WI-1354_15884_CA_MRP_20150807_GW Model

Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
TABLE OF CONTENTS
SITE INFORMATION ...................................................................................................... 1
1.0 SITE HISTORY ......................................................................................................... 3
2.0 FIELD ACTIVITIES ................................................................................................... 4
2.1 SLUG TEST RESULTS ............................................................................................ 4
3.0 THEORETICAL VS OBSERVED RATES OF GROUNDWATER TRAVEL .............. 5
3.1 Rate Based on Rising Head Test ............................................................................................................... 5
3.2 Effects of Diffusion, Dispersion, and Retardation on Rate of Contaminant Movement .............. 5
4.0 CONTAMINANT TRANSPORT MODELING ................................................................................................... 6
4.1 Empirical Modeling ....................................................................................................................................... 6
4.2 Solute Transport Modeling ......................................................................................................................... 7
5.0 CONCLUSIONS ...................................................................................................... 10
6.0 RECOMMENDATIONS ........................................................................................... 10
FIGURES
Figure 1 Site Location Map Figure 2 Aerial Site Map Figure 3 Groundwater Flow Map
Figure 4 Test Results Plot
Figure 5 Benzene Centerline Plot 10 Years Figure 6 Benzene Centerline Plot 25 Years
Figure 7 Benzene Centerline Plot 50 Years
Figure 8 Benzene Centerline Plot 100 Years Figure 9 Naphthalene Centerline Plot 10 Years Figure 10 Naphthalene Centerline Plot 25 Years
Figure 11 Naphthalene Centerline Plot 50 Years
Figure 12 Naphthalene Centerline Plot 100 Years
TABLES
Table 1 Model Parameters
Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
APPENDICIES
Appendix A Historical Groundwater Quality Data
Appendix B Field Notes and Calculations
Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
SITE INFORMATION
Date of Report:
UST Incident No.:
Land Use Category:
Site Name and Location:
Location Method:
August 7, 2015
TF-15884
Commercial
Doug’s Mini Mart
323 Swamp Fox Highway
Tabor City, Columbus County, North Carolina
Google Earth
Latitude and Longitude: 35.716456° North, -77.263433° West Contacts
Owner of UST: Unknown
Property Owner: Linda Rogers
PO Box 494
Longs, South Carolina 29568
Primary Contact: Mr. Scott Ryals
NCDENR, Division of Waste Management, UST Section 1637 Mail Service Center
Raleigh, North Carolina 27699
Consultant: ECS Carolinas, LLP 726 Ramsey Street
Fayetteville, North Carolina 28301
[phone]
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Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
Seal and Signature of Certifying Licensed Geologist
I, John M. Stewart, P.G., a Licensed Geologist for ECS Carolinas, LLP, do certify that the
information contained in this report is correct and accurate to the best of my knowledge.
________________________________
John M. Stewart, P.G.
NC License No. 1046
ECS Carolinas, LLP is permitted to practice geology | engineering in North Carolina. The
certification number of the corporation is C-406.
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Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
1.0 SITE HISTORY
The subject site is known as Doug’s Mini Mart and is located at 323 Swamp Fox Highway in Tabor City, Columbus County, North Carolina (Figures 1 and 2). The site is located within a mixed-use area of Tabor City and is developed with one building utilized as the House of Prayer
AME Zion church. The site formerly operated gasoline underground storage tanks (USTs);
however, the quantity and size of the USTs is not known. Municipal water is provided to the site and vicinity of the site by Columbus County Public Utilities; however, some properties utilize
private potable water-supply wells. One inactive water-supply well is present on the site (WSW-
Onsite).
UST closure activities at the site were reportedly conducted prior to 2008. In response to
residual-phase contamination, one two-inch monitoring well (MW-1) was installed in the vicinity
of a former UST basin (Figure 3). In November 2008, Schnabel Engineering, LLC (Schnabel) supervised the installation of give groundwater monitoring wells (MW-1R, MW-2, MW-3, MW-4,
and MW-5). Laboratory analysis of the groundwater samples collected from monitoring wells
MW-1R and MW-4 detected targeted petroleum-related compounds at concentrations that exceed their respective 15A NCAC 2L .0202 (NCAC 2L) groundwater standards. A groundwater sample was also collected from the inactive water-supply well located on the site (WSW-Onsite)
and a surface water sample was collected from a drainage ditch located west of the site (WSW-
Creek). Contaminant concentrations were detected above laboratory detection limits in surface water sample WSW-Creek.
