TYBOUTS CORNER LANDFILLRD/RA STATEMENT OF WORK PLAN

DESIGN REPORT ONALTERNATIVE GROUNDWATER CONTROLS

FORTYDOUTS CORNER LANDFILL

Prepared for:

Tybouts Corner Landfill Trust FundManagement Steering Committee

Prepared by:

DPL Consultants

July 1991

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1.TABLE OF CONTENTS

Page No.

1.0 INTRODUCTION 1-1

2.0 SITE HYDROOEOLOOY 2-1

2,1 Configuration of the Mcrchantville Formationand Undifferentiated Silt 2-1

2,2 Water Levels 2-32,3 Results of Pumping Test on Wells Tapping the

PN-1 Sand 2-32.4 Summary of Hydrogeologic Evaluation and Implications 2-4

3.0 GROUNDWATER MODEL 3-1

3,1 Model Used in RI/FS 3-13,2 Revised Model Boundaries 3-23,3 Revised Geologic Structure in the Model ' 3-33,4 Revised Assumptions 3-4

4.0 DESIGN ALTERNATIVES 4-1

4,1 Comparison of Collection Wells with Drains at South Endof Landfill 4-1

4,2 Comparison of Slurry Trench with Drain at North Endof Landfill 4-4

< 4,3 Predictions of Effectiveness of Alternatives 4-74,4 Conclusions' 4-10

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} CHAPTER 1

•'«, INTRODUCTION

DPL Consultants has recently conducted supplemental field explorations and is developingthe Remedial Design for the Tybouts Corner Landfill based on the Record of Decision(ROD), the Preliminary Agreement and (he remedial objectives of the Consent Decree(Article XXVII), The remedial objectives are, to the extent reasonably practicable to:

1. Lower the water level in the main landfill to the bottom of the waste,

2, Prevent lateral migration of groundwater into the landfill,

3. Collect leachate emanating from the landfill, and

4. Collect leachate that may continue to emanate from the west and main landfillsinto the PN-1 sand.

0HJI. The ROD specified installation of a cap and groundwater controls. Upgradient (north) and"«? downgtadlent (south) subsurface drains were described in the ROD, In addition, a system

of interceptor wells was specified to pump contaminated groundwater emanating from thedowngradient side of the site. Contaminated groundwater was to be treated prior todischarge into the local public treatment facility. The drain and well locations asconceptualized in the ROD are shown on Figure 1-1.

The ROD remedial design concept was based on the NUS1 Remedial Investigation andFeasibility Study (RI/FS) (1985), As part of the FS, various cap alternatives had beenevaluated. Conceptual approval was given in the Consent Decree for use of dredge spoilmaterial to construct a low permeability cap. The cap was to be lined with coated Typar, laidin overlapping runs without welded scams.

The upgradient drain, according to the Consent Decree, was to be located along the northeastside of the landfill, It was to extend from Route 13 at its south end to Route 71 at its north

'NUS was the EPA contractor who performed the RI/FS in 1985.

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end and was to include an unspecified length along the north (Route 71) side of the landfill,The drain on the northeast side was to be constructed to the Merchantville Formation andunderlying silt, a natural aquitard underlying much of the landfill. The upgradient drain wasto control lateral migration of groundwater into (he refuse at the upgradient end of thelandfill. In its early years of operation, it would also collect groundwater from the landfillside,

The downgradient drain was specified to collect existing leachate. It was to extend below thewater table, beginning along Route 13 at the south end of the landfill and extending aroundthe south end of the landfill and up an existing drainage swale, located along the west sideof the landfill. This drain, unlike the north drain, was not specified to be fully penetratingbut instead skimmed the upper portion of the groundwater to collect leachale, emanatingfrom the landfill.

The last element of the proposed remedy was a scries of interceptor wells to collect5> contaminated groundwaters in the PN-1 sand, Three were located along Route 13 (along they southeast side of the main landfill) and a fourth was located downgradient of the west landfill.

The wells were specified to fully penetrate the PN-1 sand. The wells were to minimize off-site migration of contaminated groundwaler in the PN-1 sand.

During the development of the RI/FS and ROD, NUS reported that the greatest saturatedthickness of refuse occurred at monitor well TY-311, where coincidcntally, the highestconcentration of contaminated groundwaler was reported. The depth of refuse there isinconsistent with the depths of refuse placement elsewhere on the site. Whatever the actualdepth of refuse is across the site, it was understood at the time of development of the RODthat not all of the refuse would be dewalered, The goal was to lower the water level in therefuse to the extent practicably feasible.

The objectives of the ROD selected remedy were based on the 1985 RI/FS. DPL hassubsequently completed the design stage field explorations specified in the EPA-approvedwork plan (Appendix C to the Consent Decree). The DPL design evaluation has the benefit

'• of the NUS RI/FS and the recent field studies, These evaluations lead DPL to recommend

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refinements and modifications to the design concept to achieve the ROD objectives. Therecommendations and their rationale that are summariKcd in this report include the following:

• Additional interceptor wells (instead of the four cited in the ROD) arerecommended to control contaminated groundwatcr in the PN-1 sand,

• Leachate emanating from the downgradicnt side of the landfill will be collectedby the interceptor wells; rather than by a subsurface drain.

• Lateral migration of groundwater into the refuse will be controlled by use ofa slurry trench rather than the north drain.

