United States

Environmental Protection

Agency

Capstone Report on the

Application, Monitoring, and

Performance of Permeable

Reactive Barriers for

Ground-Water Remediation:

Volume 1

Performance Evaluations at Two Sites

Ground Surface

Iron

Wall

Ground Water Flow
EPA/600/R-03/045a 

August 2003 

Capstone Report on the Application, 

Monitoring, and Performance of Permeable 

Reactive Barriers for Ground-Water 

Remediation: 

Volume 1 – Performance Evaluations at Two Sites


Richard T. Wilkin and Robert W. Puls


Ground Water and Ecosystems Restoration Division


National Risk Management Research Laboratory


Ada, Oklahoma 74820


National Risk Management Research Laboratory


Office of Research and Development


U.S. Environmental Protection Agency


Cincinnati, OH 45268

Notice 

The U.S. Environmental Protection Agency through its Office of Research and Development funded the research 

described here. It has been subjected to the Agency’s peer and administrative review and has been approved for 

publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement 

or recommendation for use. 

All research projects making conclusions or recommendations based on environmentally related measurements and 

funded by the Environmental Protection Agency are required to participate in the Agency Quality Assurance 

Program. This project was conducted under an approved Quality Assurance Project Plan. The procedures specified 

in this plan were used without exception. Information on the plan and documentation of the quality assurance 

activities and results are available from the Principal Investigator. 

ii 
Foreword 

The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting the Nation’s land, air, and 

water resources. Under a mandate of national environmental laws, the Agency strives to formulate and implement 

actions leading to a compatible balance between human activities and the ability of natural systems to support and 

nurture life. To meet this mandate, EPA’s research program is providing data and technical support for solving 

environmental problems today and building a science knowledge base necessary to manage our ecological 

resources wisely, understand how pollutants affect our health, and prevent or reduce environmental risks in the 

future. 

The National Risk Management Research Laboratory (NRMRL) is the Agency’s center for investigation of technological and management approaches for preventing and reducing risks from pollution that threatens human health and 

the environment. The focus of the Laboratory’s research program is on methods and their cost-effectiveness for 

prevention and control of pollution to air, land, water, and subsurface resources; protection of water quality in public 

water systems; remediation of contaminated sites, sediments and ground water; prevention and control of indoor air 

pollution; and restoration of ecosystems. NRMRL collaborates with both public and private sector partners to foster 

technologies that reduce the cost of compliance and to anticipate emerging problems. NRMRL’s research provides 

solutions to environmental problems by: developing and promoting technologies that protect and improve the 

environment; advancing scientific and engineering information to support regulatory and policy decisions; and 

providing the technical support and information transfer to ensure implementation of environmental regulations and 

strategies at the national, state, and community levels. 

This publication has been produced as part of the Laboratory’s strategic long-term research plan. It is published and 

made available by EPA’s Office of Research and Development (ORD) to assist the user community and to link 

researchers with their clients. The purpose of this document is to provide detailed performance monitoring data on 

full-scale Permeable Reactive Barriers (PRBs) installed to treat contaminated ground water at two different sites. 

This report will fill a need for a readily available source of information for site managers and others who are faced with 

the need to remediate ground water contaminated by chlorinated solvents, chromium, arsenic, nitrates, and other 

organic and inorganic compounds and are considering the use of this cost-effective technology. The PRBs 

discussed in this report are among the oldest full-scale systems available for study and provide an opportunity to 

analyze the performance of systems with more than five years of field history. In addition, the PRBs examined here 

have contrasting design and hydrogeochemical characteristics that are useful in the context of gaining insight about 

the factors that govern PRB longevity and long-term performance. The information provided in this document will be 

other interested parties. 

of use to stakeholders such as state and federal regulators, Native American tribes, consultants, contractors, and 

Ground Water and Ecosystems Restoration Division 

National Risk Management Research Laboratory 

iii 
Abstract 

Research results discussed in this report explore the geochemical and microbiological processes within zero-valent 

iron Permeable Reactive Barriers (PRBs) that may contribute to changes through time in iron reactivity and 

decreases in reaction zone permeability. Two full-scale PRBs were evaluated in this study: the U.S. Coast Guard 

Support Center PRB located near Elizabeth City, North Carolina, and the Denver Federal Center PRB in Lakewood, 

Colorado. Detailed water sampling and analysis, core sampling, and solid-phase characterization studies were 

carried out to: i) evaluate spatial and temporal trends in contaminant concentrations and key geochemical 

parameters; ii) characterize the type and nature of surface precipitates forming over time in the reactive barriers; and 

iii), identify the type and extent of microbiological activity within and around the reactive barriers. 

Trends in geochemical parameters (e.g., pH and oxidation-reduction potential) may signal changes in system 

performance, but no clear correlations between these parameters and decreased system performance have been 

observed to date at the sites studied. Long-term trends in geochemical parameters are consistent with contaminant 

removal trends observed at both sites. Spatial and temporal variations in the concentration distribution of terminal 

electron accepting species (e.g., sulfate), specific conductance, and Eh suggest that both anaerobic iron corrosion 

and microbial activity play important roles in controlling the oxidation-reduction potential in iron barriers. Low Eh 

values (≤100 mV relative to the standard hydrogen electrode) and decreases in the specific conductance of ground 

water between upgradient contaminant plumes and sampling points within reactive iron media are consistently 

observed in normally operating PRB systems. 

The rate of mineral and biomass buildup was evaluated at both sites. The principal factors that determine the amount 

of mineral precipitation in zero-valent iron PRBs are flow rate, ground-water chemistry, and microbial activity. After 

five years of operation, the Elizabeth City and Denver Federal Center reactive barriers have developed consistent 

patterns of spatially variable mineral precipitation and microbial activity. The development of precipitation and 

biomass fronts result from abrupt geochemical changes that occur at upgradient interface regions coupled with 

ground water solute transport. Upgradient regions at both sites investigated in this study have witnessed the 

greatest accumulation of mineral mass and biomass. However, neither of the sites of this study show complete filling 

of available pore space after five years, suggesting that flow characteristics should not be affected by the 

accumulation of authigenic components. For zero-valent iron systems, the reactive media is a long-term sink for C, 

S, Ca, Si, Mg, and N. Porosity loss in the iron media due to precipitation of inorganic carbon and sulfur minerals can 

be estimated by integrating the concentrations of inorganic carbon and sulfur as a function of distance in the iron and 

estimating the volume loss by using the molar volumes of zero-valent iron, calcium carbonate, iron carbonate, and 

iron sulfide. Porosity loss estimates have ranged from about 1% to 4% per year in this study. Based on these 

estimates, the average porosity of the PRB at Elizabeth City, for example, would not be expected to approach that 

of the surrounding aquifer for 15 to 30 years. As corrosion minerals form on the surface of the iron media, reactive 

surfaces are coated, presumably decreasing the effective reactive surface area. However, corrosion products 

formed include some minerals which themselves are highly reactive and capable of transforming inorganic and 

organic contaminants into immobile or non-toxic forms. This phenomenon must also be factored into lifetime 

projections. 

While long-term performance observations of the Elizabeth City and Denver Federal Center site are now past five 

years, there has still not been sufficient time to adequately predict the lifetime of these PRBs or most other PRBs. It 

is clear that lifetimes exceeding 10 years are reasonable to expect under some conditions and that PRBs may 

function adequately for much longer. Continued studies are needed to better predict longevity based on groundwater composition, flow rate and contaminant flux. 

iv 
Table of Contents 

Notice ............................................................................................................................................. ii


Foreword ....................................................................................................................................... iii


Abstract ......................................................................................................................................... iv


Table of Contents .......................................................................................................................... v


Figures ......................................................................................................................................... vii


Tables ........................................................................................................................................... xii


Acknowledgments ....................................................................................................................... xiv


1.0 Introduction ............................................................................................................................. 1


1.1 Federal Tri-Agency Initiative ............................................................................................. 2


1.1.1 DoD Studies ........................................................................................................... 2


1.1.2 DOE Studies ........................................................................................................... 3


1.1.3 EPA Studies ........................................................................................................... 3


2.0 Site Descriptions ..................................................................................................................... 5


2.1 U.S. Coast Guard Support Center .................................................................................... 5


2.2 Denver Federal Center ..................................................................................................... 5


3.0 Elizabeth City PRB Monitoring Results ................................................................................... 9


3.1 Ground Water Monitoring ................................................................................................. 9


3.1.1 Contaminant Behavior ............................................................................................ 9


3.1.2 Geochemical Parameters ..................................................................................... 13


3.1.3 Dissolved Cations and Anions .............................................................................. 18


3.2 Core Sampling at Elizabeth City ..................................................................................... 25


3.2.1 Carbon Analysis ................................................................................................... 25


3.2.2 Sulfur Analysis ...................................................................................................... 33


3.2.3 Cr Extractions ....................................................................................................... 39


3.2.4 X-ray Diffraction Analysis ..................................................................................... 39


3.2.5 Scanning Electron Microscopy ............................................................................. 41


3.2.6 Microbial Characterization .................................................................................... 46


3.3 Summary of Results from the Elizabeth City Site ........................................................... 52


4.0 Denver Federal Center Monitoring Results ........................................................................... 53


4.1 	Ground Water Monitoring ............................................................................................... 53


4.1.1 Gate 1 Contaminant Behavior .............................................................................. 53


4.1.2 Gate 2 Contaminant Behavior .............................................................................. 58


4.1.3 Gate 3 Contaminant Behavior .............................................................................. 58


4.1.4 Geochemical Parameters ..................................................................................... 58


4.1.5 Hydrogen Gas Concentrations ............................................................................. 62


4.1.6 Dissolved Cations and Anions .............................................................................. 64


4.2 Core Sampling at the Denver Federal Center ................................................................ 68


4.2.1 Carbon Analysis ................................................................................................... 71


4.2.2 Sulfur Analysis ...................................................................................................... 71


4.2.3 X-ray Diffraction Analysis ..................................................................................... 71


4.2.4 Scanning Electron Microscopy ............................................................................. 71


4.2.5 Microbial Characterization .................................................................................... 80


4.3 Summary of Results from the Denver Federal Center Site ............................................ 82


v 
5.0 Factors Affecting Longevity and Performance ...................................................................... 87


5.1 Fe0

 Dissolution ................................................................................................................ 87


5.2 Anion Composition.......................................................................................................... 89


5.2.1 Bicarbonate Reactions.......................................................................................... 89


5.2.2 Sulfate Reactions ................................................................................................. 96


5.2.3 Nitrate Reactions .................................................................................................. 97


5.2.4 Reactions with Silica ............................................................................................. 97


5.2.5 Reactions with Oxygen ......................................................................................... 98


5.3 Mineral Precipitation ...................................................................................................... 99


5.3.1 Pore Volume Reduction ......................................................................................... 99


5.3.2 Loss of Reactivity. .............................................................................................. 103


5.4 Microbial Activity ........................................................................................................... 104


5.5 Hydrogeological Issues ................................................................................................. 107


6.0 State of Permeable Reactive Barrier Technology and


Lessons Learned from Long-Term Performance Monitoring ............................................... 111


6.1 Permeable Reactive Barriers: An Accepted Remedial 

Option for Containment & Treatment of Contaminated Ground Water ......................... 111


6.2 Lessons Learned: Site Characterization and PRB Construction................................... 112


6.3 Lessons Learned: Long-term Performance Assessments of PRBs .............................. 112


6.3.1 Recommendations for Future Research. ............................................................. 113


7.0 References .......................................................................................................................... 115


Appendix A ................................................................................................................................ 121


Appendix B ................................................................................................................................ 135


vi 
Figures 

Figure 2.1	 Plan view map of the PRB at the U.S. Coast Guard Support


Center, Elizabeth City, NC ......................................................................................... 7


Figure 2.2	 Plan view map of the PRB at the Denver Federal Center,


Lakewood, CO (after McMahon et al., 1999) ............................................................. 8


Figure 2.3	 Schematic cross-section of the Denver Federal Center


funnel-and-gate system (after McMahon et al., 1999) ................................................ 8


Figure 3.1	 Concentrations of contaminants through time in monitoring


wells located hydraulically upgradient of the Elizabeth City PRB ............................. 10


Figure 3.2	 Concentrations of TCE (μg/L) through time in monitoring


wells located hydraulically downgradient of the Elizabeth City PRB ........................ 13


Figure 3.3	 Cross-sectional profiles showing total chromium


concentrations (mg/L) in transects 1, 2, and 3 (Elizabeth City PRB) ....................... 14


Figure 3.4	 Cross-sectional profiles showing TCE concentrations


(μg/L) in transects 1, 2, and 3 (Elizabeth City PRB) ................................................ 15


Figure 3.5	 Cross-sectional profiles showing cis-DCE concentrations


(μg/L) in transects 1, 2, and 3 (Elizabeth City PRB) ................................................ 16


Figure 3.6	 Cross-sectional profiles showing VC concentrations


(μg/L) in transects 1, 2, and 3 (Elizabeth City PRB) ................................................ 17


Figure 3.7	 Cross-sectional profiles showing pH distributions


in transects 1, 2, and 3 (Elizabeth City PRB) ........................................................... 19


Figure 3.8	 Cross-sectional profiles showing Eh distributions


(mV) in transects 1, 2, and 3 (Elizabeth City PRB) .................................................. 20


Figure 3.9	 Cross-sectional profiles showing specific conductance


distributions (μS/cm) in transects 1, 2, and 3 (Elizabeth City PRB) ......................... 21


Figure 3.10 Cross-sectional profiles showing calcium concentrations


(mg/L) in transects 1, 2, and 3 (Elizabeth City PRB)................................................ 22


Figure 3.11 Cross-sectional profiles showing magnesium concentrations


(mg/L) in transects 1, 2, and 3 (Elizabeth City PRB)................................................ 23


Figure 3.12 Cross-sectional profiles showing sodium concentrations


(mg/L) in transects 1, 2, and 3 (Elizabeth City PRB)................................................ 24


Figure 3.13 Cross-sectional profiles showing potassium concentrations


(mg/L) in transects 1, 2, and 3 (Elizabeth City PRB)................................................ 26


Figure 3.14 Cross-sectional profiles showing chloride concentrations


(mg/L) in transects 1, 2, and 3 (Elizabeth City PRB)................................................ 28


Figure 3.15 Cross-sectional profiles showing sulfate concentrations


(mg/L) in transects 1, 2, and 3 (Elizabeth City PRB)................................................ 29


vii 
Figure 3.16 Cross-sectional profiles showing alkalinity distributions


(mg/L) in transects 1, 2, and 3 (Elizabeth City PRB)................................................ 30


Figure 3.17 Cross-sectional profiles showing nitrate concentrations


(mg/L) in transects 1, 2, and 3 (Elizabeth City PRB)................................................ 31


Figure 3.18 Cross-sectional profiles showing silica concentrations


(mg/L) in transect 2 (Elizabeth City PRB) ................................................................ 33


Figure 3.19 Coring locations and monitoring well locations at


the Elizabeth City PRB (plan view) .......................................................................... 34


Figure 3.20 Cross-sectional profile showing concentration distribution


of inorganic carbon in the solid phase (μg/g=ppm), Elizabeth City


PRB (June 2002)...................................................................................................... 36


Figure 3.21 Concentrations of inorganic carbon (μg/g) in core materials


through time, Elizabeth City PRB ............................................................................. 37


Figure 3.22 Cross-sectional profile showing concentration distribution


of sulfur in the solid phase (μg/g=ppm), Elizabeth City PRB (June 2002) ............... 38


Figure 3.23 Powder X-ray diffraction data from fine-grained materials


removed via sonication from cores collected at the


Elizabeth City PRB: a) core EC060300-4; b) core EC050801-3 .............................. 40


Figure 3.24 Scanning electron micrographs of samples from the


Elizabeth City PRB: a) sample EC060300-4-1; b) sample


EC060300-4-3; c) EC060300-4-7 ........................................................................ 42-44


Figure 3.25 Iron concentration versus oxygen concentration in iron


grains and surface precipitates (SEM-EDX)............................................................. 46


Figure 3.26 Element concentrations in surface precipitates from the


Elizabeth City PRB ................................................................................................... 48


Figure 3.27 Cross-sectional profile showing concentration distribution


of biomass (from PLFA data) in picomoles per gram,


Elizabeth City PRB (June 2002) ............................................................................... 49


Figure 3.28 Histograms of microbial biomass concentrations


(from PLFA data) in picomoles per gram in aquifer materials,


iron from near the upgradient aquifer/iron interface, and iron


from near the downgradient aquifer/iron interface .................................................... 50


Figure 3.29 Pie graphs showing structural distribution of PLFA


compounds (average values) at the Elizabeth City site ........................................... 51


Figure 4.1	 Coring locations and monitoring well locations at the


Denver Federal Center, gate 1 (plan view) .............................................................. 54


Figure 4.2	 Coring locations and monitoring well locations at the


Denver Federal Center, gate 2 (plan view) .............................................................. 55


Figure 4.3	 Coring locations and monitoring well locations at the


Denver Federal Center, gate 3 (plan view) .............................................................. 56


Figure 4.4 Concentrations of contaminants through time in monitoring 

wells from the Denver Federal Center, gate 1 (data from FHWA): 

a) well GSA-21 (upgradient); b) well C1-I1 (iron wall); 

c) well GSA-20 (downgradient) ................................................................................ 57


viii 
Figure 4.5 Concentrations of contaminants through time in monitoring 

wells from the Denver Federal Center, gate 2 (data from FHWA): 

a) well GSA-26 (upgradient); b) well C2-I2 (iron wall); 

c) well GSA-25 (downgradient) ................................................................................ 59 

Figure 4.6 Depth-resolved concentrations of a) contaminants (μg/L) 

and b) sulfate, calcium, and iron (mg/L) in wells GSA-26 

and GSA-25 from the Denver Federal Center (gate 2) ............................................ 60 

Figure 4.7 Concentrations of contaminants through time in monitoring 

wells from the Denver Federal Center, gate 3 (data from FHWA): 

a) well GSA-31 (upgradient); b) well C3-I2 (iron wall); 

c) well GSA-30 (downgradient) ................................................................................ 61 

Figure 4.8 Average pH values through time in wells from upgradient, 

iron wall, and downgradient positions relative to gate 1, 

gate 2, and gate 3 at the Denver Federal Center ..................................................... 62 

Figure 4.9 Average specific conductance values (μS/cm) through time 

in wells from upgradient, iron wall, and downgradient 

positions relative to gate 1, gate 2, and gate 3 at the 

Denver Federal Center ............................................................................................. 63 

Figure 4.10 Average Eh (V) values through time in wells from upgradient, 

iron wall, and downgradient positions relative to gate 1, 

gate 2, and gate 3 at the Denver Federal Center ..................................................... 63 

Figure 4.11 Concentrations of dissolved hydrogen (log molar) as 

a function of sampling position and time in gate 1 at 

the Denver Federal Center. Also shown are the 

concentration ranges of dissolved hydrogen measured in 

the iron media in gate 2 and gate 3.......................................................................... 64 

Figure 4.12 Average (± 1 s.d.) concentrations of Na, K, Ca, Mg, sulfate, 

bicarbonate, chloride, and silica (mg/L) as a function of 

sampling position in gate 1 at the Denver Federal Center ....................................... 65 

Figure 4.13 Average (± 1 s.d.) concentrations of Na, K, Ca, Mg, sulfate, 

bicarbonate, chloride, and silica (mg/L) as a function of 

sampling position in gate 2 at the Denver Federal Center ....................................... 66 

Figure 4.14 Average (± 1 s.d.) concentrations of Na, K, Ca, Mg, sulfate, 

bicarbonate, chloride, and silica (mg/L) as a function of 

sampling position in gate 3 at the Denver Federal Center ....................................... 67 

Figure 4.15 Picture showing cemented nodules recovered from core 

collected at the Denver Federal Center.................................................................... 69 

Figure 4.16 Picture showing the appearance of cores collected at 

the Denver Federal Center, from gate 2 near the upgradient 

peagravel/iron interface ............................................................................................ 69 

Figure 4.17 Concentration distribution of solid phase inorganic 

carbon in angle cores collected from gate 1 at the Denver 

Federal Center ......................................................................................................... 72 

Figure 4.18 Concentration distribution of solid phase inorganic 

carbon in angle cores collected from gate 2 at the Denver 

Federal Center ......................................................................................................... 72 

ix 
Figure 4.19 Concentration distribution of solid phase inorganic 

carbon in a vertical core collected from gate 2 at the 

Denver Federal Center ............................................................................................. 73 

Figure 4.20 Concentration distribution of solid phase sulfur 

in angle cores collected from gate 1 at the Denver 

Federal Center ......................................................................................................... 73 

Figure 4.21 Concentration distribution of solid phase sulfur 

in angle cores collected from gate 2 at the Denver 

Federal Center ......................................................................................................... 74 

Figure 4.22 Inorganic carbon concentrations versus total sulfur 

a) Elizabeth City core materials; b) Denver Federal 

Center ...................................................................................................................... 75 

Figure 4.23 Powder X-ray diffraction data from fine-grained materials 

removed via sonication from cores collected at the 

Denver Federal Center PRB (core C2-3-71801) ...................................................... 76 

Figure 4.24 Scanning electron micrographs of samples from the 

Denver Federal Center PRB:a) sample C2-17-71300-2; 

b) sample C2-17-71300-7; c) sample C1-2-71000-3 ........................................... 77-79 

Figure 4.25 Concentration of microbial biomass (from PLFA data) 

in picomoles per gram in iron from near the upgradient 

peagravel/iron interface and iron from near the downgradient 

peagravel/iron interface: a) gate 1; b) gate 2 ........................................................... 82 

Figure 4.26 Concentration distribution of solid phase sulfur and microbial 

biomass (from PLFA data) in a vertical core collected from gate 2 

at the Denver Federal Center (vertical cores C2-1-71901, 

C2-2-71901, and C2-3-71901) ................................................................................. 83 

Figure 4.27 Pie graphs showing average structural distribution of PLFA 

compounds in core materials from the Denver Federal Center ................................ 84 

Figure 5.1 Redox –pH diagram showing composition of ground water 

from the Elizabeth City iron wall compared to equilibrium 

trends for the Fe0

-Fe(OH)3, Fe0-Fe3O4, and Fe2+-Fe(OH)3 couples .......................... 88 

Figure 5.2 Redox-pH diagram for the Fe-H2

speciation of iron (dashed lines) and stability fields of 

iron-bearing minerals (solid lines)............................................................................. 90 

O system at 25 oC, showing 

Figure 5.3 Redox-pH diagram for the Fe-CO2

-H2

showing speciation of iron (dashed lines) and stability fields of 

iron-bearing minerals (solid lines)............................................................................. 90 

O system at 25 oC, 

Figure 5.4	 Redox-pH diagram for the Fe-S-CO2

-H2O system at 25 oC, 

showing speciation of iron (dashed lines) and stability fields of 

iron-bearing minerals (solid lines)............................................................................. 91 

Figure 5.5	 Redox-pH diagram for the Fe-S-CO2-H2O system at 25 oC, 

showing speciation of iron (dashed lines) and stability fields of 

iron-bearing minerals (solid lines)............................................................................. 91 

Figure 5.6	 Redox-pH diagram for the Fe-CO2

-H2O system at 25 oC, 

showing speciation of iron (dashed lines) and stability fields of 

iron-bearing minerals (solid lines)............................................................................. 92 

x 
Figure 5.7	 Redox-pH diagram for the Fe-CO2

-H2O system at 25 oC,


showing speciation of iron (dashed lines) and stability fields of


iron-bearing minerals (solid lines)............................................................................. 92


Figure 5.8	 Upgradient ground-water compositions (molar ratios)


and TDS values for the Elizabeth City and Denver


Federal Center PRB sites ........................................................................................ 93


Figure 5.9	 Comparison of total dissolved solids concentrations at


PRB sites studied in the Tri-Agency initiative ........................................................... 93


Figure 5.10 Solubility diagram showing the stability field of carbonates


as a function of pH and log activities of Ca, Fe, Mg, and


dissolved inorganic carbon compared to ground-water


compositions from upgradient, iron wall, and


downgradient sampling locations (Elizabeth City PRB) ............................................ 95


Figure 5.11 Saturation indices of magnesium-bearing phases (brucite,


Mg(OH)2

; sepiolite, Mg4(OH)2Si6O15·H2O) as a function of pH


in ground water from upgradient, iron wall, and downgradient


sampling locations (Elizabeth City PRB) .................................................................. 98


Figure 5.12 Conceptual model of the impact of mineral and


biomass accumulation to PRB hydraulic performance ............................................. 99


Figure 5.13 Fractional porosity reduction as a function of inorganic


carbon concentration in the solid phase. ................................................................ 100


Figure 5.14 Fractional porosity reduction as a function of sulfur


concentration in the solid phase. ............................................................................ 102


Figure 5.15 Fractional porosity reduction as a function of the positive


molar volume change as iron metal reacts to form


magnetite, hematite, goethite, and ferrihydrite ....................................................... 102


Figure 5.16 Concentration versus time in batch tests: a) chromium; b) TCE ............................ 105


Figure 5.17 Comparison of PLFA distribution in four iron walls ................................................. 107


Figure 5.18 Water levels in Elizabeth City monitoring wells ...................................................... 109


xi 
Tables 

Table 2.1 Comparison of PRBs investigated in this study.......................................................... 6


Table 3.1	 Contaminant concentrations in ground water upgradient


of the Elizabeth City PRB ......................................................................................... 11


Table 3.2	 Contaminant concentrations in ground water downgradient


of the Elizabeth City PRB ......................................................................................... 12


Table 3.3	 Geochemical parameters in upgradient, iron wall, and


downgradient locations, Elizabeth City PRB ............................................................ 18


Table 3.4	 Changes in concentrations of metals in transect 2 as a


function of time and depth below ground surface ..................................................... 27


Table 3.5	 Changes in concentrations of anions in transect 2 as a


function of time and depth below ground surface ..................................................... 32


Table 3.6 Cores collected for analysis at the Elizabeth City PRB ............................................ 35


Table 3.7	 Results of powder X-ray diffraction analysis of core


materials from the Elizabeth City PRB ..................................................................... 41


Table 3.8	 Pearson’s correlation matrix of element concentrations


determined by SEM-EDX analysis ........................................................................... 46


Table 3.9 Summary of PLFA data from the Elizabeth City PRB .............................................. 47


Table 4.1	 Cores collected for analysis at the Denver Federal


Center PRB .............................................................................................................. 70


Table 4.2	 Results of powder X-ray diffraction analysis of core


materials from the Denver Federal Center PRB....................................................... 76


Table 4.3	 Results of SEM-EDX analysis of core materials from


The Denver Federal Center PRB ............................................................................. 80


Table 4.4 Summary of PLFA data from the Denver Federal Center PRB ................................ 81


Table 5.1 Mineral precipitates identified in iron walls ............................................................... 94


Table 5.2 Molar volume and density of mineral precipitates .................................................. 101


Table 5.3 Core samples used in batch reactivity tests ........................................................... 103


Table 5.4 Water compositions used in batch reactivity tests.................................................. 104


Table 5.5	 Summary of rate data for reactions of TCE and 1,1,1-TCA


with zero-valent iron (unreacted and from field PRBs) ........................................... 106


xii 
Appendix A-1: Inorganic carbon and sulfur concentrations in cores


collected from the Elizabeth City and Denver Federal


Center PRB sites ............................................................................................ 122-126


Appendix A-2: Reduced Sulfur Speciation in Elizabeth City and


Denver Federal Center Cores ................................................................................ 127


Appendix A-3: Inorganic Carbon and Sulfur Concentrations in


Denver Federal Center Cores ......................................................................... 128-133


Appendix B-1: Phospholipid fatty-acid (PLFA) extract data from cores


collected from the Elizabeth City and Denver Federal


Center PRB sites ............................................................................................ 136-140


xiii 
Acknowledgments 

The authors would like to acknowledge all of the participants of the Tri-Agency PRB research initiative, in particular 

Arun Gavaskar (Battelle), Bruce Sass (Battelle), Neeraj Gupta (Battelle), Woong-Sang Yoon (Battelle), Libby West 

(Oak Ridge National Laboratory), Nic Korte (Oak Ridge National Laboratory), Liyuan Liang (Cardiff University), and 

Matt Turner (Interstate Technologies Regulatory Cooperation). Members of the Permeable Reactive Barriers 

Research Team contributed to the research described in this report, especially S. Acree, F. Beck, P. Clark, K. Jones, 

M. McNeil, C. Paul, and C. Su. Mantech Environmental Research Services Corporation provided analytical support 

both in the field and in the laboratory. T. Sivavec (General Electric Corporate) provided X-ray photoelectron 

spectroscopic analysis of iron core materials. We thank R. Ford (U.S. EPA) for many discussions throughout this 

study and D. Myers (East Central University) for the scanning electron microscopy. Shim Myung-Hwa, a visiting 

student from the Kwangju Institute of Science and Technology (South Korea), is acknowledged for her help on parts 

of this study. Martha Williams (CSC) provided support in document preparation. J.P. Messier (U.S. Coast Guard 

Support Center) is thanked for providing site assistance at the Elizabeth City site and C. Eriksson (Pacific Western 

Technologies, Ltd.), J. Jordon (Pacific Western Technologies, Ltd.), and M. Gasser (Pacific Western Technologies, 

Ltd.) are thanked for site assistance at the Denver Federal Center site. We also acknowledge the U.S. Coast Guard 

for access to the Elizabeth City site and the Federal Highway Administration and General Services Administration for 

access to the Denver Federal Center site. Reviews of the document were provided by Eric Reardon (University of 

Waterloo), Liyuan Liang (Cardiff University), Steve Shoemaker (Dupont), Robert Powell (Powell and Associates), 

Carl Eriksson (Pacific Western Technologies, Ltd.), and Thomas Holdsworth (U.S. EPA); their thoughtful comments 

are appreciated. 

xiv 
1.0 Introduction 

The permeable reactive barrier (PRB) technology has gained acceptance as an effective ground-water remediation 

strategy for the treatment of a variety of chlorinated organic and inorganic compounds. The technology combines 

subsurface fluid-flow management with contaminant treatment by chemical, physical or biological processes, or by 

combinations of these three principal process categories. The PRB methodology has advantages over traditional pumpand-treat systems in that it is passive and a large plume can be treated in a cost-effective manner. More than one 

hundred implementations of the technology worldwide have proven that passive reactive barriers can be cost-effective 

and efficient approaches to remediate a variety of compounds of environmental concern. Yet, few case studies are 

available that evaluate the long-term performance of these in-situ systems, especially with respect to the long-term 

efficiency of contaminant removal, the buildup of mineral precipitates, and the buildup of microbial biomass (e.g., 

O’Hannesin and Gillham, 1998; McMahon et al., 1999; Puls et al., 1999a; Vogan, 1999; Phillips et al., 2000; Gavaskar et 

al., 2002; Liang et al., 2002; Wilkin et al., 2002). 

Granular iron is most often used as a reactive media in full-scale site remediation efforts using the PRB technology. The 

prevalent use of zero-valent iron mainly stems from its low cost (approx. $350 to $450 per ton in 2002), availability, and 

documented ability to degrade a wide variety of contaminant types. In the case of chlorinated volatile organic 

compounds such as perchloroethylene (PCE) and trichloroethylene (TCE), contaminant removal by zero-valent iron may 

be due to direct electron transfer, reaction with Fe2+ produced during anaerobic iron corrosion, or due to catalytic 

hydrogenation reactions (e.g., Matheson and Tratnyek, 1994; Gillham and O’Hannesin, 1994; Johnson and Tratnyek, 

1994; Roberts et al., 1996; Orth and Gillham, 1996; Fennelly and Roberts, 1998). In the case of inorganic contaminants 

such as U, Cr, and As, contaminant removal may be achieved through reductive precipitation or adsorption (e.g., Cantrell 

et al., 1995; Blowes et al., 1997, 2000; Powell et al., 1998; Fiedor et al., 1998; Lackovic et al., 2000; Morrison et al., 2001, 

2002). New insight regarding the reactive behavior of zero-valent iron and a more detailed understanding of reaction 

kinetics and reaction pathways involving zero-valent metals continues to emerge from laboratory studies (e.g., Burris et 

al., 1995; Agrawal and Tratnyek, 1996; Wüst et al., 1999; Deng et al., 1999; Su and Puls, 1999; Nam and Tratnyek, 2000; 

Schlicker et al., 2000; Arnold and Roberts, 2000; Farrell et al., 2000; Ruiz et al., 2000; Su and Puls, 2001; Melitas et al., 

2001; Scherer et al., 1998; Lien and Wilkin, 2002; Alowitz and Scherer, 2002; Köber et al., 2002). 

In addition to reaction processes involving contaminant species, zero-valent iron also impacts the biogeochemical 

behavior of the typically more concentrated assortment of ground-water solutes. Reaction processes that involve the 

major anionic (e.g., Cl-, SO4

2-, HCO3-

) and major cationic ground-water components (e.g., Ca2+, Mg2+) govern the kinetics 

and pathways of iron corrosion, mineral precipitation, microbial activity, and gas production within and around the 

reactive media (e.g., Reardon, 1995). The cumulative effect of these processes through time can lead to changes in the 

reactivity, porosity, and hydraulic permeability of a PRB. 

Research results described in this report explore the geochemical and microbiological processes occurring within zerovalent iron PRBs that may contribute to changes in iron reactivity and decreases in reaction zone permeability that, in 

turn, may eventually lead to system failure or plugging. Field studies were carried out at two geographically, 

hydrogeologically, and geochemically distinct iron barrier installation sites (U.S. Coast Guard Support Center, Elizabeth 

City, NC; and Denver Federal Center, Lakewood, CO). The research approach consisted of intensive ground-water 

sampling, mineralogical and microbiological characterization of core materials, and geochemical modeling to compare 

expected trends in the type and mass of mineral precipitates with observations from the field. Specific objectives of this 

research project were to: 

1)	 Evaluate spatial and temporal trends in contaminant concentrations and key geochemical parameters that 

may impact reactivity and steady system performance. 

2)	 Characterize the type and properties of surface precipitates forming over time at the upgradient aquifer/iron 

interface, within the iron media, and at the downgradient/iron interface. 

3) Develop conceptual models that predict the type and rate of precipitate formation based on iron characteristics 

and water chemistry. 

1 
4)	 Identify the type and extent of microbiological activity at the upgradient aquifer/iron interface, within the iron 

media, and at the downgradient/iron interface. 

5)	 Define practical and cost-effective protocols for long-term performance assessments at permeable reactive 

barrier installations. 

A detailed analysis of the rate of surface precipitate buildup in PRBs is critical for understanding how long these systems 

will remain effective and what methods may be employed to extend their lifetime or improve their performance (e.g., 

Geiger et al., 2002). Different types of minerals and surface coatings have been observed to form under different 

geochemical conditions that would appear to be dictated by aquifer chemistry and the composition of the permeable 

reaction zone (Powell et al., 1995; Mackenzie et al., 1999; Puls et al., 1999b; Liang et al., 2000; Phillips et al., 2000; Bonin 

et al., 2000; Roh et al., 2000; Wilkin et al., 2002; Furukawa et al., 2002). Furthermore, microbiological impacts are also 

important to understand in order to better predict how long these systems will remain effective in the subsurface 

(Matheson, 1994; Weathers et al., 1997; Till et al., 1998; Gu et al., 1999, 2002; Scherer et al., 2000; Gandhi et al., 2002). 

The presence of a large reservoir of iron coupled with plentiful substrate availability in the form of hydrogen supports the 

metabolic activity of iron-reducing, sulfate-reducing, and/or methanogenic bacteria. This enhanced microbial activity 

may beneficially influence zero-valent iron reductive dehalogenation reactions through favorable impacts to the iron 

surface or through direct microbial transformations of the target compounds. However, this enhancement may come at 

the expense of faster corrosion leading to faster precipitate buildup and potential biofouling of the permeable treatment 

zone. 

1.1 Federal Tri-Agency Initiative 

Research described in this report was carried out as part of a Tri-Agency cooperative effort between the United States 

(U.S.) Department of Defense (DoD), the U.S. Department of Energy (DOE), and the U.S. Environmental Protection 

Agency (EPA). This collaborative initiative allowed the three federal agencies to leverage technical and funding 

resources and to share experiences at over 10 PRB installations across the U.S. The primary goal of the research 

initiative was to evaluate the longevity and hydraulic performance of zero-valent iron PRBs in various hydrogeological 

and geochemical settings. Members of the Tri-Agency initiative met periodically and conducted regular conference calls 

to discuss research progress. Results of research projects conducted by members of the Tri-Agency effort are reported 

in Gavaskar et al. (2002), Liang et al. (2002), and Wilkin et al. (2002). A combined final product outlining conclusions of 

the Tri-Agency study is expected in 2003. 

This report provides a detailed exploration of long-term monitoring results obtained over the initial 5-year operation 

period of PRBs at the U.S. Coast Guard Support Center (Elizabeth City, NC) and the Denver Federal Center (Lakewood, 

CO). Results of the study are presented in two volumes. Volume 1 (this volume) presents a performance assessment 

and data on the geochemical and microbiological factors that impact the performance of zero-valent iron PRBs. Volume 

2 presents ground water monitoring practices and procedures employed in this study as well as soil core collection, 

preservation, and analysis methods. The remainder of this section presents the general scope and major conclusions of 

the DoD, DOE, and EPA contributions to the Tri-Agency collaborative effort. 

1.1.1 DoD Studies 

Results of the DoD study on PRB longevity and hydraulic performance are reported in Gavaskar et al. (2002). Field data 

were collected and analyzed from PRBs at several DoD sites. The longevity evaluation focused primarily on PRBs at the 

Moffett Field former Naval Air Station (CA) and the former Lowry Air Force Base (CO). Both PRBs have a funnel-andgate design and were installed prior to 1996. The longevity evaluation consisted of geochemical monitoring and 

modeling of ground water, iron core collection and analysis, and accelerated column tests. Hydraulic performance was 

also studied at the Seneca Army Depot (NY, installed 1998) and the Dover Air Force Base (DE, installed 1997). 

Methodologies used for the hydraulic performance testing were water level measurements and a variety of available insitu flow sensors. 

Five years after installation of the Moffett Field PRB, concentrations of TCE, PCE, and cis-1,2 dichloroethylene (cisDCE) in effluent ground water from the reactive cell were all below their respective maximum concentration levels 

(MCLs). Treatment of contaminants occurred mainly in the upgradient region of the reactive media. As of 2001, a clean 

front of ground water had not been identified in the downgradient aquifer, although there was an indication that a clean 

front would occur in the future. Similar results with respect to contaminant treatment were observed at the former Lowry 

Air Force Base (AFB). At the Moffett Field and Lowry AFB sites, concentrations of dissolved calcium, iron, magnesium, 

sulfate, nitrate, alkalinity, silica, and total dissolved solids flowing to the PRBs were significantly reduced in effluent 

ground water compared to influent ground water. At both sites, pH values within the reactive media rose to levels as high 

as 11.5 and ORP values dropped to as low as –821 mV. Solid-phase characterization studies were carried out to 

evaluate the mineralogy of precipitates that formed in the reactive media. 

2 
Careful and periodic water level measurements were found to give the best results with respect to hydraulic performance 

monitoring. In-situ, direct-flow measurements in some cases gave flow direction results that were contradictory from 

those indicated by water level measurements. This was thought to be due to the fact that in-situ techniques were point 

estimates and more indicative of localized flow conditions, whereas water level trends are more indicative of the overall 

flow regime around the PRBs studied. Estimated effective residence times varied from about 9 days at the Moffett Field 

site to about 25 days at the Lowry AFB. 

1.1.2 DOE Studies 

Results of the DOE study are presented in two reports (Liang et al., 2002; Moline et al., 2002) and recent publications 

(e.g., Kamolpornwijit et al., 2003; Liang et al., 2003). The DoE study focused on PRBs in Monticello, Utah and at the Y12 Pathway-2 in Oak Ridge, Tennessee. The Monticello site was a former vanadium and uranium ore-processing mill. 

A zero-valent iron funnel-and-gate system was installed in July 1999 to treat uranium-contaminated ground water. The 

Y-12 Pathway-2 PRB was installed in November 1997 as a trench that was intended to capture shallow ground-water 

flow contaminated with uranium. The DOE study consisted of geochemical and hydrogeologic monitoring, core 

sampling, column tests, and geochemical modeling. 

Slug tests at the Monticello barrier in wells immediately upgradient of the PRB resulted in hydraulic conductivity values 

of 3.1 to 27 m/d. Colloidal borescope measurements showed particle velocities ranging from 4.3 to 43 m/day. The 

borescope measurements showed evidence of vertical variability in flow conditions within each of the wells tested. A 

multiparameter tracer test conducted one year after the PRB was installed resulted in upgradient seepage velocities of 

0.36 to 3.6 m/d. Tracer tests were conducted by injecting anionic tracers (bromide, iodide) or dissolved gas tracers 

(helium, neon, argon), and monitoring their appearance in a network of adjacent monitoring wells. Within the PRB at 

Monticello, considerable lateral transport was observed. 

The DOE report indicates that flow directions and velocities are dependent on the type of tracer test used. A general 

conclusion presented was that an analysis of water levels provides information about average gradients but may be 

difficult to interpret within the context of site heterogeneity. Potentiometric surfaces have the most value for regional 

conceptualization of water flow patterns and for delineating gross features such as ground-water mounding. On the 

other hand, tracer tests provide definitive results on a more local scale, but the scale of the measurement needs to be 

considered when interpreting such data or when extrapolating trends to adjacent aquifer regions (Moline et al., 2002). 

Liang et al. (2002; 2003) used the geochemical equilibrium model PHREEQC to evaluate mineral saturation indices in 

waters from several sites including the Monticello and Y-12 Plant PRBs. This analysis was carried out to understand the 

types and quantities of minerals that might form as water passes through and chemically equilibrates with the reactive 

medium. The results of the modeling show that the buffering capacity and flow rate of the influent ground water is 

important in determining the equilibrium pH in the Fe0

 media and in effluent water from Fe0 columns. The predicted 

spatial distribution of secondary minerals based on pore water chemistry provides a direct indication of changing flow 

characteristics over time and has been shown to be in agreement with the results of tracer testing at the Y-12 site. 

Column tests were carried out at the Y-12 Plant site to examine iron deterioration processes, changes to influent water 

chemistry after reaction with iron metal, and mineral precipitation processes. Column tests were run for over 14 months. 

In the column experiments, in-situ ground water was used as the input fluid. Two different flow rates were used (0.09 and 

1.8 m/day) to test the effect of seepage velocity on geochemical performance. Results of the column tests showed that 

heterogeneous flow (preferential flow) conditions developed as a result of mineral precipitation and gas production in the 

column. Tracer tests in the columns showed that pH and hydrologic residence time were closely linked. Therefore, pH 

may be a key indicator of residence time in PRB installations. 

