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Blue, Karen

From: Whittle, BlakeSent: Friday, September 22, 2017 7:16 AMTo: Blue, KarenCc: Kilburn, Dianna; Sadler, Bill; Spakes, Rita; Rich, JaySubject: FW: WCA Union County Selection of RemedyAttachments: V1.0 WCA Union County Selection of Remedy-Final.pdf

Karen,

Please enter the attached report into the Solid Waste Database for Union County Landfill (Permit: 0248 S1 R4).

Thank you,

Blake

From: Lindsay James, R.G. [mailto:ljames@blackstone-env.com]Sent: Thursday, September 21, 2017 4:41 PM To: Whittle, Blake Subject: WCA Union County Selection of Remedy

Blake

Please find attached the Selection of Remedy Report for the WCA Union County Landfill (AFIN 70 00364). Please let meknow if you have any questions or concerns.

Thanks,Lindsay

Lindsay E. James, R.G.Project ManagerBlackstone Environmental, Inc.9153 West 133rd StreetOverland Park, Kansas 66213913.495.9990 office913.956.4160 direct319.541.3697 cellljames@blackstone env.com

Rec’d Digitally

AFIN:_________________________

PMT#:_________________________ SW

DOC ID#:______________________ MD

TO:___________________________

By bluek at 10:08 am, Sep 22, 2017

70-00364

0248-S1-R4

72561

BS>FILE <KMB

Selection of Remedy

Union County Landfill El Dorado, Arkansas

Arkansas Solid Waste Permit Number 0248-S1-R4 AFIN: 70-00364

Prepared for:

Waste Corporation of Arkansas, Inc. 3083 Smackover Highway El Dorado, Arkansas 71730

Prepared by:

Blackstone Environmental 9153 West 133rd Street

Overland Park, KS 66213

September 21, 2017

TABLE OF CONTENTS

Section Page TABLE OF CONTENTS ............................................................................................................... ii

1.0 INTRODUCTION ................................................................................................................ 1

1.1 Site Location and Description 1

1.2 Site Geology and Hydrogeology 2

1.3 Groundwater Monitoring Network 2

1.3.1 Site Background and Groundwater Compliance History 2

1.4 Current Groundwater Quality 3

2.0 SELECTION OF REMEDY EVALUATION ........................................................................ 4

2.1 Selection Criteria Assessment – 22.1207(b) 4

2.2 Evaluation Criteria Assessment – 22.1207(c) 6

2.3 Schedule – 22.1207(d) 10

3.0 CONCLUSIONS AND RECOMMENDATIONS ................................................................ 13

4.0 LIMITATIONS ................................................................................................................... 14

5.0 REFERENCES ................................................................................................................. 15

Figures Figure 1. Site Plan Appendices Appendix A. Trench and Vent System Design Drawings

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1.0 INTRODUCTION

Blackstone Environmental, Inc. (Blackstone) has prepared this Selection of Remedy as required by Arkansas Pollution Control and Ecology Commission Regulation 22.1207 for the Union County Landfill. An Assessment of Corrective Measures was prepared for the Union County Landfill in accordance with Reg 22.1206 and approved by the Arkansas Department of Environmental Quality (ADEQ) on January 18, 2017 (DIN 70759, 70857). A public meeting was held on April 24, 2017, and no comments were received from the public. Based on the options presented in the Assessment of Corrective Measures, the Union County Landfill has selected the installation of a passive landfill gas (LFG) trench and vent system. This remedy is intended to control LFG migration from the pre-Subtitle D area as a remediation tool. The migration of landfill gas and partitioning of VOCs from the gas into the groundwater is a recognized source of groundwater impacts at the Union County Landfill. The persistent presence of landfill gas has also created a reducing environment resulting in elevated metals concentrations such as cobalt in the groundwater. Installation of a passive landfill gas extraction system consisting of a trench and vents will help to control the migration of landfill gas from the pre-Subtitle D area. As a result, the dissolution of metals, such as cobalt, should be reduced. This remedy is intended to address the source and may be coupled with MNA or ORC application to address residual elevated cobalt concentrations in the future, if required. In selecting the remedy described above, the Union County Landfill considered the criteria presented in Regulation 22.1207(b) and Regulation 22.1207(c). The primary objective of this report is to provide an evaluation of how the selected remedy meets these criteria. To facilitate this evaluation, Blackstone has prepared an engineering design for the selected remedy that is included as part of this report.

1.1 Site Location and Description

The Union County Landfill is located at 3083 Smackover Highway, El Dorado, Union County, Arkansas (Figure 1). The Landfill is owned and operated by Waste Corporation of Arkansas, Inc. under permit number 0248-S1-R4 issued by the Arkansas Department of Environmental Quality (ADEQ) on October 22, 2001. Solid waste placement has occurred in two primary areas, an initial pre-Subtitle D trench-fill unit identified as the Old City of Norphlet Dump and the Subtitle D Union County Landfill. Limited information is available regarding the base elevations for the trenches in the Norphlet Dump area,

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but the area is believed to contain 3 trenches that are approximately 15 feet wide and 15-30 feet in depth.

