U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2014–5224

Prepared in cooperation with the U.S. Environmental Protection Agency

Hydrogeologic Framework, Hydrology, and Refined 

Conceptual Model of Groundwater Flow for Coastal Plain

Aquifers at the Standard Chlorine of Delaware, Inc.

Superfund Site, New Castle County, Delaware, 2005–12
Cover. Red Lion Creek tide control structure with reinforced dike and five outflow gates, looking from the Delaware River at low tide in 

2010. Photograph by Todd Keyser, Delaware Department of Natural Resources and Environmental Control.
Hydrogeologic Framework, Hydrology, and 

Refined Conceptual Model of Groundwater 

Flow for Coastal Plain Aquifers at the 

Standard Chlorine of Delaware, Inc. 

Superfund Site, New Castle County, 

Delaware, 2005–12

By Michael J. Brayton, Roberto M. Cruz, Luke Myers, James R. Degnan, and 

Jeff P. Raffensperger

Scientific Investigations Report 2014–5224

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the 

U.S. Environmental Protection Agency 
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Brayton, M.J., Cruz, R.M., Myers, Luke, Degnan, J.R., and Raffensperger, J.P., 2015, Hydrogeologic framework, 

hydrology, and refined conceptual model of groundwater flow for Coastal Plain aquifers at the Standard Chlorine 

of Delaware, Inc. Superfund Site, New Castle County, Delaware, 2005–12: U.S. Geological Survey Scientific 

Investigations Report 2014–5224, 61 p., http://dx.doi.org/10.3133/sir20145224.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-3901-9
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................2

Environmental Setting.................................................................................................................5

Physiography and Cultural Features ...............................................................................5

Climate..................................................................................................................................5

Site History....................................................................................................................................5

Hydrogeologic Setting and Conceptual Flow System ...........................................................7

Hydrogeology ......................................................................................................................7

Flow System.........................................................................................................................8

Surface-Water Features....................................................................................................8

Study Approach and Data-Collection Methods......................................................................................10

Exploratory Drilling .............................................................................................................................11

Geophysical Surveys..........................................................................................................................13

Water-Level Monitoring Network ....................................................................................................14

Production-Well Pumping to Evaluate Aquifer Interconnections .............................................14

Design and Equipment ..............................................................................................................14

Water-Level Measurements ....................................................................................................15

Measuring the Effects of Water Withdrawals ...............................................................................15

Hydrogeologic Framework.........................................................................................................................16

Columbia Formation, Columbia Aquifer, and Other Quaternary Age Sediments......................17

Merchantville Formation Confining Unit .........................................................................................19

Potomac Formation and Related Aquifers .....................................................................................20

Depositional Environments .............................................................................................20

Site-Specific Findings ......................................................................................................20

Site-Related Aquifer Distinctions...................................................................................20

Paleochannels and Erosional Features ..........................................................................................22

Hydrology ......................................................................................................................................................22

Recharge and Discharge Patterns ..................................................................................................23

Columbia Aquifer Recharge.....................................................................................................24

Potomac Aquifer Recharge......................................................................................................25

Groundwater Discharge ...........................................................................................................25

Water-Table Contours, Potentiometric Surfaces, and Flow Directions.....................................26

Surface-Water Influences on Groundwater ..................................................................................29

Tidal Influences..........................................................................................................................30

Red Lion Creek Stage Change Effects....................................................................................30

Response to Large Storm Events ............................................................................................31

Connection Between Aquifers .........................................................................................................32

Vertical Gradients ......................................................................................................................33

Water-Level Responses and Aquifer Interconnections During Production-Well 

Pumping ........................................................................................................................34
iv

Water-Level Drawdown...................................................................................................35

Water-Level Recovery......................................................................................................39

Water Withdrawal Influences on the Local Flow System............................................................41

Water Withdrawal Rates ..........................................................................................................41

Water-Level Changes Caused by Withdrawals....................................................................41

Production Well R-15.................................................................................................................43

Refined Local Conceptual Model of Groundwater Flow ..............................................................44

Description of Components of Conceptual Model ........................................................................44

Implications for Groundwater Contaminant Movement...............................................................45

Summary and Conclusions.........................................................................................................................46

Acknowledgments.......................................................................................................................................47

References Cited..........................................................................................................................................47

Appendix 1. Chemicals of concern detected in soils, sediment, and (or) groundwater 

at the Standard Chlorine of Delaware, Inc. Superfund Site....................................................52

Appendix 2. Characteristics of U.S. Geological Survey Potomac Aquifer Study 

monitoring sites measured at and near the Standard Chlorine of Delaware, Inc. 

Superfund Site, 2005–12.................................................................................................................53

Appendix 3. Well characteristics and water-level drawdown and recovery data 

for wells monitored as part of U.S. Geological Survey 72-hour pumping at 

well OR-6A near the Standard Chlorine of Delaware, Inc. Superfund Site, 

August 28–31, 2010.........................................................................................................................57

Appendix 4. Water-level data for non-U.S. Geological Survey network observation 

wells monitored as part of U.S. Geological Survey 72-hour pumping at well 

OR-6A near the Standard Chlorine of Delaware, Inc. Superfund Site, 

August 26–September 1, 2010......................................................................................................60

Figures

1. Maps showing location of Standard Chlorine of Delaware, Inc. Superfund 

Site study area and Delaware City industrial area in northern New Castle 

County, Delaware..........................................................................................................................2

2. Map showing location of Delaware City Industrial Area, Red Lion Creek 

watershed, water withdrawal intakes, and groundwater management 

zones related to sites designated as U.S. Environmental Protection Agency 

Superfund National Priorities List (NPL) Sites or Resource Conservation 

and Recovery Act (RCRA) Corrective Action Facilities ..........................................................3

3. Map showing location of study area including Standard Chlorine of 

Delaware, Inc. (SCD) Superfund Site, New Castle County, Delaware, 

monitoring well network, and lines of section A-A’, B-B’, and C-C’.....................................4

4. Map showing regional flow system (A) map showing surface-water 

features, subcrop areas of the Potomac aquifer system, major groundwater 

intakes near the Standard Chlorine of Delaware, Inc. Superfund Site (SCD) 

study area, and generalized flow lines for the regional groundwater system, 

and (B) generalized cross section showing recharge areas and the confined 

aquifer flow system......................................................................................................................9

5. Photograph showing Red Lion Creek tide control structure with reinforced 

dike and five outflow gates, looking from the Delaware River at low tide in 2010 ..........11
v

6. Photographs showing (A) rotosonic drilling, (B) recovered core, (C) core 

from Merchantville confining unit, and (D) core from upper Potomac 

confining unit...............................................................................................................................13

7. Cross sections showing wells, lithologic descriptions, and gamma 

responses for (A) cross section A-A’ from north to south, and (B) cross 

section B-B’ from northwest to southeast .............................................................................16

8. Cross section C-C’ showing direct-current resistivity results and 

interpretation of stratigraphy at the Standard Chlorine of Delaware, Inc. 

Superfund Site.............................................................................................................................21

9. Graph showing daily average altitude of water levels for selected upper 

Potomac A-sand aquifer wells and Red Lion Creek near the Standard 

Chlorine of Delaware, Inc. Superfund Site, 2007–12.............................................................23

10. Graphs showing (A) comparison of monthly precipitation and long-term 

average monthly precipitation in the Delaware City area, 2006–12, and 

(B) hydrograph showing water-level variation for water-table 

(Columbia aquifer) and confined aquifer (upper Potomac A-sand) wells 

compared with monthly precipitation .....................................................................................24

11. Maps showing water-table contours for the Columbia aquifer (A) May 13, 

2006 prior to completion of the barrier wall, and (B) August 8, 2012 after 

completion of the barrier wall ..................................................................................................26

12. Maps showing potentiometric surface of the upper Potomac A-sand 

aquifer during (A) limited pumping conditions (September 14, 2011), 

and (B) typical pumping conditions (August 8, 2012), and heads for 

selected wells in deeper aquifer units....................................................................................28

13. Graph showing daily water-level range in feet for the Delaware River, 

Red Lion Creek, and upper Potomac aquifer wells for selected dates 

following a 7-day period of minimal precipitation.................................................................31

14. Graphs showing water-level response in upper Potomac A-sand aquifer 

wells to tidal cycling in Red Lion Creek with no antecedent precipitation 

(A) before tide gate malfunction, and (B) after tide gate malfunction...............................32

15. Graph showing maximum water-level response, in feet, measured at 

Red Lion Creek and selected Columbia and upper Potomac aquifer wells 

for selected discrete precipitation events, 2010–12 .............................................................33

16. Graph showing hourly rainfall and normalized hydrograph showing 

recharge response in wells screened in the Columbia and upper Potomac 

aquifers during and after Hurricane Irene, August 27–30, 2011 .........................................34

17. Graph showing time-series comparison of calculated vertical gradients 

for selected nested well pairs at the Standard Chlorine of Delaware, Inc. 

Superfund Site, 2007–12 ............................................................................................................35

18. Map showing maximum observed water-level drawdown for wells 

screened in the Columbia aquifer and upper Potomac aquifer, contours 

of maximum head change in the upper Potomac A-sand aquifer, and 

general area where the confining unit is thin or absent......................................................36

19. Graph showing log/log plot of time drawdown data for selected wells 

instrumented with continuous water-level recorders during 72 hours of 

pumping by the U.S. Geological Survey at well OR-6A near the Standard 

Chlorine of Delaware, Inc. Superfund Site, August 28–31, 2010.........................................38

20. Graph showing percent recovery of water levels over elapsed time for 

selected continuously monitored upper Potomac aquifer wells after 

conclusion of 72 hours of pumping by the U.S. Geological Survey at well 

OR-6A, August 31, 2010 ..............................................................................................................39
vi

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047.0 square meter (m2)

acre 0.004047 square kilometer (km2)

square foot (ft2) 0.09290 square meter (m2)

square inch (in2) 6.452 square centimeter (cm2)

square mile (mi2) 2.590 square kilometer (km2) 

Conversion Factors and Datums

21. Map showing elapsed time in days to reach 90-percent water-level 

recovery for selected continuously monitored wells screened in the 

upper Potomac aquifer, in response to conclusion of 72 hours of 

pumping by the U.S. Geological Survey at well OR-6A, August 31, 2010...........................40

22. Graphs showing (A) hydrograph showing water-level response to 

changes in (B) pumping rate for production wells screened in the 

upper (R-15), middle (P-5B), and lower (P-6A) Potomac aquifers near 

the Standard Chlorine of Delaware, Inc. Superfund Site.....................................................42

23. Hydrograph showing upper Potomac aquifer recovery and drawdown 

for A-, B-, and C-sand intervals in response to production well R-15 

operating schedule.....................................................................................................................43

24. Cross section A-A’ showing refined conceptual model of local groundwater 

flow, including Red Lion Creek, Columbia aquifer, upper Potomac A-, B-, 

and C-sand aquifers, selected upper Potomac aquifer wells, and erosional 

channel of paleo-Red Lion Creek.............................................................................................45

Tables

1. Chronology of events at the Standard Chlorine of Delaware, Inc. 

Superfund Site, New Castle County, Delaware.......................................................................6

2. Stratigraphic correlation chart of geologic and hydrogeologic units in 

the vicinity of the Standard Chlorine of Delaware, Inc. Superfund Site..............................7

3. Characteristics for wells installed during Potomac Aquifer Study 

exploratory drilling, 2007–10 .....................................................................................................12

4. Annually averaged daily water withdrawal at major withdrawal locations 

near the Standard Chlorine of Delaware, Inc. Superfund Site, New Castle 

County, Delaware........................................................................................................................18

5. Comparison of annual and long-term mean climate conditions in the 

Delaware City area, 2005–12.....................................................................................................25
vii

Multiply By To obtain

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3) 

million gallons (Mgal) 3,785 cubic meter (m3)

cubic inch (in3) 16.39 cubic centimeter (cm3) 

cubic inch (in3) 0.01639 liter (L)

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

foot per day (ft/d) 0.3048 meter per day (m/d)

foot per year (ft/yr) 0.3048 meter per year (m/yr)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Pressure

pound per square inch (lb/in2) 6.895 kilopascal (kPa) 

Density

pound per cubic foot (lb/ft3) 16.02 kilogram per cubic meter (kg/m3)

Specific capacity

gallon per minute per foot 

[(gal/min)/ft)]

0.2070 liter per second per meter 

[(L/s)/m]

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d) 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C = (°F - 32) / 1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29). The local datum conversion from NGVD 29 to the North American Vertical Datum of 

1988 (NAVD 88) is -0.78 ft.

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). 

Hydrogeologic Framework, Hydrology, and Refined 

Conceptual Model of Groundwater Flow for Coastal 

Plain Aquifers at the Standard Chlorine of Delaware, Inc. 

Superfund Site, New Castle County, Delaware, 2005–12

By Michael J. Brayton, Roberto M. Cruz, Luke Myers, James R. Degnan, and Jeff P. Raffensperger

Abstract

From 1966 to 2002, activities at the Standard Chlorine of 

Delaware chemical facility in New Castle County, Delaware 

resulted in the contamination of groundwater, soils, and wetland sediment. In 2005, the U.S. Geological Survey (USGS), 

in partnership with the U.S. Environmental Protection Agency, 

Region 3, and the Delaware Department of Natural Resources 

and Environmental Control began a multi-year investigation of 

the hydrogeologic framework and hydrology of the confined 

aquifer system. The goals of the ongoing study at the site (the 

Potomac Aquifer Study) are to determine the hydraulic connection between the Columbia and Potomac aquifers, determine the direction of groundwater flow in the Potomac aquifer, 

and identify factors affecting the fate of contaminated groundwater. This report describes progress made towards these goals 

based on available data collected through September 2012.

The regional hydrogeologic framework indicates that the 

site is underlain by Coastal Plain sediments of the Columbia, 

Merchantville, and Potomac Formations. Two primary aquifers 

underlying the site, the Columbia and the upper Potomac, 

are separated by the Merchantville Formation confining unit. 

Local groundwater flow in the surficial (Columbia) aquifer is 

controlled by topography and generally flows northward and 

discharges to nearby surface water. Regional flow within the 

Potomac aquifer is towards the southeast, and is strongly influenced by major water withdrawals locally. Previous investigations at the site indicated that contaminants, primarily benzene 

and chlorinated benzene compounds, were present in the 

Columbia aquifer in most locations; however, there were only 

limited detections in the upper Potomac aquifer as of 2004. 

From 2005 through 2012, the USGS designed a monitoring 

network, assisted with exploratory drilling, collected data at 

monitoring wells, conducted geophysical surveys, evaluated 

water-level responses in wells during pumping of a production 

well, and evaluated major aquifer withdrawals. Data collected 

through these efforts were used to refine the local conceptual 

flow system. The refined conceptual flow system for the site 

includes: (a) identification of gaps in confining units in the 

study area, (b) identification and correlation of multiple waterbearing sand intervals within the upper Potomac Formation, 

(c) connections between groundwater and surface water, (d) 

connections between shallow and deeper groundwater, (e) new 

water-level (or potentiometric surface) maps and inferred flow 

directions, and (f) identification of major local pumping well 

influences. The implications of the revised conceptual flow 

system on the occurrence and movement of site contaminants 

are that the resulting detection of contaminants in the upper 

Potomac aquifer at specific well locations can be attributed 

primarily to either advective lateral transport, direct vertical 

contaminant transport, or a combination of vertical and lateral 

movement resulting from changes in water withdrawal rates 

over time.

Introduction

The U.S. Geological Survey (USGS), in cooperation with 

the U.S. Environmental Protection Agency, Region 3 (EPA), is 

involved in an ongoing study to characterize the flow system 

and define the hydrogeologic framework of the Columbia 

and Potomac aquifers in the vicinity of the former Standard 

Chlorine of Delaware, Inc. (SCD) Superfund Site near 

Delaware City, New Castle County, Delaware (fig. 1). The 

presence of site-related contaminants from the former chemical manufacturing facility, including benzene and chlorinated 

benzene compounds, in both the Columbia and underlying 

Potomac aquifers may pose a threat to public water-supply 

wells screened within the Potomac Formation and located 

approximately 3 miles southeast and downgradient from the 

site.

This investigation (the Potomac Aquifer Study) was 

designed to determine the extent of the hydraulic connection 

between the Columbia and underlying Potomac aquifers, to 
2 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

determine the direction of groundwater flow in the Potomac 

aquifer, and to identify factors affecting the fate of contaminated groundwater. The EPA will use the results of this study 

to help design a final remedy for groundwater at the site with a 

goal of protecting human health and the environment 

(U.S. Environmental Protection Agency, 2011).

Purpose and Scope

This report describes recent local refinements to the 

hydrogeologic framework and hydrology of the Coastal Plain 

sediments underlying the SCD site. The refinements were 

used to improve the local conceptual model of groundwater 

flow, which serves as the basis for determining the hydrogeologic and hydrologic factors that influence contaminant 

movement in groundwater at the site. Refinements were made 

possible using new data collected during 2005–12 from activities that included exploratory drilling, geophysical surveys, 

expansion of an observation-well network, and observations 

from production well pumping. Analysis of these new data are 

described in this report to provide detail on the hydrogeologic 

framework, the direction of groundwater flow, the connections between aquifers, and the effects of pumping and other 

hydraulic stresses at a local (study area) scale.

Description of Study Area

The SCD Superfund Site is located within the Delaware 

City Industrial Area (DCIA), which contains a large oil refinery built in the early 1950s, and several chemical-producing 

facilities built in subsequent decades (fig. 2). The study 

area described in this report includes Standard Chlorine of 

FALL

LINE

Base from U.S. Geological

Survey digital data, 2004,

1:24,000

DELAWARE RIVER

NEW

CASTLE

KENT

DELAWARE

COASTAL

PLAIN

SUSSEX

Wilmington

Delaware City

Dover DELAWARE

DELAWARE

MARYLAND

MARYLAND

NEW JERSEY

DELAWARE

BAY

ATLANTIC

OCEAN

39°30'

39°00'

38°30'

75°30' 75°00'

PENNSYLVANIA

Indian River

Bay

Rehoboth Bay

Georgetown

Red Lion

Creek

Dragon

Run

C&D Canal

Location of Standard

Chlorine of Delaware, Inc.

Superfund Site (SCD)

study area

0

0

6 MILES

6 KILOMETERS

DELAWARE

MARYLAND

75°45'00" 75°37'30"

39°

40'

00"

39°

35'

00"

Chesapeake and Delaware Canal

Base modified from ESRI and U.S. Geological Survey digital

data, 2013, NAD 83 State Plane Delaware Projection

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

495

295

Fort

Dupont

State

Park

Fort

Delaware

State

Park

Lums Pond

State Park

Delaware

City

Newark

Newport

New Castle

New

Castle

County

Airport

Delaware City Industrial

Area (Refer to figure 2 for

enlarged map area)

River

Pulaski Highway

Road

Dupont

Highway

Pea Patch

Island

NEW

JERSEY DELAWARE 

95 RIVER

95

9

9

9

72

72

13

N

PIEDMONT

NOAA

tide

station

8551762

National Oceanic

and Atmospheric

Administration

(NOAA)

weather station

13781

Red Lion Creek

Dragon

Run

Red Lion Creek

watershed

Christina

River

1

1

40

301

71

896

FALL

COASTAL PLAIN

University of Delaware

LINE

2

2

Tybouts

Corner

SCD study area

Red

Lion

Road 71

Figure 1. Location of Standard Chlorine of Delaware, Inc. Superfund Site study area and Delaware City industrial area in northern 

New Castle County, Delaware.
Introduction 3

Delaware, Inc., a 65-acre former chemical manufacturing 

facility, and areas extending approximately one-half mile 

beyond the site property (tax map) boundary. The SCD site 

is bounded to the north by Red Lion Creek, to the east by 

Occidental Chemical Corporation (Oxychem) Facility and 

Delaware Route 9 (River Road), to the west by the former 

Air Products, Inc. Facility and an un-named tributary of Red 

Lion Creek, and to the south by Governor Lea Road (fig. 3). 

The Chesapeake and Delaware Canal is located 3 miles to the 

south, and the Delaware River is located 1 mile to the east 

(fig. 2). A site-related monitoring-well network that includes 

wells on adjacent properties has enabled the hydrogeologic 

characterization of a broader area surrounding the site (fig. 3). 

In addition to SCD, other nearby industrial sites, including some that are no longer in operation, are also listed as 

EPA Superfund National Priorities List (NPL) sites including 

Delaware City PVC and Tybouts Corner Landfill, or listed 

as Resource Conservation and Recovery Act (RCRA) corrective action sites including Motiva Delaware City Refinery 

(currently Delaware City Refining Company or DCRC), 

Occidental Chemical Corporation (Oxychem), and Akzo 

Chemicals, Inc. Several of these sites are located within 

all or part of the 10.15-square-mile (mi2) Red Lion Creek 

watershed (fig. 2). Considering the known and potential 

contamination of groundwater resources within the existing 

industrial footprint, a Groundwater Management Zone (GMZ) 

with three sub-zones: A, B, and Tybouts was established by 

the State of Delaware, Department of Natural Resources and 

Environmental Control (Delaware Department of Natural 

Resources and Environmental Control, 2011). In this report, 

the geographic boundary of the GMZ and the DCIA are considered identical and described as bounded by the Delaware 

Figure 2. Location of Delaware City Industrial Area, Red Lion Creek watershed, water withdrawal intakes, and groundwater 

management zones related to sites designated as U.S. Environmental Protection Agency Superfund National Priorities List (NPL) Sites 

or Resource Conservation and Recovery Act (RCRA) Corrective Action Facilities (U.S. Environmental Protection Agency, 2012.).

Dragon

Run

Fort Dupont

State Park

Lums Pond

State Park

Fort

Delaware

State

Park

Delaware City

NEW

JERSEY

DELAWARE RIVER

Red Lion Creek

River

Red Lion Road

Governor

School House

Road

Lea Road

Road

9

SCD

study area

9

72

72

1

1

Pulaski Highway 40

13

13

301

71

Chesapeake and Delaware Canal

7

7

Tybouts

Area B

Area A

NOAA

tide

station

P-3B 8551762

P-16A

P-9A

ASR-1

P-1A

Dragon Run

intake

Red Lion Creek

intake

P-10A

PW-5

PW-4

R-15

P-4B

P-5B

Standard Chlorine

of Delaware, Inc.

Tybouts

Corner

Landfill

Tide

control

structure

Akzo

Chemicals,

Inc.

Delaware

City

PVC

Motiva–

Delaware City Refinery

Occidental

Chemical

Corporation

P-6A

AV-3

AV-4

AV-1

AV-2

75°37'30"

39°35'00"

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

RCRA Corrective Action Facilities

Superfund (National Priorities List) Sites

Water withdrawal locations

Groundwater Management Zones

Surface water

Groundwater–upper Potomac aquifer

Groundwater–middle Potomac aquifer

Groundwater–lower Potomac aquifer

Red Lion Creek watershed

Red Lion Creek watershed above

River Road bridge

Area A Area B Tybouts

N

Base modified from ESRI and U.S. Geological Survey digital

data, 2013, NAD 83 State Plane Delaware Projection
4 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12 39°36'20" 39°36'10" 39°36'00" 39°35'50" TY-121A TY-116A TY-116B TY-116C TYRed -121BLion Creek intake Gage

Road River

Governor Lea Road

P-5B

P-6A

PW-6S

PW-14 PW-17

PW-11 MW

MW

-5

-23

MW-22

MW-21 MW-25

EM-2D

PW-11D

MW-12

PW10D

PW-13

PS-4

MW-33

MW-52

PMW-44

TW-28

PMW-49

PZ-3

PZ-1 PZ-24

PZ-25

PZ-5

PZ-8

PZ-6

PZ-28

PZ-7

PZ-9

PZ-12 PZ-15

MW-7

MW-20

MW-24

MW-40

MW-19

PZ-10

PZ-29

PZ-14

PZ-16

PMW

PZ-13

-47

PMW-46

PMW-45

PZ-11

PZ-27

MW-31

MW-29 MW-30

MW-32

PZ-20

PZ-21

PZ-23 PZ-22

PMW-43 PMW-42

PMW-41

PMW-50 PMW-51

MW

MW

-13

-10

PW

MW

-10

-18

PW-1

PW-4DPW-4S

CW-4

PW-12

PW-9

Former Air

Products, Inc.

Facility

Occidental

Chemical

Facility

PW-2D PW-2

C-31

PW-8

Tidewater 12

PW-3

Ion MW-3

PW-16

PW-7S MW-11

CW-11

PW-5D

MW-36

MW-37

MW-38

PW-5S

OR-6C

OR-6A

PW-6D

OR-6B

75°39'00" 75°38'40" 75°38'20" 75°38'00"

Imagery from U.S. Geological Survey, Delaware Valley Regional PlanningCommission, and Department of Homeland Security, 2010, NAD 1983State Plane Delaware Transverse Mercator Projection

N

9

1

0 1,000 2,000 FEET0 400 METERS

Red Lion Creek Red Lion Creek

STANDARD CHLORINE

OF DELAWARE, INC.

(SCD) SUPERFUND SITE

STANDARD CHLORINE

OF DELAWARE, INC.

(SCD) SUPERFUND SITE

Surface-water

monitoring site

Groundwater

monitoring wells

Water withdrawal

locations

EXPLANATION

Surface-water gage Surface waterupper Potomac aquifermiddle Potomacaquifer

Groundwater–middle Potomac aquifer

lower Potomac aquifer

Groundwater–lower Potomac aquifer

Continuous water-level sites

Barrier wall

B B' Line of section B-B'

Surface-water flow directionGeneralized Columbia aquifer flow directionGeneralized Potomac aquifer flow direction

Columbia aquiferAbandoned

A

A'

B'

B

C'

C MW-9

MW-8

PZ-17

PZ-30

PZ-19

PZ-18

PMW-48

MW-16

MW-17

MW-15

MW-14

Unnamed

ributary T

Figure 3. Location of study area including Standard Chlorine of Delaware, Inc. (SCD) Superfund Site, New Castle County, Delaware, monitoring well network, and lines of section A-A’, B-B’, and C-C’.
Introduction 5

River to the east, Delaware Route 13 (approximately) to the 

west, Red Lion Creek (approximately) to the north and Dragon 

Run to the south (fig. 2). Land use is primarily industrial; however, many open fields owned by the oil refinery are leased for 

agriculture. There are no residential areas within the DCIAGMZ as currently defined (U.S. Environmental Protection 

Agency, 2012), except for a part of the town of Delaware 

City located approximately 1.5 miles (mi) southeast of the 

Delaware City Refinery. 

