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HomeMy WebLinkAbout2.0 Supplemental InformationAEC AMERICAN ENVIRONMENTAL CONSULTING, LLC August 16, 2016 David Pesnichak, AICP Senior Planner Garfield County Community Development 108 8th Street, Suite 401 Glenwood Springs, CO 81601 Robert Peterson Environmental Protection Specialist Colorado Dept. of Public Health and Environment 222 S. Sixth St., Room 232 Grand Junction, CO 81501 Re: Permitted Lined Area and Area Modification Request South Canyon Landfill City of Glenwood Springs, Colorado Dear Mr. Pesnichak and Mr. Peterson: American Environmental Consulting, LLC (AEC), on behalf of the City of Glenwood Springs (City), is submitting this plan to show the details of the Phase 4 liner design and to request a modification to the South Canyon Landfill (SCLF) Engineering Design and Operations Plan (EDOP) plan was originally prepared, dated September 30, 1994, and submitted to Garfield County by Steffen, Robertson, and Kirsten (SRK). The modification includes a change in the area initially approved to be lined with a one -foot recompacted soil barrier. The original plan included "lining of the remaining base area of the landfill" (SRK Facility Design and Operations Plan, September 30, 1994, page 6-7). This lined area included approximately 4.7 acres. A portion of this area (1.47 acres) is currently filled with refuse (see the attached drawing) and cannot be lined. The remaining base area of the landfill has also changed over time since the initial EDOP and the remaining base area to be lined is now proposed to be approximately 5.49 acres, 0.79 acres more than initially proposed. With the exception of the Phase 5 expansion, no other portions of the landfill have ever been lined. Also, a portion of this area was once owned by the Bureau of Land Management and the waste disposed on this area was removed and placed in the permitted fill. Since that time, Glenwood has purchased the property and this area is delineated as Phase 4A and is part of the remaining base area that will be lined. While the base area to be lined is requested to be changed, the permitted fill limits have not changed. 8191 Southpark Lane, Suite 107, Littleton, CO 80120 phone 303-948-7733 fax 303-948-7739 Projects/glenwood/20I6 Lined Unlined Area Modification/Modification of Lined Area Lined Base Area Modification Request South Canyon Landfill Geologic/Hydrogeologic Conditions August 16, 2016 Page 2 The unlined area with existing refuse is located north of the filling area (Drawing 1) and is approximately 1.47 acres. It is part of the approximately 27.8 acres of filled landfill that is also unlined. The 1.47 -acre area that is part of the requested modification and is proposed to remain unlined is in the northwest portion of the permitted area. As noted in the SRK EDOP document, "the site is underlain by the Cretaceous Mancos Shale Formation which serves as an aquitard in the region. Beneath the Mancos Shale Formation lies the Dakota Sandstone Formation which serves as an aquifer in the region". SRK also noted that there is greater than 3000 feet of Mancos Shale underlying the site. The base of the Phase 4A, 0.47 -acre lined area was observed on August 3 when the subgrade was prepared and proof rolled. The base is made up of a solid, fine-grained material that is likely the Mancos shale formation as this is noticeable as outcropping all around the area and very hard on the floor of the phase. According to the SRK conclusions, the subsurface characteristics are adequate to comply with "barrier layer" requirements outlined in the Colorado Solid Waste Regulations 3.2.5 (C) (1), "Natural lithology with recompaction". The subsurface in the area of the 1.47 acres of unlined subgrade indicate a insitu permeability within the Mancos Shale on the order of 10-5 and 10.6 cm/sec. At least 2 boreholes (B-4 and B-6) were placed in the area of the 1.47 -acre unlined area modification. Packer tests were conducted in boreholes B-4 and B-6 both located in the vicinity of the unlined area. The hydraulic conductivity of B-4 ranged from 3.4 x 10-5 cm/sec to a low of 4 x 10"6 cm/sec. The hydraulic conductivity of B-6 ranged from 6.9 x 10"5 cm/sec to a low of 2.5 x 10"5 cm/sec. The subgrade was noted as hard to very hard. As noted in the original SRK design document, "shallow ground water was encountered at the site in six of the ten borings completed (i.e., 7 borings completed by SRK and 3 borings completed by Gormley). This ground water generally occurs in the thin, unconsolidated surficial deposit