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EXPLANATION OF SIGNIFICANT DIFFERENCES

Site Name and Location

Bayou Bonfouca St. Tammany Parish Slidell, Louisiana

Statement of Purpose

This document presents a summary of significant differences between information presented in the project's Record of Decision and the results of recent design investigations. Associated with this are the Environmental Protection Agency's decisions concerning these differences along with a statement by the Louisiana Department of Environmental Quality and its concurrence with these findings. The following documents were reviewed for these decisions:

- Record of Decision, March 31, 1987 - Summary of Design Investigations, December 1988 - Final Supplemental Geotechnical Report, March 1989 - Pilot Testing Findings/Design Recommendation Report, April 1989 - Supplemental Sediment Exploration, June 1989 - Review of the Bayou Bonfouca Superfund Site Remedial Design, June 1989

Record of Decision and Signfficant Differences

On March 31, 1987, a Record of Decision (ROD) was signed for the Bayou Bonfouca site. The selected remedy included:

- Onsite incineration of waste piles and contaminated sediments - Placement of an engineered cap over residues from the Incinerator and

surface soils. - Pump/treatment/reinjection of contaminated ground water. - Estimated construction cost of $55 million.

During analysis of design investigations it was discovered that the following significant changes were Involved:

- The length of contaminated bayou was found to be 4,000 feet rather than 2,000 feet.

-The bayou contamination extends to a maximum depth of about 17 feet rather than 5 feet.

- The total volume of contaminated materials within the bayou was estimated to be 150,000 cubic yards rather than 46,500 cubic yards stated in the ROD.

- Excavation of contamination within the bayou would require detailed analyses of stable slopes due to the increased depth and will require leaving minimal volumes of contaminants in place due to stability concerns.

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m - The contaminated sediments near the creosote plant were in direct contact

with the materials of the shallow artesian aquifer. *̂-- The contaminated ground water was found to be in 3 distinct plumes rather

than 1 continuous plume. - Reinjection of treated ground water was determined unnecessary at this

time. - Revised estimated construction cost of $100 million vs. $55 million

estimated cost in the ROD.

Findings Concerning Significant Differences

After review of the aforementioned information and a reassessment of alternative remedies the following findings have been made:

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

- Dredging of the bayou will require consideration of stable slopes and possibly some bulkheads to maintain existing land surfaces, however, minimal volumes of contaminants wiU' remain after dredging.

- Dredging of the bayou can best be achieved by using turbidity curtains around the excavation process with silt curtains and adsorbent booms placed along the bayou.

- All dredged areas will be backfilled with clean materials to minimize the chances of contact with residual contaminants that are left in place.

- The increased volume of contaminated sediments will require an Increase in the height and areal extent of the cap. Efforts will be made to keep the maximum height of the cap at about 17 feet. *

- The contaminated ground water plumes will be considered as 3 separate areas. The 2 onsite plumes will be remediated as one operable unit.

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

Statutory Determination

The results from these evaluations have shown that although the cost for implementing the remedy outlined In the ROD has Increased from $55 million to $100 million it remains the most environmentally protective and appropriate action. Therefore, in accordance to 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 represents 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. j

t,:. t.. r

Robert E. Layton Jr., P.E. Date Regional Administrator

DRAFT

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 water to levels that are technologically achievable.

2) Excavation of 46,500 cubic yards of contaminated bayou sediments that have polynuclear aromatic hydrocarbon (PNA) concentrations in excess of 1300 ppm or to a depth which minimizes threats to aquatic biota.

3) Incineration (thermal treatment) of a total volume of 51,500 cubic yards of contaminated sediments and waste piles in ah onsite treatment unit.

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 summer and fall of 1988 showed the yolume 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 $55 million to $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 cleanup operations into two operable units, the first being ground water, the second consisting of bayou sediments and surface materials,

2) Continue design of the ground water remediation while conducting additional investigations in the bayou.

3) Conduct a value engineering analysis of the thermal destruction component of the remedy for possible cost savings.

4) Reassess the appropriateness of other remedies.

