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EXPLANATION OF SIGNIFICANT DIFFERF.NCES

Site Name and Location

Bayou Bonfouca St. Tanmany Parish Slidell, Louisiana

Statement of Puroose

This document presents a sunmary of si 9ni fi cant di fferenc:es betwt::?en the remedial action identified in the oroject's Record of Decision and the remedial action selected as a result of recent design investigations. Associated with this is a sunmary of the information on which the differences in the remedial action are based, along with a statement by the Louisiana Oeoartment of Environmental Quality and its concurrence with these differences. The following documents were reviewed for these decisions:

- Record of Decision, EPA, March 11, 1987, and its Administrative Record - Sunmary of Design Investigations, CH2M HILL, December 1988 - Final Suoplemental Geotechnical Report, CH2M HILL, March 1989 - Pilot Testing Findings/Design Recommendation Reoort, CH2M HILL,, Aoril 1989 - Supolemental Sediment Exoloration, CH2M HILL, ,June 1989 - Review of the Bayou Bonfouca Superfund Site Remedial Design, COM, ,June 1989

Record of Decision and Significant Differences

On March '.H, 1987, a Record of Decision 'ROD) was signed for the Bayou Bonfouca site. The selected remedy included:

- Excavation of contaminated bayou sediments and surface waste oiles - Onsite incineration of waste oi1es and contaminated sediments - Placement of an engineered cao over residues from the incinerator and

residual surface soils. - Pumo/treatment/reiniection of contaminated around water. - Estimated construction cost of approximately ~55 million.

During analysis of design investigations, the following new information was obtained:

- The length of bayou which is contaminated was found to be 4,000 feet rather than ?.,000 feet as indicated in the ROD.

- The bayou contamination extends to a maximum deoth of about 17 feet rather than 5 feet as stated in the ROD.

- The total volume of contaminated sediments is aoproximately 150,000 cubic yards rather than 46,~00 cubic yards as stated in the ROD.

- The contaminated sediments near the creosote olant are in direct contact with the materials of the shallow artesian aquifer.

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- The contaminated ground water is found in 3 distinct plumes rather than 1 ccmt i nuous p 1 ume as presented in the ROD.

- Reinjection of treated ground water into the shallow artesian aquifer is not considered effective because of geological propert·fes of the aquifer.

- Revised estimated construction cost of $100 million (Source $90 million and Ground Water i10 mil 1 ion) vs. $55 mil 1 ion estimated co:st in the ROD.

Decisions Concernina Sianificant Differences

After review of the aforementioned information the follow'ing decisions have been made:

- The remedy selected in the 1987 Record of Decision is still the most appropriate means of orotectina human health and the environment.

- Dredging of the bayou ~ill require consideration of stable slopes and oossibly some bulkheads to maintain existing land surfaces instead of placing sheetpiles in the middle of the bayou. This will necessitate leaving minimal volumes of contaminants after dredging.

- Dredging can best be achieved by using turbidity curtains around the excavation process with silt curtains and absorbent booms olaced along the bayou rather than sheetpiling the middle of the bayou and dewatering half of it during excavation operations.

- All dredaed areas will be backfilled with clean materials to minimize the chances of contact with residual contaminants.

- The increased volume of contaminated sediments will require an increase in the height and areal extent of the cap.

- The contaminated ground water plumes will be considered as 3 separate areas instead of 1 continuous plume. The?. onsite plumes will be remediated as one ooerable unit.

- The contaminated ground water plume adjacent to the bayou in the residential area (off-site) will be addressed after dredging of the bayou.

Statutory Determination

The results from these evaluations have shown that althou<Jh the cost for implementing the remedy outlined in the ROD has increased-from approximately ~55 million to about $100 million it remains the most environmentally orotective and appropriate action. Therefore, in accordance with the requirements of Section 117(c) of the Comprehensive Environmental Response, Compensation and Liability Act of 1980 as amended by the Superfund Amendments and Reauthorization Act of 1986, this document presents an Explanation of Significant Differences. These findings have also included considerations of the National Contingency Plan and in accordance with that plan, this document will become part of the Administrative Record.

