9209_Wake_NorthWake_Unlined_MSWLF_LFGRequest_FID1469387_20201020

Environmental Consultants & Contractors
October 20, 2020
File No. 02199312.07-2
Ms. Jackie Drummond
North Carolina Department of Environmental Quality
Division of Waste Management
Solid Waste Section
2090 U.S. Highway 70
Swannanoa, NC 2877
Subject: Request for Alternate LFG Probe Remonitoring During Groundwater Corrective Action
North Wake Landfill - Raleigh, North Carolina
Dear Ms. Drummond:
On behalf of Wake County (the County), SCS Engineers, PC (SCS) is submitting this letter to NCDEQ
as an update to an ongoing methane exceedance detected at perimeter landfill gas (LFG) monitoring
probe M-613 at the North Wake County Landfill (Permit No. 92-09, the Facility) on June 24, 2020. In
accordance with 15A NCAC 13B Section .1626(4)(c), the County must:
Immediately take all necessary steps to ensure protection of human health and notify
NCDEQ; and
Within seven days of detected exceedance, place in the operating record the
methane gas levels recorded, and a description of the steps taken to protect human
health.
Following the identification of the exceedance, the County initiated adjustments to the perimeter
migration control system, which was deemed to be the necessary steps to ensure the protection of
human health and notified NCDEQ of the exceedance via electronic mail on June 25, 2020. On June
30, 2020, which is within seven days of the initial exceedance, the County placed record of the
methane gas levels recorded at probe M-6R and a description of steps taken to protect human
health into the Facility's operating record. This report is attached for your reference.
As required by 15A NCAC 13B Section .1626(4)(c)(iii), the Facility has implemented a LFG
Remediation Plan, most recently updated on July 12, 2007. In accordance with the Plan, the
frequency for monitoring LFG probes exhibiting non -compliant methane concentrations will be
monthly until such time that the probes have demonstrated compliant concentrations for three
consecutive months or further LFG remediation activities are warranted. Due to the suspected cause
of the exceedance, SCS and the County do not believe that an update to the Plan is necessary at this
time.
It should be noted that Probe M-613 is located along the eastern property boundary on the closed
Unlined Landfill, which is currently undergoing corrective action for a groundwater exceedance
detected in the vicinity of Probe M-6R. As part of the Facility's corrective action plan, the Facility
installed an in -situ "biobarrier" using injection wells to facilitate and enhance bioremediation
("treatment") of the groundwater. This in -situ process promotes biological breakdown of Volatile
15521 Midlothian Turnpike, Suite 305, Midlothian, VA 23113 1 804-378-7440 10

Ms. Drummond
October 20, 2020
Page 2
Organic Compounds (VOCs), and yields the production of methane gas in the vadose zone as a
byproduct of the bioremediation process. Please note the following:
• The corrective action injection wells are located in close proximity to monitoring probe M-61R;
• The introduction of the specified agent(s) is expected to induce methane production; and,
• The timing of the methane exceedance at Probe M-61R occurred within several months of the
introduction of the enhanced bioremediation products via the injection wells (Probe M-6R
had not exhibited an exceedance during the past 10 years or more).
Accordingly, SCS and the County conducted further investigations to identify whether the source of
the methane in Probe M-61R was decomposition of the buried wastes within the Unlined Landfill
waste disposal unit or whether the source is the enhanced bioremediation process associated with
groundwater corrective action. This investigation focused on the following:
• Assessing the operational conditions, performance, and effectiveness of the LFG Perimeter
Migration Control System;
• Correlating the presence of methane in the corrective action injection wells; and, Evaluating
the evacuation and recharge rate of methane in Probe M-61R.
Based on the findings of these investigations, as well as the temporal and proximity considerations
noted above, the County and SCS believe that the groundwater corrective actions are the source of
the methane detected at Probe M-61R.
Essentially, the presence of methane gas in the subsurface at the Facility property boundary is
expected as a consequence of the groundwater corrective action. Considering the absence of
physical features along the eastern property boundary (the adjacent parcel is a forested area with no
homes or other occupied structures, no utility vaults, no confined spaces, etc.) in which explosive
concentrations of methane could accumulate to present a health and safety concern, it seems
unwarranted to continue gas monitoring at an accelerated frequency (when methane is expected to
be present at concentrations in excess of the threshold due to an alternate source other than
decomposition of the landfilled waste).
Therefore, the County is requesting that the required monthly probe monitoring at Probe M-613, which
is required by the site's remediation plan, be reverted to a quarterly basis. In addition, the County is
requesting that for the duration that the landfill is considered to be undergoing corrective action for
the groundwater exceedance, methane values greater than the compliance threshold of 5.0 percent
will not require further action at M-613. All other probes on the closed unlined landfill and the closed
lined landfill will continue to be monitored and any methane values recorded greater than 5.0
percent would be considered an exceedance.

Ms. Drummond
October 20, 2020
Page 3
Please document approval of this request for the Facility's files and recordkeeping. Please contact
either of the undersigned if you have questions or require additional information.
Sincerely,
Lucas S. Nachman
Project Professional
SCS Engineers, PC
LSN/RED
cc: Lee Squires, PE, Wake County
Robert E. Dick, PE, BCEE
Vice President/Project Director
SCS Engineers, PC