On March 24 2010, Terraine Environmental (Terraine) mobilized to the site to sample
groundwater monitoring wells MW-1R, MW-2, MW-3, MW-4, and MW-5; on-site water-supply well WSW-Onsite; and off-site water-supply wells, WSW-1, WSW-2, WSW-4, and WSW-5.
Samples collected from MW-1R and MW-4 contained contaminants at concentrations above
their respective 2L Standards. Historic groundwater data can be found in Appendix A. Following the sampling of site monitoring wells, a Universal Remediation, Inc. PRP® Well Boom was installed in monitoring well MW-1R. The well boom contained powdered beeswax that stimulate
indigenous microbes and promotes biodegradation of free phase product and dissolved phase
petroleum contaminants.
Water-supply wells WSW-Onsite, WSW-1, and WSW-2 were unable to be sampled since the
wells were disconnected and not in use. Water-supply well WSW-4 was not sampled due to the resident not being present. A groundwater sample was collected from a spigot associated with water-supply well WSW-5 located on the south side of the residence after purging the well by
allowing it to run for approximately 15 minutes. Laboratory analysis of the groundwater sample
did not detect targeted compounds above laboratory detection limits. The drainage ditch previously sampled (WSW-Creek) was no longer present on the site.
In January 2012, Crawford Environmental Services (CES) performed a groundwater monitoring event and a receptor survey for the site. CES identified the presence of six water-supply wells located within 1,000 feet of the site (WSW-Onsite and WSW-1 through WSW-5). Two of the
water-supply wells (WSW-4 and WSW-5) were determined to be used for potable purposes
(WSW-4 is used for livestock). CES attempted to contact the owners of the four inactive water-supply wells to determine their willingness to abandon their wells, but efforts were unsuccessful.
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Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
The groundwater monitoring event included the sampling of site monitoring wells MW-1R, MW-
3, MW-4, and MW-5 and water-supply wells WSW-4 and WSW-5. Water-supply well WSW-5 was not sampled due to the absence of the homeowner and inaccessible exterior spigots.
Laboratory analysis of groundwater samples WSW-4 and MW-3 did not detect targeted
compounds above laboratory detection limits. Laboratory analysis of groundwater samples collected from monitoring wells MW-4 and MW-5 detected targeted compounds (benzene,
naphthalene, cis-1,2-dichloroethene, and methyl-tert-butyl-ether) above laboratory detection
limits, but below their respective NCAC 2L standards. Laboratory analysis of groundwater
sample MW-1R detected various targeted compounds above laboratory detection limits, with benzene, naphthalene, and 1,2-dichloroethane detected above their respective NCAC 2L
standards. Groundwater was determined to be flowing to the southeast.
In April 2013, CES mobilized to the site to sample monitoring well MW-1R. Laboratory analysis of the groundwater sample MW-1R detected benzene and naphthalene at concentrations that
exceed their respective NCAC 2L standards with the benzene concentration increasing slightly
and the naphthalene concentration decreasing since the previous 2012 sampling event.
On January 21, 2015, ECS mobilized to the site to gauge and sample monitoring well MW-1R. The depth to water was measured approximately 0.2 feet below the top of casing. The groundwater sample was analyzed for volatile organic compounds (VOCs) using Standard Method 6200B with IPE, MTBE, and EDB. Based upon the analytical report, targeted compounds (benzene and naphthalene) were detected at concentrations exceeding the NCAC 2L groundwater standards. The concentrations did not exceed the Gross Contamination Levels (GCLs).
2.0 FIELD ACTIVITIES
On June 4, 2015, ECS personnel mobilized to the site to conduct a hydraulic conductivity test to
determine hydraulic properties of the shallow aquifer at monitoring well MW-1R. The slug test
procedure is summarized below.
1)Measure the static water level in the well from the top of casing using an electronic
water level meter.
2)Instantaneously remove a volume of water from the well.3)Upon removal of a well volume, collect water levels with the electronic water level
meter followed by readings every 10 to 30 seconds for the first 7 minutes; finishing
with readings at intervals of 1 minute for 10 minutes followed by readings every 5minutes for the last 30 minutes of the test.
2.1 TEST RESULTS
Hvorslev Method was used to determine the hydraulic conductivity from the data collected. In
the Hvorslev Method, the ratio ∆H/Hi is plotted versus the elapsed time (t) on a logarithmic scale
where ∆H is the drawdown at time t and Hi is the initial drawdown. The time T0 is determined
from the plot as the time when ∆H/Hi equals 0.37. The hydraulic conductivity (K) is calculated from the following equation:
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Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
( )K r L R
LT=
2
02
ln
where:
•r is the radius of the well casing,
•R is the radius of the sand pack, and
•L is the length of the well screen or water column if the water is below the top
of the screen.