Environmental Setting

Land in the area has been settled for more than three 

centuries, when ancestral Coastal Plain forests were cleared 

for farming, and settlements were built near waterways for 

shipping and commerce. In the areas near the Delaware River, 

water sources are abundant and the availability of surface 

water and groundwater has led to a long history of resource 

development and water-based commerce. Easy access to water 

for transportation and resource use eventually led to industrial 

development of the Delaware City area, which is also served 

by major roads and highways (fig. 2).

Physiography and Cultural Features

The SCD site is in New Castle County, Delaware, which 

contains 60 percent of the State’s population and has a population density of 1,263 persons per square mile (U.S. Census 

Bureau, 2013). Although the county is densely populated, 

there are no residences within 1 mile of the SCD site. New 

Castle County has experienced continued growth in recent 

years and current plans recommend that the SCD site should 

eventually be re-zoned for light industry or open space. Land 

cover at the site is partly wooded (approximately 20 percent), 

partly industrial (approximately 20 percent), containing 

concrete pads after infrastructure removal, and the remaining 

land cover (approximately 60 percent) is either marsh or open 

grassy field. There are other open grassy fields nearby, including agricultural fields to the west, where soybeans currently 

are grown (U.S. Environmental Protection Agency, 2011). 

Most industrial facilities in the area were built on land that had 

been previously cleared for farming, which was the primary 

land use for several hundred years until around 1950. The site 

is located on unconsolidated sediments of the North Atlantic 

Coastal Plain, approximately 8.5 mi from the Fall Line, where 

bedrock that forms the uplands of the Appalachian Piedmont 

is found in outcrops (fig. 1). Topography ranges from gentle 

slopes to steep scarps, with elevation ranging from sea level at 

Red Lion Creek to 70 feet (ft) above sea level south of the site. 

Soils present at the site are part of the Matapeake-Sassafras 

and Tidal Marsh associations (Black & Veatch, 2007). Soils 

generally are several feet thick and well-drained in upland 

areas. Thicker soils are present in marsh areas and overlie 

wetland sediments fringing Red Lion Creek. 

Climate

The climate of northern Delaware is considered humid 

temperate, and is influenced by the proximity to the Delaware 

Bay and the Atlantic Ocean. Weather and climate data 

collected at the New Castle County airport (National Oceanic 

and Atmospheric Administration [NOAA] weather station 

13781, “Wilmington New Castle Airport,” Delaware, COOP 

ID 079595), located 5.2 mi from the SCD site (fig. 1), are 

considered representative of site conditions. The mean annual 

temperature is 54.4 degrees Fahrenheit (°F), with a mean 

daily maximum of 63.5 °F and a mean daily minimum of 

45.1 °F. Mean annual precipitation is 42.8 inches (in.) and is 

fairly evenly distributed throughout the year, with each month 

receiving generally between 3 to 4 in. Snowfall averages 

19 in., occurring generally from mid-December to mid-March. 

Storm events have historically produced daily rainfall in 

excess of 2 in. in every month of the year. Large storm events 

from June through October are often associated with tropical 

storms, hurricanes, or their remnants, and have produced daily 

rainfall totals ranging from 4 to 8 in. Storm systems throughout the year are capable of causing coastal flooding (Office of 

Delaware State Climatologist, 2013; Brinson, 2013).

Site History

A chronology of events is provided for the SCD facility 

(table 1), which was constructed on farmland in 1965 and 

began producing chlorobenzene compounds in 1966. A variety 

of chlorobenzene compounds (chlorobenzene, paradichlorobenzene, orthodichlorobenzene, and lesser amounts of 

metadichlorobenzene and trichlorobenzene) were produced 

from 1966 to May 2002 by combining (through reaction and 

distillation) chlorine and benzene from adjacent industrial 

facilities. Marketable products and wastes were stored in 

above ground tanks, some of which were heated. Leakage 

from pipes and tanks was collected in catch basins and sumps 

for processing; however, a crack in Catch Basin #1 of the 

wastewater treatment plant was detected and repaired in 1976 

after leaking unknown quantities into the subsurface. A release 

of approximately 5,000 gallons (gal) of monochlorobenzene 

occurred in September 1981, while a railroad car located on 

the southwest part of the property was being filled. Efforts 

were made to contain and recover surface runoff, and contaminated soils were excavated and removed. After determining 

the extent of shallow groundwater contamination, a limited 

groundwater recovery and treatment system was installed in 

1982 to address these releases (Black & Veatch, 2007).

A second major release occurred in January 1986, when 

a single large tank failed, damaging other nearby tanks and 

leading to the release of approximately 400,000 gal of paradichlorobenzene, and approximately 169,000 gal of trichlorobenzene. Other intermediary compounds were likely part of 

this mixture but the exact quantities are unknown, complicating the understanding of subsequent contaminant degradation 

during the years since the tank failure occurred. This major 
6 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

spill covered land surface within the immediate plant area, and 

also spread to the northwest entering an unnamed tributary 

of Red Lion Creek, eventually flowing to the confluence with 

Red Lion Creek. A high and ebbing tide at the time of the 

spill caused further dispersal along the shoreline of the creek 

both upstream and downstream. Booms, dikes, and a filter 

fence were used to minimize further discharge to the creek. A 

sedimentation basin was constructed on site to store contaminated sediments, in addition to several waste piles. Following 

the initial cleanup response, the facility was stabilized and the 

production of chlorobenzenes resumed (U.S. Environmental 

Protection Agency, 1995).

As a result of the 1986 release, the SCD site was listed 

on the EPA NPL (Superfund) registry on July 22, 1987 and 

has been the subject of continuing subsurface investigations 

and site remediation. Extensive groundwater contamination 

was identified in the surficial (unconfined) Columbia aquifer, 

and has persisted (Roy F. Weston, Inc., 1992; Black & Veatch 

Special Project Corporation, written commun. to Hilary 

Thornton [EPA], 2005). Site contaminants have also more 

recently been detected at monitoring wells screened in the 

sands of the upper Potomac Formation to the north, west, and 

east of the site (U.S. Environmental Protection Agency, 2011), 

although the extent of contamination in the confined aquifer 

has not been fully characterized. Chemicals of concern that 

have been detected at SCD are listed in Appendix 1 (U.S. 

Environmental Protection Agency, 2010, 2011), and many 

of these have been detected as Dense Non-Aqueous Phase 

Liquids (DNAPLs) in the dissolved phase. Non-Arachlor polychlorinated biphenyls (PCBs) have also been detected in soils, 

sediment, and groundwater (U.S. Environmental Protection 

Agency, 2011). In addition, elevated chloride levels at the site 

have been partially attributed to spills of hydrochloric acid 

(HCl) associated with the 1986 tank failures (Black & Veatch, 

2007).

Major remedial activities at the SCD site have included 

the removal of contaminants and industrial equipment and the 

installation of a subsurface barrier “wall” or subsurface curtain 

consisting of clay and bentonite slurry installed so that the 

bottom of the wall is in contact with existing low-permeability 

clay layers. A pump and treat system was installed within the 

barrier wall to remove contaminants from groundwater, and to 

help prevent the spread of contamination by reversing groundwater gradients. A final remedy for deep groundwater 

(EPA Operable Unit-4) will be developed from results of current USGS site investigations.

Table 1. Chronology of events at the Standard Chlorine of Delaware, Inc. Superfund Site, New Castle County, Delaware.

[USGS, U.S. Geological Survey; EPA, U.S. Environmental Protection Agency; OU, operable unit, an EPA number identifying a part of a site with which remedial actions are associated; DNREC, Delaware Department of Natural Resources and Environmental Control; SCD, Standard Chlorine of Delaware, Inc.]

Event Date

Standard Chlorine of Delaware, Inc. (aka Metachem) facility in operation 1966 to May 2002

Major releases leading to Site’s National Priorities List (NPL) listing 1981 and 1986

Final Listing on the EPA NPL 07/22/1987

Remedial Investigation of shallow groundwater (OU-1), and soils and sediment (OU-2) completed 1992

Feasibility Study completed for OU-1 and OU-2 1995

Record of Decision selecting remedy for OU-1 and OU-2 signed 03/09/1995

Metachem files bankruptcy petition 05/10/2002

EPA and DNREC officially assume control of SCD site 5/14/2003

Amendment to 1995 Record of Decision 09/27/2004

USGS begins investigation of aquifer interaction and establishes long-term monitoring network in support of final 

remedy for groundwater (OU-4) August 2005

Remedial action on-site construction started 07/17/2006

Completion of subsurface barrier wall within Columbia aquifer (OU-1) May 2007

Final Remedial Investigation Report August 2007

SCD shallow (Columbia aquifer) groundwater pump and treat system active inside the barrier wall June 2008

Delaware City Refinery production well R-15 shut down 11/11/2009

Record of Decision selecting remedy for former plant area cap (OU-3) signed 09/29/2010

Red Lion Creek tide gate malfunction began during Hurricane Irene and caused a rise in creek stage, inundating 

parts of the wetland at the site 8/29/2011

Delaware City Refinery production well R-15 resumes pumping at steady 350 gallons per minute 10/26/2011

Estimated date for bringing contaminated groundwater migration under control (OU-4)* December 2017

*

 U.S. Environmental Protection Agency (2011).
Introduction 7

Hydrogeologic Setting and Conceptual Flow 

System

The study area is located in the Atlantic Coastal Plain 

Physiographic Province in central New Castle County, 

Delaware (fig. 1), near the Delaware River and approximately 

8.5 mi southeast of the Fall Line, which marks the eastern 

extent of the bedrock uplands of the Piedmont. The Atlantic 

Coastal Plain sediments consist of sequences of unconsolidated gravels, sands, silts, and clays that form a southeastdipping wedge that rests on older metamorphic, igneous, 

and consolidated sedimentary basement rocks (Cushing and 

others, 1973; Trapp and Meisler, 1992). Sediments range from 

Cretaceous to Holocene in age and were deposited in fluvial, 

deltaic, and marine environments. Later reworking by modern 

and ancestral streams and rivers has resulted in the downcutting of Cretaceous age sediments and the deposition of new 

channel and terrace deposits that constitute part of the surficial 

aquifer and control local topography. Surface-water features 

and associated ancestral river channels are also important in 

shaping the flow system due to the abundant low-lying areas 

where groundwater intersects surface water.

Hydrogeology

Three major formations exist in the vicinity of SCD: 

coarse to medium sands and gravels of the Quaternary 

Columbia Formation; a confining unit of marine deposits 

consisting of fine silt and clay of the Upper Cretaceous 

Merchantville Formation; and alternating layers of clay, 

silt, and fine to medium sand of the Cretaceous Potomac 

Formation. Due to its fluvial origin, the sediments of the 

Potomac Formation can be quite heterogeneous and stratigraphically complex (McKenna and others, 2004). Sands 

present within the various fluvial depositional environments 

of the formation are discontinuous and variable in thickness 

and extent, which limits stratigraphic correlation; however, 

the Potomac Formation can be stratigraphically partitioned 

into three major sub-formations (upper, middle, and lower), 

each approximately 250 ft thick near the SCD study area. Sand 

layers found within each of these sub-formations vary in thickness and are interbedded with silts and clays, however, their 

bulk transmissivity provides suitable public and industrial 

water supplies. Thus, the water-bearing sands found within 

these geologic formations are regionally designated as the 

upper, middle, and lower Potomac aquifers, corresponding to 

the sub-formation name. Some previous studies refer to these 

aquifers as the upper hydraulic zone, middle hydraulic zone, 

and lower hydraulic zone (Black & Veatch, 2007), but this 

report will use the aforementioned nomenclature. The base of 

the upper Potomac aquifer has been evaluated using pollen 

data indicating that it represents the Upper/Lower Cretaceous 

boundary (table 2) (McKenna and others, 2004). The upper 

Potomac aquifer has been further divided into distinct hydrogeologic units within the SCD study area and is discussed later 

in this report, along with further discussion of the Columbia 

and Merchantville Formations as they relate directly to the 

SCD study area (see Hydrogeologic Framework, this report). 

Table 2. Stratigraphic correlation chart of geologic and hydrogeologic units in the vicinity of the Standard Chlorine of Delaware, Inc. 

Superfund Site1.

System Series Geologic formation and unit Hydrogeologic unit

Quaternary

Holocene undifferentiated (Qm) Silt, clay, and peat

Upper Pleistocene Scotts Corners (Qsc)

Lynch Heights (Qlh) Surficial aquifer (Columbia aquifer)

Middle Pleistocene Columbia (Qcl)

Cretaceous

Upper Cretaceous

Merchantville (Kmv) Merchantville confining bed

upper Potomac (Kpt)

top sand aquifer (discontinuous)

confining bed

upper Potomac A-sand aquifer

confining bed

upper Potomac B-sand aquifer

confining bed

upper Potomac C-sand aquifer

Lower Cretaceous middle Potomac (Kpt) middle Potomac aquifer

lower Potomac (Kpt) lower Potomac aquifer

Lower Paleozoic to Precambrian Consolidated basement rocks

1

 Modified from Benson and McLaughlin, 2006; Martin and Denver, 1982.
8 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

The upper part of the middle Potomac aquifer contains an 

approximately 150-ft-thick sequence of fine-grained silt and 

clay, followed by alternating layers of sand and silt capable of 

producing sustained water. An approximately 50-ft-thick confining layer at the base of the middle Potomac aquifer distinguishes it from the lower Potomac aquifer, which is relatively 

more abundant in aquifer quality sands. Several industrial 

and water-supply wells are screened in the lower part of the 

Potomac Formation, particularly in the southeast part of the 

DCIA (fig. 2). 

Pleistocene erosion due to the lowering of sea level 

during the last glacial period resulted in rivers downcutting 

into the Merchantville and Potomac Formations in dendritic 

patterns that correspond with several present day surfacewater features. In some locations, these channels were refilled 

with undifferentiated sediments (sand, gravel, and clay) of the 

Columbia Formation (Phillips, 1987), or other Quaternary age 

sediments. Other more linear erosional features exist such as 

the Reybold paleochannel, which runs parallel to the present 

day Delaware River and likely formed during Pleistocene 

deglaciation from the release of floodwaters (Jengo and others, 

2013). Holocene sediments overlying the Columbia Formation 

are also present as marsh deposits and consist of black to dark 

gray organic rich silty clay with beds of peat ranging in thickness from 1 to 40 ft (Ramsey, 2005).

Flow System

Conceptualization of the flow system provides a context 

for investigations described in this report. Understanding the 

groundwater-flow system is particularly important for defining 

and delineating controls on the movement of water and 

contaminants, and is critical for the development of remediation strategies. Water in the Atlantic Coastal Plain enters the 

aquifer system as rainfall and snowmelt that moves through 

the soil and subsoil to the water table. Most of the water that 

reaches the water table in the surficial aquifer discharges to 

local streams (Cushing and others, 1973). Some of the groundwater discharges to larger streams and rivers and in coastal 

zones, may discharge to wetlands, tidal rivers, or estuaries. A 

relatively small part of groundwater recharge becomes part 

of a deeper flow system that includes confined aquifers that 

extend downdip toward the Atlantic Ocean (Shedlock and 

others, 2007).

The regional flow system within the confined aquifers 

of the Potomac Formation is characterized by relatively slow 

downdip (southeast) flow, controlled mainly by hydrostratigraphy (figs. 4a, b). Most of the regional recharge occurs to the 

northwest of the site where the upper Potomac aquifer subcrops under Quaternary surficial sediments, primarily of the 

Columbia Formation. The middle and lower Potomac aquifers 

tend to contact basement rocks in an onlapping unconformable pattern (Benson and McLaughlin, 2006; McKenna and 

others, 2004), and are likely recharged by leakage through 

confining layers (fig. 4b). Models of groundwater flow in the 

Maryland and Delaware parts of the Atlantic Coastal Plain 

estimate average regional flow rates of approximately 0.1–0.2 

feet per day (ft/d) in the aquifers (Fleck and Vroblesky, 1996). 

These rates are affected by pumping wells, which also affect 

groundwater-flow directions (fig. 4a). Historical long-term 

water use has led to documented regional and local cones 

of depression in the potentiometric surface of the Potomac 

aquifer associated with production well fields (Martin, 1984). 

Water levels within Potomac aquifer layers vary depending 

on production well demands from wells screened in each 

particular aquifer. Groundwater salinity in some locations has 

increased from pumping-induced recharge of Delaware River 

water to the aquifer system (Phillips, 1987). Vertical flow in 

the region is consistently downward from the surficial aquifer 

to the Potomac aquifer, and downward between sand layers 

(aquifers) within the Potomac Formation. Limited crossformational flow is possible, especially where confining units 

are thin or absent (fig. 4b); this may provide recharge to the 

middle and lower Potomac aquifers without extensive outcrop/

subcrop areas.

Local flow conditions in the study area are similar to 

regional flow patterns. The saturated part of the Columbia 

Formation forms a surficial aquifer (herein the Columbia aquifer), and the flow system within this aquifer is unconfined and 

conceptualized as being controlled mainly by topography and 

the location of surface-water features. Groundwater in the surficial aquifer is recharged by direct infiltration of precipitation. 

Previous studies (Black & Veatch, 2007) have shown the depth 

to water ranges from 20 ft below land surface in the uplands to 

near land surface in wetland (lowland) areas, where elevation 

approaches mean sea level. Flow is generally from higher to 

lower land-surface elevations, resulting in groundwater discharge to small streams and creeks. In the vicinity of the SCD 

site, groundwater flow in the Columbia aquifer is generally 

northward toward Red Lion Creek and away from the local 

topographic high just southeast of the barrier wall (fig. 3). 

Locally, recharge from the Columbia aquifer to the 

underlying Potomac aquifer occurs where confining units are 

thin or absent. Water level, or head, in the Potomac aquifer is 

generally considered confined although spatially distinct areas 

exist where no confining unit is present between the base of 

the surficial (Columbia) aquifer and the top of the confined 

(Potomac) aquifer. In the vicinity of SCD, groundwater flow 

in various layers of the Potomac aquifer is generally downdip from northwest to southeast, however flow directions are 

influenced by local pumping of the confined aquifers (figs. 4a, 

b) and are the subject of detailed discussion in this report.

Surface-Water Features

The most prominent nearby surface-water feature is the 

Delaware River, which conceptually serves as an eastern flow 

boundary for both surface-water and groundwater discharge. 

The Delaware River experiences semi-diurnal tides; the tide 

cycles through a high and low twice each day, with one of the 

two high tides being higher than the other, and one of the two 

low tides being lower than the other. In and around the DCIA 
Introduction 9

Chesapeake and Delaware Canal

295

Fort Dupont

State Park

Fort

Delaware

State Park

Lums Pond

State Park Delaware

City

River

Pea Patch

Island

NEW DELA

JERSEY

WARE

RIVER

9

72

Red

Lion

Creek

Dragon

Run

Red Lion

Creek

watershed

1

71

SCD study area 71

A

9

Newport River

Christina

White

Clay

Ck.

72

40

Road

Red

Lion

Road

495

9

13

301

1

DELAWARE

MARY

LAND

896

New Castle

Dupont

Highway

New

Castle

County

Airport

95

2

Tybouts

Corner

AV-3 AV-1

AV-4

P-5B

P-4B

R-15

P-3B

P-6A

P-16A

P-10A

PW-4

PW-5

P-9A

P-1A

ASR-1

AV-2

Highway

Pulaski

95

University of Delaware

2

LINE

FALL

Newark

N

75°45'00" 75°37'30"

39°40'00"

39°35'00"

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

D

upper Potomac Aquifer Subcrop Area

D'

EXPLANATION

Groundwater withdrawal locations

upper Potomac aquifer

middle Potomac aquifer

lower Potomac aquifer

Generalized groundwater-flow directions

upper Potomac aquifer

middle Potomac aquifer

lower Potomac aquifer

D D' Line of section D-D'

Base modified from ESRI andU.S. Geological Survey digital

data, 2013, NAD 83 State Plane Delaware Projection

Figure 4. Regional flow system (A) map showing surface-water features, subcrop areas of the Potomac aquifer 

system, major groundwater intakes near the Standard Chlorine of Delaware, Inc. Superfund Site (SCD) study 

area, and generalized flow lines for the regional groundwater system, and (B) generalized cross section showing 

recharge areas and the confined aquifer flow system.
10 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

on the western shore of the Delaware River, small streams 

drain into creeks (approximately 10-mi2 watersheds), which 

then drain eastward directly to the Delaware River. Dragon 

Run, located to the south, and Red Lion Creek, located to 

the north, are the primary drainages in the DCIA (fig. 2). 

Dragon Run and Red Lion Creek are occasionally used to 

provide industrial supply water to the Delaware City refinery 

(Delaware City Refining Company).

Surface water at and near the SCD site is present in Red 

Lion Creek, its tributaries, and the nearby marsh system connected to the creek. Surface-water runoff at the SCD site generally moves north into Red Lion Creek, or into small streams 

that drain north into the creek. The average depth of Red Lion 

Creek is 0.3 to 3.3 ft (Roy F. Weston, Inc., 1992). Shallow 

groundwater discharges to the creek and to wetland areas surrounding the creek. Water from Red Lion Creek drains during 

low tide through a tide control structure located approximately 

one-half mile downstream from Delaware State Route 9 

(fig. 2). The Creek is not normally subject to tidal influence 

because gates on the tide control structure close during high 

tide to prevent water from entering from the Delaware River 

(fig. 5). Tide control structures are common on the Delaware 

coastline and were installed for protection from storm surge 

and flooding, and to lower water levels along creeks to protect 

farm land and promote better drainage (Mickowski, 1986).

Study Approach and Data-Collection 

Methods

A variety of approaches and data-collection methods 

were used to create and refine a conceptual model of the local 

hydrogeologic framework, including exploratory drilling into 

the Potomac Formation, geophysical surveys, groundwater 

and surface-water-level monitoring, pumping a local production well, and examination of local pumping influences 

on groundwater. USGS involvement at the SCD site began 

in 2005 with a site assessment of the existing water-level 

monitoring network, including the evaluation of seven wells 

B D D'

(Northwest) (Southeast)

200 Land surface

FEET FEET

200

Pumpage Pumpage

Well P-5B

P-5B

Well P-6A

R-15

Standard Chlorine

of Delaware, Inc.

Superfund Site

-200 -200

-400 -400

-600 -600

-800

VERTICAL EXAGGERATION = 27X

-800

Sea

level

Sea

level

0 1 2 MILES

0 1 2 KILOMETERS

Bedrock

lower Potomac aquifer

middle Potomac aquifer

Columbia aquifer

upper Potomac aquifer

Confining unit

Confining unit

Precipitation Precipitation

Evaporation

Merchantville

confining unit

Red Lion Creek

White Clay Creek

Christina River

EXPLANATION

Pumping well,

screened interval,

and identifier

Generalized groundwaterflow direction

Recharge Geologic contact

Figure 4. Regional flow system (A) map showing surface-water features, subcrop areas of the Potomac aquifer system, major 

groundwater intakes near the Standard Chlorine of Delaware, Inc. Superfund Site (SCD) study area, and generalized flow lines 

for the regional groundwater system, and (B) generalized cross section showing recharge areas and the confined aquifer flow 

system.—Continued
Study Approach and Data-Collection Methods 11

screened in the Potomac aquifer at that time. From 2007 

through 2010, surface and borehole geophysical tools, in 

combination with exploratory drilling, were used to locate 

sand layers within the Potomac aquifer and to determine the 

presence, absence, lateral extent, and thickness of the confining clay and silt layers (see Hydrogeologic Framework, this 

report). New wells were installed in stages, expanding the 

Potomac Aquifer Study monitoring network to include wells 

screened in multiple (deeper) sand layers within the upper 

Potomac aquifer. Time-series water-level data were used to 

evaluate long-term (multi-year) changes in groundwater levels 

in response to water withdrawals and variable recharge from 

infiltration of precipitation, tidal fluctuations, and surfacewater leakage (see Hydrology, this report). Production well 

pumping was conducted to evaluate the cross-aquifer connection locally between the water table and confined flow system 

(see Hydrology, this report). Finally, the effects of water 

withdrawals on local and regional flow systems also were 

examined (see Water Withdrawal Influences on the Local 

Flow System, this report). 

Exploratory Drilling

A new site drilling program was developed in 2007 in 

response to the detection of site-related contaminants in upper 

Potomac aquifer well PW-1 in 2003, and in response to the 

need for a better understanding of the distribution of confining 

clays in the upper Potomac aquifer underlying the site. The 

USGS provided technical assistance to EPA and DNREC in 

developing a drilling approach first by investigating approximately one-quarter mile beyond the SCD site boundary, and 

then drilling adjacent to (east, north, west, and south) the 

site. Twenty-one additional monitoring wells (table 3) were 

installed as part of the Potomac Aquifer Study from 2007–10. 

Each well was completed with a 5-ft stainless steel screen, targeting specific aquifer intervals for water-quality sampling and 

long-term compliance monitoring. There has been no drilling 

into the upper Potomac aquifer in the former plant area (southern half of the area inside the barrier wall) due to the risk of 

carrying contaminants downward from the Columbia aquifer 

during the drilling process, and therefore, the distribution of 

confining clay layers in the upper Potomac Formation beneath 

Figure 5. Red Lion Creek tide control structure with reinforced dike and five outflow gates, looking from 

the Delaware River at low tide in 2010. [Photograph by Todd Keyser, Delaware Department of Natural 

Resources and Environmental Control.] 
12 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

the site is inferred from the drilling logs of wells completed 

immediately adjacent to the former plant area (fig. 3). 

Rotosonic drilling was used to achieve nearly continuous 

core recovery to a targeted depth of 200 to 250 ft below land 

surface at five locations where deep wells also were completed 

(PW-2D, PW-8, PW-10D, PW-11D, and PW-16 (table 3; fig. 

6) The drilling bit and core barrel were advanced in 10-ft 

increments from land surface downward, and descriptions of 

lithology from recovered cores were recorded by an on-site 

professional geologist. A vertical water-quality profiling 

approach was also used, whereby a temporary screen was 

installed into the borehole at each depth having water-bearing 

sands with at least a 10-ft separation from the previous sand 

interval. Water-quality samples were collected for each sand 

interval after it was pumped long enough to purge any drilling 

water introduced into the formation. Boreholes at each drilling 

location were logged by USGS using a natural gamma tool, 

and results were compared with descriptions of lithology to 

determine screen placement before final well construction 

was completed. Multiple nested wells were installed at six 

locations, and include wells screened in the Columbia aquifer 

and (or) in successive sands of the upper Potomac aquifer 

(table 3). In this study, new wells completed in the Columbia 

Formation were named “CW” (Columbia well). Multiple new 

wells completed in the upper Potomac Formation were named 

“PW” (Potomac well), and are further designated as “S,” shallow, or “D,” deep (table 3). 