1.2 Site Geology and Hydrogeology

The upper and lower Cockfield Formation immediately underlies the Landfill. These two units have been identified as Stratum I and Stratum II in historical reports. Along the northern boundary, sand lenses are present in Stratum I and may be water-bearing in some locations. Based on the available information, it is not known if the sands in Stratum I are hydraulically connected to the sands in Stratum II. Beneath Stratum II are clays of the uppermost Cook Mountain Formation, which acts as a lower confining unit. Subsurface investigations also indicate the presence of a channel-fill deposit formed by a meander cutoff in the southern portion of the site. This deposit is comprised of laminated dark grey clay and laterally extends across the southern end of the facility. Cross sections illustrating the barrier can be found in historical reports (DIN 29823 and DIN 4607). The sands of Stratum II appear to be absent between the facility’s eastern boundary and midsection of the site, at which point the barrier begins to thin and eventually pinches out as it nears the western boundary. It has been suggested that the presence of the clay channel-fill deposits acts as a hydraulic barrier and may be causing two distinct groundwater flow patterns across the permitting footprint of the landfill (DIN 69070). Groundwater flow in Stratum II, north of the channel-fill deposit is generally to the northwest (DIN 69070).

1.3 Groundwater Monitoring Network

The current assessment groundwater monitoring network at the Union County Landfill consists of the following ten monitoring wells: MW-2, MW-4, MW-5R, MW-6, MW-8, MW-9, MW-12, MW-13, MW-14, MW-18 as specified in permit 0248-S1-R4. Six additional monitoring wells identified as NE-1 through NE-6 were installed as part of the Nature and Extent Investigation (NEI). Of the six NEI monitoring wells, NE-3 and NE-4 are currently sampled on an annual basis, NE-6 is sampled on a semi-annual basis, and NE-1, NE-2 and NE-5 are gauged during each semi-annual event to aid in construction of potentiometric surface maps. The addition of these NEI wells to the monitoring network is as specified in an ADEQ letter dated April 28, 2016. 1.3.1 Site Background and Groundwater Compliance History

The Union County Landfill is currently in assessment monitoring, which was initiated on May 25, 2007 by the ADEQ-SWMD (DIN 48047). The facility was placed in assessment monitoring due to recurring statistically significant increases (SSIs) as indicated by intrawell prediction limits, statistically significant increasing trends as indicated by formal trend tests, and detections of volatile organic compounds in two monitoring wells MW-9 and MW-4.

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The SWMD requested that an NEI be completed at the facility per Regulation 22.1205(g)(1) on March 4, 2009 (DIN 54811). An NEI Work Plan was submitted on March 27, 2009 (DIN 55046). Comments on the NEI Work Plan were issued by the SWMD on February 2, 2012 (DIN 61700) and a revised NEI was submitted to the SWMD on April 22, 2012 (DIN 62189). The revised NEI Work Plan was conditionally approved by the SWMD on April 25, 2012 (DIN 62269). In accordance with the revised NEI Work Plan, six NEI groundwater monitoring wells and four gas probes were installed to determine and/or delineate possible impacts to groundwater from landfilling activities. An NEI report was submitted to the SWMD on June 14, 2013 (DIN 64568). The general conclusion of the NEI was that the presence of landfill gas (LFG) was causing deterioration of groundwater quality in some of the facility’s groundwater monitoring wells. The cobalt in monitoring well MW-18 initially prompted the preparation of an Assessment of Corrective Measures (DIN70759). The cobalt concentration increases appear to be attributable to the presence of a reducing subsurface environment due to the presence of landfill gas. Dissolution of metals such as cobalt in the saturated zone may occur when reducing conditions geochemically alter the saturated zone. Reducing conditions can be naturally occurring, but exacerbated by landfill gas migration, as evidenced by the trace concentrations of volatile organic compounds (VOCs) in MW-9. The Assessment of Corrective Measures was approved by ADEQ on January 18, 2017 (DIN 70857).

1.4 Current Groundwater Quality

The Second Half 2016 Assessment Monitoring event at the Union County Landfill provided the following information on current groundwater quality (DIN 71269).

• Measured concentrations of cobalt exceeded the GWPS of 0.006 mg/L in monitoring wells MW-2 at 0.0073 mg/L, MW-4 at 0.0327 mg/L, MW-5R at 0.0087 mg/L, MW-6 at 0.0094 mg/L, MW-9 at 0.0342 mg/L, MW-12 at 0.0312 mg/L, and MW-18 at 0.0363 mg/L. The results are generally consistent with historical observations.

• Confidence interval analyses indicated that detected cobalt statistically exceeded the GWPS of 0.006 mg/L at the 99% confidence level at MW-2, MW-4, MW-9, and MW-18.

• Appendix I VOCs detected in concentrations above laboratory reporting limits included 1,1-dichloroethane at 0.0065 mg/L in monitoring well MW-9. Trichloroethene (TCE) was detected in concentrations above laboratory reporting limits at 0.0024 mg/L in monitoring well MW-9. Cis-1,2-DCE was detected at 0.0013 mg/L in MW-9. These concentrations

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were below the GWPS for both TCE and cis-1,2-DCE. However, the concentration of 1,1-DCA was above the GWPS of 0.05 mg/L.