The hydraulic conductivity (K) will be used to calculate the average linear groundwater velocity
(v) using the following equation:
v K dh
dle
=η
where:
•ηe is the effective porosity, and
• dh
dl is the horizontal gradient.
The test data indicates that the hydraulic conductivity (K) rate of the in-situ soils at MW-1R is approximately 1.78 feet per day. An estimated effective porosity of 0.25 was assumed based on the results of the hydraulic conductivity obtained from the test, which indicated values of K found in a medium to silty sand. The assumed medium to silty-sand porosity values based on Heath Groundwater Hydrology Paper 2220 was used in the groundwater velocity (v) calculation.
3.0 THEORETICAL VS OBSERVED RATES OF GROUNDWATER TRAVEL
Groundwater at the site has historically been measured flowing to the southeast (Figure 3).
Based on historic groundwater data obtained from the Groundwater Monitoring Report dated May 11, 2010 prepared by Terraine, the average horizontal groundwater gradient between wells
MW-1R and MW-4 was calculated to be 0.0135 feet/foot.
3.1 Rate Based on Rising Head Test
Based on an average groundwater gradient measured between MW-1R, MW-4, and MW-5 of
0.0135 feet per foot, a calculated hydraulic conductivity of 1.78 feet/day from the slug tests at
MW-1R, and an assumed porosity of 25 percent for medium to silty-sand, the rate of groundwater flow was estimated to be 35.1 feet per year. This rate assumes homogenous and
isotropic conditions, and that preferred pathways for transport such as fractures, foliations or
dikes are not present. The field data and calculations are included in Appendix B. The test results are shown on figure 4.
3.2 Effects of Diffusion, Dispersion, and Retardation on Rate of Contaminant Movement
The rate of movement of a solute (contaminant) in groundwater is not only controlled by the
groundwater velocity (advective transport), but also by diffusion, mechanical dispersion, and
retardation effects.
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Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
Diffusion is the movement of a contaminant from areas of high concentration to areas of lower
concentration. Mechanical dispersion is the mixing of the contaminated groundwater with clean water, resulting in dilution of the contaminant at the leading edge of the plume. Combined,
these two processes define a parameter called hydrodynamic dispersion. Studies have shown
that hydrodynamic dispersion is an important factor in the movement of contaminants where groundwater velocities are low. As the initial concentrations of contaminants increase, the
effects of hydrodynamic dispersion may cause the plume front to reach some point
downgradient sooner than expected had only groundwater velocity been considered. At field
scales, longitudinal dispersivity may be estimated to be about one tenth of the flow length of the plume (Fetter, 1993).
As contaminants travel through the aquifer they may be adsorbed onto solid soil particles, absorbed by porous silts and clays, partitioned onto organic carbon, and undergo various chemical reactions. As a result, many contaminants may move more slowly than the
groundwater which transports them, a process termed retardation. In addition, biodegradation,
dispersion, and mixing may decrease the concentration of solute in the plume, but may not slow the rate of advance of plume movement. The ratio of the groundwater velocity to the velocity of
contaminant transport is defined as the retardation factor (R). For benzene, R is estimated to be
1.3 based on soil bulk density (rho), partition coefficient (Koc), and fraction organic carbon (foc). For naphthalene, R is estimated to be 9.8 based on the same parameters.
The observed reduction in the concentrations of benzene and naphthalene as you move
downgradient of the source area is consistent with these retardation effects.
3.3 Rate Based on Observed Conditions
Benzene was detected in MW-4 (1.4 ppb) in March 2010 and has not been detected in MW-5
since it was first sampled in 2008. Based on tax records, Doug’s Mini Mart was constructed in
1979. If the USTs were installed on the same date, and assuming a release from the UST system occurred soon after (1980), it would have taken 30 years for the benzene to travel 156
feet (distance between USTs and MW-4) or moving at a rate of 5.2 feet/year. This rate would
take into effect diffusion, dispersion, and retardation factors.