The potential for cross-formation contamination resulted 

in several design considerations for the drilling program, 

including the use of secondary (double) casings at all wells, 

and drilling depth restrictions where contaminants were 

encountered. Double casings were installed at all drilling 

locations using mud rotary techniques prior to beginning 

rotosonic profiling. As required by DNREC, double casings 

were installed at each new well location, with the bottom of 

the casing typically set at least 2 ft into the existing confining 

Table 3. Characteristics for wells installed during Potomac Aquifer Study exploratory drilling, 2007–10.

[DNREC, Delaware Department of Natural Resources and Environmental Control; DGS, Delaware Geological Survey; bls, below land surface; color coding 

reflects aquifer depth: green = Columbia and upper Potomac-top, pink = upper Potomac-A, orange = upper Potomac-B, yellow = upper Potomac-C; 

NGVD 29, National Geodetic Vertical Datum of 1929]

Local 

site 

name

DNREC 

permit

number

DGS well

identfier

Depth of 

boring

(feet bls)

Screened 

aquifer

Altitude of 

screened interval, 

NGVD 29

(feet)

Date 

well 

installed

CW-11 228525 Dc42-50 40 Columbia -12 to -17 6/28/09

CW-4 219241 Dc53-83 69 Columbia -9 to -14 5/11/07

PW-4S 218938 Dc53-84 80 upper Potomac-top sand -21 to -26 5/7/07

PW-7S 218549 Dc52-79 85 upper Potomac-top sand -26 to -31 3/31/07

PW-5D 218552 Dc43-30 110 upper Potomac-A -89 to -94 3/25/07

PW-5S 218550 Dc43-29 71 upper Potomac-A -59 to -64 3/27/07

PW-6D 218554 Dc42-44 180 upper Potomac-A -128 to -133 4/19/07

PW-6S 218553 Dc42-43 107 upper Potomac-A -58 to -63 4/20/07

PW-8 219238 Dc53-86 162 upper Potomac-A -101 to -106 5/15/07

PW-10D 228519 Dc43-37 250 upper Potomac-A -142 to -147 8/4/09

PW-11 228522 Dc42-51 135 upper Potomac-A -115 to -120 8/21/09

PW-10 228524 Dc43-36 107 upper Potomac-A -72 to -77 8/22/09

PW-2D 231209 Dc53-187 250 upper Potomac-B -175 to -180 6/6/10

PW-16 231210 Dc52-173 240 upper Potomac-B -172 to -177 6/17/10

PW-11D 228520 Dc42-52 255 upper Potomac-C -215 to -220 7/21/09

Locations where site-related contaminants have been detected

PW-4D 218725 Dc53-85 150 upper Potomac-A -83 to -88 4/26/07

PW-9 228523 Dc53-133 148 upper Potomac-A -83 to -88 7/9/09

PW-12 228521 Dc52-127 132 upper Potomac-A -79 to -84 8/8/09

PW-14 231211 Dc42-57 69 upper Potomac-A -57 to -62 5/18/10

PW-17 231212 Dc42-58 105 upper Potomac-A -83 to -88 6/2/10

PW-13 231310 Dc42-56 170 upper Potomac-A -115 to -120 6/3/10
Study Approach and Data-Collection Methods 13

units (Merchantville or upper Potomac) to limit cross-formation contamination from the Columbia into the upper Potomac 

Formation. Depth of drilling in the upper Potomac was limited 

at some locations because benzene and chlorobenzene compounds were detected during drilling (table 3) using a photo 

ionization detector (PID) for volatile gases. Concern for crossformation contamination had previously led to the abandonment of well PW-1 in 2007 due to questions regarding well 

integrity. For monitoring purposes, well PW-1 was replaced 

with well PW-4D, located approximately 100 ft to the south. 

Geophysical Surveys

The results of surface-geophysical surveys and boreholegeophysical logging from this study were used together with 

previously collected geophysical logs to better define site 

scale subsurface features and how they relate to drilling logs 

and hydrogeology. The locations of geophysical surveys 

were determined with a global positioning system (GPS) for 

geospatial referencing in a geographic information system 

(GIS). Natural gamma logs were collected for selected wells 

A

C D

B

Figure 6. Photographs showing (A) rotosonic drilling, (B) recovered core, (C) core from Merchantville confining unit, and (D) core from 

upper Potomac confining unit. [Photographs by U.S. Geological Survey.] 
14 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

in 2006, prior to barrier wall installation and for each new 

boring location during 2007–10. Electrical and seismic 

surface-geophysical methods were used at the site beginning 

in 2008 (Degnan and Brayton, 2010), with additional data 

collection and interpretation currently ongoing (2014). Natural 

gamma radiation borehole geophysical logs were collected at 

selected wells to differentiate fine-grained from coarse-grained 

sediment. Selected gamma log results (gamma responses) 

are presented in figure 7. Descriptions of standard boreholegeophysical logging methods and interpretation can be found 

in Keys (1990). Direct current (DC) resistivity measurements 

taken from core that was collected and preserved during drilling has assisted in the interpretation of data from surface DC 

resistivity surveys, enabling features to be verified at specific 

depths (Degnan and Brayton, 2010). An analysis of boring 

logs from drilling, in combination with DC resistivity data, 

has furthered the understanding of how the flow system varies 

laterally and with depth.

Water-Level Monitoring Network

A water-level monitoring network was established for the 

collection of discrete and continuous water-level data in order 

to provide a better understanding of the local flow system 

(fig. 3). Data collection was focused on monitoring water levels in wells screened in the upper Potomac aquifer; however, 

shallow water-table reference wells were measured to help 

understand aquifer recharge and a stage recorder on Red Lion 

Creek was measured to help understand the response of creek 

stage to precipitation events. Sites were established and waterlevel data were collected following standard USGS procedures 

(Cunningham and Schalk, 2011). Delaware Geological 

Survey (DGS) names were obtained for each well and well 

characteristics were added to the USGS Groundwater Site 

Inventory (GWSI) database. The vertical datum for all sites 

is the National Geodetic Vertical Datum of 1929 (NGVD 29); 

local datum conversion from NGVD 29 to the North American 

Vertical Datum of 1988 (NAVD 88) is -0.78 ft as computed 

in datum reference tables for NOAA tide station 8551762 

located near Delaware City (fig. 1). Other site characteristics 

for wells and a stage recorder at Red Lion Creek are listed in 

Appendix 2. Professional surveys were conducted at the site 

in 2006, 2008, and 2010 using differential GPS. Location and 

elevation data from these surveys were compared with existing 

data in the USGS database for consistency, and updated when 

necessary.

USGS data collection began in August 2005 with a set of 

discrete water-level measurements at existing Potomac aquifer 

wells and expanded over time to include continuous waterlevel measurements at approximately 16 sites and discrete 

monthly water-level measurements at approximately 45 sites. 

Continuous water-level measurements were made every 15 

minutes using vented pressure transducers. Instrumented sites 

included 2 wells screened in the Columbia aquifer (Ion-MW3 

and CW-11), 13 wells screened in the upper Potomac aquifer 

(MW-11, MW-12, PW-2D, PW-3, PW-4D, PW-5S, PW-6S, 

PW-7S, PW-11, PW-11D, PW-13, PW-16, and PW-17), 1 well 

screened in the lower Potomac aquifer (Tidewater-12), and 

Red Lion Creek at the Delaware Route 9 (River Road) bridge. 

Red Lion Creek was instrumented for collection of stage 

(surface-water level or height) data in June 2007, and is still 

an active data-collection site. The watershed area upstream of 

the stage recorder is 8.90 mi2 (fig. 2). Some minor data gaps 

exist for creek stage because Red Lion Creek is subject to 

freezing from mid-December through the end of February, and 

typically freezes for up to 1 week at a time. A summary of the 

period of record for sites that had continuous data collection is 

listed in Appendix 2. 

Monthly discrete water-level measurements were collected at approximately 45 sites, including sites instrumented 

with pressure transducers; site conditions or access issues 

decreased the total number of measurements during some 

months. Measurement dates were coordinated with EPA’s 

site consultant, HydroGeoLogic, Inc., which measured water 

levels in a separate network of shallow Columbia aquifer wells 

associated with the remedial pump and treat system. All waterlevel data collected by the USGS are stored in the National 

Water Information System (NWIS) database, and are publically available at http://waterdata.usgs.gov/nwis.

Production-Well Pumping to Evaluate Aquifer 

Interconnections 

A 3-day constant rate pumping test was conducted by the 

USGS from August 28–31, 2010 on well OR-6A (fig. 3) to 

evaluate the interconnection of the upper Potomac aquifer and 

overlying surficial Columbia aquifer, and the interconnection 

of different sand layers within the upper Potomac aquifer. The 

study area for this evaluation included monitoring locations 

inside and outside the SCD barrier wall and wells located on 

surrounding properties. The pumping period was preceded by 

a period of limited rainfall, mitigating potential recharge to the 

surficial aquifer and confined aquifer system. Only 0.30 in. of 

precipitation was recorded in the 14 days preceding the pumping period, and no precipitation occurred during the pumping 

period.

Design and Equipment

An evaluation of water-level responses in wells during 

pumping of well OR-6A was designed using: (a) results 

published during a previous pumping test conducted at well 

OR-6A (fig. 3) from October 1–3, 1990 by Roy F. Weston, 

Inc. (Roy F. Weston, Inc., 1992), (b) data collected from the 

existing monitoring well network, and (c) lithologic observations from exploratory drilling. Well OR-6A was selected for 

pumping because of its construction characteristics, the length 

and depth of screen, the absence of site-related contamination, 

and a discharge location near Red Lion Creek. Well OR-6A is 
Study Approach and Data-Collection Methods 15

an 8-in.-diameter, single-cased steel well screened 102–176 ft 

below land surface and located approximately 1,800 ft northwest of the site. The sampling history for this well shows 

only trace detections of volatile organic compounds (VOCs). 

During pumping, PID readings were collected approximately 

every hour for the initial 24 hours and intermittently thereafter; no detections were noted. Water levels at the pumping well 

(OR-6A) were collected from a 1-in. PVC observation tube 

within the well using an electric water-level meter. Access and 

permission to pump well OR-6A was granted by the Delaware 

City Refining Company (Aaron Vahid, Delaware City Refining 

Company, oral commun., 2010).

Prior to the 72-hour pumping period, a step-drawdown 

test was conducted on August 27, 2010 to evaluate the pumping efficiency of the well using four discharge rates ranging 

from 350–485 gallons per minute (gal/min) in step durations 

of 1 hour at each rate. A 2-stage Goulds submersible pump 

with a 30 horsepower Franklin Electric motor was installed to 

a depth of 105 ft below land surface and 2.5 feet below the top 

of the well screen. After completion of the step test, a 450 

gal/min pumping rate was selected and approved in consultation with DNREC and EPA. 

The pumping of well OR-6A was scheduled following 

the complete recovery of the upper Potomac aquifer after the 

November 2009 temporary idling of refinery production well 

R-15, located approximately one-half mile south of SCD 

(fig. 2). Water levels at monitoring wells PW-3 and MW-11, 

located to the south of the SCD site, reached full recovery 

from the pumping effects of well R-15 in June 2010 at a 

distance of approximately 2,900 ft northeast of well R-15. 

Pumping well OR-6A and observation well PW-11 screened in 

the upper Potomac aquifer and located west and east of SCD, 

respectively, reached full recovery in April 2010.

The pumping of well OR-6A began August 28, 2010 at 

13:40 EST, and a constant discharge rate of 450 gal/min was 

achieved within 2 percent (± 9 gal/min) for the duration of 

pumping. The discharge rate was measured using a Layne and 

Bowier Circular Orifice Table for a 6-in. pipe and 5-in. orifice 

(Cunningham and Schalk, 2011). The pump was powered by 

diesel generator for the duration of the 72-hour period without 

interruption or power loss. Pumped water was discharged 

northeastward towards the wetland area and Red Lion Creek 

by agreement between the USGS, EPA, and DNREC. Data 

collected by USGS personnel at the pumping well (OR-6A) 

included discrete water levels, water quality, VOCs in the 

headspace using a PID, and pump discharge and performance 

parameters throughout the duration of active pumping. 

Water-Level Measurements

Water-level measurements during the pumping period 

were collected from the USGS Potomac Aquifer Study 

well network and from the Columbia aquifer well network 

maintained by EPA site contractor HydroGeoLogic, Inc. 

Wells in the USGS Potomac Aquifer Study network, well 

characteristics and integrity, procedural methods, and instrument maintenance followed technical guidance provided 

in Groundwater Technical Procedures of the USGS 

(Cunningham and Schalk, 2011). During the evaluation of 

water-level responses in wells during pumping of well OR-6A, 

HydroGeoLogic, Inc. measured discrete water levels in the 

majority of the Columbia aquifer well network. These wells 

are not inventoried in the USGS NWIS database because of 

incomplete drilling records, however, known characteristics 

for these wells are provided in Appendix 3 (wells are identified by local site name and DGS identification number if 

available). 

Discrete and continuous water-level measurements were 

collected in 81 wells from August 26 through September 1, 

2010. Of these measured wells, 52 were completed in the 

Columbia Formation, 24 in upper Potomac A-sand, 2 in the 

upper Potomac B-sand, 2 in the upper Potomac C-sand, and 1 

well (Tidewater-12) in the lower Potomac aquifer (Appendix 

3). Continuously monitored wells were instrumented with 

In-Situ Level Troll vented pressure transducers using an observation interval of 15 minutes and were checked approximately 

every 6 hours using electric water-level meters. Monitoring 

well PW-6S was instrumented prior to the start of pumping 

to a 5-minute measurement frequency during the pumping of 

well OR-6A. Continuous measurements were calibrated using 

discrete monthly observations and verified during the pumping 

period using additional discrete measurements. 

Measuring the Effects of Water Withdrawals

The influence of water withdrawals on local and regional 

flow systems was examined because of the potential to influence flow direction and contaminant movement. Groundwater 

and surface-water withdrawal locations within approximately 

3 mi of SCD were considered for this part of the study (fig. 2; 

table 4). Groundwater is used for both industrial and public 

water supply and surface water is used only for industrial 

water supply. Water use information for drinking water and 

industrial water withdrawal locations using at least 10,000 

gallons per day (gal/d) was compiled from annual reports 

submitted to DNREC, summarized by month and year. These 

reported withdrawals were compared to water levels in monitoring wells at SCD to look for obvious relations between 

local withdrawals, and water levels and groundwater flow. 

Four supply wells (AV-1, AV-2, AV-3, and AV-4), are located 

to the north of Red Lion Creek and upgradient of SCD 

(fig. 2), and the remaining wells are located south of Red Lion 

Creek. Annually averaged daily use was calculated in million 

gallons per day (Mgal/d) and tabulated for comparison 

(table 4). Withdrawal locations having a daily water use less 

than 10,000 gal (0.01 Mgal/d), which includes most domestic 

and small business users, were not considered in this evaluation because of their limited impact on aquifers relative to 

other large users locally. 
16 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

Hydrogeologic Framework

The hydrogeologic framework of the Columbia and 

Potomac Formations underlying the SCD site is complex and 

the heterogeneity of hydraulic properties of aquifers contained 

within these formations is enhanced by channel geometry and 

discontinuous confining layers. Defining the extent, altitude 

and thickness of confining silt and clay beds is important 

because they may limit contaminant transport and distribution. 

However, paleochannel and flood-plain deposits from braided, 

anastomosed, and meandering fluvial system environments 

are also found within the formations, further complicating 

interpretation of the connections between aquifer sand layers 

(McKenna and others, 2004).

Interpretations of the hydrogeologic framework of the 

SCD site are based on exploratory drilling and geophysical 

survey data collected at the site for this study in the context 

of the regional framework interpreted by the DGS (McKenna 

and others, 2004). Data from other studies at the site were 

also used; however, previous studies were largely focused on 

the Columbia Formation due to the extent of contamination 

discovered in the Columbia aquifer. As part of those previous 

drilling explorations, permanent Columbia aquifer observation 

wells were installed throughout the 1980s and early 1990s, and 

many are still in use. Prior to the installation of the contaminant barrier wall, extensive geologic characterization of the 

Columbia Formation on site was completed using cone penetrometer technology as part of the barrier wall engineering 

A

MW-37

Land

surface

PW-17 PW-11

PW-10

PW-2

PW-11D

PW-10D

PW-2D

PW-12

PW-16 PW-9

PW-5D

PW-5S

30

Sea

level

Sea

level

30

60 60

90 90

FEET FEET

-30 -30

-60 -60

-90 -90

-120 -120

-150 -150

-180 -180

-210 -210

-240 -240

-270 -270

-300 -300

0

0

0

0

0

0

0

0

50

50

50

50

50

50

100

100

100

100

100

100

150

150

150

150

50

100

50

100

VERTICAL EXAGGERATION = 7.2X 0 500 1,000 FEET

0 125 250 METERS

A

(North)

A'

(South)

EXPLANATION

Clay

Gravel

Sand

Silt

Lithology

0

50

100

150

Gamma response

Counts per second

Geologic contact; dashed

where approximate

Red Lion

Creek

Quaternary fill

(recent sediments)

Columbia aquifer

Top

sand

A-sand

A-sand

upper Potomac aquifer (A-sand)

upper Potomac aquifer

(B-sand)

upper Potomac aquifer (C-sand)

Confining unit

Confining unit

Confining unit

upper Potomac

Merchantville

confining unit

confining unit

Confining unit Line of section B-B'

Figure 7. Wells, lithologic descriptions, and gamma responses for (A) cross section A-A’ from north to south, and (B) cross section 

B-B’ from northwest to southeast.
Hydrogeologic Framework 17

process (Black & Veatch, 2007). These data provided additional boring locations where the bottom of the Columbia and 

(or) the top of the Merchantville Formations were identified.

More recent efforts (such as this study) have focused 

on the upper Potomac Formation, and have used an iterative 

process over several years alternating between periods of drilling and subsurface exploration using geophysical resistivity 

surveys. Additional insight was gained by reviewing information from the adjacent Oxychem site to the east (lithology, 

interpretive cross sections, and borehole geophysical logs). To 

the west of SCD, lithologic information for the upper Potomac 

is limited. Drilling conducted during this study from 2007–10 

(table 3; fig. 6) has provided information on contaminant 

distribution, information on the distribution of sand and clay 

layers to a depth of 250 ft in some locations, and enabled the 

identification of three locally distinct and continuous sand 

units (aquifers) within the upper Potomac aquifer (table 2), 

and a Cretaceous age discontinuous sand layer at the top of the 

upper Potomac Formation. An understanding of the complex 

hydraulic connections between these sands, which also helps 

to distinguish them, has been developed through the analysis 

of water-level monitoring data and from observations during 

the pumping of well OR-6A (see Hydrology, this report). 

Columbia Formation, Columbia Aquifer, and 

Other Quaternary Age Sediments

Unconfined saturated sands within the Pleistocene age 

Columbia Formation (Qcl) form the uppermost unconfined 

(surficial) aquifer at the SCD site, referred to as the Columbia 

aquifer (fig. 7). Locally, the Columbia Formation is 8 to 

84 ft thick (Black & Veatch, 2007), averaging 56 ft thick 

and is composed of orange to yellow to reddish-brown sand 

with some coarse sand, gravel, and scattered beds/stringers 

?

?

EXPLANATION

B

30 30

60 60

90 90

FEET FEET

-30 -30

-60 -60

-90 -90

-120 -120

-150 -150

-180 -180

-210 -210

-240 -240

0 500 FEET

0 125 METERS

B

(Northwest)

B'

(Southeast)

0

0

0

0

50

100

150

50

100

150

50

100

150

50

100

150

PW-13 MW-30 MW-29 PW-12 PW-9 Line of section A-A'

PW-8 PW-2

PW-2D

Land surface

VERTICAL EXAGGERATION = 4.5X

Columbia aquifer

upper Potomac aquifer (A-sand)

Top sand

upper Potomac confining unit

upper Potomac aquifer (B-sand)

Confining unit

Merchantville confining unit

Sea

level

Sea

level

? 050100 150

Gamma response Geologic contact; dashed where Barrier wall

approximate, queried where

uncertain

Clay

Gravel

Sand

Silt

Lithology

Counts per second

Figure 7. Wells, lithologic descriptions, and gamma responses for (A) cross section A-A’ from north to south, and (B) cross section 

B-B’ from northwest to southeast.—Continued
18 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12 Table 4. Annually averaged daily water withdrawal at major withdrawal locations near the Standard Chlorine of Delaware, Inc. Superfund Site, New Castle County, Delaware. [DGS, Delaware Geological Survey; DNREC, Delaware Department of Natural Resources and Environmental Control; SCD, Standard Chlorine of Delaware, Inc. Superfund Site; bls, below land surface; Mgal/d, millon gallons per day; wells are color coded based on Potomac aquifer hydrologic zone: pink = upper, blue = middle, purple = lower; names in parentheses indicate previous well name used; --, no data] Name of location DNREC permit number DGS well identifier Distance from SCD (miles) Direction from SCD Date constructed Diameter of screened interval (inches) Altitude of land surface (feet) Depth of screened interval (feet bls) Altitude of screened interval (feet) Potomac aquifer hydrologic zone Hole depth (feet) Annually averaged daily water withdrawal (Mgal/d) 2001 2005 2007 2010 2012 Groundwater withdrawal locations north of Red Lion Creek AV-1 43962 Dc33-07 2.09 North 9/17/79 12 19 155–215 -136 to -196 upper 215 1.39 1.44 1.45 1.44 0.90 AV-2 43963 Dc33-08 1.56 North 3/10/79 12 28 125–225 -97 to -197 upper 225 0.29 0.29 0.25 0.15 0.31 AV-3 104641 Dc32-22 2.08 North 7/28/95 12 60 131–180 -72 to -115 upper 180 0.76 0.83 0.79 0.78 0.65 AV-4 177379 Dc32-35 1.56 North 4/11/01 10 55 117–142 -62 to -87 upper 142 0.11 0.09 0.10 0.20 0.14 Groundwater withdrawal locations south of Red Lion Creek R-15 (P-15) 10066 Dc52-24 0.72 South 4/2/56 12 70 302–333 -232 to -263 upper 364 0.52 0.49 0.44 0.01 0.49 P-1A 53065 Ec22-17 3.03 South 9/9/83 12 9 237–280 -228 to -271 upper 280 0.19 0.40 0.42 0.12 0.47 P-3B (P-3A) 216291 Eb15-17 2.19 Southwest 10/26/07 14 78 518–548 -440 to -470 middle 560 0.15 0.1 0.19 0.11 0.15 P-4B (P-4A) 216292 Dc51-43 1.55 West 9/11/07 14 37 375–483 -338 to -446 middle 500 0.17 0.13 0.02 0.10 0.11 P-5B 72826 Dc41-57 0.98 West 2/22/88 10 12 343–515 -331 to -503 middle 525 0.33 0.25 0.06 0.04 0.13 P-6A 10057 Dc42-06 0.35 Northwest 11/10/55 8 34 668–698 -634 to -664 lower 713 0.18 0.33 0.23 0.02 0.23 P-9A 163874 Ec12-25 1.89 South 9/16/99 14 45 545–570 -500 to -525 lower 590 1.00 0.56 1.07 0.08 0.89 P-10A 53066 Ec14-08 2.40 Southeast 10/24/83 14 14 630–714 -616 to -700 lower 714 1.19 1.31 1.05 0.69 0.94 P-16A 91371 Ec13-21 1.47 South 6/25/92 12 35 513–564 -478 to -529 lower 593 0.30 0.22 0.49 0.23 0.41 PW-4 036504 Ec15-27 3.08 Southeast 11/5/76 8 13 692–722 -679 to -709 lower 722 0.04 0.06 0.03 0.04 0.05 PW-5 037981 Ec15-28 2.83 Southeast 5/4/77 8 11 707–737 -696 to -726 lower 780 0.14 0.07 0.10 0.08 0.06 ASR-1 216229 Ec22-18 2.43 South 11/1/06 12 50 545–665 -495 to -615 lower 665 -- -- -- -- -- Surface-water withdrawal locations (intakes) Red Lion Creek 8013 -- 0.98 West -- -- -- -- -- -- -- 0.00 0.00 0.00 0.01 0.23 Dragon Run 8014 -- 2.05 South -- -- -- -- -- -- -- -- -- 1.29 0.20 1.34
Hydrogeologic Framework 19

of silt and clay. The Columbia Formation consists of glacial 

outwash deposited by braided rivers; sands are primarily 

medium-grained, typically cross-bedded with gravel found at 

the base (Ramsey, 2005). This basal sand and gravel layer was 

encountered at many locations drilled near the site from 2007 

through 2010. In general, the Columbia Formation is underlain 

by either the Merchantville or Potomac Formations. Previous 

investigations indicated that there may be a sink or depression 

at the base of the Columbia Formation along the east side of 

the site near well PW-12 (Black & Veatch, 2007) that would 

allow accumulation of contaminants on top of confining layers. The recent drilling program could not verify this feature 

because of the on-site risk of enabling movement of contaminants into the deeper aquifer(s) during drilling. 

The recent (2007–10) drilling confirmed previous 

conclusions about the extent and thickness of the Columbia 

Formation. Deep drilling was not practical in marsh areas; 

therefore, additional information was obtained through hand 

coring and drive point (piezometer) installation as part of the 

USGS wetland investigation (Lorah and others, 2014). As surface elevation decreases towards Red Lion Creek, the thickness of the Columbia Formation also decreases. Columbia 

Formation sands extend into the marsh towards the creek and 

are typically overlain by recent Holocene sediments, although 

in some places the Columbia Formation appears to be absent 

(Lorah and others, 2014), particularly adjacent to and beneath 

the creek. This implies a discontinuity of Columbia sand 

beneath the creek, which may limit the flow of shallow 

groundwater in the Columbia aquifer from the south to north 

side of the creek (fig. 7a). Site-related contaminants were not 

detected on the north side of the creek when drilling at site 

PW-5D, but trace amounts were detected in marsh drive points 

between PW-5D and the creek (Lorah and others, 2014). 

On the north side of Red Lion Creek, data indicate that 

some of the fine to medium sand encountered during drilling 

may be from the Lynch Heights and (or) Scotts Corners 

Formations (Tom McKenna, Delaware Geological Survey, 

oral commun., 2007). These geologic units underlie terraces 

that are parallel to the present day Delaware River and eroded 

material from them may have been reworked and deposited 

during the last sea-level decline and rise, similar to other 

Pleistocene age sediments in the vicinity of paleo-drainage 

channels like Red Lion Creek. The lithology of core material 

from the PW-5D location indicates a mix of Quaternary sediments with a similar description, primarily yellowish brown, 

fine to coarse sands, with some gravel, and discontinuous 

beds of clayey silt. This sand sequence on the north side 

was approximately 25 ft thick and no confining layers were 

encountered.