This document summarizes 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 ) . For information relating to the history of the site and the alternatives evaluated prior to choosing the original remedy, please refer to the ROD.

DRAFT

PHASE I - DESIGN INVESTIGATIONS

During the summer of 1988, design investigations were conducted to better define the limits of work and engineering properties of contaminated material related to the Bayou Bonfouca Superfund site. This information was required to obtain more data related to the specific remedy of incineration and ground water treatment for the preparation of plans and specifications. The major activities Included sediment exploration, geo­technical exploration, ground water investigations and wastewater treatment.

During these investigations it was discovered that the horizontal and vertical extent of contaminants within the bayou sediments were greater than assumed from previous information. Several of the borings near the creosote plant revealed that the upper clay layer, as shown on Figure 1, was not present as Indicated by previous investigations. This clay layer was supposedly at a maximum depth of 5 feet beneath the top of sediments, and was presumed to act as a barrier against the vertical migration of contaminants. The reason for this error appears to lie in the Incorrect interpretation of previous boring information and sub-bottom surveys within the bayou. Initial design investigation borings also revealed the horizontal extent of contamination in the downstream direction was greater than antici­pated. In response to this EPA approved an additional 10 borings above the originally planned 15 borings. After completion of this work it was decided by EPA that further sampling operations within the bayou would be postponed until all the laboratory data was complete and the changed site I conditions evaluated. ' i

While sampling was underway in the bayou, investigations were also being t conducted on water treatment aspects and a more refined delineation of I contaminated ground water within the shallow artesian aquifer (Figure 2 ) . P These studies showed that the offsite ground water plume was not as large as initially expected and that treatment of the contaminated water could be readily achieved with onsite facilities. The investigations provided valuable data for characterizing the aquifer, and showed the potentially large benefits to running a small scale pilot study.

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

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

GENERAL CROSS-SECTION

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Creosote Plant Site

Upper Cohesive Layer

Shallow Artesian Aquifer

•Bulkhead Res idental Area

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Shallow Ar t e s i an Aquifer

Lower Cohesive Layer

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- Contaminated ground water plume containing free product creosote

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2) The ground water design could proceed, although a ground water pumping/treatment pilot study would be necessary before design completion. EPA decided to run this study onsite rather than offsite as called for in the ROD because the majority of the contaminated ground water was onsite.

3) The ground water plumes would be better defined during the pilot study activities.

4) A value engineering study would be conducted on the selected remedy once the revised sediment volumes were calculated.

PHASE II - DESIGN INVESTIGATIONS

During the winter of 1988 it was decided that additional bayou borings would be necessary and that this could be achieved at the same time as the ground water pilot study. The drilling portion of this phase occurred during December 1988 with the pilot study ground water extraction and treatment process continuing into March 1989.

The second phase of bayou sampling consisted of 30 borings and 27 cone penetrometer tests. These investigations were required to supplement Information from Phase I investigations to better define the extent of contamination and to provide the necessary data for plans and specifications. The results of this activity were not available until April and May 1989 and were compiled in a Supplemental Sediment Exploration Technical Memorandum dated June 1989. The report provided valuable information on the extent of contamination and the stable slopes required for dredging operations. ,/

The bayou borings have shown that the volume of contaminated sediments is approximately 150,000 cubic yards and extends from just upstream of the site to the Chamale Cove Marina as shown on Figure 3. This reach of the bayou is approximately 4,000 feet long 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 1s considered safe could result in the undermining of trees along the bayou resulting in the possible loss of property. Therefore, it will be necessary to place bulkheads in those areas where highly contaminated sediments are near the bayou bank. In other areas it may 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 preventing contact with any residual contaminants.

Extent of contaminated sediments.