4.0 CONTAMINANT TRANSPORT MODELING
4.1 Empirical Modeling The observed groundwater flow data in the medium to silty-sand suggest an average
groundwater flow velocity of 35.1 feet/year. Assuming a retardation factor of 1.3 for benzene, a
comparative rate of flow on the order of 27 feet/year can be estimated for the bulk mass of the
plume. Assuming no biological decay or attenuation, this suggests the source of the benzene plume (MW-1R area) may reach the downgradient property boundary receptor (270 feet) in
approximately 10 years. Assuming a retardation factor of 9.8 for naphthalene, a comparative
rate flow on the order of 3.56 feet/year can be estimated for the bulk mass of the plume. Assuming no biological decay or attenuation, this suggests the source of the naphthalene
plume (MW-1R area) may reach the downgradient property boundary receptor in approximately
75 years. Weidemeier and others reported that benzene in groundwater has a half-life of 1.97
years and naphthalene has a half-life of 0.71 years. Using the concentrations of benzene and naphthalene detected in MW-1R in 2015, 18.9 and 43.3 ppb, the benzene and naphthalene
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Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
concentration should drop below its respective North Carolina 2L Groundwater Quality
Standards within 5 to 10 years. Therefore, based on the absence of any known preferential pathways, it appears the benzene and naphthalene plume above 2L Standards will not reach
the downgradient property boundary at concentrations above the 2L Standard in the
groundwater.
4.2 Solute Transport Modeling
The objective of this modeling effort was to evaluate the maximum future extent and fate of
benzene and naphthalene in the groundwater at the former Doug’s Mini Mart. Benzene and naphthalene were selected as the simulated compounds because of its presence in
groundwater above their respective NC2LGWQS. The Air Force Center for Environmental
Excellence (AFCEE) BIOSCREEN Natural Attenuation model was used to assess the fate and transport of dissolved benzene and naphthalene. This solute transport model simulates the
processes of advection, dispersion, retardation, and degradation of dissolved constituents in
groundwater.
Conservative model assumptions and parameter values were used to ensure maximum
estimates of the horizontal downgradient migration distance for the constituents. Observed
values of groundwater velocity, conservative retardation and half-life parameters, and the highest benzene and naphthalene concentrations detected in MW-1R was used in order to determine the greatest possible downgradient migration distance from the source area.
4.3 Conceptual Model
Groundwater flow and solute transport modeling require an understanding of site specific hydrogeology, constituent transport and reaction processes, and source area information, which are formulated into a conceptual model. A conceptual model for the former Doug’s Mini Mart
site requires the identification of the following elements of the aquifer system: hydrostatigraphy,
rates and directions of groundwater flow, hydraulic properties, and source release information and constituent distribution. These characteristics are incorporated into the natural attention
model to make predictions of constituent concentrations.
Field data indicate that a medium permeability, water-table aquifer is present beneath the site. This water-table aquifer is recharged primarily by precipitation events. Lithologic information
collected during the drilling of the wells indicate that the water-table aquifer consists of
predominantly silty-clay, however; based on the K value obtained from the slug test on MW-1R, the area in the proximity of MW-1R is assumed to be a medium to silty-sand. It should also be
noted that MW-1R is approximately located within the former UST basin which may have
artificial fill consisting of the types of materials that produce a K value of that observed. A
saturated thickness of approximately 10 feet was assumed for modeling purposes.
Water level data obtained from the Groundwater Monitoring Report dated February 9, 2012
prepared by CES were used to estimate groundwater flow direction by solving a three-point solution for wells MW-1R, MW-4, and MW-5. The flow direction was determined to be to the southeast. A hydraulic gradient was calculated based on the data provided in the February
2012 Groundwater Monitoring Report. The average hydraulic gradient was determined to be
0.0135 feet/foot as calculate between MW-1R, MW-4, and MW-5.
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Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
Hydraulic conductivity data obtained from the rising head test conducted in MW-1R on June 4,
2015 indicates that the water table aquifer has an average hydraulic conductivity of 1.78 feet/day (6.3E-4 cm/sec). Using an average gradient of 0.0135 feet/foot and an assumed
porosity of 25 percent in the water table aquifer, and isotropic/homogeneous conditions, an
average theoretical rate of groundwater flow in the aquifer can be estimated to be on the order of 35.1 feet/year.
4.4 Model Description and Assumptions
The three-dimensional analytical solute transport model BIOSCREEN was used to simulate
natural attenuation of dissolved benzene and naphthalene in groundwater at the site. The BIOSCREEN Natural Attenuation software is based on the Domenico (1987) three-dimensional analytical solute transport model. The original model assumes a fully-penetrating vertical plane
source oriented perpendicular to groundwater flow, to simulate the release of contaminants to
moving groundwater. In addition, the Domenico solution accounts for the effects of advective transport, three-dimensional dispersion, adsorption, and first-order decay. In BIOSCREEN, the
Domenico solution has been adapted to provide three different model types representing i)
transport with no decay, ii) transport with first-order decay, and iii) transport with “instantaneous” biodegradation reaction.
