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Geochemistry, Fate, and Three-

Dimensional Transport Modeling ofSubsurface Cyanide Contamination at aManufactured Gas Plant

Technical Repor


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EPRI Project ManagerA.J. Coleman

EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA800.313.3774 650.855.2121 [email protected] www.epri.com

Geochemistry, Fate, andThree-Dimensional Transport

Modeling of Subsurface CyanideContamination at a ManufacturedGas Plant

1001301

Final Report, January 2001

Cosponsor

Alliant Energy222 W. Washington Avenue, 2nd FloorMadison, WI 53701


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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS ANACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCHINSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THEORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I)WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, ORSIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESSFOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON ORINTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUALPROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'SCIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER(INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVEHAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOURSELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD,PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

ThermoRetec Consulting Corporation

ORDERING INFORMATION

Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins

Drive, P.O. Box 23205, Pleasant Hill, CA 94523, (800) 313-3774.

Electric Power Research Institute and EPRI are registered service marks of the Electric PowerResearch Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric PowerResearch Institute, Inc.

Copyright 2001 Electric Power Research Institute, Inc. All rights reserved.


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CITATIONS

This report was prepared by

ThermoRetec Consulting CorporationOne Monroeville Center, Suite 1015

Monroeville, Pennsylvania 15146-2121

Principal Investigators

D. Nakles

R. GhoshF. DiGnazio

This report describes research sponsored by EPRI and Alliant Energy.

The report is a corporate document that should be cited in the literature in the following manner:

Geochemistry, Fate, and Three-Dimensional Transport Modeling of Subsurface Cyanide

Contamination at a Manufactured Gas Plant, EPRI, Palo Alto, CA and Alliant Energy,

Madison, WI: 2001. 1001301.


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REPORT SUMMARY

This report documents the geochemistry, fate, and three-dimensional transport modeling of

subsurface cyanide contamination at a manufactured gas plant.

BackgroundCyanide contamination of groundwater has been observed at many former manufactured gasplant (MGP) sites, aluminum production facilities, former or active electroplating facilities, ore

heap leaching sites, and other types of industrial sites. Cyanide is present in several forms,

including free cyanide (HCN, or CN-) and metal-cyanide complexes involving cadmium, cobalt,copper, iron, nickel, and others. Cyanide toxicity to humans and aquatic life is mainly associatedwith free cyanide; the metal-cyanide complexes, especially the strong complexes with cobalt and

iron, are essentially non-toxic. However, metal-cyanide complexes can dissociate under certain

conditions (for example, low pH and in sunlight) to yield toxic free cyanide. Thus, all forms of

cyanide in groundwater may be of concern at a given site depending on site-specific conditions.

The fate and transport of cyanide in subsurface environments has received little attention in the

literature. Available data on chemical speciation of cyanide indicates that iron-cyanide

complexes are predominant in contaminated groundwater at MGP sites. These species are stablein dark, neutral- to high-pH environments, and are highly resistant to biodegradation. At neutral

pH, iron-cyanide species also exhibit little adsorption onto iron oxides and possibly greater

adsorption to aluminum oxides, both of which can be important adsorbents in groundwatersystems. This information suggests that cyanide may be mobile and persistent in groundwater

systems, but this hypothesis has not been tested against field data. The fate and transport of

cyanide compounds in groundwater must be understood to assess compliance with applicable

environmental regulations, to conduct risk assessments, and to design control and remediation

measures.

ObjectivesTo obtain an understanding of the chemistry and movement of a groundwater plume containingcyanide at an MGP site in Portage, Wisconsin; to interpret the observed plume data with a

numerical fate and transport model to investigate behavior of dissolved cyanide compounds in

the subsurface; to assess the potential for natural attenuation of the cyanide plume; and, topredict future movement of the cyanide plume under existing site conditions.

ApproachInvestigation of the cyanide groundwater plume at the Portage, Wisconsin, MGP site consisted

of field studies, laboratory studies, and modeling. Field studies characterized cyanide speciationand transport in groundwater at the site and assessed the potential for natural attenuation of

dissolved cyanide species. The project team conducted these field studies over a four-year period


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using a network of 41 monitoring wells. An additional cyanide source investigation at the site

evaluated potential continuing source areas that were identified from groundwater monitoringdata. The team designed laboratory studies to improve understanding of cyanide geochemistry, in

particular, interactions of cyanide species with aquifer material from the field site and the

potential for iron-cyanide complexes to dissociate to form free cyanide. Field study results were

interpreted using a two- and three-dimensional groundwater flow and non-reactive contaminanttransport model that incorporated findings from the laboratory studies.

ResultsSampling and analysis of site groundwater and aquifer material, including a detailed

hydrogeologic analysis, permitted delineation of the cyanide plume at the site and subsequent

characterization of the aquifer in terms of geochemical and hydraulic properties. Groundwaterspeciation studies indicated that iron cyanide complexes were dominant, accounting for greater

than 92 to 98% of the total cyanide. No native oxide box materials were discovered during the

additional source investigation at the site, confirming that the majority of oxide box materials

were removed during the remedial excavations in 1992 and 1993. Only isolated cyanide stringers

(1 to 3 inches thick) were detected on the native sand and gravel surfaces in the vadose andsaturated zones of the two areas under investigation. These stringers are likely present as a result

of the dissolution and re-precipitation of cyanide solids under excess iron conditions that exist inthese areas. These stringers represent a disperse source of iron-cyanide complexes that provide aminor contribution to the cyanide concentration in groundwater at the site. The magnitude of this

contribution is a function of the water table elevation and the pH and pE conditions of the

subsurface environment. Laboratory column tests indicate that dissolved iron cyanide complexesdo not adsorb onto the sand/gravel aquifer material in a neutral pH environment, a condition that

exists at many MGP sites. The conservative nature of the iron cyanide species in a sand/gravel

medium under neutral pH conditions was further established by interpreting the field data with anon-reactive solute transport model. The observations suggest that dilution might be the only

natural attenuation mechanism for iron cyanide complexes in sand/gravel aquifers at MGP sites.

EPRI PerspectiveThis research shows the fate and transport of cyanide compounds in sand and gravel aquifers.

The research also sheds some light on the natural attenuation of such aquifer systems. This study

provides information that illustrates and demonstrates how the stability and nonreactivity of iron

cyanide complexes in sand and gravel aquifers under different pH conditions may occur.

KeywordsMGP

Cyanide

Oxide boxGroundwater

Natural attenuation


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ABSTRACT

This report presents the results of field, laboratory, and modeling studies that were conducted tobetter understand the geochemistry and transport of a cyanide groundwater plume in asand/gravel aquifer underlying a manufactured gas plant (MGP) site. The field characterizationand continuous monitoring of the plume were conducted to delineate the cyanide movement inthe subsurface and to assess the potential for natural attenuation of dissolved cyanide species ingroundwater. An additional source investigation was performed at two areas at the site that wereidentified, based on groundwater monitoring data, as potential continuing cyanide source areas.Laboratory studies were designed to improve the understanding of cyanide geochemistry, and in

particular, to provide an understanding of cyanide speciation and chemistry in groundwater at thesite. Information obtained from the field and laboratory studies was incorporated into a two- anda three-dimensional groundwater flow and non-reactive contaminant transport model.

Cyanide was found to exist mostly as iron cyanide complexes in the groundwater at theMGP site, with less than 8% as weak acid dissociable cyanide. Free cyanide constituted almost15 to 25% of the weak-acid dissociable cyanides in the site groundwater, accounting for0.3 to 2.0% of the total cyanide. Laboratory column studies indicated that all of the cyanidespecies, including iron cyanide complexes, moved conservatively through the sand/gravel aquifermaterial from the site. Laboratory studies also indicated that iron cyanide complexes were stablein the groundwater under the neutral pH conditions, dissociating very slowly to form free

cyanide.

The additional source investigation conducted in the two potential continuing source areasrevealed no native oxide box residuals. However, the investigation did detect scattered stringers(1-3 inches thick) of precipitated blue-colored iron cyanide solids in both areas. Prior to theremoval in 1992 and 1993, the cyanide source was leaching or dissolving cyanide whichproceeded to re-precipitate at depth. This re-precipitation phenomena occurred due to excess ironconditions in the vicinity of this source material and from slight fluctuations in the pH and pE.This field observation is consistent with laboratory observations made in the CMU studyregarding the precipitation/dissolution behavior of iron cyanide solids as a function of pH and pEunder excess iron conditions.

The movement of the MGP site cyanide plume was monitored over a period of four years.The observed plume concentration data could be predicted using a two-dimensional and athree-dimensional groundwater flow and non-reactive solute transport model. The modelingresults were consistent with the nonreactive transport that was observed in the laboratory columntests using the site groundwater (which was impacted predominantly by iron cyanide complexes)and site aquifer materials. Based on the modeling and laboratory results, it appears that followingsource removal, dilution may often be the only natural attenuation mechanism for iron cyanidecomplexes in a sand/gravel aquifer under neutral pH conditions at MGP sites. It is also true that


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the continued dissolution of iron-cyanide complexes from the re-precipitated stringers ofiron-cyanide solids identified at the site is not resulting in offsite groundwater impacts thatexceed the groundwater or drinking water standards of the State environmental regulatoryagency or the U.S. EPA.