1.1.3 EPA Studies 

Preliminary results of EPA’s long-term performance study were reported in Wilkin et al. (2002). Geochemical and 

microbiological factors that control long-term performance of PRBs were evaluated at the Elizabeth City, NC and the 

Denver Federal Center, CO sites. These ground-water treatment systems use zero-valent iron granules (Peerless Metal 

Powders, Inc.) to intercept and remediate chlorinated volatile organic compounds at the Denver Federal Center (funneland-gate system) and overlapping plumes of hexavalent chromium and chlorinated compounds at Elizabeth City 

(continuous wall system). Zero-valent iron at both sites is a long-term sink for C, S, Ca, Si, N, and Mg. Based on an 

analysis of mineral precipitate abundance in core materials, after about four years of operation the average rates of 

inorganic carbon (IC) and sulfur (S) accumulation were determined to be 0.09 and 0.02 kg/m2

y, respectively, in the 

Elizabeth City PRB where upgradient waters contain <400 mg/L of total dissolved solids (TDS). At the Denver Federal 

Center site, upgradient ground water contains 1000-1200 mg/L TDS and rates of IC and S accumulation were 

determined to be as high as 2.16 and 0.80 kg/m2y, respectively. At both sites, consistent patterns of spatially variable 

mineral precipitation and microbial activity were observed. Mineral precipitates and microbial biomass accumulate the 

fastest near the upgradient aquifer-Fe0 interface. After four years, maximum net reductions in porosity, due to the 

3 
accumulation of sulfur and inorganic carbon precipitates, range from 0.032 at Elizabeth City to 0.062 at the Denver 

Federal Center (gate 2). While pore space has been lost due the accumulation of authigenic components, neither site 

showed evidence of pervasive pore clogging after four years of operation. 

The following sections of this report provide descriptions of the Elizabeth City and the Denver Federal Center PRB sites, 

and results and analysis of the first five-year monitoring period for contaminant distributions, ground-water chemistry, as 

well as mineralogical and microbiological characterization of material that has accumulated within the reactive barriers. 

4 
2.0 Site Descriptions 

The U.S. Coast Guard Support Center (USCG-SC) site near Elizabeth City, North Carolina, and the Denver Federal 

Center (DFC) in Lakewood, Colorado were evaluated in the EPA portion of the Tri-Agency PRB initiative. Both of these 

PRB installations are among the oldest full-scale systems available for study. The Elizabeth City PRB was installed in 

June 1996, and the Denver Federal Center system was installed in October 1996. The two sites are of similar age; they 

use the same type of iron (Peerless Metal Powders, Inc.), yet they have contrasting ground-water chemistry and different 

design configurations (continuous wall configuration at Elizabeth City vs. funnel-and-gate design at the Denver Federal 

Center). Table 2.1 provides a general comparison of the PRBs at Elizabeth City and the Denver Federal Center. 

2.1 U.S. Coast Guard Support Center 

The USCG-SC is located about 100 km south of Norfolk, Virginia and 60 km inland from the Outer Banks region of North 

Carolina. The base is situated on the southern bank of the Pasquotank River, about 5 km southeast of Elizabeth City, 

North Carolina. A hard-chrome plating shop was in operation for more than 30 years in Hangar 79, which is only 60 m 

south of the river (Figure 2.1). Following its closure in 1984, soils beneath the shop were found to contain chromium 

concentrations up to 14,500 mg/kg. Subsequent site investigations by U.S. EPA personnel identified a chromate plume 

extending from beneath the shop to the river. The plume has high (>10 mg/L) concentrations of chromate, elevated 

sulfate (to 150 mg/L), and minor amounts of volatile chlorinated organic compounds: TCE, cis-DCE, and vinyl chloride 

(VC). The plating shop soils and related ground-water contamination are referred to as solid waste management unit 

(SWMU) number 9 by the state of North Carolina and the USCG. Sampling results from a monitoring network consisting 

of more than 40 monitoring wells and about 100 HydropunchTM and GeoprobeTM monitoring points indicate that the 

Cr(VI) plume is about 35 m wide, extends to 6.5 m below ground surface and extends laterally about 60 m from the 

hangar to the Pasquotank River (Figure 2.1). Multilevel samplers installed near the barrier wall location indicate that the 

bulk of the contamination resides from 4.5 to 6.5 m below ground surface. 

The site geology has been described in detail elsewhere (e.g., Puls et al., 1999b), but essentially consists of typical 

Atlantic coastal plain sediments, characterized by variable sequences of surficial sands, silts and clays. In general, the 

upper 2 m of the aquifer consists of sandy- to silty-clays that pinch out toward the north, or near the Pasquotank River, 

where sandy-fill predominates. Fine-sands, with varying amounts of silt and clay, and silty-clay lenses form the rest of 

the shallow aquifer. 

Ground-water flow velocity is extremely variable with depth, with a highly conductive layer at roughly 4.5 to 6.5 m below 

ground surface. As noted above, this layer coincides with the highest aqueous concentrations of chromate. The groundwater table ranges from about 1.5 to 2.0 m below ground surface and the average horizontal hydraulic gradient varies 

from 0.0011 to 0.0033. Slug tests conducted on monitoring wells with 1.5 m screened intervals between 3 and 6 m below 

ground surface indicate hydraulic conductivity values of between 0.3 to 8.6 m/d. A multiple borehole tracer test in wells 

screened between 3.9 to 5.9 m below ground surface was conducted. Ground-water velocities between about 0.13 and 

0.18 m/d were measured in this test. Assuming an average hydraulic gradient of 0.0023 and an effective porosity of 0.38, 

these flow velocities correspond to an average hydraulic conductivity of about 26 m/d. 

In June of 1996, a 46 m long, 7.3 m deep, and 0.6 m wide permeable reactive barrier (continuous wall configuration) of 

zero-valent iron (Peerless Metal Powders, Inc.) was installed approximately 30 m from the Pasquotank River (Figure 2.1; 

Blowes et al., 1999a,b). The reactive wall was designed to remediate hexavalent chromium-contaminated ground water 

and portions of the larger overlapping plume of volatile chlorinated organic compounds. A detailed monitoring network 

of over 130 subsurface sampling points was installed in November of 1996 to provide detailed information on spatial and 

temporal changes in pore water geochemistry and hydrology (Blowes et al., 1999a; Puls et al., 1999a). 

2.2 Denver Federal Center 

The Denver Federal Center (DFC) is located about 10 km west of downtown Denver, Colorado. Aquifer materials at the 

site consist of alluvial sediments that overlie the Denver Formation. The Denver Formation is Paleocene to Late 

Cretaceous in age and consists of brown, yellowish-brown, gray, and blue-gray intercalated sandstone, claystone, 

siltstone, shale and conglomerate containing olive-brown andesitic sandstone beds. It lies about 2 to 14 m below ground 

5 
Table 2.1. Comparison of PRBs Investigated in this Study


Contaminants PRB 

Configuration 

Date 

Installed 

Iron 

Dimensions 

Iron 

Volume 

Ground 

water, 

SC 

(�S/cm) 

Ground 

water, 

pH 

Ground 

water, 

DO 

(mg/L) 

U.S. 

Coast 

Guard 

Support 

Center 

Cr(VI) 

TCE, cis-DCE 

Continuous 

wall 

6/96 	 46 m length 

7.3 m deep 

0.6 m wide 

150 m3

 307±149 

(n=18) 

5.86±0.25 

(n=18) 

0.7±0.5 

(n=17) 

Denver 

Federal 

Center 

TCE, TCA, cisDCE Funnel-andgate 

Gate 1 

Gate 2 

Gate 3 

10/96 

12.2 m 

length 

8.5 m deep 

1.8 m wide 

12.2 m 

length 

9.5 m deep 

1.2 m wide 

12.2 m 

length 

7.3 m deep 

0.6 m wide 

187 m3 1236±65 7.14±0.15 0.5±0.2 

(n=3) 

139 m3 1358±10 7.19±0.08 0.2±0.1 

(n=3) 

53 m3 1306±10 7.06±0.07 <0.05 

(n=2) 

Notes: Geochemical parameters from Elizabeth City are average values (± 1 s.d.) from upgradient monitoring well MW48. All 

parameters monitored quarterly from 2/97 to 8/01 and biannually since 8/01. Geochemical parameters from DFC gates 1, 2, 3 are 

average values of wells GSA21, GSA 26, and GSA31 from 7/00 to 7/01. SC is specific conductance. DO is dissolved oxygen. DFC 

gate 4 was not studied in this investigation. 

surface at the DFC and can attain a thickness of up to about 260 m. The Denver Formation has been divided into two 

zones, the upper weathered zone and a lower unweathered zone. These two zones are lithologically similar but differ in 

color. The upper weathered zone is up to 7 m thick and exhibits a grayish brown color with yellowish orange staining 

while the lower unweathered zone has a diagnostic blue color, commonly called “Denver Blue.” 

There are two separate deposits of alluvial sediments in the vicinity of the DFC. The Verdos Alluvium of Pleistocene age 

is a poorly sorted, stratified gravel containing lenses of sand, silt, and clay. In some surface-drainages, the Denver 

Formation may be overlain by the Piney Creek Alluvium, which consists of well-stratified sands, silts and clays with 

interbedded gravels. 

Ground water in the alluvial sediments at the site generally moves from west to east with an average hydraulic velocity 

of about 0.3 m per day and a range between about 0.03 and 0.5 m per day (Pacific Western Technologies, 2000). 

Shallow ground water is contaminated with volatile organic compounds including TCE, cis-DCE, VC, 1,1,1-trichloroethane 

(1,1,1-TCA), and 1,1-dichloroethylene (1,1-DCE). At the eastern boundary of the site, maximum concentrations entering 

gate 2 of TCE, cis-DCE, 1,1,1-TCA, and 1,1-DCE were about 80 μg/L, 1.6 μg/L, 200 μg/L, and 230 μg/L, respectively, 

when the PRB was constructed in November 1996 (FHWA, personal communication, 2002). At least one source of 

6 
0 0 m 

Approx. Scale 

MW13 

MW18 

MW35D 

MW52 

MW38 

MW46 

MW49 

MW47 MW50

MW48 

BUILDING 78 

HANGAR 79 

0.05 mg/L 

Plating 

Shop 

ML35 

ML34 

ML33 

ML32 

ML31 

ML15 

ML14 

ML13 

ML12 

ML11 

ML25 

ML24 

ML23.5 

ML22.5 

ML21 

ML 

MW Compliance well 

(5 - 10 ft. screen) 

Multilevel bundle 

(6 in. screens) 

N 

Groundwater 

flow direction 

PRB 

1 

m

g/L 

1 mg/L 

0.05

mg/L 

Cr 

Cr 

Transect 3 

Transect 2 

Transect 1 

1

Figure 2.1 Plan view map of the PRB at the U.S. Coast Guard Support Center, Elizabeth City, NC. 

these volatile organic compounds was a leaking underground storage tank located near Building 52 that was used by the 

Federal Highway Administration (FHWA) to store waste, primarily 1,1,1-TCA. Ground-water flow from the aquifer 

discharges into McIntyre Gulch (Figure 2.2). McIntyre Gulch is a deep channel that penetrates the aquifer along the 

southern edge of the contaminant plume. Downing Reservoir is too shallow to be influenced by the aquifer, but the 

reservoir stage does affect the ground-water level. 

In the fall of 1996, FHWA and the General Services Administration (GSA) installed a permeable reactive barrier at the 

eastern edge of the DFC property along north-south trending Kipling Street (Figure 2.2). In contrast to the continuous 

wall design used at the USCG-SC, the DFC PRB has a funnel-and-gate design configuration. The funnel component of 

the PRB employs metal sheet pile that was driven into unweathered bedrock of the Denver Formation or into resistant, 

weathered layers of the Denver Formation. The depth of penetration of the funnel ranged from about 7 to 10 m 

(Figure 2.3). The PRB has 4 reactive gates, each 12.2 m long, 9.5 m deep, and from 1.8 m to (gate 1) to 0.6 m (gates 

3 and 4) wide (Table 2.1). The design thickness varied because of anticipated differences of contaminant fluxes to the 

PRB at different locations. Peagravel zones (0.6 to 1.2 m) were installed immediately upgradient and downgradient of 

the reactive iron zones to improve hydraulic connection between the aquifer and the PRB. 

7 
0 m 75 

Downing

Reservoir 

McIntyre

Gate 1 Gulch

Gate 2 

Gate 3 

DENVER FEDERAL CENTER 

N 

KIPLING STREET 

Gate 4 

Ground water 

flow direction 

Figure 2.2 Plan view map of the PRB at the Denver Federal Center, Lakewood, CO (after McMahon et al., 1999). 

1700


1680


0 92 183 275 366


aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

Unweathered 

Bedrock 

Bottom of Sheetpile 

Weathered 

Bedrock 

Alluvium 

water table Alluvium 

Gate 1 

Gate 2 

Gate 3 Gate 4 

Elevation, m


Distance, m


Figure 2.3	 Schematic cross-section of the Denver Federal Center funnel-and-gate system (after McMahon et al., 

1999). 

8

3.0 Elizabeth City PRB Monitoring Results 

3.1 Ground Water Monitoring 

Ten ground-water monitoring wells (two-inch PVC) located upgradient and downgradient of the iron wall were sampled 

on a quarterly basis from February 1997 to August 2001. Beginning in August 2001, sampling of these 10 compliance 

wells switched to a biannual schedule. In order to obtain more detailed spatial information on ground-water geochemistry 

and contaminant distributions in the subsurface, up to 130 multilevel well bundles were sampled on an annual basis. The 

monitoring well network at the Elizabeth City site is described in Blowes et al. (1999b). The locations of the monitoring 

wells and the multilevel well bundles in relation to the PRB at the Elizabeth City site are shown in Figure 2.1. 

Methods of ground-water and soil core sampling, preservation and analysis, in addition to Quality Assurance/Quality 

Control (QA/QC) procedures used in this five-year investigation of PRB performance are described and discussed in 

Volume 2 of this EPA Report series. 

3.1.1 Contaminant Behavior 

Ground-water contaminants near the PRB location at the Elizabeth City site are hexavalent chromium, TCE, cis-DCE, 

and VC. Figure 3.1 shows the time-dependent variations in the concentrations of chromium, TCE, and cis-DCE in 

monitoring wells MW48 and MW13, wells located upgradient of the PRB (see Figure 2.1). The concentration trends 

evident in these upgradient wells are non-steady state, particularly for TCE in MW48 (Figure 3.1). Concentrations of 

TCE in well MW48 have increased with time suggesting that the flux of TCE to the center portion of the reactive barrier 

also has increased over the initial five-year period of operation. Average, minimum, and maximum contaminant 

concentrations determined at upgradient monitoring points are listed in Table 3.1. The greatest concentrations of 

chromium and volatile organic compounds are present in ML21, the multilevel well bundle located just upgradient of and 

near the mid-point of reactive barrier. In ML21, the maximum TCE concentration observed was 42,400 μg/L at a depth 

of 6.4 m below ground surface (Table 3.1). Note that this concentration is much greater than the average, depthintegrated concentration of 2,585 μg/L from ML21, and the average TCE concentration in the proximate well MW48 

(716 ug/L), which is screened over a 10 ft interval from 4.3 m to 7.3 m below ground surface. The detection of TCE 

degradation products, cis-DCE and VC, in upgradient monitoring wells suggests that partial anaerobic bioattenuation of 

TCE is occurring in the plume before it reaches the PRB. 

The concentration of chromium entering the PRB is greatest near ML21 (average concentration 0.82 mg/L) and least 

near ML31 on the west side of the PRB (average concentration 0.032 mg/L). Taking an average chromium concentration 

of 0.5 mg/L over the depth interval from 3 m to 6 m below ground surface and an average flow velocity of 0.16 m/d, it is 

estimated that the reactive wall removes about 4.1 kg Cr per year. Over the first five years of operation an estimated 

21 kg of chromium has been removed from the ground-water plume and sequestered into immobile forms in the solid 

phase. 

Table 3.2 lists the concentrations of contaminants in monitoring wells located downgradient of the Elizabeth City PRB. 

Over the five year period from 1996 to 2001, there were a total of 150 sampling events in downgradient monitoring wells 

MW47, MW49, MW50, and in the multilevel well bundles ML15, ML25, and ML35. Chromium was detected in 13 

sampling events or in about 9% of the sampling events of wells located in downgradient positions. The highest 

concentration of chromium observed in a well downgradient of the PRB from 1996 to 2001 was 0.005 mg/L, significantly 

below the MCL and target treatment level for chromium of 0.050 mg/L. Where influent concentrations of chromium were 

the greatest, the best performance was observed in that 0 samples of 32 from multilevel well bundle ML25 showed 

detectable concentrations of chromium. The monitoring results presented in Table 3.2 show the sustained performance 

of the PRB for removing chromium from the ground-water plume. A trend of increasing chromium concentrations at 

downgradient locations is not evident after five years of operation. 

Average, minimum, and maximum concentrations of TCE, cis-DCE, and VC in downgradient monitoring wells are shown 

in Table 3.2. Inspection of the data in Table 3.2 reveals that average TCE concentrations in wells downgradient of the 

PRB have been significantly reduced compared to locations upgradient of the PRB, although average concentrations in 

downgradient monitoring wells MW49, MW50, and multilevel well bundle ML25 are above the MCL of 5 μg/L. Monitoring 

9 
60 

g/L 

MW48 

MW13 

50 

40 

30 

20 

10 

0 

Cr, mg/L cis-DCE, 

μ

6/1/1996 10/14/1997 2/26/1999 7/10/2000 11/22/2001 

1800 

1600 

1400 

1200 

1000 

800 

600 

400 

200 

0 

MW48 

MW13 

TCE, 

μg/L 

6/1/1996 10/14/1997 2/26/1999 7/10/2000 11/22/2001 

5 

4 

3 

MW48 2 MW13

1 

0 

6/1/1996 10/14/1997 2/26/1999 7/10/2000 11/22/2001 

Time 

Figure 3.1	 Concentrations of contaminants through time in monitoring wells located hydraulically upgradient of the 

Elizabeth City PRB (screened interval in MW48 and MW13 is 4.3 to 7.3 m below ground surface). 

well MW50, in particular, has typically shown TCE concentrations above 100 μg/L, especially prior to 2000 (Figure 3.2). 

Well MW50 is screened over a 5-foot interval from 7.6 m to 9.1 m below ground surface, a depth interval that extends 

below the PRB (maximum depth 7.3 m). Some underflow of the chlorinated solvent plume, therefore, is evident in the 

central portion of the PRB. This may have been caused by the disturbance of an unrecognized TCE source near the 

PRB and/or a large influx of recharge water into the aquifer near the PRB following installation and prior to repaving of 

the parking lot covering the excavation site. 

Monitoring results from multilevel bundles ML21 and ML25 indicate that concentrations of cis-DCE have been reduced 

from an average of 154 μg/L in upgradient ground water to an average concentration of 14 μg/L in downgradient ground 

water. Similarly concentrations of VC have been reduced by the PRB. The average VC concentration in multilevel well 

bundles ML21 and ML31 was 31 μg/L and 22 μg/L, respectively, from 1997 to 2001 (Table 3.1). In downgradient 

multilevel well positions ML25 and ML35, VC was detected in about 45% to 55% of the sampling events. When detected 

the average VC concentration in multilevel well bundles ML25 and ML35 was 4.3 μg/L and 2.3 μg/L, respectively. In 

general, concentrations of cis-DCE and VC decrease by about an order of magnitude as a consequence of processes 

that take place within the PRB. 

10 
--

Table 3.1. Contaminant Concentrations in Ground Water Upgradient from the PRB at Elizabeth City


Well 

MW48 

MW13 

ML11 

ML21 

ML31 

Cr (mg/L)


TCE (μg/L)


cis-DCE (μg/L)


VC (μg/L)


Cr


TCE


cis-DCE


VC


Cr


TCE


cis-DCE


VC


Cr


TCE


cis-DCE


VC


Cr


TCE


cis-DCE


VC


ntotal ndetected Average Minimum Maximum 

18 18 0.53 0.08 1.6 

17 17 716 111 1625 

18 12 8.31 <0.1 60 

18 3 4.0 <0.1 5.8 

18 18 3.8 2.9 4.7 

18 18 17 <0.1 62 

18 6 0.8 <0.1 6.5 

18 0 <0.1 <0.1 

32 25 0.75 <0.002 2.5 

32 24 31 <0.1 89 

32 28 11 <0.1 56 

32 2 1.5 <0.1 1.5 

32 25 0.82 <0.002 3.43 

32 30 2585 <0.1 42400 

32 15 154 <0.1 384 

32 12 31 <0.1 52 

30 17 0.03 <0.002 0.08 

30 30 249 2.6 1160 

30 16 34 <0.1 103 

30 16 22 <0.1 43 

Notes: ntotal is the total number of samples analyzed; ndetected is the number of samples in which the 

contaminant was detected. Entries for ML11, ML21, and ML31 (multilevel bundles) are averages of all 

sampling depths. MW48 and MW13 data span the time period of November 1996 to February 2002. ML11 

and ML31 data were collected in 1997, 1998, and 2000 (annually). ML21 data were collected in 1997, 1998, 

1999, 2000, and 2001 (annually). Values below the analytical detection limit were excluded from the 

average calculation. Concentration units: Cr (mg/L), TCE (μg/L), cis-DCE (μg/L), VC (μg/L). 

A more detailed picture of contaminant behavior can be reached by inspecting 2-dimensional concentration profiles, 

constructed by contouring monitoring data from the multilevel well bundles. Figures 3.3 through 3.18 show crosssectional profiles of various dissolved solutes and geochemical parameters determined on an annual basis at Elizabeth 

City from 1997 to 2001. The locations of transects 1, 2, and 3 are shown in Figure 2.1. For each transect in the crosssectional profiles, the position of the PRB is referenced to the position of the furthest upgradient multilevel well bundle 

(i.e., ML11, ML21, and ML31). The distribution of subsurface sampling points is shown for transect 3 in the lower left 

hand corner of Figure 3.3 to provide an indication of the density of sampling data used to construct the contour diagrams. 

Cross-sectional profiles for dissolved chromium are shown in Figure 3.3. Long-term trends indicate that chromium 

continues to be removed from the ground-water plume after five years of operation. In general, depth-dependent 

concentration profiles have remained consistent with time. However, the dissolved chromium plume appears to be 

migrating to shallower depths at a rate of about 15 cm/y in the vicinity of transect 2 (Figure 3.3). In the vicinity of 

transect 1 there appears to be no vertical movement of the dissolved chromium plume. The upward movement of the 

plume in the vicinity of transect 2 may be linked to the emergence of the deep TCE plume as described below. 

More complicated transport behavior is evident for TCE based upon monitoring results from the multilevel well bundles. 

There are two separate TCE plumes, one shallow that coincides with the chromium plume (approx. 4 to 6 m below 

ground surface). This shallow plume containing TCE (plus cis-DCE and VC) is present in each of the three multilevel well 

transects. A second deeper plume containing TCE (minus cis-DCE and VC) appears only upgradient of the PRB in the 

vicinity of transect 2 (Figure 3.4). The shallow TCE plume is most concentrated and most shallow near transect 3, unlike 

11 
--

Table 3.2. Contaminant Concentrations in Ground Water Downgradient from the PRB at Elizabeth City


Well 

MW47 

MW49 

MW50 

ML15 

ML25 

ML35 

Cr (ppm)


TCE (μg/L)


cis-DCE (μg/L)


VC (μg/L)


Cr


TCE


cis-DCE


VC


Cr


TCE


cis-DCE


VC


Cr


TCE


cis-DCE


VC


Cr


TCE


cis-DCE


VC


Cr


TCE


cis-DCE


VC


ntotal ndetected Average Minimum Maximum 

18 2 0.002 <0.002 0.002 

18 15 6.0 <0.1 30.2 

18 17 7.0 <0.1 19.7 

18 11 5.5 <0.1 10.4 

18 4 0.003 <0.002 0.004 

18 11 9.0 <0.1 41 

18 4 1.6 <0.1 2.6 

18 8 5.0 <0.1 7.0 

18 4 0.003 <0.002 0.003 

18 18 204 <0.1 548 

18 17 15 <0.1 35 

18 14 7 <0.1 18 

32 2 0.004 <0.002 0.005 

32 8 1.8 <0.1 4.9 

32 9 1.5 <0.1 1.9 

32 8 2.1 <0.1 7.8 

32 0 <0.002 <0.002 

32 15 16 <0.1 81.6 

32 30 14 <0.1 74.6 

32 18 4.3 <0.1 10 

32 1 0.005 <0.002 0.005 

32 12 3.6 <0.1 9.1 

32 14 4.3 <0.1 13 

32 14 2.3 <0.1 4.2 

Notes: ntotal is the total number of samples analyzed; ndetected is the number of samples in which the 

contaminant was detected. Entries for ML15, ML25, and ML35 (multilevel bundles) are averages of all 

sampling depths. MW47, MW49, and MW50 data span the time period of November 1996 to February 

2002. ML15 and ML35 data were collected in 1997, 1998, and 2000 (annually). ML25 data were collected 

in 1997, 1998, 1999, 2000, and 2001 (annually). Values below the analytical detection limit were excluded 

from the average calculation. Concentration units: Cr (mg/L), TCE (μg/L), cis-DCE (μg/L), VC (μg/L). 

chromium, which is most concentrated near transect 2. These relationships suggest that rather than sharing identical 

sources, chromium and TCE are more likely being transported in a hydrologically favorable zone. Comparing both TCE 

plumes, the highest TCE concentrations are found in the deep plume at a depth range from about 6 to 7 m below ground 

surface immediately upgradient of the PRB near transect 2. Over this depth interval, TCE concentrations increased 

almost tenfold from 4,320 μg/L in 1997 to 42,400 μg/L in 2001. In downgradient multilevel well bundle ML25, TCE 

concentrations are below 10 μg/L in all but five samples collected between 1997 and 2001. However, TCE 

contamination extends beneath the maximum depth of the PRB near transect 2 and this explains the higher TCE 

concentrations detected in MW50 (Figure 3.2). 

Unlike TCE, the occurrences of cis-DCE and VC are generally restricted to the shallow plume or to depths of about 4 to 

6 m below ground surface (Figures 3.5 and 3.6). In the vicinity near transect 1, concentrations of cis-DCE are reduced 

to below detection limits as the contaminant plume passes through the PRB. In this region, the maximum concentration 

of cis-DCE observed upgradient of the PRB was 260 μg/L (Figure 3.5). The concentration of cis-DCE in regions 

immediately upgradient of the PRB increased from 1997 to 2000. The fact that neither cis-DCE nor VC are present in the 

12 
600 

500 

400 

MW47 

MW49 

MW50 

TC

E, μg/L 

300 

200 

100 

0 

6/1/1996 10/14/1997 2/26/1999 7/10/2000 11/22/2001 

Time 

Figure 3.2	 Concentrations of TCE (μg/L) through time in monitoring wells located hydraulically downgradient of the 

Elizabeth City PRB (screened intervals: MW47, 4.3-7.3 m; MW49, 4.3-7.3 m; MW50, 7.6-9.1 m). 

deeper plume near transect 2 suggests that appreciable bioattenaution of TCE in this area has not occurred. In transects 

2 and 3, concentrations of cis-DCE above 10 μg/L are sporadic in downgradient regions; however, with time, 

concentrations of cis-DCE appear to increase in regions downgradient of transect 2. 

3.1.2 Geochemical Parameters 

In this section, results for three geochemical parameters are presented and discussed: pH, Eh, and specific conductance. Table 3.3 lists average values for these parameters from the upgradient (ML11, ML21, and ML31), downgradient 

(ML15, ML25, and ML35), and iron wall (ML14, ML24, and ML34) multilevel samplers. The results of all field analyses 

are averaged to provide an overall view of the PRB impact to ground water chemistry. At Elizabeth City, the pH of 

ground-water entering the PRB is approximately 6, but varies in time and space from about 5.4 to 6.6. The pH increases 

to an average value of 9.5 within the iron media, and then decreases to a value of 7.7 in the aquifer approximately 0.5 m 

downgradient of the PRB. This trend in pH clearly shows that the PRB has an impact on the aqueous chemistry of the 

downgradient aquifer (Table 3.3). Similarly the Eh of water entering the PRB is moderately oxidizing (approx. 218 mV) 

and becomes moderately to highly reducing within the reactive media (approx. -260 mV). Moderately reducing water 

emerges from the PRB (approx. -2 mV). The decrease in Eh and increase in pH across the flow path are expected 

trends that result from the corrosion of iron in water as discussed in detail in a following section of this report. The 

specific conductance of ground water downgradient of the PRB is approximately 15% to 30% lower than that of 

upgradient ground water due to partitioning of dissolved solutes into the solid phase within the reactive media. 

Interestingly, dissolved oxygen results (data not shown) show essentially no variability between upgradient, iron, and 

downgradient positions. The reasons for this are likely due to the difficulty in obtaining highly accurate DO 

concentrations at low levels and in the presence of ferrous iron. 

Long-term trends in pH, Eh, and specific conductance are shown in Figures 3.7-3.9. In transect 2, the high pH zone 

caused by corrosion of the zero-valent iron has remained largely unchanged in space over the initial five-year period of 

operation (Figure 3.7). This result indicates that the zero-valent iron at Elizabeth City still retains some degree of 

reactivity even after five years of subsurface exposure. The pH values measured in upgradient regions have remained 

13 
14

Figure 3.3 Cross-sectional profiles showing total chromium concentrations (mg/L) in transects 1, 2, and 3, Elizabeth

City PRB.

0 1

-7

-6

-5

-4

-3

-2

PRB

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

-2

Cr, mg/L

2001

2000

-0.2

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

PRB

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

-2

Sampling point

s

PRB

1997 1998

flow

Distance, m

Transect

3

Depth,

m

Transect

2

Depth,

m

Transect

1

Depth,

m

2 3 4 1 2 3 4 1 2 3 4

1 2 3 4 2 3 4 2 3 4

2 3 1 2 3 2 3 2 3

2 3 4
15

50

200

350

50

200

350

0 1

-7

-6

-5

-4

-3

-2

PRB PRB PRB

2001

1997 1998 2000

Distance, m

Transect

3

Depth,

m

Transect

2

Depth,

m

Transect

1

Depth,

m

0

-7

-6

-5

-4

-3

-2

-5

0

5

10

15

20

25

30

35

40

45

0

-7

-6

-5

-4

-3

-2

Transect

2

Transect 1

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

-1000

500

2000

3500

5000

6500

8000

0

-7

-6

-5

-4

-3

>8 mg/L

0

-7

-6

-5

-4

-3

-2

TCE, μg/L

-100

50

200

350

500

650

800

950

1100

0

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

Transect

3

0

-7

-6

-5

-4

-3

-2

Sampling points

flow

Figure 3.4 Cross-sectional profiles showing TCE concentrations (μg/L) in transects 1, 2, and 3, Elizabeth City PRB.

2 3 4 1 2 3 4 1 2 3 4

1 2 3 2 3 2 3 1 2 3

1 2 3 4 1 2 3 4 2 3 4 1 2 3 4
16

Figure 3.5 Cross-sectional profiles showing cis-DCE concentrations (μg/L) in transects 1, 2, and 3, Elizabeth City

PRB.

10

10

70

130

190

10

10

10

70

70 130

0 1

-7

-6

-5

-4

-3

-2

PRB

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

-2

>30μμg/L

Transect 2

-2 2 6 10 14 18 22 26 30

0

-7

-6

-5

-4

-3

PRB PRB

cis-DCE, μg/L

Transect 3

Transect 1

2001

1997 1998 2000

Distance, m

Transect

3

Depth,

m

Transect

2

Depth,

m

Transect

1

Depth,

m

-50 10 70 130 190 250

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

>250 μg/L

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

-5 5 15 25 35 45 55

0

-7

-6

-5

-4

-3

-2

>55 μg/L

0

-7

-6

-5

-4

-3

-2

Sampling points

flow

2 3 4 1 2 3 4 1 2 3 4

1 2 3 1 2 3 2 3 1 2 3

2 3 4 2 3 4 1 2 3 4 1 2 3 4
-2 -2 12 50 40 

-2 

10 43 34 

-3 -3 

-3 8.8 36 29 

7.1 29 23 -4 -4 

-4 5.5 23 18 

-5 -5 3.9 16 12 

-5 2.3 8.8 6.3 

-6 -6 -6 0.63 1.9 0.63 

-1.0 -5.0 -5.0 

1997 1998 2000 

-7

PRB 

-7 -7 

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 VC,g/L 

-3 

PRB 

-3 -3 -3 2001 

-4 -4 -4 -4 

-5 -5 -5 -5 

-6 -6 -6 -6 

-7 -7 -7 -7 

0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 

-2

PRB 

-2 -2 -2 

Sampling points 

flow 

Transect 

2

-3 -3 -3 -3 

-4 -4 -4 -4 

Transect 3 

-5 -5 -5 -5 

-6 -6 -6 -6 

-7 -7 -7 -7 

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 

Distance, m 

Transect 1 

Depth, m Depth, m 

Transect 1 

Transect 3 

Transect 2


Depth, m


Figure 3.6 Cross-sectional profiles showing VC concentrations (μg/L) in transects 1, 2, and 3, Elizabeth City PRB.


17 
Table 3.3. Geochemical Parameters in Upgradient, Iron Wall, and Downgradient Locations, Elizabeth City PRB


Location Eh pH Specific Conductance 

(mV) (�S/cm) 

Upgradient 218 6.02 390 

(± 311) (± 0.24) (± 177) 

Iron wall -258 9.53 186 

(± 212) (± 0.72) (± 105) 

Downgradient -2 7.71 192 

(± 190) (± 1.14) (± 110) 

Notes: Average value and 1 s.d. reported for upgradient (ML11, ML21, and ML31), iron wall (ML14, ML24, 

and ML34), and downgradient (ML15, ML25, and ML35) well locations. 

largely unchanged in time and space. Similarly, temporal and spatial distribution of pH within the iron media and in 

downgradient regions has remained consistent since 1997 (Figure 3.7). Transects 1 and 3 also show fairly consistent 

patterns in pH. Notable is a relatively narrow zone near the influent side of the PRB where increases of 3 to 4 pH units 

are observed; a sharp gradient in pH has been observed in each sampling event since 1997. 

Iron corrosion is expected to result in moderately high pH and low Eh conditions. The contouring results shown in 

Figures 3.7 and 3.8 suggest, however, that pH and Eh values are not directly coupled (Wilkin, 2002). Values of Eh in the 

iron are gradually increasing with time (Figure 3.8), whereas pH has been consistent with time. For example, in transect 

2, the cross-sectional area represented by Eh values below –100 mV has progressively decreased from 1997 to 2001 

(Figure 3.8). There still exists after five years a sharp gradient in redox between the relatively oxidizing upgradient 

aquifer and the moderately to highly reducing reactive zone. The redox potential of upgradient waters is as high as +500 

mV and values less than 0 mV are typical within the reactive media. After the first five years of operation, the redox 

gradient continues to support efficient removal of hexavalent chromium and chlorinated compounds. Nevertheless, a 

trend of decreasing reductive capacity with time is evident. 

Corrosion of iron metal results in a moderately alkaline pH and low Eh geochemical environment that drives abiotic 

mineral precipitation and supports a variety of microbial metabolic pathways. The net effect of mineral precipitation is 

broadly captured in measurements of specific conductance (SC) that reflect an overall decrease in the concentration of 

total dissolved solids through the reactive barrier. In transect 2, the highest concentrations of ground-water solutes are 

observed at a depth of about 4 to 6 m below ground surface (Figure 3.9). This depth interval also corresponds to a 

maximum in SC and a zone within the PRB in which core analyses show enrichments in solid-phase concentrations of 

inorganic carbon, sulfur, and microbial biomass. Over the initial five-year period, variability in SC over a factor of about 

2 has been observed in upgradient regions at the Elizabeth City site. Variability of specific conductance in upgradient 

regions is also reflected in the temporal and spatial distribution of SC values within the reactive media and in 

downgradient regions (Figure 3.9). The consistent trend of decreasing SC values through the reactive media suggests 

that mineral precipitation processes are operating over the lifetime of the reactive media, i.e., that the zero-valent iron has 

remained reactive over the first five years of operation. This sustained reactivity is likely due to the presence of the steep 

pH and/or Eh gradient as described above. The region where a minimum in Eh (most reducing) is observed corresponds 

to the depth of the SC maximum in upgradient ground water. This observation suggests that the concentration of 

ground-water solutes, or more likely the concentration of terminal electron acceptors such as sulfate and microbial 

activity, is perhaps what leads to the Eh minima rather than only anaerobic iron corrosion reactions. Furthermore, the 

subtle changes in Eh within the reactive media also seem to be time-dependent and reflect temporal changes in 

upgradient Eh and SC. 

3.1.3 Dissolved Cations and Anions 

The dominant cations in ground water at the Elizabeth City site are sodium, potassium, calcium, and magnesium. On a 

molar basis, sodium is the dominant cation followed by Ca>Mg>K. From 1997 to 2001, upgradient ground water 

contained up to 5.2 mM Na, 1.1 mM Ca, 0.8 mM Mg, and 0.2 mM K. Ground water most concentrated in dissolved 

solutes was consistently found at 4 to 6 meters below ground surface in each of the multilevel well bundles (Figure 3.9). 

Inspection of cross-sectional profiles in Figures 3.10-3.12 indicates that the zero-valent iron effectively removes calcium, 

magnesium, and to some extent, sodium, from ground water at the Elizabeth City site. 

18 
0 1 

-7 

-6 

-5 

-4 

-3 

-2 

0 1 

-7 

-6 

-5 

-4 

-3 

-2 

0 1 

-7 

-6 

-5 

-4 

-3 

-2 

0 1 

-7 

-6 

-5 

-4 

-3 

0 1 

-7 

-6 

-5 

-4 

-3 

0 1 

-7 

-6 

-5 

-4 

-3 

0 1 

-7 

-6 

-5 

-4 

-3 

0 1 

-7 

-6 

-5 

-4 

-3 

-2 

PRB 

0 

-7 

-6 

-5 

-4 

-3 

-2 

0 1 

-7 

-6 

-5 

-4 

-3 

-2 

PRB PRB 

pH 

2001 

1997 1998 2000

Distance, m 

Transect 3 

Depth, m 

Transect 2 

Depth, m 

Transect 1 

Depth, m 

5 

6 

7 

8 

9 

10 

11 

0 

-7 

-6 

-5 

-4 

-3 

-2 

Sampling points 

flow 

2 3 4 2 3 4 2 3 4 

2 3 2 3 2 3 2 3 

2 3 4 1 2 3 4 2 3 4 

1 2 3 4 

Figure 3.7 Cross-sectional profiles showing pH distributions in transects 1, 2, and 3, Elizabeth City PRB.


19 
1997 1998 2000


-7 

-6 

-5 

-4 

-3 

-2 

PRB 

500


300


100


-100


-300


-500


-7 

-6 

-5 

-4 

-3 

-2 

-7 

-6 

-5 

-4 

-3 

-2 

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4


2001


PRB 

-7 

-6 

-5 

-4 

-3 

-2 

-7 

-6 

-5 

-4 

-3 

-7 

-6 

-5 

-4 

-3 

-7 

-6 

-5 

-4 

-3 

0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3


-7 

-6 

-5 

-4 

-3 

-2 

PRB 

-7 

-6 

-5 

-4 

-3 

-2 

-7 

-6 

-5 

-4 

-3 

-2 

-7 

-6 

-5 

-4 

-3 

-2 

Sampling points 

flow 

Eh, mV 

(SHE) 

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4


Distance, m 

Transect 1 

Depth, m Depth, m 

Transect 3 

Transect 2


Depth, m


Figure 3.8 Cross-sectional profiles showing Eh distributions (mV) in transects 1, 2, and 3, Elizabeth City PRB.


20

1997 1998 2000


-7 

-6 

-5 

-4 

-3 

-2 

PRB 

800 

700 

600 

500 

400 

300 

200 

100 

0 

-7 

-6 

-5 

-4 

-3 

-2 

-7 

-6 

-5 

-4 

-3 

-2 

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 

-7 

-6 

-5 

-4 

-3 2001 

-7 

-6 

-5 

-4 

-3 

-7 

-6 

-5 

-4 

-3 

-7 

-6 

-5 

-4 

-3 

PRB 

0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 

-7 

-6 

-5 

-4 

-3 

-2 

PRB 

-7 

-6 

-5 

-4 

-3 

-2 

-7 

-6 

-5 

-4 

-3 

-2 

-7 

-6 

-5 

-4 

-3 

-2

Sampling points 

flow 

Specific 

Conductance 

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 

Distance, m 

Transect 3 

Transect 2 

Transect 1


Depth, m Depth, m
 Depth, m


(S/cm)


Figure 3.9	 Cross-sectional profiles showing specific conductance distributions (μS/cm) in transects 1, 2, and 3, 

Elizabeth City PRB. 

21 
22

Figure 3.10 Cross-sectional profiles showing calcium concentrations (mg/L) in transects 1, 2, and 3, Elizabeth City

PRB.

0 1

-7

-6

-5

-4

-3

-2

PRB PRB PRB

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

Ca, mg/L

0

5

10

15

20

25

30

35

40

45

50

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

-2

Sampling points 2001

1997 1998 2000

Distance, m

Transect

3

Depth,

m

Transect

2

Depth,

m

Transect

1

Depth,

m

2 3 4 1 2 3 4 1 2 3 4

1 2 3 2 3 2 3 1 2 3

2 3 4 2 3 4 1 2 3 4 1 2 3 4
23

Figure 3.11 Cross-sectional profiles showing magnesium concentrations (mg/L) in transects 1, 2, and 3, Elizabeth

City PRB

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

Mg, mg/L

2001

2000

Distance, m

Transect

3

Depth,

m

Transect

2

Depth,

m

Transect

1

Depth,

m

-2

1

4

7

10

13

16

0

-7

-6

-5

-4

-3

-2

PRB PRB PRB

1997 1998

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0

-7

-6

-5

-4

-3

-2

Sampling points

2 3 4 2 3 4 2 3 4

1 2 3 4 1 2 3 4 2 3 4

2 3 1 2 3 2 3 2 3

1 2 3 4
24

Figure 3.12 Cross-sectional profiles showing sodium concentrations (mg/L) in transects 1, 2, and 3, Elizabeth City

PRB.

30

30

50

30

30

30

30

30

30

50

50

10

10

50

50

30

10

70

30

30 50

30

30

50

0 1 3

-7

-6

-5

-4

-3

0 1 3

-7

-6

-5

-4

-3

0 1 3

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

-2

PRB

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

-2

Na, mg/L

-10

10

30

50

70

90

110

0

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

PRB

0

-7

-6

-5

-4

-3

-2

0 2

-7

-6

-5

-4

-3

PRB

0

-7

-6

-5

-4

-3

-2

2001 Sampling points

flow

Distance, m

Transect

3

Depth,

m

Transect

2

Depth,

m

Transect

1

Depth,

m

1997 1998 2000

2 2 2

2 3 4 1 2 3 4 1 2 3 4

2 3 4 1 2 3 4 1 2 3 4

1 3

1 2 3 4
Concentrations of potassium appear to be largely unaffected by the reactive media (Figure 3.13). In transect 2, calcium 

concentrations range from about 5 to 30 mg/L in upgradient ground water and are typically less than 5 mg/L in 

downgradient ground water. Depth-dependent removal efficiencies of the major cations are presented in Table 3.4. At 

discrete depth intervals, up to 71% Ca, 73% Mg, and 60% Na are removed as ground water passes through the reactive 

media. Calcium and magnesium are consistently removed through time, although percent removal values are lower in 

2001 compared to the average value from 1997 to 2001 (Table 3.4). This trend may suggest decreasing reactivity over 

time with respect to removal of divalent cations. Sodium shows removal at some intervals and relative gains in 

concentrations at deeper levels (Figure 3.12; Table 3.4). The 2-dimensional trends in sodium and chloride concentrations suggest that there may be some component of vertical redistribution of solutes within the PRB. 