Ro er • Lay Regional Administrator

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INTRODUCTION

On March 31, 1987, a Record of Decision (ROD) for the Bayou Bonfouca Superfund site was signed. This ROD consisted of the following remedy:

1) Pump/treatment/reinjection of contaminated ground watel"' to the lowest level of contamination that is technologically achievable,

2) Excavation of 46,500 cubic yards of contaminated bayou sedimen·ts that have oolynuclear aromatic hydrocarbon {PNA) concentrat1lons in excess of 1300 ppm or to a depth which minimizes threats to aquatic biota,

3} Incineration (thermal treatment) of a total volume of fil,500 cubic yards of contaminated sediments and waste piles in an onsite treatment unit, and

4) Placement of a Resource Conservation and Recovery Act (RCRA) cap over the incinerator residue and surface soils with total PNA concentrations greater than 100 ppm.

Detailed design investigations during the su11111er and fall of 1988 showed the volume of contaminated sediments to be approximately 150,000 cubic yards, an increase of three times that estimated in the ROD. This dramatic volume increase resulted in a cost estimate for the selected remedy rising from approximately $55 million to about $150 million. Since these two factors were significantly different than the information upon which the original decision was based, the Environmental Protection Agency (EPA) decided in August 1988 to:

1) Divide the remedial operation into two operable units to allow resoonse to the contaminated ground water while the extent of contaminat•ion within the bayou was further quantified. Since then the project has bP~n defined as separate units, ground water and source control (sediments, wastepiles and surface soils).

2) Conduct an independent analysis by qualified engineers of the conceotual construction approach to indentify areas of possible cost savings (Value -Engineering (V-E) study).

This document sunmarizes these activities and represents an Explanation of Significant Differences. The EPA, in conjunction with the Louisiana Department of Environmental Quality (LDEQ), has determined the selected remedy outlined in the March 31, 1987, ROD remains the most appropriate means to protect human health and the environment (Attachment A). !=or information relating to the history of the site and the alt1~rnatives evaluated prior to choosing the original remedy, olease refer to the ROD.

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PHASE I - DESIGN INVESTIGATIONS

During the sutmler of 1988, design investigations were conducted to better define the limits of work and enaineerina orooerties of contaminated material related to the Bayou Bonfouca Suoerfund Site. This information was required to obtain more data related to the specific remedy of incinerat'ion and around water treatment for the oreparation of plans and soecifications. in addition, these studies were· also undertaken to address public concerns over the extent of contamination as discussed in previous oublic meetings and workshoos. The major activities included sediment investigations, geotechnical exoloration, 9round water studies and wastewater treatment.

Durina these investiaations it was discovered that the horizontal and vertical extent of contamination within the bayou were greater than assumed from previous information. Several of the borinas near the creosote olant revealed that the upoer clay (cohesive) layer, as shown on Figure 1, was not oresent as indicated by previous investi9ations. This clay layer was thought to be at a maximum deoth of 5 feet beneath the top of sediments, and was oresumed to act as a barrier aaainst the vertical miaration of contaminants. The reason for this error aooears to lie in the incorrect interoretation of previous borin<J information and sub-bottom surveys within the bayou. Tn addition, previous borings were limited 1n depth because of concerns with oenetrating the uooer clay layer and further contaminating the shallow artesian aquifer as shown in Figure 1. Initial design investigation borings also revealed the horizontal extent of contamination in the downstream direction was greater than anticipated. In response to this, EPA aoproved an additional 10 borings above the originally planned l~i borings. After completion of this work it was decided by EPA that further sampling ooerations within the bayou would be postponed until all the laboratory data was comolete and the changed site conditions evaluated.

While sampling was underway in the bayou, investi9ations were also being conducted on water treatment asoects and to further refine the delineation of contaminated around water within the shallow artesian aauifer (See Figure~). These studies showed that the off-site ground water plume was not as large as initially exoected and that treatment of the contaminated water could be readily achieved with conventional facilities. The investi­gations provided valuable data for characterizing the aquifer, and showed the potentially large benefits to running a small scale oilot study orior to conducting the full scale remedial action.

At the completion of field studies in the summer of 1988, EPA, in consultation with LDEQ, decided the following activities were necessary to proceed with this oroJect.