BAYOU BONFOUCA SITE

Figure 3

The ground water activities during the Phase II Investigations consisted of borings along the edge of the bayou and onsite pumping and treatment. The borings along the bayou, in conjunction with previous borings, have shown the contaminated ground water in the shallow artesian aquifer to be as defined on Figure 2. The pilot study consisted of extraction of contaminated groundwater from 2 separate well arrangements and treatment of the water onsite. The treatment process consisted of oil/water separation followed by filtration with sand, oleophilic material and activated carbon. After a 30 day trial period of this unit it was determined that the proposed process was an appropriate means to address the contaminated ground water. Since this 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 Design Investigations the offsite contaminated ground water plume will not be addressed until after dredging has been completed. This was decided due to the direct contract between the sediments and this aquifer and that the source for these contaminants would need to be remediated first.

VALUE ENGINEERING ANALYSES

In response to the increased volume of contaminated sediments it was recognized that an analysis of ways to reduce costs would be necessary. The primary emphasis of this study was to propose an effective means to reduce the cost estimate, prepared by EPA's design consultant, for the selected remedy of thermal treatment. The second part of the analysis was to evaluate other alternatives to ensure the most cost effective measure was Implemented while remaining within regulatory requirements. In accordance with Section 121 of the Superfund Amendments and Reauthorization Act, preference was given to permanent treatment.

The evaluation of design concepts by the review team identified 3 major means to reduce costs. The items consisted of reducing the flow rate of water to be treated from sediment dewatering, use of a centrifuge in dewatering processes, and the use of waste heat from the incinerator as an aid in drying sediments. The team believed that a smaller scale dewatering system would result in a large savings because of cost savings in equipment which would more than offset any additional operating costs. It was also shown

that use of centrifuges could reduce equipment costs and requires less space than other generated concepts. The final recommendation consists of utilizing the heat from cooling the offgases from the high temperature chambers and diverting it to material dryers. These material dryers would be placed after the centrifuges and before the primary combustion chamber, thus delivering a lower moisture content material to the incinerator and saving money.^

In addition to proposing methods to reduce the cost of the selected remedy, the review team evaluated 10 other alternatives. These alternatives ranged from limited action which would only treat the ground water and incinerate surface waste piles to full scale incineration or bioremediation. Of these alternatives, soil flushing/washing and no action were eliminated at the beginning of this process due to feasibility concerns on the matrix of the sediments, and the no action response was addressed in the ROD. The remaining 8 alternatives are listed in Attachment B and detailed in the following paragraphs.

All alternatives have the same ground water treatment process at a present worth cost of $10 million which includes operation and maintenance. In order to evaluate options concerning source control, the cost estimates do not include the cost of ground water remediation, the estimates presented in the report entitled "Review of the Bayou Bonfouca Superfund Site Remedial Design" (CDM, 6/89) have also been modified to include backfilling of dredged areas at a cost of approximately $3 million. The analyses below reflect costs that are rounded off and are presented to show comparative costs in a range of minus 30 percent to plus 50 percent as was also done in the ROD.

Alternatives 1 and 2 both require offsite incineration of waste piles (5,000 cubic yards), leaving 150,000 cubic yards of contaminated sediments in place, and placing a RCRA Subtitle C cap over contaminated surface soils. The difference between 1 and 2 is that the first would place clean fill over the contaminated areas (cost of fill $3,000,000) while the second would require a geotextile filled with concrete as a barrier. The costs for these remedies are about $15 million and $35 million and would potentially require extensive maintenance activities to ensure the contaminants aren't migrating. Both of these alternatives were eliminated as a final choice because they do not adequately address the goals of reducing toxicity or volume of contaminants, and the long term effectiveness remains questionable.

Alternatives 3 and 4 are similar in that 150,000 cubic yards of contaminated sediments would be removed, dewatered and placed onsite. Neither option would result in treatment of contaminants, however, both would provide benefits in that mobility of creosote is greatly reduced along with prevention of direct contact with any wastes. The difference is that Alternative 3 places a RCRA Subtitle C cap over the contaminated sediments and relies on the natural clay as a lower barrier, while Alternative 4 also includes a RCRA Subtitle C cap but places a drainage collection and detection system beneath the sediments. The advantage of such a collection system is that it provides a means to prevent any leachate from wastes migrating into the existing clay. The costs for Alternative 3 would be approximately $45 million and Alternative 4 would be about $55 million. The disadvantages of these proposals is that they do not reduce the volume or toxicity of the hazardous constituents and require long term maintenance. In addition, a similar proposal to these remedies was made prior to the March 31, 1987, ROD and the citizens were very adamant against placing untreated sediments into a landfill.

