.

A

detachable center rod was then removed from the probe and a similar gas-phase sampling procedure as used for the multilevel monitoring wells uTas employed to

sample from the

DGP.

3.2.8 Groundwater Sampling

Groundwater samples were obtained by bailing.

A

I-in stainless-steel bailer was lowered down the center of each monitoring well through the center

PVC

pipe into the water table. The bailer was then carefully retrieved and samples were collected in 40-ml borosilicate sample vials. Approximately

5

ml of 0.1

M

sodium azide were added to the sample to minimize microbiological degradation during transport and storage; the vials were then sealed with Teflon-faced septa and stored on ice.

(39)

acetone followed by numerous rinses of tap and distilled water.

In

areas where free product was observed, the bailer was first rinsed with acetone followed by washing with laboratory grade soap and rinsing with tap and distilled water. Sample system blanks were prepared by pouring distilled water into the decontaminated bailer and preparing the blank sample similarly to the groundwater samples.

3.3

Laboratory Methods

3.3.1 Materials

Controlled laboratory experiments were performed using toluene because of its high volatil- ity, prevalence in

JP-4,

and its frequent occurrence in contaminated groundwater systems. The high volatility suggests that toluene should be a major contaminant in the gas phase of the vadose zone at any gasoline spill site.

The

properties of toluene are listed in Table 1. The toluene used in the experiments was reagent grade (Fisher Scientific Co., Pittsburgh, Pennsylvania). All extraction solvents and internal standards (carbon disulfide, ethyl benzene, hexanes and iso-octane) were reagent grade or better (Mallencrofdt, Inc., Paris, Texas).

All

subsurface material that a-as used in equilibrium studies and rate studies was obtained from an uncontaminated region at the Pope

AFB

site. The sample was collected from

3

to 4 ft beneath the surface. Roots and other large organic material were floated out of the soil using de-ionized water in a 5-gal tank, washing s m d batches of soil at a time.

The

fiaction of organic carbon

(f,,)

in the soil was

0.004,

or

0.4%.

This f,,

is much higher than the d u e found in the up-gradient well

(0.01%),

but it is still low. The reason for the difference between

the

two levels is attributed to the near-surface nature of the

bulk

sample, which would normally contain higher fractions of decaying plant material and hence organic carbon.

A

similar trend has been noted by the authors based upon detailed characterization of soil profiles fiom the surface down for another research project being

performed. A gain size analysis of the washed soil is shown in Figure 4, and it shows that the solid material was a fine sand with a median grain size diameter of about

0.27

mm and

a uniformity coefficient (dso /dlo) of about 4.0.

AlI

glassware w * ~ acid washed in either nitric acid or a mixture of sulfuric acid and chromic

(40)

Figure

4.

Grain size distribution diagram of Pope

AFB

bulk sample

Bottle-Point Isotherm.

Grain

b ~ z e

(mm)

(41)

Solid samples were placed in each sample vial and then a solution of calcium chloride and sodium azide m a s added. This solution was 0.005

M

calcium chloride to help settle the solids during centrifugation and 0.045

M

sodium azide to suppress microbiological activity.

A

stock solution of toluene was prepared in a 1-1 bottle. The concentration of this solution was approximately 425 mg/l; the saturation concentration of toluene

in

water is

515

mg/l at

20°C.

The solution was stirred for

24

hr

to dissolve the toluene. Stock solution was

withdrawn with a syringe from the septum at the bottom of the bottle and then added to the isotherm vials through a Teflon septum

in

the

vial

cap.

A

second syringe needle was

inserted into the vial septum to vent displaced air as the stock solution was added.

This

technique limited volatility losses because the solution was never open t o the air but was

always contained within either the solution bottle, the syringe, or a sample vial.

During the sample preparation, three samples were taken from the stock solution. One sample was taken at the start of the sample preparation; a second was taken after half of the vials were prepared, about 20 rnin later; and the third sample was taken after the last

vial was sealed. These three samples were used to detect any change in the concentration of the stock solution during the sample preparation procedure.

At the end of the equilibration period, the samples were placed in a centrifuge for

30

min to separate the solids from the solution. Ten

ml

of the supernatant were then removed with a syringe, extracted

with

hexane as described,

and

analyzed by gas chromatography.

3.3.3 Bottle-Point Rate Studv

A

bottle-point rate study was performed to determine the nature of the approach to s o p tive equilibrium between toluene and the solid material. The method used for the bottle point rate study was very similar to that used in the isotherm experiments.

In

the rate study, the initial solute concentration was the same for

all

samples, while samples were allowed to mix for varying lengths of time to observe the approach to equilibrium.

As with the isotherm experiments, 20 g of solid material were weighed into each of sixteen

(42)

3.3.4 Sample Extraction Methods

Gas-phase sampling in the field, using either the gas-phase sampling w e b or the

DGP

method, resulted in contaminants being trapped on 150-mg activated carbon tubes. Quan- tification of the mass adsorbed required desorption of the contaminants fiom the carbon tubes. Each section of the carbon tube was analyzed separately with the larger section designated as the primary section and the second section designated as the backup sec- tion.