Anions present in ground water at the Elizabeth City site include chloride, sulfate, bicarbonate, and nitrate. On a molar 

basis, chloride and bicarbonate are the dominant anions followed by sulfate. Nitrate is commonly detected in upgradient 

ground water but always at concentrations below about 5 mg/L. In downgradient multilevel well bundles, nitrate is 

typically below the analytical detection limit (0.10 mg/L). In transect 2, sulfate concentrations typically range from about 

25 to 90 mg/L in upgradient ground water, but then decrease to less than 1 mg/L in most downgradient multi-level wells. 

Inspection of the cross-sectional profiles in Figures 3.14-3.17 indicates that anionic species, especially sulfate and 

bicarbonate, are effectively removed by zero-valent iron. Depth-dependent removal efficiencies of the major anions are 

presented in Table 3.5. Typically >90% of influent sulfate is removed as ground water passes through the reactive 

media. The high removal efficiency of sulfate does not appear to be decreasing with time (Table 3.5). Nitrate is also 

effectively removed by the PRB (Table 3.5), although the anomalous trends apparent in 2000 are not understood. 

Chloride removal occurs at depths above 5 m but reactions in the PRB appear to be sources of chloride at depths below 

5 m. The release of chloride is likely related to the degradation of TCE at depths below 5 m. 

Silica in upgradient ground water is present at concentrations up to about 16 mg/L, dominantly as the uncharged form 

H4SiO4

0

 (aq). Silica concentrations in the reactive media are typically <1 mg/L and levels then rebound in downgradient 

wells to values from about 1 to 8 mg/L (Figure 3.18). Similar behavior for dissolved silica was also reported at the Moffet 

Field and Lowry Air Force Base PRBs (Gavaskar et al., 2002). 

Over the first five years of operation, the Elizabeth City PRB has consistently removed C, S, Ca, Mg, Si, and N from 

ground water. Over this time period there appears to be a slight to no discernible loss in the capacity or efficiency of 

removal of these inorganic components. These elements are removed from ground water by mineral precipitation or by 

adsorption, processes examined in more detail in later sections of this report. 

3.2 Core Sampling at Elizabeth City 

Core samples from the PRB were collected on an annual basis at Elizabeth City to assess the extent of corrosion and 

mineral buildup on the iron surfaces. Core collection methods and analysis procedures are described in Volume 2 of this 

EPA Report series. In all cases, 5 cm inner diameter cores were collected using a GeoprobeTM. Core barrels were 

driven using a pneumatic hammer to the desired sampling location and continuous, up to 110 cm, sections of iron, iron 

+ soil, or soil were retrieved. Angle cores (30° relative to vertical) and vertical cores were collected in order assess the 

spatial distribution of mineral/biomass buildup in the reactive media. Prior to pushing the core barrel, an electrical 

conductivity profile was collected to verify the exact position of the iron/aquifer interface. Details of core collection 

procedures are described in Volume 2 of this EPA Report series. In all cases, core recovery was 50 to 92% of the 

expected value. Core materials from the Elizabeth City PRB were jet black in color without any obvious signs of 

cementation or oxidation. Immediately after collection, the cores were frozen and shipped back to the Ground Water and 

Ecosystems Restoration Division in Ada, OK for sub-sampling and analysis. The frozen cores were partially thawed and 

then placed in an anaerobic chamber with a maintained H2

-N2 atmosphere. Each core was logged and partitioned into 

5 to 10 cm segments. Each segment was homogenized by stirring in the glove box and then split into 4 sub-samples: (1) 

inorganic carbon analyses, (2) sulfur analyses/X-ray diffraction (XRD), (3) Scanning electron microscopy (SEM)/X-ray 

photoelectron spectroscopy (XPS) analyses, and, (4) microbial assays (phospholipid fatty acids, PLFA). All subsamples were retained in airtight vials to prevent any air oxidation of redox-sensitive constituents. Details of analytical 

methods and QA/QC procedures used to characterize the core materials are presented in Volume 2 of this EPA Report 

series. 

Locations of coring events at Elizabeth City are shown in Figure 3.19, and information on core recovery, core length, and 

depth of core penetration is presented in Table 3.6. Cores collected from the upgradient aquifer/iron region generally 

penetrated the PRB at depths of 4.5 to 6.5 m below ground surface. As such, the cores were retrieved from portions of 

the PRB where the highest concentrations of chromium and total dissolved solutes entered the reactive media (see, e.g., 

Figure 3.3). 

3.2.1 Carbon Analysis 

Based on long-term trends in ground-water concentrations of bicarbonate and sulfate, significant accumulations of 

inorganic carbon and sulfur precipitates might be expected in the PRB at Elizabeth City. In order to confirm this 

25 
26

Figure 3.13 Cross-sectional profiles showing potassium concentrations (mg/L) in transects 1, 2, and 3, Elizabeth City

PRB.

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

>10

mg/L

0

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

PRB

PRB PRB

1997 1998

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

-2

K, mg/L

2001

2000

0

2.0

4.0

6.0

8.0

10.0

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

-2

Sampling points

flow

Distance, m

Transect

1

Depth,

m

Transect

2

Depth,

m

Transect

3

Depth,

m

1 2 3 4 2 3 4 2 3 4

2 3 4 1 2 3 4 1 2 3 4

2 3 1 2 3 2 3 2 3

2 3 4
Table 3.4.	 Changes in Concentration of Metals in Ground Water at Transect 2 (Elizabeth City) as a Function of 

Time and Depth 

Average values from 1997 to 2001 Values from 2001 

Depth, ML21 ML25 % ML21 ML25 % 

m Upgradient Downgradient Change Upgradient Downgradient Change 

Ca, mg/L 

4 26.6 

5 8.6 

6 10.3 

7 13.7 

Mg, mg/L 

4 12.0 

5 6.3 

6 6.5 

7 8.1 

Na, mg/L 

4 52.6 

5 66.3 

6 27.6 

7 23.0 

K, mg/L 

4 4.8 

5 1.7 

6 1.3 

7 1.1 

8.1 69.5 18.8 12.4 34.0 

4.8 44.2 7.4 4.3 41.9 

4.5 56.3 10.1 5.9 41.6 

4.0 70.8 12.7 6.2 51.2 

3.2 73.3 10.0 5.2 48.0 

3.5 44.4 5.9 5.1 13.6 

2.6 60.0 6.8 4.3 36.8 

2.3 71.6 7.9 3.5 55.7 

20.9 60.3 78.8 19.8 74.9 

47.1 29.0 35.6 34.5 3.1 

42.1 -52.5 20.7 33.5 -61.8 

34.1 -48.3 23.2 24.4 -5.2 

3.5 27.1 4.4 3.9 11.4 

2.4 -41.2 3.6 3.1 13.9 

2.3 -76.9 1.8 2.1 16.7 

0.7 36.4 1.3 1.1 15.4 

% change calculated using: 


[(upgradient concentration-downgradient concentration)/(upgradient concentration)]x100 


expectation and to document the concentration distribution of inorganic precipitates within the reactive media, solid 

phase analyses of carbon and sulfur were performed. Results of these analyses on Elizabeth City cores are listed in 

Table A1 (Appendix 1). Inorganic carbon results are given in weight percent C based upon carbon that is released from 

a sample after acidification with hot 5% perchloric acid. This acid digestion procedure releases inorganic carbon present 

in minerals such as calcite (trigonal CaCO3

), aragonite (orthorhombic CaCO3), siderite (FeCO3), magnesite (MgCO3), 

rhodochrosite (MnCO3

), ferrous carbonate hydroxide (Fe2(OH)2CO3), and carbonate green rust (Fe6(OH)12CO3 ·x H2O). 

The mineral composition of authigenic precipitates formed within the reactive barrier is more fully explored in the sections 

below on X-ray diffraction analysis and SEM analysis. The bulk carbon concentrations are used to access the spaceand time-dependent quantity of inorganic carbon that has deposited in the reactive media, an understanding of which is 

necessary for estimating the extent of pore infilling through time. 

Results of all carbon analyses on Elizabeth City core materials (n = 170) are presented in Table A1. The inorganic, acidextractable carbon concentration in unreacted Peerless iron was found to be 15 μg/g; therefore, concentrations of 

inorganic carbon in core materials above this value are the result of mineral precipitation processes that have taken place 

within the reactive media as a result of continued ground-water exposure. Concentrations of inorganic carbon within 

>90% iron samples range from <1 to 5870 μg/g. In all cases, the highest concentrations were determined in samples 

collected adjacent to the upgradient aquifer/iron interface, and the lowest concentrations were detected near the 

downgradient edge of the reactive media. Accumulation of inorganic carbon precipitates does not occur at the same rate 

throughout the reactive media, rather carbon accumulation is highly spatially variable and is largely restricted to the 

upgradient portion of the reactive media (Wilkin et al., 2003). 

27 
28

Figure 3.14 Cross-sectional profiles showing chloride concentrations (mg/L) in transects 1, 2, and 3, Elizabeth City

PRB.

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

PRB PRB PRB

Cl, mg/L

0

15

30

45

60

75

90

105

0

-7

-6

-5

-4

-3

-2

2001

1998 2000

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

-2

Sampling points

flow

1997

Distance, m

Transect

3

Depth,

m

Transect

2

Depth,

m

Transect

1

Depth,

m

2 3 4 2 3 4 2 3 4

2 3 4 1 2 3 4 1 2 3 4

2 3 1 2 3 2 3 2 3

2 3 4
29

Figure 3.15 Cross-sectional profiles showing sulfate concentrations (mg/L) in transects 1, 2, and 3, Elizabeth City

PRB.

0 1

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

-2

Sulfate, mg/L

-15

5

25

45

65

85

105

0

-7

-6

-5

-4

-3

-2

PRB

PRB PRB

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

2001

1997 1998 2000

Transect

3

Depth,

m

Transect

2

Depth,

m

Transect

1

Depth,

m

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2 >105

ppm

0

-7

-6

-5

-4

-3

-2

Sampling points

flow

1 2 3 4 2 3 4 1 2 3 4

1 2 3 2 3 1 2 3 2 3

2 3 4 2 3 4 2 3 4 1 2 3 4
30

Figure 3.16 Cross-sectional profiles showing alkalinity distributions (mg/L) in transects 1, 2, and 3, Elizabeth City

PRB.

0 1

-7

-6

-5

-4

-3

-2

PRB PRB PRB

1997

0

-7

-6

-5

-4

-3

-2

2001

1998 2000

0

15

30

45

60

75

90

105

120

130

0

-7

-6

-5

-4

-3

-2

0

-7

-6

-5

-4

-3

Alkalinity, mg/L

Distance, m

Transect

3

Depth,

m

Transect

2

Depth,

m

Transect

1

Depth,

m

0

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

0 1

-7

-6

-5

-4

-3

-2

>130

mg/L

0

-7

-6

-5

-4

-3

-2

Sampling points

flow

2 3 4 1 2 3 4 1 2 3 4

1 2 3 1 2 3 2 3 2 3

2 3 4 2 3 4 2 3 4 2 4
1997 1998 2000


Transect 3 

Transect 2


PRB 

-7 

-6 

-5 

-4 

-3 

-2 

-7 

-6 

-5 

-4 

-3 

-2 

2.5 

2.0 

1.6 

1.1 

0.70 

0.25 

-0.20 

-7 

-6 

-5 

-4 

-3 

-2 

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 

2001 

PRB 

-7 

-6 

-5 

-4 

-3 

-7 

-6 

-5 

-4 

-3 

-7 

-6 

-5 

-4 

-3 

-7 

-6 

-5 

-4 

-3 

0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 

-7 

-6 

-5 

-4 

-3 

-2 

PRB 

>2.5 

ppm 

-7 

-6 

-5 

-4 

-3 

-2 

>2.5 

ppm 

-7 

-6 

-5 

-4 

-3 

-2 

-7 

-6 

-5 

-4 

-3 

-2 

flowSampling points 

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 

Distance, m 

Transect 1 

Depth, m Depth, m Depth, m 

Nitrate, mg/L


Figure 3.17	 Cross-sectional profiles showing nitrate concentrations (mg/L) in transects 1, 2, and 3, Elizabeth City 

PRB. 

31 
Table 3.5.	 Changes in Concentration of Anions in Ground Water at Transect 2 (Elizabeth City) as a Function of 

Time and Depth 

Average values from 1997 to 2001 Values from 2001 

Depth, ML21 ML25 % ML21 ML25 % 

m	 Upgradient Downgradient Change Upgradient Downgradient Change 

chloride, 

mg/L 

4 66.3 

5 53.5 

6 17.1 

7 21.4 

sulfate, 

mg/L 

4 47.9 

5 47.8 

6 27.4 

7 16.2 

alkalinity, 

mg/L 

4 85 

5 54 

6 50 

7 55 

nitrate, 

mg/L 

4 4.7 

5 1.4 

6 0.5 

7 1.2 

19.5 70.6 79.7 13.9 82.6 

39.5 26.2 22.5 25.7 -14.2 

39.2 -129.2 13.8 38.7 -180.4 

38.3 -78.9 27.1 40.5 -49.4 

7.4 84.6 79.7 10.1 87.3 

0.32 99.3 32.2 <0.1 >99.7 

0.48 97.8 27.9 <0.1 >99.6 

0.48 97.0 23.4 <0.1 >99.6 

56 34.1 65 79 -21.5 

64 -18.5 57 72 -26.3 

58 -16.0 54 56 -3.7 

33 40.0 54 28 48.1 

0.08 98.3 1.4 <0.1 >92.9 

0.08 94.3 0.3 <0.1 >66.7 

0.5 0 0.4 <0.1 >75.0 

0.4 36.1 1.6 <0.1 >93.8 

% change calculated using: 


[(upgradient concentration-downgradient concentration)/(upgradient concentration)]x100 


A cross-sectional profile showing the concentration distribution of inorganic carbon in the solid-phase is shown in 

Figure 3.20. The concentration profile was constructed based upon three angle cores collected in May 2001 that 

intercepted the upgradient edge of the PRB at depths from about 4.8 to 6.2 m below ground surface. The sampling 

transect was located approximately 3 m west of multilevel well transect 1 (see Figure 3.19). At the time of sampling, the 

highest concentrations of inorganic carbon were found at a depth of about 5 m below ground surface and 5-10 cm inside 

of the reactive media/aquifer interface. The greatest amount of carbon accumulation is localized in a rather narrow depth 

interval that corresponds to the depth where a maximum in total dissolved solids and bicarbonate enters the reactive 

media (compare Figure 3.20 with Figures 3.9 and 3.16). Not surprisingly, “hotspots” of mineral precipitation within the 

reactive media are tied to areas where influent ground water is enriched in dissolved solutes. 

The time-dependent accumulation of mineral precipitates is difficult to evaluate, in part, because the core sampling 

location, relative to ground-water solute inputs, is obviously important in governing accumulation rates. Unless core 

sampling is carried out at the exact same location through time, it may be difficult to evaluate time-resolved data sets. 

Figure 3.21 shows inorganic carbon concentrations in angle cores collected in 1997, 1998, 1999, 2000, and 2001. Each 

of these cores was collected from the same general vicinity so that the trends observed are considered to be 

comparable. Shown in Figure 3.21 is an overall regular increase in inorganic carbon concentrations with time. The 

trends clearly indicate that a front of precipitation is progressively passing through the reactive media with time. This 

observation is critically important because it suggests that complete, rapid pore-infilling does not occur in the region 

immediately adjacent to the upgradient aquifer/iron interface. Rather, mineral precipitation occurs in a larger volume of 

the reactive media than is present immediately adjacent to the upgradient interface. 

32 
33

Figure 3.18 Cross-sectional profiles showing silica concentrations (mg/L) in transect 2, Elizabeth City PRB.

0 2 4

-7

-6

-5

-4

-3

PRB

ML10

(June 2000)

Distance, m

Depth,

m

0 2 4

-7

-6

-5

-4

-3

-2

ML20

(June 2000)

T3m00

ML30

(June 2000)

0 2.0 4.0 6.0 8.0 10 12 14 16

Distance, m

0 2

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

PRB

PRB

Si, mg/L

T2m00

Distance, m

3.2.2 Sulfur Analysis

Total sulfur measurements were made with a UIC sulfur coulometer system. 2

O5

 and

combusted in the presence of oxygen at 1050 °C. 

quantitatively convert all sulfur to SO2

, which is then carried to the coulometer cell where it is absorbed and

coulometrically titrated. 

Peerless iron contains about 5 μg/g of sulfur using this combustion method. 

concentrations of acid-volatile sulfide (AVS) and chromium-reducible sulfur (CRS) were determined by chemical

extraction with hot, 6 M HCl and 1 M CrCl2

 in 0.5 M HCl, respectively (Zhabina and Volkov, 1978). 

methods determine the quantities of metal monosulfide precipitates (AVS) and iron disulfide precipitates (i.e., pyrite;

CRS). 1+xS) and mackinawite (Fe1+xS), dissolve in

dilute hydrochloric acid and subsequently release hydrogen sulfide gas which can be measured. 

pyrite (cubic FeS2

) and marcasite (orthorhombic FeS2

), and elemental sulfur are not dissolved in dilute hydrochloric acid

but are solubilized by using the more aggressive CrCl2-HCl solution. 

step (CRS) can again be quantified and related back to the concentration of disulfide-sulfur present in the core materials.

1 3 1 3 1 3

Iron samples were covered with V

Evolved gases are passed through a column of reduced Cu to

Results of total sulfur analyses on Elizabeth City cores are listed in Table A1. Un-reacted

In addition to total sulfur measurements, the

These acid extraction

Metal monosulfide precipitates, such as disordered mackinawite (Fe

Iron disulfide minerals,

The hydrogen sulfide gas released in this extraction
Groundwater flow 

Reactive Barrier 

ML12 

ML13 

ML14 

ML22.5 

ML23.5 

ML24.5 

ML32 

ML33 

ML34 

EC060300-5 

EC060500-7 

EC050801-6 

EC050801-7 

EC060300-4 

EC050801-4 

EC050801-3 EC90902 EC90903 

EC6114 EC050801-5 EC060200-1 EC030616 EC050801-1 

MW48 

ML11 ML31 ML21 

ML15 EC050901-8,9 ML35 

ML25 

MW49 

MW50 MW47 

EC060300-6 EC6101 EC010618 

Figure 3.19. Coring locations and monitoring well locations at the Elizabeth City Permeable Reactive Barrier site (plan view). 

3m 

Compliance well location 

Cluster well location 

Angle or vertical coring position 

N 

34

4.4 

5.3 

5.8 

4.9 

6.3 

5.6 

2.2 

2.2 

4.8 

4.8 

2.2 

4.0 

2.3 

5.2 

5.5 

4.8 

5.0 

4.8 

5.0 

Depth to Fe 

Table 3.6 Cores Collected for Analysis at the Elizabeth City PRB 

Sample ID Location Date Angle Core length Recovery 

EC050801-1 Upgradient May-01 30 107 88 

EC050801-3 Upgradient May-01 30 99 81 

EC050801-4 Upgradient May-01 30 109 90 

EC050801-5 Upgradient May-01 30 94 77 

EC050801-6 Upgradient May-01 30 102 83 

EC050801-7 Upgradient May-01 30 109 90 

EC050901-8 Vertical May-01 90 102 83 

EC050901-9 Vertical May-01 90 112 92 

EC060200-1 Upgradient June-00 30 61 50 

EC060300-4 Upgradient June-00 30 76 63 

EC060300-5 Vertical June-00 90 84 69 

EC060300-6 Downgradient June-00 30 94 77 

EC060500-7 Vertical June-00 90 69 56 

EC010618 Downgradient June-99 30 61 50 

EC030616 Upgradient June-99 30 61 50 

EC90902 Upgradient September-98 30 61 50 

EC90903 Upgradient September-98 30 61 50 

EC6101 Upgradient June-98 30 69 56 

EC6114 Upgradient June-98 30 66 54 

Notes: * Core EC6101 captures the upgradient iron/aquifer interface but was collected by pushing the core barrel from the


downgradient side of the reactive barrier. Depth to iron is the depth below ground surface in meters where zero-valent iron was intercepted.


35

Inorganic Carbon


2 

3 

4 

5 

6 

7 

8 

-0.6 -0.2 0 0.2 0.6 

Aquifer 

Aquifer 

Iro

Flow 

n 

4816 ppm 

Position, m 

<100 ppm 

100-1000 ppm 

1000-3000 ppm 

3000-4000 ppm 

>4000 ppm 

Depth, m


Figure 3.20	 Cross-sectional profile showing concentration distribution of inorganic carbon in the solid phase 

(μg/g=ppm), Elizabeth City PRB (June 2002). 

36 
6000
 2001 

2000 

1999 

1998a 

1998b 

Inorganic C, 

μ g/g


5000 

4000 

3000 

2000 

1000 

0 

0 10 20 30 40 50 

Distance, cm 

Figure 3.21 Concentrations of inorganic carbon (μg/g) in core materials through time (Elizabeth City PRB). 

Concentrations of total sulfur in the iron media range from about 1 to 3880 μg/g. The concentration distribution of sulfur 

broadly compares with that of inorganic carbon (Figure 3.22). The highest sulfur concentrations are found near the 

upgradient interface in regions proximal to where maximum ground-water concentrations of sulfate enter the reactive 

zone (see Figure 3.15). The lowest concentrations of sulfur in the solid-phase are found near the downgradient edge of 

the PRB. 

Sulfur may be present in the core materials as sulfide (disordered mackinawite, mackinawite, greigite), disulfide (pyrite 

or marcasite), elemental sulfur, or as sulfate (e.g., sulfate green rust). Total sulfur concentrations will reflect the sum of 

all of these sulfur forms. In order to more accurately determine sulfur partitioning and mineralogy, sequential extraction 

procedures were carried out (Wilkin et al., 2003). Results presented in Table A2 indicate that over 90% of the total sulfur 

is present as sulfide. The remaining sulfur would appear to be made up of iron disulfides (pyrite) and perhaps more acidresistant forms of iron monosulfide such as greigite (Fe3

S4). Both of these phases have been detected in minute 

quantities using high-resolution transmission electron microscopy (Furukawa et al., 2002). Sulfate green rust does not 

appear to be an important mineral form in the reactive media at Elizabeth City, which is consistent with geochemical 

modeling results. Based on the HCO3

-

/SO4

2- ratios of ground water at Elizabeth City the carbonate form of green rust is 

expected to dominate over the sulfate form (Wilkin et al., 2002). The solid-phase dominance of sulfide over sulfate in the 

reactive media and the complete loss of dissolved sulfate demonstrate that reduction of sulfate to sulfide has gone 

essentially to completion. 

The masses of IC and S deposited within the iron barriers determined from solid-phase characterization agree 

reasonably well with estimated masses based on changes in concentrations of dissolved solutes and flow rates. Mass 

balance estimates are presented and discussed in Wilkin et al. (2002). Inorganic carbon mass balance agrees to within 

37 
Sulfur


Depth, m


Aquifer 

Aquifer 

Iron 

Flow 

2680 ppm 

-0.6 -0.2 0 0.2 0.6 

Position, m 

<100 ppm 

100-500 ppm 

500-1000 ppm 

1000-2000 ppm 

>2000 ppm 

Figure 3.22	 Cross-sectional profile showing concentration distribution of sulfur in the solid phase (μg/g=ppm), 

Elizabeth City PRB (June 2002). 

38 
a factor of 1.5x for the Elizabeth City PRB. More sulfur accumulation, however, would be expected in the Elizabeth City 

PRB, based on changes in sulfate concentrations than has been observed on the core materials. Several factors lead 

to uncertainty in mass balance calculations for PRBs. Estimates of mineral accumulation based on changes in groundwater chemistry critically depend on chemical and especially hydrogeologic measurements. Determination of dissolved 

constituents may be analyzed at a high level of accuracy and precision, <5%. Similarly, estimates of mass accumulation 

based on characterization of core materials depend on the accuracy of analytical measurements, in addition to estimates 

of emplaced iron density. However, estimates of ground water flow volumes moving through PRBs are prone to large 

uncertainties, and these dominate the total error of mass balance calculations (20-50%). Spatial variability in ground 

water flow velocity, concentration of solutes, concentration of solid phase products, and emplaced iron density all factor 

into the uncertainty analysis of mass balance calculations. 

3.2.3 Cr Extractions 

To understand the distribution of solid-phase chromium, dilute hydrochloric acid (1 M) leaches were performed on 

upgradient aquifer and reactive iron material. Analysis of un-reacted Peerless iron indicated acid-extractible chromium 

concentrations of about 8.8 μg/g. Concentrations of acid-extractible chromium as high as 72 μg/g were measured in the 

reacted iron media after five years of exposure. Chromium is enriched in the solid-phase in subsurface regions near the 

upgradient iron/aquifer interface. 

3.2.4 X-ray Diffraction Analysis 

Powder X-ray diffraction scans for core samples collected in 2000 and 2001 are shown in Figure 3.23. Materials for 

analysis were prepared by sonicating iron core samples in acetone for 10 minutes, followed by filtration of the released 

particulates through 47 mm diameter, 0.2-micron filter paper (polycarbonate). The separated particles were mounted on 

a zero-background quartz plate and scanned with Cu Kα radiation from 3° to 80° 2-theta using a Rigaku Miniflex 

Diffractometer. Diffraction analysis is used to determine the bulk mineralogical composition of the materials removed 

from the reactive media. The results can be used to qualitatively evaluate the abundance of various mineral phases; no 

attempts were made to obtain quantitative results, in part, because of uncertainties regarding the separation efficiency of 

mineral precipitates during the sonication step. 

A summary of the XRD results is reported in Table 3.7. Magnetite (Fe3

O4) was observed in every iron core sample. 

Quartz (SiO2

) was sometimes detected, particularly in samples proximal to the edges of the reactive media. Aquifer 

grains had either been transported into the reactive media at some point during or after construction of the PRB, or the 

core sampling may have resulted in some limited mixing of aquifer material with the reactive iron zone. Aragonite 

(CaCO3) and calcite (CaCO3) were detected as minor components in some of the iron core samples. In addition to 

magnetite, other iron minerals detected were green rust (GR1) and iron carbonate hydroxide. Neither siderite (FeCO3) 

nor iron sulfides were detected by X-ray diffraction in any of the samples analyzed. However, using micro-analytical 

methods, Furukawa et al. (2002) detected quantities of mackinawite, greigite, and pyrite in sample EC060200-1-3, as 

well as ferrihydrite (Fe(OH)3) and lepidocrocite (FeOOH). Bulk X-ray analysis apparently captures the dominant mineral 

phases present, but the technique was not sufficiently sensitive to identify the full range of materials that form in PRBs, 

particularly those phases that have a poor degree of crystallinity. The principal authigenic phases present in the 

Elizabeth City reactive media are iron carbonate hydroxide, mackinawite, and magnetite; minor mineral products are 

aragonite, calcite, lepidocrocite, and green rust (GR1). 

Iron carbonate hydroxide was identified and described by Erdös and Altorfer (1976) as an iron corrosion product formed 

in carbonate solutions. The stoichiometry of the material is reported to be Fe2

(OH)2CO3 (powder diffraction file PDF 33-

0650). The material is related to siderite, FeCO3, and the carbonate form of green rust, Fe6(OH)12CO3 ·x H2O. Like 

siderite, iron is present only in the ferrous state in iron carbonate hydroxide, whereas in green rust, iron is present in both 

the ferrous and ferric states (McGill et al., 1976). Iron hydroxide carbonate may be a precursor to carbonate green rust 

and magnetite, these phases forming by partial oxidation. Iron carbonate hydroxide was also found to be a major 

corrosion product at the Denver Federal Center (see Section 4.2.3) and was detected at the Moffett Field PRB (Gavaskar 

et al., 2002) and the Oak Ridge Y-12 site (Liang et al., 2003). 

Mixed valence iron minerals include magnetite and carbonate green-rust compounds. Green-rust compounds are iron 

corrosion products that are expected to form under more reducing conditions than do ferric oxyhydroxides. Green rust 

precipitation is favored under moderately alkaline conditions, and transformation of these compounds to magnetite is 

expected based upon experimental evidence and thermodynamic calculations (e.g., Bonin et al., 2000). Green-rusts are 

highly susceptible to oxidation in the presence of dissolved oxygen or air. Green rust structural units consist of 

alternating positively charged tri-octahedral metal hydroxide sheets and negatively charged interlayers of anions (Taylor, 

1973). Two types of GR are distinguishable based upon X-ray analyses: GR1 in which the distance between hydroxide 

sheets is between about 0.75 and 0.80 nm (e.g., GRCO3

2-) and GR2 in which the distance between sheets is about 

1.1 nm (e.g., GRSO4

2-). X-ray diffraction results are consistent with the presence of GR1 in core materials from Elizabeth 

City. 

39 
40

Figure 3.23 Powder X-ray diffraction data from fine-grained materials removed via sonication from cores collected at

the Elizabeth City PRB: a) core EC060300-4; b) core EC050801-3.

0 10 0 0 0 0 0 0 0 0

0

500

1000

1500

2000

2500

3000

mid-barrier

mid-barrier

upgradient edge

upgradient edge

a)

EC060300-4-5

EC060300-4-2

EC060300-5-3

EC060200-1-3

Inte

n

sity

degrees 2-theta

0 10 0 0 0 0 0 0 0 0

0

500

1000

1500

2000

2500

0 10 0 0 0 0 0 0 0 0

0

500

1000

1500

2000

2500

0 10 0 0 0 0 0 0 0 0

0

500

1000

1500

2000

2500

0 10 0 0 0 0 0 0 0 0

0

500

1000

1500

2000

2500

0 10 0 0 0 0 0 0 0 0

0

500

1000

1500

2000

2500 b) EC050801-3-7

EC050801-3-5

EC050801-3-3

EC050801-3-2

EC050801-3-1

Inte

n

sity

degrees 2-theta

Inte

n

sity

degrees 2-theta

Inte

n

sity

degrees 2-theta

downgradient

edge

upgradient edge

Inte

n

sity

degrees 2-theta

Inte

n

sity

degrees 2-theta

2 3 4 5 6 7 8 9

2 3 4 5 6 7 8 9
Table 3.7. Results of Powder X-ray Diffraction Analysis of Core Materials from the Elizabeth City PRB


Sample Major Component Minor Component Trace Component 

EC060200-1-3 Magnetite, Iron Quartz, Calcite 

carbonate hydroxide 

EC060300-5-3 Magnetite, Iron Quartz, Calcite 

carbonate hydroxide 

EC060300-4-2 Magnetite, Iron Quartz, Aragonite, Lepidocrocite, 

carbonate hydroxide Calcite Mackinawite 

EC060300-4-5 Magnetite, Iron Quartz, Calcite 

carbonate hydroxide 

EC050801-3-1 Magnetite, Iron Quartz, Aragonite, 

carbonate hydroxide Calcite, Green rust 1 

EC050801-3-2 Magnetite, Iron Quartz, Calcite 

carbonate hydroxide 

EC050801-3-3 Magnetite, Iron Calcite 

carbonate hydroxide 

EC050801-3-7 Magnetite, Iron Calcite 

carbonate hydroxide 

EC050801-3-7 Magnetite, Iron Quartz, Calcite Carbon 

carbonate hydroxide 

3.2.5 Scanning Electron Microscopy 

Scanning electron microscopy (SEM) was used to evaluate the morphology and spatial relationships among mineral 

precipitates on the surfaces of zero-valent iron particles. In addition, energy dispersive X-ray spectroscopy (EDX) was 

conducted to determine on a semi-quantitative basis the composition of surface precipitates. Samples for SEM and EDX 

analysis were stored in an anaerobic glove box and then embedded in an epoxy resin. The sample mounts (1” diameter 

round mounts) were ground and polished using diamond abrasives and coated with a thin layer of carbon prior to being 

placed within the SEM sample chamber. 

SEM photomicrographs are presented in Figure 3.24a-c for three samples from core EC060300-4 from the Elizabeth City 

site collected in June 2000. These three samples were retrieved from near the upgradient aquifer/iron interface region 

(EC060300-4-1, horizontal penetration ~ 2 cm; EC060300-4-3, horizontal penetration ~8 cm), and the downgradient 

edge of the reactive iron media (EC060300-4-7, horizontal penetration ~40 cm). The micrographs in Figure 3.24 are 

representative of particles contained in each of the samples and capture a range of magnifications from about 50x to 

5000x. 

Sample EC060300-4-1 was collected from a region near the upgradient interface and shows a fairly consistent 

accumulation of mineral precipitates on the surfaces of iron particles (Figure 3.24a). In this sample, the thickness of 

surface coatings on iron grains ranges from about 20 μm to 100 μm. This surface precipitate buildup occurred over the 

first four years of operation of the PRB. An average linear rate of precipitate accumulation of about 5 to 25 μm per year 

is indicated. The SEM images exhibit two types of precipitate morphology: platy/acicular aggregates and poorly 

crystalline clusters (Furukawa et al., 2002). Individual particles in the platy aggregates are on the order of 10 μm in 

length. Platy or acicular textures are oftentimes the result of rapid precipitation from highly oversaturated solutions. 

41 
a) 

25 kV 100x 100 μm 25 kV 5000x 5 μm

25 kV 750x 25 μm

25 kV 2000x 10 μm 25 kV 350x 50 μm

25 kV 500x 50 μm

Quartz 

Iron 

Platy particles 

Poorly 

xtalline 

Figure 3.24	 (continued) Scanning electron micrographs of samples from the Elizabeth City PRB: a) sample 

EC060300-4-1 located near the upgradient iron/aquifer interface; b) sample EC060300-4-3 located in 

the midbarrier; and, c) EC060300-4-7 located near the downgradient iron/aquifer interface. 

42 
b) 

25 kV 50x 500 μm 25 kV 750x 25 μm

Iron 

25 kV 350x 50 μm

25 kV 750x 40 μm 25 kV 500x 30 μm

25 kV 200x 100 μm

Surface 

precipitates 

Figure 3.24	 (continued) Scanning electron micrographs of samples from the Elizabeth City PRB: a) sample 

EC060300-4-1 located near the upgradient iron/aquifer interface; b) sample EC060300-4-3 located in 

the midbarrier; and, c) EC060300-4-7 located near the downgradient iron/aquifer interface. 

43 
c) 

15 kV 75x 150 μm 15 kV 200x 50 μm

15 kV 1000x 10 μm

15 kV 1000x 10 μm 15 kV 1000x 10 μm

15 kV 500x 25 μm

Iron 

Iron surface 

Figure 3.24	 (continued) Scanning electron micrographs of samples from the Elizabeth City PRB: a) sample 

EC060300-4-1 located near the upgradient iron/aquifer interface; b) sample EC060300-4-3 located in 

the midbarrier; and, c) EC060300-4-7 located near the downgradient iron/aquifer interface. 

44 
Such conditions might be expected in the region where ground water passes from the aquifer into the reactive zone, i.e., 

where steep geochemical gradients are present (for example in pH and Eh). These observed morphologies are 

consistent with previous studies (e.g., Roh et al., 2000; Phillips et al., 2000). These previous studies found that acicular/ 

platy aggregates in zero-valent iron PRBs were largely composed of green rust minerals, goethite, lepidocrocite, and 

calcium carbonates. The poorly crystalline clusters more typically were composed of mackinawite and poorly 

crystallized iron oxyhydroxides (Phillips et al., 2000). Poorly crystalline clusters are typically found immediately on the 

zero-valent iron surfaces, with the elongated particles apparently growing on the clusters. In other cases, however, the 

poorly crystalline clusters are found together with the elongated particles and not directly associated with the iron 

surfaces. 

Sample EC060300-4-3 was collected near upgradient aquifer/PRB region but at a penetration depth further into the PRB 

compared to sample EC060300-4-1. Sample EC060300-4-3 shows the same particle morphologies as were noted in 

sample EC060300-4-1. However, the regularity of surface precipitates is less pronounced and the thickness of the 

surface precipitate layer is thinner, ranging from 0 to about 50 μm. In this sample, an average linear rate of precipitate 

accumulation of about 0 to 13 μm per year is indicated (Figure 3.24b). In sample EC060300-4-7, collected near the 

downgradient aquifer/iron interface, the surface precipitate thickness is very thin if present at all (Figure 3.24c). In most 

cases a 1- to 4-μm layer was observed on the iron grains in this sample collected near the downgradient edge of the 

reactive media. This observation suggests that after four years of ground-water exposure, iron particles free from 

precipitate coverage are still present at the Elizabeth City site. The PRB should still remain reactive, which is consistent 

with the previously described trends in geochemical parameters and contaminant concentrations. 

Energy dispersive analyses of iron grains from the Elizabeth City PRB indicate the presence of iron (~97 wt%), silicon 

(~2.5%), and occasionally Mn (<0.7 wt%) and Cr (<0.6 wt%). Peerless iron is also known to contain concentrations of 

carbon and other trace elements, including S, P, Ni, V, Mo, Ti, and Cu. Because the samples were coated with carbon, 

this element could not be semi-quantitatively determined; other trace elements are present at concentrations below the 

operational detection limit of the EDX method. Oxygen was not detected on freshly polished surfaces of zero-valent iron. 

However, the coatings of surface precipitates are enriched in oxygen and comparatively depleted in iron relative to the 

composition of fresh zero-valent iron (Figure 3.25). Typically the most oxygen-enriched regions are those farthest away 

from the zero-valent iron surface. Particles with a platy morphology were found to contain iron (68.9 ± 8.2 wt%, n=25), 

oxygen (28.3 ± 7.5 wt%, n=25), silicon (2.4 ± 4.2 wt%, n=25), sulfur (0.2 ± 0.6 wt%, n=25), and manganese 

(0.5 ± 0.5 wt%, n=25) (Figure 3.26). Chromium was never detected in spot analyses of the platy particles, and calcium 

was detected in only 6 of 25 analyses, always at levels below 1 wt%. The poorly crystalline clusters are similar to the 

platy particles with respect to iron concentrations (64.5 ± 8.0 wt%, n=30) and are broadly comparable in terms of the 

element distributions of Si, S, Mn, and Ca. Chromium was detected in 10 of 30 of the spot analyses of the poorly 

crystalline clusters in concentrations up to about 2 wt%. 

Chromium was detected by EDX spot or small area analysis in 72 separate measurements. Concentrations of chromium 

ranged from 0.1 wt% to 1.8 wt%. The micro-scale measurements are much greater than bulk-scale Cr concentrations 

determined by acid leaching large (~1–5 gm) quantities of material. Pearson correlation coefficients presented in 

Table 3.8 indicate significant correlation of chromium abundance with S and Mn. The high degree of correlation with Mn 

suggests that Cr uptake is in some way tied to Mn behavior; either they are present together in some discrete phase or 

as a co-precipitate. This observation is somewhat surprising as most laboratory studies have concluded that Cr is 

removed from solution through the formation of a solid solution or by adsorption of Cr(III) onto iron oxyhydroxide surfaces 

(Powell et al., 1995; Blowes et al., 1997; Pratt et al., 1997). 

XPS scans show that iron particle surfaces from Elizabeth City contain C, O, Fe, Si, S, Mg, Ca, Mn, and N. The XPS 

data indicate a surface layer dominated by iron oxyhydroxides, an intermediate layer of iron oxide, and finally, zerovalent iron at the greatest sputtering depths. Surface carbon is present predominately as carbonate with some detected 

hydrocarbon (binding energy 284.6 eV). The oxidation state of sulfur is predominantly present as sulfide (-2) but with 

minor amounts of sulfate (+6). Surface enrichment in the elements Ca, Mg, S, and Si are consistent with observed 

decreases in ground-water concentrations of these elements. Chromium was sometimes detected by XPS in iron 

samples from Elizabeth City. 

3.2.6 Microbial Characterization 

From 1999 to 2001, 117 samples were collected from the Elizabeth City site for phospholipid fatty acid (PLFA) extract 

characterization. Samples were collected from regions within the reactive iron media and from adjacent regions of the 

upgradient and downgradient aquifer. Samples for PLFA analysis were frozen immediately after collection from the 

subsurface and shipped frozen to Microbial Insights (Rockford, Tennessee). The complete PLFA data set from the 

Elizabeth City site is shown in Table B1 (Appendix B) and summarized in Table 3.9. 

Biomass contents spanned several orders of magnitude from <1 to 2614 picomoles per gram (dry weight basis), or from 

about 2.5x103

 to 5.23x107 cells per gram of dry material. The biomass is dominated by Prokaryote PLFA. The highest 

biomass concentrations were found near the upgradient aquifer/iron interface region, in the same region of the reactive 

45 
100


95 

90 

85 

80 

75 

70 

65 

60 

55 

50 

45 

40 

F 

G 

HM 

W 

Iron particles 

Precipitates directly on Iron particles 

Platy particles 

Amorphous particles 

0 5 10 15 20 25 30 35 40 45 50 

O, wt% 

Figure 3.25	 Iron concentration versus oxygen concentration in iron grains and surface precipitates (SEM-EDX); FeO compositions noted for wustite (W, FeO), magnetite (M, Fe3

O4), hematite (H, Fe2O3), goethite (G, 

FeOOH), and ferrihydrite (F, Fe(OH)3). 

Table 3.8. Pearson’s Correlation Matrix of Element Concentrations Determined by SEM-EDX Analysis 

O Si S Cl Mn Fe Cr Ca 

O 1 

Si 0.712 1 

S 0.031 -0.126 1 

Cl 0.242 0.281 0.177 1 

Mn -0.127 -0.082 0.346 0.458 1 

Fe -0.747 -0.532 -0.658 -0.392 -0.237 1 

Cr 0.057 0.014 0.254 0.292 0.655 -0.269 1 

Ca 0.597 0.471 -0.119 0.029 -0.187 -0.351 -0.162 1 

media where enrichments in inorganic precipitates are observed (Figure 3.27 and 3.28). Downgradient regions tend to 

be comparatively depleted in microbial biomass (Figure 3.28). The lower counts associated with the mid-barrier and 

downgradient samples suggest that the environment at these locations is more challenging to bacterial growth and 

survival. Examining the geochemical conditions associated with these locations supports this hypothesis. Figures 3.5 

and 3.15, for example, indicate a decrease in biologically available electron acceptors such as cis-DCE and sulfate in 

mid-wall and downgradient locations. The higher pH and lower availability of electron acceptors would also tend to 

create a more severe environment for bacterial growth. 