4.5 Model Parameters
Site-specific model parameter values were used whenever possible. Conservative values from
literature were used when site-specific data were not available.
The source release for the model was chosen to be MW-1R, the monitoring well with the
highest benzene and naphthalene concentration located in the former UST basin on the
western portion of the site. The model simulation was run for 10, 25, 50, and 100 year time frame. The modeled reduction in benzene and naphthalene concentrations was compared to observed concentrations from monitoring wells along the modeled plume flowpath.
Groundwater advection is the main mechanism that causes downgradient migration of dissolved constituents. As discussed above, a value of 6.3E-4 cm/sec was used for the water
table aquifer hydraulic conductivity based on observed field conditions, and a value of 0.0135
feet/foot was calculated as the horizontal hydraulic gradient. A saturated thickness of 10 feet was estimated at the site. ECS recognizes the apparent mixing of metric and standard units, however, the units of the individual model input parameters are set by the model.
Adsorption of constituents onto aquifer materials can cause the constituent plume center of mass to migrate at a fraction of the rate that groundwater moves. The modeling analysis used
estimated values of 25 percent porosity (Heath 1982), 1.7 kilograms per liter for the aquifer soil
bulk mass density, a Koc of 38 L/kg, and a foc of 1.0-E3 for benzene. The retardation factor for benzene was estimated to be 1.3. The modeling analysis used estimated values of 25 percent porosity (Heath 1982), 1.7 kilograms per liter for the aquifer soil bulk mass density, a Koc of
1,288 L/kg, and a foc of 1.0-E3 for naphthalene. The retardation factor for naphthalene was
estimated to be 9.8.
Dispersion is an important physical process which influences the transport of dissolved
constituents. For plume migration distances which are less than 1,000 feet, the longitudinal
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Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
dispersivity can be estimated as one tenth the mean travel distance of the plume (Pickens and
Grisak, 1981). For this site, the longitudinal dispersivity was conservatively assumed to be 12 feet, which is approximately one tenth the observed mean travel distance. The horizontal and
vertical transverse dispersivity values were estimated to be one tenth and one hundredth the
longitudinal dispersivity value, respectively (de Marsily, 1986). Higher transverse dispersivity values cause benzene and naphthalene to disperse even farther downgradient and, therefore,
conservatively overestimate the calculated migration distance.
Degradation processes also affect the transport of dissolved constituents. Degradation is a general term which includes the effects of biodegradation, oxidation-reduction, and hydrolysis.
Other physical processes such as volatilization and recharge dilution could also be considered
degradation processes. In this model all of these processes are lumped into one degradation rate constant. Benzene and naphthalene are organic compounds which are affected by degradation processes. A degradation half-life for benzene was estimated to be 1.97 years. A
degradation half-life for naphthalene was estimated to be 0.71 years.
4.6 Model Results In this model, benzene is continuously released from the source, and is simulated until it has naturally attenuated below its 2L Standard. Graphs of benzene concentrations over time with a
receptor point 270 feet downgradient from the source are shown on figures 5 through 8. These
plots show that with no degradation, benzene will migrate more than 300 feet downgradient from the source area at levels above its detection limit. The plots show that with first order
decay, benzene will migrate more than 270 feet downgradient from the source at concentration
levels above its detection limit. Instantaneous reaction shows that benzene will not migrate
more than approximately 30 feet from the source. Based on the observed benzene concentrations declining over time, it is evident that benzene is degrading in the groundwater.
In the second model, naphthalene is continuously released from the source, and is simulated until it has naturally attenuated below its 2L Standard. Graphs of naphthalene concentrations
over time with a receptor point 270 feet downgradient from the source are shown on figures 9-
12.These plots show that with no degradation, naphthalene will not migrate more than 210 feet
downgradient from the source area at levels above its detection limit in 70 years. The plot shows that with first order decay, naphthalene will not migrate more than approximately 30 feet
downgradient from the source at concentration levels above its detection limit. Based on the
observed naphthalene concentrations declining over time, it is evident that naphthalene is degrading in the groundwater.
4.7 Model Summary
The results of the groundwater flow and solute transport modeling effort indicate that the
maximum distance from the source area where benzene will be above the 2L Standard of 1 ppm with little or no attenuation is approximately 300 feet downgradient of MW-1R or slightly past the property 270 feet from the well. However, these two model assumptions are not
supported by the fact that benzene has not been detected in samples collected from MW-4 or
MW-5 at concentrations predicted by the model (See sample results in Appendix A) Furthermore, the instantaneous reaction model depicts the benzene plume reducing to a
concentration of 0.0 ppb at approximately 90 feet downgradient.