All other drilling locations that are part of the Potomac 

Aquifer Study were located south of the creek and the surficial 

material that was encountered was from the Columbia 

Formation. The thickness of the Columbia aquifer sand, and in 

many locations, the depth to the Merchantville confining unit, 

was consistent with previous investigations. No significant 

contamination was detected (concentrations were less than 

instrument detection limits) within the Columbia Formation 

at the following drilling locations, identified by the Potomac 

Formation well installed at the same location: PW-4D, 

PW-6D, PW-7S, PW-8, PW-9, PW-10, PW-12, or PW-13 

(fig. 3). The Potomac Formation well PW-11 showed trace 

detections of site-related contaminants (concentrations were 

equal to or less than lower instrument detection limits, in 

micrograms per liter) and a well screened in the Columbia 

aquifer at the same location (CW-11) has shown very lowlevel detections (concentrations were only several micrograms 

per liter).

Recent Holocene sediments, typically dark gray, organic, 

relatively uncompacted silts, are prevalent in the marsh areas 

on the fringe of the SCD site. Quaternary wetland sediments 

at the site are commonly described as Marsh Deposits (Qm) 

that are structureless to finely laminated, black to dark gray, 

organic-rich silty clay with some peat beds. They range in 

thickness from less than 1 ft to 40 ft thick near and under Red 

Lion Creek (fig. 7a), with an average thickness in the marsh 

area of approximately 6 ft (Lorah and others, 2014). The 

presence of site-related contaminants in marsh sediments is 

widespread, with some wetland areas to the northwest of the 

site having concentrations that approach solubility limits in 

water for various chlorobenzenes. 

Merchantville Formation Confining Unit

The Cretaceous Merchantville Formation (Kmv) underlies the Columbia Formation and is a discontinuous layer 

of marine silty/sandy clay forming a leaky confining unit 

(Woodruff, 1986). Primary composition is light- to darkgray, micaceous, glauconitic, silty fine-grained sand, with 

silty sand and clay (fig. 6). Roy F. Weston, Inc. (1992) found 

several locations where the Merchantville Formation is thin 

or non-existent, predominantly in the central part of the site. 

Black & Veatch (2007) also found other areas to the north of 

the site closer to Red Lion Creek where the Merchantville was 

missing. On site, the average thickness of the Merchantville 

Formation is 10 ft, and ranges from 0 to 22 ft. The thickest 

part of the unit is near the barrier wall in the southwest part of 

the study area. Drilling in the Columbia Formation for previous studies often used the top of the Merchantville as a terminus for drilling to limit downward migration of contaminants.

In previous investigations, the Merchantville was interpreted to be a low hydraulic conductivity layer continuous 

throughout the study area and sufficiently thick to prevent 

vertical contaminant migration. Whereas the physical properties of this unit may impede groundwater flow when sufficiently thick, flow through this unit is possible where it is thin 

due to its silty (rather than clayey) composition. Deposited in 

a marine environment, the original top of the formation was 

likely planar, however, it was extensively eroded prior to and 

during deposition of the Columbia Formation. Previous studies at nearby sites (Oxychem and Tybouts) indicated the presence of areas where the Merchantville was completely eroded 
20 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

by tributaries to the ancestral Delaware River, which formed 

incised valleys cutting through the Merchantville (Black & 

Veatch, 2007). These incised valleys were typically refilled 

with sands of the Columbia Formation to form paleochannels 

(see Paleochannels and Erosional Features, this report). 

One such feature was identified during test drilling (Black & 

Veatch, 2007) in the northeast part of the SCD site oriented in 

a line from well PZ-29 towards MW-22 (fig. 3). In these areas 

where the Merchantville was eroded, the Columbia Formation 

may directly overlie clay or sand of the Potomac Formation.

Potomac Formation and Related Aquifers 

The Cretaceous Potomac Formation (Kpt) underlies the 

Merchantville Formation and is composed of fluvial sediments 

including very fine- to medium-grained sands in a matrix of 

silt and clay that are generally present deeper than 41 ft below 

land surface and continue down to the crystalline bedrock 

surface, approximately 700 ft below land surface at the SCD 

site. Thick (tens of feet) layers of silty clay to clayey silt ranging in color from dark-red, purple, gray, pink, and white are 

common, with beds of gray clayey silt often containing pieces 

of charcoal and lignite (fig. 6) (Ramsey, 2005). The predominantly clay layers form both isolated and continuous confining 

layers, which create an aquifer anisotropy that inhibits vertical 

groundwater movement. Although they are separated by an 

unconformity, clays of the Potomac and silty clays of the 

Merchantville Formations act together to form a confining 

layer of varying thickness (Woodruff, 1988), which has been 

observed at the SCD site (figs. 7a, b). The Potomac Formation 

clay is thicker and more extensive than the Merchantville silt/

clay at the site and thus serves as the more effective confining 

unit in most locations, particularly to the south and east of the 

site where the upper Potomac confining unit thickens 

(figs. 7a, b).

Depositional Environments

Within a large anastomosed and meandering river system, 

such as that which formed the Potomac Formation, coarser 

material found in bed, bar, and levee deposits may result in 

preferential groundwater flow paths, and fine flood-plain material will form barriers to flow. The bulk of sediments in the 

Potomac Formation are fine-grained silts and clays. Sugarman 

and others (2005) suggested that silt and clay layers in the 

Potomac Formation originated from four paleo-environmental 

settings: (1) oxidized flood-plain soils, (2) intra-channel 

swamps, (3) oxbow and lake lacustrine sediments, and (4) 

active flood plain. These sediments form confining layers that 

were typically underlain and cut by Potomac channel sands. 

Understanding the depositional environment and distribution 

of these channel sands is the key to discriminating sitespecific flow patterns. The DGS has subdivided the Potomac 

Formation into five facies (depositional environments), of 

which the first two facies (amalgamated channel sands and 

isolated channel sands) have good permeability and are 

laterally continuous. Crevasse splay and proximal levee sands 

are relatively thin and more variable in permeability and less 

continuous than the first two facies. Distal levee/ flood-plain 

deposits contain sand, but are thin, more silty, and are poor 

yielding compared with previously described facies. The 

final facies is weathered flood-plain deposits which contain 

mottled clays and silts and form large extensive confining 

units. Facies vary based on the flow channel locations and all 

potentially have some permeable sand and (or) the potential to 

provide aquifer leakage with limited connections (McKenna 

and others, 2004). Benson and McLaughlin (2006) described 

the lateral variations of the different facies within the Potomac 

Formation that make it a heterogeneous and complex hydrogeologic system. 

Site-Specific Findings

Results from drilling and DC resistivity helped to identify 

areas where confining units were missing or conversely were 

thick. Confining units (Merchantville or Potomac) were not 

encountered north of Red Lion Creek (well PW-5), and were 

very thin adjacent to the creek to the south (well PW-17). In 

these locations, the Columbia aquifer is mostly in direct contact with sands of the upper Potomac aquifer, although some 

marsh sediments may be present. Locations to the north of the 

site where the confining units are absent correspond to areas 

where contaminant concentrations in marsh sediments are 

elevated. In areas to the east, south, and west of the SCD site, 

channel sands of the Columbia Formation are underlain by a 

varying thickness of marine silt and clay of the Merchantville 

Formation. The exact thickness of Merchantville confining 

beds was determined at borehole locations. DC resistivity 

surveys (Degnan and Brayton, 2010) were used to interpolate 

between boreholes and identify geologic contacts, although in 

many cases, a thin Merchantville unit was broadly interpreted 

as lumped together with silts and clay of the upper Potomac 

Formation because of the similar electrical conductivity 

response (fig. 8). In areas to the east of the site, an extensive 

thickness of confining silt and clay beds (well PW-8, fig. 7b) 

may limit contaminant transport and distribution to the east.

Site-Related Aquifer Distinctions

Three distinct continuous aquifers (A-, B-, and C-sand 

layers) were identified in the upper Potomac Formation at 

the SCD site based on interpretations of boring logs in cross 

sections (fig. 7a). Additional evidence for defining these 

sand layers is based on water-level response to pumping (see 

Production Well R-15, this report). An additional thin (less 

than 10 ft) discontinuous Cretaceous age sand layer was identified beneath the Merchantville confining unit at several locations near the site and is referred to in this report as the upper 

Potomac top sand (table 2). Two wells (PW-4S and PW-7S) 

were screened in this sand layer, which is part of the upper 

Potomac Formation. Water levels measured in these wells 

reflect water-table conditions and indicate a direct hydraulic 

connection to the overlying Columbia aquifer; therefore these 
Hydrogeologic Framework 21

wells are represented on site maps as Columbia aquifer wells. 

This sand layer may represent “thin sands” crevasse splay/

proximal levee sands (McKenna and others, 2004) considering 

the limited spatial distribution and thickness (less than 10 ft). 

The upper Potomac top sand aquifer was encountered during 

drilling at other well locations immediately to the east and 

south of the site (PW-9, PW-2, and MW-11). Approximately 

20–30 ft of the upper Potomac confining unit (mostly clay) 

separate the top sand from the next Potomac sand layer 

(A-sand), which tends to thicken from north to south (fig. 

7a), and from west to east (fig. 7b), although lateral variation 

is likely from the fluvial depositional environment. In other 

areas, the Merchantville confining unit is followed directly by 

Potomac clay layers, for example wells PW-6, PW-8, PW-10, 

PW-11, PW-12, and PW-13, because the top sand has either 

pinched out or become more silty in composition and functions as part of the confining unit.

The upper Potomac A-sand layer was encountered in 

all Potomac Formation borings at and near the SCD site and 

ranges in thickness from approximately 30 to 70 ft, but can be 

as thin as 10 ft (well PW-11). In most locations, it is present 

beneath the upper Potomac confining unit (figs. 7 a, b), but 

in some locations, such as near Red Lion Creek, an upper 

Potomac confining unit was not present (wells PW-17 and 

PW-5). The A-sand layer likely represents an “amalgamated 

channel sand” or “thick sand” (McKenna and others, 2004), 

which is laterally extensive as seen at the SCD site. This type 

of sand can contain more than one sand layer separated by silt 

or clay, as interpreted at drilling locations PW-10 and PW-11. 

Further distinction (or grouping) of these layers is based on 

measured hydraulic responses (see Hydrology, this report). 

Most upper Potomac Formation wells were screened in the 

A-sand layer because of the need to determine the distribution 

of contaminants detected within this sand interval. At several 

of the well locations (PW-4D, PW-9, PW-12, and PW-13), 

contaminants were detected above and within a fine-grained 

sand and silt presumed to be at the base of the A-sand layer. 

Confirming the presence of a confining clay layer below the 

contaminated interval was not possible at these locations 

because drilling was halted; however, evidence from other 

nearby drilling locations indicates that an extensive finergrained (clay) confining layer exists beneath the A-sand at 

wells PW-4D, PW-9, and PW-12, which is supported by the 

lack of contaminant detections within the B-sand layer to the 

south. Additional information is needed in the area of well 

PW-13. 

The B- and C-sand layers were typically thinner than 

the A-sand layer and were encountered beneath additional 

upper Potomac Formation confining clay layers of varying 

thickness. The B-sand was present beneath a secondary upper 

Potomac confining unit, which ranged in thickness from 40 

to 60 ft (figs. 7a, b).The B-sand layer was encountered only 

at drilling locations PW-16 and PW-2D and ranged in thickness from approximately 10 ft at well PW-16 to 15 ft at well 

PW-2D. At drilling location PW-10, a secondary sand layer 

was identified and screened (well PW-10D) beneath the primary A-sand layer (well PW-10). Initial interpretation considered this as a B-sand layer, however, hydraulic responses 

Figure 8. Cross section C-C’ showing direct-current resistivity results and interpretation of stratigraphy at the Standard 

Chlorine of Delaware, Inc. Superfund Site.

25 40 55 75 100

Resistivity, in ohm meters

185 400 780 1,605

0 0

-10

-20

-30

Elevation, in meters

Elevation, in feet

-40

-50 -164.0

-98.4

-131.2

65.6

-65.6

32.8

-32.8

0

0 164.0 328.1 492.1 656.2

50

Distance, in meters

Distance, in feet

100 150 200

10

20

channel sand, Columbia Formation

marine silt and clay, Merchantville Formation and

flood-plain silt and clay, Potomac Formation

channel sand, Potomac Formation

C C' (Northeast) (Southwest)

EXPLANATION
22 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

(see Water-Level Responses and Aquifer Interconnections 

During Production-Well Pumping, this report) indicate 

that it represents a secondary (transitional) A-sand layer that 

likely connects to the B-sand layer, as shown in figure 7a. The 

C-sand layer was encountered at drilling locations PW-10 and 

PW-11, and was less than 10 ft thick, although it may consist 

of multiple thin layers as shown in the log for well PW-11 

D (fig. 7a) Only one well was completed in the C-sand layer 

(PW-11D).

Paleochannels and Erosional Features

A variety of erosional features in terms of depth, width, 

and origin exist in the study area, with the most prominent 

site-related feature being Paleo-Red Lion Creek. Another 

prominent feature, the Reybold channel, is located east of the 

site, but has no effect on site-related groundwater flow. Less 

prominent paleochannel features also formed in the fluvial 

depositional environments of the Columbia and Potomac 

Formations. Interpretations of stratigraphy between well logs 

indicate that there are several areas where the Merchantville 

and sometimes the Potomac Formation confining layers are 

missing. The Columbia Formation is often thicker where it 

has filled paleochannels that were cut into the Merchantville 

and (or) Potomac Formations. These paleochannels result 

in preferential flow paths or areas of increased groundwater 

flow having higher than average yields (Woodruff, 1986). The 

absence of the Merchantville Formation in some locations 

resulted from fluvial erosion due to steeper gradients from the 

lowering of sea level during glacial periods in Pleistocene time 

(Phillips, 1987), which was followed by fluvial and braided 

deposition of the Columbia Formation during interglacial 

periods. Evidence from well logs and geophysical surveys 

indicates that the Pleistocene drop in sea level during one or 

more glacial periods provided a surface-water gradient that 

allowed the Paleo-Red Lion Creek (much larger drainage area) 

to cut through Merchantville and Potomac Formation confining clay layers before a confluence with the Paleo-Delaware 

River (Degnan and others, 2011). The creek was subsequently 

refilled as sea level rose. The in-filled Quaternary deposits 

include up to 60 ft of silt that may be underlain by as much 

as 40 ft of Columbia Formation or other Quaternary aged 

sands (fig. 7a). The Delaware River was cut as low as 130 ft 

below sea level during the Pleistocene near the location of 

the confluence with Red Lion Creek (Phillips, 1987) and this 

depth represents the potential limit of downcutting by PaleoRed Lion Creek. The Paleo-Red Lion Creek erosional channel 

is twice as deep as the Reybold paleochannel at its northern 

point located approximately one-half mi to the east of the site. 

Having a different origin, the north to south oriented Reybold 

channel was formed by fluvial gradients likely derived from 

a glacial lake dam burst upstream along the Delaware River 

(Jengo and others, 2013). The presence of a similar north to 

south oriented erosional feature (if it exists) in the vicinity of 

SCD would complicate the interpretation of flow patterns.

Determining the presence of smaller erosional channels 

within the former chemical plant area has been limited 

by the depth of drilling, which has typically halted at the 

Merchantville confining unit approximately 65 ft below 

land surface in most locations. Terminating drilling at the 

Merchantville, or at an altitude close to present day sea level 

when the Merchantville was not encountered, was a precautionary standard to limit cross-contamination during the drilling process. At drilling locations adjacent to the site boundary 

to the east (wells PW-12, PW-9, and PW-4D) and to the west 

(well PW-13), Merchantville and Potomac confining units 

were present and the core logging did not indicate any erosional channels. If such channels exist on site, they are narrow 

and less extensive than either the Paleo-Red Lion Creek or the 

Reybold paleochannel. 

Hydrology

Seven years of water-level data collection have enabled 

a robust characterization of the local flow system, with a 

focus on the interaction between the Columbia and upper 

Potomac aquifers at SCD. Developing a better understanding 

of groundwater flow within and between sand layers in the 

upper Potomac aquifer is driven by a need to characterize contaminant transport laterally within the A-sand and potentially 

downward to the B- and C-sands. Recharge occurs locally to 

the Columbia aquifer and continuously collected groundwaterlevel data from this study indicate that limited recharge to 

the upper Potomac aquifer also occurs locally where the 

aquifer subcrops and the confining unit is leaky. The altitude 

of groundwater was used to determine the groundwater-flow 

direction in the upper Potomac, which varied over time in 

response to both long- and short-term hydraulic stresses. 

Continuous water-level data were valuable for identifying 

stresses on the upper Potomac aquifer flow system and include 

precipitation, tidal fluctuation, creek stage change effects, and 

pumping effects (fig. 9). Patterns of long-term change in water 

levels were similar for most A-sand wells, indicating a nearly 

uniform response to various hydraulic stresses, which are 

examined in more detail in the sections that follow.

Water levels in well nests have consistently shown 

that vertical gradients at the site are downward from the 

Columbia to the Potomac aquifer and downward within the 

Potomac aquifer system, with changes in magnitude primarily controlled by water withdrawals from the upper Potomac 

aquifer. A direct connection between upper Potomac aquifers 

was identified by comparing creek stage and groundwater 

levels, whereby Red Lion Creek may promote recharge of 

the Potomac aquifer system at elevated creek stages. Aquifer 

connections were further demonstrated (and delineated) 

during pumping of a local production well (see WaterLevel Responses and Aquifer Interconnections during 

Production-Well Pumping , this report) that induced waterlevel decline in Columbia aquifer wells in the northeast part of 
Hydrology 23

the site near the creek and all upper Potomac wells. Pumping 

influences from local industrial production wells were shown 

to be a predominant stress on the upper Potomac aquifer 

system and caused much of the water-level variation that was 

observed.

Recharge and Discharge Patterns

Climate, evapotranspiration, and precipitation have a 

direct effect on available recharge to the surficial and underlying aquifers. Review of precipitation data over a multi-year 

period showed that large storm events as well as prolonged 

periods of above normal precipitation led to elevated water 

levels that imply direct recharge to both the Columbia and 

upper Potomac aquifers. Rainfall associated with Hurricane 

Irene (August 27–28, 2011; 6.94 in.) and the remnants of 

Tropical Storm Lee (September 5–8, 2011; 3.00 in.) made 

2011 one of the wettest years on record. A plot of monthly 

precipitation from NOAA station 13781 shows variation due 

to large storm events and also shows prolonged periods of seasonally above or below normal precipitation such as the “wet” 

period associated with above average snowfall in the winter of 

2010 (figs. 10a, b) and the “dry” period that began in January 

2012 and continued for several months . 

Overall precipitation from 2006 through 2012 was above 

normal (wet) during 3 of the years, normal during 4 of the 

years, and below normal (dry) during 1 year of the study 

(table 5). Non-normal is defined as deviation from the 30-year 

(1971–2000) mean annual precipitation of 42.81 in. by 

more or less than 14.7 percent (mean/ mean + 2 x variance). 

Recharge can also be affected by variations in evapotranspiration. During the study period, mean annual temperatures 

were equal to or greater than the long-term mean annual 

temperature, potentially resulting in higher evapotranspiration; 

however, these effects were not quantified.

PW-13 PW-6S

PW-17

PW-4D

MW-12

MW-11

PW-5

PW-11

Red Lion Creek

Altitude of water level, in feet relative to National Geodetic 

Vertical Datum of 1929

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

-3.5

2007 2008 2009 2010 2011 2012

Year

Aquifer recovery and limited

pumping period

U.S. Geological Survey

72-hour pumping period

at well OR-6A

Figure 9. Daily average altitude of water levels for selected upper Potomac A-sand aquifer wells and Red Lion Creek 

near the Standard Chlorine of Delaware, Inc. Superfund Site, 2007–12.
24 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

Columbia Aquifer Recharge

The effects of direct aquifer recharge from precipitation 

can be seen by examining changes in water levels within the 

Columbia aquifer, which ranged from approximately 2 ft (well 

Ion MW-3) in the uplands to only 1 ft in the lowlands closer 

to the creek (well CW-11) (fig. 10). Some seasonal variability 

occurs, with peak annual water-table levels typically from 

March to May, resulting from winter and early spring seasonal 

recharge. Rainfall distribution was normal in 2007 and 2008 

except for a prolonged (more than 3 months) dry period from 

May–September 2007, the effects of which can be seen in a 

decline in the Ion-MW-3 water level. The lowest water-table 

elevation was observed at the beginning of April 2009, following several months of minimal precipitation. Prolonged 

wet periods in recent years, notably from June 2009 to March 

2010, and from August 2011 through December 2011 led to 

a sustained higher water table, peaking in early May 2010. In 

contrast, most of 2012 was dry, with a corresponding drop in 

the water-table elevation. 

Dynamic recharge response to large storm events 

occurring over 1 to several days can be seen as spikes in the 

hydrograph on September 30, 2010 and August 27, 2011 for 

well CW-11 in the lowlands, and as moderate rises for upland 

well Ion MW-3 (fig. 10). Aquifer responses to storm events 

are discussed in detail (see Response to Large Storm Events, 

this report). Overall, the responsiveness of water levels in 

the Columbia aquifer to precipitation patterns indicates that 

direct aquifer recharge is a predominant source of water to the 

Columbia.

Jan. 1,

2006

Jan. 1,

2007

Jan. 1,

2008

Jan. 1,

2009

Jan. 1,

2010

Jan. 1,

2011

Jan. 1,

2012

Jul. 1,

2006

Jul. 1,

2007

Jul. 1,

2008

Jul. 1,

2009

Jul. 1,

2010

Jul. 1,

2011

Jul. 1,

2012

Apr. 1,

2006

Apr. 1,

2007

Apr. 1,

2008

Apr. 1,

2009

Apr. 1,

2010

Apr. 1,

2011

Apr. 1,

2012

Oct. 1,

2006

Date

Oct. 1,

2007

Oct. 1,

2008

Oct. 1,

2009

Oct. 1,

2010

Oct. 1,

2011

Oct. 1,

2012

10

9

8

Monthly precipitation, in inches

CW-11 and MW-12, altitude of water level,

in feet relative to National Geodetic 

Vertical

Datum of 1929

Ion MW-3, altitude of water level,

in feet relative to National Geodetic 

Vertical

Datum of 1929

7

Dry Wet Dry

6

5

4

3

2

1

0

7 19

6 18

5 17

4 16

3 15

2 MW-12 (upper Potomac A)

CW-11 (Columbia)

Ion MW-3 (Columbia)

14

1 13

0 12

-2 10

-3 9

-1 11

Monthly precipitation

Average monthly

precipitation,1971–2000

A

B

U.S. Geological Survey

72-hour pumping period

at well OR-6A

Aquifer recovery and limited

pumping period

Hurricane Irene

and

Tropical Storm Lee

Figure 10. (A) Comparison of monthly precipitation and long-term average monthly precipitation in the Delaware City area, 

2006–12 (precipitation data from National Oceanic and Atmospheric Administration station 13781), and (B) hydrograph showing 

water-level variation for water-table (Columbia aquifer) and confined aquifer (upper Potomac A-sand) wells compared with 

monthly precipitation. (Explanation shows screened aquifer in parentheses.)
Hydrology 25

Potomac Aquifer Recharge

Recharge to the upper Potomac aquifer occurs more 

slowly and is more limited than the direct recharge to the 

Columbia aquifer. Differences in the long-term hydrographs 

for Columbia and Potomac aquifer wells help to distinguish 

recharge influences from pumping effects in the Potomac aquifer. Overall, during the study period, water levels for upper 

Potomac A-sand wells showed much greater variation (up to 

6 ft) than Columbia wells (1 to 2 ft). Spanning both pumping 

and non-pumping periods, the annual range in water levels for 

upper Potomac A-sand wells was from 3 ft at well PW-5 near 

Red Lion Creek to approximately 6 ft at well MW-11 at the 

south end of the site (fig. 9). However, most of this variation 

can be explained by pumping effects (see Production Well 

R-15, this report). Recharge in the upper Potomac aquifer that 

is related to precipitation cannot easily be distinguished in the 

long-term hydrograph during periods of pumping influence, 

but during an approximately 2-year period (November 2009 

through October 2011) pumping was minimized and the longterm pattern of recharge that was observed is similar for both 

the Columbia and upper Potomac A-sand (fig. 10), indicating 

the presence of a coupled recharge system between aquifers. 

During this period, the annual range in water levels for the 

upper Potomac was only 1 ft, which is a muted reflection of 

changes observed in the Columbia at well Ion MW-3, which 

had a 2-ft range. 

During active pumping and aquifer recovery periods, 

changes in water levels in the upper Potomac did not mimic 

changes observed in the water table in the Columbia aquifer. 

For example, the water-table minimum in April 2009 at well 

Ion MW-3 did not have a corresponding minimum in upper 

Potomac well MW-12 (fig. 10). A reduction in pumping of the 

upper Potomac aquifer in November 2009 led to an immediate 

increase in water levels by 2 feet by January 1, whereas the 

increase in Columbia aquifer wells was less than 0.5 ft 

(fig. 10). Larger increases in Columbia aquifer water levels 

did not occur until after January 1, which is likely the result 

of above average snowfall during the winter of 2010. It is difficult to distinguish how the upper Potomac aquifer responded 

to this winter recharge because the hydrograph is masked by 

recovery from pumping.

During the period of limited pumping, more natural 

variation in water levels could be seen in the upper Potomac 

aquifer in response to recharge from the Columbia. Dry 

periods showed a similar pattern of decline in water levels for 

both the upper Potomac and Columbia aquifers. This pattern 

diverged at the end of October 2011 once pumping resumed in 

the upper Potomac. The upper Potomac water levels showed a 

steep decline, whereas Columbia water levels were maintained 

for at least 4 months, until persistent dry conditions during 

most of 2012 led to a continued water-level decline.

Groundwater Discharge

The majority of water recharging to the Columbia aquifer 

discharges to surface water and a minor amount recharges 

the upper Potomac aquifer. In order to assess the potential 

significance of groundwater discharge to tidal Red Lion 

Creek, Lorah and others (2014) compared estimates of vertical 

groundwater fluxes integrated over the area of the tidal creek 

(based on measured heads and hydraulic properties) with an 

estimate of total discharge from the watershed draining to tidal 

Red Lion Creek (based on historical streamflow data from a 

nearby streamgage). These estimates, with some limitations, 

Table 5. Comparison of annual and long-term mean climate conditions in the Delaware City area, 2005–12.