The following chapters summarize the design explorations and evaluations, and the rationalefor the above recommendations. Chapter 2 describes the current evaluation of sitehydrogeology and provides a comparison with the RI/FS and ROD. Chapter 3 describesrevisions to the groundwater model used in the RI/FS to compare remedial alternatives.Design alternatives are developed, followed by evaluation of the alternatives in Chapter 4.

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t

CHAPTER 2

s SITE IIYDROGEOLOGV

Hydtogcologic explorations were conducted to obtain supplemental site specific Informationto develop plans and specifications for completion of the remedial design. The tasks specifiedby the EPA approved Work Plan were completed, The explorations focused on informationneeded for design of the subsurface drains and the interceptor well system.

Chapter 2 presents a summary of the hydrogeologic explorations and evaluations with respectto the remedial design. The NUS interpretation is essentially unchanged except that we havebeen able to refine the hydrogeology, The following arc discussed;

• Configuration of the Merchantville Formation and undifferentiated silt

• Water levels

**) • Results of Pumping Tests on wells lapping the PN-1 Sanda!!/

2.1 Configuration of the Merchuntvlllc Formation and UndltTerentlittd Flit

The RI/FS stales that "The Merchantville Formation is located immediately above thePotomac Formation beneath Tybouts Corner Landfill and forms much of the base of theMain Landfill at the site". The Merchantville Formation on the site consists of a dark gray,micaceous, glauconitic sandy silt. As mapped by NUS, the Merchantville Formation isunderlain by a white to gray sill bed on, and around, the site which NUS concluded has a lowhydraulic conductivity similar to the Merchantville Formation (1 x ID'7 cm/sec) and thus NUSincluded this underlying silt unit with the Merchantville Formation as a continuous aquitard.

These two units are the underlying aquitard on Plate 1. Borings drilled by New CastleCounty in 1968 and by E,H, Richardson Associates, Inc. in 1974 confirm NUS's interpretationthat this aquitard occurs beneath the landfill and except, as noted below, DPL's designexplorations and evaluations confirm these findings.o

2-1

The southern drain design objective is to collect Icachate emanating from the landfill. Thisdowngradicnl drain is to encompass the southern (downgradient) edge of the landfill andwould have been constructed above the lop of the aquitard to collect Icachate discharginglaterally from the landfill. The aquitard was believed to be shallow (less than 20 ft belowgrade) with the saturated refuse resting on it. The design intent of the conceptual plandescribed in the ROD was to "skim" Icachalc contamination from the upper portion of thegroundwater as it leaves the landfill.

During the predesign exploration, the aquitard was found to be missing at the southern edgeof the landfill where the downgradient drain was to be constructed. In portions of thesouthern section of the landfill, a brown silt sandy unit occurs. This unit extends in someplaces from the ground surface to the top of the PN-1 Sand. This silt unit is above the watertable and is not continuous beneath the refuse in this section of the landfill, DPL considersthis unit to be part of the Columbia Formation. Figure 2-2 is a geologic cross-section throughthe southern sections of the landfill (see Figure 2-1 for cross-section location). The absenceof the aquitard under the proposed downgradient drain raises concerns relative to the designand function of the proposed downgradient drain system.

The southern drain was conceptualized to collect existing leachate believed by NUS to beperched at the south end of the landfill by the aquilard. However, the indicated effectivenessof the drain in skimming Icachate as it leaves the landfill is reduced since the aquitard ismissing beneath the proposed southern drain. Furthermore, nearby interceptor wells (in thePN-1 Sand) would impact and probably dewater the drain,

Supplemental design exploration for the northern drain also focused on the lateral extent anddepth to the aquitard. The depth below ground surface to the aquitard ranges from 20 to 35ft. Along Route 71, the upper surface of the Merchantville slopes down to the west, Adepression in the aquitard surface at PZ-47 and a small rise in the aquitard at PZ-43 (Plate1) are indicated by the recent exploration data. These features influence the groundwaterflow in the refuse and limit the extent of refuse dewatering achieved by the remedy at thenorth end of the landfill.

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2,2 Water Levels

Current water level elevations in the water (able and the PN-1 Sand were determined by DPLfrom the existing wells and from recently installed piezometers and monitoring wells, Lateralgroundwater movement at the water table and in the PN-1 Sand is generally toward thesouth, which is consistent with the NUS interpretation,

Water table elevations have also been measured with Telra Tech Richardson in February1991 on the property north of Route 71. Those data show the control of the Pigeon Runtributary on the water table.

2,3 Results of Pumping Tests on Wells Tapping the PN-1 Sand

Controlled pump tests were conducted in the southern (TW-05 and TW-06) and northern(TW-01) portions of the Landfill.

Transmissivity values obtained from the pumping tests conducted on the PN-1 lest interceptorwells, TW-06 and TW-05, are estimated to be 28,500 gpd/ft and 43,000 gpd/ft, respectively.Because of the absence of the aquitard beneath the southern drain, DPL Consultantsconstructed its groundwater model on the basis of conservative assumptions, concerning theabsence of this aquitard in part of the southern portion of the site, Even based on theseassumptions, a system of interceptor wells will effectively control groundwater movement inthe PN-1 along the downgradient perimeter of the Me. Based on the revised groundwatermodel, results suggest that at least one additional well is needed to create a capture zone.According to the groundwater model, the capture «m« created by the interceptor wells willoverlap in the PN-1 Sand and will collect any Icnchaie emanating from the landfill.Additional design analysis is presently being conducted to optimize the design of theinterceptor well system.