overlying the Mancos Shale bedrock and in fractures in the upper weathered portion of the Mancos Shale. Ground water has been encountered at various depths ranging from approximately 16 to 42 feet." Initial groundwater depths noted on the borehole logs range from 42.5 feet in B-6 to below 38 feet in B-4 (the total depth of the boring). The static water levels then equilibrated at around 21.5 below ground surface in B-6 and 20 feet in B-4. There was no indication of groundwater during the construction of Phase 4A. Additionally the SRK investigation determined that groundwater is "likely locally occurring in non -hydraulically connected perched ground water zones. Subsurface investigation at the property indicates that these zones may be confined to fracture zones less than 1 -foot in thickness". AEC Projects/glenwood/2016 Lined Unlined Area Modification/Modification of Lined Area Lined Base Area Modification Request South Canyon Landfill August 16, 2016 Page 3 The SRK report also noted that, "Given that the site is underlain by the Mancos Shale aquitard and that surface water controls as well as leachate collection measures are included in the facility design, there is a very low potential for a significant impact from the facility to surface water or ground water quality. Although some shallow ground water exists beneath the site, investigation indicates it is locally occurring. No seeps were encountered during the preparation of the subgrade for the Phase 4A area. CD Boundary Amendment The request by Glenwood to Garfield County to amend the CD boundary to include the entire permitted fill area was approved on August 8, 2016 which will allow the Phase 4 area to be filled after lining is installed, certified and approved by the CDPHE. Phase 4 Construction Details The original permit requires that the remaining base of the landfill be constructed with one -foot of recompacted soil. The originally outlined base (floor) of the landfill has changed since the plan was first approved. The current base is larger as a result of soil excavation activities conducted since 1994, however; as noted above, there are 1.47 acres that have been previously filled. The intent of the plan to place a recompacted soil liner on the base of the landfill will not change. A survey was conducted to define the base of the landfill by surveying points at the base of the existing fill and at the base of slope of the existing ground surface north and east of the fill. The plates (1 through 4) show the extent of the landfill base in Phase 4A and the permitted fill limit. Construction in the permitted Phase 4 area will take place sequentially in modules 4A, 4B, and 4C in order to quickly develop landfill capacity and continue uninterrupted disposal. The entire permitted remaining unused filling area includes approximately 11.23 acres with approximately 5.49 acres being on the base (floor) of the landfill and the remaining area including the sidewalls of the fill area. The approved disposal plan includes placing a one -foot thick compacted soil barrier on the base of the landfill prior to beginning filling. The initial plans include constructing approximately 1.77 acres of the base of the landfill by installing the permitted one -foot thick liner over two areas, Phases 4A (0.47 acres, Plates 3 and 4 attached) and 4B (1.30 acres, Plates 5 and 6, to follow). The construction will take place in two consecutive events in order to ensure capacity for continued filling at the landfill. The initial construction, Phase 4A, will take place on the landfill base north of the existing fill area (-0.47 acres) in a portion of the landfill property that was once owned by the U.S. Bureau of Land Management (BLM). The City acquired this property in 2013 after the refuse filled on this portion of the landfill was removed. Once liner over this area is installed, construction will immediately move to the next area, Phase 4B, which is a 1.30 -acre area east of the existing fill. During construction of Phase 4B, the certification report for the Phase 4A area will be completed and submitted for review and approval. AEC Projects/glenwoodl2016 Lined Unlined Area Modification/Modification of Lined Area Lined Base Area Modification Request South Canyon Landfill August 16, 2016 Page 4 It should be noted that the intent is to construct one foot of recompacted clay on the floor area of the remaining, permitted landfill. It is likely that some of the subgrade and top of liner points may have to be field fit against the toes of the fill area and toes of the natural ground slopes. Therefore, some of the acreages may change and there may be additional survey points added or survey point moved slightly. These will be documented within the liner construction area to ensure a minimum of one -foot of clay is placed over the subgrade floor. Construction of the base floor areas will include the following: 1. Preparation of the subgrade including surveying the floor and field fitting survey points as necessary. 