[°F, degrees Fahrenheit; --, no data; temperature and precipitation data from National Oceanic and Atmospheric Administration, weather station 13781]

Mean 

temperature

Mean 

maximum 

temperature 

(°F)

Mean 

minimum 

temperature 

(°F)

Precipitation 

(inches)

Precipitation 

percent 

difference 

compared with 

30-year mean

Annual 

wetness 

condition

(1971–2000)1 54.4 63.5 45.1 42.81 -- --

2005 54.6 63.8 45.4 40.25 -5.98 normal

2006 56.2 65.6 46.9 49.41 15.42 wet

2007 55.7 65.0 46.3 41.81 -2.34 normal

2008 55.6 65.1 46.0 40.44 -5.54 normal

2009 54.4 63.0 45.8 52.06 21.61 wet

2010 56.0 65.3 46.8 43.96 2.69 normal

2011 56.3 65.5 47.0 56.58 32.17 wet

2012 57.4 66.9 48.0 36.30 -15.21 dry

1

 National Oceanic and Atmospheric Administration, 2004.
26 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

indicate that much, if not most, of the net recharge to the 

surficial aquifer in the sub-watershed of tidal Red Lion Creek 

discharges from groundwater to the tidal creek and wetland.

Localized groundwater discharge to the creek and marsh 

from the upper Potomac aquifer likely occurred during the 

limited pumping period (November 2009 through October 

2011) when water levels in all monitored upper Potomac wells 

were higher than the creek stage and similar to Columbia aquifer water levels near the marsh (fig. 9). Thus, vertical gradients 

decreased during the limited pumping period (see Vertical 

Gradients, this report). Conversely, recharge of the A-sand 

aquifer from Red Lion Creek is possible when water levels 

drop below the level of the creek during pumping periods. It 

is unclear to what extent this occurs, because A-sand aquifer 

water levels dropped below creek stage in wells located south 

of the site, but not in wells adjacent to the creek.

Water-Table Contours, Potentiometric Surfaces, 

and Flow Directions

Monthly synoptic measurements of altitudes of water 

levels in wells were used to determine flow direction within 

the Columbia and upper Potomac aquifers for a range of dates 

throughout the period of study. Water-level data from specific 

dates were used to create water-table contour maps for the 

Columbia aquifer to illustrate the effect of the barrier wall on 

flow patterns. Water-level data from specific dates were used 

to create potentiometric surface maps for the upper Potomac 

A-sand aquifer to show pumping and limited pumping conditions, and their effect on groundwater-flow direction. 

Groundwater flow in the Columbia aquifer is topographically driven and flow direction is consistent from year to year, 

and is generally south to north (fig. 11), as shown by previous 

OR-6C

(1.28)

MW-7

(5.13)

MW-15

(11.79)

C-31

(15.14)

Ion MW-3

(15.59)

1

3

5

7

9

11

13

15

Governor Lea Road

39°36'20"

39°36'10"

39°36'00"

39°35'50"

75°38'20" 75°38'00"

River Road

9

Imagery from U.S. Geological Survey, Delaware Valley Regional Planning

Commission, and Department of Homeland Security, 2010, NAD 1983

State Plane Delaware Transverse Mercator Projection

0 1,000 FEET

0 200 METERS

N

EXPLANATION

May 13, 2006

altitude of water table

Columbia aquifer well

and identifier (Number

in parentheses is

altitude of water level,

in feet.)

Water-table contour

(Interval is 2 feet;

dashed where

approximate)

Generalized groundwater-flow direction

C-31

(15.14)

15

A

Red Lion Creek

Figure 11. Water-table contours for the Columbia aquifer (A) May 13, 2006 prior to completion of the barrier 

wall, and (B) August 8, 2012 after completion of the barrier wall.
Hydrology 27

studies at the site (Roy F. Weston, Inc., 1992; Black & Veatch, 

2007) The altitude of the water table near the site ranges from 

approximately 17 ft at the south end, to consistently just above 

sea level (0 to 1 ft) at the north end of the site near Red Lion 

Creek. Completion of the barrier wall in 2007 caused a divergence in flow direction near the south end of the wall so that 

groundwater is diverted to the west and east locally around the 

wall; however, the overall flow pattern outside the wall has 

remained consistently towards Red Lion Creek (fig. 11). 

Groundwater inside the barrier wall is isolated from the 

natural local flow system and is pumped as part of remedial 

operations. Water-level measurements inside the barrier wall, 

collected by EPA site consultant HydroGeoLogic Inc., are 

used to optimize the operation of the remedial pump and treat 

system. The typical aggregate pumping rate for extraction 

wells located inside the barrier wall was 50 gal/min during 

the study period (Chris Wolfe, HydroGeoLogic, Inc., written 

commun., 2012). Groundwater pumped from inside the barrier wall is treated and discharged as overland runoff outside 

the barrier wall to the east (U.S. Environmental Protection 

Agency, 2011), and has little effect on the altitude of the water 

table outside the barrier wall.

As previously described in this report, the prevailing 

groundwater-flow direction for aquifers in the Potomac 

Formation is from northwest to southeast, following the 

regional bedrock dip direction. Flow direction was determined 

for the A-sand of the upper Potomac aquifer at SCD using 

water levels from wells having an altitude of screened interval 

from approximately -80 to -120 feet. The altitude of water 

level and flow direction are influenced by groundwater withdrawal wells in the study area. Two dates were selected to 

compare conditions between limited pumping 

Red Lion Creek

3

1

1

3

5

7

9

11

11

13

15

17

13

15

17

9

7

5

MW-10

(10.03)

PZ-7

(7.96)

MW-8

(5.22)

MW-19

(2.81)

OR-6C

(1.34)

MW-20

(2.83)

MW-24

(1.69)

MW-40

(2.23)

MW-25

(2.12)

MW-21

(2.57)

MW-22

(2.64)

MW-23

(2.21)

PZ-12

(3.08)

PZ-15

(3.47)

PZ-18

(6.84)

PZ-17

(6.74)

MW-5

(6.17)

CW-11

(5.26)

EM-2D

(2.36)

MW-36

(2.11)

MW-37

(1.15) MW-38

(1.29)

EM-4D

(3.88)

EM-5D

(2.66)

EM-6D

(1.86)

PMW-47

(3.95)

PZ-5

(8.82)

PZ-4

(11.42)

PZ-1

(17.26)

PZ-24

(17.78)

PZ-23

(16.82)

MW-16

(12.36)

MW-18

(10.32) MW-17

(9.96)

PMW-48

(10.51)

MW-14

(11.88) PW-4S

(12.49)

CW-4

(12.53)

Ion MW-3

(16.46)

PZ-22

(15.68)

MW-9

(8.74)

MW-33

(13.21)

N

Governor Lea Road

River Road

9

EXPLANATION

Columbia aquifer well

and identifier (Number

in parentheses is

altitude of water level,

in feet.)

Water-table contour

(Interval is 2 feet;

dashed where

approximate)

Generalized groundwater-flow direction

Barrier wall

MW-10

(10.03)

15

B

August 8, 2012

altitude of water table

Imagery from U.S. Geological Survey, Delaware Valley Regional Planning

Commission, and Department of Homeland Security, 2010, NAD 1983

State Plane Delaware Transverse Mercator Projection

0 1,000 FEET

0 200 METERS

39°36'20"

39°36'10"

39°36'00"

39°35'50"

75°38'20" 75°38'00"

Figure 11. Water-table contours for the Columbia aquifer (A) May 13, 2006 prior to completion of the barrier 

wall, and (B) August 8, 2012 after completion of the barrier wall.—Continued
28 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

(September 14, 2011) and pumping (August 8, 2012) periods. 

Water levels during limited pumping conditions were up to 5 

ft higher at the south end of the site, and up to 2 ft higher near 

Red Lion Creek (fig. 9). Flow direction in the upper Potomac 

A-sand was to the east towards the Delaware River during 

limited pumping (fig. 12a), and to the south when influenced 

by pumping (fig. 12b). Prior to November 2009, flow direction 

was consistently to the south during typical pumping conditions. Comparison of hydrographs from wells screened in 

the upper Potomac A-sand showed that well PW-3 is likely 

screened in a “perched” or disconnected sand interval, and 

water levels for this well were not used in the creation of the 

potentiometric surface maps.

Flow direction within B- and C-sands cannot be accurately determined due to an insufficient number of wells 

screened at the appropriate depths. Additional wells screened 

within these sand intervals, at altitudes of approximately -180 

and -220 ft, would greatly assist in determining flow direction 

and would improve the understanding of the spatial extent of 

interconnections between the A-, B-, and C-sand layers in the 

upper Potomac aquifer.

Figure 12. Potentiometric surface of the upper Potomac A-sand aquifer during (A) limited pumping 

conditions (September 14, 2011), and (B) typical pumping conditions (August 8, 2012), and heads for selected 

wells in deeper aquifer units.

PW-5D

(2.57)

PW-6D (3.02)

OR-6A

(2.99)

PW-14

(2.52) PW-17

(2.59)

PW-11

(2.24)

PW-10

(2.38)

PW-12

(2.39)

PW-13

(2.97)

PW-4D

(2.15)

PW-9

(2.30) PW-8

(1.86)

Tidewater 12

(-88.1)

PW-2

(2.13)

PW-2D

(-4.55)

PW-16

(-6.38)

MW-11

(2.70)

MW-12

(1.85)

PW-11D

(-15.58)

PW-6S (3.04)

OR-6B

(3.01)

PW-5S 3.0

(2.54)

2.5

2.0

2.0

2.5

3.0

N

Governor Lea Road

River Road

9

EXPLANATION

upper Potomac A-sand

and identifier (Number

in parentheses is

potentiometric surface,

in feet.)

upper Potomac B-sand

upper Potomac C-sand

lower Potomac

Potentiometric surface

contour (Interval is

0.5 foot; dashed where

approximate)

Generalized groundwater-flow direction

Barrier wall

3.0

A

September 14, 2011

potentiometric surface

Wells (aquifer)

PW-13

(2.97)

Imagery from U.S. Geological Survey, Delaware Valley Regional Planning

Commission, and Department of Homeland Security, 2010, NAD 1983

State Plane Delaware Transverse Mercator Projection

0 1,000 FEET

0 200 METERS

39°36'20"

39°36'10"

39°36'00"

39°35'50"

75°38'20" 75°38'00"

Red Lion Creek
Hydrology 29

Surface-Water Influences on Groundwater

Changes in the surface-water level near the SCD site 

caused by tides and precipitation have various influences 

on the groundwater system. Tidal fluctuations within the 

Delaware River minimally affect water levels in the Potomac 

aquifer system, and because of a tide control structure at 

the mouth of Red Lion Creek (figs. 2 and 5), creek stage 

is also minimally affected by tides in the Delaware River. 

Malfunction of the tide control structure provided insight 

into how tidally influenced changes in creek stage affect 

groundwater levels (see Tidal Influences, this report). 

Although Red Lion Creek and connected wetlands adjacent 

to the SCD site are a receiving source for overland runoff and 

shallow groundwater discharge, elevated creek stage within 

these areas can raise groundwater levels and contribute to 

aquifer recharge. Large increases in creek stage from precipitation associated with storm events results in a short-term 

increase in groundwater levels, particularly in wells adjacent 

to the creek. The potential to induce recharge from these storm 

events was examined.

PW-5D

(1.27)

PW-6D (0.31)

OR-6A

(0.31)

PW-17

(1.03)

PW-11

(0.37)

PW-10

(-1.54)

PW-12

(-1.64)

PW-13

(-1.1)

PW-4D

(-1.69)

PW-9

(-1.69) PW-8

(-1.72)

Tidewater 12

(-99.6)

PW-2

(-1.72)

PW-2D

(-29.47)

PW-16

(-43.47)

MW-11

(-1.75)

MW-12

(-0.95)

PW-11D

(-44.9)

PW-6S (0.31)

PW-5S

(1.27)

1.0

-1.0

-2.0

0.0

N

Governor Lea Road

River Road

9 EXPLANATION

upper Potomac A-sand

and identifier (Number

in parentheses is

potentiometric surface,

in feet.)

upper Potomac B-sand

upper Potomac C-sand

lower Potomac

Potentiometric surface

contour (Interval is

1 foot; dashed where

approximate)

Generalized groundwater-flow direction

Barrier wall

-2.0

B

August 8, 2012

potentiometric surface

Wells (aquifer)

PW-13

(-1.1)

Imagery from U.S. Geological Survey, Delaware Valley Regional Planning

Commission, and Department of Homeland Security, 2010, NAD 1983

State Plane Delaware Transverse Mercator Projection

0 1,000 FEET

0 200 METERS

39°36'20"

39°36'10"

39°36'00"

39°35'50"

75°38'20" 75°38'00"

Red Lion Creek

Figure 12. Potentiometric surface of the upper Potomac A-sand aquifer during (A) limited pumping 

conditions (September 14, 2011), and (B) typical pumping conditions (August 8, 2012), and heads for selected 

wells in deeper aquifer units.—Continued
30 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

Tidal Influences

Water levels in wells screened in the upper Potomac 

aquifer show some tidal influences from both the Delaware 

River and Red Lion Creek. A 6-ft tidal range in the Delaware 

River produces very limited pressure loading and unloading 

on confined parts of the upper Potomac aquifer, affecting A-, 

B-, and C-sands uniformly across a broad area. Although it 

is isolated from direct tidal effects from the Delaware River 

because of the tide control structure, the creek does experience 

stage changes resulting from backwater effects related to 

discharge through the tide control structure (for example, 

the stage does not continue to fall, and may increase when 

the gates are closed during high tide in the Delaware River). 

Stage changes in the creek are much smaller in magnitude 

than changes in the Delaware River but more directly affect 

ambient groundwater levels in the upper Potomac aquifer near 

the SCD site. Malfunction of at least one tide gate is known 

to have occurred during Hurricane Irene, August 27–28, 

2011, which resulted in tidal cycling within the creek and in 

the marsh adjacent to the SCD site. The center gate (out of 

five) remained stuck open after attempts to remove debris and 

repair it, which resulted in a gradual increase in base stage in 

Red Lion Creek (fig. 9) since water could not effectively be 

drained from the creek. Flow reversals due to tidal inflow have 

been observed at the Route 9 bridge and were measured most 

recently on December 19, 2012.

Variation in surface-water levels at different time scales 

was evaluated in this study. The typical monthly stage fluctuation in Red Lion Creek before the tide gate malfunction 

ranged from 1.8 ft below sea level to 0.0 ft above sea level, 

and after the malfunction ranged from 1.0 ft below sea level 

to 0.5 ft above sea level (Lorah and others, 2014). Daily stage 

fluctuation (range) before the malfunction was typically 0.18 

ft, whereas after the malfunction, it was approximately 0.75 

ft. Water levels in unconfined wells generally are not affected 

by tidal fluctuation in the Delaware River or the creek, 

however small episodic variations in water levels in shallow 

drive points (piezometers) in the fringing marshes have been 

observed, but are commonly less than 0.5 ft (Lorah and others, 

2014). Tidal effects have not been observed in water-table 

wells upgradient of the fringing marsh. 

Water levels in wells screened in upper Potomac aquifer 

sands (A, B, and C) are minimally affected by stage changes 

caused by tidal fluctuation, but did show a greater daily 

water-level range after the tide gate malfunction (fig. 13). The 

range of average daily water-level variation was examined for 

selected wells during time periods with minimal antecedent 

precipitation. Dates were selected at a similar point in time for 

each year from 2009 to 2012, during which less than 0.10 in. 

of precipitation had fallen for 7 days prior to water-level measurement. Tidal effects from the Delaware River, measured at 

NOAA tide station 8551762, accounted for minor (less than 

0.04 ft) daily variations in water levels for the upper Potomac 

A- and B-sands prior to the tide gate malfunction. The C-sand 

(well PW-11D) is more confined and has a greater response 

to pressure loading and therefore shows a greater daily range 

(0.10 ft), although part of this variation may be attributed to 

deeper aquifer pumping influences from the cycling of production well pumps (see Production Well R-15, this report). 

After the tide gate malfunction, wells located closer to 

the creek had an approximately fourfold increase in waterlevel variation (up to 0.16 ft) whereas wells located farther 

away from the creek to the south showed little change in water 

levels (fig. 13). The range of daily water levels in well PW-2D 

(upper Potomac B-sand) increased from 0.03 to 0.06 ft after 

the tide gate malfunction, indicating a limited hydraulic connection to the upper Potomac A-sand. There is no apparent 

effect from the tide gate malfunction on the daily water-level 

range for well PW-11D (upper Potomac C-sand), which is 

reasonable considering its depth and likely poor hydraulic 

connection to the creek. 

Semi-diurnal water-level variations in the upper Potomac 

A-sand wells were not apparent before the tide gate malfunction but were easily distinguished after the tide gate malfunction. This evidence also reinforces the concept that the 

Delaware River itself has little effect on water-level variations 

for wells in the upper Potomac aquifer system. A comparison 

of water-level response over similar 3-day periods with no precipitation showed that responses in the upper Potomac A-Sand 

were greatest closest to the creek (fig. 14), but were evident as 

far as 1,700 ft away from the creek (well PW-13), indicating 

a good connection between the creek and the upper Potomac 

A-sand. This connection resulted from the erosion of confining 

layers near the creek and marsh. Tidal response variations in 

the upper Potomac B- and C-sands were apparent but muted, 

accounting for less than 0.05 ft of daily variation.

Red Lion Creek Stage Change Effects

The extent of interconnection between Red Lion Creek 

and the upper Potomac aquifer was further explored by 

examining water-level response in wells with evidence of correlation between water levels and precipitation-runoff-driven 

changes in creek stage. Red Lion Creek is very responsive 

to rainfall and will typically begin to show stage increases 

with as little as 0.50 in. of rainfall. Corresponding changes 

in groundwater levels have been observed and are typically 

20–40 percent of maximum stage change for wells located 

close to the creek, and 10–20 percent of maximum stage 

change for wells father from the creek (fig. 15). Increased 

water levels due to elevated stage likely represent a combination of pressure loading from Red Lion Creek and some 

recharge from the creek to the aquifer. 

Stage changes caused by precipitation are commonly 

larger than those that result from tidal effects. Precipitation 

events were selected from 2010–12 to examine stream stage 

and groundwater-level response. Discrete events were chosen 

when precipitation occurred over no more than a 2-day period, 

and was preceded by at least 4 days with no precipitation. The 

results showed that creek stage and water-level response in 

wells due to large storms (described in the next section) are 
Hydrology 31

generally proportional to the total amount of precipitation for a 

specific event (fig. 15). 

One in. of precipitation causes a 1-ft rise in creek stage 

and can be approximated using a linear empirical formula 

derived from 14 precipitation events (2010–12):

Stage = 0.62 * p + 0.48 (1)

where 

p = precipitation, and the constant 0.48 is the 

average tidal stage fluctuation before and 

after the tide gate malfunction.

This stage-rainfall relation was calculated using daily 

rainfall data from ten dates (fig. 15) and data from the following additional dates (10/14/10, 8/14/11, 5/14/12, and 9/18/12). 

The formula can be extended for daily rainfall amounts greater 

than 7.58 in.; however, some variability in peak stage is possible due to the timing and spatial distribution of precipitation. 

Most precipitation events cause a relatively quick rise and fall 

in creek stage, although larger events can lead to a multiple 

day stage recession following peak stage.

Response to Large Storm Events

The response to storm events was more easily observed 

during minimal aquifer pumping periods, from November 

2009 through October 2011. Hurricane Irene in August 2011 

provided a good example of aquifer response for multiple 

aquifer sand intervals. Water levels for several sites were 

normalized to pre-event water levels (by setting the minimum 

to zero) and plotted together to analyze storm response to 6.94 

in. of rainfall from Hurricane Irene (fig. 16). Following this 

storm event, higher water levels were sustained at many sites 

(from precipitation recharge), which differs from the response 

to stage change effects (from pressure loading) that typically 

last 1 to 2 days, are cyclical (show tidal fluctuation), and are 

not sustained as creek stage is lowered. Water-level responses 

to large storms differed depending on the location and depth 

of the screened interval for wells screened in the Columbia 

aquifer, and the upper Potomac A-, B-, and C-sands (fig. 16). 

The Columbia aquifer response to Hurricane Irene 

showed that recharge in the uplands was minimal (well Ion 

MW-3), whereas recharge that occurred closer to the creek 

at well CW-11 resulted in a water-level increase of 1.0 ft that 

Figure 13. Daily water-level range in feet for the Delaware River, Red Lion Creek, and upper Potomac aquifer wells for selected 

dates following a 7-day period of minimal precipitation. (Explanation shows screened aquifer in parentheses.) Range of daily water levels for Delaware River and Red Lion Creek, in feet

Range of daily water levels for upper Potomac aquifer wells, in feet

9.0

8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

9/23/09 10/13/10

Date

10/11/11 9/17/12

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

Red Lion Creek

PW-5S (upper Potomac A-sand)

PW-6S (upper Potomac A-sand)

PW-17 (upper Potomac A-sand)

PW-11 (upper Potomac A-sand)

PW-13 (upper Potomac A-sand)

MW-11 (upper Potomac A-sand)

PW-2D (upper Potomac B-sand)

PW-16 (upper Potomac B-sand)

PW-11D (upper Potomac C-sand)

Delaware River

Tide gate

malfunction

8/29/11
32 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

was sustained for several days (fig. 16). Recharge of confined 

aquifer layers, as shown by water-level increases that are sustained in response to storms, occurs through leakage between 

aquifer layers. This indicates partial connectivity to the surficial system. Storm response increases in water levels within 

the upper Potomac B-sand (wells PW-16 and PW-2D) and the 

upper Potomac C-sand (well PW-11D) were not sustained over 

a several day period, although water levels at well PW-2D 

(closer to the creek) were higher than at well PW-16. Although 

trending downward, water levels remained 0.6 ft above prestorm levels in the upper Potomac A-sand nearly 2 days after 

precipitation ended, indicating that large storm events can 

effectively recharge the upper Potomac aquifer.

Connection Between Aquifers

The extent of connectivity between aquifers is directly 

related to the presence and relative thickness of confining 

units; where confining units are absent, direct recharge from 

overlying aquifers is possible. To help quantify potential water 

exchange, vertical gradients were calculated from water levels 

directly measured at several locations having nested pairs of 

wells. The changes in vertical gradients between aquifers over 

time also have led to a better understanding of the effects of a 

variety of short- and long-term hydraulic stresses, in particular, the effects of long-term industrial water withdrawals. The 

responses to recharge that were previously described have 

shown that aquifer layers are connected. Observed water-level 

3.5 3.5

3.0 3.0

2.5 2.5

2.0 2.0

1.5 1.5

1.0 1.0

0.5 0.5

0.0 0.0

-0.5

Date Date

-0.5

-1.0 -1.0

-1.5

10/12/10 10/13/10 10/14/10 10/15/10 10/10/11 10/11/11 10/12/11 10/13/11

-1.5

Altitude of water level, in feet relative to National Geodetic 

Vertical Datum of 1929

A. Before tide gate malfunction B. After tide gate malfunction

PW-5S (290 feet)

PW-17 (395 feet)

PW-6S (600 feet)

PW-11 (1,060 feet)

PW-13 (1,725 feet)

MW-11 (2,855 feet)

Red Lion Creek

Figure 14. Water-level response in upper Potomac A-sand aquifer wells to tidal cycling in Red Lion Creek with no antecedent 

precipitation (A) before tide gate malfunction, and (B) after tide gate malfunction. (Explanation shows distance from Red Lion 

Creek in parentheses.)
Hydrology 33

responses during the pumping of well OR-6A allowed the 

identification of more specific areas of the site that have a 

good connection between aquifers.

Vertical Gradients

Vertical hydraulic gradients provide insight on the threedimensional nature of the flow system and help to distinguish 

the relative importance of vertical flow compared to lateral 

flow. Vertical gradients measured in nested well pairs were 

consistently downward from the Columbia to the Potomac 

aquifers and downward within upper layers of the Potomac 

aquifer, except for well pairs located adjacent to Red Lion 

Creek (fig. 17). The magnitude of vertical gradients was 

affected by nearby industrial withdrawals. Hydraulic gradients 

were calculated using water levels measured in September 

of each year from 2007 through 2012 (except August 2012). 

The vertical gradients from the Columbia to the Potomac 

A-sand are typically downward and ranged from 0.3 (ft/ft or 

dimensionless) downward south of the site to slightly upward 

near the creek (at wells PW-6S and PW-5S) in September 

2011, when upper Potomac A-sand water levels reached their 

peak when production wells were idled during the refinery 

shutdown. Along the north end of the site, gradients are 

slightly but consistently downward at well PW-17. Generally, 

during the limited pumping period from November 2009 

through October 2011, the magnitude of vertically downward 

gradients decreased. At well pair MW-37/ PW-5S north of 

the creek, gradients were slightly upward to no gradient (0.02 

to 0.00). Two pairs of wells (PW-5S/ PW-5D and PW-6S/ 

PW-6D) that are screened in nearly continuous upper Potomac 

A-sand showed no measurable gradients and are not shown as 

well pairs in fgure 17. Gradients between another well pair 

(PW-10 and PW-10D) screened in the upper Potomac A-sand, 

but separated by a confining unit, showed slightly downward 

gradients during the study period, even during the limited 

pumping period. Gradients from the upper Potomac A-sand 

to the upper Potomac C-sand (PW-11 and PW-11D) were 

Figure 15. Maximum water-level response, in feet, measured at Red Lion Creek and selected Columbia and upper 

Potomac aquifer wells for selected discrete precipitation events, 2010–12. (Explanation shows screened aquifer and 

distance from Red Lion Creek in parentheses.)

CW-11 (Columbia) (1,070 feet)

Ion MW-3 (Columbia) (3,170 feet)

PW-5S (upper Potomac A-sand) (290 feet)

PW-6S (upper Potomac A-sand) (600 feet)

PW-11 (upper Potomac A-sand) (1,060 feet)

PW-17 (upper Potomac A-sand) (395 feet)

MW-12 (upper Potomac A-sand) (1,590 feet)

PW-13 (upper Potomac A-sand) (1,725 feet)

PW-4D (upper Potomac A-sand) (2,140 feet)

MW-11 (upper Potomac A-sand) (2,855 feet)

PW-2D (upper Potomac B-sand) (2,750 feet)

PW-16 (upper Potomac B-sand) (3,155 feet)

PW-11D (upper Potomac C-sand) (1,050 feet)

Red Lion Creek (0 feet)

Water-level response, in feet

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

5/6/11

0.22"

6/28/11

0.98"

11/4/10

1.67"

7/8/11

2.48"

Date and precipitation, in inches

8/27/11

6.94"

9/30/10

7.58"

8/5/12

0.79"

6/12/12

1.48"

4/22/12

1.99"

11/21/11

2.24"
34 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

downward during limited pumping periods (0.14 and 0.17), 

and strongly downward during pumping periods (0.54 and 

0.65).