Each

carbon section was removed from the glass

tube

and

placed

in

a 4-ml glass

vial.

One

ml

of carbon disulfide with an internal standard of I-chlorodecane was added to the carbon. The sample was then sealed and placed on a vibrating shaker and agitated for 30 min.

A

1-4 aliquot of the resulting carbon disulfide solution was then analyzed by capillary gas chromatography. White et al. (1970) reported recoveries of between 90%

and 100% for a variety of non-polar compounds by use of this method. In this research, a recovery of 100% was assumed.

A

liquid-liquid extraction method was used for extracting groundwater samples taken at the site. Pentane was used as the extraction solvent because of its low boiling point (33"

C)

and low solubility in water, which prevented interference with target compounds during chromatographic analysis.

Three

25-ml pentane extractions were completed on the January set of goundwater samples. The solvent from the extractions was combined and reduced to 10-ml by the Kuderna-Danish method. The March and May groundwater samples were prepared by completing three 2-ml pent ane ex tract ions and did not require the

find

process of reducing the final amount of solvent. The extracts were 'then analyzed

by

GC

methods.

In

the sorption equilibrium and rate experiments, aqueous solutions had to be analyzed for toluene.

In

each case 10 ml of solution were used for the extraction procedure.

The

10 rnl

of solution were placed in a 20-ml sample vial

with

3

rnl

of hexane, spiked with iso-octane as an internal standard. The concentration of the internal standard was approximately

200 pg/l. The vial was shaken vigorously on an orbit shaker for several minutes. After the two phases were separated, 3 ml of the organic phase were drawn off using a syringe. The organic phase was then dried in a

3-ml

sample vial with 0.25 g of sodium sulfate to remove any residual water from the solution.

3.3.5 Gas Chromatog.raphv Methods

A

Hewlett Packard model 5890A G C equipped with a flame ionization detector

(FID)

and

a Hewlett Packard

3390

integrator were used to analyze

all

samples collected &om the Pope

AFB

field site.

A

50-m, DB-5

(R

&

W)

capillary column was used for

all analyses.

The temperature program was (1) 40°C initial temperature for

8

min; (2) 5°C per min fiom 40" to 120°C; (3) 25°C per min from 120" to 250°C; and (4) 5 rnin at 250°C.

(43)

were established fiom analyzing a set of five external standards containing various concen- trations of each of the target compounds. An internal standard of 1-chlorodecane with a concentration of approximately

55

mg/l was used in both standards and unknowns. The method is explained in more detail in Miller (1987) and is common for most chromatogra- phy analysis.

Laboratory sorption rate and equilibrium samples were analyzed for toluene concentra- tion by injecting a 1-p1 sample of the extraction solvent, hexane, into a Varian 3700 gas

(44)

4

Results

4.1

Field

Results

4.1.1 Overview

The field aspects of this project included observations of groundwater heads and gas-phase concentrations. The bulk movement of contaminants is strongly influenced by the direction and velocity of groundwater %ow. Therefore, interpreting the hydrocarbon distribution data required characterizing the groundwater hydraulics at the site. Monthly water-level measurements from monitoring wells were used to determine the gradient of hydraulic head and direction of groundwater %ow.

Hydrocarbon distributions of targeted contaminants in the subsurface were determined by collecting and analyzing gas-phase samples from multilevel monitoring wells and DGPs, collected as a function of varying horizontal and vertical locations and time. Sampling

was conducted during January, March, and June 1989. Each sampling trip took two to

t,hree days to complete. Samples were then returned to the laboratory and analyzed, and

concentrations were plotted on a twedimensional z

-

y plot. Concentration contours were developed using the geostat,istical method of kriging. The kriging method estimates the concentration based on the theory that the variable is correlated spatially. The technique is based on the regional \+able theory and requires an est h a t e of a variogram (Marsily, 1986). Concentration contour intends were based on duplicate sample variance. The computer software package Surfer (Golden Software, Inc., Golden, Colorado) was used to perform the kriging

and

create contoured plots.

4.1.2 Groundwater Flow

The unconfined aquifer at the site is influenced by infiltration of precipitation. Base weather records indicate that before the initial sampling trip, Pope

AFB

experienced drought conditions. Groundwater elevation measurements made in January 1989 were the lowest recorded during the monitoring period. The piezometric surface data for January

1989 are shown in Figure

5.

Between January

and

March,

the base received approximately

I0

in of precipitation,

3

in

above normal for that period.

The

water table elevations rose 1.5 to

2 ft

in response to the increased precipitation. The piezometric surface data for

March

1989 are shown in Figure 6.

(45)

Figure

5.

January 1989 observed piezometric surface

POPE

AIR

FORCE

BASE

~ l d l s h ~d

NORTH CAROLINA

Arrows indlcate direction of deweasing

FT2

hydreullc head

SS HDnitorlhg W e l l

I

FT 1

0 WC Hwrltoring W e l l

I

PIEZOMETRIC SURFACE

(Elevation in Feet EJ4SU

occurred between May and June. The piezometric surface data for June are shown in Figure

7.