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ACKNOWLEDGMENTS

We thank Dr. Ishwar Murarka of Ish Inc. (formerly of the Electric Power Research Institute[EPRI]), Ms. Adda Quinn of EPRI, and Mr. Joseph Shefchek and Mr. Bruce Greer ofAlliant Energy Corporation for their support and thoughtful suggestions during the course of thisresearch. We also acknowledge valuable assistance from Ms. Carol McKee, Mr. Anthony Como,Mr. Jonathan Murer, Ms. Michelle McGovern, Mr. Andrew Kirkman, Mr. Christopher Ahrendt,and Dr. Alessandro Battaglia of ThermoRetec and from Dr. David Dzombak,Mr. Brian Blashich, Mr. Joseph Robinson, and Mr. Anping Zheng of Carnegie MellonUniversity. Finally, we would like to thank Dr. Carol McCartney, Dr. Galen Kenoyer, and

Mr. John Rice of RMT, Inc., for their thorough review of the groundwater flow and contaminanttransport models developed in this study.


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CONTENTS

1INTRODUCTION.................................................................................................................. 1-1

1.1 Background .................................................................................................................. 1-1

1.2 Organization of the Report............................................................................................ 1-2

2FIELD, LABORATORY AND MODELING STUDIES000000000 ......................................... 2-1

2.1 Methods........................................................................................................................ 2-1

2.1.1 Field Studies ......................................................................................................... 2-1

2.1.1.1 Monitoring Well Network................................................................................ 2-1

2.1.1.2 Site Hydrogeological Conditions .................................................................... 2-3

2.1.1.3 Plume Monitoring........................................................................................... 2-4

2.1.1.4 Additional Source Investigation...................................................................... 2-4

2.1.2 Laboratory Studies................................................................................................ 2-5

2.1.2.1 Column Studies ............................................................................................. 2-5

2.1.2.2 Batch Studies ................................................................................................ 2-6

2.1.3 Modeling Studies................................................................................................... 2-6

2.1.3.1 Two-Dimensional Groundwater Flow and Contaminant Transport Model....... 2-6

2.1.3.2 Three-Dimensional Flow and Transport Model .............................................. 2-7

Groundwater Flow Model (MODFLOW) ................................................................ 2-7

Particle Tracking Transport Model (MT3D) ......................................................... 2-11

2.2 Results and Discussion............................................................................................... 2-12

2.2.1 Field Studies ....................................................................................................... 2-12

2.2.2 Laboratory Studies ..............................................................................................2-19

2.2.2.1 Column Tests .............................................................................................. 2-19

2.2.2.2 Dissociation Studies .................................................................................... 2-21

2.2.3 Modeling Studies................................................................................................. 2-22

2.3 Summary of Observations .......................................................................................... 2-32

3CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK ................................ 3-1


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3.1 Major Findings of this Research.................................................................................... 3-1

3.2 Engineering and Field Scale Implications...................................................................... 3-2

3.3 Recommendations for Future Work .............................................................................. 3-2

4REFERENCES..................................................................................................................... 4-1

AAPPENDIX A GEOPROBE SURVEY FOR PLUME DELINEATION ................................A-1

BAPPENDIX B MONITORING WELL LOCATIONS AND DEPTHS...................................B-1

CAPPENDIX C GROUNDWATER HEAD DATA ................................................................C-1

DAPPENDIX D HYDRAULIC CONDUCTIVITY ESTIMATES FROM SLUG TESTS...........D-1

EAPPENDIX E COMPILATION OF CYANIDE MONITORING DATA OBTAINEDFROM THE ANALYSIS OF GROUNDWATER SAMPLES AT THE PORTAGE,WISCONSIN MGP SITE..........................................................................................................E-1

FAPPENDIX F COMPILATION OF METALS AND MAJOR ION DATA OBTAINEDFROM THE ANALYSIS OF GROUNDWATER SAMPLES AT THE PORTAGE,WISCONSIN MGP SITE.......................................................................................................... F-1

GAPPENDIX G COMPILATION OF PH, PE, CONDUCTIVITY DISSOLVED OXYGEN(DO) AND TEMPERATURE DATA OF GROUNDWATER SAMPLES AT THEPORTAGE, WISCONSIN MGP SITE ..................................................................................... G-1

HAPPENDIX H METHODS EMPLOYED FOR THE CENTER OF MASSCALCULATIONS....................................................................................................................H-1

References .........................................................................................................................H-2

IAPPENDIX I FATE AND TRANSPORT MODEL FOR CYANIDE INGROUNDWATER .................................................................................................................... I-1

Flow Model .......................................................................................................................... I-1

MT3D Transport Model........................................................................................................ I-6

References.......................................................................................................................... I-6

JAPPENDIX J PUMPING DATA FROM THE MUNICIPAL WELL...................................... J-1

KAPPENDIX K DISSOCIATION OF COMPLEX IRON CYANIDE SPECIES IN THEDARK......................................................................................................................................K-1


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LIST OF FIGURES

Figure 2-1 Geoprobe and Monitoring Well Locations at Portage, Wisconsin MGP Site ........... 2-2

Figure 2-2 Schematic Vertical Cross Section of the MGP Site (1 ft = 0.3048 m)......................2-3

Figure 2-3 Approximate Boring Locations for Additional Source Characterization at thePortage MGP Site........................................................................................................... 2-5

Figure 2-4 Model Boundary Conditions Under Pumping and Non-Pumping Conditions(1 ft = 0.3048 m) .............................................................................................................. 2-8

Figure 2-5 Model Finite Difference Grid with the Location of the Municipal Well

(1 ft = 0.3048 m) ............................................................................................................ 2-10Figure 2-6 Steady State Flow Model Calibration with Observed Heads Versus Simulated

Heads (1 ft = 0.3048 m) ................................................................................................ 2-10

Figure 2-7 Schematic Showing the Plan and Cross-Sectional Area of the ContaminationScenario Used for Three-Dimensional Modeling Purposes (1 ft = 0.3048 m)................. 2-11

Figure 2-8 Water Table Elevation Contours at Portage, Wisconsin MGP Site(from average groundwater elevations measured between July 1996 andNovember 1999) [1 ft = 0.3048 m] ................................................................................. 2-12

Figure 2-9 Total Cyanide Plume in Groundwater at Portage, WI MGP Site,June/July 1996 (1 ft = 0.3048 m) .................................................................................. 2-14

Figure 2-10 Total Cyanide Plume in Groundwater at Portage, WI MGP Site,

November 1999 (1 ft = 0.3048 m) ................................................................................. 2-14

Figure 2-11 Vertical Profile of Total Cyanide Concentration Along the Centerline of thePlume at Well Nests near the Portage, Wisconsin MGP Site, November 1999(1 ft = 0.3048 m) ............................................................................................................ 2-16

Figure 2-12 Vertical Profile of Total Cyanide Concentration along the Centerline of thePlume at Well Nests near the Municipal Well, Downgradient of the Site(1 ft = 0.3048 m) ............................................................................................................ 2-16

Figure 2-13 WAD Cyanide Plume in Groundwater at Portage, Wisconsin MGP Site,June/July 1996 (1 ft = 0.3048 m) .................................................................................. 2-17

Figure 2-14 WAD Cyanide Plume in Groundwater at Portage, Wisconsin MGP Site,November 1999 (1 ft = 0.3048 m) ................................................................................. 2-18

Figure 2-15 Total Cyanide Breakthrough in Column Study with MGP Site Groundwaterand Aquifer Material....................................................................................................... 2-19

Figure 2-16 Breakthrough in Column Study with Nickel Cyanide and Clean Ottawa Sand..... 2-20

Figure 2-17 Breakthrough in Column with a Nickel Cyanide and Iron Cyanide Mixtureand Clean Ottawa Sand................................................................................................. 2-21


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Figure 2-18 Profile of Complex and Free Cyanide Concentrations During BatchDissociation Experiments with Ferrocyanide Solutions in Dark. Initial ferrocyanideconcentration = 15.03 mg/l ............................................................................................ 2-22

Figure 2-19 Total Cyanide Isoconcentration Map in the Top Saturated Layer(K = 13.87 ft/day) for Simulation of 49.6 Years of Transport. (40 years pre-pumpingconditions plus pumping from 1986 to July 1996) [1 ft = 0.3048 m]................................ 2-23

Figure 2-20 Total Cyanide Isoconcentration Map in the Top Saturated Layer(K = 13.87 ft/day) for Simulation of 53 Years of Transport. (40 years pre-pumpingconditions plus pumping from 1986 to November 1999) [1 ft = 0.3048 m]...................... 2-23

