Preliminary Evaluation

Potential Impacts ofRemedial ConstructionNewport Superfund SiteNewport, Delaware

Prepared for:

Ciba^Geigy

Prepared by:DAMES & MOOREAugust 26, 1992

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1.0 INTRODUCTION

This report presents Dames & Moore's preliminary evaluation of the physicaland economic impacts related to the construction of an engineered physical vertical barrierat the Ciba-Geigy pigment manufacturing plant in Newport, Delaware. The barrier isproposed to inhibit the tnigration of ground water from the DuPont-Newport Superfund Site(referred to herein as "DuPont Site" or the "SITE") to human, ecological, and environmentalreceptors. The Ciba-Geigy pigment manufacturing pilant is situated within the definedboundaries of the DuPont Site, and is referred to as the Ciba-Geigy Plant Area. Theremaining portions of the DuPont Site include two inactive landfills, designated the NorthDisposal Site and the South Disposal Site, the DuPont Holly Run Plant Area, a portion ofthe Christina River, wetlands associated with the inactive landfills, and the Ballpark arealocated northwest of the plant areas. Figure 1 of this report is a map that shows each of theportions of the DuPont Site. This figure is a photocopy of Figure 1-2, which was previouslyprovided in the Final Focused Feasibility Study report for the SITE.

This evaluation presents Dames & Moore's assessment of the physical(engineering and hydrplogic), operational, and financial constraints and considerationspertaining to the construction and maintenance of a physical vertical barrier adjacent to theboundaries of the DuPont Site. The evaluation focuses on impacts associated with theconstruction of a circumscribing vertical barrier and a riverbank cover system.

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Chapter 2.0 of this report provides background information for the DuPontSite and Ciba-Geigy's involvement with the SITE. Dames & Moore's evaluation of theconstruction and hydrologic impacts pertaining to the installation of the previouslyreferenced types of physical vertical barriers are presented in Chapters 3.0 and 4.0,respectively. Conclusions made based on the evaluation are discussed in Chapter 5.0.

2.0 BACKGROUND

2.1 SITE HISTORY AND OWNERSHIP

Pigment manufacturing operations were initiated at the area now defined asthe DuPont Site in 1902, when Mr. Henrik J. Krebs commenced the manufacture oflithopone, a white inorganic pigment. Lithopone was manufactured primarily by blendingzinc sulfate and barium sulfide. Waste materials and pigment slurries, consisting primarilyof zinc and barite ores, were disposed of by pumping from the manufacturing area to thearea currently defined as the South Disposal Site via a pipeline that was situated beneaththe Christina River. Mr, Krebs continued lithopone manufacture until approximately 1932,when E.I. du Pont de Nemours and Company, Inc. (DuPont) purchased the property andmanufacturing operations.

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Following the purchase of the property in 1932, DuPont continued pigmentmanufacture, in addition to other processes and research. Over the years, DuPont modifiedand expanded the plant several times and increased the number of pigment productsmanufactured. During DuPont's ownership, waste materials and off-specification productswere disposed of in the North and South Disposal Sites. Disposal in the South Disposal Sitewas ceased in 1953, and disposal in the North Disposal Site ceased in 1974.

In 1984, Ciba-Geigy purchased a portion of the property and associatedmanufacturing operations from DuPont. The portion of the property purchased, with regardto the DuPont Site, is referred to as the Ciba-Geigy Plant Area. DuPont retained ownershipof the western plant area, currently designated the DuPont Holly Run Plant Area, andcontinued to manufacture chromium dioxide and chromium dioxide based ma'gnetic tape.Since the 1984 property purchase, Ciba-Geigy has exclusively manufactured pigmentproducts, primarily the red pigment Quinacridone (QA). Wastes generated by Ciba-Geigyhave not been disposed of within the boundaries of the DuPont Site.

2.2 REGULATORY COMPLIANCE

DuPont was required to conduct a Remedial Investigation/Feasibility Study(RI/FS) for the DuPont Site in accordance with an Administrative Order by Consent issuedby the United States Environmental Protection Agency (USEPA), Region III, on July 22,1988 (Docket No. III-DC-88-6). The RI/FS was implemented at the DuPont Site toevaluate impacts to human health and the environment associated with current and historicoperations conducted.

To meet the requirements established by the consent order, DuPont retainedWoodward-Clyde Consultants (WCC) in 1987 to develop and implement an RI/FS WorkPlan for the SITE. WCC prepared an RI/FS Work Plan for the SITE in July 1987, andsince that time has conducted several multi-phased investigations to evaluate SITEconditions and identify constituents of concern related to historic operations. WCC hasimplemented and reported the results of three separate phases of the RI for the DuPontSite that has included soil sampling and analysis, ground water sampling and analysis,geophysical surveys, and several phases of wetlands and ecological investigations.

