NC0025348_Technical Memo_20180111

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GMA
GROVN"AnY MANAGIMANT ASSCKIAT[S, INC.
TECHNICAL MEMORANDUM
January 11, 2018
To: Mr. Blaine Humphrey, P.E., Rivers and Associates, Inc.
107 East 2"d Street, Greenville, North Carolina 27858
4300 Sapphire Court, Suite 100
Greenville, North Carolina 27834
Telephone [phone]
www.amo-nc.com
Subject: Groundwater Flow and Transport Modeling Results for the New Bern Wastewater
Reclamation Facility
This Technical Memorandum (TM) presents a concise summary of the major findings of a
groundwater modeling effort completed by Groundwater Management Associates, Inc. (GMA) for
the City of New Bern Wastewater Reclamation Facility. The intent of this TM is to provide
preliminary data that may be presented to the NC Division of Environmental Quality (NCDEQ) for
their review and consideration. We anticipate that the NCDEQ provide comments and questions
about the model results, and those responses can be addressed in the final model report.
GMA has developed a three-dimensional, six -layer groundwater flow and transport model for the
East Lake Wastewater Reclamation Facility operated by the City of New Bern. Modeling was
performed by applying the United States Geological Survey (USGS) MODFLOW-2000 code, and we
used the Groundwater Modeling System (GMS 10.2.3) software as the graphical user interface to
input model parameters and facilitate integration of the simulations for output presentation.
MODFLOW is a modular, three-dimensional groundwater flow model code that simulates
groundwater flow using a finite -difference method applied to a block -centered rectangular grid.
Model assumptions were based upon a wealth of regional and local hydrogeologic data available to
GMA. Model input assumptions and conceptual model design elements were derived
predominantly from the following sources: GMA's prior work on the City of New Bern Castle Hayne
Aquifer Wellfield, data provided by Martin Marietta Aggregates from the Clark's Quarry, design data
provided by Rivers and Associates for the East Lake Wastewater Reclamation facility, drilling and
testing data collected by GMA from the Mackilwean Turf Farm, published data from an ECU
master's thesis prepared by Sara Matyiko, results of prior regional modeling performed by Ken
Pohlig of the NCDENR, and 10 years of groundwater and effluent monitoring data provided by the
City of New Bern.
GMA calibrated the groundwater model to simulate steady-state flow based on average elevations
of groundwater and lake levels reported for 2016. The model calibration has an acceptably high

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correlation coefficient (r =0.92) and the root mean square of residuals was low (RMSR=0.79 ft). The
groundwater flow model provides spatial representations of head pressure variations and
groundwater flow for the Castle Hayne Aquifer System and the Surficial Aquifer for an
approximately 44 mi2 area. After model calibration, GMA used MT3DMS (Modular Transport, 3-
Dimensional, Multi -species model) in conjunction with MODFLOW to predict nitrogen
concentrations moving from East Lake into and through the groundwater system.
Results from the MODFLOW groundwater flow model and MT3DMS simulations indicate the
following:
• Figure 1 illustrates the calibrated steady-state head distribution for the uppermost
limestone layer (River Bend) of the Castle Hayne Aquifer. This unit is the most permeable
aquifer layer, and it is in direct connection with the water in East Lake.
• Groundwater pumping associated with the nearby Martin Marietta Clarks Quarry and the
City of New Bern production wells were determined to not have an effect on the
groundwater flow from the East Lake Wastewater Reclamation Facility.
• The East Lake is functioning as a recharge boundary that drives groundwater radially and
downward under the induced, artificial head created by the wastewater facility.
• The induced head beneath the East Lake results in a groundwater divide in the upper layers
of the Castle Hayne Aquifer beneath the lake. Groundwater recharge on the east side of
that divide discharges to the Neuse River, and groundwater recharge west of the divide
discharges to the West Lake (Figure 2).
• The majority of groundwater flow out of East Lake is concentrated at the sides of the basin
where hydraulic gradients are the steepest and where aquifer permeability is the highest.
An estimated 62% of the groundwater recharge exiting the East Lake ultimately discharges
to the Neuse River. The remaining 38% discharges to the West Lake, where it then flows as
surface water back to the Neuse River, or it is lost as evapotranspiration. Travel times for
the majority of groundwater flow from the recharge at East Lake to the areas of discharge
to the Neuse River or to the West Lake range from 66 days to 5 years.
• Model results indicate that approximately 4%of the total groundwater recharge entering
from the East Lake is directed downward through deeper, less permeable, layers of the
Castle Hayne Aquifer System before eventually discharging at the Neuse River or the West
Lake. Travel times for this deeper portion of the groundwater flow from the East Lake are
on the order of 100+ years.
• Model simulations were also run without the effects of the mounded surface water feature
in the East Lake, and these model runs indicate that there would not be any deep
groundwater recharge in the area of the East Lake if this mounded surface water feature did
not exist. Instead, the area would function as a groundwater discharge area for the River
Bend and Upper Spring Garden portions of the Castle Hayne Aquifer.
• Historical water -quality monitoring of wells surrounding the East Lake facility do not
indicate an increase in total nitrogen (TN) concentrations above the background
concentrations that were measured prior to the facility's Hirst use. Further, base -level (i.e.,