The highest biomass contents detected were found in samples collected in 2001, four years after installation of the iron 

wall (sample EC050801-5-2). However, nearly equivalent values were detected in samples collected in 1999 (sample 

EC90903), only two years after installation. These data suggest non-constant microbial growth rates, or they may Fe, wt% 

46 
Table 3.9. Summary of PLFA Data from the Elizabeth City PRB


Number of samples 

Average PLFA concentration 

(pmoles/g) 

PLFA range 

PLFA Structural Groups 

average % (range %) 

Monoenoic 

Found in Gram-negative 

bacteria 

Terminally Branched 

Saturated 

Found in many Gram-positive 

bacteria, and in some Gramnegative bacteria 

Branched Monoenoic 

Common in obligate 

anaerobes, such as sulfatereducing and iron-reducing 

bacteria 

Mid-Chain Branched 

Saturated 

Common in actinomycetes and 

sulfate-reducing bacteria 

Normal Saturated 

Ubiquitous in prokaryotes and 

eukaryotes 

Polyenoic 

Found in fungi, protozoa, 

algae, higher plants, and 

animals 

Fe(0) Fe(0) Aquifer 

Upgradient Downgradient Elizabeth City 

Elizabeth City Elizabeth City 

27 33 46 

611 26 33 

6-2614 <1-260 <1-348 

36.9 35.9 28.9 

(14.0-73.5) (<1-74.8) (<1-63.0) 

19.1 4.1 4.5 

(5.3-42.1) (<1-28.3) (<1-19.2) 

11.5 3.1 3.9 

(<1-30.3) (<1-37.8) (<1-29.8) 

7.8 0.6 2.4 

(<1-38.9) (<1-7.9) (<1-17.1) 

22.6 50.4 59.1 

(5.2-74.5) (<1-100) (15.6-100) 

2.0 2.9 1.3 

(<1-49.6) (<1-53.7) (<1-9.5) 

suggest that comparisons of microbial biomass concentrations must be made in the context of the sample location 

relative to the ground-water plume (i.e., the input of electron acceptors). The enrichments in microbial biomass at 5 to 

6 meters below ground surface correspond to the depth where the highest concentrations of terminal electron accepting 

species enter the reactive media via ground-water transport (Figure 3.27). 

PLFA profiles from the Elizabeth City site are enriched in fatty acid biomarkers indicative of anaerobic sulfate- or ironreducing bacteria (Dowling et al., 1986; Edlund et al., 1986; Tunlid and White, 1991; Parkes et al., 1993). The high 

proportions of terminally branched and branched monoenoic PLFA specifically indicate anaerobic metabolism. Terminally branched PLFA are typical of Gram-positive bacteria, but can also be present in the cell membranes of some 

anaerobic Gram-negative bacteria. However, because high proportions are present of branched monoenoic PLFA 

47 
10
 Si 

Mn 

Ca 

8 

6 

4 

2 

0 

wt%


Iron Platy Amorphous Surfaces Particles Particles 

Figure 3.26	 Element concentrations in surface precipitates from the Elizabeth City PRB; iron surfaces=precipitates 

directly in contact with iron grains. 

indicative of anaerobic metal reducing bacteria, the terminally branched PLFA are likely to be mainly from sulfate or iron 

reducing bacteria. Where biomass is most concentrated (i.e., near the upgradient aquifer/iron interface), the distribution 

of PLFA overall appears to be distinct from the PLFA distribution observed in the native aquifer materials (Figure 3.29). 

Near the upgradient aquifer/iron interface, the proportion is greater of branched monoenoic PLFA and PLFA indicative of 

sulfate-reducing bacteria compared to the PLFA signature of native aquifer materials (Figure 3.29). Where biomass is 

least concentrated (i.e., near the downgradient aquifer/iron interface), the distribution of PLFA overall appears to match 

the PLFA distribution observed in the native aquifer materials (Figure 3.29). 

3.3 Summary of Results from the Elizabeth City Site 

Results of the long-term performance evaluation at the Elizabeth City site indicate that the reactive barrier there 

continues to remove contaminants from ground water after five years of operation. The PRB at Elizabeth City is 

expected to retain an effective level of reactivity and hydraulic performance for at least another five year time period. The 

salient results of the Elizabeth City site study are summarized below: 

� Removal of contaminants, Cr, TCE, cis-DCE, and VC, continues after five years of PRB operation. In all 

cases, chromium concentrations have been reduced to below the MCL, and in the majority of sampling 

events, Cr was undetected in monitoring wells located downgradient from the PRB. Concentrations of 

volatile organic compounds have been significantly reduced, but TCE concentrations above the MCL have 

been observed in some downgradient wells. 

� After five years, ground water in the PRB is moderately alkaline (pH>9) and moderately reducing (Eh<-100 

mV). Time trends in pH suggest quasi-steady-state conditions. Time trends in Eh, however, suggest that 

the PRB is gradually losing the capacity to produce reducing conditions due to progressive exposure to 

ground water. Time trends in specific conductance values indicate that influent solutes continue to be 

48 
PLFA


2 

3 

4 

5 

6 

7 

8 

-0.6 -0.2 0 0.2 0.6 Aqui

f

er 

Aquifer 

Iron 

Flow 

2614 pmoles/g 

Position, m 

<50 pmoles/g 

50-500 pmoles/g 

500-1000 pmoles/g 

1000-2000 pmoles/g 

>2000 pmoles/g 

Depth, m


Figure 3.27	 Cross-sectional profile showing concentration distribution of biomass (from PLFA data) in picomoles per 

gram, Elizabeth City PRB (June 2002). 

49 
16 

14 

12 

10 

8 

6 

4 

2 

0 

20 

16 

12 

8 

4 

0 

3 

2 

1 

0 

0 1000 2000 3000 

Fe(0) upgradient zone 

Fe(0) downgradient zone 

Aquifer 

PLFA, picomoles/g 

Frequency Frequency Frequency 

Figure 3.28	 Histograms of microbial biomass concentrations (from Elizabeth City PLFA data) in picomoles per gram 

in aquifer materials, iron from near the downgradient aquifer/iron interface, and iron from near the 

upgradient aquifer/iron interface. 

50 
51

Figure 3.29 Pie graphs showing structural distribution of PLFA compounds (average values) at the Elizabeth City

site.

Fe(0)

Downgradient

2.5%

0.6% 49%

3.1%

36% 4.1%

Fe(0)

Upgradient

2.0%

23%

12% 7.8%

36%

19%

Aquifer

1.3%

60%

2.4%

3.9%

29% 4.5% Gram+/Gram- (Ter. Br. Sat.)

Gram- (Monoenoic)

Anaerobic (Br. Mono.)

SRB, Actinomycetes

Genera Nsats

Polyenoic
removed within the reactive medium and partitioned into the solid phase after five years of PRB operation. 

The geochemistry of ground water downgradient of the PRB is impacted by the PRB, in that higher pH and 

lower Eh conditions are observed in wells located within 2 m of the downgradient iron/aquifer interface. 

� The Elizabeth City PRB has consistently removed inorganic carbon, sulfate, calcium, magnesium, silicon, 

and nitrogen from influent ground water. These components have either been precipitated out in the PRB, 

adsorbed to iron granules or secondary precipitates within the PRB, or have been chemically transformed by 

biotic or abiotic processes. 

� Mineralogical characterization of soil core materials indicates the formation of calcite/aragonite, iron 

carbonate hydroxide, magnetite, lepidocrocite, mackinawite, and carbonate green rust in the PRB at 

Elizabeth City. Inorganic carbon is present in several calcium- and iron-containing minerals. Sulfur is 

dominantly present in the iron sulfide mackinawite. Mineral precipitation mainly occurs near the upgradient 

edge of the PRB, although there is an indication that a precipitation front is progressively moving through the 

PRB. Iron core collected near the downgradient edge of the PRB contains very little if any mineral 

precipitate mass. After five years of operation, less than 10% of the available pore space has been lost due 

to mineralization near the upgradient edge. Near the downgradient edge, <1% of the available pore space 

has been lost. 

� Microscopic characterization of core materials indicates that mineral accumulation is occurring mainly on the 

surfaces of iron granules. After five years, coverage of iron granules near the upgradient edge is regular and 

approximately 20 to 100 μm thick. Near the downgradient edge, coverage of the iron grains is less 

consistent and where present mineral coatings are generally <5 μm thick. Although the available reactive 

surface area of Fe0

 has been reduced through time, some of the secondary mineral precipitates identified 

(magnetite, green rust, mackinawite) also support contaminant transformation and uptake, thus potentially 

compensating for the loss in iron metal reactivity due to surface precipitation. 

� Microbial characterization results, based on PLFA profiles, from the Elizabeth City PRB and adjacent aquifer 

materials showed a diverse microbiological community dominated by Gram-negative bacteria. Iron core 

samples from near the upgradient edge of the PRB are typically enriched in microbial biomass (up to 

5.23x107 cells/g) and contained elevated proportions of biomarkers indicative of metal-reducing and sulfatereducing bacteria. Aquifer materials (up to 6.96x106

 cells/g) and iron from near the downgradient PRB edge 

(5.20x106

 cells/g) were comparatively depleted in total biomass and in biomarkers indicative of metalreducing and sulfate-reducing bacteria. 

52 
4.0 Denver Federal Center PRB Monitoring Results 

4.1 Ground-water Monitoring 

Ground-water monitoring wells in and around gates 1, 2, and 3 at the Denver Federal Center (DFC) were sampled on an 

annual basis starting in May of 1999, approximately 2.5 years after the funnel-and-gate system was installed. Up to 20 

monitoring wells were sampled to obtain contaminant and geochemical profiles in upgradient, iron media, and 

downgradient positions. The Federal Highway Administration provided a more extensive ground-water data set, 

covering the entire history of the DFC PRB (FHWA, 2002 pers. commun.; Pacific Western Technologies, 2000). These 

secondary data were collected under an approved QA/QC program (USGS, 1999). The locations of the monitoring wells 

in relation to the reactive iron media in gates 1-3 at the DFC are shown in Figures 4.1-4.3. 

Methods of ground-water and soil core sampling, preservation and analysis, in addition to Quality Assurance/Quality 

Control procedures used in this five-year investigation of PRB performance, are described and discussed in Volume 2 of 

this EPA Report series. 

4.1.1 Gate 1 Contaminant Behavior 

Time-dependent concentrations of volatile organic compounds (1,1-DCE, 1,1,1-TCA, TCE, and 1,1-DCA) in monitoring 

wells located in upgradient, iron wall, and downgradient positions near gate 1 are shown in Figure 4.4. Data plotted in 

Figures 4.4, 4.5, and 4.7 were provided by FHWA. It should be noted that not all monitoring data collected at the DFC 

are discussed in this report. The data presented correspond to wells that overlap between EPA’s performance study and 

monitoring efforts conducted by FHWA and GSA. The concentrations of 1,1-DCE and 1,1,1-TCA have gradually 

decreased with time in upgradient well GSA-21 (Figures 4.4a). This trend in contaminant concentrations suggests that 

natural attenuation processes are taking place in regions of the plume upgradient from the southern most extension of 

the funnel-and-gate system at the DFC. Concentrations of TCE and 1,1-DCA in ground water entering gate 1 at the DFC 

have remained fairly constant since January 1998 at 23 ± 7 μg/L and 7 ± 2 μg/L, respectively. Concentrations of vinyl 

chloride and cis-DCE have not been detected in upgradient monitoring points in the vicinity of gate 1. 

Within the iron media of gate 1 (well C1-I2), concentrations of TCE and 1,1,1-TCA have remained at or below nominal 

quantification limits (Figure 4.4b). Concentrations of 1,1-DCE were not detected until November 1999. At that time, 

concentrations of 1,1-DCE began to increase at a time-averaged rate of about 5 μg/L per year. Similarly, concentrations 

of 1,1-DCA were not detected until November 1999. However, since that time 1,1-DCA has remained at a fairly constant 

value of 7 ± 0.8 μg/L. Concentrations of vinyl chloride and cis-DCE have not been detected in monitoring wells located 

within the reactive media of gate 1. Although lower contaminant concentrations are present, parallel trends in 

contaminant levels are observed in well C1-I2, located approximately 3 m to the south of well C1-I1 along the mid-point 

axis of the reactive media in gate 1 (see Figure 4.1). 

Based on laboratory studies to evaluate contaminant removal rates in the presence of zero-valent iron, degradation rates 

of chlorinated ethenes and ethanes decrease in the order 1,1,1-TCA>TCE>1,1-DCE>cis-DCE>VC (see Johnson et al., 

1996). In general, with each successive dehalogenation the degradation reaction proceeds more slowly (Matheson and 

Tratnyek, 1994; but also see Arnold and Roberts, 2000). In a performance scenario where zero-valent iron progressively 

loses reactivity over time relative to an initial reactivity defined by laboratory batch or column studies, it might be 

expected that less reactive contaminants such as VC, cis-DCE, and 1,1-DCE would appear or “break through” prior to 

more rapidly degraded compounds such as TCE and 1,1,1-TCA. These trends are in fact observed in gate 1 at the DFC. 

It should be noted that monitoring points C1-I1 and C1-I2 are located at the approximate mid-point of the reactive media 

so that contaminants should continue to degrade in the downgradient direction. 

Ground-water seepage velocities in gate 1 have been measured using heat-flow sensors and range from about 0.40 to 

0.82 m/d (McMahon et al., 1999; Pacific Western Technologies, 2000). Based on this range of flow rates, an average 

saturated thickness of 4.5 m, and a saturated gate throughput area of 55 m2

, gate 1 has removed approximately 

0.8-1.6 kg of TCE, 2-4 kg of 1,1,1-TCA, and 3.2-6.4 kg of 1,1-DCE over the first five-year period of operation. By 

February 1998, a clean front was observed in downgradient well GSA-20. At that time, concentrations of 1,1-DCE and 

1,1, DCA increased to above detectable limits and have remained at detectable levels to the present time. The reasons 

for the increase in the concentrations of these compounds is not certain but would appear to relate to a loss of reactivity 

or change in residence time of contaminants in the reactive media. 

53 
N


GSA-21 

C1-2-71000 

C1-1-71701 

C1-2-71701 

C1-4-71801 

C1-GU1 

C1-GD1 

C1-I1 

C1-I2 

Gate 1 

Groundwater flow 

C1-3-71701 

GSA-20 

C1-3-71100 

Pea 

G

ravel 

Pea 

G

ravel

Reactive

media 

1.5 m 

Monitoring well location 

Angle or vertical 

coring position 

Figure 4.1. Coring locations and monitoring well locations at the Denver Federal Center, gate 1 (plan view). 

54

N


Gate 

USGS-10 

USGS-12 C2-I2 

C2-GU2 

C2-I1 

USGS-5 

USGS-4 USGS-9 

C2-12-71300 

C2-13-71300 

C2-14-71300 

Gravel 

Gravel 

Reactive 

media 

2 

Groundwater flow C2-3-71801 

C2-4-71801 

GSA-26 

GSA-25 

C2-16-71300 

C2-17-71300 

C2-1-71801 

C2-1-71901 

C2-2-71901 

C2-3-71901 

Monitoring well location 

Angle or vertical 

coring position 

1.5 m 

Pea 

Pea 

Figure 4.2. Coring locations and monitoring well locations at the Denver Federal Center, gate 2 (plan view).


55 
N


GSA-31 

C3-GU2 

C3-I2 

C3-I1 

Gate 3 

1.5 m 

Groundwater flow 

GSA-30 

C3-2-71801 

Monitoring well location 

Angle or vertical 

coring position 

Pea 

Gravel 

Pea 

Gravel

Reactive

media 

Figure 4.3. Coring locations and monitoring well locations at the Denver Federal Center, gate 3 (plan view). 

56

180 

concentration, 

μg/L concentration, 

μg/L 

160 

140 

120 

100 

80 

60 

40 

20 

0 

10/10/1995 2/21/1997 7/6/1998 11/18/1999 4/1/2001 8/14/2002 

a) 1,1-DCE 

1,1,1-TCA 

TCE 

1,1-DCA 

40 

30 

20 

10 

0 

b) 

GSA-21 

Gate 1 

Upgradient 

C1-I1 

Gate 1 

Fe(0) 

concentration, 

μg/L 

10/10/1995 2/21/1997 7/6/1998 11/18/1999 4/1/2001 8/14/2002 

20 

c) GSA-20 

16 Gate 1 

Downgradient 12 

8 

4 

0 

10/10/1995 2/21/1997 7/6/1998 11/18/1999 4/1/2001 8/14/2002 

Figure 4.4	 Concentrations of contaminants through time in monitoring wells from the Denver Federal Center, gate 1 

(data from FHWA): a) well GSA-21 (upgradient); b) well C1-I1 (iron wall); c) well GSA-20 

(downgradient). Symbol key shown in Figure 4.4a applies to each graph; note change in concentration 

plotting range to show trends through time. 

57 
4.1.2 Gate 2 Contaminant Behavior 

Gate 2 is located approximately 120 meters north of gate 1. Of the four treatment cells at the DFC, the highest 

concentrations of total volatile organic compounds are found in the vicinity nearby gate 2. Since November 1996, 

concentrations of 1,1-DCE and 1,1,1-TCA have gradually decreased in the upgradient well, GSA-26, to values of about 

110 μg/L and 50 μg/L, respectively (Figure 4.5a). In this same well, concentrations of TCE, 1,1-DCA, and cis-1,2-DCE 

have been steady since November 1996 at 72 ± 12 μg/L, 8 ± 1 μg/L, and 1 ± 0.5 μg/L, respectively (Figure 4.5a). The 

highest TCE concentrations observed, in either upgradient or downgradient monitoring locations, are in downgradient 

well GSA-25 (Figure 4.5c). A maximum TCE concentration of 600 μg/L was measured in ground water collected from 

GSA-25 in January 1997. From January 1997 to November 1998, TCE concentrations rapidly dropped in GSA-25, and 

since November 1998, TCE and all other volatile organic compounds have remained at relatively constant concentrations (Figure 4.5c). 

Because TCE concentrations are comparatively low in wells located within the iron media of gate 2 and in the pea gravel 

immediately downgradient of gate 2, the TCE observed in downgradient well GSA-25 is thought to be derived from 

residual bedrock contamination from a proximal release of TCE (Pacific Western Technologies, 2000). Yet, concentrations of 1,1-DCE have been consistently present in wells both within iron media of gate 2 and in downgradient pea gravel 

wells (Figure 4.5b). The increase beginning in November 1998 of 1,1-DCE levels both in downgradient well GSA-25 and 

in the iron well C2-I2 corresponds to the installation of a distribution trench placed on the downgradient side of gate 2. 

The trench was installed with the goals of increasing the flow through gate 2 and lowering the hydraulic head differential 

across the gate. Although the timing of trench construction matches the point at which 1,1-DCE concentrations increase, 

the mechanism linking these events is uncertain. Alternatively, the increasing 1,1-DCE levels could be from incomplete 

degradation within the iron media due either to loss of iron reactivity or to changing flow regimes within gate 2 (decrease 

in residence time). Iron reactivity tests and mineral precipitate/microbial biomass characterization studies on the iron 

media in gate 2 are discussed in following sections of this report. Equally equivocal is the trend of decreasing 

concentrations in C2-I2 beginning in September 2001 (Figure 4.5b), which would be an unexpected outcome if 

channeling was occurring or if the iron media had lost reactivity. 

A previous study proposed that ground-water mounding upgradient of gate 2 was likely driving flow underneath of the 

funnel-and-gate system near gate 2 (McMahon et al., 1999). Multi-level sampling using low-volume diffusion samplers 

in well GSA-25 shows that TCE concentrations in this well are highly depth dependent with the greatest concentrations 

detected near the bottom of the screened interval (Figure 4.6). These concentration trends cannot be used to reliably 

confirm or refute underflow of contaminants beneath the gate. The necessary hydraulic data are not available to make 

this assessment. However, examination of [TCE]/[1,1-DCE] ratios suggests that ground water upgradient of gate 2 

(1,1-DCE-rich) is distinct from and not likely to be a source of ground water present in downgradient well GSA-25. 

4.1.3 Gate 3 Contaminant Behavior 

The concentrations of volatile organic compounds in monitoring wells around gate 3 of the DFC are shown in Figure 4.7. 

In general, the concentrations of individual contaminants are at levels below 5 μg/L in well C3-I2 located within the 

treatment cell, with the exceptions of late-summer sampling events in August 2000, September 2001, and August 2002. 

In gate 3, temporal trends in contaminant distributions are clearly intermittent. Concentrations of cis-DCE, 1,1-DCE, VC, 

and 1,1-DCA in upgradient well GSA-31, for example, spiked in August of 2000. At the same time, elevated contaminant 

concentrations are found in well C3-I2 located within the reactive media. However, co-temporal spikes are not observed 

in downgradient well GSA-30. As in gate 2, contaminants detected in downgradient monitoring points are thought to be 

derived from residual bedrock contamination (Pacific Western Technologies, 2000). Correlations have been noted 

between high concentrations of volatile organic compounds and low ground water levels, but the mechanism linking 

these observations is unclear. 

4.1.4 Geochemical Parameters 

At the Denver Federal Center, trends in pH have followed consistent patterns from May 1999 to July 2001. Gates 1 

through 3 show comparable trends in pH from upgradient to downgradient sampling locations (Figure 4.8). Trends in 

contaminant degradation behavior among the reactive cells do not clearly correlate with pH, i.e., pH values in gate 2 are 

similar to pH values measured in gate 1 and gate 3. One notable feature is the continued decrease in pH of about 0.1 pH 

units per year in ground water from the iron media in gates 1, 2, and 3. This trend is marginally significant with respect 

to the precision and accuracy of the pH measurements but might be related to an overall decrease in residence time of 

ground water in the reactive media, related to pore-infilling by mineral precipitates and microbial biomass. 

The specific conductance (SC) of ground water at the Denver Federal Center shows variable patterns (Figure 4.9). In 

gate 1, SC values in the reactive iron media decrease by an average of about 31% relative to upgradient ground water; 

in gate 3 SC values in the reactive media decrease by about 28% relative to upgradient ground water. As described 

previously, decreases in SC are expected as ground water passes through zero-valent iron reaction zones due to 

mineral precipitation. On the other hand, trends in SC in gate 2 are anomalous. In 1999 and 2000, SC values in the iron 

58 
250 

GSA-26 

200 Gate 2 

150 

a) 

Upgradient 

100 

50 

0 

10/10/1995 2/21/1997 7/6/1998 11/18/1999 4/1/2001 8/14/2002 

150 

C2-I2 

120 Gate 2 

90 

b) 

Fe(0) 

60 

30 

0 

concentration, 

μg/L concentration, 

μg/L 

10/10/1995 2/10/1997 6/10/1998 10/10/1999 2/10/2001 6/10/2002 

600 

500 

c) 1,1-DCE 

1,1,1-TCA 

TCE 

1,1-DCA 

cis-1,2-DCE 

GSA-25


Gate 2 

Downgradient 

concentration, 

μg/L 

400 

300 

200 

100 

0 

10/10/1995 2/21/1997 7/6/1998 11/18/1999 4/1/2001 8/14/2002 

Figure 4.5	 Concentrations of contaminants through time in monitoring wells from the Denver Federal Center, gate 2 

(data from FHWA): a) well GSA-26 (upgradient); b) well C2-I2 (iron wall); c) well GSA-25 

(downgradient). Symbol key shown in Figure 4.5c applies to each graph; note change in concentration 

plotting range to show trends through time. 

59 
10 

1,1-DCE 

1,1,1-TCA 

TCE 

GSA-26 (upgradient, gate 2) 

10 

a) 

15 15


20 20


25 25


30 30


GSA-25 (downgradient, gate 2) 

1,1-DCE 

1,1,1-TCA 

TCE 

0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180


concentration, μg/L concentration, μg/L 

10 

sulfate 

calcium 

iron 

GSA-26 (upgradient, gate 2) 

10


b) 

15 15


20 20


25 25


30 30


GSA-25 (downgradient, gate 2) 

sulfate 

calcium 

iron 

0 50 100 150 200 250 300 350 400 450 0 50 100 150 200 250 300 350 400 450


concentration, mg/L concentration, mg/L 

depth, fbgs depth, fbgs 

Figure 4.6	 Depth-resolved concentrations of a) contaminants (μg/L) and b) sulfate, calcium, and iron (mg/L) in wells 

GSA-26 and GSA-25 from the Denver Federal Center (gate 2). Data collected in May 1999. 

60 
75 

70 

65 a) 1,1-DCE 

1,1,1-TCA 

TCE 

1,1-DCA 

cis-1,2-DCE 

GSA-31 

60 

55 Gate 3 

50 

45 Upgradient 

40 

35 

30 

25 

20 

15 

10 

5 

0 

-5 

10/10/1995 2/21/1997 7/6/1998 11/18/1999 4/1/2001 8/14/2002 

30 

b) C3-I2 

25 

Gate 3 

20 Fe(0) 

15 

10 

5 

0 

concentration, 

μg/L concentration, 

μg/L 

6/16/1996 10/29/1997 3/13/1999 7/25/2000 12/7/2001 

40 GSA-30 

35 Gate 3 

30 

25 

c) 

Downgradient 

20 

15 

10 

5 

0 

-5 

10/10/1995 2/21/1997 7/6/1998 11/18/1999 4/1/2001 8/14/2002 concentration, 

μg/L 

Figure 4.7	 Concentrations of contaminants through time in monitoring wells from the Denver Federal Center, gate 3 

(data from FHWA): a) well GSA-31 (upgradient); b) well C3-I2 (iron wall); c) well GSA-30 

(downgradient). Symbol key shown in Figure 4.7a applies to each graph; note change in concentration 

plotting range to show trends through time. 

61 
pH 2001


pH 2000


pH 1999


Upgrd. 

Gate 3 Fe(0) 

Dwngrd. 

Upgrd. 

Gate 2 Fe(0) 

Dwngrd. 

Upgrd. 

Gate 1 Fe(0) 

Dwngrd. 

0 2 4 6 8 10 

pH 

Figure 4.8	 Average pH values through time in wells from upgradient, iron wall, and downgradient positions relative 

to gate 1, gate 2, and gate 3 at the Denver Federal Center. 

media were actually greater than the values obtained from upgradient ground water. In 2001, the trend reversed to what 

is considered to be normal behavior, i.e., SC values were low within the reactive media (Figure 4.9). The high values of 

SC in Cell 2 in 1999 and 2000 indicate that the iron media in gate 2 was a source of dissolved solutes rather than a sink. 

Such anomalous trends in SC may be indicative of decreased performance or at least of non-typical reactive behavior. 

In gate 3, downgradient ground water has a significantly higher SC than ground water upgradient and within the 

treatment cell. This result suggests that the ground-water chemistry and contaminant distributions in ground-water from 

well GSA-30 are not greatly influenced by water emerging from the reactive cell (gate 3). 

At the Denver Federal Center, ground-water upgradient from the funnel-and-gate system is progressively more reducing 

moving northward from gate 1 to gate 3. Fairly typical trends in Eh values are observed in gate 1 (Figure 4.10). Eh 

values in ground water collected from upgradient and downgradient compliance wells range from about -75 mV to +150 

mV. In the reactive iron media of gate 1, Eh values are negative (-175 to -250 mV), and in downgradient locations, Eh 

values generally rebound to positive values. Somewhat lower and more variable Eh values are apparent in gate 2, 

reinforcing the overall anomalous behavior in this iron wall (Figure 4.10). 

4.1.5 Hydrogen Gas Concentrations 

The concentration of dissolved hydrogen gas is another key indicator of redox conditions that may be useful in PRB 

monitoring programs. Figure 4.11 shows trends in dissolved hydrogen concentrations in gate 1 at the DFC. 

Comparatively high concentrations of hydrogen are observed within the reactive media of gates 1-3. High dissolved 

hydrogen concentrations are an expected consequence of iron corrosion. Hydrogen concentrations are the greatest in 

62 
SC 2001


SC 2000


SC 1999


Upgrd. 

Gate 3 Fe(0) 

Dwngrd. 

Upgrd. 

Gate 2 Fe(0) 

Dwngrd. 

Upgrd. 

Gate 1 Fe(0) 

Dwngrd. 

0 200 400 600 800 1000 1200 1400 1600 1800 2000 

Specific Conductance, μS/cm 

Figure 4.9	 Average specific conductance values (μS/cm) through time in wells from upgradient, iron media, and 

downgradient positions relative to gate 1, gate 2, and gate 3 at the Denver Federal Center. 

Eh 2001 

Eh 2000 

Upgrd. 

Gate 3 Fe(0) 

Dwngrd. 

Upgrd. 

Gate 2 Fe(0) 

Dwngrd. 

Upgrd. 

Gate1 Fe(0) 

Dwngrd. 

Eh 1999 

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 

Eh, V 

Figure 4.10	 Average Eh (V) values through time in wells from upgradient, iron media, and downgradient positions 

relative to gate 1, gate 2, and gate 3 at the Denver Federal Center. 

63 
-5.5


-6.0 

-6.5 

-7.0 

-7.5 

-8.0 

-8.5 

-9.0 

-9.5 

-10.0 

Gate 2 

Fe(0) 

Gate 3 

Fe(0) 

2001 

2000 

1999 Gate 1 

log 

H2 (molar) 

Upgradient Upgradient Fe(0) Downgradient Downgradient 

peagravel peagravel 

Position 

Figure 4.11	 Concentrations of dissolved hydrogen (log molar) as a function of sampling position and time in gate 1 at 

the Denver Federal Center. Also shown are the concentration ranges of dissolved hydrogen measured 

in the iron media in gate 2 and gate 3. 

the reactive media of gate 3 and the least in gate 2. In gate 1, measured concentrations of hydrogen in the iron media 

and downgradient positions have progressively decreased from 1999 to 2001. The lowest hydrogen levels were 

observed in gate 2, which is consistent with the lower redox potentials indicated by platinum electrode measurements in 

this iron cell. The decreased reducing potential in gate 2 indicated by Eh and H2 measurements seems to correspond 

with the lower degree of contaminant removal in this reactive cell. Dissolved H2 concentrations observed within the 

reactive iron media correspond to equilibrium hydrogen gas partial pressures of about 0.05 to 1 mbar. In all cases, H2 

concentrations within the iron zones are greater than those typically encountered in methanogenic aquifers (5 to 30 nM, 

see Chapelle et al., 1996). 

4.1.6 Dissolved Cations and Anions 

The cation compositions of ground water upgradient from gates 1, 2, and 3 are broadly comparable. On a molar basis, 

sodium is the most abundant cation, followed by calcium, magnesium, and potassium. Cation concentrations in ground 

water from upgradient wells (GSA-21, GSA-26, GSA-31) range from 5.7-7.9 mM Na, 2.7-2.9 mM Ca, 0.8-1.8 mM Mg, 

and about 0.01 mM K. Ground water upgradient of gate 1 is slightly more enriched in Na, but depleted in Ca and Mg 

compared to ground water from regions upgradient of gate 3 (Figures 4.12-4.14). In the reactive cells, concentrations of 

calcium and magnesium are greatly reduced compared to upgradient wells, whereas concentrations of sodium are 

largely unchanged relative to upgradient regions. Potassium concentrations increase slightly in the upgradient pea 

gravel probably due to the dissolution of potassium-bearing aluminosilicates that are present in the pea gravel material. 

From 1999 to 2001, average reductions in the concentrations of calcium and magnesium between upgradient and midwall positions were greater than 95% and 75%, respectively, in gate 1. Similarly, average reductions in the 

concentrations of calcium and magnesium between upgradient and mid-wall positions were greater than 95% and 50%, 

respectively, in gate 2 between 1999 and 2001. 

64 
DFC Gate 1 

Ca, mg/L Na, mg/L 200 1.5 

175 

1.2 

150 

125 0.9 

100 

0.6 75 

50 

0.3 

25 

0 0.0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

120 20 

18 

100 16 

14 80 

12 

60 10 

8 

40 6 

4 20 

2 

0 0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

350 500 

300 

250 

200 

150 

100 

50 

0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

120 18 

100 

80 

60 

40 

20 

0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Mg, mg/L K, 

mg/L 

chloride, mg/L sulfate, mg/L 

0 

100 

200 

300 

400 

bicarbonate, mg/L 

0 

2 

4 

6 

8 

10 

12 

14 

16 

silica, mg/L 

Figure 4.12	 Average (± 1 s.d.) concentrations of Na, K, Ca, Mg, sulfate, bicarbonate, chloride, and silica (mg/L) as a 

function of sampling position in gate 1 at the Denver Federal Center. 

65 
DFC Gate 2 300 1.5 

250 1.2 

200 

0.9 

150 

0.6 

100 

0.3 50 

0 0.0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

120 100 

100 80 

80 

60 

60 

40 

40 

K, 

mg/L 20 20 

0 0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

500 

400 

350 

300 

250 

200 

150 

100 

50 

0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

150 18 

120 

90 

60 

30 

0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

Ca, mg/L Na, mg/L 

Mg, mg/L 

0 

100 

200 

300 

400 

bicarbonate, 

mg/L 

chloride, 

mg/L sulfate, 

mg/L 

0 

2 

4 

6 

8 

10 

12 

14 

16 

silica, mg/L 

Figure 4.13	 Average (± 1 s.d.) concentrations of Na, K, Ca, Mg, sulfate, bicarbonate, chloride, and silica (mg/L) as a 

function of sampling position in gate 2 at the Denver Federal Center. 

66 
DFC Gate 3 

300 3.5 

3.0 250 

2.5 200 

2.0 

150 

1.5 

100 1.0 

50 0.5 

0 0.0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

280 50 

45 240 

40 

200 35 

30 160 

25 

120 

silica, mg/L 20 

80 15 

10 

40 5 

0 0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

500 1500 

450 

400 

350 

300 

250 

200 

150 

100 

50 

0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

200 18 

16 

14 150 

12 

10 

100 

8 

6 

50 4 

2 

0 0 

Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. Upgrd. Upgrd. PG Fe(0) Dwngrd. PG Dwngrd. 

Ca, mg/L Na, mg/L 

Mg, mg/L K, 

mg/L 

0 

300 

600 

900 

1200 

bicarbonate, mg/L 

chloride, 

mg/L sulfate, mg/L 

Figure 4.14	 Average (± 1 s.d.) concentrations of Na, K, Ca, Mg, sulfate, bicarbonate, chloride, and silica (mg/L) as a 

function of sampling position in gate 3 at the Denver Federal Center. 

67 
In downgradient wells (GSA-20, GSA-25, and GSA-30), concentrations of sodium, calcium, magnesium, and potassium 

were broadly comparable to those measured in upgradient-monitoring locations (Figures 4.12-4.14). Calcium concentrations are slightly depleted (30% reduction) in downgradient well GSA-25 (gate 2) compared to Ca concentrations in 

upgradient well GSA-26. In well GSA-30, located downgradient from gate 3, concentrations of sodium and calcium are 

greater than those in upgradient well GSA-31. As noted previously, specific conductance values are also greater in well 

GSA-31 compared to well GSA-30 indicating that gate 3 only partially impacts the chemistry of downgradient ground 

water. Iron concentrations in downgradient monitoring points have always been below 0.5 mg/L (<0.01 mM). 

Interestingly, influent ground water to gate 3 is elevated in ferrous iron (>15 mg/L), perhaps due to an influence of anoxic 

bottom waters in Downing Reservoir. Dissolved iron concentrations, however, are expected to decrease within the 

reactive media due to the corrosion-induced pH increase, consequent development of oversaturated conditions with 

respect to iron hydroxide precipitates, and iron partitioning into the solid phase. 

In contrast to the cationic compositions of ground water collected from upgradient wells at the DFC that show only a 

moderate amount of variability, anionic compositions are more variable. In gates 1-3, the predominant anion on molar 

basis is bicarbonate, which ranges in concentration from about 5.9 mM (gate 2) to 11.9 mM (gate 3). Sulfate 

concentrations are fairly consistent in upgradient wells from gate 1 (2.5 mM) and gate 2 (2.9 mM), but sulfate was not 

detected in ground water upgradient of gate 3 (<0.01 mM). Chloride concentrations are fairly uniform, ranging from 

1.5 mM upgradient of gate 1 to 2.1 mM upgradient of gate 3. Concentrations of nitrate entering the funnel-and-gate 

system at the DFC are low to below detection limits (<0.03 mM). Alkalinity concentrations are between 26% and 55% 

lower in the reactive cells than in upgradient sampling wells. Sulfate concentrations are often below detection limits 

within the reactive cells; however, higher concentrations of sulfate (relative to upgradient points) were observed in wells 

located within the reactive media of gate 2 in 1999 and 2000 (up to 154% higher). This trend in sulfate concentrations 

in gate 2 is highly unusual compared to other PRB systems and suggests that some re-oxidation of sulfide precipitates 

was occurring, perhaps due to infiltration of oxidizing ground water. In 2001, however, the trend in sulfate concentrations 

in gate 2 reversed, i.e., lower concentrations were detected within the reactive media, but not to the >95% depletion 

levels typical in gate 1 or at the Elizabeth City PRB as previously described. Nitrate was never detected in mid-wall 

monitoring wells or in downgradient wells. 

Chloride concentrations are significantly higher in downgradient wells compared to upgradient wells. Up to about two 

times as much chloride is found in downgradient wells GSA-20, GSA-25, and GSA-30 as compared to upgradient wells 

GSA-21, GSA-26, GSA-31 (Figures 4.12-4.14). Similarly, sulfate concentrations are greater in the downgradient regions 

of gate 1 and gate 3, whereas concentrations of bicarbonate are lower. In general, the geochemical trends at the DFC 

are fairly atypical of other PRB sites investigated in the Tri-Agency initiative. Well transects across the various reactive 

cells do not appear to show geochemical connectivity between upgradient and downgradient regions. As was observed 

in the contaminant distributions, the geochemistry of downgradient ground water is only partially represented by the 

chemistries of ground water emerging from the various treatment cells. 

4.2 Core Sampling at the Denver Federal Center 

Core samples were collected at the DFC from gate 1, gate 2, and gate 3 in July 2000 and 2001. Similar core collection 

methods to those used at the Elizabeth City site were adopted at the DFC. Core collection methods and analysis 

procedures are described in Volume 2 of this EPA Report series. In all cases 5 cm inner diameter cores were collected 

using direct-push methods (GeoprobeTM). Angled cores (30° relative to vertical) and vertical cores were collected in 

order assess the spatial distribution of mineral/biomass buildup in the reactive media. Prior to pushing the core barrel, 

an electrical conductivity profile was collected to verify the exact position of the iron/pea gravel interface. 

Core materials from the DFC PRB were jet black in color. Cores collected in July 2000 showed no obvious signs of 

cementation. Several cores collected in July 2001, however, contained sub-spherical welded nodules of iron grains up 

to about 3 cm in diameter (Figure 4.15). Iron grains from the upgradient interface of DFC gate 2 were noticeably enriched 

in a black-colored, gel-like material (mixture of biomass and fine-grained mineral precipitates). This core consistency 

was not observed at other DFC gates or at the Elizabeth City PRB (Figure 4.16). 

Immediately after collection, the cores were frozen and shipped back to the Ground Water and Ecosystems Restoration 

Division in Ada, OK for sub-sampling and analysis. The frozen cores were partially thawed and then placed in an 

anaerobic chamber maintained with a H2

-N2 atmosphere. Each core was logged and partitioned into 5 to 10 cm 

segments. Each segment was homogenized by stirring in the glove box and then split into 4 sub-samples: (1) inorganic 

carbon analyses, (2) sulfur analyses/X-ray diffraction (XRD), (3) Scanning electron microscopy (SEM)/X-ray photoelectron spectroscopy (XPS) analyses, and, (4) microbial assays (phospholipid fatty acids, PLFA). All sub-samples were 

retained in airtight vials to prevent any air oxidation of redox-sensitive constituents. Details of analytical methods used 

to characterize the core materials are presented in Volume 2 of this EPA Report series. 

Locations of coring events at the DFC are shown in Figures 4.1-4.3 and information about core recovery, core length, 

and depth of core penetration is presented in Table 4.1. Cores collected from the upgradient aquifer/iron region generally 

penetrated the PRB at depths of 4.5 to 6.5 m below ground surface. 

68 
Figure 4.15	 Picture showing cemented nodules recovered from a gate 1 core collected at the Denver Federal Center 

(core C1-1-71701). 

Figure 4.16	 Picture showing the appearance of a core collected at the Denver Federal Center, from gate 2 near the 

upgradient pea gravel/iron interface. 

69 
5.3 

6.3 

5.3 

5.3 

5.8 

5.8 

6.1 

4.2

4.2

4.2

5.5

4.8 

5.3 

5.8 

4.3 

4.2

4.2

4.2

Depth to iron is the depth 

Depth to Fe 

below ground surface in meters at which zero-valent iron was intercepted. The percent recovery was calculated as (length compacted core/push length)x100. 

m bgs 

Notes: All gate 2 and gate 3 cores (C2, C3) were collected by pushing the core barrel from the downgradient side of the reactive barrier. 

Recovery 

% 

83 65 

67 

92 

96 

79 

79 

94 

88 

69 

67 

73 

69 

71 

46 

83 

75 

62 

Core length 

cm 

102 

112 

117 

114 

107 

102 

79 

81 

97 

97 

91 

81 

82 

81 

43 

28 

91 

71 

Angle 

30 30 

30 

30 

30 

30 

30 

90 

90 

90 

30 

30 

30 

30 

30 

90 

90 

90 

Table 4.1. Cores collected for analysis at the Denver Federal Center PRB 

July-01 July-01 

July-01 

July-01 

July-01 

July-01 

July-01 

July-01 

July-01 

July-01 

July-01 

July-00 

July-00 

July-00 

July-00 

July-00 

July-00 

July-00 

Date 

Downgradient 

Downgradient 

Downgradient 

Location Upgradient Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Vertic

Upgradient 

Upgradient 

Upgradient 

al 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

C2-17-71300 

C2-16-71300 

C2-12-71300 

C2-13-71300 

C2-14-71300 

C1-1-71701 C1-2-71701 

C1-3-71701 

C1-4-71801 

C2-1-71801 

C2-3-71801 

C2-4-71801 

C2-1-71901 C2-2-71901 

C2-3-71901 

C3-2-71801 

C1-2-71000 

Sample ID 

C1-3-71100 

70

4.2.1 Carbon Analysis 

Results of all carbon analyses of DFC core materials (n = 251) are presented in Table A3 (Appendix A). Concentrations 

of inorganic carbon within >95% iron samples range from about 10 to 11,600 μg/g. In all cases the highest 

concentrations were found in samples collected adjacent to the upgradient or downgradient pea gravel/iron interfaces 

and the lowest concentrations were detected near the center regions of the iron gates. 

The concentration distribution of inorganic carbon in cores collected from DFC gate 1 is shown in Figure 4.17. The 

maximum concentration near the upgradient edge of gate 1 was observed in core C1-1-71701 (8700 μg/g) in material 

that contained a mixture of Fe0

 and pea gravel (Figure 4.17). A concretion was recovered in this core at the location 

corresponding to the maximum carbonate concentration (Figure 4.17). Inorganic carbon concentrations fell below 

300 μg/g in gate 1 at horizontal penetration depths >25 cm. Core materials were not collected from the mid-barrier 

regions, but the trends shown in Figure 4.17 suggest that carbonate accumulation near the center of the iron wall is 

negligible. Surprisingly, elevated inorganic carbon concentrations were observed in two cores collected at the 

downgradient pea gravel/Fe0 interface region (Figure 4.17). Concentrations in core C1-4-71801 were even higher than 

those detected near the upgradient region of the reactive media. Concretions up to 2-cm in diameter were recovered in 

samples that contained between about 8,000 and 11,600 μg/g inorganic carbon. 