The RI/FS Work Plan was prepared to provide an analytical databasesufficient in quality and quantity to establish baseline risk concentrations for human,ecological, and environmental receptors of SITE constituents of concern. Based on theresults of the RI, WCC developed two separate risk assessments for human health andenvironmental and ecological receptors. The results of the Human Health Evaluation andthe Ecological Evaluation were presented under separate cover. The risk-basedconcentrations for constituents of concern determined in the risk assessment and the resultsof the RI were utilized to prepare the Focused Feasibility Study (FFS).

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2.3 CIBA-GEIGY INVOLVEMENT

Ciba-Geigy retained Dames & Moore to provide environmental consultingservices that consisted of evaluating the potential ramifications to Ciba-Geigy of the findingsand recommendations made as a result of the RI/FS process at the DuPont Site. Dames& Moore's evaluation was based on a review of the following WCC reports:

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Remedial Investigation Reports

• Work Plan (Phase II), Remedial Investigation/Feasibility Study, DuPont-Newport Site, Volumes I, II, III, July 28, 1988

• Phase HI Remedial Investigation, Data Sufficiency Report, DuPont-NewportSite, Volumes 1 of 4 and 3 of 4, May 17, 1991

• Draft Remedial Investigation Report, DuPont-Newport Site, Volume 1 of 2,May 11, 1992

• Final Remedial Investigation Report, DuPont-Newport Site, Volumes 1 of 2and 2 of 2, May 28, 1992

Risk Assessment Reports

• Draft Risk Assessment, Human Health Evaluation, October 14, 1991

• Draft Risk Assessment, Environmental Evaluation, DuPont-Newport Site,June 11, 1991

• Final Risk Assessment, Human Health Evaluation, DuPont-Newport Site,March 12, 1992

• Final Risk Assessment, Environmental Evaluation, DuPont-Newport Site,April 29, 1992

Feasibility Study

• Draft Focused Feasibility Study, DuPonii-Newport Site, December 23, 1991i

• Preliminary Draft Focused Feasibility Study, DuPont-Newport Site, May 1,1992

• Final Focused Feasibility Study, DuPont-Newport Site, Volumes 1 of 2 and2 of 2, May 28, 1992

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Correspondences

• USEPA comment letter, February 11, 1992

• DuPont response letter to USEPA comments, February 24, 1992

During a project meeting to discuss Dames & Moore's preliminary findings,representatives of Ciba-Geigy indicated that USEPA, Region III, requested an evaluationof the potential technical and economic ramifications to Ciba-Geigy associated with theconstruction and maintenance of a physical vertical barrier. Based on this request, Ciba-Geigy directed Dames & Moore to assess the potential engineering, operational, andeconomic impacts to Ciba-Geigy related to the construction of a physical vertical barrier.

3.0 CIRCUMSCRIBING VERTICAL BARRIER CONSTRUCTIONRELATED IMPACTS

The impacts associated with the construction of two types of physical verticalbarriers were assessed by Dames & Moore. The two types of barriers considered for thisassessment were a circumscribing physical barrier placed near the boundaries of the DuPontSite and a riverbank cover system placed atop the north bank of the Christina River. Eachof these physical barriers would be accompanied by a complete plant area cover, whichconsists of asphaltic covering to reduce infiltration in plant areas that are currently exposed.The details and specifications related to the development and evaluation of each of thevertical barrier remedial alternatives are presented in Chapter 4.0 of the WCC FocusedFeasibility Study (FFS) report for the SITE. The circumscribing vertical barrier and theriverbank cover system are designated Alternative PA-6 and Alternative PA-3, respectively,in the FFS.

Impacts associated with the circumscribing physical barrier are presented inSection 3.1. An assessment of the impacts related to the construction of a riverbank coversystem is provided in Section 3.2.

3.1 CIRCUMSCRIBING VERTICAL BARRIER

The construction and maintenance of a circumscribing physical vertical barrierwould have substantial engineering, operational, and financial impacts to manufacturingoperations at the Ciba-Geigy Newport plant. Engineering impacts are related to theconstruction and physical constraints of the vertical barrier. Operational and financialimpacts relate to the interruption of normal plant manufacturing processes and the costsassociated with the reduced production capacity resulting from these interruptions. Otherfinancial ramifications pertaining to the restructuring of underground process and utilitylines must also be considered.