• The Secondary Drinking Water Standard (SDWS) was exceeded for chloride in MW-9.

• The SDWS was exceeded for iron in all wells except MW-13 and MW-14.

• The SDWS was exceeded for manganese in all wells except MW-2, MW-6, MW-13, and MW-14.

• The SDWS was exceeded for TDS in MW-9.

• The SWDS of 6.5 to 8.5 standard units (S.U.) for pH was below 6.5 S.U. in the samples collected from all wells.

2.0 SELECTION OF REMEDY EVALUATION

Regulation 22.1207 specifies that based on the results of the corrective measures assessment, a remedy must be selected that addresses the selection and evaluation criteria specified in Regulation 22.1207(b) and (c), respectively. As previously discussed, the Union County Landfill has selected the installation of a passive landfill gas (LFG) trench and vent system. The selection criteria and evaluation assessment is presented below.

2.1 Selection Criteria Assessment – 22.1207(b)

1. Be protective of human health and the environment The passive trench and vent system will be located out of waste and will allow removal of landfill gas from the pre-Subtitle D area by using a lined cut-off trench with a system of passive vertical vents. Passive trench and vent systems are a widely used and accepted method to control landfill gas migration. This system is protective of human health and the environment as it controls the source of contamination. A comprehensive review was conducted to locate dwellings and property owners within a ½-mile radius of the Union County Landfill as part of the previously conducted NEI that may have a well (DIN 64568). Findings indicated that three property owners within the study area have a well, but none are used for domestic purposes. Furthermore, these wells are located south and upgradient of the Landfill’s permitted boundary. An additional search of the Arkansas Geological Commission well records did not indicate there were any wells located within the search area other than the monitoring wells located at the facility. The results of this comprehensive review suggest there are no known off-site users applicable to this limited extent of impact from the site. However, groundwater will continue to be monitored on a semi-annual basis for Union County Landfills’ AMC list, which includes VOCs and metals. Furthermore, because the source of contamination

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will be controlled by the passive trench and vent system, residual groundwater contamination levels are anticipated to decline over time. 2. Attain the groundwater protection standard as specified pursuant to Reg. 22.1205(h) or (i) Assessment Monitoring at the Union County Landfill was initiated on May 25, 2007 by the ADEQ-SWMD (DIN 48047). The facility was placed in assessment monitoring due to recurring statistically significant increases (SSIs) as indicated by intrawell prediction limits, statistically significant increasing trends as indicated by formal trend tests, and detections of volatile organic compounds in two monitoring wells MW-9 and MW-4. Cobalt in monitoring well MW-18 prompted the preparation of an Assessment of Corrective Measures (DIN70759). Historically, exceedances of the groundwater protection standard (GWPS) for cobalt have generally been identified in samples collected from monitoring wells MW-9 and MW-18. Additional exceedances of the GWPS for cobalt that were identified during the First and Second Half 2016 monitoring events are due to the recent reduction of the cobalt GWPS from 0.0031 mg/L to 0.006 mg/L to meet the criteria of lifetime exposure as required by APC&EC Regulation 22.1205(i)(4). The selected remedy is designed to control the landfill gas migration, which will also control the enhanced dissolution of cobalt. Because cobalt can be naturally occurring in soils, Blackstone has proposed to collect soil samples during the installation of upgradient replacement monitoring well MW-13R to evaluate the concentration of naturally occurring cobalt and allow for the calculation of site-specific background concentrations (DIN70050 and DIN70506). If cobalt concentrations remain above the GWPS after the landfill gas migration has been controlled, a Work Plan will be submitted to ADEQ for groundwater restoration using in-situ groundwater treatment in accordance with the Assessment of Corrective Measures. 3. Control the source(s) of release so as to reduce or eliminate, to the maximum extent practicable, further release of Appendix 2 constituents into the environment that may pose a threat to human health or the environment The source of the release is believed to be landfill gas generated from the decomposition of solid waste from the pre-Subtitle D trench-fill unit identified as the Old City of Norphlet Dump. The increased cobalt concentrations in groundwater appear to be attributable to the presence of a reducing subsurface environment due to the presence of landfill gas from this pre-Subtitle D area. Dissolution of metals such as cobalt in the saturated zone may occur when reducing conditions geochemically alter the saturated zone. Reducing conditions and the presence of these metals can be naturally occurring, but exacerbated by landfill gas migration, as evidenced by the trace concentrations of volatile organic compounds (VOCs) in MW-9. Installation of the passive trench and vent system will allow the source to be controlled and safety managed so that the leaching of metals from soil to groundwater is no longer exacerbated by the presence of landfill gas.

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4. Comply with standards for management of wastes as specified in Reg.22.1208(d) Because the cut-off trench will be located out of the waste mass, it is not anticipated that any solid waste will be encountered during construction. Based on the trench design specifications, the generation of appreciable amounts of condensate is also not anticipated. However, should any wastes be encountered or managed during construction and operation of the trench and vent system, those materials will be handled in a manner protective of human health and the environment and in compliance with applicable RCRA requirements.