The fluvial depositional environment within the upper 

Potomac aquifer system at SCD creates multiple pathways for 

groundwater flow and contaminant movement. The interconnection of sands is complex, and changes in the magnitude 

of vertical gradients alter which pathways are favored over 

others. For example, a strongly downward component of 

flow at the north end of the site near well PW-17 resulted in 

contaminant migration downward through sand and silt layers. 

Subsequent decreases in the magnitude of vertical gradients 

may then favor lateral migration (spreading) of contaminants 

away from areas of known higher concentration. 

Water-Level Responses and Aquifer 

Interconnections During Production-Well 

Pumping 

Pumping for a 72-hour continuous period was conducted 

by USGS at well OR-6A and resulted in water-level data available for USGS network wells (Appendix 3) and non-USGS 

network wells (Appendix 4). Data for USGS network wells are 

also available at the USGS NWIS database (http://waterdata.

usgs.gov/nwis), accessible using the site identifiers that appear 

in Appendix 2. Results of the step-drawdown test used to 

determine the pumping rate for the 72-hour pumping period 

are on file in the USGS Maryland-Delaware-D.C. Water 

Science Center office in Baltimore, Maryland. 

CW-11 (Columbia) (1,070 feet)

Ion MW-3 (Columbia) (3,170 feet)

PW-5S (upper Potomac A-sand) (290 feet)

PW-17 (upper Potomac A-sand) (395 feet)

PW-6S (upper Potomac A-sand) (600 feet)

PW-11 (upper Potomac A-sand) (1,060 feet)

PW-13 (upper Potomac A-sand) (1,725 feet)

MW-11 (upper Potomac A-sand) (2,855 feet)

PW-2D (upper Potomac B-sand) (2,750 feet)

PW-16 (upper Potomac B-sand) (3,155 feet)

PW-11D (upper Potomac C-sand) (1,050 feet)

Red Lion Creek (0 feet)

5.0

4.5

4.0 2.0

2.25

2.5

3.5 1.75

3.0 1.5

2.5 1.25

2.0 1.0

1.5 0.75

1.0 0.5

0.5 0.25

0.0 0.0

-0.5

8/27/11

0:00

8/27/11

12:00

8/28/11

0:00

8/28/11

12:00

8/29/11

0:00

8/29/11

12:00

8/30/11

0:00

Water-level response, in feet

Hourly rainfall, in inches

Date and time

EXPLANATION

Figure 16. Hourly rainfall and normalized hydrograph showing recharge response in wells screened in the Columbia and upper 

Potomac aquifers during and after Hurricane Irene, August 27–30, 2011. (Explanation shows screened aquifer and distance from 

Red Lion Creek in parentheses; precipitation data from National Oceanic and Atmospheric Administration station 13781.)
Hydrology 35

Water-Level Drawdown

Water-level drawdown was observed in some Columbia 

aquifer wells and in all upper Potomac aquifer wells and 

revealed anisotropy in the confined aquifer flow system. 

Pumping-induced drawdown was observed in 12 Columbia 

aquifer wells and in all 28 wells screened within the upper 

Potomac aquifer demonstrating hydraulic connectivity 

between the upper Potomac aquifer A-, B-, and C-sands underlying the site (fig. 18). Pumping at well OR-6A did not affect 

water levels within the lower Potomac aquifer, as measured 

at the Tidewater-12 well (shown in figure 3). Drawdown 

response in the upper Potomac aquifer was similar to that 

observed during the 1990 aquifer test, however, there were 24 

more observation wells screened within the upper Potomac 

aquifer system in 2010 in addition to the 4 wells (OR-6A, 

OR-6B, MW-11, and MW-12) monitored during both pumping 

periods (Appendix 3).

The majority of Columbia wells showed no response to 

the 72-hour pumping of OR-6A; however, drawdown was 

induced in the surficial (Columbia) aquifer at the northeast 

end of the site near the wetlands and north of Red Lion Creek, 

confirming the absence of effective confining layers in the 

northeast area of the site near the creek (fig. 18). Observed 

drawdown greater than 0.07 ft was attributed to pumping; 

otherwise, changes less than 0.07 ft were attributed to a slow 

decline in unconfined water levels resulting from a lack of 

recent recharge, with only 0.30 in. of precipitation recorded 

during 14 days prior to the pumping period. Over 1 ft of 

drawdown was measured north of the creek, whereas closer 

to the site, drawdown measured less than 0.34 ft even though 

this area was closer to the pumped well (Appendixes 3 and 

4). This supports interpretation from drilling logs that the 

Merchantville confining unit outside the barrier wall thins to 

the northeast and is completely eroded near Red Lion Creek 

and areas to the north of the creek where the greatest drawdown was observed (fig. 18). During the pumping period, 

Figure 17. Time-series comparison of calculated vertical gradients for selected nested well pairs at the Standard Chlorine of 

Delaware, Inc. Superfund Site, 2007–12. 

Downward vertical flow

Upward vertical flow

Hydraulic gradient, dimensionless

0.70

0.65

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

-0.05

-0.10

9/18/2007 9/18/2008 9/15/2009 9/22/2010

Date

9/14/2011 8/8/2012

0.00

Active pumping period Limited pumping Active pumping period

PW-7, MW-11

Ion MW-3, PW-3

CW-4, PW-4D

MW-18, PW-10

CW-11, PW-11

MW-10, PW-13

MW-23, PW-17

OR-6C, PW-6S

MW-37, PW-5S

PW-10, PW-10D

PW-2, PW-2D

PW-11, PW-11D
36 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

10

5

4

3

6

7

8

9

TY-121A

(0.16)

TY-116A

(0.15)

TY-116B

(1.32) TY-116C (3.24)

TY-121B

(0.27)

Road River

Governor Lea

PW

Road

-6S

(28.18) PW-14

(9.24)

PW-11

(5.91)

PW-11D

(0.18)

MW-12

(4.32) PW-10D (5.22) PW-13

(10.29)

PW-10

(6.43)

PW-4D

(5.96)

PW-12

(6.73)

PW-9

(6.44)

PW-2D

(2.44) PW-2

(6.28)

PW-8

(4.83)

PW-3

(3.65)

PW-16

(0.65)

PW-7S

(0.48)

MW-11

(7.36)

CW-11

(0.08)

PW-5D

(3.21)PW-5S

(3.13)

MW-37

(1.18) OR-6A

(43.98) PW-6D

(28.25)

OR-6B

(24.50)

N

9

1

Red Lion Creek Red Lion Creek

STANDARD CHLORINE

OF DELAWARE, INC.

(SCD) SUPERFUND SITE

STANDARD CHLORINE

OF DELAWARE, INC.

(SCD) SUPERFUND SITE

EXPLANATION

Pumping well(Number is water-leveldrawdown, in feet)

Columbia aquifer wells

0.00-0.17 feet0.18-1.00 feet

1.01-3.00 feet

3.01-5.26 feet

upper Potomac-A aquifer wellsupper Potomac-B aquifer wells

0.10-5.00 feet

0.10-5.00 feet

5.01-10.00 feet

10.01-44.00 feet

upper Potomac-C aquifer well

0.10-5.00 feet Barrier wall

Upper Potomac A-sand equipotential contour, in feet(Interval is 1.0 foot; dashed where approximate)

5

Area with thin or

missing confining unit

Area with thin or

missing confining unit

OR-6A

(43.98)

MW-38

(1.76) MW-22

(0.24)

MW-21

(0.17)

MW-23

(0.34)

PW-17

(7.76 )

PZ-29

(5.28)

PZ-12

(0.1)

PZ-16

(0.1)

Former Air

Products, Inc.

Facility

Occidental

Chemical

Facility

1,000

feet from

pumping

well

Imagery from U.S. Geological Survey, Delaware Valley Regional PlanningCommission, and Department of Homeland Security, 2010, NAD 1983State Plane Delaware Transverse Mercator Projection

0 1,000 2,000 FEET0 400 METERS

39°36'20"39°36'10"39°36'00"39°35'50"

75°39'00" 75°38'40" 75°38'20" 75°38'00"

Figure 18. Maximum observed water-level drawdown for wells screened in the Columbia aquifer and upper Potomac aquifer, contours of maximum head change in the upper Potomac A-sand aquifer, and general area where the confining unit is thin or absent. 
Hydrology 37

water levels in the Columbia aquifer did not drop below the 

level of the creek, which supports the conclusion that the creek 

and marsh serve as a discharge area for topographically driven 

flow within the water-table aquifer. In one Columbia aquifer 

well, OR-6C, located 74 ft from pumping well OR-6A, the 

water level increased during the pumping because of water 

being discharged at land surface from well OR-6A.

The absence of a confining layer near the creek also 

indicates that the creek itself may be a source of recharge to 

underlying aquifers, primarily the upper Potomac A-sand and 

by extension implies that a hydraulic connection therefore 

exists between the Columbia aquifer, Red Lion Creek, and the 

upper Potomac A-sand. Drilling logs from wells PW-5D and 

PW-17 further support this conclusion because Merchantville 

and Potomac Formation clays were not present. During the 

pumping of OR-6A, vertically downward flow gradients from 

the Columbia aquifer to the upper Potomac aquifer were 

enhanced at all locations. Red Lion Creek did not show any 

water-level changes in response to pumping at well OR-6A; 

however, water from the pumped well was discharged to the 

marsh and creek, so a decline was not expected. The tide 

control structure was functioning properly during the 72-hour 

period of pumping and fluctuations in creek stage during the 

pumping were less than 0.22 ft.

Surficial aquifer wells within the barrier wall did not 

show responses to the pumping of well OR-6A, except 

for well PZ-29, which is located in an area where the 

Merchantville Formation confining unit is very thin. This 

confining unit was likely penetrated during well installation, 

resulting in cross-screening into the upper Potomac aquifer. 

This well has since been properly abandoned by grouting so 

that it is not a pathway for vertically downward contaminant 

transport. The lack of drawdown in other Columbia aquifer 

wells within the barrier wall demonstrates a poor vertical 

hydraulic connection from the Columbia to the underlying 

upper Potomac aquifer. This is an important finding and 

implies that the barrier wall has been an effective tool for 

containing and limiting contaminated groundwater flow 

laterally towards the wetland and areas where confining units 

are absent. 

Outside the barrier wall, there was some evidence of 

limited hydraulic connection between the Columbia and 

upper Potomac aquifers. Two wells (PW-4S and PW-7S) are 

screened in the upper Potomac top sand aquifer and longterm water-level data show correlation to water-table conditions observed in nearby Columbia aquifer wells. During the 

pumping of well OR-6A, drawdown was not observed at well 

PW-4S, however, drawdown of 0.48 ft was measured at well 

PW-7S, indicating that this sand layer may be connected to the 

upper Potomac A-sand southwest of the SCD site. The extent 

of this connection is unknown but may be better understood 

if additional hydrogeologic and water-level monitoring data 

were to become available from new wells to the west of well 

PW-7S.

In general, the maximum drawdown response in the 

upper Potomac A-sand was proportional to the distance from 

the pumped well; however, maximum drawdown was greater 

in a southward rather than eastward direction from the pumping well, indicating anisotropy, differences in permeability 

and (or) transmissivity, and a better hydraulic connection 

to sand layers to the south of the pumped well. The initial 

drawdown response to pumping supports this conclusion 

because well MW-11 located to the south began to show 

a response sooner than wells PW-4D and MW-12, which 

are located to the southeast at a similar distance (fig. 19). 

Maximum observed drawdown was greater in well PW-10 

(6.43 ft) than in well PW-10D (5.22 ft), which is screened 70 

feet lower than well PW-10 within a secondary sand layer. 

The magnitude of drawdown within this secondary sand layer 

indicates it is part of the upper Potomac A-sand aquifer, and 

not part of the upper Potomac B-sand aquifer, which was less 

responsive to pumping at well OR-6A. Towards the end of the 

72-hour pumping period, drawdown began to decrease (the 

curve flattened) for wells PW-5S, PW-17, and PW-11, which 

indicates a boundary condition, interpreted as recharge from 

Red Lion Creek limiting the magnitude of drawdown. Water 

levels during the pumping dropped below creek stage for all 

wells located south of the creek, but did not drop below creek 

stage at well PW-5S, which is north of the creek. For continuously monitored wells, the time of maximum drawdown 

generally occurred between 1 hour prior to and 3 hours after 

the pumping finished, indicating a good hydraulic connection 

to the pumped well. The exception was well PW-3, where 

maximum drawdown occurred 9 hours after the pumping 

finished, indicating a poor hydraulic connection. A weaker 

drawdown response for wells located near Route 9 (MW-12, 

PW-8, and PW-3) also indicates a poor connection to other 

A-sand wells located to the west (fig. 19; Appendix 3.). This 

may be explained by a greater clay/silt fraction described in 

the lithologic logs for wells closer to Route 9. Wells located 

to the west of well OR-6A near the Tybouts Landfill showed 

drawdown response in four wells screened in upper Potomac 

A-sand (TY-116B, TY-116C, TY-121A, and TY-121B), but no 

response was greater than 3.24 ft, due to their greater distance 

from the pumped well (Appendix 3).

Pumping effects were observed in the upper Potomac 

B- and C-sand layers beneath the pumped interval (A-sand), 

confirming a measurable although limited hydraulic connection. Wells screened in the B-sand responded to pumping 

at well OR-6A, resulting in a drawdown of 2.44 ft at well 

PW-2D, and 0.65 ft at well PW-16. The stronger drawdown 

response at PW-2D indicates a stronger hydraulic connection 

to the A-sand at well PW-2D than at well PW-16. Two wells 

screened in the C-sand were minimally affected by pumping at 

well OR-6A, which resulted in a maximum drawdown of 0.18 

ft at well PW-11D, and 0.15 ft of drawdown at well TY-116A. 

The extent of the connection between the upper Potomac 

A-sand and B- and C-sands is spatially variable and difficult to 

determine due to the limited number of wells screened lower 

in the B- and C-sands. The installation of additional wells 

would aid in understanding the response to pumping stresses 

within these aquifers.
38 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12 Drawdown, in feet

Time after pumping started, in minutes

100.00

10.00

1.00

0.10

0.01

1 10 100 1,000 10,000 Increasing time of initial response

OR-6A (Pumping well)

PW-6S (44 feet South)

OR-6B (95 feet South)

PW-17 (1,288 feet East)

PW-13 (1,440 feet South)

PW-11 (1,865 feet Southeast)

MW-11 (2,724 feet South)

PW-5S (2,050 feet East)

PW-4D (2,654 feet Southeast)

MW-12 (2,513 feet Southeast)

PW-3 (3,570 feet Southeast)

PW-2D (3,039 feet Southeast)

Figure 19. Log/log plot of time drawdown data for selected wells instrumented with continuous water-level 

recorders during 72 hours of pumping by the U.S. Geological Survey at well OR-6A near the Standard Chlorine 

of Delaware, Inc. Superfund Site, August 28–31, 2010 (Explanation shows distance and direction from pumping 

well in parentheses.)
Hydrology 39

Water-Level Recovery

Water-level recovery to 50 percent of pre-pumping levels 

occurred in less than 24 hours for affected Columbia aquifer 

wells. The exact time for full recovery was not documented 

because the affected Columbia wells were not instrumented 

for continuous data collection. The recovery pattern in the 

A-sand was similar in magnitude and time delay to the 

observed drawdown pattern. Water-level recovery of 90 

percent was achieved in upper Potomac A-sand wells within 

12 days, for wells having continuous measurements (fig. 20). 

Recovery at other A-sand wells was similar, but periodic measurements were not frequent enough to permit direct comparison. Wells screened in the B-sand and C-sand showed a slow 

recovery time, indicating a weak connection to the A-sand, 

which is consistent with the small magnitude of observed 

drawdown in response to pumping.

Water-level recovery time generally increased farther 

away from the pumped well, although some differences were 

also noted for wells located a similar distance from well 

OR-6A (fig. 21). Recovery was quicker at well PW-17 than at 

well PW-13, which is likely due to a thin or missing confining 

unit and the proximity to Red Lion Creek as a source of aquifer recharge. A relatively quick recovery was expected at well 

PW-5S; however, water levels did not drop below creek stage 

during pumping at well OR-6A. Rather than being recharged 

from the creek, recovery in this well was likely due to upgradient recharge from a northwest direction, which would occur 

more slowly. Recovery time at well MW-11 was quicker than 

at well MW-12 even though they are similar distances from 

well OR-6A, indicating a good hydraulic connection to the 

south of the site. A relatively fast recovery time at well PW-13 

indicates that more transmissive sands are located in the 

Figure 20. Percent recovery of water levels over elapsed time for selected continuously monitored upper Potomac aquifer 

wells after conclusion of 72 hours of pumping by the U.S. Geological Survey at well OR-6A, August 31, 2010. (Distance from 

pumping well shown in parentheses.) Percent recovery

Time, in days

100

90

80

70

60

50

40

30

20

10

0

0 1 2 3 4 5 6 7 8 9 10 11 12

90-percent recovery

OR-6A (Pumping well)

PW-6S (44 feet)

PW-17 (1,288 feet)

PW-13 (1,440 feet)

PW-11 (1,865 feet)

MW-11 (2,724 feet)

PW-5S (2,050 feet)

PW-4D (2,654 feet)

MW-12 (2,513 feet)

PW-3 (3,570 feet)

PW-2D (3,039 feet)
40 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

southwest area of the site, where lithology at depth is not as 

well characterized as other areas of the site that have multiple 

nested monitoring wells (fig. 3; table 3). The slow recovery 

time at well PW-3 supports the conclusion that this well is 

poorly connected to the upper Potomac A-sand and is likely 

perched. 

During the 72-hour period, pumping in the upper 

Potomac A-sand caused vertically downward gradients to 

increase from the Columbia aquifer to the upper Potomac 

A-sand aquifer, as shown for several well pairs by comparing the increase from typical gradients to those that occurred 

during the pumping of well OR-6A: MW-23/PW-17 (from 

0.02 to 0.11), CW-11/PW-11 (from 0.05 to 0.09), MW-10/

PW-13 (from 0.09 to 0.15), CW-4/PW-4D (from 0.19 to 0.24), 

and PW-7/MW-11 (from 0.27 to 0.39). After the pumping of 

well OR-6A was complete, vertically downward gradients 

decreased to nearly the same magnitude calculated prior to 

beginning the pumping. A large recharge event approximately 

1 month after completion of the pumping of well OR-6A 

raised A-sand water levels higher than pre-pumping levels, 

which can be seen in the long-term hydrograph (fig. 9).

OR-6A

(1.69)

PW-6S

(2.88)

PW-5S

(4.99)

PW-17

(3.33) 1,000 feet from

pumping well

PW-11D

(15.8)

PW-11

(4.31)

PW-4D

(6.05)

PW-2D

(12.3)

PW-3

(11.32)

MW-11

(5.46)

MW-12

(6.36)

PW-13

(4.13)

EXPLANATION

Pumping well

(Number is elapsed

time, in days)

upper Potomac A-sand

Days to 90-percent recovery

3.01-6.00 days

1.69-3.00 days

6.01-9.00 days

9.01-12.00 days

Days to 90-percent recovery

Days to 90-percent recovery

upper Potomac B-sand

12.30 days

upper Potomac C-sand

15.80 days

Barrier wall

OR-6A

(1.69)

N

Governor Lea Road

River Road

9

Red Lion Creek

Imagery from U.S. Geological Survey, Delaware Valley Regional Planning

Commission, and Department of Homeland Security, 2010, NAD 1983

State Plane Delaware Transverse Mercator Projection

0 1,000 FEET

0 200 METERS

39°36'20"

39°36'10"

39°36'00"

39°35'50"

75°38'20" 75°38'00"

Figure 21. Elapsed time in days to reach 90-percent water-level recovery for selected continuously monitored 

wells screened in the upper Potomac aquifer, in response to conclusion of 72 hours of pumping by the U.S. 

Geological Survey at well OR-6A, August 31, 2010.
Hydrology 41

Water Withdrawal Influences on the Local Flow 

System

The influence of groundwater withdrawals on the confined aquifer flow system is well documented and primarily 

attributed to pumping by industrial production wells. Head 

declines on the order of tens of feet have been measured in 

a cone of depression that extends radially from the DCIA. 

Changes in vertical and lateral flow gradients induced by 

aggregate pumping in the confined aquifer strongly control 

the flow system near SCD and may influence the potential 

movement of site-related contaminants. Pumping of watertable wells is limited, and related mostly to pump and treat 

extraction systems that have site-specific localized effects on 

the water table and little to no effect on the confined aquifer 

system. 

Water Withdrawal Rates

The surficial aquifer in the Columbia Formation 

(Columbia aquifer) is infrequently used for small domestic 

water supplies and has historically been pumped at times for 

contaminant recovery. Five shallow recovery wells at the SCD 

site were pumped from 1992 through 2001 as part of mitigation efforts related to the 1986 spill. There are approximately 

five domestic wells located 1.5 mi to the north and northwest 

of the SCD site (Black & Veatch, 2007), with typical usage of 

less than 10,000 gal/d. These wells do not affect the shallow 

groundwater-flow system at SCD. In contrast, the current SCD 

shallow groundwater pump and treat system typically extracts 

50,000 gal/d, causing drawdown of the water table within the 

barrier wall. No pumping effects have been observed in watertable wells outside the barrier wall, except during the USGS 

72-hour pumping of well OR-6A in 2010.

Groundwater in the confined aquifers is primarily used 

for industrial purposes in the DCIA, with some withdrawal 

for public drinking-water supply. A series of exploratory 

wells were drilled to bedrock basement as part of a water 

supply study commissioned by the Tide Water Associated Oil 

Company (Leggette & Brashears, 1955) for estimating potential industrial use. Water levels for all Potomac aquifer layers 

in the area prior to 1955 were typically close to sea level, 

even within the deeper sands of the lower Potomac aquifer as 

measured at the Tidewater-12 well (Dc53-07). Industrial withdrawals began in 1955, sharply increased to approximately 

3.5 Mgal/d in 1957, and reached 4.0 Mgal/d by 1980 (Martin 

and Denver, 1982). The typical total annual withdrawal from 

groundwater wells within 3 mi of SCD is approximately 

6.7 Mgal/d. The largest user, DCRC, operates a network 

of withdrawal wells having a combined permitted usage of 

5.5 Mgal/d. Production has ranged from 4 to 5 Mgal/d over 

the past 30 years, with annual production over the last 10 

years averaging approximately 4 Mgal/d, except during the 

2010 refinery shutdown, when it was 1.5 Mgal/d (Delaware 

City Refining Company, 2005–2012). Local drinking-water 

withdrawals from groundwater collectively amount to approximately 2.7 Mgal/d. 

Surface water is also used for industrial processes by 

DCRC, but withdrawals have negligible effects on groundwater resources. The use of surface water is less desirable 

because it requires more treatment due to a higher mineral 

content than local groundwater. Sources of surface-water 

supply in decreasing order of average daily use during 2012 

included: the Delaware River (358 Mgal/d), Dragon Run 

(1.34 Mgal/d), and Red Lion Creek (0.23 Mgal/d). The Red 

Lion Creek intake is located one-half mile upstream from 

SCD, and is the only surface intake that may potentially affect 

surface and groundwater interaction at the site. Given that 

the average daily total surface-water discharge near SCD is 

approximately 6.60 Mgal/d, flow in Red Lion Creek is only 

reduced on average by 3.5 percent when the intake is operational. Prior to July 2009, the Red Lion Creek intake was idle 

for a period of 9 years (Delaware City Refining Company, 

2005–2012). The intake was used from July–December 2009, 

but then idled again until August 2011. The relative effect of 

upstream withdrawals on creek stage is minimal compared to 

the effects from tidal fluctuation due to the broken tide control 

structure.

Water-Level Changes Caused by Withdrawals

The relative effects of pumping wells screened in various 

Potomac aquifer layers on groundwater flow at the site were 

evaluated based on corresponding changes in water levels in 

monitoring network wells. The depth of the screened interval 

and changes in the withdrawal rate are more important than 

pumping well location relative to the site for understanding the 

effects on groundwater flow (table 3; fig. 2). Refinery production wells are periodically serviced to maintain yield, which 

requires temporary idling, removal of the pump, and re-development of the screened interval. Monitoring well responses 

during these multi-week-long service periods provided additional insight on how the local flow system operates. Pumping 

effects were identified in late 2007 and early 2008 (fig. 9), 

when observed recovery-drawdown curves for upper Potomac 

aquifer wells coincided with a servicing period for production 

well R-15 (discussed in the next section).

Water-level declines up to 200 ft below sea level in parts 

of the Potomac aquifer in New Castle County, Delaware 

have been documented since withdrawals began in the 1950s 

(Martin, 1984). Most of the decline near SCD is due to 

long-term sustained use by the DCRC. Lower head values in 

the Potomac aquifer relative to the water-table aquifer have 

enhanced a vertically downward flow gradient. A fairly constant aggregate groundwater use by the refinery over time has 

led to a steady state “pumping” condition for measured water 

levels within the upper, middle, and lower Potomac aquifers. 

This status was interrupted when most refinery production 

wells were idled during an ownership transition period from 

May 2010 to January 2011. During this time, selected wells 

were occasionally pumped to ensure that the pumps remained 
42 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

in good working condition, with only very limited withdrawals 

to support industrial plant maintenance (Rebecca Gudgeon, 

Delawere City Refining Company, oral commun., 2011). 

The ownership transition period occurred within the “aquifer 

recovery and limited pumping period” from November 2009 

to October 2011. A widespread water-level recovery was 

observed during this period in local monitoring wells near the 

SCD site and in Potomac aquifer monitoring wells that are 

part of the regional DGS monitoring network (Tom McKenna, 

Delaware Geological Survey, oral commun., 2012).

The altitude of water levels in the lower Potomac aquifer 

near the site (Tidewater-12 monitoring well [Dc53-07]), typically range from -50 to -120 ft below sea level and show the 

aggregate long-term withdrawal effects from multiple deep 

withdrawal wells in the lower Potomac aquifer (P-6A, P-9A, 

P-10A, P-16A, and to a much lesser extent, PW-4, PW-5, 

and ASR-1 due to their relatively low pumping rates) (fig. 2). 