Figure 2-21 Total Cyanide Isoconcentration Map in the Top Saturated Layer(K = 13.87 ft/day) for Simulation of 40 Years of Transport (Pre-Pumping Conditions)[1 ft = 0.3048 m]............................................................................................................. 2-24

Figure 2-22 Total Cyanide Isoconcentration Map in the Second Saturated Layer(K = 106.67 ft/day) for Simulation of 40 Years of Transport (Pre-PumpingConditions) [1 ft = 0.3048 m].......................................................................................... 2-25

Figure 2-23 Total Cyanide Isoconcentration Map in the Second Saturated Layer

(K = 106.67 ft/day) for Simulation of 49.6 Years of Transport (40 years pre-pumpingconditions plus pumping from 1986 to July 1996) [1 ft = 0.3048 m]................................ 2-25

Figure 2-24 Total Cyanide Isoconcentration Map in the Second Saturated Layer(K = 106.67 ft/day) for Simulation of 53 Years of Transport (40 years pre-pumpingconditions plus pumping from 1986 to November 1999) [1 ft = 0.3048 m]...................... 2-26

Figure 2-25 Total Cyanide Isoconcentration Map in the Third Saturated Layer(K = 98.76 ft/day) for Simulation of 40 Years of Transport. (Pre-PumpingConditions) [1 ft = 0.3048 m].......................................................................................... 2-27

Figure 2-26 Total Cyanide Isoconcentration Map in the Third Saturated Layer(K = 98.76 ft/day) for Simulation of 49.6 Years of Transport (40 years pre-pumpingconditions plus pumping from 1986 to July 1996) [1 ft = 0.3048 m]................................ 2-27

Figure 2-27 Total Cyanide Isoconcentration Map in the Third Saturated Layer(K = 98.76 ft/day) for Simulation of 53 Years of Transport (40 years pre-pumpingconditions plus pumping from 1986 to November 1999) [1 ft = 0.3048 m]...................... 2-28

Figure 2-28 Three-Dimensional Representation of the 0.01 PPM Total Cyanide PlumeIso-Surface.................................................................................................................... 2-29

Figure 2-29 Two-Dimensional Vertical Representation of the 0.01 PPM Total CyanidePlume Iso-Surface........................................................................................................ 2-29

Figure 2-30 Model Prediction for Total Cyanide Concentration at the Municipal Well ............ 2-32

Figure A-1 Geoprobe and Monitoring Well Locations at Portage, Wisconsin MGP Site ...........A-2

Figure A-2 Main features of the Geoprobe screen point sampler (obtained fromTechnical Bulletin No. 94-440, Standard Operating Procedure, Geoprobe Systems,KS) ..................................................................................................................................A-3

Figure H-1 Center of Mass Movement for the Midwest Total Cyanide Plume(1 ft = 0.3048m)...............................................................................................................H-2

Figure I-1 Model Conceptualization of Saturated Thickness of the Portage Aquifer(1 ft = 0.3048 m) ............................................................................................................... I-1

Figure I-2 Model Finite Difference Grid (1 ft = 0.3048 m).......................................................... I-2


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Figure I-3 Model Potentiometric Surface Map Under Pre-Pumping Conditions(prior to 1986)................................................................................................................... I-3

Figure I-4 Model Potentiometric Surface Map Under Pumping Conditions ............................... I-3

Figure I-5 Two-Dimensional Flow Vectors During Non-Pumping Conditions............................. I-4

Figure I-6 Two-Dimensional Flow Vectors During Pumping Conditions .................................... I-4

Figure I-7 Vertical flow Vector Cross-Sector Locations Steady State Flow Model UnderPumping Conditions.......................................................................................................... I-5

Figure I-8 Vertical Flow Vector Cross-Section A-A Steady State Flow Model UnderPumping Conditions.......................................................................................................... I-5

Figure K-1 Kinetic dissociation data for the first set of batch experiments, with anaverage initial pH of 6.7 and an initial total cyanide concentration of 15.03 mg/L.............K-1

Figure K-2 Kinetic dissociation data for the second set of batch experiments, with anaverage initial pH of 6.5 and an initial total cyanide concentration of 15.5 mg/L...............K-2


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LIST OF TABLES

Table 2-1 Geometric Average Hydraulic Conductivity Values for the Four SaturatedLayers in the Site Aquifer (1 ft = 0.3048 m)...................................................................... 2-4

Table 2-2 Average Major Ion and Metal Concentrations Observed in Groundwater atPortage, Wisconsin MGP Site (data averaged over 5 sampling events)......................... 2-13

Table 2-3 Cyanide Speciation Results from November 1997 Sampling of Groundwater atPortage, Wisconsin MGP Site........................................................................................ 2-17

Table 2-4 Parameter Values Used to Model Existing Cyanide Plume at Portage,Wisconsin MGP Site...................................................................................................... 2-30

Table 2-5 Comparison Between Observed (Field) and Simulated (Model) Movement of0.04 mg/l Total Cyanide Contour in the Uppermost Aquifer Layer BetweenJuly 1996 And November 1999...................................................................................... 2-31

Table B-1 Monitoring Well Locations and Screen Interval Depths at the Portage,Wisconsin MGP Site........................................................................................................B-1

Table C-1 Piezometric Head Data Obtained During Sampling of the Monitoring Wells atthe Portage, Wisconsin MGP Site....................................................................................C-1

Table D-1 Hydraulic Conductivity Estimates from July 1996 Sampling Period .........................D-1

Table D-2 Hydraulic Conductivity Estimates from September 1997 Sampling Period ..............D-2

Table E-1 Cyanide in Geoprobe Groundwater Samples November 1995 .............................E-1

Table E-2 Cyanide in Monitoring Well Samples November 1995.........................................E-4

Table E-3 Cyanide in Monitoring Well Samples June 1996/July 1996 ..................................E-5

Table E-4 Cyanide in Monitoring Well Samples September 1996.........................................E-7

Table E-5 Cyanide in Monitoring Well Samples January 1997..............................................E-8

Table E-6 Cyanide in Monitoring Well Samples April 1997 .................................................E-10

Table E-7 Cyanide in Monitoring Well Samples September 1997.......................................E-12

Table E-8 Cyanide in Monitoring Well Samples November 1997........................................E-14

Table E-9 Cyanide in Monitoring Well Samples June 1998 ................................................E-17

Table E-10 Cyanide in Monitoring Well Samples September/October 1998 .......................E-18

Table E-11 Cyanide in Monitoring Well Samples December 1998......................................E-21

Table E-12 Cyanide in Monitoring Well Samples January 1999..........................................E-22

Table E-13 Cyanide in Monitoring Well Samples April 1999 ...............................................E-23

Table E-14 Cyanide in Monitoring Well Samples July 1999................................................E-25

Table E-15 Cyanide in Monitoring Well Samples November 1999......................................E-26

Table F-1 Major Ion and Metal Concentrations in Groundwater June 1996/July 1996 ..........F-1


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Table F-2 Major Ion and Metal Concentrations in Groundwater September 1996.................F-2

Table F-3 Major Ion and Metal Concentrations in Groundwater January 1997 ..................... F-2

Table F-4 Major Ion and Metal Concentrations in Groundwater April 1997........................... F-3

Table F-5 Major Ion and Metal Concentrations in Groundwater September 1997.................F-3

Table G-1 Groundwater Sampling Data June 1996/July 1996 ............................................. G-1

Table G-2 Groundwater Sampling Data September 1996.................................................... G-2

Table G-3 Groundwater Sampling Data January 1997 ........................................................ G-3

Table G-4 Groundwater Sampling Data April 1997.............................................................. G-4

Table G-5 Groundwater Sampling Data September 1997.................................................... G-5

Table G-6 Groundwater Sampling Data November 1997..................................................... G-6

Table G-7 Groundwater Sampling Data June 1998 ............................................................. G-7

Table G-8 Groundwater Sampling Data September/October 1998 ...................................... G-8

Table G-9 Groundwater Sampling Data January 1999 ........................................................ G-9

Table G-10 Groundwater Sampling Data April 1999.......................................................... G-10

Table G-11 Groundwater Sampling Data July 1999........................................................... G-11

Table G-12 Groundwater Sampling Data November 1999................................................. G-12

Table J-1 Municipal Well Pumping Data 1986 ...................................................................... J-1









Table J-10 Municipal Well Pumping Data 1995 .................................................................. J-10

Table J-11 Municipal Well Pumping Data 1996 .................................................................. J-11

Table J-12 Municipal Well Pumping Data 1997(1)

................................................................ J-12

Table J-13 Model Calculation of Average Daily Pumping Volume and Daily PumpingRate............................................................................................................................... J-13


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1INTRODUCTION

1.1 Background

Cyanide has been observed as a groundwater contaminant at various types of current and formerindustrial sites. The presence of cyanide in these aquifers has occurred as a result of the leakageof cyanide solutions from baths and storage tanks in electroplating processes and from oreleaching basins, as well as, by leaching of cyanide-bearing solid residuals that are present atindustrial facilities (e.g., aluminum manufacturing or manufactured gas plant [MGP] sites).

