7401_PittCounty_MSWLF_PilotStudy_FID1323159_20190621

GMA
The Groundwater Experts
June 21, 2019
Ms. Jaclynne Drummond
Hydrogeologist
NCDENR Solid Waste Section
2090 US Highway 70
Swannanoa, North Carolina 28778
Groundwater Management Associates, Inc.
4300 Sapphire Court, Suite 100
Greenville, North Carolina 27834
Telephone: [phone]
Facsimile: [phone]
www.cimo-nc.com
Apex and Greenville, NC
Re: Proposal for 1,4-Dioxane Assessment and Pilot Study at the Pitt County Municipal Solid
Waste Landfill
Dear Ms. Drummond,
Analytical results from the January 2019 semi-annual monitoring event at the Southern Unit of
the inactive Pitt County Solid Waste Municipal Landfill (Facility Permit #74-01) confirmed
exceedances of the 1,4-dioxane NCAC 02L standard in groundwater at the facility. On behalf of
Pitt County, Groundwater Management Associates (GMA) submitted a '14-Day Notification of
Groundwater Protection Standard Exceedance' for 1,4-dioxane at the Southern Unit on February
27, 2019. The Southern Unit of the Pitt County Landfill is currently under assessment for low-
level exceedances of benzene, but 1,4-dioxane was previously not being addressed as part of
the assessment.
North Carolina Administrative Code 15A NCAC 13B .1634 requires the County to characterize
the nature and extent of any release. During the January 2019 monitoring event, GMA sampled
additional wells that were no longer part of the groundwater monitoring program. Thus, the
County has already begun to assess the horizontal extent of 1,4-dioxane. The vertical extent of
1,4-dioxane in the groundwater at the landfill is undetermined. Whether or not the 1,4-dioxane
originates from waste placed in the Northern Unit, the Southern Unit, or both also remains
unknown.
GMA suspects that the detection of 1,4-dioxane in monitoring wells adjacent to the Southern
Landfill Unit relates to degradation of the integrity of one or both of the landfill caps. Pitt
County emplaced the clay caps above the Landfill in the mid-1990s. Over time, erosion of the
cap, as well as cracking due to heating/cooling, desiccation, and/or subsidence due to
decomposition of fill material can increase the permeability of this type of barrier. Cap
degradation may allow infiltration of water that can leach contaminants downward into the
underlying groundwater. Eliminating infiltration of water through the cap is a primary goal for

Ms. Jaclynne Drummond
June 21, 2019
Page 3
reducing, or eliminating, the release of additional contaminants into the groundwater.
To address the 1,4-dioxane in groundwater at the Landfill, GMA, on behalf of the County,
proposes to: 1) fully delineate the vertical and horizontal extent of 1,4-dioxane, and 2) initiate a
pilot study to document and establish the effectiveness of cap upgrades and repairs. We
provide details for each of these two proposed tasks below.
1 4-Dioxane Delineation
The County began proactively assessing the horizontal extent of 1,4-dioxane during the January
2019 sampling event by collecting additional groundwater samples from existing monitoring
wells around the Southern Unit that were removed from routine sampling requirements. To
better assess the extent of 1,4-dioxane in groundwater, and to improve our understanding of
local groundwater flow patterns at the site, the County proposes to install two additional
monitoring wells and six surface water gages at the landfill facility (Figure 1). One of the
proposed monitoring wells will be a Type -III well (up to 60 feet deep) that will allow the County
to assess the vertical extent of 1,4-dioxane detected at the facility. The other groundwater
monitoring well will be a Type -II monitoring well (15 feet deep) placed between the Northern
and Southern mounds. This well will be located north of well P16R on the south side of the
Northern mound, and the well will be hydrologically separated from well P16R by two drainage
ditches along the railroad.
The proposed surface water staff gauges and the monitoring wells will provide a better
understanding of groundwater and surface water flow patterns between the two landfill units.
The additional assessment will attempt to determine the location(s) of the source(s) of 1,4-
dioxane. Groundwater monitoring wells will be installed according to 15A NCAC 2C .0100 Well
Construction standards. A North Carolina Registered Land Surveyor will survey the horizontal
locations and elevations of the new monitoring wells and the six staff gauges. Once the
surveying is completed, GMA, on behalf of Pitt County, will provide an updated site map to the
NC Solid Waste Section.
Cap Assessment and Initiation of a Pilot Study
The County proposes to conduct geophysical testing of the Southern Unit to evaluate landfill
cap integrity. We propose to conduct geophysical testing as part of an approximately 1 year
pilot study to be conducted in conjunction with cap repairs intended to limit the infiltration of
rainwater into the landfill mounds and reduce leachate volume and concentrations. Geophysical
testing will include an electromagnetic (EM) survey over the Southern Unit followed by Ground -
Penetrating Radar (GPR). The proposed geophysical surveying will attempt to identify areas
where the landfill cap has been breached and to document the locations of any groundwater