2.2 Evaluation Criteria Assessment – 22.1207(c)

1. The long- and short-term effectiveness and protectiveness of the potential remedy(s), along with the degree of certainty that the remedy will prove successful based on consideration of the following: i. Magnitude of reduction of existing risks The magnitude of reduction of existing risks will be evaluated using semi-annual groundwater monitoring data. Monitoring well NE-3 will be located between the pre-Subtitle D area and the passive trench as shown on the design drawings. Monitoring wells MW-9, MW-18, and MW-4 are located downgradient from the trench and have shown increased cobalt concentrations and MW-9 has also shown VOC impacts. Groundwater quality data in each of the downgradient wells will be evaluated for trends and overall reduction in metals and VOC concentrations and compared to data from NE-3 on a semi-annual basis. It is anticipated that wells downgradient of the trench will show decreasing trends in metals and VOC concentrations following trench installation, while monitoring well NE-3 will remain consistent with historical results. Monitoring well NE-3 is currently sampled on an annual basis and it is recommended that this well be sampled semi-annually in conjunction with the regularly scheduled semi-annual groundwater monitoring going forward to allow for evaluation of trench performance in both the short and long-term. ii. Magnitude of residual risks in terms of likelihood of further releases due to waste remaining following implementation of a remedy The risk of further LFG release impacting downgradient groundwater quality is minimal since the trench is designed to intercept LFG along the entire length of the southern pre-Subtitle D landfill boundary. Decomposition of solid waste within the pre-Subtitle D landfill area will continue, but LFG generation should decline over the long-term. The passive trench and vent system is

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expected to remain in operation for the duration of the period where landfill gas generation occurs as measured by sampling the landfill gas probes. iii. The type and degree of long-term management required, including monitoring, operation, and maintenance No long-term maintenance of the passive trench and vent system is anticipated. The maximum depth of the trench (20-feet) has been designed to avoid contact with groundwater to reduce the potential for the trench to become watered-in and require maintenance such as removal of liquid from the trench. Any necessary operation and maintenance of the passive trench and vent system will be overseen by experienced technicians. Groundwater monitoring to evaluate trench performance is anticipated to continue until groundwater quality has been restored. iv. Short-term risks that might be posed to the community, workers, or the environment during implementation of such a remedy, including potential threats to human health and the environment associated with excavation, transportation, and re-disposal or containment Routine health and safety procedures will be used to avoid and/or eliminate short-term risks to human health of site workers and the community. The proposed trench construction will be performed by experienced and trained personnel and limited to the confines of the facility property. Because the trench will be installed outside the waste footprint, handling of waste material is not anticipated as part of this project. v. Time until full protection is achieved It is anticipated that the passive system would require six months for implementation after ADEQ approval of the selected remedy. The time required to achieve full protection including decline of cobalt concentrations is dependent on the amount of landfill gas that can be diverted to the passive trench and vent system. Measurement of LFG probes and monitoring of groundwater quality in proximity to the pre-Subtitle D landfill area and passive trench and vent system will provide a means to measure progress with time. vi. Potential for exposure of humans and environmental receptors to remaining wastes, considering the potential threat to human health and the environment associated with excavation, transportation, re-disposal, or containment All activities associated with the passive trench and vent system will be performed by experienced and trained personnel in accordance with routine health and safety procedures. Because the trench will be installed outside the waste footprint, contact with waste is not anticipated. As such,

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the potential for exposure to humans and the environment is anticipated to be low and can be managed. vii. Long-term reliability of the engineering and institutional controls Engineering and institutional controls are designed and overseen by professional engineers licensed in the State of Arkansas. Installation of engineering and institutional controls will be performed by experienced and qualified contractors. The passive trench and vent system is designed to operate in an automatic manner and any necessary maintenance will be conducted by experienced technicians. As such, these controls are considered to be highly reliable. viii. Potential need for replacement of the remedy The potential need to replace the passive trench and vent system is considered to be low. System performance will be monitored, and if it is determined that additional steps are necessary, a work plan will be submitted for ADEQ review and approval. 2. The effectiveness of the remedy in controlling the source to reduce further releases based on consideration of the following factors: i. The extent to which containment practices will reduce further releases The pre-Subtitle D landfill area that has been identified as the likely source of LFG is closed and no longer accepting waste. The installation of the passive trench and vent system is anticipated to control gas migration along the southern border of the pre-Subtitle D landfill area, which is intended to greatly reduce the potential for further migration. ii. The extent to which treatment technologies may be used The use of treatment technologies is not anticipated for the passive trench and vent system. 3. The ease or difficulty of implementing a potential remedy(s) based on consideration of the following types of factors: i. Degree of difficulty associated with constructing the technology The degree of difficulty associated with constructing a passive trench and vent system is generally low. Implementation issues associated with installation of the passive trench and vent system are not anticipated. Confounding variables may include stability of the trench walls depending on the final depth of excavation and subsurface materials encountered (i.e. sands, gravels, etc.). The