At MGP sites, boxes (commonly called oxide boxes) containing wood shavings or crushedblast-furnace slag mixed with a chemically active form of hydrated iron oxide were used to scrubhydrogen sulfide (H

2S) from the product gas; hydrogen cyanide in the gas was removed

simultaneously (Lowry, 1945). The resultant oxide box material, which was regenerated, reused,and then finally managed on- and off-site, contained iron cyanide solids. As a result of theleaching of this material by infiltrating rainwater or direct contact with groundwater, dissolvedforms of cyanide have been released to groundwater. Cyanide can also infiltrate the groundwaterfrom iron cyanide anti-caking agents in road salt that is used for de-icing purposes during thewinter (Paschka et al., 1999).

The geochemistry, transport, and treatment of cyanide in groundwater systems have not beenextensively studied. Available information indicates that cyanide usually exists as metal-cyanide

complexes in groundwater systems, with iron cyanide complexes often being the dominantform at MGP and aluminum production sites (Meeussen et al., 1992b; Theis et al., 1994;Dzombak et al., 1996). Iron cyanide complexes are stable in the dark (Meeussen et al., 1992a)and are highly resistant to biodegradation (Aronstein et al., 1994; Laha and Luthy, 1991).Limited available sorption data (Alessi and Fuller, 1976; Theis and West, 1986) indicate that ironcyanide complexes may not exhibit significant sorption onto oxide minerals in the neutral pHrange. These data suggest that cyanide in the subsurface may be mobile and recalcitrant in manycases, a hypothesis which has not been tested against field data.

The research described in this report had four principal objectives. These were: i) to obtain anunderstanding of the chemistry and movement of a groundwater plume containing cyanide at an

MGP site in Portage, Wisconsin; ii) to interpret the observed plume data with a numerical fateand transport model to investigate the behavior of the dissolved cyanide compounds in thesubsurface; iii) to assess the potential for natural attenuation of the cyanide plume; and iv) topredict the future movement of the cyanide plume under existing site conditions. Specific goalsand activities associated with each of these overall objectives are outlined below.

To study the speciation and movement of cyanide compounds in groundwater, a groundwatermonitoring program was conducted at the Site. A network of 41 groundwater monitoring wells


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1-2

was established and quarterly sampling was performed over a period of four years to characterizethe cyanide species in groundwater and to monitor their movement over time. Field workincluded the collection of aquifer hydraulic property data and contaminant distribution data. Anadditional source investigation was also conducted as part of the field studies at two areas of theSite that represent potential continuing source areas for cyanide impacts to groundwater.Laboratory studies were also performed to help understand the stability of cyanide species in thesubsurface environment and the nature of their movement through actual and synthetic aquifermedia.

Two numerical models were used to analyze the subsurface fate and transport of the cyanidespecies. Initially, a two-dimensional particle tracking solute transport model was used to modelthe shallow plume data. Later, the modeling effort was enhanced with the three-dimensionalGMS-V2.1 numerical transport model. This model was used to interpret the shallow plume dataand the vertical distribution of the contaminants. The hydraulic data and the cyanide chemistrydata that were collected during the field and laboratory studies were used to calibrate thetwo- and three-dimensional models. These models were then used to predict the fate andtransport of cyanide in the subsurface at the Site.

Lastly, the three-dimensional numerical transport modeling coupled with laboratory column testdata associated with the site groundwater and aquifer material were used to evaluate the potentialfor natural attenuation of cyanide in the site groundwater. This was accomplished by comparingthe predicted movement of the cyanide plume, which assumed non-reactive transport of thecyanide, to the actual movement observed in the field monitoring data.

1.2 Organization of the Report

The results of this research project are presented entirely in Section 2 of this report. This sectionconsists of three major subsections that address the field studies, the laboratory studies, and thegroundwater modeling efforts. The final section of the report, Section 3, presents the overallconclusions of this research and provides recommendations for future work. Raw data and othersupplementary information are provided in Appendices A through K.


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2-1

2FIELD, LABORATORY AND MODELING STUDIES

This project was conducted as a tailored collaboration (TC) project between EPRI and AlliantEnergy Corporation (Alliant Energy). It addresses the chemical speciation, fate, and transport

of cyanide compounds in groundwater at an MGP site in Portage, Wisconsin (hereafter referred

to as Site). MGP operations at the Site began in 1887 and ceased in 1947. Because of the on-sitemanagement of oxide-box wastes as fill during the course of site operations, cyanide is present in

the soil and groundwater at the Site.

The investigation of the cyanide groundwater plume at the Portage MGP site involved laboratory

studies, field studies, and fate and transport modeling. The field studies were designed tocharacterize cyanide speciation and transport in groundwater at the Site and to assess the

potential for natural attenuation of the cyanide plume. Laboratory studies were designed to

improve the understanding of the cyanide geochemistry, in particular, the interactions of cyanide

species with the field site aquifer material. Information from the field and laboratory studies wasused in a fate and transport model to predict the movement of cyanide in groundwater at the Site.

The overall objective of the field, laboratory, and modeling investigations was to gain an

understanding of the factors affecting the fate and transport of cyanide in a sand/gravel aquifer,to assess the potential for natural attenuation of cyanide in such an environmental setting, and to

predict the future movement of the cyanide plume under existing site conditions.

2.1 Methods

2.1.1 Field Studies

A cyanide-impacted groundwater plume at the Sitewas studied for a period of four years. Field

studies focused on a detailed delineation of the plume through a geoprobe survey, quarterly

groundwater elevation and water-quality monitoring, and an additional source investigation.

2.1.1.1 Monitoring Well Network

An initial survey to delineate the boundaries of the plume was conducted using a Geoprobe

Screen Point Ground Water Sampler (See Appendix A for Geoprobe Sampler details). Elevenwells were already located at the Site to monitor the extent and movement of the plume at the

time of the survey. Following the survey, 30 additional monitoring wells were installed on-site.

The additional well installations occurred in June 1996 and again in November 1997. The

locations of these wells are shown in Figure 2-1.


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Field, Laboratory and Modeling Studies

2-2

Figure 2-1Geoprobe and Monitoring Well Locations at Portage, Wisconsin MGP Site

The wells were drilled using the rotasonic drilling method. Well casings were constructed ofPVC with an internal diameter of 2 inches (5 cms). The screened interval for each well wasconstructed of slotted PVC, and the screened length varied from a length of 10 ft (3 m) for wellslocated near the water table to a length of 2 ft (0.61 m) for deeper wells. The screen intervals of

the monitoring wells for the Portage monitoring well network are listed in Appendix B. The welllocations were largely confined to city streets and alleys where there was ready access to preventdisruption of current residential land use.

Most of the additional wells were installed to sample the top 10 ft (3 m) of the aquifer where themajority of the cyanide mass was identified in the samples collected from the geoprobe survey.However, multilevel well nests were also installed at eight locations to delineate the verticalextent of impacts and the vertical hydraulic gradients. Five of these multilevel well nests werelocated along the centerline of the plume and include the following: Well nest MW-422A/B/C;Well nest MW-423A/B/C; Well nest MW-408A/B and MW-421; Well nest MW-426A/B/C/D;and Well nest MW-427A/B/C/D. The three-well well nests (MW-422, MW-423, and

MW-408/MW-421) permitted sampling at depths of 25 to 30 feet (7.6 to 9.1 m), 40 feet(12.2 m), and 60 feet (18.3 m). The four-well well nests (MW-426 and MW-427) permittedsampling at depths of 30 feet (9.1 m), 60 feet (18.3 m), 90 feet (27.4 m), and 130 feet (39.6 m).Three other multi-level wells were installed at the plume edges perpendicular to the direction offlow to delineate the transverse extent of the plume. These multi-level well nests included:MW-409A/B; well nest MW-410A/B; and well nest MW-425A/B/C/D. The multi-level wellnests MW-409 and MW-410 permitted sampling at depths of 25 feet (7.6 m) and 45 feet(13.7 m). The multi-level well nest MW-425 permitted sampling at depths of 30 feet (9.1 m),60 feet (18.3 m), 90 feet (27.4 m), and 130 feet (39.6 m).


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2.1.1.2 Site Hydrogeological Conditions

The aquifer underlying the Site and the area downgradient of the Site consists of sand and gravelwith a saturated thickness of approximately 110 feet (33.5 m). A schematic representation of thesubsurface geological profile is presented in Figure 2-2. Potentiometric head measurements

indicate that the average water table occurs at a depth of approximately 25 feet (7.6 m). Allgroundwater head data that were obtained during the study are provided in Appendix C.