2. Placement of one foot of recompacted soil to a permeability of 1 x 10"7 cm/sec or less 3. Installation of a termination berm along the downgradient limit of each constructed area 4. Installation of a temporary leachate collection area and conveyance pipe in Phase 4B that will tie into the existing leachate collection pipe (detail Plate 7 attached). An additional leachate control mechanism will be added in Phase 4A by installing a pipeless leachate drain consisting of a geotextile-wrapped shredded tire aggregate at the base of the toe of the current landfill (the southern edge of Phase 4A) to route any leachate from the lined portion of the landfill and to keep it from entering the unlined portion of the SCLF. The drain will be a minimum of 16 inches thick above the liner and extend a minimum of 16 inches out from the toe of the fill. Plate 7 includes a detail drawing of the pipeless drain. The leachate will then be routed into Phase 4B and 4C where it will ultimately be piped to the leachate collection pond. Construction will be completed according to the attached Construction Quality Assurance Plan (CQAP). The CQAP includes preconstruction soil testing and construction soil testing. The plan does not include the preparation of a test pad since that was completed during the Phase 5 construction and materials from the same borrow area will be used. The temporary leachate conveyance pipe will be installed following completion of the recompacted soil liner placement in Phase 4B. The pipe will be corrugated or smooth high density polyethylene (HDPE) pipe that will extend from a low point inside Phase 4B to tie into the existing leachate pipe located near the manhole. It will tie in before the valve so the valve can continue to be used to segregate and remove clean runoff and leachate. The pipe will be placed in a trench of a minimum of one -foot deep and covered with lightly compacted soil for protection. A termination berm will be constructed on the edge of the liner as shown in the details (Plate 7). Care will be taken to lightly compact the berm in the vicinity of the leachate pipe. Inside the berm, a four -foot section of the leachate pipe will be perforated and wrapped with a geotextile to facilitate leachate flow from the Phase 4A and 4B areas. Upon completion of Phase 4C, the termination berm and pipe will be removed. Phase 4C has not been designed as yet since it will require the construction of a leachate sump that will tie into the existing leachate removal system. Once it is designed, the plan will be submitted and construction will begin. AEC Projects/glenwood/2016 Lined Unlined Area Modification/Modification of Lined Area Lined Base Area Modification Request South Canyon Landfill Conclusions August 16, 2016 Page 5 Given the original findings in the subsurface investigation and the configuration of the existing fill versus the original design, the SCLF respectfully requests a modification to the area lined on the remaining base of the landfill to be as shown in the attached drawing. We request the action based on the following: 1. The unlined area is 1.47 acres and is already a part of a larger 27.8 acre unlined area. 2. The remaining base of the landfill will continue to be lined with a one -foot thick recompacted clay following an approved CQA Plan. 3. Leachate from the Phase 4A area will be routed away from the unlined area using a toe drain. Leachate from the entire remaining base liner will be collected as a part of Phase 4C construction and be piped to the leachate collection pond. 4. The ultimate lined area will be larger than the originally anticipated floor area. Please let me know if you have any questions or require any further information. Thank you. Respectfully, Reviewed by: AMERICAN ENVIRONMENTAL CONSULTING, LLC Curtis Ahrendsen Michael H. Stewart, P.E. Principal/Project Manager Senior Engineer Cc: King Lloyd, Solid Waste Superintendent, City of Glenwood Springs Attachment: CQA Plan 0041 e C�6 • Kat j (,i" •••°°.o•k°may V\• . ng°y AEC Projects/glenwood/2016 Lined Unlined Area Modification/Modification of Lined Area