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contractor will place the trench as close as possible to the design location to reduce disturbance to existing structures and wells. Trench elevations will follow existing topographic elevations in the vicinity of the excavation area. Standard erosion control practices will be applied to all disturbed areas. Excavated material will be placed away from the trench in a manner that will not impose stress on the sideslopes, causing a cave-in. Routine health and safety precautions will be taken. ii. Expected operational reliability of the technologies Passive trench and vent systems are a widely used and accepted method to control landfill gas migration. The passive trench and vent system has been designed to operate in an automatic manner. The maximum depth of the trench (20-feet) has been designed to avoid contact with groundwater to reduce the potential for the trench to become watered-in and require maintenance such as removal of liquid from the trench. Operation and maintenance of the passive trench and vent system will be overseen by experienced technicians. iii. Need to coordinate with and obtain necessary approvals and permits from other agencies The ADEQ Air Division issues permits related to emissions from solid waste facilities. The Union County Landfill already has a permit for operation of their Gas Control and Collection System (GCCS). As this project is considered an extension of the existing GCCS, no additional permit requirements are anticipated. iv. Availability of necessary equipment and specialists Necessary equipment and specialists are available from a number of vendors and qualified contractors. v. Available capacity and location of needed treatment, storage, and disposal services The Union County Landfill has the necessary resources to manage the operation and maintenance needs of the passive trench and vent system. Because the cut-off trench will be located out of the waste mass, it is not anticipated that any solid waste will be encountered during construction. Based on the trench design specifications, the generation of appreciable amounts of condensate is also not anticipated. However, should any wastes be encountered or managed during construction and operation of the trench and vent system, solid waste will be disposed of at the Union County Landfill in the active Subtitle D waste disposal cell. Liquid waste will be treated in a similar manner as the leachate as part of the routine operation of the site.

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4. Practicable capability of the owner or operator, including a consideration of the technical and economic capability The Union County Landfill has trained staff and financial resources to hire qualified consultants and contractors, as necessary, to maintain the passive trench and vent system and conduct ongoing groundwater monitoring activities. 5. The degree to which community concerns are addressed by a potential remedy(s) There are no known community concerns at this time. A public meeting was held on April 24, 2017, and no comments were received from the public.

2.3 Schedule – 22.1207(d)

Regulation 22.1207(d) requires that a schedule be provided for initiating and completing the selected remedy. The schedule requires the initiation of remedial activities within a reasonable period of time and is presented below. Task Date or Estimated Time Frame ACM Report to ADEQ January 10, 2017 ADEQ Approval of ACM Report January 18, 2017 Public Meeting to discuss ACM April 24, 2017 Final Remedy Selection September 11, 2017 ADEQ Approval of Remedy Selection 60-days from Final Remedy Selection Implement Remedy Within 6-months of ADEQ Approval of Remedy

Selection Corrective Action Status Reports Following submittal of each Second Half

Semi-Annual Monitoring Report Corrective Action Completion To be Determined

As specified in 22.1207(d), consideration was also given to the following factors during schedule development: 1. Extent and nature of contamination. An NEI report was submitted to the SWMD on June 14, 2013 (DIN 64568). The general conclusion of the NEI was that the presence of landfill gas (LFG) was causing deterioration of groundwater quality in some of the facility’s groundwater monitoring wells. The NEI has been approved by ADEQ.

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2. Practical capabilities of remedial technologies in achieving compliance with groundwater protection standards established under Reg. 22.1205(g) or (h) and other objectives of the remedy Passive trench and vent systems are a widely used and accepted method to control landfill gas migration. The pre-Subtitle D landfill area has been identified as the likely source of LFG. The installation of the passive trench and vent system is anticipated to control gas migration along the southern border of the pre-Subtitle D landfill area, which is intended to greatly reduce the potential for further LFG migration. The time required to achieve compliance including decline of cobalt concentrations below the GWPS is dependent on the amount of landfill gas that can be diverted to the passive trench and vent system. Measurement of LFG probes and monitoring of groundwater quality in proximity to the pre-Subtitle D landfill area and passive trench and vent system will provide a means to measure progress with time. It is anticipated that operation of the passive trench and vent system will show improving water quality results with each semi-annual groundwater monitoring event. 3. Availability of treatment or disposal capacity for wastes managed during implementation of the remedy The Union County Landfill has the necessary resources to manage construction, operation, and maintenance needs of the passive trench and vent system. Because the cut-off trench will be located out of the waste mass, it is not anticipated that any solid waste will be encountered during construction. Based on the trench design specifications, the generation of appreciable amounts of condensate is also not anticipated. However, should any wastes be encountered or managed during construction and operation of the trench and vent system, solid waste will be disposed of at the Union County Landfill in the active Subtitle D waste disposal cell. Liquid waste will be treated in a similar manner as the leachate as part of the routine operation of the site. 4. Desirability of utilizing technologies that are not currently available, but which may offer significant advantages over already available technologies in terms of effectiveness, reliability, safety, or ability to achieve remedial objectives Because passive trench and vent systems are a widely used and accepted method to control landfill gas migration, it is not desirable to use other technologies at this time. Based on the performance evaluation of the system over time, it may be desirable to investigate other measures to compliment the passive trench and vent system in the future, if required. 5. Potential risks to human health and the environment from exposure to contamination prior to completion of the remedy