Alternative 5 calls for insitu solidification of the contaminated bayou material at a cost of approximately $60 million. This option would Include offsite incineration of the waste piles (5,000 cubic yards) at a unit cost of $800 per ton or a total of about $2 million. This charge for offsite incineration represents a cost 5 times higher than onsite Incineration as discussed in the following paragraph. This alternative would require the addition of a solidification agent such as portland cement and flyash to the sediments and would also Include a RCRA Subtitle C cap on surface soils similar to previous discussions. The primary reason for eliminating this proposal is that the long term effectiveness of solidification of the organics remains questionable.

Alternative 6 is the selected remedy as stated in the ROD. This option calls for dredging the bayou on stable slopes, backfilling dredged areas with clean fill, onsite incineration of contaminated sediments and waste piles, and placing a RCRA compliant cap over the residues of incineration and contaminated surface soils. The benefits of this option are that it significantly reduces the volume, mobility, and toxicity of the contamin­ation and provides long term effectiveness and permanence. The major change between this option and the ROD is that the volume has increased by a factor of 3 which resulted in a construction cost estimate rising from $55 million to approximately $90 million. The concern of this alternative, which is also associated with all dredging alternatives, is that of air emissions during dredging. This issue will be examined through a pilot study and will require provisions in contract documents for controlling sUch emissions.

Alternative 7 is similar to Alternative 6, however, it would involve only removing apprpximately one fourth the volume of contaminated sediments. This effort would concentrate on the most heavily contaminated areas and would cost apprpximately $40 million. This alternative would provide benefits in reducing the volume of contamination, however, it is only a partial solution.

The final alternative is bioremediation which is Identical to Alternative 6, except that Instead of treating the contaminants by incineration it involves bioreactors. These bioreactors would treat contaminants through use of bacteria to degrade creosote in a series of mixing tanks. This alternative is discussed in more detail in the ROD and has an estimated cost of about $90 million. This alternative would require a great deal of additional studies and preliminary review has indicated that treatment of sediments down to incineration treatment levels may not be achievable. Due to these concerns and that it was the same cost as incineration, this alternative was not investigated further.

In conclusion, a review of the V-E study has shown that the most appropriate means of protecting human health and the environment is through continuation of the selected remedy as called for in the March 31, 1989 Record of Decision.

DECISIONS CONCERNING SIGNIFICANT DIFFERENCES

It has been determined by EPA and LDEQ that the selected remedy as presented in the March 31, 1989, 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 respond 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 paragraphs.

It was stated in the ROD that sediments would be excavated by driving piles down the middle of the bayou and dewatering half of it while maintain­ing steam flow In the remaining half. This was based on contamination being at a maximum depth of 5 feet and that it hadn't penetrated 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 depth of about 17 feet. Therefore, the stability of dredged slopes down to these greater depths becomes an Important factor along with the usefullness of sheet piles. It has been decided that the most appropriate

pr

means to address this is through elimination of sheet piles in the middle |§ of the bayou and performing dredging with turbidity curtains, silt curtains and adsorbent booms along the bayou. The turbidity curtains would act to minimize suspension of material during dredging activities with the silt curtains and adsorbent booms providing secondary means to aid in controlling migration of contaminants.

As noted, the increased depth of sediment contamination presents 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 slopes 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 dredged will be backfilled with clean material which will provide a barrier against any contamination that is not removed. This minimizes the chances of direct contact and provides an opportunity for aquatic biota to reestablish itself.

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 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 -rei njection process. Since the ROD the need for a slurry wall and reinjection have been determined to be unnecessary at this time. In addition, the investigations have shown the shallow artesian aquifer to to be contaminated in 3 sep€(rate plumes as shown on Figure 2.

