9001_UnionCoMSWLF_LFGAssmtResp_DIN27207_20170105

January 5, 2017

Sent Via Email – [e-mail]

Mr. Ron Gilkerson
Solid Waste Director
Union County Department of Public Works
500 N. Main Street, Suite 500
Monroe, NC 28112

Re: Soil Gas Assessment Report
Union County Landfill
Permit ID # 9001
Union County
DIN 27207

Dear Mr. Gilkerson:

The Solid Waste Section (Section) has reviewed the Soil Gas Assessment report (DIN 27054) for the Union
County Landfill prepared by Civil and Environmental Consultants, Inc. (CEC). Union County operates a
construction and demolition landfill on top of a closed unlined unit that received approval for a corrective
action plan (DIN 6670) on January 28, 2009. The strategy of phytoremediation coupled with MNA was
proposed to remediate constituents of concern such as trichloroethene, tetrachloroethene, vinyl chloride,
1,4-dichlorobenzene; aldrin, alpha-BHC, beta-BHC, 4,4-DDE, delta-BHC, endrin aldehyde, kepone;
heptachlor; cobalt, and thallium in monitor wells MW-1A, MW-5, MW-7, MW-8, MW-11, and MW-11A.
Over the past several years, the constituents of concern have been reduced to primarily include benzene,
tetrachloroethene, and tricholorethene in monitor well MW-1A. Since MW-1A is an upgradient well, CEC
performed a landfill gas investigation to determine if landfill gas is causing groundwater contamination.

Three temporary soil gas monitoring points were installed within the vadose zone between the waste
disposal boundary and MW-1A. Geoprobe direct push technology was utilized to install the borings at
depths ranging from eight to twelve feet below ground surface. Soil gas samples were collected from the
three monitoring points and the MW-1A headspace. The soil gas samples were analyzed for methane,
carbon dioxide, carbon monoxide, hydrogen, nitrogen, oxygen, and volatile organic compounds(VOCs).

The VOCs detected in the soil gas probes were consistent with the VOCs detected in the MW-1A headspace
sample. VOC concentrations were highest in the two monitoring points (SGP-1 and SGP-3) nearest MW-
1A. The third monitoring point(SGP-2) reported the lowest VOC concentrations. VOC gas concentrations
measured in the MW-1A headspace were found to be one to two orders of magnitude lower than the soil
gas VOC concentrations reported in the temporary monitoring points. CEC determined the difference in
concentration was most likely caused by dilution within the well headspace or the well screen being located
below the water table. CEC calculated equilibrium gas concentrations for several VOCs and compared
those values to the observed groundwater concentrations for the same constituents in MW-1A to evaluate

the partitioning of VOCs from soil gas to groundwater. In certain instances, the calculated equilibrium gas
concentrations were less than the observed gas concentrations, which indicated the potential for partitioning
of VOCs from the gas to aqueous phase.

Based on the investigation, CEC concluded that VOC concentrations are present in the soil gas at
concentrations that could contribute to the reported groundwater exceedances. CEC proposes to install
permanent landfill gas probes along the southeastern landfill boundary to further evaluate the presence of
landfill gas and its potential impact. In addition, CEC plans to use the data from the probes to aid in the
design of a landfill gas control system if one is deemed necessary. The Section approves of this
recommendation and Union County should submit a proposed schedule for landfill gas probe installation
and an estimated timeline for determining if a landfill gas control system is needed. The proposal and
timeline should be submitted with 60 days of receiving this letter. Please contact me at [phone] or
by e-mail at [e-mail] if you have any questions or concerns regarding this letter. Thank you
in advance for your cooperation in this matter.

Sincerely,

Ervin Lane
Compliance Hydrogeologist
Solid Waste Section

cc: Edward H. Stephens, P.G., Civil and Environmental Consultants
Jason Watkins, Field Operations Branch Head
Deb Aja, Western District Supervisor
Teresa Bradford, Environmental Senior Specialist