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The passive trench and vent system will be located out of waste and therefore exposure to solid waste is not anticipated during trench construction. The depth of the trench has been designed to avoid contact with groundwater to reduce the potential for the trench to become watered-in and require removal of liquids. This system is protective of human health and the environment as it controls the source of contamination. A comprehensive review was conducted to locate dwellings and property owners within a ½-mile radius of the Union County Landfill as part of the previously conducted NEI that may have a well (DIN 64568). Findings indicated that three property owners within the study area have a well, but none are used for domestic purposes. Furthermore, these wells are located south and upgradient of the Landfill’s permitted boundary. An additional search of the Arkansas Geological Commission well records did not indicate there were any wells located within the search area other than the monitoring wells located at the facility. The results of this comprehensive review suggest there are no known off-site users applicable to this limited extent of impact from the site. However, groundwater will continue to be monitored on a semi-annual basis for Union County Landfills’ AMC list, which includes VOCs and metals. Furthermore, because the source of contamination will be controlled by the passive trench and vent system, residual groundwater contamination levels are anticipated to decline over time. 6. Resource value of the aquifer The resource value of the aquifer is considered limited in the vicinity of the site. There are no known current uses as the nearest property owners that have wells do not use them for domestic purposes. Furthermore, these wells are located south and upgradient of the Landfill’s permitted boundary. Potential impact caused by waste exposure is considered unlikely as any waste removal and relocation activities would occur within the facility boundaries. Site specific and regional hydrogeological characteristics have been documented in previous reports including the NEI and semi-annual groundwater monitoring reports submitted to ADEQ. Groundwater removal and treatment costs cannot be accurately calculated at this time as they are dependent on the level of treatment desired and residual levels of contamination. Costs of alternate water supplies have not been evaluated as there are no known domestic water users within the area. 7. Practicable capability of the owner or operator The Union County Landfill is fully capable of constructing, operating, and maintaining the selected remedy. 8. Other relevant factors There are no other known relevant factors.

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3.0 CONCLUSIONS AND RECOMMENDATIONS

The following conclusions and recommendations are based upon an evaluation of the selected remedy presented in this report in meeting the criteria described in Regulation 22.1207. Cobalt in monitoring well MW-18 initially prompted preparation of an Assessment of Corrective Measures. The cobalt concentration increases appear to be attributable to the presence of a reducing subsurface environment due to the presence of landfill gas. Dissolution of metals such as cobalt in the saturated zone can occur when reducing conditions geochemically alter the saturated zone. Reducing conditions and metals can be naturally occurring, but exacerbated by landfill gas migration, as evidenced by the trace concentrations of VOCs in MW-9 and the cobalt concentrations in MW-18 and MW-9. Because landfill gas presumably from the pre-Subtitle D area is considered the dominant source of groundwater issues at the Union County Landfill, installation of a passive trench and vent system is proposed to control landfill gas migration from the pre-Subtitle D area. Efficacy of the passive trench and vent system will be assessed by monitoring and evaluating landfill gas probe and groundwater monitoring well data from upgradient and downgradient of the passive trench. ADEQ’s policy states that groundwater remediation shall protect, enhance, and restore groundwater conditions based on current and reasonably anticipated future groundwater usage. The proposed passive trench and vent system is designed to control the landfill gas migration, which will also control the enhanced dissolution of cobalt. Because cobalt can be naturally occurring in soils, Blackstone has proposed to collect soil samples during the installation of upgradient replacement monitoring well MW-13R to evaluate the concentration of naturally occurring cobalt and allow for the calculation of site-specific background concentrations (DIN70050 and DIN70506). If cobalt concentrations remain above the GWPS after the landfill gas migration has been controlled, a Work Plan will be submitted to ADEQ for groundwater restoration using in-situ groundwater treatment to address the residual groundwater contamination. If required, the specific in-situ groundwater treatment will be selected after residual cobalt concentrations are evaluated to select the most appropriate in-situ technique based on the most current concentration data.

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4.0 LIMITATIONS

This report was prepared in accordance with that level of skill and care ordinarily exercised by other members of Blackstone’s profession practicing in the same locality and under similar conditions when the services were provided. No warranties, express or implied, are intended or made.

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5.0 REFERENCES

1. FTN (June 2013). Nature and Extent Summary Report, Union County Landfill, El Dorado, Arkansas.

2. FTN (March 2016). Second Half 2015 Groundwater Assessment Monitoring Report, Union County Landfill, El Dorado, Arkansas.

3. RUST (1994). Final Report of the Hydrogeological/Geotechnical Investigation, Union

County Landfill, El Dorado, Arkansas.

4. RUST (1995). Hydrogeological /Geotechnical Investigation for the Horizontal Expansion onto Contiguous Property, Union County Landfill, El Dorado, Arkansas.

5. RUST (1997). Evaluation of Clay Plug Structure Southern Expansion Tract, Union County

Recycling and Disposal Facility, El Dorado, Arkansas.