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Fate and Transport Modeling Report Task Authorization No 2 – Doug’s Mini Mart Tabor City, Columbus County, North Carolina
NCDENR Incident:TF-15884 ECS Project: 33-3322 August 7, 2015
The model indicates that the maximum distance from the source area where naphthalene will
be above the 2L Standard of 6 ppm is approximately 30 feet downgradient of MW-1R. The maximum constituent migration distance is supported by the fact that naphthalene has not been
detected in a sample collected from MW-3 at concentrations above detection limits. The
modeling results indicate that the concentrations of benzene and naphthalene will eventually decrease below their detection limits through natural processes.
5.0 CONCLUSIONS
Based on the results of this assessment, ECS concludes the following:
•Modeling using conservative assumptions indicates that concentrations of naphthalene
at the site are not likely to reach the property line or any man-made or natural receptors
at concentrations above their respective 2L Standards.
•No degradation and first order decay models using conservative assumptions predicts
that concentrations of benzene at the site are likely to reach the property line or any
man-made or natural receptors at concentrations above their respective 2L Standards.
However, the instantaneous reaction model using conservative assumptions predicts
that concentrations of benzene at the site are not likely to reach the property line or any
man-made or natural receptors at concentrations above their respective 2L Standards.
•It is ECS opinion that the “no degradation” and “first order decay” models under predict
the biodegradation and attenuation of the benzene. This opinion is supported by the
empirical modeling and the actual field data ( benzene has not been detected in
downgradient wells (MW-4 and MW-5) at the predicted model concentrations (no
degradation and first order decay).
•Observed concentrations of benzene and naphthalene in downgradient wells suggest
the contaminants have not migrated off-site and have generally decreased in
concentrations since monitoring of the release began.
6.0 RECOMMENDATIONS
Based on the findings of the models and observed constituent concentrations;
•ECS recommends the site be considered for closure.
•ECS recommends that a copy of report be provided to the property owner.
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REFERENCES
15A NCAC North Carolina Department of Environment and Natural Resources, Division of Water
Quality, Subchapter 2L Classifications and Water Quality Standards Applicable to the
Groundwaters of North Carolina.
Fetter, C.W., 1993, “Contaminated Hydrology”, MacMillan, New York, New York, p.458.
Heath, Ralph C. “Basic Ground-water Hydrology”. Washington, D.C.: United States Government
Printing Office; 1983. Print.
Howard, P.H., 1991, Handbook of Environmental Fate and Exposure Data for Organic Chemicals,
Volume III, Lewis Publishers, Syracuse, New York, p.712.
Wiedemier, T.H., Rifai, H.S., Newell, C.J., & WIlson, J.T., 1999, Natural Attenuation of Fuels and
Chlorinated Solvents in the Subsurface, John Wiley, New York, p. 601.
FIGURES
FIGURE 1
SITE LOCATION MAP
DOUG’S MINI MART
323 SWAMPFOX HIGHWAY
TABOR CITY, COLUMBUS COUNTY, NORTH CAROLINA
NCDENR INCIDENT: TF-15889
ECS PROJECT NO. 33-3322
SOURCE:
2013 USGS TOPOGRAPHIC MAP:
TABOR CITY, NORTH CAROLINA
QUADRANGLE
SCALE: AS SHOWN
SITE
LEGEND
Property Boundary
FIGURE 2
SITE MAP
DOUG’S MINI MART
323 SWAMPFOX HIGHWAY
TABOR CITY, COLUMBUS COUNTY, NORTH CAROLINA
NCDENR INCIDENT: TF-15889
ECS PROJECT NO. 33-3322
SOURCE:
COLUMBUS COUNTY GIS
SCALE: AS SHOWN
LEGEND
Property Boundary
FIGURE 3
GROUNDWATER FLOW MAP
DOUG’S MINI MART
323 SWAMPFOX HIGHWAY
TABOR CITY, COLUMBUS COUNTY, NORTH CAROLINA
NCDENR INCIDENT: TF-15889
ECS PROJECT NO. 33-3322
SOURCE:
TERRAINE
GROUNDWATER
MONITORING REPORT
DATED MARCH 24, 2010
SCALE: AS SHOWN
0.1001.0000 5 10 15 20 25 30 35 40 45 50H/HiTime in MinutesFigure 4Test Results Plot - Doug's Mini MartMW-1R, June 4, 2015 0.3711.5 Min.