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The impacts associated with the construction and maintenance of acircumscribing physical vertical barrier are based on a review of the remedial alternativespresented in the WCC FFS and a visit and walkthrough of the Ciba-Geigy Plant Areaconducted by Dames & Moore personnel on August 7, 1992. During the visit, Dames &Moore representatives observed the Ciba-Geigy plant area with regard to the proposedorientation and location of the circumscribing barrier, as proposed in the FFS. Table 1provides a summary of specific areas that require engineering and constructionconsiderations related to the installation of a circumscribing physical vertical barrier.Engineering and construction impacts associated with the installation of this type of aphysical barrier are discussed in subsection 3.1.1. Operational and financial impacts aredetailed in subsections 3.1.2 and 3.1.3, respectively.

3.1.1 Engineering Considerations

There are significant engineering constraints and impacts related to theconstruction and long-term maintenance and performance of a circumscribing physicalvertical barrier in the Ciba-Geigy Plant Area. The proposed circumscribing physical verticalbarrier would consist of a combination of three different kinds of vertical barriers due toSITE constraints. A Deep Soil Mixing (DSM) cutoff wall would be constructed along aportion of the north bank of the Christina River, a permanent steel sheet pile wall extendingfrom the vicinity of Building A47 to the James Street Bridge would be placed along thenorth bank of the river at the east end of the Ciba-Gei.gy Plant Area, the DSM cutoff wallwould resume along the east and north boundaries of the Ciba-Geigy Plant Area, and theconstruction of a slurry trench cutoff wall is proposed to circumscribe the west portion ofthe Ciba-Geigy Plant Area. The sheet pile wall is proposed along the eastern portion of theChristina River north bank due to proximity of Building A47 to the river which inhibits theplacement of a DSM cutoff wall there. The proposed locations for the DSM cutoff wallwere based on the limited availability of space, a greater amount of which is needed toconstruct the slurry tretich cutoff wall, and access restrictions adjacent to the railroad tracksand James Street. Figiire 2 of this report, which is a photocopy of Figure 4-14 provided inthe WCC FFS report, presents the proposed orientation, location, and extent of each typeof vertical barrier at the SITE.

3.1.1.1 Permanent Steel Sheet Pile

A proposed steel sheet pile would be installed into the river bank in thevicinity of Building A47 of the Ciba-Geigy Plant Area. The sheet piles would be driventhrough the fill deposits and Columbia formation, until i;hey have penetrated the upper semi-confining unit of the Potomac formation. As detailed in the FFS, several engineeringproblems associated with sheet pile walls commonly occur, including installation problemssuch as breaking or ripping of the sheet pile interlocks by natural and artificial fill andalluvium obstructions ;and loss of verticality, low efficiency (high permeability) at theinterlocks, and corrosion of the sheet pile walls. A gujhJh J taUjitipn of the sheet pilewall would reduce the rate of intertidal zone ground water discharge to the river, it would*

* * «v r~*- fV = - -Tuer.-* r-"T jiPS3g gg i*:*Ctii'gg--i-*- -'- *-,- _ wnof.inhibit discharges completely in this area of the SITE.* A P ? I 7 7 ft I

Installation of the permanent sheet pile wall would be performed using riverbarges that would house crawler cranes with pile driving heads. The majority of theinstallation procedure could be accomplished from the river barges. However, it is expectedthat material excavation would be required due to obstructions at the ground surface, whichwould most likely have to be conducted from in-land portions of the Ciba-Geigy Plant Area.In addition, the delivery of construction vehicles and supply materials would have to be aland-based operation. These two factors would require a working construction area andplatform that would disrupt Ciba-Geigy operations in the vicinity of Building A47.

The proposed area for the sheet pile wall contains various obstacles that mayimpede construction traffic during installation of the barrier. A fence is located at thesouthwest comer of Building A47 which would need to be removed to provide access forconstruction traffic and supply materials and provide sufficient room for building aconstruction platform. A natural gas line oriented adjacent to the perimeter of the outsidewall of Building A47 would also require relocation. Overhead power lines present west ofthe building may also need to be rerouted to accommodate sufficient room for the operationof heavy construction vehicles.