Short-term changes in water levels at well Tidewater-12 often 

correlate to rate changes at production well P-6A, located 

nearby (fig. 3). Production wells P-6A (lower Potomac) and 

P-5B (middle Potomac) are located close to the SCD site but 

an analysis of hydrographs showed that changes in withdrawal rate have little to no effect on water levels at monitoring wells at SCD, which are screened in the upper Potomac 

aquifer (fig. 22). Withdrawal rates at production well R-15 

(upper Potomac), however, do affect water levels in the upper 

Potomac aquifer at the site and are discussed further in the 

next section.

Industrial groundwater withdrawal wells located closest 

to SCD (R-15, P-5B, and P-6A) (fig. 2) typically pump a combined 1 Mgal/d during normal operation. These wells were 

operated in very limited capacity (pumping less than 100,000 

gal/d) from December 2009 to May 2010, and pumping was 

negligible while the refinery was idled from May 2010 to 

January 2011. A groundwater recovery was observed during 

this period, with a corresponding reduction in the magnitude 

of the vertically downward gradients, as previously described.

Analysis of hydrographs and pumping rates shows that 

other nearby production wells had little to no effect on water 

levels in the upper Potomac A-sand at the site. Production 

wells P-3B and P-4B are located west of the SCD site (fig. 2) 

and have little if any effect on water levels in the upper 

Potomac at SCD due to their relatively low combined pumping rates (less than 0.25 Mgal/d), and the depth of the screened 

interval (middle Potomac). Wells PW-4 and PW-5 (fig. 2) are 

screened in the lower Potomac aquifer and have a combined 

withdrawal of 0.13 Mgal/d that is consistent over the study 

period. These wells likely do not affect water levels in the 

Altitude of water level for wells MW-11 and MW-12,

in feet relative to National Geodetic

Vertical Datum of 1929

Monthly average pumping rate,

in million gallons per day

Altitude of water level for well Tidewater-12,

in feet relative to National Geodetic

Vertical Datum of 1929

4.0

3.0

2.0

1.0

0.0

-1.0

-2.0

-3.0

-4.0

-5.0

-6.0

-7.0

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1/1/07 1/1/08

Date

1/1/09 1/1/10 1/1/11 1/1/12 1/1/13

-140

80

60

40

20

0

-20

-40

-60

-80

-100

-120

MW-11 (upper Potomac A-sand: -74 to -84 feet)

MW-12 (upper Potomac A-sand: -76 to -86 feet)

Tidewater-12 (lower Potomac: -479 to -484 feet)

A

B

R-15 (upper Potomac: -232 to -263 feet)

P-5B (middle Potomac: -331 to -503 feet)

P-6A (lower Potomac: -634 to -664 feet)

Figure 22. (A) Hydrograph showing water-level response to changes in (B) pumping rate for production wells screened in the upper 

(R-15), middle (P-5B), and lower (P-6A) Potomac aquifers near the Standard Chlorine of Delaware, Inc. Superfund Site. (Aquifer name 

and altitude of screened interval in parentheses.)
Hydrology 43

upper Potomac at SCD due to their downgradient location and 

the depth of the screened interval. Well ASR-1 is an aquifer 

storage and recovery well, where water input and output is 

fairly equal, although some storage loss to the lower Potomac 

aquifer likely occurs. Wells AV-1, AV-2, AV-3, and AV-4 are 

screened in the upper Potomac aquifer but do not appear to 

affect SCD water levels, even though the pumping rate of 

1.4 Mgal/d at AV-1 is greater than the pumping rate at any of 

the refinery production wells (fig. 2). Their lack of influence 

on the flow system is likely due to their upgradient location 

within the regional flow system. AV wells have been pumped 

at fairly constant rates over time, and a review of the pumping history since 2001 did not reveal any notable period of 

influence on SCD hydrographs, even when selected wells were 

shut down for servicing. In contrast, water-level changes were 

not observed at AV observation wells during the USGS pumping of well OR-6A in August 2010, or during the shutdown of 

the refinery wells during 2010. The AV well pumping network 

is likely hydraulically disconnected from upper Potomac aquifer wells located south of Red Lion Creek, further indicating 

that the creek serves as a flow boundary for the upper Potomac 

aquifer.

Production Well R-15

Evaluation of water levels in wells screened in the A-, B-, 

and C-sand aquifers of the upper Potomac Formation near the 

SCD site has shown that hydraulic heads in the upper Potomac 

aquifer system are significantly affected by the operating 

schedule of refinery production well R-15, located south of 

the site (fig. 4a) and screened within the C-sand aquifer of 

the upper Potomac Formation (figs. 4b and 23). Correlation 

between head changes and pumping was first observed in 

2007 and early 2008 during two distinct periods of time when 

the well was idled for servicing. A longer period of shutdown 

began on November 11, 2009, and data from USGS monitoring wells showed aquifer recovery within the A- and C-sands 

in the upper Potomac aquifer (fig. 23) (B-sand was not 

monitored during recovery). Complete recovery was achieved 

approximately 6 months after pumping in the C-sand stopped. 

In contrast, 90-percent recovery was achieved in the A-sand 

14 days after USGS pumping at well OR-6A.

To further assess the effects of pumping well R-15, an 

aquifer pumping step test was designed for the well during the 

restarting period, late in 2011. Approximately equal changes 

in rate (steps) were made every 2 weeks, beginning with 240 

Figure 23. Hydrograph showing upper Potomac aquifer recovery and drawdown for A-, B-, and C-sand intervals in response to 

production well R-15 operating schedule. Altitude of water level, in feet relative to National Geodetic 

Vertical Datum of 1929

5

0

-5

-10

-15

-20

-25

-30

MW-11: upper Potomac A-sand (-74 to -84)

Well number, aquifer, and altitude of

screened interval, in feet

PW-11: upper Potomac A-sand (-115 to -120)

PW-16: upper Potomac B-sand (-172 to -177)

PW-2D: upper Potomac B-sand (-179 to -184)

PW-11D: upper Potomac C-sand (-215 to -220)

-35

-40

-45

-50

-55

-60

7/1/09 1/1/10

Production well R-15

idled, 11/11/09

U.S. Geological Survey

72-hour pumping period

at well OR-6A

Production well R-15

restarted, 10/26/11

7/1/10

Date

1/1/11 7/1/11 1/1/12 7/1/12 1/1/13
44 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

gal/min on October 26, 2011 until reaching 350 gal/min on 

December 7, 2011. The pumping rate was limited (350 

gal/min) by DNREC, in part due to the known effects of well 

R-15 on upper Potomac water levels. A change in hydrograph 

slope corresponding to each rate change during the restart 

period was observed for B- and C-sand monitoring wells, 

indicating a good hydraulic connection with well R-15, which 

is screened in the C-sand. A change in the slope of the waterlevel drawdown curve was not observed for A-sand wells, but 

water levels in all A-sand wells showed a decline in response 

to pumping at well R-15. Drawdown was not observed in 

continuously monitored Columbia aquifer wells (CW-11 

and Ion MW-3). Similar to the recovery time period, waterlevel drawdown was completed in approximately 6 months, 

and represents a new steady state flow condition within and 

between the upper Potomac sand layers under pumping influence. The magnitude of drawdown response measured in the 

observation wells indicates the extent of hydraulic connection 

to the pumped interval (C-sand). C-sand well PW-11D showed 

a clear response, as did B-sand wells (PW-16 and PW-2D). 

The response was greater at well PW-16 than at well PW-2D, 

which may reflect the relative distance to well R-15, and may 

also be indicative of differences in hydraulic properties. This 

is counter to what was observed during the USGS pumping 

of well OR-6A, when well PW-2D showed a greater response 

to pumping in the A-sand than well PW-16, indicating that 

PW-2D is better connected to the A-sand. Another indication 

of the connection between B-sand and C-sand wells and well 

R-15 was the short-term response from a 1-week emergency 

shutdown of R-15 that occurred on August 8, 2012, which can 

be seen on the hydrograph in fgure 23. A small rise is evident 

at A-sand well MW-11.

Generally, drawdown response in the A-sand was similar 

to the B-sand in that wells located farthest from the pumping 

center showed the smallest drawdown (well PW-5 in fg. 9). 

The cone of depression steepens closer to the pumping center, 

with large declines measured near the south end of the site 

(wells MW-11, PW-4D, MW-12, and PW-13), whereas wells 

located closer to the creek (PW-5 and PW-17) showed less 

decline (fig. 9), and did not drop below creek stage. Water 

levels in wells near the creek have remained higher than 

creek stage since pumping at well R-15 resumed in October 

2011, which is similar to the pumping scenario in 2007–09; 

however, water levels in wells to the south have dropped 

below creek stage and the flow direction has shifted toward 

the south. Water-level declines with well R-15 pumping at 

a steady rate (350 gal/min) have not dropped below levels 

previously observed in 2009 (fig. 9). At well MW-11, the 

previous maximum low was approximately -3.0 ft, compared 

to -1.75 ft, which was observed more recently. Although water 

levels in wells adjacent to the creek are currently higher than 

creek stage, potential recharge is still possible because water 

levels at some wells are lower than creek stage. Potential 

recharge from the creek/marsh may be mitigated by restoring 

creek stage to levels observed before the tide gate malfunction 

(approximately -1.75 ft), which is similar to the lowest current 

water level observed in any upper Potomac aquifer well. 

Refined Local Conceptual Model of 

Groundwater Flow

The identification of areas of the SCD site where confining units are absent in conjunction with hydrologic evidence 

of aquifer interconnection has led to refinement of the local 

conceptual model of groundwater flow at the SCD site. The 

Merchantville confining unit was initially thought to be a 

nearly continuous low permeability layer that limited contamination from migrating into deeper aquifers at the site. This 

conclusion was partially drawn from the lack of water-level 

response in Columbia wells to an aquifer test conducted in 

1990 (Roy F. Weston, Inc., 1992); however, recent detection of 

benzene and chlorobenzene compounds in sand layers of the 

upper Potomac aquifer indicates that the clay is discontinuous 

and drilling and geophysical data indicate that holes may have 

been eroded through the clay by paleochannels prior to and 

during the deposition of the Columbia Formation 

(U.S. Environmental Protection Agency, 2004, 2006). Erosion 

of the upper Potomac clay also occurred, mostly in the vicinity 

of Red Lion Creek, which created a focused area of aquifer 

connection between the Columbia and upper Potomac aquifers. This area is of particular concern at the site because of 

current and potential future contaminant movement downward 

into the upper Potomac aquifer.

Description of Components of Conceptual Model

Major features that influence groundwater-flow direction 

and potential contaminant transport at the SCD site are presented as a refined conceptual model of groundwater flow 

(fig. 24). This conceptual model represents groundwater 

flow during the prevailing condition of steady groundwater 

withdrawal (pumping) from the upper Potomac C-sand layer 

at production well R-15 (fig. 2). Groundwater flow within the 

surficial aquifer is generally from topographic high points 

towards and discharging to Red Lion Creek, although some 

water recharges the underlying upper Potomac A-sand. In turn, 

water from the A-sand recharges the B- and C-sand layers, 

although the connection is not as direct. Flow direction within 

the A- and B-sands is influenced by pumping and is generally 

from north to south, differing from the regional flow direction 

from northwest to southeast. Flow direction within the C-sand 

is also believed to be from north to south, but cannot be determined from a single well screened within the C-sand. The erosion of confining layers near Red Lion Creek and the resulting 

extent of aquifer connection near the creek has been demonstrated by looking at various hydraulic stresses on the aquifer 

system. The dynamics of groundwater flow in the vicinity of 

Red Lion Creek are complicated by local pumping effects, 

and by the inter-aquifer connections created by the paleo-Red 

Lion Creek erosional channel. Recharge to the A-sand is likely 

enhanced by pumping at production well R-15, but discharge 

from the A-sand to the creek likely occurs during periods of 

limited pumping when water levels in the A-sand are higher 

than creek stage.
Hydrology 45

Implications for Groundwater Contaminant 

Movement

The connections between aquifers that were identified 

have major implications for the long-term movement of 

contaminants in groundwater near the SCD site. Contaminants 

detected within the upper Potomac A-sand at several locations 

have the potential for further movement, depending on the prevailing groundwater-flow direction and head magnitude within 

the upper Potomac sand layers. The presence of benzene and 

various chlorobenzene compounds in A-sand wells at the 

north end of the site (wells PW-14 and PW-17) is attributed to 

vertically downward transport from overlying contaminated 

marsh sediments and contaminated Columbia aquifer water. 

This vertical mixing is due, in part, to strong vertically downward gradients induced by local pumping combined with the 

absence of confining units in this area. At other A-sand well 

locations near the SCD site to the east (wells PW-4D, PW-9, 

and PW-12) and to the west (well PW-13), an effective confining unit (upper Potomac clay) was identified during drilling; 

however, contaminants were detected below this layer. Their 

presence is likely a result of lateral groundwater advective 

transport from other areas to the north where confining units 

are absent, but may also be attributed to movement of DNAPL 

downward through microfractures within the confining units. 

Contaminants have not been detected in the upper Potomac Band C-sands, indicating that the hydraulic connection from the 

A-sand to the B- and C-sands is weaker than the connection 

between the Columbia aquifer and the upper Potomac A-sand. 

The relation between creek stage and water levels in the 

upper Potomac aquifer is important for understanding potential contaminant transport from the creek/marsh to underlying 

aquifers. The marsh area is a nexus for aquifer mixing where 

shallow contaminated Columbia aquifer groundwater discharges to surface water, but also recharges the subcropping 

upper Potomac aquifer in areas where confining units have 

A A'

(North) (South)

FEET FEET

Sea

level

Sea

level

-50

PW-5D PW-17

PW-11

PW-11D

PW-10D PW-12

PW-2D PW-16 PW-13

50

-50

50

-100 -100

-150 -150

-200 -200

-250 -250

VERTICAL EXAGGERATION = 4.9X 0 500 1,000 FEET

0 125 250 METERS

EXPLANATION

Confining unit

Confining unit ?

Confining unit

Confining unit

Merchantville

confining unit

Land surface

Paleo-Red Lion Creek

erosional channel

Red Lion

Creek

Columbia aquifer

upper Potomac aquifer (A-sand)

Top

sand

upper Potomac aquifer (B-sand)

upper Potomac aquifer (C-sand)

Standard Chlorine

of Delaware, Inc.

Superfund Site

upper Potomac

aquifer (A-sand)

A-sand

Generalized groundwater-flow direction in the Columbia aquifer

Generalized groundwater-flow direction in the upper Potomac aquifer Well location,

screened interval,

and identifier Geologic contact; dashed where approximate

Recharge

Water table

Organics

Organics/silt

Sand

PW-17

Figure 24. Cross section A-A’ showing refined conceptual model of local groundwater flow, including Red Lion Creek, Columbia 

aquifer, upper Potomac A-, B-, and C-sand aquifers, selected upper Potomac aquifer wells, and erosional channel of paleo-Red 

Lion Creek.
46 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

been eroded (such as near well PW-17, which showed contamination during vertical profiling from the water table to 100 ft 

below land surface). Recharge is driven by higher water levels 

in the creek relative to the aquifer, thus, the ability to regulate 

creek stage changes due to tidal cycling may help to limit the 

dynamics of both physical and chemical mixing in the marsh 

area. In particular, a sustained lower creek stage would reduce 

the potential aquifer recharge of water (and contaminants). 

Short-term variability in upper Potomac water levels can also 

be reduced by controlling tidal influences to Red Lion Creek 

using fully functional tide gates. Changes in aquifer water levels due to increases in creek stage resulting from precipitation 

runoff are typically of short duration (1-2 days), lasting until 

the water is discharged to the Delaware River.

Considerable control over the flow system at the SCD site 

may be achieved by carefully managing the local groundwater 

pumping regime within the upper Potomac aquifer, particularly at production well R-15. Sustained pumping has resulted 

in decreased water levels in the upper Potomac aquifer and 

vertically downward gradients throughout the site, including 

inside the barrier wall where the Columbia aquifer pump and 

treat system must maintain a positively upward gradient (with 

respect to the upper Potomac aquifer) for effective contaminant recovery. Groundwater-flow direction within the upper 

Potomac aquifer is also largely controlled by pumping at 

well R-15, with a southward flow toward the production well 

when operating. Potential long-term contaminant transport in 

this direction is a concern and contamination detected south 

of contaminated wetland areas indicates that lateral transport 

is an issue at the site. Further delineation of the extent of 

contamination to the southwest of the site (and south of well 

PW-13) is needed, and is in keeping with the recommendations from the 1995 Record of Decision (U.S. Environmental 

Protection Agency, 1995). In addition, a characterization of 

lithology to a depth of 300 ft in this area would be needed to 

confirm the suspected presence of thick sands (sand channels) 

inferred from the water-level responses during pumping of 

well OR-6A. These higher hydraulic conductivity zones may 

serve as preferential pathways for contaminant transport, and 

their presence combined with pumping influence will factor 

into the design of any remedy for addressing deeper groundwater (EPA Operable Unit 4). 

Groundwater modeling may prove useful for testing various pumping scenarios within the upper Potomac aquifer to 

determine the effects on flow direction and changes in leakage 

(recharge) due to enhanced vertical gradients, which are up to 

two to three times greater during pumping periods (fig. 17). 

Future monitoring of changes in vertical gradients may be 

necessary to evaluate the sensitivity of the flow system to such 

changes, and to predict potential lateral migration of contaminants resulting from periodic decreases in vertical gradients. 

Groundwater modeling would also provide insight on the 

complex recharge mechanisms for this Coastal Plain setting 

located near the Fall Line.

Summary and Conclusions

The U.S. Geological Survey (USGS), in partnership 

with the U.S. Environmental Protection Agency, Region 3, 

and the Delaware Department of Natural Resources and 

Environmental Control is continuing a multi-year investigation 

of the hydrogeologic framework and hydrology of the confined aquifer system near the Standard Chlorine of Delaware, 

Inc. (SCD) Superfund Site. The goals of the ongoing study at 

the site are to determine the hydraulic connection between the 

Columbia and Potomac aquifers, determine the direction and 

rate of groundwater flow in the Potomac aquifer, and identify 

factors affecting the fate of contaminated groundwater. 

Geologic and hydrologic investigation of the upper 

Potomac Formation near the SCD site from 2005–12 has 

resulted in a revised site hydrogeologic framework, subdividing the upper Potomac aquifer into three continuous sand 

layers (A-sand, B-sand, and C-sand), based on lithology 

and hydraulic responses to precipitation, tidal effects, creek 

stage changes, and aquifer pumping. A discontinuous sand 

layer also was identified at the top of the upper Potomac 

Formation. Interpretations of the flow system in the upper 

Potomac A-sand were made based on water-level data from 

a monitoring well network that was installed because chlorobenzene contaminants had been previously detected at the site, 

typically at an altitude of -85 feet (ft) at multiple locations. 

Drilling to an altitude of -220 ft at non-contaminated locations 

provided additional lithologic information to help classify 

sediments within a complex fluvial environment that includes 

channel sands ranging in thickness from 10 to 70 ft that may 

function as preferential groundwater flow paths affecting 

contaminant transport, and multiple confining units within the 

upper Potomac Formation ranging in thickness from 20 to 60 

ft that may function as effective barriers to contaminant movement. The Merchantville Formation clay was not shown to be 

an effective confining unit in some areas based on the similarity of water-level responses in the upper Potomac top sand 

aquifer and the water table. Areas under Red Lion Creek to the 

northeast of the site show thick sand sequences, an absence of 

confining units, and a direct hydraulic connection to overlying 

contaminated sediments. The USGS evaluation of water-level 

responses in wells during pumping of well OR-6A in August 

2010 confirmed the absence of effective confining units near 

Red Lion Creek, and also demonstrated that shallow groundwater within the barrier wall is mostly isolated from external 

stresses. Drawdown and recovery responses within the upper 

Potomac aquifer to the south and west of the site indicate a 

preferential hydraulic connection in this area. 

Efforts to further refine site hydrogeology using well 

borings paired with geophysical surveys are ongoing. 

Additional borings to approximately 300 ft below land surface targeting the Potomac Formation would enable a more 

complete geologic characterization in the vicinity of the SCD 

site and would also enable further contaminant delineation. 
References Cited 47

These efforts would also help to classify (locally) the depositional environments of Potomac Formation sediments into 

various facies, but more importantly, they would help identify 

paleochannel features formed by amalgamated sands (amalgamated channels) and thick sands (isolated channels), which 

collectively may form areas of preferential contaminant transport at and near the site.

A variety of hydraulic stresses on the local flow system 

at the SCD site were identified and evaluated in terms of their 

relative effect on groundwater flow. Recharge from precipitation to the Columbia and upper Potomac aquifers occurs 

locally, but water-level variations show greater response to 

stage loading from Red Lion Creek than to recharge from 

precipitation, except during large rainfall events. Water levels 

in the Columbia aquifer varied less than 2 ft during the study, 

whereas water levels varied up to 6 ft in the upper Potomac 

aquifer because of pumping. The tide gate malfunction on 

August 28, 2011 raised the base stage of Red Lion Creek and 

resulted in a fourfold increase in daily water-level variation 

within the upper Potomac aquifer. Red Lion Creek and 

fringing marshes receive discharging groundwater from the 

surficial aquifer and from the underlying semi-confined upper 

Potomac aquifer when industrial withdrawals are limited, and 

water levels are higher than the creek. When major groundwater withdrawals are occurring, water levels in several A-sand 

wells drop below Red Lion Creek stage, and aquifer recharge 

from Red Lion Creek may be possible. Groundwater withdrawals also increase vertically downward gradients from 

the Columbia aquifer to underlying sand layers in the upper 

Potomac aquifer, and between upper Potomac sand layers. 

Vertical gradients ranged from slightly upward (0.02) from 

the A-sand to the Columbia aquifer near Red Lion Creek to 

strongly downward from the A-sand to C-sand (0.65, during 

pumping periods).

Continued monitoring of withdrawal rates for nearby 

production wells, in particular well R-15, will assist in understanding site-related water-level responses and improve future 

remedial design strategies for deep groundwater. Future data 

collection and analyses that would yield a more refined understanding of the hydrologic system at the site include: exploratory drilling to a depth of 300 ft, particularly southwest of the 

site; further analysis of existing surface geophysical data in the 

context of new lithologic information; and instrumentation of 

new wells to measure and understand water-level responses 

in multiple sand intervals of the upper Potomac aquifer. The 

understanding of the hydrogeologic system and potential 

contaminant pathways in the vicinity of the SCD site would 

be further enhanced with development of a groundwater-flow 

model that incorporates a revised understanding of site stratigraphy and local pumping conditions. This model could be used 

to predict groundwater traveltimes, assess Red Lion Creek as a 

flow boundary, and be used to evaluate appropriate withdrawal 

rates for nearby production wells to align industrial pumping 

with site remedial goals.

Acknowledgments

The authors wish to thank Hilary Thornton, Bernice 

Pasquini, Kathy Davies, and Brad White, U.S. Environmental 

Protection Agency (EPA) Region III for logistical support, 

data, and explanations of site history and hydrogeology. 

Todd Keyser, Delaware Department of Natural Resources 

and Environmental Control (DNREC), and Chris Wolfe, 

HydroGeoLogic, Inc., were essential in conducting this 

investigation because of their site-related experience. Data 

collection would not have been possible without field assistance from Jessica Teunis, U.S. Geological Survey (USGS) 

Maryland-Delaware-D.C. Water Science Center. Site visits and 

discussions of regional and nearby site hydrology and geology 

with Doug Rambo (DNREC), Tom McKenna, Delaware 

Geological Survey, John Jengo from MWH (engineering firm), 

and Stephen Zahniser, URS Corporation (engineering firm) 

helped conceptualize the hydrogeologic system.

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Martin, M.M., and Denver, J.M., 1982, Hydrologic data for 

the Potomac Formation in New Castle County, Delaware: 

U.S. Geological Survey Open-File Report 81–916, 148 p.

McKenna, T.E., McLaughlin, P.P., and Benson, R.N., 2004, 

Characterization of the Potomac aquifer, an extremely 

heterogeneous fluvial system in the Atlantic Coastal Plain of 

Delaware: Delaware Geological Survey Open-File Report 

45, 1 p., 3 sheets, accessed August 29, 2014, at http://www.

dgs.udel.edu/publications/ofr45-characterization-potomacaquifer-extremely-heterogeneous-fluvial-system-atlantic-.

Mickowski, R.G., 1986, Tidegate Study for New Castle 

County, Delaware: New Castle Conservation District, 

Newark, Del., 94 p. 

National Oceanic and Atmospheric Administration, 2004, 

Climatography of the United States No. 20, 1971–2000, for 

station Wilmington New Castle AP, Del., accessed January 

15, 2013, at http://hurricane.ncdc.noaa.gov/climatenormals/

clim20/de/079595.pdf.

Office of Delaware State Climatologist, 2013, Delaware 

climate information, accessed January 15, 2013, at http://

climate.udel.edu/delawares-climate.

Phillips, S.W., 1987, Hydrogeology, degradation of groundwater quality, and simulation of infiltration from the Delaware 

River into the Potomac aquifers, Northern Delaware: U.S. 

Geological Survey Water-Resources Investigations Report 

87–4185, 86 p.
References Cited 49

Ramsey, K.W., 2005, Geologic map of New Castle County, 

Delaware: Delaware Geological Survey, Geologic 

Map No. 13, 1 pl., scale 1:100,000, accessed August 

29, 2014, at http://www.dgs.udel.edu/publications/

gm13-geologic-map-new-castle-county-delaware.

Roy F. Weston, Inc., 1992, Remedial investigation report for 

the Standard Chlorine of Delaware, Inc. Site, Delaware 

City, Delaware: v. 1, [variously paged], accessed September 

16, 2014, at http://loggerhead.epa.gov/arweb/public/

pdf/100721.pdf.

Shedlock, R.J., Bolton, D.W., Cleaves, E.T., Gerhart, J.M., 

and Nardi, M.R., 2007, A science plan for a comprehensive 

regional assessment of the Atlantic Coastal Plain aquifer 

system in Maryland: U.S. Geological Survey Open-File 

Report 2007–1205, 25 p., accessed August 29, 2014, at 

http://pubs.usgs.gov/of/2007/1205/.

Sugarman, P.J., Miller, K.G., Browning, J.V., Kulpecz, A.A., 

McLaughlin, P.P., Jr., and, Monteverde, D.H., 2005, 

Hydrostratigraphy of the New Jersey Coastal Plain: 

Sequences and facies predict continuity of aquifers and confining units: Stratigraphy, v. 2, no. 3, p. 259–275, accessed 

August 29, 2014, at http://rockbox.rutgers.edu/kgmpdf/05-

Sugarman.Strat.PDF.