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2.4 Summary of ilydrogeologlc Evaluation and Implications

A review of the NUS data in conjunction with new data from the DPL hydrogeoiogicexploration, indicates that the aquitard is missing beneath the proposed location of thesouthern drain. Lateral groundwater movement at the water (able and in the PN-1 Sand isgenerally to the south, coincident with the NUS findings, The design pumping tests indicatethat overlapping cones of influence can be achieved by the system.

The logical alternative in controlling the hydrogeoiogic conditions in the northern portion ofthe landfill is a barrier to flow. The complexities of drain design, construction, health andsafety, and operation led DPL to consider a simpler, cost effective alternative. Incombination with the landfill capping (as detailed in Section 4,2,2) a slurry trench isrecommended for evaluation as an alternative to form a barrier to groundwater flow andlocalized recharge in the northern portion of the landfill.

The groundwater model for the site has been revised and expanded based on thesupplemental data and conservative assumptions concerning hydrogeoiogic conditions at thesite. This model has been used to develop the remedial design and to evaluate alternativeremedial elements with respect to the remedial objectives. The changes made to the modelare described in Chapter 3.

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O

CHAPTER 3

GROUNDWATER MODEL

3.1 MODEL USED IN RI/FS

NUS applied the U.S. Geological Survey finite-difference model (MODFLOW) developedby McDonald and Harbaugh (1984) to the site. The model is a lime-dependent numericalmodel that uses multiple layers to simulate three dimensional groundwater flow patterns. AsNUS simulated in its 1985 modeling effort, the upper layer of the model (layer 1) is theColumbia Formation and the refuse. The lower layer (layer 2) simulates the PN-1 Sand. TheMerchantville Formation and associated sills are treated as an aquitard between layers 1 and2.

In the upper layer, the model northern boundary conditions were defined by NUS as constantheads to simulate the Pigeon Run tributary and variable (general) heads elsewhere to thenorth (see Figure 3-1). Inactive (no flow) cells were used to establish the boundary of themtJel several hundred feet east of the landfill. To the west of the landfill, the modelboundary was set at the edge of the refuse, as a row of constant heads and a row of inactivecells. South of Route 13 the model's boundary extended nearly to Red Lion Creek and itstributary as constant head and inactive cells. Figure 3-1 shows these boundaries.

In the second layer (PN-1), the northern boundary was set at the same location as in layer1, to the north of Route 71, as constant heads. To the east, inactive cells were used to limitthe layer to the immediate vicinity of the landfill. On the west side, the layer 2 flow wascontrolled by constant heads along Pigeon Run and Red Lion Creek and by general headconditions further to the west, The model extended to the southeast to the Red Liontributary shown in the southeast corner of Figure 3-2. The southern boundary was controlledby constant heads located in the Red Lion marsh area to the south of Route 13 and constantheads with inactive cells beyond Red Lion Creek. The boundaries in layer 2 are illustratedin Figure 3-2.

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Wetlands on the west side of the Main Landfill were not simulated. The extent of theaquitard simulated in the NUS 1985 modeling effort is illustrated on Plate 1. NUS describeda groundwater table flow regime from the landfill northward beneath Route 71 and thenflowing to the west. NUS indicated that as water levels in the refuse declined, ambientgroundwater flow from the north side of the landfill would enter the refuse laterally. On thenortheast side of the landfill NUS also simulated potential flow into the refuse after thelandfill was capped. Further evaluation of these flow patterns was developed during the DPLdesign evaluation and is summarized in Chapter 4.

3.2 Revised Model Boundaries

The model has been revised to include several changes to both the types and locations of themodel boundary conditions. General head conditions were used at several locations whereNUS used constant head boundaries, The model's authors recommend this change as a morerealistic method for simulation. Changing these conditions allows the model heads to

( f l u c t u a t e and respond more realistically to the remedial alternatives. NUS used constantheads along Pigeon Run and Red Lion Creek, but the model had been refined byincorporating a recently developed river package to simulate these surface water bodies.

Water level data for the Columbia Formation (layer 1) north of Route 71 (supplied by theproperty owner) show that the tributary of Pigeon Run, which runs parallel to Route 71,functions as a groundwater control for layer 1 (Columbia Formation). A new subroutineallows simulation of the tributary as a river controlling groundwater levels there. Therefore,layer 1 has been extended to beyond the tributary and the water level in the tributary hasbeen set as the revised northern boundary condition for layer 1. The western boundaryremains, as before, the western limit of the Main Landfill refuse. At the southwest end ofthe landfill layer 1 extends with very thin saturated thicknesses down to the south end of thelandfill. As discussed in Chapter 2 of this report, the thickness of saturated zone in refuseat the south end of the landfill is uncertain. On the east side of the landfill, boundaryconditions have been modified to use general head boundary conditions where NUS usedinactive cells. South of Route 13 inactive cells were used at the limits of the Columbia

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Formation (layer 1). Infiltration was reduced to better simulate the effect of the Route 7interchange, The layer 1 boundaries of DPI are shown on Figure 3-3.