Based on available data, contaminated ground water plumes onsite will be remediated through extraction, onsite treatment and then discharge to the bayou. This process will also include a monitoring system to ensure any drawdown of the aquifer does not effect surrounding structures. The offsite 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 design investigations have also identified 3 areas of subsurface contamination that were not previously known as shown on Figure 4. These areas represent relatively small quantities of materials and will be approached as follows. Areas A and B have shown small seams of contaminants at 5 feet with uncontaminated material above and below. These seams will be excavated and the material incinerated, with all excavated areas back­filled with clean soils. Area C consists of a small seam of contamination with a maximum concentration of 517 ppm total PNAs and is found at a depth of 11 feet below clean clay material. It has been decided this small seam of material at this depth surrounded by clay and at this concentration does not pose a health threat and therefore will not be removed. The EPA is recommending institutional controls to prevent excavation in this area and in areas to be occupied by the RCRA cap.

The remaining issue is that contamination along the Western Creek and the Eastern Drainage Channel 1s now estimated at 2,400 cubic yards compared to 2,000 cubic .yards presented in the ROD. The location of these water/ways is shown on Figure 4 and the contamination has been found to be limited to areas within the property boundaries and excavation materials will be incinerated as specified in the ROD.

REVISED SCHEDULE

Ground Water Design Completion - June 1989

Award of Ground water Construction Contract - October 1989 { I-

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Completion of Source Control Design - September 1990 j:

Award of Construction Contract for Source Control - April 1991 I

Completion of Source Control Construction - December 1995

9T. TAMMAMV PATOSH LOUISIANA

Areas A thru C represent locations of subsurface seams of contaminates The soils above and below these seams are uncontaminated.

SOO 400

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State of Loumana Department of^nyiroiiiriental Quality

BUDDY ROEMER Govemor

^AZARDO 'iiltnn.

PAUL TEMPLET Secretary

September 22, 1989

Dr. Allyn M. Davis Director Hazardous V7aste Management Division U.S. EPA, Region 6 1445 Ross Avenue Dallas, Texas 75202

RE: Bayou Bonfouca Remedial Action

Dear Dr. Davis:

The purpose 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 -cooperation of EPA Region 6, and looks forward to progress in the remediation of Bayou Bonfouca.

Sincerely,

Paul H. Templet, PMD Secretary •

cc: Harold Ethridge LDEQ IAS Division

OFFICE OF LEGAL AFFAIRS AND ENFORCEMENT P.O. BOX 44066 BATON ROUGE, LOUISIANA 70804

AN EQUAL OPPORTUNITY EMPLOYER

Attachment A

BAYOU BONFOUCA EVALUATION OF POSSIBLE ALTERNATIVES

ALTERNATIVE

1) Bayou Sand Layer (RCRA CAP)

2) Bayou Concrete Liner (RCRA CAP)

BENEFITS

Removes direct contact over short duration.

Removes direct contact, duration unknown.

CONCERNS '

Containment remedy. Does not reduce the long term t h r e a t .

COST (MILLION)

; • • ' . • • 1 5 ' : •

35

3) On-Site Consolidation (RCRA CAP)

Reduces the rate of migration of wastes.

Containment remedy. Long term threat exists.

45

4) On-Site Consolidation (RCRA VAULT)

5) In-SI tu Bayou Sol i f i c a t i o n

(RCRA CAP)

6) Sediment Incineration (RCRA CAP)

7) Partial (1/4) Sediment Incineration (RCRA CAP)

Provides a slightly more engineered approach to reduce the migration of waste than Alternative

Reduces the short term mobility of wastes.

Reduces the volume, mobility and toxicity of the contaminants.

Reduces and treats a portion of the waste materials.

Containment remedy. 55 Requires perpetual O&M.

Long term effectiveness 58 is questionable.

Potential for air 87 emissions during dredging and stockpiling needs to be evaluated. .

Only addresses a portion 41 of the problem.

8) Sediment Bioremediation (RCRA CAP)

Reduces the volume, mobility and toxicity of the contaminants.

The effectiveness of this technology is questionable along with the potential air emissions.

88

Attachment B ^jrfwsp-'r^-'''''^^wmn-Tm-