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preexisting) concentrations in some monitoring wells (e.g. EL1 and EL3) prior to use of the
facility exceed the measured TN concentrations in the East Lake after the start of facility
use. The field monitoring data indicate, on average, that the addition of wastewater into
the East Lake is not resulting in a recognizable increase in TN concentrations in the
groundwater monitoring wells surrounding the facility.
• During 2016, effluent was discharged to the East Lake reclamation facility during the months
of May and November (with a small amount discharged during October). Consistent with
historical water -quality monitoring, MT3DMS groundwater transport simulations under this
usage scenario do not predict a measurable increase over base level TN concentrations at
the groundwater monitoring well locations (Figure 3). These MT3DMS simulations were
performed without applying a retardation factor, a very conservative assumption. The
results suggest that under present usage patterns TN added to the East Lake is being rapidly
diluted in the groundwater system to concentrations that cannot be distinguished from the
naturally occurring background concentrations.
• Per request of the City of New Bern and Rivers and Associates, Inc., GMA ran additional
MT3DMS simulations that modeled effluent discharge to the East Lake at a rate of 5.0
million gallons per day (MGD) and a concentration of 5.0 mg/L of TN for four equally spaced
months per year. This scenario represents the realistic maximum time period that the City
would discharge to the East Lake. MT3DMS simulations predict that after discharging under
this scenario for 15 years, TN loading to the Neuse River would only be 91 pounds of TN per
year.
• As an additional run, GMA also modeled effluent discharge to the East Lake at a rate of 5.0
MGD and a concentration of 5.0 mg/L of TN every day for 15 years (Figure 4). Under this
usage scenario, after 15 years the model predicts an annual TN load to the Neuse River of
265 pounds per year.
• Because the MT3DMS runs assume no retardation factor for TN movement through the
groundwater, all reductions in concentration in these simulations are due to dilution only.
The concentrations predicted to be discharging to the river are quite low. Thus, GMA
concludes that a significant portion of the TN load to the East Lake has a high retention time
in the groundwater system. The induced downward flow beneath the East Lake forces a
significant portion of the TN mass to migrate to deeper portions of the flow system where
travel velocities are very slow.
Available field data show close agreement with the MODFLOW 2000 groundwater flow model
simulations and the MT3DMS results. GMA asserts that the groundwater flow model that we have
produced is a reasonable representation of the groundwater conditions associated with the City of
New Bern Wastewater Reclamation Facility. However, groundwater models must incorporate
simplifying assumptions about the nature of groundwater flow, and as a result some local
heterogeneities of the groundwater system may not be accurately represented. For example, this
model assumes that groundwater flow through the aquifer beneath the lake occurs solely as
homogeneous flow and does not reflect preferential flow paths (i.e., heterogeneities) that might