Gate 2 shows trends in inorganic carbon concentrations that are similar to those observed in gate 1. Concentrations as 

high as 7,700 μg/g were detected in core C2-3-71801; concretions were also identified in this core (Figure 4.18). Vertical 

cores were collected in gate 2 (C2-1-71901, C2-2-71901, and C2-3-71901) to examine the depth-dependent distribution 

of carbon and sulfur accumulation in the reactive media. The vertical sampling position was located approximately 15 cm 

downgradient of the upgradient pea gravel-Fe0

 interface (Figure 4.2). Results show that there is a depth interval between 

about 5.5 m and 8.0 m below ground surface where very little inorganic carbon accumulation is occurring (Figure 4.19); 

significant amounts of carbonate precipitation has occurred only near the bottom and the top of the reactive zone. The 

implication of this trend, which is also mirrored in the concentration profiles of sulfur and microbial biomass, is that little 

ground water is entering gate 2 over the depth interval from about 5.5 m to 8 m below ground surface. Flow would appear 

to be occurring near the very top and the bottom of the reactive cell. This result could help explain the anomalous 

behavior of this gate with respect to contaminant removal performance. 

4.2.2 Sulfur Analysis 

Results of sulfur analyses of DFC core materials strongly correlate with the inorganic carbon results (Figures 4.20-4.21). 

Concentrations of sulfur in core materials from the DFC range from about 100 μg/g to 7,500 μg/g (Table A3). Chemical 

extractions indicate that over 90% of the total sulfur present in the core materials is as sulfide, in acid-volatile sulfide 

materials such as poorly crystalline to crystalline mackinawite (Table A2; Wilkin et al., 2003). The remaining sulfur is 

likely present as iron disulfides (pyrite) and perhaps as sulfate associated with iron corrosion products (Furukawa et al., 

2002). Figure 4.22 shows the good correlation between concentrations of carbon and sulfur in the solid phase in cores 

collected from the Elizabeth City and Denver Federal Center PRBs. The average S/C ratio in core materials from the 

DFC is about 0.42, which is slightly greater than the S/C ratio observed in Elizabeth City core materials (S/C=0.33). The 

difference is apparently related to a higher average ground-water S/C ratio at the Denver Federal Center site compared 

to the Elizabeth City site. 

4.2.3 X-ray Diffraction Analysis 

Powder X-ray diffraction scans for samples from DFC core C2-3-71801, collected in 2001, are shown in Figure 4.23. 

Materials for analysis were obtained by sonicating iron core samples in acetone for 10 minutes followed by collection of 

the released particulates on a 0.2-micron filter paper (polycarbonate). The separated particles were then mounted on a 

zero-background quartz plate and scanned with Cu Kα radiation from 3° to 80° 2-theta using a Rigaku Miniflex 

Diffractometer. 

A summary of the XRD analysis results is reported in Table 4.2. Qualitative abundances of the mineral phases identified 

are reported based upon observed peak intensities (Table 4.2). Magnetite (Fe3

O4) and iron carbonate hydroxide were 

observed in every iron core sample. Graphite and quartz were detected in one of the samples (C2-3-71801–3). 

Aragonite (CaCO3

) was not detected in the diffraction analysis, although the presence of calcite was confirmed using 

XRD, SEM and optical microscopy. Using micro-analytical diffraction methods, Furukawa et al. (2002) also detected 

quantities of mackinawite, greigite, ferrihydrite, and goethite in core materials collected from the DFC. 

4.2.4 Scanning Electron Microscopy 

SEM photomicrographs for three samples from the DFC collected in July 2000 are shown in Figure 4.24. These samples 

were retrieved from near the upgradient pea gravel/iron interface region from gate 2 (C2-17-71300-2, horizontal 

penetration ~4 cm; Figure 4.24a, C2-17-71300-7, horizontal penetration ~16 cm; Figure 4.24b) and gate 1 (C1-2-71000-3, 

horizontal penetration ~ 6 cm; Figure 4.24c). The SEM micrographs in Figure 4.24 are representative of particles 

contained in each of the samples and capture a range of magnifications from about 50x to 2000x. 

71 
10000 

12000 

14000 

16000 

C1-1-71701 

C1-2-71000 

C1-3-71100 

C1-4-71801 

upgradient pea gravel 

downgradient 

pea gravel 

Fe(0) + pea gravel 

Concretions observed 

Concretions observed 

Inorganic Carbon, 

μg/g 

8000


6000


4000


2000


0


0 50 100 150 200 

Distance, cm 

Figure 4.17	 Concentration distribution of solid phase inorganic carbon in angle cores collected from gate 1 at the 

Denver Federal Center. 

C2-17-71300 

C2-3-71801 

C2-4-71801 

upgradient pea gravel 

8000


7000


6000


g/g Inorganic Carbon, 

μ

5000


4000


3000


2000


1000


0


0 15 30 45 60 

Distance, cm 

Figure 4.18	 Concentration distribution of solid phase inorganic carbon in angle cores collected from gate 2 at the 

Denver Federal Center. 

72 
5


Iron Zone 

C2-[1-3]-71901 

6 

7 

8 

9 

10 

0 2000 4000 6000 8000 

Inorganic Carbon, μg/g 

Figure 4.19	 Concentration distribution of solid phase inorganic carbon in a vertical core collected from gate 2 at the 

Denver Federal Center. 

8000 

7000 

6000 

5000 

4000 

3000 

2000 

1000 

0 

C1-1-71701 

C1-2-71000 

C1-3-71100 

C1-4-71801

downgradient 

pea gravel 

De

pth 

B

G

S, m 

Total Sulfur, μg/g 

0 50 100 150 200 

Distance, cm 

Figure 4.20	 Concentration distribution of solid phase sulfur in angle cores collected from gate 1 at the Denver 

Federal Center. 

73 
4000
 C2-17-71300 

C2-3-71801 

C2-4-71801 

g/g

Total Sulfur, 3000 

2000 

1000 

0 

0 15 30 45 60 

Distance, cm 

Figure 4.21	 Concentration distribution of solid phase sulfur in angle cores collected from gate 2 at the Denver 

Federal Center. 

Sample C2-17-71300-2, collected from a region near the upgradient interface in gate 2, shows a fairly regular 

accumulation of mineral precipitates on the iron surfaces (Figure 4.24a). The thickness of the surface coating ranges 

from about 10 μm to 100 μm. This precipitate accumulation took place over the initial four years of operation of the PRB, 

or at a rate of about 3 to 25 μm per year, similar to the average linear rate of precipitate accumulation observed at the 

Elizabeth City site. As noted before, ground-water chemistry is different between these two sites as is the total mass of 

mineral precipitate accumulation. Rates of inorganic carbon and sulfur accumulation are between 2 and 10 times greater 

within the DFC reactive media as compared to the Elizabeth City reactive media (Wilkin et al., 2003). The similar 

average thickness of mineral precipitates found on iron grains near the upgradient edge at the two sites suggests that 

mineral precipitation at the DFC must occur over a wider range of penetration depths, i.e., with time a mineral precipitate 

front moves through the reactive barrier. 

The DFC samples contain a significant proportion of free grains, grains not directly bound to the iron surfaces 

(Figure 4.24a). In all cases, these large free grains (up to 300 μm in diameter) are composed of calcium carbonate. At 

greater horizontal penetration depths in gate 2, surface coatings were thinner, 10 to 50 μm, but the abundance of free 

calcium carbonate grains persisted (Figure 4.24b). It cannot be determined conclusively whether the free grains actually 

grew within pore spaces, or whether they were at one point attached to the iron surfaces, but were subsequently 

detached, for example, during vibratory coring or during sample handling. In gate 2, surface coverage and particle 

morphologies are similar to those observed in gate 1 (Figure 4.24c). The micrographs clearly reveal an abundance of 

calcium carbonate grains associated with the iron surfaces. 

Energy dispersive X-ray analyses of iron grains from the DFC (Table 4.3) indicate the presence of iron (~97 wt%), silicon 

(~2.3%), and to some extent Mn (<1.1 wt%) and Cr (<1.2 wt%). Oxygen was not detected on freshly polished surfaces 

of zero-valent iron. The compositions of Ca- and O-rich grains (presumably calcium carbonate) associated with iron 

surfaces are identical to those found as free grains (Table 4.3). These particles appear to be enriched in calcium 

compared to ideal CaCO3

 (40.0 wt% Ca), and this is probably related to quantitative inaccuracies of the EDX method. 

Surface precipitates, i.e., materials that are generally fine-grained and coat the iron surfaces, are enriched in oxygen and 

depleted in iron, respectively, compared to the composition of fresh zero-valent iron (Table 4.3). In addition to iron and 

74 
4000 

3500 

a) Elizabeth City cores 

S = 128.6 + 0.33 * IC 

R2 

3000 = 0.48, n=170

Total s

ulfur, 

u

g/g 

Total S

ulfur, 

u

g/g 

2500 

2000 

1500 

1000 

500 

0 

-500 

0 1000 2000 3000 4000 5000 6000 

Inorganic carbon, ug/g 

-1000 

10000 

b) Denver Federal Center cores 

S = 712.9 + 0.42 * IC 

R = 0.62, n=251 

9000 

8000 

7000 

6000 

5000 

4000 

3000 

2000 

1000 

0 

-2000 0 4000 8000 12000 16000 20000 

Inorganic Carbon, ug/g 

Figure 4.22	 Inorganic carbon concentrations versus total sulfur: a) Elizabeth City core materials; b) Denver Federal 

Center core materials. 

75 
3500


3000 

2500 

2000 

1500 

1000 

500 

0 

0 10 20 30 40 50 60 70 80 90 

mid-barrier 

upgradient edge 

DFC core C2-3-71801 

-15 

-7 

-5 

-3 

-2 

-1 

degrees 2-theta 

Figure 4.23	 Powder X-ray diffraction data from fine-grained materials removed via sonication from cores collected at 

the Denver Federal Center PRB (core C2-3-71801). 

Table 4.2. Results of Powder X-ray Diffraction Analysis of Core Materials from the Denver Federal Center PRB Inte

n

sity


Sample Major Component Minor Component 

C2-3-71801-1 Magnetite, Iron Green rust 1, Calcite 

carbonate hydroxide 

C2-3-71801-2 Magnetite, Iron Mackinawite, Calcite 

carbonate hydroxide 

C2-3-71801-3 Magnetite, Iron Mackinawite, Calcite 

carbonate hydroxide 

C2-3-71801-5 Magnetite, Iron Mackinawite, Calcite 

carbonate hydroxide 

C2-3-71801-7 Magnetite, Iron Mackinawite, Calcite 

carbonate hydroxide 

C2-3-71801-15 Magnetite, Iron Calcite 

carbonate hydroxide 

Trace Component 

Goethite 

Quartz, Graphite 

76 
15 kV 150x 100 μm BSE

Iron 

(backscattered 

electron image) 

Surface precipitates 

μ

15 kV 200x 100μm 

15 kV 500x 50 m 

15 kV 100x 100μm 

15 kV 2000x 10μm 

Calcium 

carbonate 

15 kV 350x 50μm 

Figure 4.24	 Scanning electron micrographs of samples from the Denver Federal Center PRB:a) sample C2-17-

71300-2 located near the upgradient edge of gate 2; b) sample C2-17-71300-7 located in a midbarrier 

region of gate 2; and, c) sample C1-2-71000-3 located near the upgradient edge of gate 1. 

77 
15 kV 350x 50 μm 15 kV 100x 100 μm

15 kV 350x 50 μm

15 kV 150x 100 μm 15 kV 50x 500 μm

15 kV 1000x 10 μm

Surface 

carbonate 

Free 

carbonate 

Free 

carbonate 

Figure 4.24	 (continued) Scanning electron micrographs of samples from the Denver Federal Center PRB:a) sample 

C2-17-71300-2 located near the upgradient edge of gate 2; b) sample C2-17-71300-7 located in a 

midbarrier region of gate 2; and, c) sample C1-2-71000-3 located near the upgradient edge of gate 1. 

78 
15 kV 75x 100 μm 15 kV 350x 50 μm

15 kV 1000x 10 μm

15 kV 200x 100 μm 15 kV 1500x 10 μm

15 kV 350x 50 μm

Surface 

carbonate 

Figure 4.24	 (continued) Scanning electron micrographs of samples from the Denver Federal Center PRB:a) sample 

C2-17-71300-2 located near the upgradient edge of gate 2; b) sample C2-17-71300-7 located in a 

midbarrier region of gate 2; and, c) sample C1-2-71000-3 located near the upgradient edge of gate 1. 

79 
Table 4.3. Results of SEM-EDX Analysis of Core Materials from the Denver Federal Center PRB


Iron 

(n=25) 

Ca-rich 

precipitates, 

on iron 

surfaces 

(n=23) 

Ca-rich 

precipitates, 

Fe 

(wt%) 

97.4 

(±0.7) 

0.2 

not 

O 

(wt%) 

not 

detected 

54.7 

(±3.5) 

54.8 

free detected (±1.5) 

(n=11) 

Surface 

precipitates 58.3 31.6 

(n=57) (±10.5) (±5.5) 

S Mn Cr Ca Mg Si 

(wt%) (wt%) (wt%) (wt%) (wt%) (wt%) 

not 

detected 

0.15 

(±0.14) 0.3 not 

detected 

not 

detected 

2.30 

(±0.4) 

not not not 44.5 not not 

detected detected detected (±3.4) detected detected 

not not not 44.6 not not 

detected detected detected (±1.9) detected detected 

3.9 

(±3.0) 

0.34 

(±0.7) 0.04 1.2 

(±5.2) 

0.11 

(±0.3) 

3.89 

(±3.0) 

oxygen, the surface precipitates also contain silicon (3.4 ± 1.9 wt%, n=58), sulfur (3.89 ± 3.0 wt%, n=58), manganese 

(0.34 ± 0.70 wt%, n=58), chromium (0.04 ± 0.2 wt%, n=58), calcium (1.2 ± 5.2 wt%, n=58), and magnesium 

(0.11 ± 0.30 wt%, n=25). The presence of Ca, Mg, and Si in the surface coatings is consistent with their removal from 

ground water. Other than CaCO3, the identity of mineral species could not be confirmed by using EDX measurements. 

4.2.5 Microbial Characterization 

Eighty-one samples from 2000 to 2001 were collected at the DFC for phospholipid fatty acid (PLFA) extract 

characterization. Samples were collected mainly from within the reactive iron media near the upgradient iron/pea gravel 

interface and near the downgradient iron/pea gravel interface. Samples for PLFA analysis were shipped frozen on dry 

ice to Microbial Insights (Rockford, Tennessee). The complete PLFA data set for samples from the DFC is shown in 

Table B2 (Appendix B) and summarized in Table 4.4. 

In samples collected at the DFC, biomass contents spanned several orders of magnitude from 15 to 4924 picomoles per 

gram (dry weight basis), or from about 3.85x105 to 9.85x107 cells per gram. The highest biomass contents observed in 

this study were in gate 2 of the DFC, where maximum PLFA concentrations were about 1.5 to 3 times the maximum 

concentrations observed in DFC gate 1 or in the Elizabeth City PRB. 

In gate 1 and gate 2, the highest biomass concentrations were found near the upgradient pea gravel/iron interface region, 

in the same portion of the reactive media where enrichments in inorganic precipitates are observed (Figure 4.25). This 

same trend was observed at the Elizabeth City PRB and appears to be repeated at other PRB sites (Gu et al., 2002). 

Downgradient regions are comparatively depleted in microbial biomass. The lower counts associated with the midbarrier and downgradient samples suggest that the environment at these locations is more challenging to bacterial 

growth and survival. Vertically resolved biomass concentrations and total sulfur concentrations from gate 2 are plotted 

in Figure 4.26. The results parallel those previously shown for inorganic carbon (Figure 4.19). The depth interval 

between about 5.5 and 8.0 m below ground surface is characterized by low accumulation of sulfur and microbial biomass 

suggesting stagnant conditions in this portion of gate 2. 

PLFA profiles from DFC are typically dominated by fatty acid biomarkers indicative of anaerobic sulfate- or iron-reducing 

bacteria (Figure 4.27). High proportions of terminally branched and branched monoenoic PLFA specifically indicate 

anaerobic metabolism. Terminally branched PLFA are typical of Gram-positive bacteria, but can also be present in the 

cell membranes of some anaerobic Gram-negative bacteria. Because high proportions are present of branched 

80 
Table 4.4. Summary of PLFA Data from the Denver Federal Center PRB


Number of samples


Average PLFA


concentration


(pmoles/g)


PLFA range


PLFA Structural


Groups


average % (range %)


Monoenoic 

Found in Gram-negative 

bacteria 

Terminally Branched 

Saturated 

Found in many Grampositive bacteria, and in 

some Gram-negative 

bacteria 

Branched Monoenoic 

Common in obligate 

anaerobes, such as 

sulfate-reducing and 

iron-reducing bacteria 

Mid-Chain Branched 

Saturated 

Common in 

actinomycetes and 

sulfate-reducing bacteria 

Normal Saturated 

Ubiquitous in 

prokaryotes and 

eukaryotes 

Polyenoic 

Found in fungi, 

protozoa, algae, higher 

plants, and animals 

Fe(0) 

Upgradient 

DFC - Gate 1 

9 

841 

95-1904 

59.6 

(10.1-71.2) 

7.0 

(2.7-11.4) 

8.3 

(2.0-11.1) 

4.2 

(1.7-6.2) 

18.5 

(4.6-23.2) 

2.4 

(0.2-6.7) 

Fe(0) 

Downgradient 

DFC – Gate 1 

9 

49 

15-127 

77.4 

(9.2-87.8) 

2.1 

(<1-6.9) 

3.7 

(<1-8.9) 

1.3 

(<1-4.7) 

12.2 

(3.8-17.3) 

3.3 

(<1-9.4) 

Fe(0) 

Upgradient 

DFC - Gate 2 

38 

1440 

50-4924 

52.2 

(31.6-85.3) 

14.2 

(3.4-26.4) 

10.1 

(<1-16.5) 

6.1 

(<1-22.2) 

15.4 

(8.9-30.6) 

2.0 

(<1-8.2) 

Fe(0) 

Downgradient 

DFC - Gate 2 

6 

98 

24-332 

69.6 

(54.9-77.7) 

6.8 

(3.1-26.4) 

2.6 

(<1-8.4) 

2.4 

(<1-10.6) 

11.2 

(8.6-13.2) 

7.4 

(1.7-18.4) 

81 
PLFA,picomoles/gm 

PLFA, picomoles/g 

1000 

1500 

2000 

2500 

3000 

a) Gate 1, Fe(0) upgradient 

Gate 1, Fe(0) downgradient 

500 

0 

1 2 3 4 5 6 7 8 9 

Ranked Sample Distribution 

1000 

2000 

3000 

4000 

5000 

b) Gate 2, Fe(0) upgradient 

Gate 2, Fe(0) downgradient 

0 

5 10 15 20 25 30 35 

Ranked Sample Distribution 

Figure 4.25	 Ranked concentration distribution of microbial biomass (from Denver Federal Center PLFA data) in 

picomoles per gram in iron from near the upgradient pea gravel/iron interface and iron from near the 

downgradient pea gravel/iron interface: a) gate 1; b) gate 2. 

monoenoic PLFA indicative of anaerobic metal reducing bacteria, the terminally branched PLFA are likely to be mainly 

from sulfate- or iron-reducing bacteria. Where biomass is most concentrated (i.e., near the upgradient pea gravel/iron 

interface), the distribution of PLFA overall appears to be distinct from the PLFA distribution observed in regions further 

downgradient (Figure 4.27). Near the upgradient pea gravel/iron interface, the proportion is greater of branched 

monoenoic PLFA and PLFA indicative of sulfate-reducing bacteria compared to the PLFA signature of materials 

collected at the downgradient pea gravel/iron interface (Figure 4.27). 

4.3 Summary of Results from the Denver Federal Center Site 

The Denver Federal Center permeable reactive barrier is a funnel-and-gate system with four reactive gates, each 

separated by up to about 120 m of metal sheet pile. In this study, ground-water sampling, core collection, and solid 

phase characterization studies were carried out in gates 1, 2, and 3. After five years of operation, gate 1 and gate 3 have 

been effective in significantly decreasing concentrations of TCE, 1,1,1-TCA, 1,1-DCE, and cis-DCE from influent ground 

water. A noticeable clean ground-water front is identifiable in the downgradient aquifer near gate 1. The performance of 

gate 2 has been more difficult to assess, in part, because of an apparent source of residual bedrock TCE contamination 

82 
5

6 

7 

8 

9 

10 

0 500 1000 1500 2000 2500 3000 3500 4000 

DFC Gate 2 

Vertical Core 

Total Sulfur,g/g 

Biomass (PLFA), picomoles/g 

Total Sulfur, Biomass 

Figure 4.26	 Concentration distribution of solid phase sulfur and microbial biomass (from PLFA data) in a vertical core 

collected from gate 2 at the Denver Federal Center (vertical cores C2-1-71901, C2-2-71901, and C2-3-

71901). 

located just downgradient of gate 2 and because of the large hydraulic head differential across the gate. Concentrations 

of volatile organic compounds in gate 2, especially 1,1-DCE, have shown increases with time. Results of the Denver 

Federal Center site study are summarized below: 

� Trends in pH in gates 1, 2, and 3 were fairly consistent from May 1999 to July 2001. There is some 

indication that pH in the iron zone is slowly increasing with time, perhaps due to increasing residence time. 

Measurements of Eh and hydrogen gas concentrations indicate consistent reducing conditions in gates 1 

and 3. Gate 2 shows evidence of having decreased reducing potential, based on increased Eh and 

decreasing hydrogen gas concentrations. 

� The reactive gates at the Denver Federal Center have removed most of the dissolved calcium, magnesium, 

sulfate, and silicon in the water flowing through the PRB. Levels of alkalinity and total dissolved solids were 

reduced. Ground water from the DFC is comparatively (~3x) more enriched in concentrations of total 

dissolved solids compared to ground water from the Elizabeth City site. Consequently, more mineral 

precipitate mass has accumulated over five years in the DFC gates compared to the Elizabeth City PRB. 

After five years, core sampling revealed the presence of cemented iron nodules in some of the cores 

collected at the DFC. 

� Mineralogical characterization of soil cores indicates the formation in the iron gates of calcite, iron carbonate 

hydroxide, magnetite, mackinawite, carbonate green rust, and goethite. Overall, this assemblage of mineral 

precipitates is very similar to that observed at the Elizabeth City site and at other PRB installations. Mineral De

p h t B

G

S, m

83 
8.3%


4.2%
 3.7%


1.3%


12%


19%
 77%


60%
 3.3%
 2.4%


7%
 2.1%


Gate 1 upgradient
 Gate 1 downgradient


10%
 2.6%
 6.1%
 2.4%


11%


7.4%


15%
 70%


52%


2%


6.8%
 14%


Gate 2 upgradient Gate 2 downgradient


Gram+/Gram- (Ter. Br. Sat.)


Gram- (Monoenoic)


Anaerobic (Br. Mono.)


SRB, Actinomycetes


Genera Nsats


Polyenoic


Figure 4.27	 Pie graphs showing average structural distribution of PLFA compounds in Iron core materials from the 

Denver Federal Center. 

84 
precipitates are enriched in gate 1 and gate 2 at both the upgradient and downgradient edges of the reactive 

medium. After five years of operation, up to 50% of the available pore space has been lost due to infilling of 

mineral precipitates near the leading edges of the iron treatment zones. The high infilling rate has led to 

partial cementation of the iron zone. Because of the high influent TDS concentrations and rates of 

mineralization, hydraulic performance of the system is expected to degrade over the next five years. 

� Vertical coring in gate 2 reveals a zone from 5.5 to 8 m below ground surface with little mineral or microbial 

biomass accumulation, suggesting a substantial zone of reduced flow through the PRB. This reduced flow 

zone may be related to the low permeability zone or smear zone that developed during construction of the 

gate. 

� Microbiological results, based on PLFA analyses, indicate enriched biomass in gate 2 of the Denver Federal 

Center (up to 9.85x107

 cell/g). Biomass concentrations in gate 2 were a factor of about 2x greater compared 

to gates 1 and 3, and the Elizabeth City PRB. The higher biomass concentrations may be linked to low-flow 

conditions in the gate and high sulfate concentrations. Concentrations of iron sulfide precipitates are also 

comparatively high in gate 2 of the DFC (up to 5650 μg/g). Gram-negative bacteria dominate the microbial 

community. Present in the iron samples are elevated proportions of biomarkers indicative of metal-reducing 

and sulfate-reducing bacteria. 

85 
86

5.0 Factors Affecting Longevity and Performance 

Permeable Reactive Barriers are principally designed to remove contaminants from ground water, yet it has been shown 

that zero-valent iron reactive barriers are long-term sinks for other dissolved components in ground water. In particular, 

inorganic carbon, sulfate, calcium, magnesium, nitrate, and silica are either entirely or partially removed from influent 

waters to PRBs. Removal processes for inorganic constituents include mineral precipitation, adsorption, and biochemical transformation. Mineral precipitation, in particular, can lead to changes in porosity and permeability, or both, in 

addition to affecting the reactivity of zero-valent iron media by forming coatings on the reactive grains. In this way, 

geochemical reaction processes can have an effect on the hydraulic and reactive performance of PRB systems. A major 

research effort, therefore, has gone into 1) identifying the nature of element removal processes in PRBs, 2) linking these 

processes to site geochemistry and hydrogeology, and 3) linking these processes to declining remedial performance. 

Considerable progress has been made on the first question relating to identifying element removal process. Observations from numerous PRB installations and laboratory studies are used below to develop a framework for understanding 

how ground-water chemistry may be evaluated for the purpose of predicting longevity and barrier performance through 

time. This framework will be useful in constructing remedial designs and in developing performance-monitoring 

programs. 

Iron metal dissolution, microbial sulfate reduction, microbial nitrate reduction, adsorption, gas production, and mineral 

precipitation (oxides, hydroxides, carbonates, and sulfides) are processes that have been recognized as important 

geochemical and biogeochemical processes in zero-valent iron walls. The lifetime of an iron wall will essentially be 

determined by the extent that these processes impact the reactive barriers' ability to remove contaminants from ground 

water. These processes are discussed in the following sections. 

5.1 Fe0 Dissolution 

The corrosion of zero-valent iron in aqueous environments has been widely studied (e.g., Davies and Burstein, 1980; 

Reardon, 1995; Blengino et al., 1995; Gui and Devine, 1994; Odziemkowski et al., 1998). In water, zero-valent iron may 

be oxidized to ferrous- or ferric-iron, leading to dissolution and volume loss of the metal. Under oxic conditions, dissolved 

oxygen acts as the electron acceptor and can lead to an increase in pH and the production of ferrous-iron and/or ferriciron: 

2Fe0

 + O2 + 2H2O = 2Fe2+ + 4OH- (5.1) 

4Fe2+ + O2 + 2H+ = 4Fe3+ + 2OH- (5.2) 

Ferric-iron is not expected to remain soluble and will precipitate, for example, as ferric hydroxide: 

Fe3+ + 3OH- = Fe(OH)3

(s) (5.3) 

As oxygen is consumed, iron corrosion reactions result in the production of hydrogen. 

Fe0

 + 2H2O = Fe2+ + H2 + 2OH- (5.4) 

Both aerobic and anaerobic iron corrosion reactions lead to an increase in pH. Aerobic corrosion is a more rapid 

process, as evidenced by the rapid loss of dissolved oxygen in iron-water systems. As iron corrosion proceeds, iron 

mineral precipitates form on or near the surface of corroding iron grains, which increases the thickness of an iron oxide 

passivation layer already present at the metal surface (e.g., Ritter et al., 2002). Under anaerobic conditions, hydrogen 

gas produced through reaction (5.4) may also temporarily passivate the iron surface. In general, low pH and the 

presence of oxidants result in more rapid iron corrosion rates. However, many species abundant in ground water can 

affect the rate and pathway of iron corrosion. For example, chloride, carbonate, and sulfate can all affect the corrosion 

rate of iron metal. 

The major consequences of iron dissolution that follow from reactions (5.1)-(5.4) are the production of OH- (pH increase), 

decrease in oxidation-reduction potential (Eh decrease), increase in hydrogen concentration, release of ferrous iron, and 

possible precipitation of sparingly soluble iron precipitates. Laboratory and field studies have shown that pH values in 

iron walls are typically >9 and <11. Eh values measured in zero-valent iron systems are oftentimes below –0.50 V. 

87 
Measured Eh values in PRBs are likely governed by redox reactions involving Fe(0), Fe(II), and Fe(III). For example, 

reactions that might bracket measured Eh values in iron walls are: 

Fe(OH)3 + 3H+ + 3e- = Fe0

 + 3H2O (5.5) 

and, 

Fe(OH)3

 + 3H+ + e- = Fe2+ + 3H2O (5.6) 

The Eh-pH equation for the ferrihydrite-zero-valent iron couple (E0

=0.053 V) is: 

Eh = E0

 + αRT / nF log[H+]

3 (5.7) 

where R is the gas constant, T is temperature, F is Faraday’s constant, and n is the number of electrons in the balanced 

half-cell reaction. Using a ∆Gf

0

 for Fe(OH)3 of -696.3 kJ/mol (Langmuir, 1997) and ∆Gf

0

 for H2O(l) of -237.18 kJ/mol, we 

calculate E0

 = -∆G 0/nF = 0.053V, so that equation (5.7) reduces to: r 

Eh = 0.053 – 0.0592 pH (5.8) 

Similarly, the Eh-pH equation for the ferrihydrite-ferrous iron couple is 

Eh = 0.975 + 0.0592 log [H+

]

3

 [Fe2+]

-1 (5.9) 

Assuming ideal behavior and Fe2+ = 0.001 mg/L, equation (5.9) reduces to: 

Eh = 1.43 - 0.178 pH (5.10) 

Figure 5.1 shows equilibrium trends in Eh and pH for the Fe0

-Fe(OH)3 couple (eqn. 5.8), the Fe0-Fe3O4 couple, and the 

Fe2+-Fe(OH)3 couple (eqn. 5.10) compared to field measurements of ground-water pH and Eh from Elizabeth City 

monitoring wells located within the zero-valent iron media. The scattering of measured data points generally falls in 

between the two equilibrium trends plotted on Figure 5.1. This observation suggests that the measured Eh values in Fe0 

PRBs are not equilibrium potentials resulting from one redox pair, but are likely the result of mixed potentials from 

multiple redox reactions. These redox reactions are generally bracketed by the Fe0-Fe(OH)3 couple and the 

Fe2+-Fe(OH)3 couple. 

100 

0 

-100 

-200 

-300 

-400 

-500 

-600 

-700 

8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 

Fe0-Fe 3 O 4 

Fe2+-Fe(OH)

3 

Fe0-Fe(OH)

3 

ML14, Transect 1 Fe 0 

ML24, Transect 2 Fe 0 

ML34, Transect 3 Fe 0 

pH 

Eh, 

mV 

Figure 5.1	 Redox –pH diagram showing composition of ground water from the Elizabeth City iron wall compared to 

equilibrium trends for the Fe0-Fe(OH)3, Fe0-Fe3O4, and Fe2+-Fe(OH)3 couples (FeT=0.001 mg/L). 

88 
Eh-pH diagrams showing equilibrium relations in the system Fe-H2

O-C-S are shown in Figures 5.2-5.7. These Eh-pH 

diagrams were constructed using the EQ3/6 thermodynamic database (Wolery, 1979), modified to include data for green 

rust (Drissi et al., 1995; Bourrié et al., 1999; Refait et al., 2003), iron sulfides (Benning et al., 2000), and iron metal. The 

diagrams show both predominance areas for aqueous species (dashed lines) and solid phases (bold lines). All diagrams 

were drawn with the activity of iron equal to 10-5. Figures 5.2-5.4 were constructed with no suppressed minerals, i.e., 

they represent the equilibrium description of the systems based on the inclusion of all possible iron phases and aqueous 

species in the EQ3/6 database. In some cases, suppressing phases that are not expected to form based upon kinetic 

reasoning is useful in constructing and interpreting Eh-pH diagrams. Figures 5.5-5.7 were constructed with selected 

phases suppressed as described below. 

Equilibrium relations in the system Fe-H2

O at 25 °C are shown in Figure 5.2. Within the stability field of liquid water, iron 

metal dissolution (corrosion) is expected below pH 8. Note this pH boundary depends on the specified iron activity. At 

lower iron activities, the solution-solid boundary will shift to the left so that the Fe2+ field will shrink in size. Above pH 8, 

iron metal is expected to corrode and be replaced by FeO and magnetite. At higher redox potentials and over a wide pH 

range, hematite (Fe2

O3) is a stable mineral. Figure 5.3 shows equilibrium relations in the system Fe-H2O-C (ΣC = 10-2) 

at 25 °C. With carbon in the system, stability fields appear for siderite and carbonate green rust. At the specified 

conditions for Figure 5.2, green rust is a stable phase and not metastable as is sometimes assumed, i.e., no mineral 

phases were suppressed in order for green rust to appear on the equilibrium diagram. When sulfur is exchanged for 

carbon, a broad stability field for pyrite (FeS2) appears at low redox potentials (Figure 5.4). 

Note that although there is a strong thermodynamic driving force for forming pyrite, kinetic factors may limit the formation 

of pyrite in PRBs. Solid-phase characterization studies indicate that mackinawite (Fe1+xS) is the dominant iron sulfide in 

Fe0

 PRBs with only trace quantities of greigite and pyrite detected (Wilkin et al., 2003; Furukawa et al., 2002). Because 

pyrite is not a major corrosion product in iron walls, suppression of pyrite from the Eh-pH diagram is reasonable (Figure 

5.5). As pyrite is removed from the thermodynamic database, other iron sulfides: pyrrhotite, greigite, troilite, and 

mackinawite, appear as the stable solid phase at low redox potentials. Figure 5.6 was constructed with all sulfides 

suppressed. In this case, magnetite and FeO reappear at pH>8 and low redox potentials, as does a narrow window of 

sulfate green rust stability. 

Hematite is typically not identified as a corrosion product in iron walls, although Fe(III)-bearing corrosion products are 

common. Suppression of hematite from the thermodynamic database leads to the appearance of goethite (Figure 5.6) 

in the Fe-H2

O-C system. Further suppression of goethite, magnetite, FeO, and siderite leads to broad fields of carbonate 

green rust and ferrihydrite. There is in fact very little kinetic hindrance for the precipitation of these phases (Figure 5.7). 

Note that the carbonate green rust field expands as the more insoluble phases hematite and goethite are replaced by 

ferrihydrite. 

5.2 Anion Composition 

The anionic composition of ground water is a critical factor in governing the rate of Fe0 corrosion and in directing the types 

of mineral precipitates that form within Fe0 reactive walls. Table 5.1 lists the authigenic minerals that have been identified 

in Fe0 barriers; these include an assortment of oxides, sulfides, and carbonates. The tendency of ferrous iron to form 

complexes with the common anions present in ground water increases in the order Cl-

>HCO3

-

>SO4

2->>OH-

. Therefore, 

it might be reasonably expected that iron metal corrosion rates will be the fastest in chloride-rich ground water and 

slowest in bicarbonate- or sulfate-rich ground water. Ground-water chemistry at the Elizabeth City and Denver Federal 

Center PRB sites is perhaps typical of many contaminated sites where PRBs might be used in that they contain a mixture 

of chloride, bicarbonate, and sulfate (Figure 5.8). If mineral precipitates form as surface coatings on the reactive iron 

grains (see Figures 3.24 and 4.24), the effectiveness of iron to degrade halogenated hydrocarbons may be reduced. In 

addition, mineral precipitation may result in porosity and permeability reductions (Reardon, 1995). On the other hand, 

some mineral coatings may be advantageous for the removal of both organic (e.g., Butler and Hayes, 2000; Lee and 

Batchelor, 2002a,b) and inorganic contaminants (e.g., Furukawa et al., 2002), as long as the hydraulic integrity of the 

system is retained to prevent ground water bypass of the reactive media. 

The amount of mineral precipitation expected in Fe0 barriers is linked to the site-specific distribution of anionic species 

and to the concentration of total dissolved solids (TDS) in influent waters. High TDS concentrations in influent waters to 

Fe0 barriers will in most cases result in high mineral accumulation rates. Figure 5.9 shows the range in TDS values 

reported from several PRB systems examined in the Tri-Agency research initiative. Note that TDS values are 

comparatively low at the Elizabeth City PRB, and this correlates with comparatively low rates of mineral accumulation at 

this site. 

5.2.1 Bicarbonate Reactions 

Inorganic carbon can have an effect on the longevity of zero-valent iron PRBs in two different ways: 1) by accelerating 

2 the corrosion rate of Fe0

 and the production of dissolved iron and H , and 2) by forming metal carbonate precipitates 

89 
1.0

2 4 6 8 10 12 

FeOH+ 

H O2 

O2 

H2 

H O2 

FeO 

Hematite 

Iron 

Magnetite 

Fe3+ 

FeOH2+ 

Fe(OH)2 

+ 

Fe(OH)3 

0 

Fe(OH)4 

-

Fe2+ 

0.5 

0 

-0.5 

0 14 

pH Fe(OH)2

0 

Figure 5.2 Redox-pH diagram for the Fe-H2

and stability fields of iron-bearing minerals (solid lines). Diagram is drawn assuming iron species 

activities of 10-5 (blue shaded area shows field for iron in solution). 

O system at 25 oC, showing speciation of soluble iron (dashed lines) 

1.0 

0.5 

0 

-0.5 

H O2 

O2 

H2 

HO 2 

Hematite 

FeCO3 

0 

FeO 

GR-CO3 

Iron Magnetite

Siderite 

Fe3+ 

FeOH2+ 

Fe(OH)2 

+ 

Fe(OH)3 

0 

Fe(OH)4

-

Fe2+ 

0 2 4 6 8 10 12 14 

pH 

E

H, volts 

E

H, volts 

Figure 5.3 Redox-pH diagram for the Fe-CO2-H2

lines) and stability fields of iron-bearing minerals (solid lines). Diagram is drawn assuming iron species 

activities of 10-5 and carbon species activities of 10-2 (blue shaded area shows field for iron in solution). 

O system at 25 oC, showing speciation of soluble iron (dashed 

90 
1.0�

H O2 

O2 

H2 

H O2 

Fe(OH)2 

0 

FeOH+ 

FeSO4 

0 

FeSO4 

+ 

Hematite 

Magnetite 

Pyrite 

Fe(OH)2 

+ 

Fe(OH)3 

0 

Fe(OH)4 

-

0.5 

0 

-0.5 

0 2 4 6 8 10 12 14 

pH 

Figure 5.4	 Redox-pH diagram for the Fe-S-CO2-H2O system at 25 °C, showing speciation of soluble iron (dashed 

lines) and stability fields of iron-bearing minerals (solid lines). Diagram is drawn assuming iron species 

activities of 10-5, carbon species activities of 10-2, and sulfur species activities of 10-3 (blue shaded area 

shows field for iron in solution). 

1.0 

0.5 

0 

-0.5 

H O2 

O2 

H2 

H O2 

FeSO4 

0 

FeSO4 

+ 

Fe(OH)2 

+ 

Fe(OH)3 

0 

Fe(OH)4 

-

FeO 

GR-SO4 

Hematite 

25C Iron 

Magnetite 

HO 2

0 2 4 6 8 10 12 14 

pH 

E

H, volts 

E

H, volts 

Figure 5.5	 Redox-pH diagram for the Fe-S-CO2

-H2O system at 25 °C, showing speciation of soluble iron (dashed 

lines) and stability fields of iron-bearing minerals (solid lines). Diagram is drawn assuming iron species 

activities of 10-5, carbon species activities of 10-2, and sulfur species activities of 10-3 (blue shaded area 

shows field for iron in solution). Diagram drawn by suppressing all sulfide minerals. 

91 
1.0�

Goethite 

GR-CO3

Iron 

Siderite Magnetite

Fe2+ 

Fe3+ 

Fe(OH)4 

-

Fe(OH)3 

0 

Fe(OH)2 

+ 

FeOH2+ 

FeCO3 

0 

O2 

H O2 

H2 

H O2 

0.5 

0 

-0.5 

0 2 4 6 8 10 12 14 

pH 

Figure 5.6	 Redox-pH diagram for the Fe-CO2-H2O system at 25 oC, showing speciation of soluble iron (dashed 

lines) and stability fields of iron-bearing minerals (solid lines). Diagram is drawn assuming iron species 

activities of 10-5 and carbon species activities of 10-2. Diagram drawn by suppressing hematite and 

FeO(s). 

1.0 

0.5 

0 

-0.5 

Fe(OH)3 

0 

GR-CO3 

Iron 

Fe2+ 

Fe3+ 

Fe(OH)4 

-

Fe(OH)3 

Fe(OH)2 

FeOH

+ 

2+ 

FeCO3 

0 

O2 

H O2 

H2 

HO E

2 

H, volts 

E

H, volts 

0 2 4 6 8 10 12 14 

pH 

Figure 5.7	 Redox-pH diagram for the Fe-CO2-H2O system at 25 oC, showing speciation of soluble iron (dashed 

lines) and stability fields of iron-bearing minerals (solid lines). Diagram is drawn assuming iron species 

activities of 10-5 and carbon species activities of 10-2. Diagram drawn by suppressing hematite, goethite, 

magnetite, FeO(s), and siderite. 

92 
-

HCO3 

Elizabeth City ML10 

Denver Federal Center 

2- SO - 4 Cl 

2+ Ca

Mg 

TDS, mg/L 

0-300 

900-1200 

600-900 

300-600 

2+ + +

Na +K 

Figure 5.8	 Upgradient ground-water compositions (molar ratios) and TDS values for the Elizabeth City and Denver 

Federal Center PRB sites. 

TDS, mg/L 

0 100 1,000 10,000 

Moffett Field (820) 

Lowry AFB (2900) 

Elizabeth City (250-350) 

Y-12 (470-3225) 

Monticello (1300) 

Denver Fed Ctr (900-1200) 

Total Dissolved Soilds 

Data source: Tri-Agency PRB Initiative, Combined Report 

Figure 5.9 Comparison of total dissolved solids concentrations at PRB sites studied in the Tri-Agency initiative. 

93 
which may coat iron metal surfaces, block reactive sites, fill pore space, and through time, impact hydraulic performance. 

Abundant field and laboratory evidence indicates that inorganic carbon is removed from ground water during transit 

through zero-valent iron PRBs (Vogan et al., 1999; Phillips et al., 2000; Roh et al., 2000). The decrease in dissolved 

inorganic carbon is also accompanied by decreases in the concentrations of calcium and magnesium. Aragonite, calcite, 

siderite, ferrous carbonate hydroxide, and the carbonate form of green rust have all been identified in Fe0

 reactive 

barriers (Table 5.1). 