The permanent steel sheet pile wall would prevent the flow of a portion ofdischarged water from National Pollution Discharge Elimination System (NPDES) permittedoutfalls 008 and 009. Although discharge from outfall 009 would be affected by the sheetpile wall, the outfall orientation will most likely be realigned prior to the sheet pile wall dueto the installation of the DSM cutoff wall that would also intersect the pipeline for outfall009 along the northern boundary for the Ciba-Geigy Plant Area. As previously mentioned,the anticipated high permeability of the sheet pile wall would allow the majority of thedischarges to enter the Christina River. However, if SITE conditions warrant, these outfallsmay have to be temporarily realigned during construction. If realignment is not feasible,design specifications will need to be developed for modification of the current system. Themodification may include temporary containment of discharges from these outfalls into aholding vessel, the contents of which would be pumped into the river. This procedure mayneed to be instituted for those other areas of the SITE where the DSM and slurry trenchcutoff walls intersect permitted outfalls.

3.1.1.2 Deep Soil Mixing Cutoff Wall

The circumscribing vertical barrier evaluated in the FFS for the DuPont Siteindicates that a DSM cutoff wall is the preferred barrier type along the north bank of theChristina River (with the exception of the sheet pile wall area) and the eastern and northernproperty boundaries. The DSM barrier is emplaced by advancing a series of vertical shaftsto the desired depth below ground surface and injecting a grout mixture into the hollow-stems of the mixing shafts. The vertical shaft columns are continuously overlapped toensure a proper seal. The DSM cutoff wall would be installed to an average depth ofapproximately 35 feet below ground surface, keyed into the top of the Potomac formationdeposits. Installation of the vertical barrier to this depth will effectively inhibit migrationof shallow aquifer ground water to the Christina River along the southern pqrtjpiKof lha QCiba-Geigy Plant Area. In addition, the DSM barrier will isolate the SITE,

shallow ground water, by reducing the volume of ground water entering the shallow aquiferfrom the east and north.

Construction of the DSM vertical barrier is a land-based operation thatrequires the use of lafge construction vehicles and a substantial amount of materials.Although the DSM cutoff wall is proposed to be situated approximately 10 feet from theedge of the sloped barjk of the Christina River, the construction of working platforms atvarious locations may be required to support the weight and volume of constructionequipment. \

Installation of the DSM cutoff wall at the proposed areas will necessitate thererouting of numerous overhead and underground utilities and pipelines, several of whichare critical to daily manufacturing operations at Ciba-Geigy. The intrusive nature of DSMcutoff wall installations will require the realignment or modification of five NPDES- tpermitted outfall discharge lines, relocation of water lines that supply river water to the fire ?water reservoirs, realignment or modification of sanitary and process sewer lines, relocationof nitrogen feed lines, located in an overhead metal barricade oriented parallel to the river,relocation of numerous overhead power lines, and realignment of railroad tracks enteringthe Ciba-Geigy Plant Area at the northern property perimeter. Additionally, disruptions andrealignments of unknown magnitude for the area referred to as the "Lift Station" and theaboveground tank farm adjacent to Building A101 would be required.

Underground Obstructions

Dames & Moore identified five NPDES permitted outfalls that would requirerealignment, reconstruction, or modification during the construction of the DSM verticalbarrier. Each of the invert elevations of the outfall piping is less than the anticipated depthof the barrier. The outfalls observed to be in the prciposed path of the DSM cutoff wallinclude 001, 003, 005, 006, and 009t which was previously discussed in the sheet pile wallsubsection. Outfall 009, the integrity of which is questionable based on a recent videosurvey conducted on behalf of Ciba-Geigy. .Realignment of this outfall must consider theeffects to the City of Newport.

As discussed in the sheet pile wall engineering evaluation, water currentlydischarged from outfalls to the Christina River may be diverted into a central holding vesseland pumped to the river during installation of the bairier. Although this may be suitableas a temporary measure during installation of the DSM cutoff wall, modification of outfalldischarge pipelines directly to the river would need to be addressed after installation of thebarrier. This could be accomplished by building a special containment to span the widthof the barrier before the bentonite injection mixture hardens completely and reinstalling theoutfall discharge pipe; However, this construction may provide a conduit for seepagethrough the wall if a proper seal is not secured around the discharge pipe. Note that Ciba-Geigy recently installed a relatively impermeable, sealed liner within the discharge pipelinefor outfall 001, through the process of "Insituform", at a cost of approximately $1,000,000.

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In addition to the NPDES permitted oujfall pipelines, several undergroundwater and process lines would require relocation during installation of the DSM cutoff wall.The orientation of three underground water lines "mat deliver water to the fire reservoirsfrom the Christina River intersects the proposed location for the DSM barrier. Anunderground sewer line and a wastewater line, situated in the vicinity of Buildings A22 andA52, respectively, would also require relocation. Consideration would also need to be givento a 6-inch diameter watermain that bisects the proposed path of the DSM vertical barrieradjacent to Building A17, near the north property boundary. .——

Overhead Obstructions

The use of heavy construction equipment to install the DSM cutoff wallvertical barrier requires overhead clearance to effectively maneuver machinery. DuringDames & Moore's reconnaissance of the Ciba-Geigy Plant Area, several overheadobstructions were observed that would most likely inhibit equipment mobility. Therefore,relocation or realignment of overhead utility and process lines would most likely be requiredprior to construction operations.