DISSOLVED HYDROCARBON CONCENTRATION ALONG PLUME CENTERLINE (mg/L at Z=0)
Distance from Source (ft)
TYPE OF MODEL 0 30 60 90 120 150 180 210 240 270 300
No Degradation 0.571 0.571 0.565 0.548 0.526 0.496 0.457 0.401 0.328 0.244 0.160
1st Order Decay 0.571 0.404 0.283 0.194 0.133 0.090 0.061 0.040 0.026 0.016 0.009
Inst. Reaction 0.514 0.514 0.355 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Field Data from Site 0.040 0.000
Time:
10 YearsNext Timestep
Prev Timestep
Replay Recalculate This
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 50 100 150 200 250 300 350Concentration (mg/L) Distance From Source (ft)
1st Order Decay Instantaneous Reaction No Degradation Field Data from Site
Return to
Figure 5
Benzene 10 Year
Model
DISSOLVED HYDROCARBON CONCENTRATION ALONG PLUME CENTERLINE (mg/L at Z=0)
Distance from Source (ft)
TYPE OF MODEL 0 30 60 90 120 150 180 210 240 270 300
No Degradation 0.571 0.571 0.565 0.550 0.530 0.510 0.490 0.472 0.455 0.440 0.426
1st Order Decay 0.571 0.404 0.282 0.194 0.133 0.090 0.061 0.042 0.029 0.019 0.013
Inst. Reaction 0.428 0.429 0.278 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Field Data from Site 0.040 0.000
Time:
25 YearsNext Timestep
Prev Timestep
Replay Recalculate This
0.00
0.10
0.20
0.30
0.40
0.50
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0 50 100 150 200 250 300 350Concentration (mg/L) Distance From Source (ft)
1st Order Decay Instantaneous Reaction No Degradation Field Data from Site
Return to
Figure 6
Benzene 25 Year Model
DISSOLVED HYDROCARBON CONCENTRATION ALONG PLUME CENTERLINE (mg/L at Z=0)Distance from Source (ft)TYPE OF MODEL0 30 60 90 120 150 180 210 240 270 300No Degradation0.571 0.570 0.564 0.549 0.530 0.510 0.490 0.472 0.455 0.440 0.4261st Order Decay0.571 0.403 0.282 0.194 0.133 0.090 0.061 0.042 0.029 0.019 0.013Inst. Reaction0.286 0.287 0.137 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Field Data from Site0.0400.000Time:50 YearsNext TimestepPrev TimestepReplayRecalculate This 0.000.100.200.300.400.500.600 50 100150200250300350Concentration(mg/L)Distance From Source (ft)1st Order DecayInstantaneous ReactionNo DegradationField Data from SiteReturn to Figure 7Benzene 50 Year Model
DISSOLVED HYDROCARBON CONCENTRATION ALONG PLUME CENTERLINE (mg/L at Z=0)Distance from Source (ft)TYPE OF MODEL0 30 60 90 120 150 180 210 240 270 300No Degradation0.570 0.570 0.564 0.549 0.530 0.509 0.490 0.472 0.455 0.440 0.4251st Order Decay0.570 0.403 0.282 0.194 0.133 0.090 0.061 0.042 0.028 0.019 0.013Inst. Reaction0.006 0.007 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Field Data from Site0.0400.000Time:100 YearsNext TimestepPrev TimestepReplayRecalculate This 0.000.100.200.300.400.500.600 50 100150200250300350Concentration(mg/L)Distance From Source (ft)1st Order DecayInstantaneous ReactionNo DegradationField Data from SiteReturn to Figure 8Benzene 100 Year Model
DISSOLVED HYDROCARBON CONCENTRATION ALONG PLUME CENTERLINE (mg/L at Z=0)Distance from Source (ft)TYPE OF MODEL0 30 60 90 120 150 180 210 240 270 300No Degradation0.267 0.160 0.042 0.003 0.000 0.000 0.000 0.000 0.000 0.000 0.0001st Order Decay0.267 0.005 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Inst. Reaction0.212 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Field Data from SiteTime:10 YearsNext TimestepPrev TimestepReplayRecalculate This 0.0000.0500.1000.1500.2000.2500.3000 50 100150200250300350Concentration(mg/L)Distance From Source (ft)1st Order DecayInstantaneous ReactionNo DegradationField Data from SiteReturn to Figure 9Naphthalene 10 Year Model