Layer 2 boundaries used by DPL are shown on Figure 3-4. The layer 2 boundary to thenorth was placed 300 ft further north and inactive cells were used rather than constant headcells. The boundary to the west has also moved westward expanding the area simulated by600 ft to the west. As for layer 1, layer 2 now uses the new river package to simulate PigeonRiver and Red Lion Creek rather than using the constant heads specified by NUS. Theeastern limit of layer 2 is 450 to 600 ft further east of the landfill than was simulated by NUSto reduce the influence of that boundary.

As part of revised model formulation, consideration was given to coupling the wetlands andon-site ponds with the water table in the PN-1 Sand where there is no layer 1. However,after review of the piezometric head data, it was concluded that the pond at the south endof the Main Landfill and the wetlands to the west are perched and are not hydrologicallyconnected with either layer 1 or layer 2 of the model. In the wetlands area along the westside of the Main Landfill, these perched conditions required a reduction in this simulatedinfiltration since infiltration apparently does not reach layer 2 of the model (the PN-1 Sand)in this area.

3.3 Revised Geologic Structure In The Model

The major structural feature that was revised internal to the model is the simulation of theaquitard. The 1990 field explorations have differentiated the Merchantville Clay Formationfrom associated clayey sills to fine sands that also act as an aquitard. The clay is of morelimited extent beneath the refuse than the extent of the aquitard described by NUS. At thesouth end of the landfill borings revealed areas with no aquitard. The NUS formulation wasrevised to reflect contact in this area between refuse and the PN-1 Sand in the DPL modelruns. The extent of the aquitard as evaluated by NUS and DPL is shown on Plate 1.

In addition to the refined interpretation of the aquitard, vertical permeabilities have also beenmodified in the new model runs. Specifically, while the aquitard is present in this area,

3-3

vertical permeabilities on the northwest side of the landfill were increased. In several areasof the model reduced vertical permeabilities have been used to account for silly layers orlenses which were observed within the PN-1 Sand.

3.4 R«vl«d Assumptions

Figure 3-3 illustrates the measured current conditions water table map (layer 1). Figure 3-4illustrates measured current conditions for model layer 2 (PN-1 Formation). The boringswhich encountered refuse below the water table were used to define the extent and thicknessof saturated refuse. The depth to the base of the refuse at the NUS boring for well TY-311is anomalous, reporting refuse detection to elevation 18.6 ft whereas elsewhere in the landfillthe refuse base is typically at elevation 27 to 35 ft. Without the TY-311 "pocket" of refuse,Figure 3-5 illustrates the thickness of saturated refuse; the average thickness is 2.5 ft If the"pocket" near TY-311 is confirmed to extend down to elevation 18,6 ft, up to 1.4 ft ofsaturated refuse is added to the mean values.

Predesign explorations, as discussed in Chapter 2, suggest alternatives to the planned remedy.At the north end of the main landfill aquitard elevations along the proposed north drainalignment indicate a high point which creates a groundwater divide. These features wouldreduce the effectiveness of the proposed drain with respect to lowering the water level withinthe refuse, beneath the "dog leg" at the north end of the landfill. The model simulations todate have revealed that small quantities of leachate which were to be intercepted by the southdrain could be intercepted by interceptor wells since vertical flow in this portion of the siteis sufficient to be collected in the interceptor wells.'

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-N CHAPTER 4. i

DESIGN ALTERNATIVES*i

This chapter discusses design alternatives and modifications considered to address the currentunderstanding of site conditions. The function of additional interceptor wells instead of thesouth drain Is discussed in Section 4.1. The wells will collect leachate as well as control flowin the PN-1 formation. Use of a slurry trench instead of the north drain is evaluated inSection 4.2. Either will control lateral flow and lower the water level in the refuse. Sincethe groundwater model incorporates the entire site hydrogeology, it was used to compareexisting conditions to the predicted effects of the ROD design concept, and to the predictedeffects of the alternative design concept (incorporating the changes at the north and southends of the landfill). The model predictions are described in Section 4.3.

4.1 COMPARISON OF COLLECTION WELLS WITH DRAINS AT SOUTH END OFk LANDFILL

The ROD, Consent Decree, and Preliminary Agreement specify use of a subsurface drain atthe south end of the Main Landfill to collect existing leachate. Absence of the aquitard inthis area indicates that the proposed interceptor wells will probably dewater most of the drainand that, with the addition of supplemental interceptor wells the well system will collectleachate emanating from the landfill.

4.1,1 Description of the Alternatives

The south drain, described in the ROD and the NUS Feasibility Study (FS) (1985), consistedof a 10 to 20 ft deep drain with the effluent being pretreated prior to discharge to the countysewer system. The alignment of the drain as conceptualized in the ROD is shown in Figure1-1. The drain was provided to collect shallow leachate evidenced along the banks of theswale (Figure 1-1) and present In the subsurface at the southwest end of the landfill. Whileit was recognized that after the water level had been lowered in the refuse, leachate

' '\ formation would essentially cease, it was assumed that even after capping some infiltration

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~N through the cap might occur over lime and that provisions for continued collection of"" leachale were therefore necessary, The south drain was planned to intercept or "skim" the'̂ lop of the water-bearing zone (the zone discharging the leachate), assist in dewalering refuse

at the south end of the landfill, and collect any long term leachate that might be released.Since surficial Icachalc seeps were observed around the toe of the landfill and in the swale,the use of a continuous shallow drain to collect the leachate was an obvious choice, Thesouth drain segments were planned to operate at control water levels 3-5 ft below the presentwater table.