Iron corrosion generally leads to a pH increase and a consequent increase in the [CO3

2-/HCO3-

] ratio in solution. 

Generally, influent ground water to iron barriers is saturated to undersaturated with respect to various carbonate 

minerals. Increases in pH and the [CO3

2-/HCO3-

] ratio will impact reaction affinity and favor precipitation of carbonates 

such as aragonite and siderite. At the Elizabeth City PRB, ground water upgradient of multilevel well transect 2 is 

undersaturated with respect to both aragonite and calcite (Figure 5.10), based upon the reaction: 

Ca2+ - + HCO3 = CaCO3

 (ar, cc) + H+ (5.11) 

Ground-water compositions from within the iron wall clusters near the saturation point of both aragonite and/or calcite, 

and this observation is in agreement with: 1) the measurement of inorganic carbonate in the solid phase, and 2) the 

identification of aragonite by powder X-ray diffraction in core materials. Ground water downgradient of the iron wall is 

saturated to undersaturated with respect to CaCO3

 (Figure 5.10). A similar analysis shows that upgradient ground water 

is undersaturated with respect to magnesite and siderite, and that ground water within the Fe0

 media is near–saturated 

to undersaturated with respect to these minerals. Even though ground-water compositions cluster near the saturation 

point for Mg and Fe carbonates, we were unable to detect magnesite or siderite in core materials collected from Elizabeth 

City. Conclusions about the identity of phases accumulating in reactive barriers that are based solely on the analysis of 

mineral saturation states can be misleading. Both magnesite and siderite are known to exhibit very slow 

dissolution/precipitation kinetics (e.g., Langmuir, 1997). Solid-phase characterization studies indicate that iron hydroxy 

carbonate is, in fact, the dominant mineral carbonate at Elizabeth City, and calcium carbonate and iron hydroxy 

carbonate dominate mineral carbonate forms at the Denver Federal Center. There are presently no thermodynamic data 

for iron hydroxy carbonate that can be used, for example, to estimate solution saturation indices. 

The factors that govern the distribution and form of carbonate precipitates are still unclear. Iron hydroxy carbonate 

appears to be a common precipitate documented in both lab and field studies (e.g., this study, Gavaskar et al., 2002; 

Kamolpornwijit et al., 2002). The formation of Ca vs. Fe carbonates could in part be controlled by the Ca concentration 

of influent water to PRBs. For example, following the exchange reaction: 

CaCO3 (cc) + Fe2+ = FeCO3 + Ca2+ (5.12) 

Table 5.1. Mineral Precipitates Identified in Iron Walls 

Mineral precipitate type 

Oxides and Hydroxides 

Carbonates 

Sulfides 

Minerals identified in 

zero-valent iron PRBs 

Ferrihydrite, Fe(OH)3


Lepidocrocite, γ-FeOOH 


Goethite, α-FeOOH 


Hematite, Fe2O3


Maghemite, Fe2O3


Green rust 1, Fe6(OH)12CO3·xH2O 


Magnetite, Fe3O4


Calcite, CaCO3


Aragonite, CaCO3


Iron carbonate hydroxide, Fe2(OH)2CO3


Siderite, FeCO3


Mackinawite, Fe1+xS 


Greigite, Fe3S4


Pyrite, FeS2


94 
log [DIC+Fe] log [DIC+Mg] 

-12 

-10 

-8 

-6 

-4 

-2 

Solution 

Magnesite 

c) 

ML24, Transect 2, iron wall 

ML21, Transect 2, upgradient 

ML25, Transect 2, downgradient 

4 6 8 10 12�

pH 

-12 

-10 

-8 

-6 

-4 

-2 

Solution 

Siderite 

b) 

ML24, Transect 2 - iron wall 

ML21, Transect 2 - upgradient 

ML25, Transect 2 - downgradient 

4 6 8 10 12�

pH 0 

log [DIC+Ca] -10 

-8 

-6 

-4 

-2 

a) 

Solution 

Aragonite 

ML24, Transect 2 - iron wall 

ML21, Transect 2 - upgradient 

ML25, Transect 2 - downgradient 

4 6 8 10 12�

pH 

Figure 5.10	 Solubility diagram showing the stability field of a) aragonite, b) siderite, and c) magnesite as a function of 

pH and log activities of dissolved inorganic carbon, Ca2+, Fe2+, and Mg2+. Also plotted are ground-water 

compositions from upgradient, iron wall, and downgradient sampling locations (Elizabeth City PRB). 

95

with log K[5.12] = 2.2 and ideal behavior in the solid phase, solution concentration ratios of [Ca2+]/[Fe2+]>158 should favor 

the formation of calcite over siderite. In batch laboratory tests devoid of calcium, siderite is often found to be the primary 

carbonate phase as might be expected (e.g., Agrawal et al., 2002). As the concentration of Ca increases, calcium 

carbonates are likely to become more important than siderite. Both aragonite and calcite are observed in iron walls. The 

formation of aragonite over calcite may be linked to the [Mg2+/Ca2+] ratio in solution in addition to the absolute Mg2+ 

concentration. In experiments designed to explore the growth of calcite and aragonite in seawater, Berner (1975) 

showed that calcite growth was sensitive to the Mg2+ in solution. In a seawater matrix, the growth rate of calcite was 

depressed at Mg2+ concentrations >70 ppm and aragonite growth was favored from supersaturated solutions. It is also 

possible that other ground-water solutes have an effect on directing the nucleation and growth of aragonite over calcite 

and subsequent transformation. 

Agrawal et al. (2002) recently examined the effect of carbonate precipitation on the reaction between Fe0

 and 

1,1,1-trichloroethane (TCA) in model experiments. Time-dependent trends in degradation half-lives suggest an initial 

increase followed by a decrease. The exposure of iron surfaces to bicarbonate solution apparently results in an initial 

period of inhibited corrosion due to the presence of a film of iron oxide, followed by bicarbonate-enhanced iron corrosion, 

and eventually passivation occurs as a result of carbonate precipitation (siderite and carbonate green rust). These three 

regimes yield different contaminant degradation kinetics and perhaps different degradation mechanisms. 

5.2.2 Sulfate Reactions

Decreases in sulfate concentrations in Fe0 barriers are attributed to the microbial activity of sulfate-reducing bacteria. At 

both the Elizabeth City and DFC PRBs, sulfate concentrations are near completely removed as ground water moves 

through the reactive media. Accompanying the reduction of sulfate concentrations is the accumulation in the solid-phase 

of mackinawite (Fe1+xS) and microbial biomass with a phospholipid fatty extract signature consistent with the presence of 

sulfate-reducing bacteria. Abiotic reduction of sulfate to sulfite, thiosulfate, elemental sulfur, or hydrogen sulfide are 

known to be a sluggish processes at temperatures below 100 °C (Trudinger et al., 1985). The slow kinetics of the abiotic 

process is related to the eight-electron transfer required to reduce sulfate to sulfide. 

In many subsurface systems, microbial reduction of sulfate is accompanied by oxidation of reduced organic carbon, e.g., 

- SO42- + 2CH2

O = 2HCO3 + HS- + H+ (5.13) 

In Fe0

 barriers the amount of reduced organic carbon present would appear to be limiting and insufficient to account for 

the amount of sulfate removed from solution and the quantity of iron sulfide precipitates observed in the solid phase. 

Sulfate-reducing bacteria, such as Desulfovibrio desulfuricans, can also utilize hydrogen as a substrate for the reduction 

of sulfate, e.g., 

SO4

2- + 4H2

 + H+ = HS- + 4H2O (5.14) 

In gate 2 of the DFC, we detected high concentrations of iron sulfide within the iron wall and comparatively low 

concentrations of hydrogen. These observations are consistent with sulfate-reduction being an important sink for 

hydrogen in Fe0

 barriers. A potential negative consequence of the presence of bacteria is biofouling. Proliferation of 

bacteria could reduce the hydraulic conductivity of the barrier (Roh et al., 2000; Wilkin et al., 2003). On the other hand, 

iron sulfide can be effective in enhancing the degradation of TCE and other halogenated aliphatic compounds (e.g., 

Butler and Hayes, 2000, 2001) and sulfur impurities in iron metal may also influence reactivity (e.g., Hassan, 2000). The 

formation of iron sulfides in Fe0

 barriers, therefore, may be beneficial to their continued operation. In addition, compostbased reactive barriers have been used to remove metal contaminants from ground water (Waybrant et al., 1998, 2002; 

Benner et al., 1997). This treatment strategy is analogous to the use of anaerobic solid-substrate bioreactors for 

removing metals from solution (e.g., Dvorak et al., 1992; Drury, 1999). These systems rely on carbon-based media as 

a substrate for sulfate reduction, sulfide production, and precipitation of insoluble metal sulfides. Sulfate-reduction could 

also serve to increase the removal of metals in Fe0 barriers through direct precipitation of metal sulfides, co-precipitation 

with FeS, and/or adsorption onto iron sulfide surfaces. 

Complete reduction of sulfate to bisulfide could lead to high levels of aqueous sulfide species in the absence of any 

removal process. At near-neutral pH, iron monosulfides are insoluble so that all sulfide produced is likely to be 

precipitated and removed from solution. At the DFC, we observed total dissolved sulfide concentrations of <1 ppm; 

concentrations of sulfide in the solid-phase were as high as about 0.5 wt% S. 

Field evidence suggests that microbial sulfate reduction is most effective in decreasing sulfate concentrations in low-flow 

regimes. Iron barriers that experience higher flow-rates tend to show less effective removal of sulfate (e.g., Morrison et 

al., 2001; Kamolpornwijit et al., 2003). 

Several studies have suggested that another possible sink for sulfate in Fe0

 barriers is the formation of sulfate green rust. 

Geochemical modeling studies of iron corrosion described in Wilkin et al. (2002) suggest that the carbonate form of 

96 
green rust is favored over the sulfate form at the Elizabeth City and DFC sites. X-ray diffraction results from these sites 

are consistent with the formation of the carbonate form of green rust; sulfate green rust has not been detected. These 

observations can be understood by considering the following exchange equilibrium based upon the anhydrous GR 

components: 

Fe6

(OH)12SO4 + CO3

2- = Fe6

(OH)12CO3 + SO4

2- (5.15) 

so that, 

K[5.15] = [SO4

2- / CO32-] / [Fe6

(OH)12CO3 / Fe6(OH)12SO4] (5.16) 

Assuming ideal mixing relations in the solids and taking thermodynamic data from Bourrié et al. (1999) for the anhydrous 

carbonate and sulfate forms of green rust, we estimate K[5.15] = 103.1. Consequently, sulfate green rust (GR2) is expected 

to be a primary iron corrosion product only when sulfate-rich and bicarbonate-poor waters interact with zero-valent iron. 

High-sulfate conditions are most likely to be present in areas impacted by the oxidative weathering of metal sulfide 

deposits. 

5.2.3 Nitrate Reactions 

Field-deployed zero-valent iron systems effectively remove nitrate from ground water (e.g., Gillham et al., 1994; 

McMahon et al., 1999; Liang et al., 2002). In this study, nitrate removal was consistently observed at the Elizabeth City 

and Denver Federal Center PRBs over the initial five-year period of operation (Blowes et al., 1999b, Puls et al., 1999a, 

Wilkin et al., 2002; this report). Laboratory investigations suggest that nitrate can inhibit the reduction of TCE and other 

chlorinated ethenes through competition for reducing equivalents (e.g., Schlicker et al., 2000), however, as of yet, this 

behavior has not been documented in the field. 

At the Y-12 Pathway 2 PRB (Oak Ridge, TN), influent water contains nitrate at concentrations approaching 1,000 mg/L; 

nitrate concentrations in effluent waters and in wells adjacent to the zero-valent iron zone are near or below analytical 

detection limits (Liang et al., 2002). Unlike sulfate reduction that only proceeds via microbial respiration, reduction of 

nitrate may proceed by either abiotic or biotic pathways (Siantar et al., 1996; Huang et al., 1998; Till et al., 1998; Gandhi 

et al., 2002). Abiotic reduction is a pH-dependent process that results in the formation of ammonium and possibly nitrite 

as an intermediate product. Rates of nitrate reduction in sterile systems are fast at pH � 4 (Huang et al., 1998), but 

apparently reduction rates can be significant even at circumneutral pH (Siantar et al., 1996; Alowitz and Scherer, 2002, 

but also see Huang et al., 1998). The overall reaction may follow: 

NO -

3 + 4Fe0

 + 10H+ = NH4

+ + 4Fe2+ + 3H2

O (5.17) 

Huang et al. (1998) propose that electrons necessary for nitrate reduction are supplied directly from Fe0

 or indirectly 

2 through an iron corrosion product (H ).

Data presented in Gandhi et al. (2002) suggest that the effective removal of nitrate observed in field PRB systems is 

likely the result of significant biotic contributions. In biologically mediated systems, reduced organic carbon is frequently 

the electron donor for the redox transformation (Postma et al., 1991). In zero-valent systems, biotic reduction of nitrate - occurs by denitrifying bacteria that likely use cathodic H2

 as an electron donor to respire NO3 (e.g., Till et al., 1998; 

Gandhi et al., 2002). Nitrogen gas is the principal product of biotic nitrate reduction rather than ammonia (Till et al., 

1998). A significant result of nitrate reduction from both abiotic and biotic mechanisms is increased iron corrosion, 

leading to secondary mineral precipitation (Kamolpornwijit et al., 2003). 

5.2.4 Reactions with Silica 

Field evidence indicates that dissolved silica is removed by Fe0 barriers (Gavaskar et al., 2002; Wilkin et al., 2003). 

Ground-water concentrations of silica typically range between saturation values with respect to quartz and amorphous 

silica (3 to 54 mg/L Si). Within iron walls, concentrations of Si typically fall below 1 mg/L. Using high-resolution 

microscopy, Furukawa et al. (2002) observed silica predominantly associated with iron corrosion products. The role that 

silica might play in passivated iron surfaces is not clear. However, recent long-term column studies suggest that silica, 

carbonate, and natural organic matter co-solutes reduce the reactivity of Fe0

 (Vikesland et al., 2002). 

Forms of SiO2

 are not likely precipitating in iron walls because of their slow precipitation kinetics and because increasing 

pH increases rather than decreases SiO2

 solubility. One possibility is that silica is associated with magnesium in the clay 

mineral sepiolite, Mg4(OH)2Si6O15 ·H2O. Sepiolite typically occurs as finely fibrous aggregates, but is less frequently 

encountered in natural systems compared to the layered clay minerals. Sepiolite has a chain-like crystal structure of 

continuous Si2O5 sheets with ribbons of Mg octahedra leaving channels that can incorporate water or organic molecules. 

Figure 5.11 shows saturation indices of magnesium-bearing phases, sepiolite and brucite (Mg(OH)2), as a function of pH 

for Elizabeth City ground water. With increasing pH, ground water approaches or clusters near the saturation points of 

both sepiolite and brucite. Both phases, therefore, represent possible sinks for magnesium in iron walls (pH>9), and in 

addition, sepiolite is a possible sink for silica. Appreciable buildup of these phases might only be expected in PRB 

97 
-12

-10

-8

-6

-4

-2

0

2

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

undersaturated

oversaturated

ML21 upgradient

ML24 midbarrier

ML25 downgradient

Saturation Index, brucite Saturation Index, sepiolite

5 6 7 8 9 10

pH 

undersaturated

oversaturated

ML21 upgradient

ML24 midbarrier

ML25 downgradient

5 6 7 8 9 10

pH 

Figure 5.11 Saturation indices of magnesium-bearing phases (brucite, Mg(OH)2

function of pH in ground water from upgradient, iron wall, and downgradient sampling locations 

(Elizabeth City PRB). 

; sepiolite, Mg4(OH)2Si6O15 ·H2O) as a 

systems with greater influent magnesium concentrations than are encountered at the Elizabeth City or Denver Federal 

Center sites. 

5.2.5 Reactions with Oxygen 

Concentrations of dissolved oxygen in influent ground water to Fe0 barriers are rapidly consumed. Low concentrations 

and the presence of dissolved ferrous iron complicate the accurate quantitation of dissolved oxygen in ground water 

samples collected around iron walls. Oxygen is incorporated into a complex mixture of mineral oxides formed near the 

surface of Fe0

 particles. The oxide film that develops on iron surfaces could be composed of magnetite, green rust, 

maghemite (γ-Fe2

O3), hematite, goethite, lepidocrocite, or ferrihydrite. The oxide layer at the metal surface is expected 

to evolve from a predominantly Fe(III) phase where oxygen-containing solutions enter the Fe0

 zone to a mixed-valent or 

pure Fe(II) phase under highly-reducing conditions expected in mid-barrier regions (e.g., Scherer et al., 1999; 

Odziemkowski et al., 1998). High concentrations of dissolved oxygen entering Fe0

 barriers are especially problematic 

due to rapid oxidation reactions, cementation by Fe(III) materials, and plugging (e.g., Liang et al., 2000). 

98 
5.3 Mineral Precipitation 

Mineral precipitates in zero-valent iron PRBs can be classified by formation processes into three groups: 1) those that 

result from changes in chemical conditions (i.e., change in pH, e.g., calcite); 2) those that are a consequence of microbial 

activity (i.e., sulfate reduction, e.g., mackinawite); and 3) those that are the result of iron metal instability and corrosion 

(e.g., magnetite). As noted in many publications, the formation of mineral precipitates in PRBs can impact system 

performance through time. As minerals precipitate in iron walls they occupy volume and, therefore, reduce porosity and 

permeability of the reactive zone. In this way, the hydraulic performance of PRBs (e.g., residence time, capture zone) 

could degrade through time as the effective porosity of the iron wall approaches or exceeds that in the adjacent aquifer. 

For example, preferential mineral accumulation in regions of a PRB resulting from higher inputs of dissolved solutes may 

lead to increases in ground-water residence times. However, adjacent regions of the reactive barrier may experience 

greater throughput and decreased residence times, potentially leading to contaminant breakthrough (Figure 5.12). 

A second effect of mineral precipitation relates to the reactivity of materials placed in PRBs. As mineral precipitates 

accumulate on iron surfaces (see Figures 3.24 and 4.24) it is expected that electron transfer processes, critical for the 

degradation of chlorinated organic compounds, become less efficient. The formation of mineral precipitates has been 

viewed generally as a process that limits the long-term performance of reactive barriers for ground-water cleanup. Yet 

some corrosion products that deposit on the surfaces of iron particles may also contribute to the overall treatment 

effectiveness of reactive barriers (e.g., Butler and Hayes, 1998, 1999, 2000; Lee and Batchelor, 2002; Furukawa et al., 

2002). For example, iron sulfides, magnetite, and green rust minerals can chemically transform chlorinated organic 

compounds. 

5.3.1 Pore Volume Reduction 

The infilling of pore space by mineral precipitates can be assessed by theoretical modeling efforts and through direct field 

measurements. The development of models of mineral precipitation in PRBs is of great interest because these models 

can be used as predictive tools during remedial investigations at contaminated sites (e.g., Blowes and Mayer, 1999; 

Mayer et al., 2001; Yabusaki et al., 2001; Liang et al., 2003). Field measurements are critical in verifying model 

predictions at specific sites. Based upon the results of this and other studies, volume loss in zero-valent iron systems 

results primarily from the formation of mineral precipitates containing carbon, sulfur, and iron. 

PRB

Zone of increased

buildup, increased τ

Decreased τ

Decreased τ

Figure 5.12 Conceptual model of the impact of mineral and biomass accumulation to PRB hydraulic performance 

(ρ =effective porosity; K=hydraulic conductivity; τ=residence time). e

99 
0.0

0.1

0.2

0.3

0.4

0.5

After five years of mineral accumulation, concentrations of inorganic carbon in core materials from the Elizabeth City and 

Denver Federal Center PRBs range from <1 to about 14,100 μg/g. This concentration range can be directly related to 

a fractional porosity reduction, i.e., the reduction in the fraction of the total volume occupied by pore space. In 

Figure 5.13, fractional porosity reduction resulting from formation of carbonate minerals is plotted as a function of the 

concentration of inorganic carbon. An inorganic carbon concentration of 15,000 μg/g would result in a decrease of the 

fractional porosity by 0.13 to >0.5, depending on how the carbon is distributed in the solid phase (i.e., siderite or 

carbonate green rust, respectively). The least amount of pore infilling occurs when carbon is partitioned into minerals 

with low molar volumes, such as siderite, aragonite, and calcite (Table 5.2). Greater porosity loss is a consequence of 

the formation of carbonate green rust and iron hydroxy carbonate, materials with comparatively high molar volumes 

(Figure 5.13; Table 5.2). After the first five years of operation at Elizabeth City, the maximum loss of fractional pore 

space near the upgradient iron/aquifer interface due to the formation of inorganic carbon precipitates is estimated to be 

about 0.07. At the Denver Federal Center, the maximum loss of fractional pore space due to the formation of inorganic 

carbon precipitates is estimated to be about 0.17 after the first five years of operation. 

The main sulfur-bearing mineral in zero-valent iron PRBs is mackinawite (FeS) as seen in this study and at other sites 

(e.g., Phillips et al., 2000; Roh et al., 2000). The low molar volume of mackinawite results in little loss of porosity (<0.05) 

even at the highest concentrations of total sulfur observed in this study (7,520 μg/g; Figure 5.14). If through time 

mackinawite were to completely transform to pyrite, the loss of pore space would be even lower due to the low molar 

volume of pyrite (Table 5.2). Sulfur partitioned into sulfate green rust would result in much more significant porosity 

reductions (Figure 5.14). 

Fractional

Poro

sity

Reductio

n

0 10000 20000 30000 40000

Denver Federal Center PRB after 5 y

Elizabeth City PRB after 5 y

Measured inorganic carbon, μg/g

Aragonite

Calcite

Siderite

Carbonate green rust

Iron carbonate hydroxide

0 10000 20000 30000 40000

Inorganic Carbon, μ g/g

Figure 5.13	 Fractional porosity reduction as a function of inorganic carbon concentration in the solid phase. The 

lines represent volume loss due to the accumulation of a carbon-bearing phase assuming that all carbon 

is present in that phase. Fractional porosity reduction is calculated assuming an initial porosity of 0.50 

and iron density of 7.0 g/cm3

. The range of observed inorganic carbon concentrations after 5 years at 

the Elizabeth City and the Denver Federal Center PRBs are shown. 

100 
Table 5.2. Molar Volume and Density of Mineral Precipitates


Phase Formula Molecular Density 

Weight 

(g/mole) (g/cm3

) 

Calcite CaCO3 100.09 2.71a 

Aragonite CaCO3 100.09 2.95 a 

Siderite FeCO3 115.86 3.96 a 

Green Rust – Fe6(OH)12CO3 599.17 3.5 b 

CO3 

Iron Hydroxy Fe2(OH)2CO3 205.72 3.65 c 

Carbonate 

Mackinawite FeS 87.91 4.1d 

Pyrite FeS2 119.98 5.02 a 

Green Rust – Fe6(OH)12SO4 635.23 3.5 b 

SO4 

Iron Fe 55.85 7.0 a 

Magnetite Fe3O4 231.54 5.18 a 

Hematite Fe2O3 159.69 5.26 a 

Goethite FeOOH 88.85 4.37 a 

Ferrihydrite Fe(OH)3 106.87 3.1 b


Molar volume = MW/density
 a

 Hurlbut and Klein (1977); b estimated; c Erdös and Altorfer (1976); d Vaughan and 


Craig (1978) 


Molar Volume 

(cm3

/mole) 

36.93 

33.93 

29.26 

171.19 

56.36 

21.44 

23.90 

181.49 

7.98 

44.70 

30.36 

20.33 

34.47 

Corrosion reactions in which iron reacts to form iron oxides such as magnetite, hematite, goethite, or ferrihydrite also 

result in volume increases and porosity reductions. For example, the molar volume change of the reaction (∆Vr

to magnetite can be computed from the reaction: 

) from Fe0 

Fe0

 + 4/3H2O = 1/3 Fe3O4 + 4/3 H2(g) (5.18) 

by using the molar volumes (Vm) of Fe0 and magnetite listed in Table 5.2. The molar volume change of reaction is given 

by: 

∆Vr = 1/3(Vm magnetite) – (Vm Fe0) = 6.9 cm3/mol (5.19) 

Given the assumption that iron is conserved in the solid phase, transformation reactions of iron metal to iron hydroxides, 

oxyhydroxides, and oxides all have positive molar volume changes. The fractional porosity reduction associated with 

various iron transformations are plotted in Figure 5.15 as a function of extent of reaction or extent of transformation. 

Although it is well documented that magnetite, for example, is a product formed in iron walls, the overall extent of the 

transformation has not been directly determined or estimated. Based upon microscopic examination, iron particles 

collected from the Elizabeth City and DFC PRBs (after five years) are dominantly composed of Fe0 and not iron oxidation 

products. An average rate of iron metal corrosion in ground water is estimated to be 0.6 mmol/kg d (Reardon, 1995). At 

this rate of corrosion, after five years the extent of reaction to magnetite is estimated to be 0.02. If ferrihydrite is the end 

product instead of magnetite, the extent of reaction is estimated to be 0.06. Consequently, the extent of reaction after 

5 years is likely to be less than 0.3 and probably much less than 0.1. As shown in Figure 5.15, the volume loss 

accompanying the transformation to magnetite is not expected to exceed 0.1 after five years. It should be noted that 

where rapid oxidation occurs due to the influx of ground water with high levels of dissolved oxygen (>2 mg/L), reaction 

to form goethite or ferrihydrite has been documented. Figure 5.15 shows that even partial conversion of Fe0

 to 

ferrihydrite can dramatically impact the porosity of zero-valent iron systems. 

101 
Elizabeth City PRB after 5 y 

Denver Federal Center PRB after 5 y 

0 10000 20000 30000 40000 50000 

Measured sulfur, μg/g 0.5 

0.4 

0.3 

0.2 

0.1 

0.0 

0 10000 20000 30000 40000 50000 

Mackinawite 

Sulfate green rust 

Pyrite 

Sulfur, μg/g 

Figure 5.14	 Fractional porosity reduction as a function of sulfur concentration in the solid phase. The lines represent 

volume loss due to the accumulation of a sulfur-bearing phase assuming that all sulfur is present in that 

phase. Fractional porosity reduction is calculated assuming an initial porosity of 0.50 and iron density of 

7.0 g/cm3. The range of observed sulfur concentrations at the Elizabeth City and the Denver Federal 

Center PRBs are shown. 

0.5 

0.4 

0.3 

0.2 

0.1 

0.0 

0.0 0.2 0.4 0.6 0.8 1.0 

Fe-to-Magnetite 

Fe-to-Hematite 

Fe-to-Goethite 

Fe-to-Ferrihydrite 

Extent of Reaction 

Fractional 

P

orosity 

Reduction 

Fractional 

P

orosity 

Reduction 

Figure 5.15	 Fractional porosity reduction as a function of the positive molar volume change as iron metal reacts to 

form magnetite, hematite, goethite, and ferrihydrite. 

102 
5.3.2 Loss of Reactivity

The previous calculations examining porosity reductions in iron walls as a consequence of mineral precipitation relate 

primarily to potential impacts to the hydraulic performance of PRBs. However, these calculations provide no direct 

insight about how mineral precipitation affects or could interfere with contaminant removal processes. In order to 

examine changes in the reactivity of zero-valent iron as a function of exposure time, we conducted batch experiments 

using fresh iron (Peerless Iron) and materials that were collected from the Elizabeth City and Denver Federal Center 

PRBs after five years of exposure to ground water. In the batch tests, iron samples used were collected from upgradient 

and midbarrier locations as summarized in Table 5.3. The water composition used in the batch tests was prepared using 

reagent grade sodium bicarbonate, calcium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, and 

hydrochloric acid to approximately match ground-water compositions encountered at the two PRB sites (Table 5.4). 

Solutions were deoxygenated by purging with high-purity nitrogen gas for one hour. 

Stock solutions were prepared using high purity trichloroethylene (99+%, Aldrich), 1,1,1-trichloroethane (anhydrous, 

99+%, Aldrich), and 1,1-dichloroethene (99%, Aldrich). Experiments to determine the kinetics of VOC degradation and 

Cr removal were carried out in 45-mL glass VOA vials, each containing 10 g of iron and filled with freshly prepared 

synthetic ground water. The VOA vials were sealed without headspace with Teflon-lined screwcaps. Next the reaction 

vessels were injected with a volume of the VOC stock solution and placed in a rotary shaker at 100 rpm at room 

temperature (23 ± 1 °C). Visual observations suggest that the gentle mixing did not result in any physical abrasion of the 

iron particles or corrosion products. Periodically vials were withdrawn for sample collection and measurement of pH, 

oxidation-reduction potential, sulfate, chloride, calcium, magnesium, iron, sodium, potassium, chromium, and VOCs. 

Control experiments (without iron) showed no loss of organohalides or metals over the relevant experimental timescales. 

Results of the batch experiments show that field-exposed iron samples from both midbarrier and upgradient locations are 

able to remove contaminants from solution at rates comparable or, even better, to those observed in systems containing 

unreacted zero-valent iron (Figure 5.16; Table 5.5). The initial removal rate of chromium is actually greater for fieldTable 5.3. Samples Used in Batch Reactivity Tests 

Sample Location Inorganic Sulfur, �g/g PLFA, pm/g

Carbon, �g/g

Unreacted -8/+50 mesh 

Peerless Iron size 

EC050801-3-1 Elizabeth City, 

upgradient 

EC050901-9-1 Elizabeth City, 

midbarrier 

C1-1-71701-19 Denver Federal 

Center, Gate 1, 

midbarrier 

C2-3-71801-4 Denver Federal 

Center, Gate 2, 

upgradient 

<15 <5 ND 

4633 3880 ND 

100 100 ND 

87 318 ND 

2100 1785 455 

103 
Table 5.4. Composition of Water Used in Batch Reactivity Tests 

Component	 Synthetic Elizabeth City 

Ground Water (mg/L) 

Synthetic Denver Federal 

Center Ground Water (mg/L) 

Ca 10 110 

Mg 7 32 

Na 31 163 

K 4 4 

Cl 17 68 

SO4 30 300 

HCO3 82 433 

pH 6.0 7.2 

exposed iron compared to unreacted iron (Figure 5.16). The added component of mineral precipitates (carbonates and 

sulfides) and microbial biomass to zero-valent iron apparently increases the uptake rate of chromium, probably due to an 

increase in the number of available sorption sites. Increased chromium removal was also observed in experiments with 

field-exposed iron materials from the DFC (where chromium is not a ground-water contaminant). 

Table 5.5 presents a summary of rate data for reactions of TCE and 1,1,1-TCA. Values of log kSA in Table 5.5 are based 

on pseudo-first-order reaction kinetics. In general, log kSA values for TCE are independent of whether the iron used was 

fresh or contained quantities of mineral precipitates and microbial biomass. The linearity of ln[TCE] vs. time plots 

decreases in the order unreacted iron>midbarrier iron>upgradient iron, as indicated by the standard regression 

coefficient. This trend suggests that removal processes are more complicated in materials containing authigenic 

components. Results for 1,1,1-TCA give log kSA values that range from –3.1 to –4.0. The effect that mineral precipitates 

play is still unclear as the slowest and fastest kinetics were observed, respectively, in systems containing upgradient iron 

from the DFC and Elizabeth City PRBs. 

The batch experiments indicate that zero-valent iron retains reactivity and the ability to remove chlorinated ethenes, 

ethanes, and hexavalent chromium even after long-term exposure times to ground water. Interestingly, the ability of 

zero-valent iron to remove chromium from ground water appears to improve with time. The Elizabeth City PRB is 

expected to remain effective for chromium removal for another five-year period at a minimum. Observations of 

contaminant breakthrough in field PRBs may be more directly tied to decreases in hydraulic performance (i.e., system 

residence time, plume bypass) rather than loss of reactivity of zero-valent iron. 

5.4 Microbial Activity 

The oxidation or corrosion of zero-valent iron may be stimulated or inhibited by microorganisms. From a subsurface 

ecological perspective, metallic iron represents a significant energy reservoir. Due to the limited solubility of oxygen in 

ground water and the rapid reduction of molecular oxygen by Fe(0) and Fe(II), PRBs usually exist as anaerobic 

environments. Under anaerobic conditions molecular oxygen-driven chemical corrosion rates may be reduced, but 

biologically mediated anaerobic corrosion may occur at rates exceeding those seen under oxygenated conditions. In the 

absence of oxygen, protons may serve as electron acceptors and allow for the formation of oxidized iron species such 

as Fe(II). 

The enhancement of anaerobic corrosion and the formation of dissolved Fe(II) and hydrogen gas is not necessarily 

detrimental to PRB performance. If the target contaminant such as Cr(VI) is reduced by Fe(II) as well as Fe(0), 

production of aqueous Fe(II) could increase the size of the reaction/treatment zone as compared to the surface contact 

area of Fe(0) alone. However, the utilization of dissolved hydrogen may result in bacterial growth and biofilm formation. 

The development of this biofilm in a PRB may be detrimental to performance through several mechanisms. Biofilm 

growth in a porous medium may reduce the total volume and the average size of the pores (e.g., Taylor et al., 1990a; 

Thullner et al., 2002). Changes in PRB hydraulic conductivity, the masking of active sites, the removal of active chemical 

species, mineral precipitation, production of gas bubbles, and the competition for reducing equivalents are processes 

mediated by bacteria that could negatively affect PRB performance. Conversely some microbial processes could 

enhance PRB performance. In some instances, bacteria may be more effective at contaminant transformation or may 

degrade compounds unaffected by PRBs. It is, therefore, evident that a clear understanding is needed of microbial/PRB 

interactions for the design and efficient operation of PRBs. 

104 
4000 

3500 

3000 

2500 

2000 

1500 

1000 

500 

0 a) 

Unreacted Peerless Iron 

Elizabeth City Iron, downgradient 

Elizabeth City Iron, upgradient 

TCE, μg/L Cr, μg/L 

0 5 10 15 20 25 30 

time, h 

1800 

1600 

1400 

1200 

1000 

800 

b) 

Unreacted Peerless Iron 

DFC Iron, midbarrier 

DFC Iron, upgradient 

600 

400 

200 

0 

0 10 20 30 40 50 60 70 80 90 100 110 

time, h 

Figure 5.16 Concentration versus time in batch tests: a) chromium; b) TCE. 

The possibility of exploiting microorganisms to enhance reductive treatment has been explored in laboratory studies 

(e.g., Till et al., 1998; Weathers et al., 1997; Gu et al., 1999; Oh et al., 2001; Gandhi et al., 2002). Scherer et al. (2000) 

provide a detailed review of laboratory investigations on combined microbial-Fe0

 systems. Bacterial improvements are 

possible, for example, by iron-reducing bacteria that could reductively dissolve Fe(III) oxide layers, participate in forming 

reactive mineral species such as green rust, or directly degrade contaminants. Gandhi et al. (2002) conclude that 

bioaugmentation with specialized strains could enhance contaminant removal in some instances or increase the range 

of pollutants that could be treated in iron systems. Yet the effect of microorganisms on the long-term permeability and 

reactivity of PRB systems is not completely understood. 

Studies that evaluate the microbiology of field Fe0

 PRBs are few. Gu et al. (2002) investigated the microbial population 

and community structure at the Oak Ridge Y-12 Plant site. They found a diverse microbial community in the Fe0 media 

despite mildly alkaline conditions (pH up to 10). Concentrations of microbial biomass were from one to three orders 

magnitude greater within the Fe0 media compared to values found in adjacent aquifer materials. DNA analysis indicated 

the presence of sulfate-reducing and denitrifying bacteria in the iron wall. The activity of methanogenic bacteria was 

found to be relatively low, apparently due to competition by sulfate- and metal-reducing bacteria. 

The results of this study at the Elizabeth City and Denver Federal Center PRBs are in broad agreement with those of Gu 

et al. (2002). At both sites elevated concentrations of microbial biomass were found within the iron wall as compared to 

adjacent aquifer materials (up to three orders of magnitude greater in biomass). The highest biomass accumulations 

were found near the upgradient aquifer/iron interface where electron donors enter the reactive treatment zone. 

105 
Table 5.5.	 Summary of Rate Data for Reactions of TCE and 1,1,1-TCA with Zero-valent Iron (Unreacted and 

Collected from Field PRBs) 

System 

Elizabeth City


(GW)


Unreacted 


Peerless Fe


Elizabeth City


(GW)


EC Iron


midbarrier


Elizabeth City


(GW)


EC Iron


upgradient


DFC (GW)


Unreacted 


Peerless Fe


DFC (GW)


DFC Iron 


midbarrier


DFC (GW)


DFC Iron 


upgradient


TCE 1,1,1-TCA


log kSA (L/h·m2

) R 2 log kSA (L/h·m2) R 2 n

-3.77 0.885 -3.86 0.949 10 

-3.67 0.841 -3.66 0.935 7 

-3.72 0.781 -3.12 0.978 6 

-3.83 0.988 -3.52 0.998 7 

-3.81 0.962 -3.82 0.990 9 

-3.93 0.906 -4.06 0.884 8 

Midbarrier and downgradient regions of the reactive barrier are, in most cases, free of microbial biomass. PLFA analysis 

suggests that a diverse assemblage of microorganisms colonize zero-valent iron systems at Elizabeth City and the 

Denver Federal Center. The formation of comparatively high-density, contaminant-reactive iron sulfides is one indirect 

consequence of microbiological activity (sulfate reducing bacteria). 

The visual appearance and results of PLFA analyses from iron core materials collected from DFC gate 2 suggests that 

high levels of microbial biomass could significantly reduce permeability of PRBs. Previous laboratory investigations have 

documented this effect (e.g., Taylor and Jaffé, 1990b; Vandevivere and Baveye, 1992). Low-flow systems may be 

problematic in this regard. Although it is clear that microorganisms colonize zero-valent iron systems, an outstanding 

question remains as to whether microorganisms play a direct role in reducing contaminant concentrations. Additional 

research is needed to address this question. 

A comparison of the average PLFA distribution among samples within reactive iron materials collected from Elizabeth 

City, DFC gate 1, DFC gate 2, and the Moffett Field PRB (data from Gavaskar et al., 2002) is shown in Figure 5.17. PLFA 

distributions in these four PRBs are broadly comparable. The Elizabeth City PRB is comparatively enriched in normal 

saturated (Nsats) structural groups that are ubiquitous in both prokaryotic and eukaryotic organisms. The broad 

similarity suggests that similar microbial populations colonize zero-valent iron systems. PLFA profiles in the reactive 

media tend to mirror profiles found in the aquifer materials sampled immediately upgradient to the reactive media; 

however, biomass concentrations are significantly greater within the iron zones. Indigenous microbial communities 

(especially anaerobes) appear to be stimulated by the placement of iron in the subsurface. 

106 
Microbial Biomass – PLFA Distribution

<4920 pm/g 

N=44 

<2600 pm/g 

N=40 

Gram+/ anaerobic 

GramGram- (Monos) 

Anaerobic metal 

reducers 

SRB/ 

Actinomycetes 

Genera Nsats 

<240 pm/g 

N=2 

From Gavaskar et al., 2002 

Figure 5.17 Comparison of PLFA distribution in four iron walls. 

5.5 Hydrogeological Issues 

The majority of recent research concerning PRBs has been directed at understanding geochemical processes involving 

different reactive materials as well as issues relating to predicting long-term performance. In comparison, fewer studies 

have explored issues of ground-water hydrology and flow behavior in PRBs. Clearly hydraulic and geochemical 

processes in PRBs are interrelated (see Tratnyek et al., 1997; Yabusaki et al., 2001; Mayer et al., 2001; Das, 2002). An 

understanding of the hydrologic properties of aquifers and reactive media, such as porosity, hydraulic conductivity 

distribution, hydraulic gradients, and flow velocities, heavily factor into the design and ultimately the success of PRB 

installations (Gavaskar et al., 1998). Two critical issues in system design that require an understanding of the spatial and 

temporal aspects of site hydrology are: 1) plume capture, i.e., ensuring that contaminated ground-water plumes are 

directed through the reactive material without bypass, underflow, or overflow; and, 2) residence time, i.e., ensuring that 

ground water remains in contact with the reactive media for an adequate period of time to allow for the removal or 

transformation of contaminant compounds. 

Aquifer heterogeneity in chemical and physical properties is typically present to some degree at all sites. Such 

heterogeneities will result in variable contaminant flux across the influent area of the PRB and variable residence time 

requirements for contaminant treatment. Reliance on the use of bulk or averaged geochemical and hydraulic parameters 

may potentially result in inadequate system designs (Gavaskar et al., 2002). Several studies provide some insight into 

how aquifer heterogeneity can impact the performance of PRBs. Eykholt et al. (1999) concluded that seepage velocities 

within a homogeneous reactive media are principally controlled by heterogeneity in the downgradient aquifer. They 

showed that variability in hydraulic conductivity spanning two orders of magnitude resulted in variability in flow velocities 

and residence times within the barrier that span one order of magnitude. Studies by Tri-Agency partners (Kamolpornwijit 

et al., 2003; Moline et al., 2002) show that treatment of high nitrate and TDS ground water has lead to increased mineral 

precipitation, which facilitated the development of heterogeneous flow, in addition to the initial heterogeneity present. 

This additional preferential flow over a year of PRB operation could cause ground-water bypass. In another modeling 

study, Benner et al. (2001) showed that localized or narrow high conductivity zones within the aquifer lead to greater 

preferential flow within the reactive media (see also Gupta and Fox, 1999). Benner et al. (2001) suggest that less 

variable flow will be attained using thicker, homogeneous barriers. 

One of the most critical issues that must be addressed in the design of PRBs is the selection of the appropriate barrier 

width. The barrier width must provide sufficient contact time to insure that contaminants are degraded to target levels. E. City DFC 

Cell 1 

<3500 pm/g 

N=18 

Moffett DFC 

Cell 2 

107 
The barrier width must be selected based upon the range of ground-water flow velocities expected and the desired extent 

of contaminant removal for a given reactive material. Tratnyek et al. (1997) used laboratory derived kinetic data 

pertaining to the degradation of various chlorinated halogens by zero-valent iron and a one-dimensional transport model 

to estimate the minimum barrier width required for specified contaminant reductions as a function of ground-water flow 

velocity. This type of analysis is critical for providing a quantitative basis for system design to accompany laboratory 

feasibility testing. 

Gavaskar et al. (1998) discuss the tools and methods that may be used to maximize the probability of success in 

hydraulic aspects of PRB performance. They emphasize the importance of thorough site characterization and groundwater flow modeling. A principal goal of site characterization is the development of a detailed understanding of site 

geology, hydrogeology, and contaminant distributions. Site characterization efforts should be to the level of capturing 

seasonal and spatial variability. Ground-water modeling efforts incorporate site characterization data to simulate water 

and contaminant transport. Modeling results will directly feed into a system design that ensures plume capture and 

adequate residence time for contaminant removal. 