Not drawn to scale

Figure 2-2Schematic Vertical Cross Section of the MGP Site (1 ft = 0.3048 m)

Slug tests were conducted at various locations in the flow field in July 1996 and September 1997and hydraulic conductivities were determined for the aquifer from these tests using the Bouwerand Rice solution method (Bouwer and Rice, 1976; Bouwer, 1989). Both falling and rising headdata were acquired during the course of the slug testing. The time-displacement data wereinterpreted using the software Aqtesolv (Geraghty and Miller, Reston, VA). A tabulation ofhydraulic conductivity estimates for the two sampling periods and the Aqtesolv output for the allof the wells that were tested are provided in Appendix D.

During the September 1997 event, slug tests were also conducted in wells screened at multipledepths to determine if horizontal hydraulic conductivity varied with depth in the aquifer. Basedon the calculated hydraulic conductivities, four distinct hydrogeological layers were identified in

the saturated zone. The geometric average hydraulic conductivity values for these four layers(using both rising head and falling head slug test data) are listed in Table 2-1. From this table, itis observed that the top layer, comprising the first 10 feet of the saturated zone, has asignificantly lower hydraulic conductivity than the underlying layers. The second layer has thehighest hydraulic conductivity in the aquifer and hydraulic conductivity values becomeprogressively lower in the third and fourth layers. From these data, it was determined that theplume would likely travel fastest in the middle two layers of the aquifer and slowest in the mostshallow and the deepest layers.


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Table 2-1Geometric Average Hydraulic Conductivity Values for the Four Saturated Layers in the SiteAquifer (1 ft = 0.3048 m)

Hydraulic Conductivity, ft/day (m/day)Layer Number Depth Interval from WaterTable, ft (m)

Falling Head Rising Head

Layer 1 0-10 (0-3) 77.7 (23.7) 197.4 (60.2)

Layer 2 10-40 (3-12.2) 597.35 (182.1) 689.33 (210.1)

Layer 3 40-70 (12.2-21.3) 553.03 (168.5) 491.79 (149.9)

Layer 4 70-110 (21.3-33.5) 131.45 (49.2) 114.13 (34.8)

2.1.1.3 Plume Monitoring

Sampling of the groundwater monitoring wells at the Site was conducted on nine occasionsbetween June 1996 and November 1999. Groundwater samples from the Site were analyzedfor total cyanide and weak-acid-dissociable cyanide using standard distillation methods(Method 4500-CN-C and I, APHA, 1995). Free cyanide was also measured for all the samplingrounds by the microdiffusion method (Method D-4282, ASTM, 1989). Total cyanide provides ameasure of all the cyanide present in a sample, including strong and weak metal-cyanidecomplexes, as well as free cyanide. Weak-acid-dissociable cyanide encompasses free cyanideplus weak metal-cyanide complexes. Free cyanide encompasses cyanide that diffuses as HCNat room temperature from a solution at pH 6. On five occasions between June 1996 andSeptember 1997, samples from the wells exhibiting elevated total cyanide concentrations wereanalyzed for metal-cyanide complexes by ion chromatography (Dionex Corporation, 1989). Acomplete compilation of all well monitoring data for cyanide concentrations is provided inAppendix E. In addition to the cyanide measurements taken during these same five monitoringevents, samples from a subset of the 41 monitoring wells were analyzed for metals (Ca, Na, Mg,K, Fe) and major ions (SO

4

2-, Cl

-, NO

3

-, HCO

3

-) by atomic absorption spectroscopy and ion

chromatography, respectively. A detailed record of the major ion and metal concentration dataare presented in Appendix F. Field measurements of groundwater were also made for pH, pE,conductivity, dissolved oxygen, and temperature at different monitoring well locations duringeach of the nine sampling events. A detailed record of the groundwater conditions during eachsampling period is presented in Appendix G.

The center of mass for the cyanide plume was calculated for six of the nine rounds of samplingwith the purpose of analyzing the plume transport on the basis of center of mass movement

(Freyberg, 1986). Methods employed for the center of mass calculations are provided inAppendix H.

2.1.1.4 Additional Source Investigation

An additional source investigation was conducted in January 2000 at the Site to evaluate areasthat were potentially acting as continuing sources of dissolved-phase cyanide to the aquifer.Based on quarterly groundwater monitoring data for the Site and previously acquired soil boring


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2-5

data acquired during Phase I and II remedial investigations, two areas that represented the

maximum potential to contain cyanide-bearing residuals were identified. These areas aredesignated as Area A and Area B in Figure 2-3. Area A is located in the vicinity of the MW-423

well cluster and Area B is located in the vicinity of monitoring well MW-419 on Alliant

Energy

property. Eighteen borings were completed in Area A and 9 borings in Area B (Figure

2-3). The soil borings were completed using a geoprobe groundpoint sampling device andadvanced to depths ranging from 24 to 40 feet below ground surface. Continuous soil cores were

collected for visual inspection of cyanide impacts and selected soil boring samples were

submitted for analysis of total cyanide.

MW-404

MW-402

MW-403

MW-407

MW-408A&B

MW-410A&B

MW-409A&B

MW-418

MW-415

MW-414

MW-413

MW-419

MW-416

MW-424

MW-422B

MW-422A

MW-423C

MW-423B

MW-423A

MW-421

MW-422C

MW-417

MW-417MONITORING WELL

STRUCTURES

RAILROAD

ROADWAYS

LEGEND

AREAA

AREA B

GEOPROBE SOIL

SAMPLING LOCATIONS

E.EM

METT

STRE

ET

MARI

ONSTRE

ET

JEFF

ERSO

NST

REET

MGP SITE

PRIVA

TEPR

OPER

TY

SB-17

SB-25SB-24SB-23

SB-22

SB-21SB-20

SB-19SB-18

SB-27SB-14

SB-16 SB-15

SB-9SB-6

SB-3

SB-2SB-1

SB-12SB-11

SB-7

SB-13

SB-26SB-5

SB-8

SB-10

CYANIDE IMPACTED AREAS(VERTICAL EXTENT OF

CONTAMINATION = 0' - 30')

Figure 2-3Approximate Boring Locations for Additional Source Characterization at the PortageMGP Site

2.1.2 Laboratory Studies

Factors influencing the transport and transformation of cyanide in aquifer media were studied vialaboratory column and batch tests using combinations of synthetic solutions, site groundwater,

synthetic porous media, and site aquifer materials.

2.1.2.1 Column Studies

The potential for retardation of cyanide transport in the subsurface at the MGP site wasinvestigated through laboratory column studies. These experiments involved passage ofcyanide-bearing groundwater collected from the Site, as well as some synthesized cyanide

solutions, through a 2.5 cm diameter, variable length (5 to 10 cm) glass column containing


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2-6

aquifer material. A constant flow rate of 1 ml/min was maintained with the use of an HPLCpump. Tests were performed with the actual field-site sand and also clean Ottawa sand.Complete details of the column tests are provided in Ghosh et al. (1999b).

Some column tests were conducted with synthetic solutions of iron (II)- and iron (III)-cyanide

complexes in de-aerated water as the influent. Column tests were also performed with a nickelcyanide solution, which is a weak-acid dissociable cyanide compound, and with a solutioncomprising a mixture of nickel cyanide and ferricyanide. The tests with the iron cyanidesolutions were conducted with columns packed with field-site aquifer material and Ottawa sand,while the column tests involving nickel cyanide used Ottawa sand, only.

2.1.2.2 Batch Studies

Batch experiments were conducted to examine the stability of iron-cyanide complexes in thedark under neutral pH conditions. These tests were conducted with ferrocyanide, which isreported to decompose somewhat more rapidly than ferricyanide (Meeussen et al., 1992a). A

15.5 mg/l stock solution of potassium ferrocyanide was prepared with deaerated, deionizedwater. Centrifuge vials of 40 mL capacity were filled with the ferrocyanide solution to capacity,capped, covered with aluminum foil, and placed in a vial rack in a constant temperature bath(Temperature = 22.00.5C). At pre-selected times over a period of several weeks, individualvials were removed from the bath and were analyzed for total cyanide (Method 4500-CN-C,APHA, 1995), iron-cyanide complex via UV spectrophotometry (Dionex Corporation, 1989),and free cyanide using the microdiffusion technique (Method D-4282, ASTM, 1989). Thecomplete data set for these batch dissociation studies is presented in Appendix K.

2.1.3 Modeling Studies

2.1.3.1 Two-Dimensional Groundwater Flow and Contaminant Transport Model

A two-dimensional groundwater flow and solute transport model was used to fit the shallowgroundwater plume data from June 1996 through November 1997. The model was used todescribe the observed plume movement and to predict future plume movement. Based on theresults from the groundwater sample analyses and the laboratory column tests, it was determinedthat total cyanide at the field site could be modeled as a single, nonreactive, nondegradingchemical species. This model consisted of a modified version of the random walk particletracking numerical model (Prickett et al., 1981) which incorporates two-dimensional flow in thex and y directions and two-dimensional solute transport in the x-y plane. The particle trackingalgorithm for solute transport is similar to those employed in other groundwater solute transport

models including MODPATH and PATH3D used in conjunction with the USGS MODFLOWmodel (Pollock, 1988, 1989; Zheng, 1989). The flow model uses the iterative, alternatingdirection, implicit method to calculate the heads in a finite difference grid (Pricket andLonnquist, 1971).