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exist within the limestone. Although there is some degree of uncertainty in the model predictions,
this model reasonably simulates groundwater flow and advective transport of dissolved added to
the East Lake of the New Bern Wastewater Reclamation facility based on the best available data.
Please contact us if you would like to further discuss this summary. We are actively working on the
full report of the model, and we will provide a draft report for your review prior to finalization.
Best regards,
Groundwater Management Associates, Inc.
9
Emma H. Shipley, P.G.
Project Hydrogeologist
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James K. Holley, P.G
Senior Hydrogeologist

FIGURE 1: MODELED POTENTIOMETRIC SURFACE FOR THE RIVER BEND AQUIFER
-LEGEND- HEAD (FT MSL)
,4
-2
-18
-34
-50
NOTE: MODEL DIMENSIONS ARE 40,000 FT BY 30,500 FT
File: ORAWMG52134& RB POT SURF I NEW BERN WASTEWATER RECLAMATION FACILITY
PROJECT 21342 1 CITY OF NEW BERN, CRAVEN CO., NC
GMA
DATE: 12/08/2017

FIGURE 2: MODELED GROUNDWATER FLOW WITHIN A VERTICAL SECTION THROUGH THE EAST LAKE
WEST LAKE EAST LAKE
NEUSE RIVER
PRO
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!
I
1 Ox VERTICAL EXAGGERATION
500 FT
�—y
-LEGEND-
HEAD (FT MSL) NOTE: VIEW IS LOOKING NORTH
10
+ RIVER CELL
♦ GENERAL HEAD CELL
R
GMA
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4
0.00,....o.....o..w,..w 5. ll.
2
0
File: DRAWINGS/21342/
NEW BERN WASTEWATER RECLAMATION FACILITY
DATE: 12/6/2017
VERT PROFILE Fig
PROJECT21342
CITY OF NEW BERN, CRAVEN CO., NC

FIGURE 3: MAXIMUM PREDICTED TOTAL NITROGEN (TN) CONCENTRATIONS ABOVE BACKGROUND DURING 2016
TN: 12/1/201612:00:01 AM
0.0005 mg/L
0.0004 mg/L
0.0003 mg/L
0.0002 mg/L
0.0001 mg/L
-
EASTLAKE
0.0 mg/L
WEST LAKE �1 REUSE RIVER
I Ox VERTICAL EXAGGERATION
-LEGEND-
GM w
V M
4 RIVER CELL
♦ GENERAL HEAD CELL
.,
File. DRAWINGS/21342/
NEW BERN WASTEWATER RECLAMATION FACILITY
DATE: 12/8/2017
TN MT3DMS Fig
PROJECT 21342
CITY OF NEW BERN, CRAVEN CO., NC

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FIGURE 4: PREDICTED TOTAL NITROGEN (TN) CONCENTRATIONS ABOVE BACKGROUND AFTER 15 YEARS OF
DAILY DISCHARGE OF 5 MGD OF EFFLUENT CONTAINING 5.0 MG/L OF TN TO THE EAST LAKE
TN: 12/1/201612:00:01 AM
0.008 mg/L
0.006 mg/L
0.004 mg/L
0.002 mg/L
EAST LAKE
0.0 mg/L
WEST LAKE NEUSE RIVER
10x VERTICAL EXAGGERATION
-LEGEND-
GM w
V /�
+ RIVER CELL
♦ GENERAL HEAD CELL
-�
n,
File. DRAWINGS/213421
NEW BERN WASTEWATER RECLAMATION FACILITY
DATE: 12/13/2017
TN MT3DMS Fi
PROJECT 21342
CITY OF NEW BERN, CRAVEN CO., INC