Ms. Jaclynne Drummond
June 21, 2019
Page 4
seeps along the landfill sides. The County will repair any areas identified by the geophysical
survey and will inspect surface water drainage structures on the mound to ensure that they are
operating as designed. Once repairs are made, geophysical surveying will be repeated to
document hydrologic benefits from cap repairs. Because groundwater velocities are relatively
slow at the site, contaminant concentrations may not decrease quickly after cap repairs are
made and less water infiltrates into the mound. By repeating the geophysical surveying
following the cap repairs, the County may be able to more quickly determine if the repairs have
significantly reduced rainfall infiltration.
At this time, the County only proposes to conduct geophysical testing of the Southern Unit,
which is subject to the 15A NCAC 13B .1600 rules. If the method proves successful, the County
can later use geophysical testing to assess cap integrity at the Northern Unit. Although the
Northern Unit of the Pitt County Landfill is not subject to the 15A NCAC 13B .1600 rules, the
Northern unit lies adjacent to, and upgradient of, the Southern Landfill Unit and may serve as a
contributing source of the 1,4-dioxane detected in the groundwater at the facility.
In -stream piezometers
Since 2017, Pitt County has collected pore -water samples beneath the southern boundary ditch
in two locations downgradient of monitoring well P20R (Figure 1). This pore -water sampling is
part of the on -going assessment to determine whether benzene is migrating off -site. Historical
pore -water analyses have not indicated that benzene is migrating off the property at
concentrations above the 2L standard. Per verbal instruction from the NC SWS, GMA continues
to collect pore -water samples during each monitoring event. The pore -water samples will now
also be used to assess the horizontal extent of 1,4-dioxane.
Because of the nature of pore -water sampling collection, it is difficult to ensure that samples
are collected from identical locations and depths between events. Additionally, the pore -water
samples are often very turbid and have erroneously high concentrations of metals that are not
indicative of dissolved levels. To address both of these issues, GMA proposes to install two in -
stream piezometers approximately 1.5 feet below the ditch bottom at the locations of currently
collected pore -water samples (Figure 1). These piezometers will be used to collect
representative groundwater samples from repeatable locations just below the stream bed. We
will also measure water levels within the piezometers to determine if we can see any
potentiometric head differences between this shallow groundwater and the surface water in the
ditch. Head differences over such a small interval may not be measurable using this technique,
but others have successfully used this approach to document discharge relationships in small
NC streams. The piezometers will eliminate the need for continuing the pore -water sampling
techniques that have been used since 2017.

Ms. Jaclynne Drummond
June 21, 2019
Page 5
The next semi-annual monitoring event for the Southern Unit of the Pitt County Solid Waste
Municipal Landfill is scheduled for July of 2019. This monitoring event will include the analyses
of water samples for Appendix II parameters plus 1,4-dioxane. If the proposed assessment
actions and pilot study are approved by the NC SWS, then the County will sample the two new
wells for Appendix II parameters plus 1,4-dioxane at that time. If you have any questions, or
require additional information, please contact us at [phone].
Sincerely,
Groundwater Management Associates, Inc.
1
Emma H. Shipley, P.G.
Project Hydrogeologist
Enclosures: Figure 1 Site Map
CC: Mr. Tim Corley, Pitt County Engineer
Mr. John Demary, Pitt County Landfill
Dr. Richard Spruill, GMA-Greenville
Mr. Jay Holley, GMA-Greenville
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File: DRAWINGS/25101
SITE LAYOUTAND EXISTING MONITORING NETWORK
DATE: 3/22/2019
FIG 1
PROJECT 25101
PITT COUNTY LANDFILL, GREENVILLE, PITT CO., NC
FIGURE 1