Overhead electric power lines and process lines are oriented adjacent to andparallel with the Christina River at several locations at the south boundary of the Ciba-Geigy Plant Area. Overhead electrical power lines and several telephone poles are situatedalong the riverbank near outfall 001. Power lines continue parallel to the river from outfall001 to outfalls 003 and 005. Powerline relocation would most likely be required in thevicinity of outfall 005, as well as the area adjacent to Building A22 and the wasteneutralization area. Several overhead power line obstructions to heavy equipment operationwere also observed adjacent to the fence line that separates the Ciba-Geigy Plant Area fromthe Amtrak railroad right-of-way at the north property boundary. In addition to theoverhead electrical utility constraints to DSM construction, a nitrogen feed line in thevicinity of outfall 005 would require relocation.

Table 1 provides a summary of the structures that may require relocation,realignment, or repktg y pstiniated costs associated with the modification of these*structures are presented in the table also."

3.1.1.3 Slurry Trench Cutoff Wall

A slurry trench cutoff wall is proposed as the vertical barrier type to beinstalled as the western perimeter of the circumscribing barrier. The western perimeter ofthe barrier is defined on the basis of soil borings and shallow ground water monitoring welldata. The proposed alignment of the slurry wall intersects only a limited portion of theCiba-Geigy Plant Area. The slurry trench cutoff wall extends into the DuPont Holly RunPlant Area and continues west and south, surrounding the North Disposal Site.

Due to the limited extent of the slurry wall in relation to the Ciba-Geigy PlantArea, engineering considerations are not as extensive as those for the permanent steel sheetpile or the DSM cutoff wall. The only considerations for the slurry wall constr*c o« kirfiwtathe modification of existing water lines situated beneath Water Street and thereidcaiion'oP

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overhead steam lines that cross Water Street, oriented north-south. Although overheadelectrical power lines adjacent to the Amtrak railroad right-of-way would probably requirerealignment, this would be addressed during installation of the DSM cutoff wall at thenorthern SITE boundary.

3.1.2 Operational Considerations

Installation and construction of a circumscribing physical vertical barrier at theCiba-Geigy Plant Area would have significant impacts to the operations at Ciba-Geigy theHolly Run Plant Area. The volume of construction equipment and supplies necessary toimplement this type of remedial construction would most likely inhibit daily traffic flow inand out of the Ciba-Geigy and DuPont manufacturing plants, which share a commonentrance. As the construction of the barrier progressed around the perimeter of the Ciba-Geigy Plant Area, roadway and building access would be limited in the immediate vicinityof construction. Access limitations would also be associated with the realignment,modification, or reconstruction of the various underground and overhead obstructions. Theeffect of the limited access and increased traffic capacity could be significant.

Installation of the DSM cutoff wall adjacent to James Street, along the eastboundary for the Ciba-Geigy Plant Area, may impact Local traffic. Any substantial impactto local traffic patterns on James Street may effect surrounding business operations situatedin several small industrial and commercial complexes in the SITE vicinity. In addition, theinstallation of the DSM vertical barrier across the entrance to the Ciba-Geigy and DuPontmanufacturing plants could substajjlialj effect options by limiting access to the plantareas. Although there are two separate entrance roadways to the plant areas, they are

~" "B1i B.L. iSls?jf- T~ "-V •"£**!•»• -, CM-- .proximally situated and access to and from the plant areas may be severely restricted duringconstruction activities adjacent to James Street.

Another factor to consider when evaluating the potential operationalramifications related to the construction of a circumscribing vertical barrier is the potentialneed to implement shut-down procedures for specific plant areas when relocating ormodifying existing underground and overhead obstructions. The realignment of overheadpower lines may require the temporary termination of electrical service to various plantbuildings, which may result in substantial loss of production and revenue. Loss ofproduction capacity may also occur if the rerouting of permitted outfall discharge pipesnecessitates that production be reduced or halted in those areas that generate water.Similar situations would occur for the interruption of process lines, water supply lines,and/or sewer lines.