Hydraulic aspects of performance monitoring generally include water level surveys, hydraulic conductivity measurements, and determinations of seepage velocity. Water level surveys provide information on ground-water gradients and 

capture zones of PRBs. Water level surveys were regularly conducted at the Elizabeth City site on a quarterly basis. 

The results of water level surveys from June 1997, 1998, and 1999 are shown in Figure 5.18. Inspection of the water 

level map shows that the primary flow direction is across the PRB. In general, the hydraulic gradient in June varies from 

about 0.001 to 0.004, and this range further captures annual variability in the hydraulic gradient at this site. Based on 

hydraulic conductivities measured from slug tests and the hydraulic gradient obtained from water level measurements, 

a typical ground-water velocity of 0.5 ft/day and a typical residence time of four days are estimated. 

McMahon et al. (1999) discussed the hydraulic performance of the Denver Federal Center PRB. Installation of the 

funnel-and-gate system at the DFC resulted in the mounding of ground water on the upgradient side of the sheet pile due 

to insufficient flow through the system. The buildup of ground-water levels on the upgradient side of the PRB raised 

concerns about the increased potential for ground-water bypass; flow under, over, or around the PRB. Follow-up studies 

suggest that underflow and overflow are not occurring, but that some bypass occurs around the southern side of gate 1 

(Pacific Western Technologies, 2000). A downgradient distribution ditch was installed in late 1998, connecting gates 1 

and 2, to decrease the ground-water mound. Although this attempt was unsuccessful in decreasing the head difference 

across the gates, the trench did greatly decrease the water levels within gates 1 and 2 (Pacific Western Technologies, 

2000). Before and after installation of the distribution trench, the head differential across gate 2 has averaged 

approximately 7 ft. The lowering of water level in gates 1 and 2 suggested that only partial hydraulic connectivity existed 

at the upgradient aquifer/iron interface. Further studies by FHWA and the GSA were initiated to understand the hydraulic 

and performance issues at the DFC, including parts of the investigation described in this report. 

It is believed that the head differential in gate 2 is a result of low permeability zone that was produced by backfilling preexcavation trenches with muddy material. Pre-excavation was required in order to install the sheet pile. Alternatively, a 

smear zone of fine materials could have resulted as a consequence of the installation and removal of sheet piling 

installed for gate construction. In either case, flow velocity through gate 2 is reduced compared to that through gate 1. 

Characterization studies described in this report suggest that flow through gate 2 is reduced over a depth interval from 

about 5.5 to 8 m below ground surface. This reduced flow zone may be related to the low permeability zone or smear 

zone that developed during construction of the gate. 

108 
Pasquotank River

MW46

Iron Wall

MW18

MW13

MW38

1.2

1.1 1.0

0.9

1.2

MW47

MW48

MW49

June 1997 Former Chrome Plating Shop

(contour interval = 0.05 ft)

Pasquotank River

MW46

Iron Wall

MW18

MW38

1.7

1.6

1.5

1.4

1.7

MW47

MW48

MW49

June 1998

(contour interval = 0.05 ft) MW13

Former Chrome Plating Shop

Pasquotank River

MW46

Iron Wall

MW13

MW18

MW38

1.1

1.0

0.9

MW47

MW48

MW49

MW13

Former Chrome Plating Shop June 1999

(contour interval = 0.05 ft)

Figure 5.18 Water levels in Elizabeth City monitoring wells.


109 
110

6.0 State of Permeable Reactive Barrier Technology and Lessons Learned 

from Long-Term Performance Monitoring 

6.1	 Permeable Reactive Barriers: An Accepted Remedial Option for Containment & 

Treatment of Contaminated Ground Water 

The use of permeable reactive barriers for the restoration of contaminated ground water has evolved from innovative to 

accepted, standard practice, for the containment and treatment of a variety of contaminants in ground water. Like any 

remedial technology, the decision to use PRBs will be conditioned by the nature of the natural system, the target 

contaminants, and the treatment objectives. In the past 10 years, more than 100 sites have implemented this technology 

to treat chlorinated solvent compounds, fuel hydrocarbons, and various inorganic contaminants in ground water. As with 

any technology used to treat or extract contaminants in the subsurface, successful implementation will be contingent on 

effective site characterization, design, and construction. Our studies on long-term performance of the technology at a 

number of sites in the United States have shown the following with respect to ensuring (designing) and verifying 

(monitoring) that the PRB meets performance objectives: 

•	 Adequate site characterization is necessary on the scale of the PRB. Site characterization approaches, 

typical of remedial feasibility investigations, are oftentimes not adequate. Additional localized characterization of the plume distribution in four dimensions (including time), understanding of local hydrogeology, and 

knowledge of the geochemistry of the site is required. 

•	 Understanding of site hydrology has emerged as the most important factor for successful implementation. 

This is not surprising given the nature of the technology. The PRB must be located to intercept the plume. 

Once located in the subsurface, it cannot be moved, so an understanding of how the PRB will impact the 

prevailing flow patterns is important. It is imperative that the selected design allow for capture of the plume 

in its present configuration, as well as allow for variations in flow direction, depth, velocity, and concentrations of contaminants, which may vary over time. 

•	 There is a need to develop contingency plans in case a system fails to meet design objectives. This requires 

specification of design criteria, performance objectives, and what constitutes a failure in order to clearly 

trigger the activation of contingency plans, i.e., alternative technologies or remedies to the installed PRB 

system. 

Performance goals should target the adequacy of plume capture and contaminant treatment such that acceptable 

downgradient water quality is achieved in a reasonable time frame. Short-term objectives generally involve establishment of adequate residence time of the contaminant(s) in the reactive media to achieve treatment goals, while long-term 

objectives revolve around longevity or lifetime expectations for the system, which in turn, affect cost. 

Specific criteria need to be established for all these concerns, and both parties (site owner and regulator) need to be 

clear on what triggers contingency plans. Maximum contaminant level (MCL) concentrations for contaminants in ground 

water are often used as criteria at points of compliance. This approach becomes complicated when contaminant levels 

already exceed goals at the point of compliance, and meeting these goals is contingent on desorption of residual 

contaminants or “flushing” over time. A time period needs to be specified in such a case that is reasonable given site 

characteristics and known contaminant behavior. 

Performance goals are usually developed for the site as a whole and contingent upon plume and barrier location relative 

to compliance points and/or site boundaries. The performance goals can be numeric, regulatory-driven targets and may 

have system design features including remedial measures, such as natural attenuation, downgradient of the barrier 

location. The time horizon for sustaining performance goals will depend on site-specific factors related to chemical, 

physical and microbiological processes, but also such factors as the extent of source removal, source containment or 

expected lifetime of the source as a plume generator. 

In many cases, contingency plans are required in the event that the PRB fails to meet performance criteria. Such plans 

may range from minor modifications of the PRB to use of an alternative technology. If the PRB fails to capture a portion 

of the plume, an extension to the PRB may be prescribed. If concentrations of contaminants exiting the PRB are higher 

111 
than expected, then an additional wall may be required downgradient of the first wall. In some installations, monitored 

natural attenuation of contaminants downgradient of the wall is expected and designed into the system to help meet 

compliance goals. 

6.2 Lessons Learned: Site Characterization and PRB Construction 

As with any ground-water remediation technology, adequate hydrogeologic characterization must be done to understand 

flow patterns and the distribution of the contaminant plume. This is particularly important for PRBs as the treatment 

system is immovable or passive, yet it must intercept and capture the contaminant plume for effective treatment. 

Information to be obtained includes advective velocity parameters such as the gradient, the hydraulic conductivity, 

porosity, and other parameters collected as part of a hydrogeologic characterization program. It is also important to 

understand temporal changes in flow direction and flux due to processes such as recharge, pumping of adjacent wells or 

other disturbances. Observed changes in flow direction at the Elizabeth City site, for example, have ranged as high as 

25 degrees from time to time. 

In addition to the hydrology, the stratigraphy and lithology of the site is important to understand and will dictate effective 

PRB design. If a low permeability layer exists at the site, the PRB can be keyed into this layer. If it does not exist, then 

a “hanging wall” design must be chosen which may add to the uncertainty of plume capture. If the site has low 

permeability layers through which the PRB must be constructed, care must be taken during construction to avoid 

smearing of such layers. This could impact hydraulic contact between the formation and the reactive media. A thorough 

understanding of site stratigraphy is especially important or helpful in choosing a particular construction method. Use of 

sheet piling to construct a reactive “gate” may not be a good choice where low permeability layers exist because of the 

smearing potential and increased difficulty in reestablishing good hydraulic contact between aquifer sediments and the 

reactive zone. 

Characterization of contaminant concentrations in four dimensions is required for successful implementation of a PRB. 

In addition to knowledge of the plume in the three-dimensional space, it is also imperative to understand variability in 

plume shape and direction over time. Plumes deviate in direction and location over time and may change shape due to 

attenuation, degradation, mixing with other plumes, dilution, recharge, and other natural and anthropogenic-induced 

disturbances. 

PRBs are often located within plumes. This requires some understanding of the impact of construction on plume 

behavior, both upgradient and downgradient of the barrier. For example, it is essential to verify that hydraulic contact 

between the plume and reactive media is established. If a PRB is located below an impermeable surface structure such 

as a parking lot, will the surface be repaved immediately or will recharge be allowed to occur over the PRB? Some 

understanding of natural attenuation processes at the site is important in being able to interpret the subsequent response 

of the natural system to the presence of the PRB. This is most often manifested in trying to estimate how long the 

downgradient aquifer will require to achieve cleanup goals. A key question to address is the length of time required 

before contaminants located downgradient of the barrier flush out of the sediments or degrade naturally. 

6.3 Lessons Learned: Long-term Performance Assessments of PRBs 

Geochemical characterization of sites for PRBs is important for optimizing the design and performance of a PRB and for 

predicting longevity. The lifetime of a PRB will depend on the hydrogeochemical nature of the site, flow rate, and 

contaminant flux, among other factors. It is known that high carbonate waters, high nitrate waters, high dissolved organic 

carbon waters, or waters with generally high total dissolved solids will have shorter life expectancies than what might be 

considered for “typical” or “average” composition waters. Decreased life expectancy may be caused by competition for 

reaction sites, loss of reactive sites due to rapid corrosion or fouling, or precipitation of inorganic minerals due to changes 

in geochemistry caused by the presence of the reactive media with subsequent loss of permeability. 

If zero-valent iron is the reactive media, corrosion reactions together with mineral precipitation will eventually result in 

loss of permeability and/or reactivity resulting in decreased performance to the point where performance goals are no 

longer met. Long-term performance studies have documented decreased reactivity at some sites over time as well as 

loss of porosity, which can affect residence time of contaminants in the reactive media. For inorganic contaminants, 

such as chromium and arsenic, removal capacities have been calculated for zero valent iron PRBs. These capacities 

can be quickly reached if waters are rich in species that compete for reaction sites. 

Monitoring of geochemical parameters (e.g., pH, Eh, dissolved oxygen [DO], specific conductance, terminal electron 

acceptors) in iron-based PRBs will verify that the system functions as designed once installed. Within the zero valent 

iron barriers, the change in these parameters is strikingly different than the natural system (e.g. Eh ~ -400 mV, pH > 9, 

DO = 0). Trends in these parameters may signal changes in system performance, but no clear correlations between 

these parameters and decreased performance have been observed to date. Long-term trends for these parameters are 

consistent with contaminant trends observed in both sites studied and reported on in this document. Spatial and 

temporal variations in the concentration distribution of terminal electron accepting species (e.g., sulfate), specific 

112 
conductance, and Eh suggest that both anaerobic iron corrosion and microbial activity play important roles in controlling 

the oxidation-reduction potential in iron barriers. Low Eh values (<-100 mV relative to the SHE) and decreases in the 

specific conductance of ground water between upgradient contaminant plumes and sampling points within reactive iron 

media are consistently indicative of normally operating PRB systems. Anomalous behavior in these parameters may be 

useful indicators of problems associated with construction, as observed in gate 2 at the Denver Federal Center. 

Performance monitoring also focuses on the rate of mineral buildup within the reactive media. This may lead to 

decreased permeability and clogging. For zero valent iron systems, the reactive media is a long-term sink for C, S, Ca, 

Si, Mg, and N. The buildup of mineral precipitates is related to influent ground-water chemistry and flow rate. Mineral 

precipitates and microbial biomass accumulate the fastest near the upgradient aquifer-Fe0

 interface. Porosity loss in the 

iron media due to precipitation of inorganic carbon and sulfur minerals can be estimated by integrating the concentrations 

of inorganic carbon and sulfur as a function of distance in the iron and estimating the volume loss by using the molar 

volumes of zero-valent iron, calcium carbonate, iron carbonate, and iron sulfide. The rate of mineral accumulation and 

the rate of iron corrosion varies spatially, therefore, so does the rate of porosity infilling. Porosity loss estimates have 

ranged from 1 to 4 % per year in this study. Based on these estimates, the average porosity of the PRB at Elizabeth City 

would not be expected to approach that of the surrounding aquifer for 15 to 30 years. The highest concentrations of 

mineral precipitates and rates of porosity loss are found adjacent to upgradient interfaces, and at Elizabeth City, there is 

a vertically localized zone which corresponds to the higher specific conductance portion of the plume. As these zones 

lose porosity, flow may be diverted to other locations along the face of the wall, thus extending lifetime estimates based 

on worst-case scenarios. As corrosion minerals form on the surface of the iron media, reactive surfaces are coated, 

presumably decreasing the effective reactive surface area. However, corrosion products formed include some minerals 

which themselves are highly reactive and capable of transforming inorganic and organic contaminants into immobile or 

non-toxic species. This phenomenon must also be factored into lifetime projections. 

Microbiological impacts are important to understand in order to better predict how long PRB systems will remain 

effective. The presence of a large reservoir of iron, coupled with abundant substrate supports the metabolic activity of 

iron-reducing, sulfate-reducing, and/or methanogenic bacteria. These populations may have either beneficial or 

detrimental effects on system performance. Enhanced biodegradation of contaminants is possible where growth is 

stimulated by the presence of the reactive media, but biofouling may lead to permeability reduction within the reactive 

media or immediately upgradient. Additional research is required to assess changes in microbial ecology associated with 

the installation of these systems. Indications from Elizabeth City are that enhanced treatment some distance away from 

the PRB occurs, and it will be interesting to follow this development over extended periods of time. 

While long-term performance observations of the Elizabeth City and Denver Federal Center site are now approaching 

seven years, there has still not been sufficient time to adequately predict the lifetime of these PRBs. It is clear that 

lifetimes exceeding 10 years are reasonable to expect, and they may function adequately for much longer. Continued 

studies are needed to better predict longevity based on ground-water composition, flow rate, and contaminant flux. 

6.3.1 Recommendations for Future Research 

The research conducted to date on the long-term performance of permeable reactive barriers has uncovered a number 

of new questions that require further investigation. In addition, several complex outstanding issues still require further 

study. Topical areas of research are outlined below where continued efforts will lead to more successful implementation 

of PRB technology for ground-water remediation. 

•	 Tools are needed to more quantitatively relate ground-water chemistry to porosity loss, changes in hydraulic 

performance, and changes in Fe0

 reactivity. 

•	 An improved understanding is needed of how the reactivity and removal capacity of iron-based media 

changes with time and continued ground-water exposure, with respect to various inorganic and organic 

contaminants of concern, and over a range of hydrogeological and geochemical conditions. 

•	 Research is needed on the microbial ecology of iron-based PRBs, especially with respect to potential 

benefits and deleterious effects, and the factors that control biomass accumulation. 

•	 A better understanding and development of approaches are needed to determine flow patterns through 

reactive barriers in three-dimensions. 

•	 Application of the PRB technology in conjunction with other subsurface treatment technologies is needed 

(e.g., monitored natural attenuation, source zone treatment). 

•	 New methods are needed for regeneration of the reactive medium in situ, without having to excavate and 

entirely or partially replace the PRB. 

113 
114

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120 
Appendix A


121

Table A1. Inorganic Carbon and Sulfur Concentrations in Elizabeth City Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-1 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-3 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

EC050801-4 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

5 

4 

3 

2 

1 

-1 

-2 

-3 

-4 

-5 

-6 

-7 

-8 

-9 

-11 

5 

4 

3 

2 

1 

-1 

-2 

-3 

-4 

-5 

-6 

-7 

-8 

-9 

5 

4 

3 

2 

1 

-1 

-2 

-3 

-4 

-5 

-6 

-7 

-8 

-9 

-10 

0-10.2 

10.2-20.4 

20.4-30.6 

30.6-40.8 

40.8-51.0 

51.0-56.1 

56.1-61.2 

61.2-66.3 

66.3-71.4 

71.4-76.5 

76.5-81.6 

81.6-86.7 

86.7-91.8 

91.8-96.9 

102-107.1 

0-10.2 

10.2-20.4 

20.4-30.6 

30.6-40.8 

40.8-51.0 

51.0-56.1 

56.1-61.2 

61.2-66.3 

66.3-71.4 

71.4-76.5 

76.5-81.6 

81.6-86.7 

86.7-91.8 

91.8-99.4 

0-10.2 

10.2-20.4 

20.4-30.6 

30.6-40.8 

40.8-51.0 

51.0-56.1 

56.1-61.2 

61.2-66.3 

66.3-71.4 

71.4-76.5 

76.5-81.6 

81.6-86.7 

86.7-91.8 

91.8-96.9 

96.9-107.1 

May-01 56 80 

May-01 31 110 

May-01 83 170 

May-01 121 330 

May-01 315 620 

May-01 1850 468 

May-01 384 240 

May-01 264 90 

May-01 153 70 

May-01 151 70 

May-01 23 100 

May-01 44 70 

May-01 71 50 

May-01 106 60 

May-01 7 100 

May-01 4 130 

May-01 53 410 

May-01 176 1510 

May-01 14 190 

May-01 10 160 

May-01 4633 3880 

May-01 5867 1100 

May-01 4000 590 

May-01 2100 340 

May-01 1086 210 

May-01 381 150 

May-01 453 110 

May-01 176 80 

May-01 149 90 

May-01 5 60 

May-01 7 100 

May-01 29 140 

May-01 20 170 

May-01 500 690 

May-01 1672 2140 

May-01 3337 2080 

May-01 1167 570 

May-01 401 240 

May-01 174 216 

May-01 136 232 

May-01 129 285 

May-01 155 189 

May-01 60 1 

May-01 46 1 

122 
Table A1. (continued) Inorganic Carbon and Sulfur Concentrations in Elizabeth City Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

EC050801-5 

EC050801-5 

EC050801-5 

EC050801-5 

EC050801-5 

EC050801-5 

EC050801-5 

EC050801-5 

EC050801-5 

EC050801-5 

EC050801-5 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-6 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

EC050801-7 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

5 

4 

3 

2 

1 

-1 

-2 

-3 

-4 

-5 

-6 

5 

4 

3 

2 

1 

-1 

-2 

-3 

-4 

-5 

-6 

-7 

5 

4 

3 

2 

1 

-1 

-2 

-3 

-4 

-5 

-6 

-7 

-8 

-9 

0-12.7 

12.7-25.4 

25.4-38.1 

38.1-50.8 

50.8-63.5 

63.5-68.6 

68.6-73.7 

73.7-78.8 

78.8-83.9 

83.9-89.0 

89.0-94.1 

0-12.7 

12.7-25.4 

25.4-38.1 

38.1-50.8 

50.8-63.5 

63.5-68.6 

68.6-73.7 

73.7-78.8 

78.8-83.9 

83.9-89.0 

89.0-94.1 

94.1-101.7 

0-12.7 

12.7-25.4 

25.4-38.1 

38.1-50.8 

50.8-63.5 

63.5-68.6 

68.6-73.7 

73.7-78.8 

78.8-83.9 

83.9-89.0 

89.0-94.1 

94.1-99.2 

99.2-104.3 

104.3-109.4 

May-01 274 768 

May-01 150 620 

May-01 1023 720 

May-01 126 420 

May-01 258 640 

May-01 468 1370 

May-01 4491 1830 

May-01 2370 2680 

May-01 4000 2210 

May-01 4520 709 

May-01 1824 303 

May-01 0 10 

May-01 2 10 

May-01 20 20 

May-01 28 10 

May-01 69 20 

May-01 231 180 

May-01 223 160 

May-01 172 100 

May-01 186 110 

May-01 110 90 

May-01 76 80 

May-01 121 70 

May-01 65 151 

May-01 8 149 

May-01 45 93 

May-01 78 280 

May-01 55 129 

May-01 1542 1610 

May-01 3615 926 

May-01 3636 278 

May-01 1219 212 

May-01 383 168 

May-01 162 122 

May-01 206 95 

May-01 158 76 

May-01 256 147 

123 
Table A1. (continued) Inorganic Carbon and Sulfur Concentrations in Elizabeth City Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

EC050901-8 

EC050901-8 

EC050901-8 

EC050901-8 

EC050901-8 

EC050901-8 

EC050901-8 

EC050901-8 

EC050901-8 

EC050901-8 

EC050901-8 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC050901-9 

EC060200-1 

EC060200-1 

EC060200-1 

EC060200-1 

EC060200-1 

EC060200-1 

EC060200-1 

EC060200-1 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Vertical 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

Upgradient 

-1 

-3 

-5 

-8 

-11 

-13 

-15 

-17 

-18 

-19 

-20 

-1 

-3 

-5 

-7 

-9 

-11 

-13 

-15 

-17 

-18 

-19 

-20 

-22 

4 

3 

2 

1 

-1 

-2 

-3 

-4 

3 

2 

1 

-1 

-2 

-3 

-4 

-5 

-6 

-7 

-8 

-9 

0-5.1 

10.2-15.3 

20.4-25.6 

35.8-40.9 

51.1-56.2 

61.3-66.4 

71.5-76.6 

81.7-86.8 

86.8-91.9 

91.9-97.0 

97-102.1 

0-5.1 

10.2-15.3 

20.4-25.6 

30.7-35.8 

40.9-46.0 

51.1-56.2 

61.3-66.4 

71.5-76.6 

81.7-86.8 

86.8-91.9 

91.9-97.0 

97.0-102.1 

107.2-112.3 

0-10.2 

10.2-20.4 

20.4-30.6 

30.6-40.8 

40.8-45.9 

45.9-51.0 

51.0-56.1 

56.1-61.2 

0-10.2 

10.2-20.4 

20.4-30.6 

30.6-35.7 

35.7-40.8 

40.8-45.9 

45.9-51.0 

51.0-56.1 

56.1-61.2 

61.2-66.3 

66.3-71.4 

71.4-76.5 

May-01 59 98 

May-01 80 87 

May-01 51 70 

May-01 112 88 

May-01 49 80 

May-01 46 58 

May-01 90 84 

May-01 98 84 

May-01 201 90 

May-01 333 93 

May-01 285 101 

May-01 100 100 

May-01 122 103 

May-01 118 121 

May-01 204 93 

May-01 160 95 

May-01 242 104 

May-01 272 132 

May-01 276 116 

May-01 513 165 

May-01 517 174 

May-01 523 169 

May-01 854 121 

May-01 355 70 

Jun-00 7 50 

Jun-00 4 69 

Jun-00 3 70 

Jun-00 8 141 

Jun-00 308 756 

Jun-00 1740 588 

Jun-00 244 230 

Jun-00 175 168 

Jun-00 9 147 

Jun-00 8 143 

Jun-00 65 187 

Jun-00 2339 1179 

Jun-00 3210 466 

Jun-00 2141 279 

Jun-00 234 301 

Jun-00 194 166 

Jun-00 119 161 

Jun-00 73 130 

Jun-00 72 104 

Jun-00 107 128 

124 
Table A1. (continued) Inorganic Carbon and Sulfur Concentrations in Elizabeth City Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

EC060300-6 Downgradient 9 0-7.6 Jun-00 <1 265 

EC060300-6 Downgradient 8 7.6-15.2 Jun-00 <1 296 

EC060300-6 Downgradient 6 22.8-30.4 Jun-00 <1 194 

EC060300-6 Downgradient 4 38.0-45.6 Jun-00 1 157 

EC060300-6 Downgradient 2 53.2-60.8 Jun-00 41 125 

EC060300-6 Downgradient 1 60.8-68.4 Jun-00 81 239 

EC060300-6 Downgradient -1 68.4-73.5 Jun-00 92 196 

EC060300-6 Downgradient -2 73.5-78.6 Jun-00 66 106 

EC060300-6 Downgradient -3 78.6-83.7 Jun-00 81 163 

EC060300-6 Downgradient -4 83.7-88.8 Jun-00 80 161 

EC060300-6 Downgradient -5 88.8-93.9 Jun-00 242 218 

EC060300-5 Vertical 8 0-7.6 Jun-00 2 151 

EC060300-5 Vertical 7 7.6-15.2 Jun-00 10 97 

EC060300-5 Vertical 6 15.2-22.8 Jun-00 16 155 

EC060300-5 Vertical 5 22.8-30.4 Jun-00 12 138 

EC060300-5 Vertical 4 30.4-38.0 Jun-00 4 126 

EC060300-5 Vertical 3 38.0-45.6 Jun-00 10 114 

EC060300-5 Vertical 2 45.6-53.2 Jun-00 19 79 

EC060300-5 Vertical 1 53.2-60.8 Jun-00 7 133 

EC060300-5 Vertical -1 60.8-68.4 Jun-00 66 239 

EC060300-5 Vertical -2 68.4-76.0 Jun-00 57 173 

EC060300-5 Vertical -3 76.0-83.6 Jun-00 80 174 

EC060500-7A Vertical 5 0-7.6 Jun-00 na 88 

EC060500-7A Vertical 4 7.6-15.2 Jun-00 na 81 

EC060500-7A Vertical 3 15.2-22.8 Jun-00 na 166 

EC060500-7A Vertical 2 22.8-30.4 Jun-00 na 104 

EC060500-7A Vertical 1 30.4-38.0 Jun-00 na 160 

EC060500-7A Vertical -1 38.0-43.1 Jun-00 na 106 

EC060500-7A Vertical -2 43.1-48.2 Jun-00 na 148 

EC060500-7A Vertical -3 48.2-53.3 Jun-00 na 76 

EC060500-7A Vertical -4 53.3-58.4 Jun-00 na 269 

EC060500-7A Vertical -5 58.4-63.5 Jun-00 na 165 

EC060500-7A Vertical -6 63.5-68.6 Jun-00 na 161 

EC030616 Upgradient 1 0-10.2 Jun-99 571 289 

EC030616 Upgradient -1 10.2-20.4 Jun-99 1392 303 

EC030616 Upgradient -2 20.4-30.6 Jun-99 1345 292 

EC030616 Upgradient -3 30.6-40.8 Jun-99 1500 231 

EC030616 Upgradient -4 40.8-51.0 Jun-99 97 101 

EC030616 Upgradient -5 51.0-61.2 Jun-99 274 94 

125

Table A1. (continued) Inorganic Carbon and Sulfur Concentrations in Elizabeth City Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

EC90903 Upgradient 2 0-7.6 Sep-98 4 18 

EC90903 Upgradient 1 7.6-15.2 Sep-98 55 211 

EC90903 Upgradient -1 15.2-22.8 Sep-98 2350 1019 

EC90903 Upgradient -2 22.8-30.4 Sep-98 357 315 

EC90903 Upgradient -3 30.4-38.0 Sep-98 174 137 

EC90903 Upgradient -4 38.0-45.6 Sep-98 117 183 

EC90903 Upgradient -5 45.6-53.2 Sep-98 110 178 

EC90903 Upgradient -6 53.2-60.8 Sep-98 50 142 

EC6101 Upgradient -1 0-7.6 Jun-98 1800 228 

EC6101 Upgradient -2 7.6-15.2 Jun-98 792 195 

EC6101 Upgradient -3 15.2-22.8 Jun-98 550 157 

EC6101 Upgradient -4 22.8-30.4 Jun-98 451 117 

EC6101 Upgradient -5 30.4-38.0 Jun-98 65 64 

EC6101 Upgradient -6 38.0-45.6 Jun-98 106 96 

EC6101 Upgradient -7 45.6-53.2 Jun-98 208 39 

EC6101 Upgradient -8 53.2-60.8 Jun-98 419 34 

EC6101 Upgradient -9 60.8-68.4 Jun-98 541 36 

126

Table A2.	 Reduced Sulfur Speciation in Elizabeth City and Denver Federal Center Cores 

Sample ID Site Section Interval Date AVS CRS Total S 

cm μg/g μg/g μg/g 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

EC060300-4 

C2-12-71300 

C2-12-71300 

C2-12-71300 

C2-12-71300 

C2-12-71300 

C2-13-71300 

C2-13-71300 

C2-13-71300 

C2-13-71300 

C2-13-71300 

C2-14-71300 

C2-14-71300 

C2-14-71300 

C2-14-71300 

C2-17-71300 

C2-17-71300 

C2-17-71300 

C2-17-71300 

EC 1 20.4-30.6 Jun-00 17 6 187 

EC -1 30.6-35.7 Jun-00 1200 nd 1179 

EC -2 35.7-40.8 Jun-00 612 39 466 

EC -3 40.8-45.9 Jun-00 303 nd 279 

EC -4 45.9-51.0 Jun-00 351 nd 301 

EC -6 56.1-61.2 Jun-00 222 nd 161 

EC -7 61.2-66.3 Jun-00 155 nd 130 

EC -9 71.4-76.5 Jun-00 145 nd 128 

DFC -1 0-10.2 Jul-00 3089 120 3239 

DFC -3 20.4-30.6 Jul-00 4333 198 4597 

DFC -5 40.8-51.0 Jul-00 3019 55 3356 

DFC -7 61.2-71.4 Jul-00 3976 230 4084 

DFC -9 81.6-91.8 Jul-00 3746 250 3833 

DFC -1 0-10.2 Jul-00 3382 504 3844 

DFC -3 20.4-30.6 Jul-00 3434 130 3527 

DFC -5 40.8-51.0 Jul-00 2807 227 3172 

DFC -7 61.2-71.4 Jul-00 4033 422 3935 

DFC -9 81.6-91.8 Jul-00 3254 105 3189 

DFC -1 0-10.2 Jul-00 1538 205 1627 

DFC -3 20.4-30.6 Jul-00 1275 193 1519 

DFC -5 40.8-51.0 Jul-00 2128 25 1957 

DFC -7 61.2-71.4 Jul-00 1829 25 1878 

DFC -1 0-5.1 Jul-00 3590 197 3799 

DFC -3 10.2-15.3 Jul-00 3365 262 3366 

DFC -5 20.4-25.5 Jul-00 2445 337 2324 

DFC -7 30.6-35.7 Jul-00 1752 148 1867 

Notes: nd, not determined. 

127 
Table A3. Inorganic Carbon and Sulfur Concentrations in Denver Federal Center Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

C1-1-71701 Upgradient -1 0-5.1 Jul-01 8700 2203 

C1-1-71701 Upgradient -2 5.1-10.2 Jul-01 2500 2066 

C1-1-71701 Upgradient -3 10.2-15.3 Jul-01 2200 1966 

C1-1-71701 Upgradient -4 15.3-20.4 Jul-01 1444 1545 

C1-1-71701 Upgradient -5 20.4-25.5 Jul-01 395 928 

C1-1-71701 Upgradient -6 25.5-30.6 Jul-01 442 820 

C1-1-71701 Upgradient -7 30.6-35.7 Jul-01 392 1187 

C1-1-71701 Upgradient -8 35.7-40.8 Jul-01 348 874 

C1-1-71701 Upgradient -9 40.8-45.9 Jul-01 629 889 

C1-1-71701 Upgradient -10 45.9-51.0 Jul-01 352 899 

C1-1-71701 Upgradient -11 51.0-56.1 Jul-01 162 448 

C1-1-71701 Upgradient -12 56.1-61.2 Jul-01 125 463 

C1-1-71701 Upgradient -13 61.2-66.3 Jul-01 95 393 

C1-1-71701 Upgradient -14 66.3-71.4 Jul-01 102 359 

C1-1-71701 Upgradient -15 71.4-76.5 Jul-01 115 347 

C1-1-71701 Upgradient -16 76.5-81.6 Jul-01 95 326 

C1-1-71701 Upgradient -17 81.6-86.7 Jul-01 111 282 

C1-1-71701 Upgradient -18 86.7-91.8 Jul-01 146 242 

C1-1-71701 Upgradient -19 91.8-102 Jul-01 87 318 

C1-2-71701 Upgradient 4 0-10.2 Jul-01 10300 390 

C1-2-71701 Upgradient 3 10.2-20.4 Jul-01 358 279 

C1-2-71701 Upgradient 2 20.4-30.6 Jul-01 31 72.6 

C1-2-71701 Upgradient 1 30.6-40.8 Jul-01 42 151 

C1-2-71701 Upgradient -1 40.8-45.9 Jul-01 9000 3570 

C1-2-71701 Upgradient -2 45.9-51.0 Jul-01 3700 2790 

C1-2-71701 Upgradient -3 51.0-56.1 Jul-01 1800 1450 

C1-2-71701 Upgradient -4 56.1-61.2 Jul-01 4400 3160 

C1-2-71701 Upgradient -5 61.2-66.3 Jul-01 2500 3180 

C1-2-71701 Upgradient -6 66.3-71.4 Jul-01 3407 2440 

C1-2-71701 Upgradient -7 71.4-79.0 Jul-01 1347 1940 

C1-3-71701 Upgradient 3 0-7.6 Jul-01 10 <5 

C1-3-71701 Upgradient 2 7.6-15.2 Jul-01 19 <5 

C1-3-71701 Upgradient 1 15.2-22.8 Jul-01 38 137 

C1-3-71701 Upgradient -1 22.8-27.9 Jul-01 8 138 

C1-3-71701 Upgradient -2 27.9-33.0 Jul-01 726 149 

C1-3-71701 Upgradient -3 33.0-38.1 Jul-01 2600 4321 

C1-3-71701 Upgradient -4 38.1-43.2 Jul-01 8900 5680 

C1-3-71701 Upgradient -5 43.2-48.3 Jul-01 4200 4860 

C1-3-71701 Upgradient -6 48.3-53.4 Jul-01 3000 4400 

C1-3-71701 Upgradient -7 53.4-58.5 Jul-01 3000 3290 

C1-3-71701 Upgradient -8 58.5-63.6 Jul-01 3600 5110 

C1-3-71701 Upgradient -9 63.6-68.7 Jul-01 3200 4640 

C1-3-71701 Upgradient -10 68.7-73.8 Jul-01 2000 2570 

C1-3-71701 Upgradient -11 73.8-81.4 Jul-01 775 1570 

128

Table A3. (continued) Inorganic Carbon and Sulfur Concentrations in Denver Federal Center Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

C1-4-71801 Downgradient 3 0-5.1 Jul-01 187 144 

C1-4-71801 Downgradient 2 5.1-10.2 Jul-01 692 142 

C1-4-71801 Downgradient 1 10.2-15.3 Jul-01 1500 237 

C1-4-71801 Downgradient -1 15.3-20.4 Jul-01 1700 787 

C1-4-71801 Downgradient -2 20.4-25.5 Jul-01 5800 2780 

C1-4-71801 Downgradient -3 25.5-30.6 Jul-01 9800 5290 

C1-4-71801 Downgradient -4 30.6-35.7 Jul-01 14100 7520 

C1-4-71801 Downgradient -5 35.7-40.8 Jul-01 8900 5450 

C1-4-71801 Downgradient -6 40.8-45.9 Jul-01 9500 5210 

C1-4-71801 Downgradient -7 45.9-51.0 Jul-01 11600 4430 

C1-4-71801 Downgradient -8 51.0-56.1 Jul-01 10400 4170 

C1-4-71801 Downgradient -9 56.1-61.2 Jul-01 7400 4690 

C1-4-71801 Downgradient -10 61.2-66.3 Jul-01 7000 3720 

C1-4-71801 Downgradient -11 66.3-71.4 Jul-01 3100 3010 

C1-4-71801 Downgradient -12 71.4-76.5 Jul-01 707 1096 

C1-4-71801 Downgradient -13 76.5-81.6 Jul-01 206 472 

C1-4-71801 Downgradient -14 81.6-86.7 Jul-01 124 580 

C1-4-71801 Downgradient -15 86.7-91.8 Jul-01 80 274 

C1-4-71801 Downgradient -16 91.8-96.9 Jul-01 226 280 

C1-4-71801 Downgradient -17 96.9-102 Jul-01 117 197 

C1-4-71801 Downgradient -18 102-107.1 Jul-01 65 220 

C1-4-71801 Downgradient -19 107.1-112.2 Jul-01 154 217 

C2-1-71801 Upgradient 2 0-5.1 Jul-01 224 1580 

C2-1-71801 Upgradient 1 5.1-10.2 Jul-01 13700 4930 

C2-1-71801 Upgradient -1 10.2-15.3 Jul-01 7500 5650 

C2-1-71801 Upgradient -2 15.3-20.4 Jul-01 4700 4460 

C2-1-71801 Upgradient -3 20.4-25.5 Jul-01 5200 3950 

C2-1-71801 Upgradient -4 25.5-30.6 Jul-01 5200 3320 

C2-1-71801 Upgradient -5 30.6-35.7 Jul-01 2200 3420 

C2-1-71801 Upgradient -6 35.7-40.8 Jul-01 3000 2820 

C2-1-71801 Upgradient -7 40.8-45.9 Jul-01 3500 3550 

C2-1-71801 Upgradient -8 45.9-51.0 Jul-01 3300 4260 

C2-1-71801 Upgradient -9 51.0-56.1 Jul-01 3300 2880 

C2-1-71801 Upgradient -10 56.1-61.2 Jul-01 1800 2590 

C2-1-71801 Upgradient -11 61.2-66.3 Jul-01 1404 2090 

C2-1-71801 Upgradient -12 66.3-71.4 Jul-01 1383 2460 

C2-1-71801 Upgradient -13 71.4-76.5 Jul-01 2600 3330 

C2-1-71801 Upgradient -14 76.5-81.6 Jul-01 3300 3530 

C2-1-71801 Upgradient -15 81.6-86.7 Jul-01 2700 2270 

C2-1-71801 Upgradient -16 86.7-91.8 Jul-01 2200 2200 

C2-1-71801 Upgradient -17 91.8-96.9 Jul-01 1035 1750 

C2-1-71801 Upgradient -18 96.9-102 Jul-01 774 1450 

C2-1-71801 Upgradient -19 102-107.1 Jul-01 499 1210 

C2-1-71801 Upgradient -20 107.1-112.2 Jul-01 189 580 

C2-1-71801 Upgradient -21 112.2-117.3 Jul-01 1146 2130 

129

Table A3. (continued) Inorganic Carbon and Sulfur Concentrations in Denver Federal Center Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

C2-3-71801 Upgradient -1 0-5.1 Jul-01 7700 2718 

C2-3-71801 Upgradient -2 5.1-10.2 Jul-01 5900 2041 

C2-3-71801 Upgradient -3 10.2-15.3 Jul-01 4100 2111 

C2-3-71801 Upgradient -4 15.3-20.4 Jul-01 2100 1785 

C2-3-71801 Upgradient -5 20.4-25.5 Jul-01 1324 1464 

C2-3-71801 Upgradient -6 25.5-30.6 Jul-01 331 582 

C2-3-71801 Upgradient -7 30.6-35.7 Jul-01 137 495 

C2-3-71801 Upgradient -8 35.7-40.8 Jul-01 150 561 

C2-3-71801 Upgradient -9 40.8-45.9 Jul-01 128 520 

C2-3-71801 Upgradient -10 45.9-51.0 Jul-01 118 506 

C2-3-71801 Upgradient -11 51.0-56.1 Jul-01 91 547 

C2-3-71801 Upgradient -12 56.1-61.2 Jul-01 115 300 

C2-3-71801 Upgradient -13 61.2-66.3 Jul-01 90 320 

C2-3-71801 Upgradient -14 66.3-71.4 Jul-01 108 440 

C2-3-71801 Upgradient -15 71.4-76.5 Jul-01 122 425 

C2-3-71801 Upgradient -16 76.5-81.6 Jul-01 144 501 

C2-3-71801 Upgradient -17 81.6-86.7 Jul-01 111 736 

C2-3-71801 Upgradient -18 86.7-91.8 Jul-01 107 637 

C2-3-71801 Upgradient -19 91.8-96.9 Jul-01 135 375 

C2-4-71801 Upgradient 2 0-7.6 Jul-01 147 <5 

C2-4-71801 Upgradient 1 7.6-15.2 Jul-01 4100 <5 

C2-4-71801 Upgradient -1 15.2-20.3 Jul-01 5900 1110 

C2-4-71801 Upgradient -2 20.3-25.4 Jul-01 2500 1460 

C2-4-71801 Upgradient -3 25.4-30.5 Jul-01 1036 758 

C2-4-71801 Upgradient -4 30.5-35.6 Jul-01 398 727 

C2-4-71801 Upgradient -5 35.6-40.7 Jul-01 435 611 

C2-4-71801 Upgradient -6 40.7-45.8 Jul-01 296 580 

C2-4-71801 Upgradient -7 45.8-50.9 Jul-01 1600 1550 

C2-4-71801 Upgradient -8 50.9-56.0 Jul-01 1205 1250 

C2-4-71801 Upgradient -9 56.0-61.1 Jul-01 1385 1414 

C2-4-71801 Upgradient -10 61.1-66.2 Jul-01 1498 1485 

C2-4-71801 Upgradient -11 66.2-71.3 Jul-01 1161 1234 

C2-4-71801 Upgradient -12 71.3-76.4 Jul-01 1308 1189 

C2-4-71801 Upgradient -13 76.4-81.5 Jul-01 1012 1062 

C2-4-71801 Upgradient -14 81.5-86.6 Jul-01 755 925 

C2-4-71801 Upgradient -15 86.6-91.7 Jul-01 438 716 

C2-4-71801 Upgradient -16 91.7-96.8 Jul-01 382 619 

130

Table A3. (continued) Inorganic Carbon and Sulfur Concentrations in Denver Federal Center Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