The flow module was calibrated using the average piezometric head levels that were obtainedfrom the six rounds of sampling between June 1996 and November 1997, including the effect ofa municipal supply well located 1600 ft (487.68 m) to the southeast of the Site. The municipal


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well was constructed in 1982 and started pumping on an intermittent basis from 1986 to thepresent. As an approximate representation of the actual pumping data (Appendix J), the well wasconsidered to operate under a schedule of 7 days on followed by 7 days off, with an averagepumping rate of 560 gpm when operating. It was assumed that a steady state head distribution isreached within a short time of initiation of pumping from the well (i.e., the transient responseof the water table to the onset and cessation of pumping was neglected). The rate ofcyanide-contaminated leachate input to the saturated zone was estimated using the averageprecipitation rate of 78.26 cms/yr for the Site (Hydro-Search, 1991) and assuming a runoffcoefficient of 0.5 (Ven Te Chow, 1964). The average leachate concentration was estimated to be6 mg/l, based on groundwater analysis of cyanide near the apparent source location (MW-423A).The discrete hydraulic conductivity data (Appendix D) were interpolated over the simulationdomain. These interpolated values were thereafter used in the flow model to generate the steadystate head distribution. Simulations, performed with the geometric average value of hydraulicconductivity data in the top 10 ft (3 m) of the aquifer, yielded similar head distributions that wereobserved using the interpolated values. This can be attributed to the homogenous nature of thesand/gravel aquifer near the water table.

Two-dimensional modeling required the assumption of uniform cyanide concentration across thesaturated thickness of the aquifer (i.e., a vertical cyanide concentration gradient of zero wasassumed). Insufficient vertical concentration resolution prior to November 1997 limited themodeling to this two-dimensional approach. The complete details of this model, the associatedflow calibration, and the plume prediction data and figures are presented in an earlier EPRIreport (EPRI, 1998).

2.1.3.2 Three-Dimensional Flow and Transport Model

As previously mentioned, additional monitoring wells were installed in November 1997. Thesewells included multi-level well nests to permit sampling of the aquifer across the entire length ofits saturated thickness for improved vertical delineation of the plume. Following the acquisitionof multilevel data from these wells, it was possible to interpret the field data with a moresophisticated three-dimensional fate and transport model. Hence, the state-of-the-art Departmentof Defense Groundwater Modeling System (GMS v2.1) software (Brigham Young University,1998) was used to interpret field data and to predict future plume movement at the Site. Theentire GMS system consists of a graphical user interface and a number of analysis modules(MODFLOW, MT3D, RT3D, MODPATH, SEEP2D, FEMWATER). The analysis modules usedin the Portage MGP site modeling included the USGS MODFLOW module to generate athree-dimensional head distribution and the MT3D module to determine advective-dispersivesolute transport. Details of the numerical model, including descriptions of flow and transportmodels, and relevant figures from the flow and transport simulations are provided in Appendix I.

Groundwater Flow Model (MODFLOW)

To accurately represent the regional flow conditions, a finite unconfined aquifer was modeledusing the specified head boundaries shown in Figure 2-4. These head boundaries weredetermined from a USGS 7.5-minute topographic map that included the area of the Site. Twosteady state flow models were used in the simulation: one model incorporating the effect fromthe municipal well pumping (1986 to present); and the other model incorporating forty years of


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flow under the natural hydraulic gradient (i.e., the time period following the cessation of plantoperations and prior to operation of the municipal pump in 1986). As in the 2-D modeling, thetransient response of the water table to the onset and cessation of the pumping was neglected.The model boundary conditions shown in Figure 2-4 were used for the steady models under bothpumping and non-pumping conditions.

Portage MGP

Site

788.5 ft amsl

786 ft amsl

785 ft amsl783 ft amsl

783 ft amsl

780 ft amsl

Note: Orange lines represent specified head boundaries. Elevations of nodes along the specified head boundaries are denoted with a black arrowand the associated elevation. Green outlined areas indicate approximate extent of Alliant Energy property.

MunicipalWell No. 7

Figure 2-4Model Boundary Conditions Under Pumping and Non-Pumping Conditions(1 ft = 0.3048 m)

Based on the Site data, the groundwater flow model was constructed to include four distinctsaturated layers with varying hydraulic conductivities and thickness as shown in Table 2-1.Vertical flow between the four saturated layers in the groundwater flow model was accounted forthrough a vertical leakance factor. The following formula was used to calculate the leakancebetween the layers:

ZL

L

ZU

U

K2

Z

K2

Z

1

+=

where,

= leakanceZ

U= thickness of the upper layer (ft)

KZU

= hydraulic conductivity of the upper layer (ft/day)

ZL

= thickness of the lower layer (ft)K

ZL= hydraulic conductivity of the lower layer (ft/day)


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2-9

An identical finite difference grid was used for each saturated layer. A series of four refinepoints was used to achieve improved grid resolution in the Site area within the model domain.The base cell size in a grid varied from a minimum value of 50 ft (15.24 m) in the x direction and50 ft (15.24 m) in the y direction at the Site and in the vicinity of the municipal well to amaximum of 500 ft (152.4 m) in the x direction and 500 ft (152.4 m) in the y direction elsewherein the flow domain. The depth of the cells in the vertical (z) direction were 30 ft (9 m) for all thecells in the first layer (of which 10 feet was saturated), 30 ft (9 m) for the second and third layerand 40 ft (12.2 m) for the fourth layer. Figure 2-5 shows the typical model finite difference gridfor a layer with the location of the municipal well.

A complete list of the hydraulic parameters used in the development of the three-dimensionalgroundwater flow model are listed in Table 2-4 (Page 2-30 of this report). The average dailypumping rate of 363 gpm was calculated for the Portage municipal well from pumping logsprovided by the city of Portage. The detailed calculation of the average pumping volume isprovided in Appendix J. During calibration of the model, it was necessary to reduce the layerhydraulic conductivities (reported previously in Table 2-1) uniformly by a factor of 5.6 toachieve calibration. Slug tests provide a good estimate of the relative difference in hydraulic

conductivities between locations and depths with an aquifer, however, the slug test values are notalways accurate with respect to the true hydraulic conductivity of the aquifer material. As such,this uniform reduction of the slug test-derived hydraulic conductivity values is believed to bevalid. A plot showing the model calibration between the observed heads and the simulated headsat each monitoring well is presented in Figure 2-6. From this figure it is observed that theobserved heads and simulated heads are in good agreement which confirms the validity of thesteady state flow model for use in the transport simulations.

Except for the municipal well location, groundwater piezometric elevation data were notavailable for the flow domain prior to the establishment of the municipal well. However, usingthe boundary conditions previously described, the piezometric surface generated by the flow

model during the non-pumping period predicted a piezometric elevation of 785.26 ft amsl(239.35 m) at the municipal well location (Figure I-3 in Appendix I), which is very close to theactual water level measurement of 785 ft amsl (239.27 m) measured at that location prior topumping.


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Portage MGPSite

Note: Orange lines represent specified head boundaries. Green outlined areas indicate approximate extent of Alliant Energy property.

MunicipalWell No. 7

Figure 2-5Model Finite Difference Grid with the Location of the Municipal Well (1 ft = 0.3048 m)

781.00

781.50

782.00

782.50

783.00

783.50

784.00

784.50

785.00

785.50

786.00

781.00 781.50 782.00 782.50 783.00 783.50 784.00 784.50 785.00 785.50 786.00

Average Observed Head (ft amsl)

PredictedHead(ftamsl)

1:1 Line

Figure 2-6Steady State Flow Model Calibration with Observed Heads Versus Simulated Heads(1 ft = 0.3048 m)


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2-11

Particle Tracking Transport Model (MT3D)

Two distinct sources of cyanide input to the saturated zone were considered in the contaminanttransport modeling. A schematic representation of these areas are shown in Figure 2-7 whichshows one in the vicinity of MW-423A and the other near the down-gradient well MW-419. Theexistence of these two sources is well supported by the observed shallow plume contours(Figures 2-9 and 2-10) which show two distinct epicenters around those areas. Following theintroduction of the cyanide input to the first and second saturated layer of the simulation domain,a particle tracking approach was used to advance individual particles (particle masses per gridvolume = cyanide concentration in that grid) through advection and dispersion. Assuming thatthe cyanide input to the saturated aquifer began in 1947 (i.e., the cessation of plant operations),the particle transport was performed with continuous source input for 40 years under anon-pumping hydraulic gradient until 1986 when the well began operating. In 1986, transportwas simulated with the municipal well pumping 365 days a year under the daily average flowrate of 363 gpm. The three parameters that were used to fit the observed plume were longitudinaldispersivity, transverse dispersivity, and time since initiation of cyanide discharge. Table 2-4(Page 2-30 of the report) lists the different transport parameters that were used in the simulation.