3.1.3 Economic Considerations

Table 1 provides a summary of specific structures and areas identified withinthe Ciba-Geigy Plant Area that require special construction and consideration prior to theinitiation of construction related to the installation of the circumscribing vertical barrierremedial system. As previously indicated, the identification of these items is based onDames & Moore's reconnaissance of the Ciba-Geigy Plant Area on August 7, 1992 icb oobserve the proposed locations of the barrier. Costs related to the specialized constructiorr*

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are also estimated in Table 1. As the table indicates, Dames & Moore estimates tnat costsrelated to special construction prior to vertical barrier installation total approximately$3,000,000.

The loss of revenue related to reduced production capacity, if any, to Ciba-,Geigy cannot be estimated by Dames & Moore. However, based on the informationpresented in this evaluation, Ciba-Geigy can most likely provide a conservative estimate ofpotential revenue ramifications related to the special construction necessitated byinstallation of the circumscribing vertical barrier.

3.2 RIVERBANK COVER SYSTEM

Although there are engineering, operational, and financial impacts associatedwith the construction and maintenance of a riverbank cover system, they are not asnumerous or as costly as those related to the installation of a circumscribing vertical barrier.Because the riverbank cover system, consisting of a concrete revetment mattress placed atopthe north bank of the Christina River, requires no intrusive construction, the magnitude ofthe impacts is reduced. The non-intrusive construction procedures for the riverbank coversystem reduce the engineering, operational, and economic ramifications to Ciba-Geigyassociated with the restructuring of underground process and utility lines.

3.2.1 Engineering Considerations

Engineering considerations associated with the riverbank cover systemconstruction are similar to those items discussed for the circumscribing vertical barrier. Thepotential rerouting and modification of overhead power lines, the construction of workingplatforms along the riverbank, and the temporary relocation or modification of NPDESpermitted outfalls would be required. However, because this type of remedial design doesnot involve the hydrologic isolation of the Ciba-Geigy Plant Area and a circumscribingperimeter barrier to ground water migration, the magnitude of the impacts is restricted toone small area at the Ciba-Geigy Plant Area. The NPDES outfall discharge pipe may berealigned through the revetment mattress during construction.

3.2.2 Operational Considerations

Impacts to Ciba-Geigy's daily operations caused by the installation of ariverbank cover system are greatly reduced when evaluated in comparison to the installationof a circumscribing physical vertical barrier. Disruptions to plant operations would occuralong the south property boundary for the Ciba-Geigy Plant Area only, adjacent to theChristina River. Temporary termination of power, process, and other utility lines mayimpact operations on a short-term basis. Construction and material supply traffic woulcrbedirected to the south perimeter of the Ciba-Geigy Plant Area, potentially disruptingoperations there. However, the entrance to Ciba-Geigy and DuPont's Holly Runmanufacturing operations would not be greatly affected by the construction of the riverbankcover system, and the potential impact to local traffic patterns and commercial businessesis minimal. &R3I7786

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3.2.3 Economic Considerations

Because the impact to normal daily operations would not be as significant fora riverbank cover system as compared to the circumscribing barrier, the associated economicimpact is not as substantial. The non-intrusive constniction procedures and limited extentrelated to installation of the concrete revetment mattress reduce the costs for specializedconstruction. Additionally, the construction of the riverbank cover system affects only alimited portion of the Ciba-Geigy Plant Area; therefore, the potential for disruptions tomanufacturing operations at the SITE are reduced, and the economic impacts are limited.The financial impact to Ciba-Geigy associated with the installation of the riverbank coversystem is substantially reduced when compared to the circumscribing vertical barrier.

4.0 HYDROLOGIC IMPACT OF PROPOSED REMEDIAL SYSTEMS

Dames & Moore was requested to evaluate the potential impact to hydrologicconditions related to the installation of two types of physical vertical barriers at the Ciba-Geigy Plant Area. The two types of physical vertical barriers evaluated include thecircumscribing vertical barrier and a riverbank cover system. As previously indicated in thisreport, the development and evaluation of each of these remedial alternatives for theDuPont Site were presented as Alternatives PA-6 (circumscribing vertical barrier) andAlternative PA-3 (riverbank cover system) in Chapter 4.0 of the FFS. The materialreviewed to conduct this evaluation included the results and conclusions presented in theWCC reports detailing the various phase of the RI conducted at the SITE, and theconstruction and technical specifications for the circumscribing barrier and concreterevetment mattress (riverbank cover system), provided in the final FFS, dated May 28,1992.The impact to current hydrologic conditions at the Ciba-Geigy Plant Area resulting from theconstruction of a circumscribing vertical barrier and a riverbank cover system is discuss inSections 4.1 and 4.2, respectively.