6. Krupka, K.M. and R. J. Serne, (December 2002). Geochemical Factors Affecting the Behavior of Antimony, Cobalt, Europa, Technetium, and Uranium in Vadose Sediments, Pacific Northwest National Laboratory.

7. ITRC, (December 2010). Decision Framework for Applying Monitoring Natural Attenuation Processes to Metals and Radionuclides in Groundwater.

8. United States Environmental Protection Agency, (December 2010). Use of Monitored

Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites.

FIGURES

UNION COUNTY LANDFILL Site Plan

GMP-09

GMP-11R

MW-06MW-07

MW-04

GMP-01

GMP-10

GMP-7R

GMP-08R

GMP-02

GMP-05

NE-6

GMP-04

GMP-03

MW-08

MW-12

MW-02

MW-05R

MW-09

MW-13

MW-14

NE-3

NE-2

MW-18

NE-4

NE-5

NE-1

GMP-12

GMP-13

GMP-14

GMP-15

GMP-06R

MW-13R

LEGEND

MW-08

NE-3

GMP-05

PROPERTY BOUNDARY

GROUNDWATER MONITORING WELL

INVESTIGATIVE GROUNDWATER WELL

EXISTING GAS MONITORING PROBES

FENCE

APPROXIMATE AREA WHERE STRATUM II

IS ABSENT (BASED ON RUST 1995)

APPROXIMATE LIMITS OF CLAY CHANNEL-FILL

(BASED ON RUST 1995)

1

SHEET

Project M

gr LJ

Designed B

y A

Q

Draw

n B

y A

Q

Checked B

y LJ

Job N

o. 1673

Date 07--22-2016

Rev.

Rev.

Rev.

Rev.

400' 800'0'

BL

AC

KST

ON

EEN

VIR

ON

MEN

TAL

MO

NIT

OR

IN

G W

EL

L S

IT

E M

AP

WC

A W

AS

TE

C

OR

PO

RA

TIO

N

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M

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UN

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DF

IL

L

30

83

S

MA

CK

OV

ER

HW

Y

EL

D

OR

AD

O, A

R 7

17

30

APPROXIMATE LIMITS

OF THE OLD CITY OF

NORPHLET DUMP

STORMWATER

POND

CLAY CHANNEL-FILL

DEPOSIT THINS

WESTWARD

STORMWATER

POND

STORMWATER

POND

LEACHATE

STORAGE

AREA

SCALE HOUSE

& OFFICE

APPENDIX A

UNION COUNTY LANDFILL Passive Trench and Vent Design Drawings

GMP-10

GMP-01

GMP-09

GMP-11R

GMP-08R

GMP-7R

GMP-02

GMP-05

GMP-14

GMP-04

GMP-03

MW-06

MW-07

MW-04 MW-08

MW-05R

MW-02

MW-12

GMP-15

MW-18

MW-14

GMP-05

GW-32

DL-1

DL-1

GW-19

GW-21

GW-22

GW-23GW-25

GW-24

GW-20

GW-17

GW-16

GW-14

GW-15

GW-28

GW-27

GW-26

GW-29

GW-31

GW-30

GW-8

GW-12

GW-7

GW-6

GW-5

GW-9

GW-10

GW-11

GW-32

GW-33

GW-40

GW-41

GW-13

GW-4

H

P

HP

H

P

6"

6"

6"6"

6"

6"

6"

6"

6"

6"

6"

6"

6"

6"

6"

6"

6"

12"

12"

12"

12"

12"

12"

12"

12"

12"

12"12"

8"

8"

8"

8"

8"8"

6"

6"

GW-12

DL-1

NE-6

NE-3

NE-2

NE-4

NE-5

NE-1

GMP-13

APPROXIMATE LIMITS OF THE

OLD CITY OF NORPHLET DUMP

MW-13

MW-14

MW-13R

GMP-06R

250

225

200

200

175

1

7

5

200

2

2

5

2

5

0

2

0

0

225

2

5

0

2

0

0

1

7

5

175

2

0

0

2

2

5

2

5

0

CELL 11

CELL 12

CELL 10

CELL 3

C

E

L

L

2

CELL 1

CELL 9

CELL 8

CELL 7 CELL 6

CELL 13

EAST

FUTURE

CELL 13

WEST

CELL 4

FU

TU

RE

CE

LL 16

FU

TU

RE

CE

LL 17

FU

TU

RE

CE

LL 15

CELL 5

180

1

7

5 1

8

0

190

1

8

5

1

9

0

1

8

5

1

8

0

1

8

0

175

170

6"

12"

12"

MW-09

GW-3

GW-2

GW-1

8"

8"

LEGEND

PROPERTY BOUNDARY

CELL LIMITS

MW-09

EXISTING 5' CONTOURS

EXISTING 1' CONTOURS

GROUNDWATER MONITORING WELL

INVESTIGATIVE GROUNDWATER MONITORING WELL

PROPOSED GROUNDWATER MONITORING WELL

GAS MONITORING PROBE

STORMWATER POND

CLOSED PRE-SUBTITLE D LANDFILL

NE-2

MW-13R

GMP-02

ACCESS ROAD

NOTE:

SURFACE CONTOURS FROM SURVEY CONDUCTED BY TUKUH

TECHNOLOGIES, FEBRUARY 27, 2017. COORDINATES FOR THIS

PROJECT ARE ARKANSAS STATE PLANE (NAD83 ARKANSAS SOUTH ZONE).