Details about the particle transport package are provided in Appendix I.

Layer 1

Layer 2

Layer 3

Layer 4

MW-423

cluster

Constant

Source Input

in 1st and

2nd layers

MW-419 20 ft

10 ft

30 ft

30 ft

40 ft

Municipal

Well

Ground surface

1600 ft

Flow Direction

Vertical Section

Municipal WellMGP Site

Layer 1

Layer 2

Constant

Source Input

in 1st and 2nd

Layers

MW-423

Layer 3

Bedrock

Layer 4

MW-419

Vadose Zone

3-D Aquifer Unit

xy

z

415 ft

Note: Source in

each layer isintroduced along a

single cell at the welllocations 423A and 419.

Water table

Figure 2-7

Schematic Showing the Plan and Cross-Sectional Area of the Contamination ScenarioUsed for Three-Dimensional Modeling Purposes (1 ft = 0.3048 m)


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2.2 Results and Discussion

2.2.1 Field Studies

Measured groundwater elevations observed at the Site indicate that groundwater flows towardthe ESE (Figure 2-8). Groundwater flow at the Site is influenced significantly by the pumping ofthe municipal water supply well located approximately 1600 ft (487.68 m) SE of the Site.Figure 2-8 shows the slope of the water table at the Site as determined from the averagegroundwater elevation measurements between the July 1996 and November 1999 samplingperiods. The average hydraulic gradient observed during the study period was approximately0.0014 ft/ft (m/m) when the pump was in operation. The average hydraulic conductivity in theaquifer, as determined from the falling head slug test data, is 13.87 ft/day (4.23 m/day) in the top10 ft (3 m) of the aquifer with higher values obtained at greater depths. Based on the hydraulicgradient and average hydraulic conductivity, and considering an aquifer material porosity ofabout 36% as measured for samples of the Site aquifer material, the average linear groundwatervelocity in the shallow aquifer at the Site is approximately 19.69 ft/yr (6.00 m/yr) when the

pump is operating. Variations of flow velocity across the flow field occur, however, due tovariations in the hydraulic conductivity, and range from 109 to 848 ft/yr (33.3 to 258.4 m/yr)with depth as calculated using Darcys law.

781.60

781

.80

782.0

0

782.20

782.40

782

.60

782

.80

783

.00

783

.40

783

.80

78

4.

00

78

4.2

0

78

4.4

0

784

.60

784

.80

784

.80

Figure 2-8Water Table Elevation Contours at Portage, Wisconsin MGP Site (from averagegroundwater elevations measured between July 1996 and November 1999) [1 ft = 0.3048 m]


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Groundwater at the Site has a total dissolved solids concentration of approximately 600 mg/L,with Ca

2+and Mg

2+as the major cations and HCO

3

-and SO

4

2-as the major anions. The pH ranges

from 6.5 to 8 (Appendix G). Groundwater pE is between 4 and 5 (anoxic) in the top portion ofthe aquifer, and has a pE < 3 at greater depths (Appendix G). Average major ion and metalconcentration data are provided in Table 2-2; a complete listing of all the major ion and metalconcentration data are provided in Appendix F.

Table 2-2Average Major Ion and Metal Concentrations Observed in Groundwater at Portage,Wisconsin MGP Site (data averaged over 5 sampling events)

WellNumbers

Cl-

(mg/l)SO

4

2-

(mg/l)NO

3

-

(mg/l)HCO

3

-

(mg/l)Na

(mg/l)Mg

(mg/l)K

(mg/l)Ca

(mg/l)Fe

(mg/l)

411 112.74 15.43 20.59 222.21 40.13 35.07 4.75 61.61 0.17

417 53.1 13.02 9.21 226.21 36.14 29.26 3.49 57.72 0.12

419 37.72 42.85 7.58 256.32 19.4 28.73 12.72 71.61 1.72

420 80.26 47.29 20.21 342.31 27.58 43.49 12.85 93.44 0.09

421 38.65 77.41 5.02 320.32 23.02 39.77 2.53 86.12 0.17

423A 64.09 56.93 30.77 284.32 22.37 44.61 5.17 97.71 2.51

423B 17.65 66.25 25.71 339.33 14.71 42.57 3.78 101.62 0.21

423C 169.35 53.64 15.31 313.31 70.35 41.24 7.69 91.43 0.01

Nine rounds of quarterly sampling have yielded a good representation of the plume geometry intwo-dimensional plan view. Figures 2-9 and 2-10 show the shape of the shallow total cyanideplume as defined by the groundwater analysis results for the June/July 1996 and November 1999sampling rounds, respectively. As shown in these figures, the measured total cyanideconcentration in the groundwater near the on-site source (i.e., MW-423 cluster) is approximately7.0 mg/L, consistent with the solubility of Prussian Blue/Turnbulls Blue solid in presence ofexcess iron and under the observed pH and pE conditions (Ghosh et al., 1999a). Total cyanideconcentration decreases continuously with distance from the source (i.e., from 7 mg/l to less than0.01 mg/l) suggesting that the isolated stringers of re-precipitated iron-cyanide solids observedduring the additional investigation are serving as a minor continuing source of cyanide impactsto groundwater. The overall geometry of the plume shows more significant longitudinal

dispersion as compared to transverse dispersion (Figures 2-9 and 2-10). Also, as previouslynoted, two distinct potential source regions are evident in these two figures, one around theMW-423 cluster and the other further down-gradient around MW-419.


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Figure 2-9Total Cyanide Plume in Groundwater at Portage, WI MGP Site, June/July 1996(1 ft = 0.3048 m)

Figure 2-10Total Cyanide Plume in Groundwater at Portage, WI MGP Site, November 1999(1 ft = 0.3048 m)


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Calculations for locating the center of mass for the total cyanide plume in the shallow aquifer didnot show any appreciable movement of the center of mass (Appendix H), although the leadingedge of the plume has shown some movements over the course of the last three years. Center ofmass analysis is most useful for assessing migration of a fixed mass of solute in the ground(i.e., cases in which solute is injected into the aquifer in the form of a pulse input orinstantaneous point source) (Freeze and Cherry, 1979; Freyberg, 1986). It is of limited use whena continuous source is present. At the MGP site, the release of cyanide to the groundwaterappears to be based on the fact that the total amount of cyanide mass in the aquifer increasedover the entire period of monitoring.

Vertical profiles of cyanide concentration were obtained for the MW-423A/B/C, theMW-422A/B/C, and the MW-408/MW-421 well nests during the November 1999 samplingperiod. These data show that total cyanide concentrations decrease with depth but are stilldetectable at 60 ft (18.29 m) (Figure 2-11). However, when vertical profiles of cyanideconcentrations were plotted for well nests immediately upgradient of the municipal well(MW-425A/B/C/D and MW-426A/B/C/D for the same sampling period, cyanide concentrationswere found to increase with a depth down to 60 ft (18.3 m) after which it decreased to

non-detectable levels (Figure 2-12). The reason for this variation in the vertical concentrationprofile throughout the saturated thickness can be attributed to the heterogeneity of the aquifer interms of the variable hydraulic conductivity, which is further confirmed by the three dimensionalfate and transport model (i.e., there are preferential flow pathways of higher hydraulicconductivities at depths through which the cyanide compounds migrate more readily).

Total and weak acid dissociable cyanide analyses by distillation, free cyanide analyses bymicrodiffusion, and iron-cyanide complex analyses by ion chromatography indicate that cyanidein the groundwater is mostly complexed with iron. The data in Table 2-3, which show theNovember 1997 speciation of cyanide in the Site groundwater at selected well locations, confirmthis observation. Concentration data for iron-cyanide complexes that are presented in the table

were based on the difference between total and weak acid dissociable cyanide concentrations andby direct measurement using ion chromatography. A complete compilation of all cyanidemonitoring data is provided in Appendix E.

Figures 2-13 and 2-14 show the shallow weak acid dissociable (WAD) cyanide concentrations ingroundwater for the June/July 1996 and November 1999 sampling events. As shown in thesefigures, the overall dimensions of the weak acid dissociable cyanide plume are similar to the totalcyanide plume, with longitudinal dispersion being more significant than transverse dispersion.However, the concentrations of weak acid dissociable cyanide were much lower than totalcyanide concentrations, ranging from 0.002 mg/l to 0.086 mg/l. As a point of comparison forthese cyanide concentrations, consider that the relevant state groundwater enforcement standard(ES) for free cyanide is 0.2 mg/L, and the U.S.EPA maximum contaminant limit (MCL) forcyanide in drinking water is 0.2 mg/l as free cyanide. None of the on-site or off-site weak aciddissociable cyanide concentrations measured during these two events exceed the ES or the MCL(weak-acid dissociable cyanide is a conservatively high estimate of the free cyanideconcentration).