4.1 CIRCUMSCRIBING VERTICAL BARRIER

The proposed circumscribing vertical barrier is intended to isolate the DuPontSite, including the Ciba-Geigy Plant Area, as a ground water management zone. Thebarrier would effectively prevent the lateral migration (horizontal transport) of the shallowaquifer ground water from the fill and Columbia formation at the SITE from dischargingdirectly to the Christina River.

Although the construction of a circumscribing vertical barrier that is keyedinto the Potomac forajation would prevent the discharge of shallow aquifer ground waterto the river, potential adverse effects associated with this type of a remedial system include:

• An increase of the ground water table elevation within the boundary of thebarrier.

j• An increase in the ground water table elevation upgradient (north) of the

north boundary of the barrier. A R 3 I 7 7 8

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• An increase in the area of the SITE where a downward vertical hydraulicgradient from the water table aquifer to the underlying, semi-confined deeperaquifers.

An increased downward vertical hydraulic gradient would result in theincreased potential for contaminated ground water in the shallow aquifer to migrate to theunderlying semi-confined aquifers of the Potomac formation. The potential for this type ofinteraction between the shallow and deeper water-bearing hydrostratigraphic units at theSITE is probable where the semi-confining units of the Potomac formation are either verythin or absent, as shown in stratigraphic cross-section H-H' (Figure 1-3 of the FFS). Thiscross-section indicates that the upper semi-confining unit of the Potomac formation(designated Hydrostatigraphic Unit II) is discontinuous across the Ciba-Geigy Plant Area,as evidenced by the thinning and possible wedge out to the north. Thus, this semi-confiningunit may be absent in areas under the north portion of the Ciba-Geigy Plant Area.

An increased water table elevation created by the circumscribing verticalbarrier would cause swamp-like conditions to occur both within the boundaries of thebarrier and upgradient (north) of the barrier. Assuming an average annual recharge rateof approximately 1 foot and specific yield of approximately 0.10 in the water table aquifer(values common for the Columbia formation), the installation of a circumscribing verticalbarrier at the DuPont Site would result in an increase in the elevation of the water tableaquifer of approximately 10 feet within the boundaries of barriers. However, in conjunctionwith the circumscribing vertical barrier, an asphaltic plant cover is proposed to be placedacross the entire Ciba-Geigy Plant Area. This would effectively reduce the volume ofprecipitation infiltration water that enters the ground, thereby reducing the degree of groundwater table elevation increase.

Assuming an estimated area of 1 million square feet within the confines of thecircumscribing vertical barrier and an annual rainfall recharge rate of 1 foot, the averagerate of recharge within the barrier would be about 15 gallons per minute (gpm). Becauscgof this, approximately 15 gpm would need to be extracted from the shallow aquifer withinthe area surrounded by the vertical barrier on a constant basis to prevent an increase in thewater table elevation and to minimize potential downward migration of water from theshallow aquifer to the deeper water-bearing units. This value does not account forreductions jaiBfiltration r%tes resulting after the completion of a plant area cover (asphalt).The estimated area of water table elevation increase on the upgradient side of thecircumscribing vertical barrier is indicated in the FFS report (page ES-17) to beapproximately 45 acres (2 million square feet). Assuming that this area is correct, rainfallrecharge for this area would be approximately .30 gpm. Therefore, ground water would haveto be continously extracted in this area at a constant rate of approximately 30 gpm tomaintain the static water table at an elevation similar to that which currently exists for thisarea. This would require the development and installation of a continously operatingground water extraction system.

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The placement of hydraulic controls within the circumscribing vertical barrierwould help to minimize the potential impacts related to the structural integrity of building •foundations at the DuPpnt Site. Without hydraulic control, the estimated increase in groundwater table elevations (approximately 10 feet) would increase the pore water pressure within |the soil matrix. This increase may reduce the effective stress in the soil matrix, and portionsof building loads may be supported in part by pore water. The potential instability of plant *buildings and structure related to an increase in the ground water table elevation within (the barrier may require stability shoring for building foundation support. Additionally,buildings A17, A47, and A60, which have basements below ground surface, may experienceflooding. /

Although the buildings with basements, would most likely be affected bychanges in the ground water table elevation, the structural stability of the remainingbuildings and structures at the Ciba-Geigy Plant Area and surrounding vicinity may also beaffected. However, as-built construction specification drawings were not reviewed by Dames& Moore, and therefore, an accurate assessment cannot be made regarding the affect ofwater level fluctuations on structural stability.