Throughout the sampling period, bulging of the piezometric head contour lines occurred in the area of the burn pit. This phenomenon is believed to be related to groundwater recharge. As previously mentioned, during an exercise the fire pit is filled with contami- nated jet fuel and water that slowly infiltrate into the underlying aquifer, thus raising the water table elevations near the burn pit. This increase in head is evident as a groundwater mound in the piezometric surface beneath the burn pit.

(46)

Figure 6. March 1989 observed piezometric surface

AFB

was assumed to be homogeneous and isotropic. This assumption allowed for the analysis of the groundwater %ow field based on measured water levels from monitoring wells. In a homogeneous, isotropic aquifer, the hydraulic conductivity and porosity are independent of location and the direction of groundwater flow. Therefore, kom Darcy's law, the direction of groundwater flow under such assumptions is identical to the direction of the gradient of hydraulic head. The components of the piezometric head gadient in twdimensions may be determined by a method described by Abriola and Pinder (1982).

The method estimates the value of the piezometric head by using the method of a linear interpolation function. The linear interpolation function allows the z

-

y components of the hydraulic head gradient to be calculated from t h e e known piezometric head values.

(47)

Figure

7.

June

1989

observed piezometric surface

POPE

A I R

FORCE

BASE

NORTH CAROLINA

Arrows lndlcate

dlrectlon of decreasing

FIRE TRAININC AREA QS hy&arllc head

\\

FT2

v1

SS b i t o r k g W e l l

FT 1

0 WC Hariiorfq W e l l

PIEZOMETRIC

SURFACE

(Elevatlm in Feet U4SL)

C o n t w Interval 1' h 1589

Scale

measured piezometric head. A computer code, based on the Abriola and Finder (1982)

linear interpolation method, was used to solve the z and y components of the gradient of piezometric head for each of the triangular elements. The resulting vectors of the piezometric head were calculated and plotted on Figures

5

to

7.

Analyses of piezometric contours and the velocity vector field indicate that in the January sampling period groundwater flow at the site was predominantly in an east-west flow pattern, converging on the groundwater seeps near monitoring well

V3. In

March

and

June,

(48)

4.1.3 Gas-Phase Solute Distributions

4.1

.$.I

Sample Variabilitq

To determine the precision and accuracy of the sampling methods using the

DGP

and .

multilevel gas wells, duplicate samples were taken at

15

multilevel gas wells and 13

DGP

sample sites during the three sampling periods. At each location two samples were taken using the same method and sample device. The duplicate samples were taken at various locations covering a wide distribution of concentrations.

Figure

8

shows the concentration variation about the mean for octane as a function of concentration and sampling method. This figure shows that at low concentrations the duplicate samples had a 220% difference from the mean concentration, although a few samples had greater than

20%

difference from the mean. At higher concentrations, the difference from the mean decreased to less than

f

10%.

In

general, monitoring well con- centrations had less variability than the probe samples.

4.1.3.2

Nonaoueow

Phase

Liquid Zone

The initial fd-round of sampling was performed on

23

January 1989.

In

this initial sampling trip a significant amount of

JP-4

jet fuel was found above the water table as a separate nonaqueous phase liquid or

NAPL.

The thickness and distnbution of

NAPL

could not be determined quantitatively because of the lack of monitoring wells near the fire pit. Measurable amounts of

NAPL

were found in monitoring wells, V1,

V2,

FT6, and

V3.

A

maximum thickness of about 1 in was measured in

March.

Heavy amounts of hydrocarbons were found in drill cuttings from probe holes PH5, PHIO, and PHI 1. This indicated that

NAPL

probably extended intermittently from the burn pit to the groundwater leachate outflow area, located on the western limit of the study area. Figure 9 illustrates the estimated extent of the

NAPL

zone.

A

total of 59 gas-phase samples was analyzed from the site in January. The distribution of gas-phase concentrations for all targeted compounds was spatially correlated to the location of the

NAPL

zone. The concentration profiles, shown in Figures 10 through 13, exhibit a general east-west distribution starting near the burn pit and extending toward the area where leachate was exposed on the western side of the study area. The highest concentrations of

all

targeted compounds were located from between 100 to 200 feet down- gradient of the burn pit.

The gas-phase concentration value plotted at each sampling location is the concentration located midway between the gound surface and the top of the water table.

This

point

(49)

Octane concent,ration sample variability.

0 0 0 0 0

Vapor-Phase Wells

0 0 o

Probe Holes

Vapor concentration

(mg octane/m3)

The gas-phase concentrations above the

NAPL

phase were expected to be related to the compound's vapor pressure. The compounds with the higher vapor pressure should there- fore, have a greater concentration. Hexane has a significantly higher vapor pressure than the other compounds analyzed, and hexane concentrations were consistently

2

to 10 times greater in magnitude than the other targeted compounds.

A

similar areal distribution was

Figure

Figure 1. Topographic map of the Pope AFB Fire Protection Training Area 4 site
Figure 2. Monitoring station location map
Figure 3. Schemat,ic of gas-phase monitoring well I - design
Figure 5. January 1989 observed piezometric surface
+7

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