Exploration borings drilled in 1990 (described in Chapter 2) have shown that the aquilard isabsent along most of the south drain alignment. The water table there varies from gradelevel (i.e., seep areas) to a depth of 8-ft, The pumping test at the southern-most testinterceptor well along Route 13 indicates that its operation would dewater the south drain.This section discusses potential advantages and disadvantages of substituting interceptor wellsfor the south drain.

«? 4.1.2 Evaluation of the Alternatives

4.1.2.1 Overall Effectiveness. A system of wells to collect leachate emanating from the landfillmust be designed and operated to have overlapping zones of withdrawal which complementthe interceptor wells specified in the ROD. In the long term, pumping from multiple wellswith overlapping zones of withdrawal potentially provides belter control of groundwater flowin the PN-1 Sand. Conceptual locations of the proposed interceptor wells are shown on theattached map (Figure 4-1). Flow rates and water table predictions are described in Section4.3 of this chapter.

4.1.2.2 Operation and Maintenance. The drain will require maintenance to service liftstation(s) and remove sediment. If precipitation or biofouling clog the formation, expensivecleaning (if feasible at all) will be required, or the system will have to be replaced, Cloggingwithin the drain system or its piping can be avoided by periodic cleaning with standardmechanical methods. It is recognized (hat the interceptor wells will also require some

'•\ periodic maintenance. The cost for well replacement, however, is small compared to the

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construction costs for the drains, as wells can economically be reinstalled on short notice andas needed, Furthermore, the technology for redevelopment of wells is well-established,

4,1.2,3 Short-Term Effectiveness. The short term effectiveness of wells versus drains includespotential impacts on workers, considering the effectiveness and reliability of protectivemeasures available. The drains are anticipated to involve placement of piping and beddingmaterial with construction personnel and inspectors in the trench, thereby posing a potentialhazard to workers. While there are adequate health and safety precautions for workers inthe trench, this would be a fairly hazardous task which would be avoided by installation ofwells instead,

In the short term two sources for environmental impacts accrue from the drain alternative:excavation of soil downgradient of the landfill will likely include residual aqueouscontaminants which will volatilize after the removal, and the drain will accumulategroundwaler which will emit volatile organic compounds prior to being pumped to thetreatment system. Installation of the well system, on the other hand, will not involve removalof any significant volumes of soils, but will involve the pumping of contaminated groundwaterin developing the wells. The development water, as agreed to by the E?A, may be dischargedback into the landfill.

4.1,2,4 Costs. With an overall length of approximately 1600 ft, the construction cost for thesouth drain (with administrative and other costs) would total $1.5 million. Incremental costsfor installation and operation of one or more additional interceptor wells on the other handwould likely only total two hundred thousand dollars. Use of wells instead of the south drainpresents savings of about $1 million for the remedial construction program.

The costs to maintain the drain and the interceptor wells are difficult to estimate. However,if the formation surrounding the drain becomes clogged with precipitated iron or otherminerals, or with bacterial slime, it may be more difficult and costly to remove than it wouldbe to redevelop or replace a well(s), If the drain suffered clogging or physical damage thatrequired excavation of the pipe, there would be a considerable cost. If a well becamedamaged beyond repair it could be replaced nearby at relatively low cost. Although amounts

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cannot be confidently estimated, maintenance of the wells would cost less than maintenanceof the drain system should serious plugging or physical damage occur. Table 4-1 summarizesadvantages of the wells over the south drain.

In conclusion, the interceptor well system proposed will act to control the groundwater in thePN-1 Sand and additionally collect Icachatc emanating from the landfill more effectively andeconomically the southern drain.

4.2 COMPARISON OF SLURRY TRENCH WITH DRAIN AT NORTH END OFLANDFILL

The design concept in the ROD was use of a subsurface drain at the north end of the MainLandfill to control lateral migration of groundwater into the landfill, Evaluation of thehydrogeologic data for the north end of the landfill, including the off-site area on the northside of Route 71, has led to a modified design concept using a slurry trench instead of thenorthern drain to control lateral groundwater flow. The slurry trench is a more effective andeconomical remedy at the north end of the landfill.

4,2.1 Description of the Alternatives

The function of the drain at the north end of the landfill is to intercept ambient groundwalerflow which may otherwise enter refuse, hence to reduce the production of leachate, At thenorth end of the landfill the drain described in the ROD and the FS consisted of a 30-40 footdeep drain (keyed into the confining layer) constructed along the property line and thencutting across the "dog leg" and extending an unspecified distance along Rl, 71. The effluentwould be treated prior to discharge to the county sewer system on the assumption that theinitial effluent would be contaminated, The drain was expected at some unspecified futuretime to discharge ambient groundwaler, whose quality would be adequate to allow directdischarge to Pigeon Run. The plan for the drain as conceptualized in the ROD Is shown inFigure 1-1.

The exploration borings performed in 1990 at the north end of the landfill have providedadditional data with respect to the configuration and structure of the upper surface of the

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TABLE 4-1

SUMMARY COMPARISON OF WELLS VS DRAINS AT SOUTH END OF LANDFILL ' (j$,.