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Creosote Plant Site

GENERAL CROSS-S~CTION

( not to scale)

-----------------------~Bulkhead Fill Material

Previously assumed/

Upper Cohesive Layer depth of / contamination

I I

Shallow Artesian Aquifer

Lower Cohesive Layer

Residental Area

urface Soil

Upper Cohesive Layer

Shallow Artesian Aquifer

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Deep Artesian Aquifer

BAYOU BONFOUCA

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Original limits of contaminated water - within the shallow artesian aquifer

as presented .in the RCD. 400

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1) All the data from the bayou sampling would need to be compiled and evaluated prior to continuing additional investigations.

2) A ground water pumping/treatment pilot study would be necessary before design completion. EPA decided to conduct this stud_y at the creosote plant {onsite) rather than offsite as called for in the ROD because the majority of the contaminated ground water was onsite.

. .

3) A value engineering study would be conducted on the selected remedy and the conceptual aporoach once the revised sediment vo'lumes were calculated.

PHASE II - DESIGN INVESTIGATIONS

During the winter of 1988 it was decided to implement a ground water pilot study and to conduct additional borings which would be necessary to adequately define the limits of contamination. The drillino oortion of this ohase occurred during December 1988 ~th the pilot stu~y ground ~ater extraction and treatment orocess continuina into March 1989. This second phase of bayou samol i no consisted of 30 bori nos and ?..7 cone oenetrometer tests. These investigations were required to supolement information from Phase I investigations to better define the extent of contamination and to provide the necessary data for clans and soecifications. The results of this activity were not available until APril and May 1989 and were comPiled in a Supolemental Sediment Exploration Technical Memorandum dated June 1989. The report orovided valuable information on the extent of contamination and the stable slopes required for dredging ooerations.

The bayou borings have shown that the volume of contaminated sediments is aooroximately 150,000 cubic yards and extends from just upstream of the site to the Chamale Cove Marina as shown on Fiaure 3. This reach of the bayou is approximately 4,000 feet lon~ with contamination at a maximum depth of about 17 feet as indicated by the generalized cross section on Figure 1. The field and laboratory data have also shown what "safe slopes" would be required for dredging.· Any excavation on slopes greater than what is considered safe could result in the undermining of trees along the bayou resulting in the oossible loss of property and harm to the environment. Therefore, it will be necessary to evaluate the use of bulkheads in those areas where highly contaminated sediments are near the bayou bank. In other areas it will be necessary to leave small quantities of contaminants above the 1300 ppm PNA concentration to reduce chances of unstable excavations. However, EPA and LDEQ have determined that in all dredged areas the bayou would be backfilled with clean material oreventin9 contact with any residual contaminants and oroviding an area for restoration of aquatic life.

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The around water activities durina the Phase II Investigations consisted of borings i'llong the edge of the bayou and onsite pumoing and treatment. The borings along the bayou, in conjunction with orevious borings, have shown the contaminated ground water in the shallow artesiian aquifer to be as defined on Fiaure 2. The onsfte treatment orocess ccinsisted of oil/water separation followed by filtration with sand, oleoohilic material and activated carbon. After a 30 day trial period of this unit it was, determined that the proposed process was an aporopriate means to treat the contaminated ground water. Since that time the ground water design has been completed and a construction contract has been awarded.

Based on the results of both the Phase I and II Desian Investiaations the small contaminated ground water olume in the residential area will not be addressed until after dredaina has been completed. This was decided due to the direct contact between-the sediments in the bayou and the shallow artesian aquifer and that these sediments would need to be remediated first.

VALUE ENGINEERING ANALYSES

Due to the increased volume of contaminated sediments it was decided that an independent analysis of design concepts and their associated costs would be beneficial. To accomolish this, EPA contracted with Camp Dresser and McKee Inc. (COM) to indentify and analyze areas of ootential cost savings. In the evaluation, the review team identified two ootential processes for cost reductions. These were use of a centrifuae for dewaterina sediments and utilizing waste heat from the incinerator for ~dditional -drying of these sediments.