GW-30

EXISTING LANDFILL GAS COLLECTION SYSTEM

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\C

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esign_eg (24X

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150' 300'0'

BL

AC

KST

ON

EEN

VIR

ON

MEN

TAL

LEACHATE STORAGE TANK

Image courtesy of USGS © 2017 Microsoft Corporation

GMP-02

GMP-04

GMP-03

H

P

12"

NE-3

NE-2

GMP-13

CONCRETE PAD

CONTRACTOR TO

REPAIR AS NECESSARY

TO MATCH EXISTING

LEACHATE

STORAGE TANK

ANTICIPATED

WORKING AREA

25' WIDTH

APPROXIMATE LIMITS OF THE

OLD CITY OF NORPHLET DUMP

180

1

7

5

1

8

0

190

1

8

5

1

9

0

1

8

5

1

8

01

8

0

175

170

6"

12"

12"

CUT-OFF

TRENCH

615' LENGTH

SEE DETAIL A

MW-09

GW-3

GW-2

GW-1

8"

8"

BEGIN CUT-OFF TRENCH

NORTHING=239187.50

EASTING=1791328.64

END CUT-OFF TRENCH

NORTHING=239187.50

EASTING=1791943.80

TYPICAL GAS VENT

SEE DETAIL B

LEGEND

PROPERTY BOUNDARY

EXISTING 5' CONTOUR

EXISTING 1' CONTOUR

ACCESS ROAD

CLOSED PRE-SUBTITLE D LANDFILL

STORMWATER POND

CUT-OFF TRENCH

APPROXIMATE WORKING AREA

GROUNDWATER MONITORING WELL

INVESTIGATIVE GROUNDWATER

MONITORING WELL

GAS MONITORING PROBE

LANDFILL GAS COLLECTION SYSTEM

MW-09

NE-2

GW-30

GMP-02

GAS VENT

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40' 80'0'

BL

AC

KST

ON

EEN

VIR

ON

MEN

TAL

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BL

AC

KST

ON

EEN

VIR

ON

MEN

TAL

3'

17'

20'

6" HDPE RISER TO

GAS VENT SEE

DETAIL B

1'

NOMINALLY

COMPACTED CLAY

FILL

20 MIL PVC LINER &

GEOTEXTILE OR

ENGINEER-APPROVED

EQUAL

20-MIL PVC LINER & GEOTEXTILE

WRAPPED OVER ROCK

NOT TO SCALE

TYPICAL CUTOFF TRENCH

SECTION

NOT TO SCALE

TYPICAL PASSIVE

GAS VENT

6" HDPE

RISER

6" HDPE

ELBOW

6" HDPE

ELBOW

B

-A

GENERAL NOTES:

1. CONTRACTOR TO PLACE TRENCH AS SHOWN ON SHEET 2 AS CLOSE AS POSSIBLE TO THE DESIGN LOCATION TO REDUCE DISTURBANCE TO EXISTING STRUCTURES AND

WELLS. IF FIELD CONDITIONS ALLOW OR REQUIRE A SHIFT IN TRENCH LOCATION TO REDUCE DISTURBANCE OF EXISTING STRUCTURES, THE VARIATION SHALL BE

APPROVED BY THE ENGINEER FIRST.

2. ANY DISTURBED STRUCTURES SHALL BE RE-CONSTRUCTED AFTER THE TRENCH IS COMPLETED TO MATCH EXISTING CONDITIONS AS CLOSELY AS POSSIBLE.

3. TRENCH ELEVATIONS WILL FOLLOW EXISTING TOPOGRAPHIC ELEVATIONS IN THE VICINITY OF THE EXCAVATION AREA.

4. ANY WASTE ENCOUNTERED DURING TRENCH EXCAVATION SHALL BE TRANSPORTED TO THE ACTIVE FACE OF THE LANDFILL AND WILL BE THE RESPONSIBILITY OF THE

CONTRACTOR.

5. PVC PANELS SHALL BE OVERLAPPED A MINIMUM OF 6 INCHES. PVC PANELS SHOULD BE GLUED WITH VINYL CEMENT ADHESIVE (OR ENGINEER-APPROVED EQUAL).

6. GEOTEXTILE PANELS SHALL BE OVERLAPPED A MINIMUM OF 6 INCHES. GEOTEXTILE PANELS DO NOT NEED TO BE SEAMED TOGETHER.

7. STANDARD EROSION CONTROL PRACTICES WILL BE APPLIED TO ALL DISTURBED AREAS.

8. EXCAVATED MATERIAL WILL BE PLACED AWAY FROM THE TRENCH IN A MANNER THAT WILL NOT IMPOSE STRESS ON THE SLIDESLOPES CAUSING A CAVE-IN.

1

1

2

" TO 3"

WASHED STONE,

NONCALCAREOUS

-

PIPE BOOT

CONSTRUCTED

AROUND VENT