DRAINS

A, Overall ProtectionB, Compliance with ARARsC. Long-Term Effectiveness and Permc lence

1. Residential risk2. Adequate and Reliable

Adequate collectionReliable operation

D, Reduction Through TreatmentE, Short-Term Effectiveness

1. Risks during implementation2. Impacts on workers3. Environmental Impacts4. Time to Achieve Protection

F. Implementability1. Technical feasibility2, Administrative feasibility3. Available services and materials

G, Cost1. Capital2. O&M3, Net worth

H. State AcceptanceI, Community Acceptance j.TOTAL 0

WELLS

pd)-

-

P••

•PP-

•-P

PPP•E9

p('): Preferred alternative

' > aquitard, Its upper surface drops to a low point of approximately elevation 25 ft (aboveMSL) midway along the northeaiit side of the landfill and then rises to an elevation of nearly

•̂ 35 ft midway across the "dog leg" of the landfill. Because of this configuration of theaquitard, the northern drain as specified in the ROD would of necessity consist of twosegments. One segment would run from Rt. 13 to the low point of the aquitard, and on tothe "dog-leg," A second segment would run along Rt. 71. Because of the depths involvedtwo pumping stations would be required: one for the north-east section and one for theRoute 71 section. Each discharge would be pumped to the pretreatment facility, nowproposed to be located at the south end of the landfill,

Recent groundwater data to the north of Route 71 on the so called Lester Property indicatesthat groundwater table control on that parcel is exerted by a westward flowing tributary toPigeon Run. This tributary controls the water table elevation and hence ambientgroundwater flow in the area. As a result, the model indicates that after the landfill iscapped, ambient water table (layer 1) flow beneath Route 71 into the landfill to the south is

.-v predicted to be minimal. The model also indicates that the water table north of Route 71 will' ,J.,J drop below the elevations predicted by NUS.

Some uncertainty exists with respect to the flow regime that will be established on thenortheast side of the landfill, where residences were present when NUS performed the RI/FS,The property is now owned by DelDOT, and the residential structures have been removed,Infiltration in the area to the east of the Main Landfill has been somewhat reduced by thesurface drainage facilities installed with the new Route 7 interchange.

The predesign exploration of the site hydrogcology led to a reevaluation of the effectivenessof the drains versus other effective alternatives, Specific attention was focused on a slurrytrench as an alternative. The NUS feasibility study had considered a slurry trench on thenortheast and the north sides of the landfill, The current design work has led to designmodifications including use of a passive slurry trench rather than actively pumping thesegments of a subsurface drain.

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4.2,2 Evaluation of (he Alternatives

4.2,2.1 Overall Effectiveness. Along Route 71 some groundwatcr control is required, especiallyto the cast. Along this side of the landfill the water table is closer to the bottom of therefuse than in the landfill in general; therefore, less lowering of the water table in the refuseIs needed. However, aiong this side of the landfill the aquitard thins. Even where theaqultard thins, the slurry trench controls the lateral flow of groundwater because verticalpermeabilities are lower than the horizontal permeabilities. The model simulations describedin Section 4.3, indicate the effectiveness of the slurry trench, even with this condition. Thepreliminary extent of the slurry trench concept is shown on Figure 4-1.

While the north drain would be effective in terms of collection and treatment of alreadycontaminated groundwater, the quantities of flow are not large (see Section 4.3). Projectedreliability of the north drain is good. Unless it became clogged or pumps failed to operate,it would reliably intercept groundwater flow. However, the slurry trench would be apermanent, passive seal and therefore would be more reliable in controlling ambientgroundwater flow to allow water levels in the refuse to be lowered.

4,2.2.2 Operation and Maintenance. The operation and maintenance (O&M) costs aremoderate for the north drain but have considerable uncertainty. If the drains clog they willhave to be treated or require intensive inspection or monitoring, therefore, O&M costs willbe fairly high, The slurry trench requires virtually no maintenance and there are nooperational elements for this passive system.

4,2,2,3 Short-Term Effectiveness. The major construction problem for the drain is the lack ofspace. The edge of the refuse is close to the property line, so that limited width Is availablefor drain construction, This problem is most apparent along Route 71, where the refuse hasbeen found within a few feet of the existing fence along the highway right-of-way.

During the construction period waslewater from the drain and handling of contaminated soilswill lead to air emissions which could not be totally prevented, Workers would have to bein the trench to install the drain piping and to place drainage material around the pipe. A «"'!"'!

4-6

flROQ2i>76

slurry trench on the other hand requires n designation of areas for slurry mixing and handling,but it has effectively no air emissions and requires only grade level construction.

4.2.2.4 Costs. The cost estimates of NUS (1985) associated with the slurry trench versussubsurface drain construction did not indicate large differences in the cost between the twotechnologies. Sinuc that time, a number of hazardous waste sites as well as non-hazardouslandfills have successfully used slurry trench construction to economically and effectivelycontrol groundwatcr flow, Table 4-2 compares the estimated costs of the slurry trench to thecosts of the northern drain. As indicated, the capital cost estimate for the northern drain isapproximately $2.4 million assuming that level D personnel protection only is needed. IfLevel B personnel protection is required, a 100% increase in this cost is projected. The slurrytrench alternative is estimated to cost approximately $1.25 million, a savings of approximately$1.15 million (at Level D) and of $3.55 million if Level B work is required for the drain.Because the drain would require some operation and maintenance, the comparison of thepresent worth of the two alternatives is even more divergent.

The declining cost since 1985 of slurry trench construction and the escalating costs of deepsubsurface drain construction lead to the modified design concept, the slurry trench, as aneffective design stage modification to effectively lower the water level in the refuse. Table4-3 summarizes the overall comparison between the northern drain and the slurry trenchalternatives at the northern end of (he landfill.