The operational ramifications associated \vith the potential structural instabilityof buildings and structures at the Ciba-Geigy Plant .Area related to ground water tableelevation increases cannot be ascertained at this time. The potential for flooding inBuildings A17, A47, and A60 exist if water level increaises occur. Impacts to Building A47,the on-site laboratory, may be substantial due to the delicate laboratory instrumentation andchemical storage.

A reduction in the existing ground water table elevation created by hydrauliccontrols may potentially cause "Soil dewatering and compaction, especially for fine-grainedsoil. The compaction of soil may result in differential settlement of overlying structures,potentially resulting irj structural damage or instability. The degree and extent of thedifferential settlement is dependent on the water table elevation reduction, the nature of theunderlying soils that re being dewatered, and structural foundation construction. Toprovide a quantitative estimate of the extent of soil compaction that may result from adecreased water table, the reduction in the water table elevation, the structural constructionspecifications, and analytical soil consolidation data for potentially affected soil is required.

4.2 RIVERBANK COVER SYSTEM

4.2.1 Proposed Designj

The FFS prepared for the DuPont Site included an evaluation of a remedialsystem that consists of the placement of a concrete revetment mattress atop the north bankof the Christina River, extending from the North Disposal Site to the Ciba-Geigy PlantArea. The proposed design was evaluated as a means of preventing the discharge ofintertidal zone ground water and a portion of the shallow aquifer directly to the ChristinaRiver. ' ~i - . ; /

ftR3!7789

Although the theoretical basis for the riverbank cover system is valid, potentialphysical constraints and effects related to the construction of a concrete revetment mattressinclude:

• An increase of the water table elevation of the shallow aquifer north of thebarrier

• An increased downward vertical hydraulic gradient from the shallow watertable aquifer to the deeper aquifers of the Potomac formation, increasing thepotential for constituents present in the shallow aquifer to migrate to thedeeper water-bearing zones.

• The potential fracturing of the concrete riverbank cover system caused by thebuildup of soil water pore pressure behind the barrier.

Because of the potential for structural failure of such a barrier, this system ofcontrolling the contaminated ground water flow is not recommended.

4.2.2 Alternative Design

To minimize the potential physical effects related to the construction of ariverbank cover system, modifications to the proposed design can be instituted. Analternative design for the concrete revetment mattress involves the inclusion of an internalpermeable drain to prevent the buildup of soil water pore pressure behind the barrier. Thepermeable internal drain would be constructed to prevent the buildup of soil water porepressure on the downstream slope of the earth fill embankment, similar to the drains placedin earth fill dams. In addition to reducing soil water pore pressure buildup, the internaldrain would provide a pathway for shallow ground water to migrate downward, beneath thetoe of the revetment mattress where it could discharge directly into the sediment on thebase of the river. Sediment on the base of the river is proposed to be isolated by theplacement of a separate concrete revetment mattress atop the base of the area of theChristina River that bisects the DuPont Site, as detailed in the FFS review of remedialalternatives for the Christina River. Isolation of the river bottom sediments effectivelyreduces the potential for sediment migration. Therefore, shallow ground water dischargesdirected beneath the river revetment mattress poses no threat to ecological or environmentalreceptors.

5.0 CONCLUSIONS

The number and magnitude of engineering and operational constraints andconsiderations associated with the installation of a circumscribing vertical barrier at theDuPont-Newport Superfund Site would substantially impact Ciba-Geigy operations andphysical conditions. In addition to engineering and operational considerations, the potentialeconomic impact to Ciba-Geigy related to vertical barrier construction could be excessive.Due to the construction, physical, and operational constraints that would be experiencedduring barrier installation, the installation of a circumscribing physical vertical barrietijLnpt, 7 Q «

14

recommended for the DuPont Site. In addition to the above indicated considerations, theincreased ground water table elevations in the shallow aquifer at the SITE would requirethe development and installation of a ground water extraction system that would have tooperate continuously. An increase in the water table elevation could have ramificationsassociated with structural stability and flooding at the Ciba-Geigy Plant Area. In addition,the potential reduction in the ground water table elevation related to the operation ofhydraulic controls may impact structural stability at the Ciba-Geigy Plant Area as well.

The recornmended alternative for the control of shallow aquifer and intertidalground water discharges to the Christina River is the construction of a riverbank coversystem, consisting of a modified concrete revetment mattress with an internal permeabledrain system. The construction of this system has significantly less potential adverse effectsto operational and physical conditions at the Ciba-Geigy Plant Area.

This report was prepared by:

DAMES & MOORE, INC.

Ralph T. GoliaManaging Associate, Geoscience

AAW02A42

AAW02A42

Mark Piaz2:aProject Hydrogeologist

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