DISSOLVED HYDROCARBON CONCENTRATION ALONG PLUME CENTERLINE (mg/L at Z=0)Distance from Source (ft)TYPE OF MODEL0 30 60 90 120 150 180 210 240 270 300No Degradation0.267 0.252 0.207 0.128 0.053 0.014 0.002 0.000 0.000 0.000 0.0001st Order Decay0.267 0.005 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Inst. Reaction0.129 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Field Data from SiteTime:25 YearsNext TimestepPrev TimestepReplayRecalculate This 0.0000.0500.1000.1500.2000.2500.3000 50 100150200250300350Concentration(mg/L)Distance From Source (ft)1st Order DecayInstantaneous ReactionNo DegradationField Data from SiteReturn to Figure 10Naphthalene 25 Year Model
DISSOLVED HYDROCARBON CONCENTRATION ALONG PLUME CENTERLINE (mg/L at Z=0)Distance from Source (ft)TYPE OF MODEL0 30 60 90 120 150 180 210 240 270 300No Degradation0.267 0.266 0.261 0.245 0.215 0.170 0.114 0.063 0.028 0.010 0.0031st Order Decay0.267 0.005 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Inst. Reaction0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Field Data from SiteTime:50 YearsNext TimestepPrev TimestepReplayRecalculate This 0.0000.0500.1000.1500.2000.2500.3000 50 100150200250300350Concentration(mg/L)Distance From Source (ft)1st Order DecayInstantaneous ReactionNo DegradationField Data from SiteReturn to Figure 11Naphthalene 50 Year Model
DISSOLVED HYDROCARBON CONCENTRATION ALONG PLUME CENTERLINE (mg/L at Z=0)Distance from Source (ft)TYPE OF MODEL0 30 60 90 120 150 180 210 240 270 300No Degradation0.267 0.267 0.264 0.257 0.248 0.238 0.227 0.215 0.201 0.181 0.1561st Order Decay0.267 0.005 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Inst. Reaction0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Field Data from SiteTime:100 YearsNext TimestepPrev TimestepReplayRecalculate This 0.0000.0500.1000.1500.2000.2500.3000 50 100150200250300350Concentration(mg/L)Distance From Source (ft)1st Order DecayInstantaneous ReactionNo DegradationField Data from SiteReturn to Figure 12Naphthalene 100 Year Model
TABLES
ConstituentSeepage Velocity Estimated Plume LengthRetardation FactorSolute Half-lifeModel DimensionsSource Zone ThicknessSimulation TimeBenzene 35.1.5 ft/yr 120 Feet 1.301.97 300'X150' 10 Feet 100 YearsNaphthalene 35.1 ft/yr 120 Feet 9.800.71 300'X150' 10 Feet 100 YearsNCDENR Incident # TF-15889ECS Project 33-3322Table 1Model parametersDoug's Mini Mart 323 Swampfox HighwayTabor City, Columbus County, North Carolina
APPENDIX A
APPENDIX B
HYDRAULIC CONDUCTIVITY TEST DATA AND CALCULATIONS - HVORSLEV METHOD
DOUGS MINI MART, TABOR CITY, NORTH CAROLINA
ECS PROJECT 33.3322
MONITORING WELL MW-1R
4/10/2013
Total Depth 12 ft
Screened Length 10 ft
Static Water Level 2.1 ft
Well Radius (r)2 in =5.08 cm
Anulus Radius (R)6 in =15.24 cm
If rise/fall is in sand pack replace r with re = {r2 + 0.3(R2 - r2)}1/2 :re=9.37 cm
Interpolation
DH / Hi 0.388 Time 9.000 closest T above H/Hi = .37
DH / Hi 0.379 T(0) =10.00 (interpolates T at H / Hi is .37)
DH / Hi 0.366 Time 10.000 closest T below H/Hi = .37
DH / Hi 0.37 T(0) = 11 from graph
Hydraulic Conductivity = r2 [ln(Leff/R)]/2*Leff*T(0)
L(eff) =9.9 ft (depth - static water level
L(eff) =301.8 cm or screen length)
Results -based on T(0) from interpolation
T(0) =570.0 sec
Hydraulic Conductivity =7.62E-04 cm/sec
Hydraulic Conductivity =2.16 ft/day
Hydraulic Conductivity =787.9 ft/yr
Results - based on T (0) from Graph
T(0) =690.0 sec
Hydraulic Conductivity =6.29E-04 cm/sec
Hydraulic Conductivity =1.78 ft/day
Hydraulic Conductivity =650.9 ft/yr