The design concepts oresented in the Summary of Design Investigations in December 1988 showed a conceotual dewaterina orocess consistina of covered drying beds for the basis of the cost estimate. The review team identified that this orocess could be altered to use conventional centrifuges which would require significantly less space than drying beds and save cleanuo costs. The potential savinas from this chanae were estimated at aooroximately $20 million. The second it;m which offered ~ossible reductions in remedial action costs was shown to be the use of waste heat from the incinerator to further dry the material prior to entering the incinerator. This orooosal recommends placing a rotary drum material dryer in the treatment process to provide a lower moisture content material to the incinerator and to caoture any fugitive emissions during this orocess. The estimated savings from using this waste heat was over $20 million. Both of these orooosals are being evaluated further and will be incoroorated in the design to the maximum extent oossible.

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DEr.ISIONS CONCERNING SIGNIFICANT DIFFF.RENCES

It has been determined by EPA and LDEQ that the selected remedy as presented in the March :n, 1987, ROD remains the most effective means to address the changes as discovered through design investigations. The most significant change deals with the increased volume of sediments to be removed and the associated costs of remediation. To resoond to this change additional funds have been requested to ensure that this project may proceed. In addition, there are other minor differences between what was stated in the ROD and what is now known, or how the remedy will be implemented. These differences are detailed in the following pa1ragraohs and are sul'l1Tlarized in Table 1.

lt was stated in the ROD that sediments would be excavated by dr·iving sheet­piles down the middle of the bayou and dewatering half of it while maintain­ing steam flow in the remaining hilf. This was based on contamination being at a maxi mum depth of 5 feet and that it had not penetrate.d into the shallow artesian aquifer. However, design investigations have shown that the upper clay layer is not continuous across the bayou and that contaminants have reached a deoth of about 17 feet. Therefore, the stability of dredged slopes down to these areater depths becomes an imoortant factor alono with the usefullness of sheet oiles. lt has been decided that the most aoorooriate means to address this is throuah elimination of sheet oiles in the middle of the bayou and oerforming dredging with turbidity curtains, silt curtains and absorbent booms along the bayou. These curtains are manufactured from synthetic materials which allow the oassaqe of water but act to orevent the the flow of soil particles. The turbidity curtains would minimize susoension of material durina dredaina activities with the silt curtains and absorbent booms providing s~conda~y ~eans to aid in controlling migration of contaminants.

As noted, the increased deoth of sediment contamination oresents difficulties in slope stability caused by dredging. In order to deal with this the material will be excavated on "safe slopes" which may result in small amounts of contamination being left in place. However, in areas where it is found stable slooes would result in leaving significant volumes of contaminants, bulkheads will be placed and the material removed. It has been determined that all areas to be dredaed will be backfilled with clean material which will orovide a barrier aaainst any contam·fnation that is unable to be removed. This barrier act; to minimize the chances of direct contact and provides an opportunity for aquatic biota to reestablish itself.

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Another item needing clarification is that of contaminated ground water. The ROD identified 3 aquifers, the surficial, shallow artesian, and deep artesian. As presented in the ROD, the deep artesian aquifer is not contaminated and, therefore, not considered in this discussion. The surficial aquifer has been found not to be an actual aquifer in that it only yields water from recent rainfall events and thus does not provide a pumpable zone. The placement of a Resource Conservation and Recovery Act (RCRA) compliant Subtitle C cap over the contaminated surface soils would prevent any rainfall into this zone and as such addresses this area. The remaining aquifer is the shallow artesian in which the contamination was thought to be one continuous plume. This plume was to be remediated through use of a slurry wall and an extraction-treatment-reinjection process. Since the ROD, tht! need for a slurry wal 1 and reinjection have been determined to be unnecessary because of questions concerning the effectiveness of these items and the need to obtain data from full scale operations. In addition, the investigations have shown the shallow artesian aquifer to be contaminated with pockets of creosote in 3 separate plumes as shown on Figure 2. Based on available data, contaminated ground water plumes on-site will be remediated through extraction, on-site treatment and then discharged to the bayou. This process will include a monitoring system to ensure any drawdown of the aquifer does not effect surrounding structures. The need to remediate the off-site plume (residential area) will be addressed after the removal of contaminated sediments. The need for a slurry wall will also be considered at a future date.