4.3 PREDICTIONS OF EFFECTIVENESS OF ALTERNATIVES

The groundwaler flow regimes for various conditions have been evaluated in the revisedversion of the three dimensional model used in 1985 by NUS. As discussed above in Chapter3, the current version of the model has refined boundary conditions, a refined and expandedgrid, the addition of a third layer and is recalibrated, based on data obtained during the designexplorations and conservative assumptions. Figure 3-5 shows the estimated thickness ofsaturated refuse based on current water table conditions and the elevation of the base ofrefuse, The mean thickness of saturated refuse is 2,5 ft for the existing conditions.

4-7 -

HR002l»77

TABLE 4-2

Tybouts Corner Landfill Remediation

;, Summary of Alternative Upgradient Control Costs

1. Drains

a. Capital:

Drain Construction (1500 ft)1 $2,200,0002 Pumping Stations 80,0002 Force Mains (2100 ft + 3400 ft) 90,000

Total Capital Costs: $2,370,000

b. Total O&M Costs: 1st Year $120,0002nd Year $40,000

2, Slurry Trench (2,462 LF)

a. Capital:

Basic Trench (2,462 ft)Area @ average Depth of 29 ft 71100 sfKey to Merchantville @ 3 ft 7386 sf

Total - Bounded by Merchantville 78486 sf $1,020,000(@ $13/sf)

Extension Trench:600 ft south along Rte 71: 18000 sf $230,000600 ft x 30 ft depth (@ $13/sf)

Total with Extension $1,250,000

b. Total O&M Costs:

• 'Assumes all construction at Level D; field conditions may dictate Level B protection atsubstantially greater costs, AR002478

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The model simulations of the ROD design concept were developed using a set of drains atthe north end of the landfill (Figure 4-2) which had inverts at the top of the aquitard, Alongthe northeast side of the landfill one drain segment (A) runs from Route 13 to the low pointin the aquitard about halfway across the northeast side of the landfill. A second segment (B)runs from the landfill "dog leg" and flows southward to the low point in the aquitard. A liftstation would be installed at the low point and pump the effluent to the pretreatmenl facility.The drain was not extended across the "dog leg" because (he aquilard elevations rise so thatthe drain would collect very small quantities of water and that water in any case would becollected by segment B referenced above or would flow to the north to the drain segments(C and D) along Route 71, Along Route 71 various drain lengths were simulated extendingalong the length of segments C and D shown in Figure 4-2. These drains segments alsofollow the elevation of the top of the aquitard and flow to the west along Route 71. At theirterminus a lift station would pump the effluent Into a force main which would continue alongRoute 71 to the pretreatment facility.

The landfill cap as presented in the ROD was simulated allowing 2% of the Incidentprecipitation to infiltrate,

The drain at the south end of the landfill was simulated with the alignment shown on Figure1-1. The inverts were set to slope downward along Route 13 in segment E to a low pointapproximately 3 ft below sea level where segment E meets segment F. Segment G slopesdown to segment F.

Interceptor wells were simulated for three locations along Route 13 and one location nearthe west landfill as conceptualized in the ROD. Flow rates from these wells were adjusteduntil the model could run to the five year time step without the wells going dry. Becauseof the lower transmlssiviry near the wells to the northeast (1-1 and 1-2) their yields were setto 5 and 1 gpm respectively. Near the south end of the landfill where the pumping testsrevealed more transmlssive formations, wells 1-3 and 1-4 were simulated at IS gpm. Table 4-4lists the constant flow rates for each well and the variable drain flow rates for the RODremedy. These rates are approximate, since the model simulation of field stratigraphy andpermeabilities cannot be precise. Actual flows that could be pumped from the wells and the

4-8

TABLE 4-4

ROD DESIGN CONCEPT

Tybouls Corner Landfill

WELLS

1-11-21-31-4

Total (0PM)

FLOW1WEEK

5.01.0

15,015,036

GPM1 MONTH

5,01,0

15.015.036

1YEAR

5.01.0

15.015.036

5 YEARS

5,01.0

15.015,036.

NORTH DRAIN

Segment A Totals (GPM)Segment B Totals (GPM)Segment D Totals (GPM)North Drain Total (GPM)

FLOW1WEEK

44.963334.7

142.8

GPM1 MONTH

15,221.112.648.9

1YEAR

1.00.60.382.0

5 YEARS

0.80,50,101,4

SOUTH DRAIN

Segment E Totals (GPM)Segment F Totals (GPM)Segment G Totals (GPM)South Drain Total (GPM)

FLOW1WEEK

7.15.883

21.2

GPM1 MONTH

5.25.07.6

17.8

1YEAR 5YEARS

3.5 2.83.5 2.96.2 4.113.2 9.6

o Total now (GPM) 200.1 102.7 5U 47.0

July 1991DPL CONSULTANTS

AR002lt82

yield of drains could easily be 50% greater (or less) than the modeled rates, The modelsimulation of the ROD design concept was run in a lime dependent mode of the model.Table 4-4 lists the decline in yield from each of the drains through the five year time period.