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0

20

40

60

M W 423 cluster M W 422 cluster M W 408 cluster

C N (tot) conc.,ppm CN (tot) conc.,ppm CN (tot) conc.,ppm

gw table

0 1 2 3 4 5 6 7 3.02.01.00.0 1.00.50.0

D

ept

h,

ft

260 ft 372 ft

Not drawn to scale

Figure 2-11Vertical Profile of Total Cyanide Concentration Along the Centerline of the Plume at WellNests near the Portage, Wisconsin MGP Site, November 1999 (1 ft = 0.3048 m)

0

20

40

60

80

100

120

1400.0 0.05 0.1

CN (tot) conc., ppm

0.0 0.1 0.2 0.3 0.4 0.5

CN (tot) conc., ppm

gw table

M W 425 cluster M W 426 cluster

M unicipal W ell

245 ft 180.5 ft

D

ept

h,

ft

N ot drawn to scale

Figure 2-12Vertical Profile of Total Cyanide Concentration along the Centerline of the Plume at WellNests near the Municipal Well, Downgradient of the Site (1 ft = 0.3048 m)


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Table 2-3Cyanide Speciation Results from November 1997 Sampling of Groundwater at Portage,Wisconsin MGP Site

WellNumbers

Total Cyanideby Distillation

Method, mg/l(2)

Iron Cyanide byDistillation (total

cyanide minusWAD cyanide), mg/l

(2)

IronCyanide

by I.C.(1)

,mg/l

(2)

Free Cyanideby

MicrodiffusionMethod, ppb

% Differencebetween

Distillation andI.C. Results

MW 402 3.69 3.61 3.54 18 +2.0

MW 407 1.66 1.63 1.61 3 +1.2

MW 408A 1.13 1.10 1.24 6 -12.7

MW 411 0.04 0.04 0.05(3)


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Figure 2-14WAD Cyanide Plume in Groundwater at Portage, Wisconsin MGP Site, November 1999(1 ft = 0.3048 m)

The additional source characterization study conducted at the site in January of 2000 revealed nocyanide-bearing oxide box residuals at Area A and Area B (locations of Area A and Area B arepresented in Figure 2-3). The major portion of the cyanide-bearing oxide box residuals at the site(Area A) appear to have been removed during the excavations in 1992 and 1993. However, a

dissolution and re-precipitation event appears to have occurred at disperse depth intervals(1 3 inch stringers) throughout the vadose zone and in the upper portions of the saturated zone.Of the 33 soil samples that were collected during this study and were submitted for the analysisof total cyanide, 16 samples showed detectable cyanide with concentrations ranging from0.8 to 95 mg/Kg. Most of the detects were located in Area A around the MW-423 cluster wherethe highest cyanide concentrations in groundwater are reported, with fewer detects in Area B.This observation of isolated cyanide stringers is consistent with the recent study performed atCarnegie Mellon University regarding the precipitation/dissolution behavior of iron cyanidesolids as a function of pH and pE in the laboratory under excess iron conditions (Ghosh et al.,1999a). The studies suggest that cyanide will dissolve from the iron cyanide solids at certaincombinations of pH and pE only to re-precipitate further down in the soil column as the pH

and/or pE conditions change. However, the occurrence of this re-precipitation requires thepresence of excess iron. The scattered stringers of precipitated cyanide solids in Area A andArea B appear to be serving as a minor continuing source of cyanide contamination to thegroundwater around these areas.


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2.2.2 Laboratory Studies

2.2.2.1 Column Tests

Column tests performed with contaminated site groundwater samples and site aquifer media

under anoxic conditions, consistent with the actual aquifer conditions, indicated no retardation ofcyanide movement relative to the water transport. Results of the test with near-sourcecontaminated site groundwater and the field site sand from uncontaminated portions of theaquifer are shown in Figure 2-15. From this figure it is observed that dissolved iron-cyanide, thepredominant form of cyanide in the field site groundwater, clearly moved through the site sandwithout retardation in absence of excess iron conditions. Breakthrough of influent total cyanideoccurred in about one pore volume, consistent with the transport characteristics of a nonreactivesolute. The absence of retardation can be attributed to the lack of sorption in the porous media. Inother column tests using the uncontaminated site sand/gravel aquifer material (Ghosh et al.,1999b), solutions of NaCl and ferri-/ferrocyanide also demonstrated breakthrough in about onepore volume, confirming the nonreactive transport of the total dissolved cyanide at the site.

Figure 2-15

Total Cyanide Breakthrough in Column Study with MGP Site Groundwater and AquiferMaterial

To investigate the extent of retardation, if any, of weak acid dissociable cyanide species throughsand aquifer material, column tests were conducted with synthetic solutions of nickel cyanideand Ottawa sand. Some tests with synthetic solutions comprising mixtures of nickel cyanide andferricyanide were also performed. These tests were performed because monitoring the


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breakthrough of weak acid dissociable cyanide using actual site contaminated groundwater wasimpractical due to the very low concentrations of these species in the site groundwater. Nickelcyanide concentrations higher than those observed at the site were employed in the column tests.Figure 2-16 shows the breakthrough profile for the nickel cyanide column tests. It can be seenfrom this figure that the nickel cyanide, a weak-acid dissociable cyanide, also came through thesand medium within one pore volume, showing no signs of retardation.

Figure 2-16Breakthrough in Column Study with Nickel Cyanide and Clean Ottawa Sand

The results for the test with the nickel and iron cyanide mixture are presented in Figure 2-17. Theweak-acid dissociable nickel cyanide, which constituted 12% of the total cyanide in the mixture,came through the column within one pore volume in this case also. These results indicate that,like iron-cyanide complexes, weak metal-cyanide complexes are not retarded by a sand mediumunder neutral pH conditions.


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Figure 2-17Breakthrough in Column with a Nickel Cyanide and Iron Cyanide Mixture and Clean OttawaSand

2.2.2.2 Dissociation Studies

Studies to examine the dissociation of iron cyanide to free cyanide were performed over apH range of 6.5 to 6.7 with solutions of potassium ferrocyanide prepared in deaerated, deionized

water. Time-series results of these batch tests showed little conversion of ferrocyanidecomplexes into free cyanide (3 to 5%). Figure 2-18 presents the results from one of these batchdissociation studies. Complete data from the batch dissociation studies, all of which are similarto that shown in Figure 2-18, are presented in Appendix K. As shown in Figure 2-18, theconcentration of free cyanide gradually increased and the concentration of complex cyanidegradually decreased over approximately 9,000 hours. Equilibrium was not achieved over thecourse of the experiments, although the dissolved concentrations were progressing towardequilibrium conditions. Further long-term batch experiments are recommended to identify theexact nature and extent of dissociation of iron-cyanide complexes in the subsurface.


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Figure 2-18Profile of Complex and Free Cyanide Concentrations During Batch DissociationExperiments with Ferrocyanide Solutions in Dark. Initial ferrocyanideconcentration = 15.03 mg/l

2.2.3 Modeling Studies

Modeling efforts were targeted at representing the observed cyanide plume using thethree-dimensional model described previously, assuming two continuous source input areas. Thelongitudinal and transverse dispersion coefficients and the time since initiation of cyanidedischarge to the saturated zone were adjusted using a trial and error approach to match theobserved field data. Results of fitting the June/July 1996 and November 1999 shallow plume(depth = 20 ft to 30 ft or 9.14 m to 18.29 m) data are presented in Figure 2-19 and Figure 2-20,respectively, where fits for the 0.04 mg/l total cyanide leading edge is shown. Note that for thepredicted plume, a continuous gradation in concentration is presented with color coding whereasfor the observed plume, selected isoconcentration lines are shown in red. From these figures, it isobserved that the predicted plume leading edge and the observed plume leading edge matchreasonably well.


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Note: Predicted plume extent displayed as a shaded range of colors.

Observed plume extext displayed as red linear contours.

Concentration(mg/L)

14.12

12.36

10.59

8.83

7.06

5.30

3.53

1.77

0.04

MW408A

MW419MW422A

MW402

MW423A

MW427A

Municipal Well No. 7

MW426A

MW425A

Figure 2-19Total Cyanide Isoconcentration Map in the Top Saturated Layer (K = 13.87 ft/day) for

Simulation of 49.6 Years of Transport. (40 years pre-pumping conditions plus pumpingfrom 1986 to July 1996) [1 ft = 0.3048 m]

Note: Predicted plume extent displayed as a shaded range of colors.

Observed plume extext displayed as red linear contours.

Concentration (mg/L)14.12

12.36

10.59

8.83

7.06

5.30

3.53

1.77

0.04

MW408A

MW419MW422AMW402

MW423A

MW427A

Municipal Well No. 7

MW426A

MW425A

Figure 2-20Total Cyanide Isoconcentration Map in the Top Saturated Layer (K = 13.87 ft/day) forSimulation of 53 Years of Transport. (40 years pre-pumping conditions plus pumping from

1986 to November 1999) [1 ft = 0.3048 m]


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In absence of real mon

modelo cianuro

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