The remaining issue is that contamination along the Western Creek and the Eastern Drainage Channel is now estimated at l,400 cubic yards comoared to 2,000 cubic yards presented in the ROD. The location of these waterways is shown on Fiaure 4 and the contamination has been found to be limited to areas within the prooerty boundaries and excavation materials will be incinerated as specified in the ROD.

RISK ASSESSMENT

In addition to the aforementioned issues, EPA also reevaluated the action levels presented in the ROD to ensure the risk assessments reflected the more recent criteria concernina CERCLA cleanups. These analyses have shown that the 1987 ROD action-level of 100 porn total polynuclear-aromatic­hydrocarbons (PNAs) for surface soils is equivalent to apgroximately 9 ppm carcinogenic PNAs. This level presents less than a 3xl0- lifetime increased cancer risk to a oerson residina on the site all their life. In addition, the 1,300 ppm PNA action level for sediments was examined for recreational exposure and fish consumpti2n and found to present a lifetime increased cancer risk of less than lxlo- • Since both of these action levels conform to the acceptable health risk criteria contained in the National Contingency Plan, they were not altered.

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ISSUE AS PRESENTED IN THE RECORD OF DECISION

Length of contaminated bayou 2,000 feet

Maximum depth of contaminated sediments is 5 feet'

Total volume of contaminated sediments in the Western Creek and Eastern Drainage Ditch is

2,000 cubic yards

Total volume of contaminated sediments ·is 46,500 cubic yards

The surficial aquifer is contaminated

The shallow artesian aquifer is contaminated in 1 continuous olume

The bayou will be divided in half for excavation and dewatering

No plans for placing clean backfill in the bayou

The project will be addressed as one ooerable unit

The estimated remedial action cost would be $55 million

(range of -30% to +50%)

TABLE l

SIGNIFICANT DIFFERENCES ANO RECOMMENDED DESIGN APPROACH

4,000 feet

17 feE!t

2,400 cubic yards

150,000 cubic yards

There is no surficial aquifer, this zone is only saturated durina recent rain. This will be prevented by a cap

There are 3 distinct pure oroduct olumes of creosote

Dredaina will be accomolished within-turbidity curtains without

dewatering the bayou

After dredging, the contaminated oortion of the bayou will be backfilled with clean materials to reduce the chances of contact with any residual materials

The oroject is divided into two operable units

$100 mi 11 ion (range of -30% to .+50%)

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REVISED SCHEDULE

SCHEDULE SHOWN REVISED SCHEDULE ITEM IN ROD GROUND WATER SOURCE CONTROL

Design Start June 1987 June 1987 June 1987

Design Completion February 1988 June 1989 SeptE?mber 1990

Interagency Agreement to February 1988 June 1989 November 1990 USACE for Construction -0\

tn Award of Remedial May 1988 October 1989 March 1991 tr)

0 Action Contract 0

Completion of Approximately 1994 March 1991 December 1996 Remedial Action

Startup of Operations and April 1991 January 1997 Maintenance

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PAUL TEMPLET": BUDDY ROEMER Governor Secretary ----'~;f: ·

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Dr. Allyn M. Davis Director

September 22, 1989

Hazardous Waste Management Division U.S. EPA, Region 6 1445 Ross Avenue Dallas, Texas 7~202

RE: Bayou Bonfouca Remedial Action

Dear Dr. Davis:

The pu:t·pose of this letter is to confirm our decisions from the July meeting between LDEQ and Steve Gilrein of your staff. In that meeting we expressed that LDEQ will support continuation of the selected remedy, as outlined in the March 1987 Record of Decision and as discussed in the July meeting. As required, LDEQ will also seek funding to provide the 10% match, or $9 million, for this $90 million project.

LDEQ appreciates the continued and looks forward to progress Bonfouca.

cooperation of EPA Region 6, in the remediation of Bayou

Sincerely,

62/fll~-cc: Harold Ethridge

LDEQ IAS Division

Paul. H. Templet, Secretary

OFFICE OF LEGAL AFFAIRS AND ENFORCEMENT P.O. BOX 44066 BATC>N ROUGE:, LOUISIANA 70804

AN EQUAL OPPORTUNITY EMPLOYER

Attachment A

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