C2-1-71901 Vertical -1 0-7.6 Jul-01 417 920 

C2-1-71901 Vertical -2 7.6-15.2 Jul-01 567 1192 

C2-1-71901 Vertical -3 15.2-22.8 Jul-01 384 1220 

C2-1-71901 Vertical -4 22.8-30.4 Jul-01 960 1742 

C2-1-71901 Vertical -5 30.4-38.0 Jul-01 1203 2150 

C2-1-71901 Vertical -6 38.0-45.6 Jul-01 1523 2807 

C2-1-71901 Vertical -7 45.6-53.2 Jul-01 1116 2621 

C2-1-71901 Vertical -8 53.2-60.8 Jul-01 1349 1879 

C2-1-71901 Vertical -9 60.8-68.4 Jul-01 1283 2291 

C2-1-71901 Vertical -10 68.4-76.0 Jul-01 2600 2995 

C2-1-71901 Vertical -11 76.0-83.6 Jul-01 2050 3108 

C2-1-71901 Vertical -12 83.6-91.2 Jul-01 2850 3175 

C2-1-71901 Vertical -13 91.2-98.8 Jul-01 2700 3803 

C2-1-71901 Vertical -14 98.8-106.4 Jul-01 2600 2493 

C2-1-71901 Vertical -15 106.4-114.0 Jul-01 4500 3374 

C2-2-71901 Vertical -1 0-7.6 Jul-01 212 814 

C2-2-71901 Vertical -2 7.6-15.2 Jul-01 184 755 

C2-2-71901 Vertical -3 15.2-22.8 Jul-01 217 546 

C2-2-71901 Vertical -4 22.8-30.4 Jul-01 249 628 

C2-2-71901 Vertical -5 30.4-38.0 Jul-01 365 499 

C2-2-71901 Vertical -6 38.0-45.6 Jul-01 352 380 

C2-2-71901 Vertical -7 45.6-53.2 Jul-01 1700 613 

C2-2-71901 Vertical -8 53.2-60.8 Jul-01 253 633 

C2-2-71901 Vertical -9 60.8-68.4 Jul-01 291 210 

C2-2-71901 Vertical -10 68.4-76.0 Jul-01 176 694 

C2-2-71901 Vertical -11 76.0-83.6 Jul-01 172 551 

C2-2-71901 Vertical -12 83.6-91.2 Jul-01 124 687 

C2-2-71901 Vertical -13 91.2-98.8 Jul-01 216 232 

C2-2-71901 Vertical -14 98.8-106.4 Jul-01 146 782 

C2-3-71901 Vertical 1 0-7.6 Jul-01 2500 122 

C2-3-71901 Vertical 2 7.6-15.2 Jul-01 7900 269 

C2-3-71901 Vertical -1 15.2-22.8 Jul-01 1299 831 

C2-3-71901 Vertical -2 22.8-30.4 Jul-01 1113 971 

C2-3-71901 Vertical -3 30.4-38.0 Jul-01 2500 1590 

C2-3-71901 Vertical -4 38.0-45.6 Jul-01 5600 1910 

C2-3-71901 Vertical -5 45.6-53.2 Jul-01 4500 1750 

C2-3-71901 Vertical -6 53.2-60.8 Jul-01 4100 1660 

C2-3-71901 Vertical -7 60.8-68.4 Jul-01 2800 1730 

C2-3-71901 Vertical -8 68.4-76.0 Jul-01 3000 1840 

C2-3-71901 Vertical -9 76.0-83.6 Jul-01 4000 1490 

C2-3-71901 Vertical -10 83.6-91.2 Jul-01 3300 1420 

131

Table A3.	 (continued) Inorganic Carbon and Sulfur Concentrations in Denver Federal Center Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

C3-2-71801 Upgradient -1 0-5.1 Jul-01 1800 231 

C3-2-71801 Upgradient -2 5.1-10.2 Jul-01 594 203 

C3-2-71801 Upgradient -3 10.2-15.3 Jul-01 452 152 

C3-2-71801 Upgradient -4 15.3-20.4 Jul-01 429 133 

C3-2-71801 Upgradient -5 20.4-25.5 Jul-01 693 146 

C3-2-71801 Upgradient -6 25.5-30.6 Jul-01 1037 149 

C3-2-71801 Upgradient -7 30.6-35.7 Jul-01 1900 881 

C3-2-71801 Upgradient -8 35.7-40.8 Jul-01 640 401 

C3-2-71801 Upgradient -9 40.8-45.9 Jul-01 1200 547 

C3-2-71801 Upgradient -10 45.9-51.0 Jul-01 2000 649 

C3-2-71801 Upgradient -11 51.0-56.1 Jul-01 2800 1180 

C3-2-71801 Upgradient -12 56.1-61.2 Jul-01 3900 1940 

C3-2-71801 Upgradient -13 61.2-66.3 Jul-01 6300 2320 

C3-2-71801 Upgradient -14 66.3-71.4 Jul-01 4900 2290 

C3-2-71801 Upgradient -15 71.4-76.5 Jul-01 4100 1051 

C3-2-71801 Upgradient -16 76.5-81.6 Jul-01 5800 2120 

C1-2-71000 Upgradient -1 0-5.1 Jul-00 593 702 

C1-2-71000 Upgradient -2 5.1-10.2 Jul-00 854 1141 

C1-2-71000 Upgradient -3 10.2-15.3 Jul-00 1390 1377 

C1-2-71000 Upgradient -4 15.3-20.4 Jul-00 1559 1328 

C1-2-71000 Upgradient -5 20.4-25.5 Jul-00 1056 1107 

C1-2-71000 Upgradient -6 25.5-30.6 Jul-00 563 681 

C1-2-71000 Upgradient -7 30.6-35.7 Jul-00 493 648 

C1-2-71000 Upgradient -8 35.7-40.8 Jul-00 361 437 

C1-2-71000 Upgradient -9 40.8-45.9 Jul-00 202 431 

C1-2-71000 Upgradient -10 45.9-51.0 Jul-00 306 460 

C1-2-71000 Upgradient -11 51.0-56.1 Jul-00 235 421 

C1-2-71000 Upgradient -12 56.1-61.2 Jul-00 118 306 

C1-2-71000 Upgradient -13 61.2-66.3 Jul-00 144 267 

C1-2-71000 Upgradient -14 66.3-73.9 Jul-00 125 278 

C1-2-71000 Upgradient -15 73.9-81.5 Jul-00 167 284 

C1-3-71100 Downgradient 3 0-7.6 Jul-00 2656 4143 

C1-3-71100 Downgradient 2 7.6-15.2 Jul-00 906 2518 

C1-3-71100 Downgradient 1 15.2-22.8 Jul-00 769 1199 

C1-3-71100 Downgradient -1 22.8-27.9 Jul-00 245 717 

C1-3-71100 Downgradient -2 27.9-33.0 Jul-00 218 464 

C1-3-71100 Downgradient -3 33.0-38.1 Jul-00 161 412 

C1-3-71100 Downgradient -4 38.1-43.2 Jul-00 154 413 

C1-3-71100 Downgradient -5 43.2-48.3 Jul-00 158 303 

C1-3-71100 Downgradient -6 48.3-53.4 Jul-00 160 232 

C1-3-71100 Downgradient -7 53.4-58.5 Jul-00 174 233 

C1-3-71100 Downgradient -8 58.5-63.6 Jul-00 126 169 

C1-3-71100 Downgradient -9 63.6-68.7 Jul-00 139 128 

C1-3-71100 Downgradient -10 68.7-73.8 Jul-00 128 143 

C1-3-71100 Downgradient -11 73.8-81.4 Jul-00 143 106 

132

Table A3. (continued) Inorganic Carbon and Sulfur Concentrations in Denver Federal Center Cores 

Sample ID Location Section Interval Date Total IC Total S 

cm μg/g μg/g 

C2-17-71300 Upgradient -1 0-5.1 Jul-00 5987 3799 

C2-17-71300 Upgradient -2 5.1-10.2 Jul-00 4324 3546 

C2-17-71300 Upgradient -3 10.2-15.3 Jul-00 4772 3366 

C2-17-71300 Upgradient -4 15.3-20.4 Jul-00 4817 2838 

C2-17-71300 Upgradient -5 20.4-25.5 Jul-00 3704 2324 

C2-17-71300 Upgradient -6 25.5-30.6 Jul-00 3481 2092 

C2-17-71300 Upgradient -7 30.6-35.7 Jul-00 2367 1867 

C2-17-71300 Upgradient -8 35.7-43.3 Jul-00 2769 1678 

C2-16-71300 Downgradient 1 0-7.6 Jul-00 871 913 

C2-16-71300 Downgradient -1 7.6-12.7 Jul-00 793 1320 

C2-16-71300 Downgradient -2 12.7-17.8 Jul-00 2299 2034 

C2-16-71300 Downgradient -3 17.8-22.9 Jul-00 4243 3830 

C2-16-71300 Downgradient -4 22.9-28.0 Jul-00 5949 3362 

C2-12-71300 Vertical -1 0-10.2 Jul-00 5470 3239 

C2-12-71300 Vertical -2 10.2-20.4 Jul-00 6048 3966 

C2-12-71300 Vertical -3 20.4-30.6 Jul-00 7233 4597 

C2-12-71300 Vertical -4 30.6-40.8 Jul-00 7201 3886 

C2-12-71300 Vertical -5 40.8-51.0 Jul-00 7391 3356 

C2-12-71300 Vertical -6 51.0-61.2 Jul-00 7506 3493 

C2-12-71300 Vertical -7 61.2-71.4 Jul-00 4613 4084 

C2-12-71300 Vertical -8 71.4-81.6 Jul-00 6556 4453 

C2-12-71300 Vertical -9 81.6-91.8 Jul-00 6692 3833 

C2-12-71300 Vertical -10 91.8-102 Jul-00 8969 2044 

C2-13-71300 Vertical -1 0-10.2 Jul-00 5178 3844 

C2-13-71300 Vertical -2 10.2-20.4 Jul-00 6786 3761 

C2-13-71300 Vertical -3 20.4-30.6 Jul-00 6602 3527 

C2-13-71300 Vertical -4 30.6-40.8 Jul-00 5603 2923 

C2-13-71300 Vertical -5 40.8-51.0 Jul-00 4035 3172 

C2-13-71300 Vertical -6 51.0-61.2 Jul-00 3850 3354 

C2-13-71300 Vertical -7 61.2-71.4 Jul-00 7058 3935 

C2-13-71300 Vertical -8 71.4-81.6 Jul-00 5073 3324 

C2-13-71300 Vertical -9 81.6-91.8 Jul-00 5637 3189 

C2-14-71300 Vertical -1 0-10.2 Jul-00 1523 1627 

C2-14-71300 Vertical -2 10.2-20.4 Jul-00 476 1207 

C2-14-71300 Vertical -3 20.4-30.6 Jul-00 799 1519 

C2-14-71300 Vertical -4 30.6-40.8 Jul-00 1360 1474 

C2-14-71300 Vertical -5 40.8-51.0 Jul-00 2495 1957 

C2-14-71300 Vertical -6 51.0-61.2 Jul-00 3577 1883 

C2-14-71300 Vertical -7 61.2-71.4 Jul-00 3284 1878 

133

134

Appendix B


135

Table B1. Phospholipid Fatty-acid (PLFA) Extract Data from Elizabeth City Cores 

Biomass Community Structure (% of total PLFA) 

Prokaryote Eukaryotic Prokaryote/ Gram+/ Gram- Anaerobic SRB/ Genera Eukaryotes Physiological 

Sample ID Section Date PLFA PLFA PLFA PLFA Eukaryote Anaerobic Gram- (Monos) Metal Reducers Actinomycetes Nsats (Polyenoics) Status 

pM/g cells/g pM/g pM/g (TerBrSats) (BrMonos) (MidBrSats) 

EC050801-5 3 May-01 155 3.11E+06 154 1 174 19.2 27.7 28.2 6.9 17.5 0.6 0.00 

EC050801-5 2 May-01 248 4.95E+06 245 2 100 17.7 30.6 26.1 7.0 17.7 1.0 0.00 

EC050801-5 1 May-01 348 6.96E+06 3341 7 52 18.3 28.9 27.2 5.5 18.3 1.9 0.00 

EC050801-5 -1 May-01 782 1.56E+07 782 ND NC 28.3 29.4 23.6 0.4 18.4 0.0 0.03 

EC050801-5 -2 May-01 2614 5.23E+07 2614 ND NC 42.1 17.1 30.3 0.0 10.6 0.0 0.00 

EC050801-5 -3 May-01 1809 3.62E+07 1809 ND NC 38.1 19.0 29.3 2.8 10.9 0.0 0.00 

EC050801-5 -4 May-01 2406 4.81E+07 2406 ND NC 41.1 18.8 28.5 0.0 11.7 0.0 0.00 

EC050801-5 -5 May-01 260 5.20E+06 260 ND NC 28.3 20.9 37.8 0.9 12.1 0.0 0.00 

EC050801-5 -6 May-01 24 4.72E+05 24 ND NC 21.9 33.1 27.9 0.0 17.2 0.0 0.00 

EC050801-6 3 May-01 27 5.38E+05 27 ND NC 0.0 0.0 0.0 0.0 100.0 0.0 0.00 EC050801-6 2 May-01 8 1.66E+05 8 ND NC 0.0 59.9 0.0 0.0 40.1 0.0 0.00 

EC050801-6 1 May-01 5 1.06E+05 5 ND NC 0.0 52.6 0.0 0.0 47.4 0.0 0.00 EC050801-6 -1 May-01 54 1.07E+06 50 4 13 12.8 41.9 16.6 2.0 19.4 7.2 0.00 

EC050801-6 -2 May-01 9 1.86E+05 9.3 ND NC 0.0 69.0 0.0 0.0 31.0 0.0 0.00 

EC050801-6 -3 May-01 5 1.09E+05 5.4 ND NC 0.0 69.0 0.0 0.0 31.1 0.0 0.00 

EC050801-6 -4 May-01 6 1.24E+05 6.2 ND NC 0.0 71.5 0.0 0.0 28.5 0.0 0.00 EC050801-6 -5 May-01 4 8.77E+04 4.4 ND NC 0.0 50.8 0.0 0.0 49.3 0.0 0.00 

EC050801-6 -6 May-01 3 6.95E+04 3.5 ND NC 0.0 53.8 0.0 0.0 46.2 0.0 0.00 EC050801-6 -7 May-01 ND ND ND ND NC 0.0 0.0 0.0 0.0 0.0 0.0 0.00 

EC050801-7 3 May-01 25 5.05E+05 25 ND NC 16.5 30.6 26.4 0.0 26.6 0.0 0.00 

EC050801-7 2 May-01 5 9.28E+04 5 ND NC 0.0 22.2 29.8 0.0 48.1 0.0 0.00 

EC050801-7 1 May-01 18 3.56E+05 17 1 16 10.1 37.8 20.7 0.0 25.6 5.8 0.00 

EC050801-7 -1 May-01 913 1.83E+07 913 ND NC 18.1 31.3 26.5 0.0 24.2 0.0 0.00 

EC050801-7 -2 May-01 196 3.92E+06 196 ND NC 16.0 35.2 27.4 1.1 20.4 0.0 0.00 

EC050801-7 -3 May-01 25 5.00E+05 25 ND NC 14.1 45.0 20.7 3.3 16.9 0.0 0.00 

EC050801-7 -4 May-01 9 1.77E+05 9 ND NC 5.3 73.5 0.0 0.0 21.2 0.0 0.00 

EC050801-7 -7 May-01 6 1.19E+05 6 ND NC 0.0 74.8 0.0 0.0 25.2 0.0 0.00 EC050801-7 -9 May-01 6 1.15E+05 6 ND NC 0.0 58.9 0.0 0.0 41.1 0.0 0.00 

EC060200-1 4 Jun-00 4 7.10E+04 3 Trace 17 0.0 34.2 0.0 6.3 53.9 5.7 0.00 

EC060200-1 3 Jun-00 3 5.44E+04 3 ND 19 0.0 29.8 0.0 7.0 58.2 5.0 0.00 

EC060200-1 2 Jun-00 9 1.87E+05 9 ND NC 0.0 34.7 0.0 0.0 65.3 0.0 0.00 

EC060200-1 1 Jun-00 89 1.78E+06 88 1 148 5.8 37.2 0.5 6.0 49.9 0.7 0.10 

EC060200-1 -1 Jun-00 875 1.75E+07 875 ND NC 7.4 57.7 3.6 4.0 27.3 0.0 0.10 

EC060200-1 -2 Jun-00 309 6.18E+06 309 1 555 12.6 50.9 12.7 9.5 14.2 0.2 0.20 

EC060200-1 -3 Jun-00 44 8.73E+05 44 Trace 832 7.5 30.1 4.8 3.9 53.6 0.1 0.04 EC060200-1 -4 Jun-00 51 1.02E+06 51 Trace 178 3.8 16.5 2.5 2.2 74.5 0.6 0.18 

EC060200-4 3 Jun-00 4 7.56E+04 4 ND NC 0.0 37.0 0.0 0.0 63.0 0.0 0.00 

EC060200-4 2 Jun-00 5 9.52E+04 5 Trace 27 8.1 34.7 0.0 12.3 41.3 3.6 0.00 

EC060200-4 1 Jun-00 51 1.02E+06 51 Trace 138 4.5 15.1 0.4 3.7 75.6 0.7 0.00 EC060200-4 -1 Jun-00 570 1.14E+07 569 1 434 24.8 17.6 8.0 18.5 30.9 0.2 0.38 

EC060200-4 -2 Jun-00 203 4.07E+06 203 Trace 525 32.5 17.4 12.7 30.3 7.0 0.2 0.94 EC060200-4 -3 Jun-00 173 3.46E+06 173 Trace 666 27.7 14.4 13.6 38.9 5.2 0.2 0.85 EC060200-4 -4 Jun-00 41 8.24E+05 41 ND NC 22.0 20.4 12.8 28.9 15.9 0.0 0.51 EC060200-4 -5 Jun-00 13 2.67E+05 13 ND NC 12.4 35.8 4.8 7.9 39.1 0.0 0.27 

EC060200-4 -6 Jun-00 4 7.73E+04 4 ND NC 13.2 41.4 2.6 4.1 38.8 0.0 0.00 EC060200-4 -7 Jun-00 3 6.55E+04 3 ND NC 2.7 22.7 0.0 0.0 74.6 0.0 0.00 

EC060200-4 -8 Jun-00 42 8.46E+05 42 Trace 249 2.2 10.1 0.4 1.0 85.9 0.4 0.00 EC060200-4 -9 Jun-00 5 9.63E+04 5 ND NC 16.8 30.7 0.0 0.0 52.6 0.0 0.00 

136

Table B1. Phospholipid Fatty-acid (PLFA) Extract Data from Elizabeth City Cores 

Biomass Community Structure (% of total PLFA) 

Prokaryote Eukaryotic Prokaryote/ Gram+/ Gram- Anaerobic SRB/ Genera Eukaryotes Physiological 

Sample ID Section Date PLFA PLFA PLFA PLFA Eukaryote Anaerobic Gram- (Monos) Metal Reducers Actinomycetes Nsats (Polyenoics) Status 

pM/g cells/g pM/g pM/g (TerBrSats) (BrMonos) (MidBrSats) 

EC060300-5 8 Jun-00 4 8.51E+04 4 Trace 14 4.1 21.8 0.0 0.0 67.4 6.8 0.00 

EC060300-5 7 Jun-00 12 2.33E+05 11 Trace 43 8.6 18.1 0.0 0.0 71.0 2.3 0.00 EC060300-5 6 Jun-00 2 4.18E+04 2 Trace 10 0.0 21.3 0.0 0.0 69.2 9.5 0.00 

EC060300-5 5 Jun-00 12 2.40E+05 12 Trace 38 4.2 26.1 0.0 0.8 66.3 2.6 0.00 EC060300-5 4 Jun-00 11 2.12E+05 10 1 20 2.6 12.1 0.0 2.6 78.0 4.7 0.00 

EC060300-5 3 Jun-00 9 1.80E+05 9 Trace 34 9.8 11.1 0.0 2.7 73.6 2.9 0.00 

EC060300-5 2 Jun-00 6 1.20E+05 6 Trace 24 16.7 20.9 0.0 7.2 51.2 4.1 0.00 EC060300-5 1 Jun-00 57 1.13E+06 56 1 59 11.3 16.0 0.4 3.5 67.2 1.7 0.13 EC060300-5 -1 Jun-00 14 2.79E+05 14 Trace 146 3.1 13.7 0.0 0.4 82.1 0.7 0.00 EC060300-5 -2 Jun-00 7 1.42E+05 7 ND NC 0.0 23.5 8.2 0.0 68.4 0.0 0.00 

EC060300-5 -3 Jun-00 21 4.25E+05 21 ND NC 0.6 12.5 0.0 0.0 86.9 0.0 0.00 

EC060300-6 9 Jun-00 22 4.39E+05 22 Trace 56 2.8 25.7 0.0 0.0 58.6 1.8 0.20 

EC060300-6 8 Jun-00 41 8.13E+05 40 1 64 2.7 17.2 2.1 9.1 74.2 1.6 0.00 

EC060300-6 7 Jun-00 1 2.98E+04 1 ND NC 0.0 0.0 1.3 3.0 100.0 0.0 0.00 

EC060300-6 6 Jun-00 0 2.51E+03 0 ND NC 0.0 0.0 0.0 0.0 100.0 0.0 0.00 EC060300-6 5 Jun-00 0 6.92E+03 0 ND NC 0.0 0.0 0.0 0.0 100.0 0.0 0.00 EC060300-6 4 Jun-00 1 1.88E+04 1 ND NC 0.0 0.0 0.0 0.0 100.0 0.0 0.00 

EC060300-6 3 Jun-00 6 1.14E+05 6 ND NC 0.0 17.2 2.8 0.0 80.1 0.0 0.00 

EC060300-6 2 Jun-00 54 1.07E+06 54 ND NC 12.0 63.0 5.2 4.2 15.6 0.0 0.00 EC060300-6 1 Jun-00 5 1.09E+05 5 ND NC 5.2 25.0 0.0 0.0 69.8 0.0 0.00 

EC060300-6 -1 Jun-00 15 3.06E+05 15 Trace 75 0.0 14.1 0.6 0.0 84.6 1.3 0.00 EC060300-6 -2 Jun-00 25 4.91E+05 24 Trace 121 1.7 32.7 0.0 0.0 64.2 0.8 0.10 EC060300-6 -3 Jun-00 9 1.82E+05 9 Trace 50 0.0 33.1 0.0 0.0 64.9 2.0 0.19 

EC060300-6 -4 Jun-00 6 1.23E+05 6 ND NC 0.0 17.5 0.0 0.0 82.5 0.0 0.00 

EC060300-6 -5 Jun-00 92 1.85E+06 92 1 131 2.1 13.3 1.5 1.5 80.9 0.8 0.26 

EC90902 8 Sep-99 5 9.19E+04 5 ND NC 0.0 50.9 0.0 0.0 49.1 0.0 0.00 

EC90902 7 Sep-99 1 2.99E+04 1 ND NC 0.0 50.1 0.0 0.0 49.9 0.0 0.00 EC90902 6 Sep-99 1 1.93E+04 1 ND NC 0.0 33.6 0.0 0.0 66.4 0.0 NC 

EC90902 5 Sep-99 2 4.19E+04 2 ND NC 0.0 46.6 0.0 0.0 53.4 0.0 0.00 

EC90902 4 Sep-99 1 1.88E+04 1 ND NC 0.0 30.4 0.0 0.0 69.6 0.0 NC 

EC90902 3 Sep-99 1 1.77E+04 1 ND NC 0.0 39.7 0.0 0.0 60.3 0.0 NC EC90902 2 Sep-99 3 5.79E+04 3 ND NC 0.0 38.9 0.0 0.0 61.2 0.0 0.00 EC90902 1 Sep-99 4 7.77E+04 4 ND NC 0.0 47.7 0.0 0.0 52.3 0.0 0.00 EC90903 2 Sep-99 51 1.02E+06 559 ND NC 13.9 38.7 6.3 17.1 24.1 0.0 0.39 

EC90903 1 Sep-99 219 4.38E+06 1541 ND NC 15.6 40.0 2.2 8.2 34.2 0.0 0.13 EC90903 -1 Sep-99 2610 5.22E+07 2610 ND NC 40.1 23.5 6.8 17.3 12.4 0.0 NC 

EC90903 -2 Sep-99 114 2.28E+06 114 ND NC 22.6 38.4 3.3 6.3 29.5 0.0 0.30 EC90903 -3 Sep-99 53 1.06E+06 53 ND NC 10.1 63.3 2.8 1.2 22.7 0.0 0.38 EC90903 -4 Sep-99 22 4.40E+05 22 ND NC 21.1 50.9 2.4 0.0 25.6 0.0 0.00 EC90903 -5 Sep-99 11 2.26E+05 11 ND NC 0.0 66.9 0.0 0.0 33.1 0.0 0.00 

EC90903 -6 Sep-99 15 3.03E+05 15 ND NC 13.6 60.0 1.2 0.0 25.3 0.0 0.00 

EC01061899 -1 Jun-99 128 2.55E+06 125 3 47 15.8 30.6 12.3 8.1 31.1 2.1 0.07 

EC01061899 -2 Jun-99 39 7.82E+05 38 1 34 4.5 10.3 3.1 2.3 75.8 2.8 0.10 

EC01061899 -3 Jun-99 38 7.54E+05 33 3 12 2.2 5.5 1.3 1.1 77.6 7.5 0.00 

EC03061699 -1 Jun-99 474 9.48E+06 463 11 41 20.2 36.3 11.7 11.6 17.9 2.4 0.07 

EC03061699 -2 Jun-99 1171 2.34E+07 1159 11 101 25.5 32.4 14.2 14.1 12.8 1.0 0.07 EC03061699 -4 Jun-99 130 2.60E+06 61 64 1 1.7 14.0 1.0 1.0 29.5 49.6 0.00 

EC03061699 -5 Jun-99 69 1.38E+06 30 37 1 2.1 10.0 1.2 1.4 28.4 53.7 0.00 

137

Physiological 

Status 

NC 

NC NC 

NC 

NC 

NC 

NC NC 

Table B1. Phospholipid Fatty-acid (PLFA) Extract Data from Elizabeth City Cores 

Biomass Community Structure (% of total PLFA) 

Prokaryote Eukaryotic Prokaryote/ Gram+/ Gram- Anaerobic SRB/ Genera Eukaryotes 

Sample ID Section Date PLFA PLFA PLFA PLFA Eukaryote Anaerobic Gram- (Monos) Metal Reducers Actinomycetes Nsats (Polyenoics) 

pM/g cells/g pM/g pM/g (TerBrSats) (BrMonos) (MidBrSats) 

EC6102 4 Jun-99 1 1.99E+04 1 ND NC 0.0 31.0 1.0 0.0 69.0 0.0 EC6102 3 Jun-99 3 5.21E+04 3 ND NC 0.0 45.9 1.0 0.0 54.1 0.0 EC6102 2 Jun-99 1 1.32E+04 1 ND NC 0.0 57.0 1.0 0.0 43.0 0.0 EC6102 1 Jun-99 2 3.06E+04 2 ND NC 0.0 0.0 0.0 0.0 100.0 0.0 

EC6101 -1 Jun-99 795 1.59E+07 795 ND NC 15.7 39.8 7.0 7.9 29.7 0.0 

EC6101 -2 Jun-99 85 1.70E+06 85 ND NC 9.9 59.5 4.8 6.5 19.2 0.0 

EC6101 -3 Jun-99 14 2.72E+05 14 ND NC 7.1 77.2 0.0 0.0 15.7 0.0 EC6101 -4 Jun-99 6 1.26E+05 6 ND NC 0.0 66.9 0.0 0.0 33.1 0.0 

EC6101 -5 Jun-99 5 1.05E+05 5 ND NC 0.0 65.9 0.0 0.0 34.1 0.0 EC6101 -6 Jun-99 2 3.59E+04 2 ND NC 0.0 29.3 0.0 0.0 70.7 0.0 EC6101 -7 Jun-99 1 1.25E+05 1 ND NC 0.0 0.0 0.0 0.0 100.0 0.0 EC6101 -8 Jun-99 3 6.90E+04 3 ND NC 0.0 37.5 0.0 0.0 62.5 0.0 EC6101 -9 Jun-99 1 2.03E+04 1 ND NC 0.0 18.1 0.0 0.0 81.9 0.0 EC6101 -10 Jun-99 96 1.92E+06 70 26 3 1.4 51.9 0.0 0.1 19.4 27.3 

EC6114 4 Jun-99 412 8.24E+06 412 ND NC 27.5 37.9 7.6 12.1 14.9 0.0 EC6114 3 Jun-99 39 7.84E+05 39 ND NC 11.5 56.1 4.2 5.4 22.8 0.0 EC6114 2 Jun-99 8 1.62E+05 8 ND NC 21.1 54.4 0.0 0.0 24.4 0.0 

EC6114 1 Jun-99 6 1.17E+05 6 ND NC 0.0 58.1 0.0 0.0 41.9 0.0 

EC6114 -1 Jun-99 21 4.21E+05 21 ND NC 3.3 58.5 0.0 2.2 36.0 0.0 

EC6114 -2 Jun-99 8 1.59E+05 8 ND NC 1.6 31.6 0.0 7.4 59.5 0.0 

EC6114 -3 Jun-99 2 3.27E+04 2 ND NC 0.0 0.0 0.0 0.0 100.0 0.0 

EC6114 -4 Jun-99 3 6.11E+04 3 ND NC 14.4 18.6 0.0 0.0 66.9 0.0 

The number of cells is calculated based on 2.0x1012 cells per gram dry weight of cells and 108 picolmoles of phospholipid per gram dry weight of cells. Microbial Insights (G. Davis, pers. comm.) 


points out that the number of cells/gram of dry weight may vary and is dependent on the environmental conditions from which the microorganisms were recovered. Physiological status is based on ratios
of fatty acid biomarkers that indicate a metabolic response to environmentally induced stress evidenced by decreased membrane permeability (w7/w7c). The biomarker indicators generally suggest that the samples 


have slow to moderate rates of turnover. Sample locations are shown in Figure 3.19. ND, not detected; NC not calculated.


138


0.17 

0.14 

0.00 

0.00 

0.00 

0.00 0.00 

0.30 

0.10 

0.00 

0.00 

0.00 

0.20 

0.00 
Table B2. Phospholipid Fatty-acid (PLFA) Extract Data from Denver Federal Center Cores 

Biomass Community Structure (% of total PLFA) 

Prokaryote Eukaryotic Prokaryote/ Gram+/ Gram- Anaerobic SRB/ Genera Eukaryotes Physiological 

Sample ID Section Date PLFA PLFA PLFA PLFA Eukaryote Anaerobic Gram- (Monos) Metal Reducers Actinomycetes Nsats (Polyenoics) Status 

pM/g cells/g pM/g pM/g (TerBrSats) (BrMonos) (MidBrSats) 

C1-1-71701 -1 Jul-02 1533 3.07E+07 1499 34 44 9.3 51.6 9.8 6.2 20.9 2.2 0.26 C1-1-71701 -2 Jul-02 979 1.96E+07 954 25 38 8.1 55.7 7.9 4.8 21.0 2.6 0.26 C1-1-71701 -3 Jul-02 775 1.55E+07 758 17 44 10.0 51.5 8.0 4.8 23.4 2.2 0.24 C1-1-71701 -4 Jul-02 858 1.72E+07 838 20 42 11.4 47.1 10.9 6.0 22.2 2.3 0.21 C1-1-71701 -6 Jul-02 222 4.43E+06 213 9 24 9.1 53.5 10.7 5.9 16.8 4.1 0.17 

C1-1-71701 -8 Jul-02 127 2.55E+06 123 4 28 6.5 60.0 8.1 4.7 17.3 3.5 0.17 

C1-1-71701 -12 Jul-02 27 5.42E+05 26 1 28 4.0 72.2 4.3 0.0 16.0 3.5 0.00 C1-1-71701 -18 Jul-02 22 4.33E+05 20 2 10 2.5 71.0 0.0 0.0 17.1 9.4 0.00 

C2-1-71801 2 Jul-02 68 1.36E+06 66 1 46 8.7 51.9 10.3 8.5 18.5 2.1 0.16 C2-1-71801 -1 Jul-02 1439 2.88E+07 1410 29 50 17.1 41.4 12.9 10.9 15.8 2.0 0.42 

C2-1-71801 -2 Jul-02 1091 2.18E+07 1066 25 42 18.3 43.2 12.2 9.0 15.1 2.3 0.32 C2-1-71801 -3 Jul-02 1100 2.20E+07 1084 17 67 20.5 42.2 12.4 8.3 15.0 1.5 0.32 C2-1-71801 -4 Jul-02 1561 3.12E+07 1527 34 45 17.8 47.8 10.4 6.0 15.8 2.2 0.27 C2-1-71801 -5 Jul-02 1517 3.03E+07 1477 40 37 17.6 46.9 11.3 6.4 15.2 2.6 0.28 C2-1-71801 -7 Jul-02 1556 3.11E+07 1533 24 64 13.8 55.3 8.9 5.0 15.6 1.5 0.26 

C2-1-71801 -11 Jul-02 984 1.97E+07 970 14 69 14.2 55.8 9.6 5.2 13.8 1.4 0.15 

C2-1-71801 -15 Jul-02 1372 2.74E+07 1348 24 57 14.1 54.1 7.8 4.3 18.0 1.7 0.24 

C2-1-71801 -19 Jul-02 332 6.63E+06 324 8 43 11.9 54.9 8.4 10.6 11.8 2.3 0.13 

C2-3-71801 -1 Jul-02 1869 3.74E+07 1853 16 119 26.4 31.6 12.2 9.4 19.7 0.8 0.25 

C2-3-71801 -2 Jul-02 773 1.55E+07 767 6 124 22.1 39.4 11.9 7.9 17.9 0.8 0.23 C2-3-71801 -3 Jul-02 533 1.07E+07 526 6 82 20.3 44.3 10.3 6.5 17.4 1.2 0.18 C2-3-71801 -4 Jul-02 463 9.25E+06 455 8 60 17.2 51.5 8.6 5.6 15.4 1.6 0.19 C2-3-71801 -6 Jul-02 125 2.51E+06 121 4 27 8.5 72.2 3.0 2.0 10.9 3.5 0.05 C2-3-71801 -9 Jul-02 111 2.22E+06 105 6 18 5.5 56.3 1.8 21.2 10.1 5.1 0.06 C2-3-71801 -12 Jul-02 58 1.17E+06 58 1 68 3.4 85.3 0.0 0.0 9.9 1.5 0.04 

C2-3-71801 -15 Jul-02 61 1.23E+06 59 3 21 4.0 78.2 1.1 0.0 12.2 4.6 0.05 C2-3-71801 -17 Jul-02 114 2.29E+06 112 2 47 6.8 81.2 1.0 0.0 8.9 2.1 0.04 

C2-3-71801 -19 Jul-02 50 1.00E+06 49 1 52 7.2 79.1 1.3 0.0 10.6 1.9 0.06 

C2-1-71901 -2 Jul-02 363 7.26E+06 345 17 20 9.3 62.7 9.4 4.4 9.4 4.8 0.06 

C2-1-71901 -4 Jul-02 1326 2.65E+07 1303 23 56 20.3 42.4 15.2 6.2 14.3 1.7 0.34 

C2-1-71901 -6 Jul-02 1075 2.15E+07 1053 22 48 15.3 43.9 14.8 9.1 14.9 2.1 0.13 

C2-1-71901 -10 Jul-02 1909 3.82E+07 1883 25 74 19.4 36.1 16.5 10.0 16.7 1.3 0.17 

C2-1-71901 -12 Jul-02 2874 5.75E+07 2835 38 74 20.2 41.2 15.3 4.6 17.4 1.3 0.46 

C2-1-71901 -14 Jul-02 1527 3.05E+07 1497 30 49 15.5 44.1 15.4 8.9 14.1 2.0 0.21 

C2-2-71901 -2 Jul-02 42 8.50E+05 39 3 13 4.0 77.7 1.0 0.0 10.2 7.2 0.00 C2-2-71901 -6 Jul-02 24 4.73E+05 19 4 4 3.1 61.7 3.6 0.0 13.2 18.4 0.00 

C2-2-71901 -9 Jul-02 60 1.20E+06 55 5 12 3.6 76.1 0.0 3.7 8.6 8.0 0.05 

C2-2-71901 -12 Jul-02 42 8.38E+05 39 3 14 4.3 74.9 1.1 0.0 12.9 6.9 0.06 C2-2-71901 -14 Jul-02 86 1.72E+06 85 1 59 14.0 72.5 1.4 0.0 10.5 1.7 0.16 C2-3-71901 -1 Jul-02 208 4.16E+06 191 17 11 17.8 49.6 2.6 2.2 19.6 8.2 0.12 C2-3-71901 -2 Jul-02 100 2.00E+06 94 6 16 10.1 67.0 3.7 2.6 10.8 5.8 0.08 C2-3-71901 -6 Jul-02 337 6.74E+06 331 6 57 17.8 32.3 9.6 8.2 30.6 1.7 0.17 C2-3-71901 -9 Jul-02 656 1.31E+07 617 38 16 17.8 38.5 9.9 14.3 13.8 5.8 0.15 

C3-2-71801 -5 Jul-02 234 4.68E+06 220 14 16 13.8 52.4 11.7 3.4 12.9 5.8 0.32 C3-2-71801 -10 Jul-02 655 1.31E+07 638 17 37 18.4 47.0 12.8 5.6 13.6 2.6 0.44 C3-2-71801 -12 Jul-02 977 1.95E+07 965 12 82 21.4 34.0 15.8 8.9 18.7 1.2 0.56 

C3-2-71801 -14 Jul-02 1439 2.88E+07 1431 8 178 22.5 30.6 15.9 7.4 23.0 0.6 0.50 C3-2-71801 -16 Jul-02 1301 2.60E+07 1294 7 195 22.3 30.9 14.5 5.3 26.6 0.5 0.36 

139

Physiological 

Status 

0.59 

NC 

Table B2. Phospholipid Fatty-acid (PLFA) Extract Data from Denver Federal Center Cores 

Biomass Community Structure (% of total PLFA) 

Prokaryote Eukaryotic Prokaryote/ Gram+/ Gram- Anaerobic SRB/ Genera Eukaryotes 

Sample ID Section Date PLFA PLFA PLFA PLFA Eukaryote Anaerobic Gram- (Monos) Metal Reducers Actinomycetes Nsats (Polyenoics) 

pM/g cells/g pM/g pM/g (TerBrSats) (BrMonos) (MidBrSats) 

C1-2-71000 -1 Jul-00 1904 3.81E+07 1776 128 14 3.5 62.4 2.0 2.1 23.2 6.7 

C1-2-71000 -2 Jul-00 95 1.89E+06 93 1 69 3.4 68.9 11.1 3.4 12.0 1.4 

C1-2-71000 -3 Jul-00 693 1.39E+07 691 1 499 5.3 68.3 8.5 3.0 14.8 0.2 

C1-2-71000 -4 Jul-00 506 1.01E+07 505 1 624 2.7 77.3 5.6 1.9 12.3 0.2 

C1-2-71000 -5 Jul-00 103 2.07E+06 103 0.4 262 1.7 80.2 4.6 1.3 11.9 0.4 C1-2-71000 -8 Jul-00 15 3.03E+05 15 0.4 38 1.5 71.8 8.9 2.7 12.7 2.6 C1-2-71000 -13 Jul-00 22 4.48E+05 22 1 36 0.0 87.8 1.7 0.0 7.8 2.7 

C1-3-71100 3 Jul-00 124 2.49E+06 122 2 62 8.5 52.0 11.7 6.7 19.5 1.6 

C1-3-71100 1 Jul-00 306 6.13E+06 304 2 127 7.7 59.2 10.9 6.7 14.8 0.8 

C1-3-71100 -1 Jul-00 80 1.59E+06 79 1 82 2.1 82.8 3.3 1.8 8.7 1.2 C1-3-71100 -2 Jul-00 19 3.85E+05 19 1 30 0.8 84.3 1.9 1.2 8.6 3.2 C1-3-71100 -4 Jul-00 30 6.00E+05 29 1 28 0.0 86.2 0.6 0.0 9.7 3.5 

C2-13-71300 -1 Jul-00 2752 5.50E+07 2752 ND NC 11.2 52.9 13.5 7.0 15.4 0.0 

C2-13-71300 -3 Jul-00 4182 8.36E+07 4177 5 768 15.3 48.5 16.1 6.7 13.2 0.1 

C2-13-71300 -5 Jul-00 1827 3.65E+07 1821 7 262 11.2 57.0 11.9 5.3 14.2 0.4 

C2-13-71300 -7 Jul-00 1115 2.23E+07 1115 ND NC 16.8 45.0 14.3 7.0 16.9 0.0 

C2-13-71300 -9 Jul-00 2296 4.59E+07 2297 ND NC 10.7 54.7 10.5 4.8 19.4 0.0 

C2-14-71300 -1 Jul-00 601 1.20E+07 599 2 356 14.8 59.6 9.5 5.3 10.5 0.3 

C2-14-71300 -3 Jul-00 576 1.15E+07 574 2 269 17.0 66.2 4.8 1.6 10.1 0.4 

C2-14-71300 -5 Jul-00 283 5.66E+06 282 1 453 10.3 53.4 13.0 6.6 16.4 0.2 

C2-14-71300 -7 Jul-00 916 1.83E+07 913 3 344 11.2 57.8 10.5 5.2 15.1 0.3 

C2-17-71300 -1 Jul-00 3851 7.70E+07 3842 9 416 11.8 52.7 13.2 4.7 17.5 0.2 

C2-17-71300 -2 Jul-00 3760 7.52E+07 3752 8 454 10.2 53.4 13.3 5.3 17.7 0.2 

C2-17-71300 -3 Jul-00 4066 8.13E+07 4061 6 666 11.3 51.8 13.8 4.9 18.1 0.2 

C2-17-71300 -5 Jul-00 3916 7.83E+07 3905 10 384 11.3 51.2 14.3 4.7 18.3 0.3 

C2-17-71300 -7 Jul-00 1820 3.64E+07 1819 ND NC 9.7 54.3 13.1 4.9 18.1 0.0 

C1-2-52199 1 May-99 592 1.18E+07 559 33 17 12.1 24.3 3.5 26.9 23.0 5.5 

C1-3-52199 -1 May-99 2584 5.17E+07 2537 48 53 10.4 56.9 9.4 4.0 17.5 1.9 

C1-3-52199 -3 May-99 3554 7.11E+07 3476 78 44 13.4 50.9 11.0 4.7 17.9 2.2 

C1-3-52199 -4 May-99 3455 6.91E+07 3395 59 57 13.0 52.9 10.6 4.1 17.6 1.7 

C2-1-52299 4 May-99 1629 3.26E+07 1541 89 17 13.2 43.2 3.2 15.6 19.4 5.4 

C2-1-52299 -1 May-99 4523 9.05E+07 4485 38 118 6.0 62.1 4.8 1.8 24.4 0.8 

C2-1-52299 -2 May-99 4924 9.85E+07 4859 65 75 8.8 54.8 6.3 2.7 26.1 1.3 

The number of cells is calculated based on 2.0x1012 cells per gram dry weight of cells and 108 picolmoles of phospholipid per gram dry weight of cells. Microbial Insights (G. Davis, pers. comm.) 


points out that the number of cells/gram of dry weight may vary and is dependent on the environmental conditions from which the microorganisms were recovered. Physiological status is based on ratios
of fatty acid biomarkers that indicate a metabolic response to environmentally induced stress evidenced by decreased membrane permeability (w7/w7c). The biomarker indicators generally suggest that the samples 
have slow to moderate rates of turnover. Sample locations are shown in Figures 4.1 to 4.3. ND, not detected; NC not calculated.


140


0.08 

0.08 

0.02 

0.04 0.00 

0.00 0.00 

0.03 

0.12 0.00 

0.00 

0.34 

0.32 

0.32 

0.27 

0.22 

0.25 

0.08 

0.29 

0.34 

0.19 

0.22 

0.14 

0.17 0.16 

0.17 

0.10 

0.10 

0.10 

0.03 

0.10 
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