The effect of the ROD design concept with respect to saturated refuse thickness is shown onFigure 4-2, For all practical purposes, the landfill cap, drains and wells conceptualized in theROD is predicted to decrease the mean saturated thickness of the refuse from 2,5 ft to 0.3 ft.Much of the residual saturated refuse exists in thin layers at the base of the landfill and willcontinue to be depleted over time. In addition, saturated refuse (if present) would remainin the vicinity of TY-311 with the ROD design concept after 5 years of operation. Additional

'' dewatering in this area could be echieved by placement of an additional interceptor well inthat area,

At the south end of the landfill, well 1-3, the well furthest south along Route 13, depressedthe water table sufficiently to dewater one or more cells along the south drain. Since thedrain is continuous, this model result indicates a redundancy between the south drain and theinterceptor well as simulated; the interceptor well would collect leachate emanating from thelandfill.

Based on the groundwater model for the ROD design concept a modified concept wasdeveloped. It incorporates one additional interceptor well to replace the southern drain andan interceptor well in the vicinity of TY-311, if deep refuse is present there, In addition,preliminary design for the north drain has identified a number of issues relating to health,safety, reliability, and has defined the high cost associated with its construction operation andmaintenance as discussed above in Section 4,2, For these reasons a passive slurry trenchsystem was subjected to evaluation in the groundwater model as an alternative to the drain.The slurry trench alternative consists of a slurry trench keyed into the aquitard from Route13 up to the "dog leg" of the landfill in its northeast corner, The slurry trench then extendsaround the "dog leg" following the property line and extends westward along Route 71 to thewestern limit of refuse (see Figure 4-1). The depth of the slurry trench along Route 71 wasassumed to extend to the bottom of layer 1 (the approximate elevation of aquitard). Along

4-9

AR002l»83

this alignment a set of cells in the model was specified to be impermeable (inactive) tosimulate the slurry trench.

The modified design concept, replacing the south drain with supplemental interceptor wellsand the north drain with a slurry trench, is illustrated on Figure 4-1, Model simulations wererun for long term conditions (approximating steady slate after five years); flows are given inTable 4-5, For these simulations 2% of precipitation was assumed to infiltrate through thecap, as had been assumed in the ROD concept simulations.

The modified remedial plan was simulated with the conditions described above out to thesame time period (rive years) as the ROD design concept evaluation. Result! indicate nosignificant difference in the thickness of saturated refuse associated with the modified concept(Figure 4-3) as compared with ROD concept (Figure 4-2). The mean saturated refusethickness is simulated as 0.4 ft. At the south end of the landfill the use of multipleinterceptor wells can achieve overlapping cones of influence along the downgradlent side ofthe landfill in the PN-1 Formation.

This well plus slurry trench alternative, comparable to the ROD remedy in lowering the waterlevel in the refuse, might further lower the water level in the refuse for either the ROD ormodified remedy. The mean thicknesses of saturated refuse are listed in Table 4-6.

4,4 CONCLUSIONS

The goals of the remedial plan for the site are to lower water levels in the refuse to theextent practicable and to collect leachate emanating from the landfill.

A primary concern besides the effectiveness in lower water levels is the reliability of theoverall system. The modified remedy exerts more positive control over the PN-1 aquifer onthe downgradient side of the landfill by use of additional interceptor wells. The absence ofan aquitard at the south end of the landfill has raised some questions regarding the hydrauliccompatibility of the southern drain with the interceptor wells, both being located in the PN-1Formation. Considering the information available and reasonable estimates of future

4-10

TABLE 4-5

MODIFIED DESIGN CONCEPT

Tybouls Corner Landfill

WELL

1-11-21-31-41-5

1-6D1-6S

Grand Totals (GPM)

FLOW1WEEK

5.01.015.010.015.00,52.549

GPM1 MONTH

5.01.0

15.010.015.00.52.549

1YEAR

5.01.015.010.015.00.52.549

5 YEARS

5.01.015.010.015.00,52.549

July 1991DPL CONSULTANTS

TABLE 4-6

AVERAGE SATURATED REFUSE THICKNESSUSING DIFFERENT REMEDIAL ALTERNATIVES

Tybouls Corner Landfill

CONDITION

Initial Condition

ROD Concept (after 5 years)

Modified Concept (after 5 years)

AVERAGE SATURATEDREFUSE THICKNESS

2.5ft

.3ft

.4ft

% DEWATERED

88-

84

July 1991DPL CONSULTANTS

AR0021f86

conditioiu, the construction of additional interceptor wells is a more effective remedy at thesouth end of the landfill than the subsurface drain. The northern drain remains viable formeeting the ROD objectives, but a slurry trench would be more effective and economical andcould be constructed with less hazard potential.

4-11

AR0021J88

REFERENCES CITED

Civil Action No. 80-489 (LON), 1989. Consent decree of the United Slates and the Slateof Delaware and New Castle County, Stauffer Chemical Company and ICI America! Inc,

Tybouls Corner Landfill, RD/RA statement of work plan, Appendix C of Consent Decree.

Decision Document Preaulhorizalion of CERCLA Section 111 (a) Claim. 1988 TyboulsCorner Landfill • New Castle County Delaware.

McDonald, M.G., and A.W. Harbaugh. 1988. Techniques of water resources investigationof the USGS. Book 6, Chap Al, Open file report No, 83-875.

NUS Corporation. 1985. Remedial investigation/feasibility study report, Tybouts CornerLandfill Volume I-V: Appendices.

Record of Decision (ROD). 1986. Remedial Alternative Selection, Tybouts Comer Landfill.

R-lLawler, Matusky & Skelly Engineers

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