Trapp, Henry, Jr., and Meisler, Harold, 1992, The regional 

aquifer system underlying the Northern Atlantic Coastal 

Plain in parts of North Carolina, Virginia, Maryland, 

Delaware, New Jersey, and New York–Summary: U.S. 

Geological Survey Professional Paper 1404-A, 33 p., 11 

pls., accessed August 29, 2014, at http://pubs.er.usgs.gov/

publication/pp1404A.

U.S. Census Bureau, 2013, State and county quick facts, New 

Castle County, Delaware, accessed January 18, 2013, at 

http://quickfacts.census.gov/qfd/states/10/10003.html.

U.S. Environmental Protection Agency, 1995, EPA Superfund 

Record of Decision: Standard Chlorine of Delaware, Inc., 

March 1995, [variously paged], accessed September 16, 

2014, at http://www.epa.gov/superfund/sites/rods/fulltext/

r0395193.pdf.

U.S. Environmental Protection Agency, 2004, U.S. 

Environmental Protection Agency Fact Sheet #14 Standard 

Chlorine of Delaware (aka Metachem) Site, accessed 

October 2, 2009, at http://www.epa.gov/reg3hwmd/npl/

DED041212473/fs/FactSheet12-6-04.pdf.

U.S. Environmental Protection Agency, 2006, U.S. 

Environmental Protection Agency Fact Sheet #16 Standard 

Chlorine of Delaware (aka Metachem) Site, accessed 

October 2, 2009, at http://www.epa.gov/reg3hwmd/npl/

DED041212473/fs/09-2006.pdf.

U.S. Environmental Protection Agency, 2010, EPA Superfund 

Record of Decision Operable Unit 3: Standard Chlorine 

of Delaware, Inc., September 2010, [variously paged], 

accessed September 16, 2014, at http://www.epa.gov/

reg3hwmd/npl/DED041212473/rod/OU3_Record_of_

Decision.pdf.

U.S. Environmental Protection Agency, 2011, First FiveYear Review Report for Standard Chlorine of Delaware, 

Inc. Site, New Castle County, Delaware, 22 p., accessed 

September 16, 2014, at http://www.epa.gov/reg3hwmd/npl/

DED041212473/fyreview/SCD_5YR_Final.pdf.

U.S. Environmental Protection Agency, 2012, Map of 

Standard Chlorine of Delaware, Inc., accessed December 

4, 2012, at http://www.epa.gov/reg3hwmd/npl/

DED041212473.htm.

Woodruff, K.D., 1986, Geohydrology of the Chesapeake and 

Delaware Canal Area, Delaware (Hydro Map No. 6, Sheet 

1—Basic Geology), accessed October 1, 2009, at http://

dspace.udel.edu:8080/dspace/bitstream/19716/3085/2/

Hydro6%2csht1.pdf.

Woodruff, K.D., 1988, Geohydrology of the Chesapeake and 

Delaware Canal Area, Delaware (Hydro Map No. 6, Sheet 

2—Thickness of the confining unit beneath the water-table 

aquifer), accessed October 1, 2009, at http://dspace.udel.

edu:8080/dspace/bitstream/19716/3085/1/Hydro6%2csht2.

pdf.

Appendixes 51

Appendixes

Appendix 1. Chemicals of concern detected in soils, sediment, and (or) groundwater at the Standard 

Chlorine of Delaware, Inc. Superfund Site.

Appendix 2. Characteristics of U.S. Geological Survey Potomac Aquifer Study monitoring sites 

measured at and near the Standard Chlorine of Delaware, Inc. Superfund Site, 2005–12.

Appendix 3. Well characteristics and water-level drawdown and recovery data for wells monitored as 

part of U.S. Geological Survey 72-hour pumping at well OR-6A near the Standard Chlorine of Delaware, 

Inc. Superfund Site, August 28–31, 2010.

Appendix 4. Water-level data for non-U.S. Geological Survey network observation wells monitored as 

part of U.S. Geological Survey 72-hour pumping at well OR-6A near the Standard Chlorine of Delaware, 

Inc. Superfund Site, August 26–September 1, 2010.
52 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

Appendix 1. Chemicals of concern detected in soils, sediment, and (or) groundwater at the Standard Chlorine of Delaware, Inc. 

Superfund Site.

[EPA, U.S. Environmental Protection Agency]

Chemicals of concern identified in 1995 EPA Record of Decision

Benzene 1,3,5-trichlorobenzene

Chlorobenzene 1,2,3,4-tetrachlorobenzene

1,2-dichlorobenzene 1,2,4,5-tetrachlorobenzene

1,3-dichlorobenzene Pentachlorobenzene

1,4-dichlorobenzene Hexachlorobenzene

1,2,3-trichlorobenzene Nitrobenzene

1,2,4-trichlorobenzene Toluene

Additional chemicals of concern identified in 2010 EPA Record of Decision

Dioxin PCE (Tetrachloroethylene)

Carbon tetrachloride TCE (Trichloroethylene)

Chloroform
Appendixes 53

Appendix 2. Characteristics of U.S. Geological Survey Potomac Aquifer Study monitoring sites measured at and near the Standard Chlorine of Delaware, Inc. Superfund Site, 2005–12.—Continued[USGS, U.S. Geological Survey; DNREC, Delaware Department of Natural Resources and Environmental Control; DGS, Delaware Geological Survey; EPA, U.S. Environmental Protection Agency; Motiva, Motiva Delaware City Refinery; NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929; bls, below land surface; toc, top of casing; --, no data; TIC, top of inner casing; TPVC, top of polyvinyl chloride (PVC) casing; TSC, top of steel casing; TPVCcollar, top of PVC collar; 4sq., 4-inch square; flush, flush mounted with land surface; up., upper; psi, pounds per square inch; Δ, difference]

Local site 

name

USGS station 

number

DNREC 

permit 

number

DGS 

identification 

number

Latitude 

(NAD 83)

(decimal 

degrees)

Longitude 

(NAD 83)

(decimal 

degrees)

Northing 

(NAD 83)

(Delaware 

State 

Plane, 

feet)

Easting 

(NAD 83)

(Delaware 

State 

Plane, 

feet)

Measuring point 

elevation 

(NGVD 29)

(feet)

Measuring 

point 

reference

Outer 

casing 

diameter

(inches)

Inner 

casing 

diameter

(inches)

Depth of 

boring

(feet bls)

Red Lion 

Creek 01482320 -- -- 39.6053 -75.6300 584732.08 596052.17 6.78 shelter floor -- -- --

Ion-MW03 393541075375302 188967 Dc53-93 39.5966 -75.6314 581572.91 595655.61 52.84 TIC 3 2 39

C-31 393552075375601 72650 Dc52-68 39.5979 -75.6323 582025.31 595397.17 62.03 TPVC 8 4 82

CW-4 393558075375503 219241 Dc53-83 39.5995 -75.6318 582631.72 595534.28 55.87 TIC 4sq. 2 69

MW-14 393559075375201 89170 Dc53-68 39.5997 -75.6311 582714.84 595743.80 48.26 TPVC 6 4 60MW-15 393559075375801 89171 Dc53-67 39.5997 -75.6318 582710.33 595544.15 48.33 TPVC 6 4 67MW-16 393559075375803 89172 Dc53-66 39.5998 -75.6327 582725.73 595280.04 47.06 TPVC 6 4 66MW-17 393602075375401 89173 Dc43-28 39.6006 -75.6317 583017.43 595584.79 52.62 TPVC 6 4 72MW-18 393602075375801 89174 Dc43-27 39.6007 -75.6328 583045.37 595276.16 32.24 TPVC 6 4 54

PMW-48 393602075380101 218228 -- 39.6006 -75.6337 583032.83 595001.35 49.56 TIC 4 2 --PMW-49 393602075380102 218229 -- 39.6006 -75.6340 583033.95 594933.39 49.42 TIC 4 2 --

CW-11 393603075375901 228525 Dc42-50 39.6024 -75.6324 583681.30 595390.32 17.47 TIC 6 2 40

SCD-EM-2S 393611075375602 -- Dc43-39 39.6031 -75.6323 583936.51 595406.15 3.36 TSC -- 2 3.78

SCD-EM-2D 393611075375601 -- Dc43-38 39.6031 -75.6323 583936.51 595406.15 4.66 TSC -- 2 9.1

PZ-29 393611075380601 -- -- 39.6031 -75.6350 583931.60 594645.93 48.41 TIC 4 1.25 --

OR-6C 393616075381802 -- -- 39.6048 -75.6382 584475.47 593743.77 47.09 TPVC 6 4 52

MW-37 393622075375203 178327 Dc43-31 39.6061 -75.6312 585021.39 595729.27 6.03 TPVC 6 4 82

MW-7 393652075380901 81372 Dc42-19 39.6032 -75.6356 583990.06 594473.60 48.70 TPVC 6 4 61.3

PW-7S 393551075381102 218549 Dc52-79 39.5975 -75.6359 581915.77 594373.92 53.60 TIC 4sq. 2 85PW-4S 393558075375501 218938 Dc53-84 39.5995 -75.6318 582621.11 595549.22 56.46 TIC 4sq. 2 80

PW-3 393541075375301 202312 Dc53-51 39.5966 -75.6313 581563.36 595695.87 51.74 TIC 4 2 147

MW-11 393551075381101 81385 Dc52-56 39.5975 -75.6358 581907.85 594405.97 53.41 TPVCcollar 6 4 136

PW-2 393552075375602 202206 Dc53-50 39.5978 -75.6323 581991.24 595394.64 62.47 TIC 4 2 167PW-8 393554075375001 219238 Dc53-86 39.5984 -75.6306 582236.65 595885.79 53.38 TIC 4sq. 2 162PW-9 393555075375701 228523 Dc53-133 39.5987 -75.6325 582322.51 595336.19 52.23 TIC 6 2 148

PW-4D 393558075375502 218725 Dc53-85 39.5995 -75.6318 582616.07 595557.15 56.15 TIC 4sq. 2 150
54 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12 Appendix 2. Characteristics of U.S. Geological Survey Potomac Aquifer Study monitoring sites measured at and near the Standard Chlorine of Delaware, Inc. Superfund Site, 2005–12.—Continued [USGS, U.S. Geological Survey; DNREC, Delaware Department of Natural Resources and Environmental Control; DGS, Delaware Geological Survey; EPA, U.S. Environmental Protection Agency; Motiva, Motiva Delaware City Refinery; NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929; bls, below land surface; toc, top of casing; --, no data; TIC, top of inner casing; TPVC, top of polyvinyl chloride (PVC) casing; TSC, top of steel casing; TPVCcollar, top of PVC collar; 4sq., 4-inch square; flush, flush mounted with land surface; up., upper; psi, pounds per square inch; Δ, difference] Local site name USGS station number DNREC permit number DGS identifica- tion number Latitude (NAD 83) (decimal degrees) Longitude (NAD 83) (decimal degrees) Northing (NAD 83) (Delaware State Plane, feet) Easting (NAD 83) (Delaware State Plane, feet) Measur- ing point elevation (NGVD 29) (feet) Measuring point reference Outer casing diameter (inches) Inner casing diameter (inches) Depth of boring (feet bls) PW-1 393559075375802 195516 Dc53-49 39.5997 -75.6319 582706.16 595517.06 48.21 TIC 4 2 154 PW-12 393559075380101 228521 Dc52-127 39.5996 -75.6336 582656.32 595036.66 47.97 TIC 6 2 132 PW-11 393603075375902 228522 Dc42-51 39.6024 -75.6323 583692.57 595397.80 17.48 TIC 6 2 135 PW-13 393603075381501 231310 Dc42-56 39.6009 -75.6375 583128.59 593943.42 45.62 TIC flush 2 170 MW-12 393604075375301 81386 Dc43-20 39.6009 -75.6308 583148.16 595825.88 55.63 TPVCcollar 6 4 148 PW-10 393609075375601 228524 Dc43-36 39.6008 -75.6329 583089.67 595233.65 30.05 TIC 6 2 107.4 PW-10D 393609075375602 228519 Dc43-37 39.6008 -75.6330 583096.61 595220.57 29.25 TIC 6 2 250 PW-17 393615075380101 231212 Dc42-58 39.6042 -75.6337 584328.55 595006.75 12.73 TIC 6 2 105 PW-14 393616075380501 231211 Dc42-57 39.6044 -75.6346 584398.31 594756.14 9.81 TIC 6 2 69 OR-6A 393616075381801 45620 Dc42-41 39.6048 -75.6382 584548.68 593739.22 43.63 TSC 8 -- 180 OR-6B 393616075381803 45619 Dc42-42 39.6045 -75.6382 584453.93 593731.55 47.43 TSC 2 -- -- PW-6S 393616075381804 218553 Dc42-43 39.6047 -75.6382 584505.98 593746.57 45.79 TIC 4sq. 2 107 PW-6D 393616075381805 218554 Dc42-44 39.6047 -75.6382 584513.48 593741.26 45.57 TIC 4sq. 2 180 PW-5S 393622075375201 218550 Dc43-29 39.6061 -75.6312 585037.15 595729.45 7.00 TIC 4sq. 2 71 PW-5D 393622075375202 218552 Dc43-30 39.6061 -75.6312 585039.95 595722.96 6.94 TIC 4sq. 2 110 TY-116B -- 58514 9058514 39.6047 -75.6488 584522.43 590756.86 32.66 TPVC 6 4 128 TY-116C -- 58515 9058515 39.6046 -75.6487 584486.22 590779.83 36.48 TPVC 6 4 127 TY-121A -- 58511 Dc41-21 39.6053 -75.6525 584753.33 589718.06 16.16 TPVC 6 4 98 TY-121B -- 58512 9058512 39.6053 -75.6525 584737.37 589720.19 17.12 TPVC 6 4 57 PW-16 393548075380101 231210 Dc52-173 39.5966 -75.6337 581553.07 595032.66 63.24 TIC 6 2 240 PW-2D 393552075375603 231209 Dc53-187 39.5978 -75.6324 581993.71 595374.66 62.81 TIC 6 2 250 PW-11D 393603075375903 228520 Dc42-52 39.6024 -75.6323 583699.69 595406.56 17.14 TIC 6 2 255 TY-116A -- 58513 Dc42-18 39.6046 -75.6487 584498.43 590781.66 35.29 TPVC 6 4 247 Tidewater-12 393551075375301 -- Dc53-07 39.5977 -75.6306 581953.12 595876.79 58.40 TIC 8 3 701
Appendixes 55

Appendix 2. Characteristics of U.S. Geological Survey Potomac Aquifer Study monitoring sites measured at and near the Standard Chlorine of Delaware, Inc. Superfund Site, 2005–12.—Continued[USGS, U.S. Geological Survey; DNREC, Delaware Department of Natural Resources and Environmental Control; DGS, Delaware Geological Survey; EPA, U.S. Environmental Protection Agency; Motiva, Motiva Delaware City Refinery; NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929; bls, below land surface; toc, top of casing; --, no data; TIC, top of inner casing; TPVC, top of polyvinyl chloride (PVC) casing; TSC, top of steel casing; TPVCcollar, top of PVC collar; 4sq., 4-inch square; flush, flush mounted with land surface; up., upper; psi, pounds per square inch; Δ, difference]

Local site 

name

Mesured 

well 

depth

(feet toc)

Δ

outer 

to 

inner 

casing

(feet)

Outer 

casing 

stickup

(feet)

Inner 

casing 

stickup

(feet)

 Land 

surface 

elevation 

(NGVD 

29)

(feet)

Depth 

to top 

of open 

interval

(feet bls)

Depth to 

bottom 

of open 

interval

(feet bls)

Altitude 

of top of 

screened 

interval

(feet)

Altitude 

of bottom of 

screened 

interval

(feet)

Screened 

aquifer

Date 

installed

Responsible 

agency or 

company

Period of continous 

data collection1,2 

Red Lion 

Creek

-- -- -- -- -- -- -- -- -- (surface water) 12/1/06 USGS 6/20/07-9/30/12*

Ion-MW03 41.5 -0.21 3.16 2.95 49.89 29 39 21 11 Columbia 9/1/04 Ion Power 10/27/06-9/30/12*

C-31 74.0 -0.45 2.39 1.94 60.09 32 72 28 -12 Columbia 2/5/88 Motiva --

CW-4 69.4 0.06 2.80 2.86 53.01 62 67 -9 -14 Columbia 5/11/07 EPA --

MW-14 61.9 -0.29 2.70 2.41 45.85 49 59 -3 -13 Columbia 10/18/91 EPA --MW-15 66.8 -0.34 2.42 2.08 46.25 55 65 -9 -19 Columbia 10/21/91 EPA --MW-16 67.2 -0.31 2.63 2.32 44.74 55 65 -10 -20 Columbia 10/22/91 EPA --MW-17 72.8 -0.19 3.13 2.94 49.68 60 70 -10 -20 Columbia 10/24/91 EPA --MW-18 52.6 -0.35 3.02 2.67 29.57 40 50 -10 -20 Columbia 10/23/91 EPA --

PMW-48 60.2 -0.13 2.11 1.98 47.58 48 58 0 -10 Columbia -- EPA 10/20/07-9/30/12*PMW-49 72.0 -0.39 2.12 1.73 47.69 60 70 -12 -22 Columbia -- EPA 10/20/07-9/30/12*

CW-11 35.0 -0.23 2.09 1.86 15.61 28 33 -12 -17 Columbia 6/28/09 EPA --

SCD-EM-2S 6.56 -- 2.78 -- 0.58 3 4 -2 -3 Columbia 5/7/08 EPA --

SCD-EM-2D 13.4 -- 4.31 -- 0.35 9 10 -9 -10 Columbia 5/7/08 EPA --

PZ-29 74.5 -0.30 1.00 0.70 47.71 69 74 -21 -26 Columbia -- EPA 9/1/10-5/11/11

OR-6C 51.1 -0.22 3.00 2.78 44.31 41 51 3 -7 Columbia -- Motiva --

MW-37 51.5 -0.41 2.33 1.92 4.11 30 50 -26 -46 Columbia 5/25/01 EPA 6/23/11-9/30/12

MW-7 58.0 -0.40 2.48 2.08 46.62 47 57 0 -10 Columbia 1/17/90 EPA --

PW-7S 85.0 -0.03 2.23 2.20 51.40 77 82 -26 -31 up. Potomac–top 3/31/07 EPA 6/6/07-11/18/07 and 6/23/11-9/30/12*PW-4S 83.8 -0.15 2.74 2.59 53.87 75 80 -21 -26 up. Potomac–top 5/7/07 EPA 6/6/07-9/15/09*

PW-3 138.0 -0.41 2.19 1.78 49.96 125 135 -75 -85 up. Potomac–A 8/5/04 EPA --

MW-11 138.0 -0.15 1.78 1.63 51.78 126 136 -74 -84 up. Potomac–A 2/24/90 EPA 3/26/07-9/30/12*

PW-2 155.5 -0.27 2.35 2.08 60.39 140 150 -80 -90 up. Potomac–A 7/30/04 EPA 3/17/06-6/5/07PW-8 159.3 -0.09 2.89 2.80 50.58 152 157 -101 -106 up. Potomac–A 5/15/07 EPA --PW-9 140.0 -0.25 2.31 2.06 50.17 133 138 -83 -88 up. Potomac–A 7/9/09 EPA --

PW-4D 145.0 -0.07 2.08 2.01 54.14 137 142 -83 -88 up. Potomac–A 4/26/07 EPA 6/6/07-9/30/12*
56 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12 Appendix 2. Characteristics of U.S. Geological Survey Potomac Aquifer Study monitoring sites measured at and near the Standard Chlorine of Delaware, Inc. Superfund Site, 2005–12.—Continued [USGS, U.S. Geological Survey; DNREC, Delaware Department of Natural Resources and Environmental Control; DGS, Delaware Geological Survey; EPA, U.S. Environmental Protection Agency; Motiva, Motiva Delaware City Refinery; NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929; bls, below land surface; toc, top of casing; --, no data; TIC, top of inner casing; TPVC, top of polyvinyl chloride (PVC) casing; TSC, top of steel casing; TPVCcollar, top of PVC collar; 4sq., 4-inch square; flush, flush mounted with land surface; up., upper; psi, pounds per square inch; Δ, difference] Local site name Mesured well depth (feet toc)

Δ

outer 

to 

inner 

casing

(feet)

Outer 

casing 

stickup

(feet)

Inner 

casing 

stickup

(feet)

 Land 

surface 

elevation 

(NGVD 

29)

(feet)

Depth 

to top 

of open 

interval

(feet bls)

Depth to 

bottom 

of open 

interval

(feet bls)

Altitude 

of top of 

screened 

interval

(feet)

Altitude 

of bottom of 

screened 

interval

(feet)

Screened 

aquifer

Date 

installed

Responsible 

agency or 

company

Period of continous 

data collection1,2 

PW-1 150.1 -0.12 2.59 2.47 45.74 138 148 -92 -102 up. Potomac–A 11/11/03 EPA --

PW-12 132.0 -0.33 2.62 2.29 45.68 125 130 -79 -84 up. Potomac–A 8/8/09 EPA --

PW-11 137.5 -0.22 2.37 2.15 15.33 130 135 -115 -120 up. Potomac–A 8/21/09 EPA 10/1/09-9/30/12

PW-13 165.0 -0.73 0.00 -0.73 46.35 161 166 -115 -120 up. Potomac–A 6/3/10 EPA 8/20/10-9/30/12

MW-12 141.5 -0.33 2.26 1.93 53.70 130 140 -76 -86 up. Potomac–A 6/20/90 EPA 12/15/05-9/30/12*

PW-10 107.4 -0.30 2.76 2.46 27.59 100 105 -72 -77 up. Potomac–A 8/22/09 EPA --

PW-10D 175.7 -0.21 2.08 1.87 27.38 169 174 -142 -147 up. Potomac–A 8/4/09 EPA --

PW-17 98.9 -0.35 2.65 2.30 10.43 92 97 -82 -87 up. Potomac–A 6/2/10 EPA 8/27/10-9/30/12*PW-14 71.5 -0.24 2.57 2.33 7.48 64 69 -57 -62 up. Potomac–A 5/18/10 EPA --

OR-6A -- 0.00 1.21 1.21 42.42 102 176 -61 -134 up. Potomac–A -- Motiva --OR-6B 183.0 0.00 3.20 3.20 44.23 175 180 -131 -136 up. Potomac–A -- Motiva --

PW-6S 108.4 -0.10 1.79 1.69 44.10 102 107 -58 -63 up. Potomac–A 4/20/07 EPA 5/15/07-9/30/12*

PW-6D 180.0 -0.12 1.90 1.78 43.79 172 177 -128 -133 up. Potomac–A 4/19/07 EPA 5/15/12-10/31/07

PW-5S 71.3 -0.03 3.21 3.18 3.82 63 68 -59 -64 up. Potomac–A 3/27/07 EPA 5/15/07-10/12/07 and 10/1/09-9/30/12*

PW-5D 101.8 -0.07 3.12 3.05 3.89 93 98 -89 -94 up. Potomac–A 3/25/07 EPA 4/21/07-5/12/09*

TY-116B 124.7 -- -- -- -- 113 123 -78 -88 up. Potomac–A 10/3/84 Tybouts Trust --TY-116C 73.1 -- -- -- 34.80 60 70 -32 -42 up. Potomac–A 9/25/84 Tybouts Trust --

TY-121A 89.5 -0.34 2.38 2.04 13.78 77 87 -63 -73 up. Potomac–A 12/4/84 Tybouts Trust --TY-121B 53.8 -0.33 2.50 2.17 14.62 43 53 -28 -38 up. Potomac–A 12/5/84 Tybouts Trust --

PW-16 243.3 -0.25 2.43 2.18 61.06 233 238 -172 -177 up. Potomac–B 6/17/10 EPA 6/23/11-9/30/12*

PW-2D 246.9 -0.48 3.27 2.79 60.02 239 244 -179 -184 up. Potomac–B 6/6/10 EPA 5/11/11-9/30/12*

PW-11D 236.7 -0.24 2.31 2.07 15.07 230 235 -215 -220 up. Potomac–C 7/21/09 EPA 10/1/09-9/30/12*

TY-116A 82.1 -- -- -- -- 221 231 -186 -196 up. Potomac–C 9/18/84 Tybouts Trust --

Tidewater-12 701.0 0.00 3.04 3.04 55.36 534 539 -479 -484 lower Potomac late 1954 Motiva 3/20/07-9/30/12*

1 Continuous data collected at selected wells using In-Situ, Inc. Level-Troll 500, or 700 pressure transducers with pressure ranges between 0–5 and 0–30 psi. Other equipment occasionally used, but no 

longer in service, included Waterlog DH-21, In-Situ, Inc. mini troll, and Solinst level logger pressure transducers.

2 Dates with * indicate some missing periods of data collection.
Appendixes 57

Appendix 3. Well characteristics and water-level drawdown and recovery data for wells monitored as part of U.S. Geological Survey 72-hour pumping at well OR-6A near the Standard Chlorine of Delaware, Inc. Superfund Site, August 28–31, 2010.—Continued[DGS, Delaware Geological Survey; NGVD29, vertical elevation referenced to National Geodetic Vertical Datum of 1929; bls, below land surface; N/A, data not available; N/C, data not calculated; EST, eastern standard time; %, percent; Note: Wells are identified by local site name and DGS identification number if available.]

Local 

site 

name

DGS 

identification 

number

Distance from 

pumping well 

(feet)

Hydrologic unit

Well 

depth 

(feet 

bls)

Land 

surface 

elevation 

(feet 

NGVD29)

Screened 

interval 

(feet bls)

Pre-pumping static 

water 

level 

(NGVD29)

Maximum 

drawdown 

(feet)

Date and time 

of maximum 

drawdown 

(EST)

Date and time 

of 90% recovery 

(EST)

Time of 90% 

recovery 

(days)

Discrete measurements

CW-4 Dc53-83 2,626 Columbia 67 53.01 62–67 13.32 0.05 8/31/2010 11:28 N/C N/C

MW-10 1,423 Columbia N/A 46.33 N/A 10.60 0.04 8/31/2010 12:59 N/C N/CMW-13 1,931 Columbia N/A 46.56 N/A 15.40 0.02 8/31/2010 12:56 N/C N/CMW-14 Dc53-68 2,717 Columbia 58 45.85 48–58 12.65 0.06 8/31/2010 17:02 N/C N/CMW-16 Dc53-66 2,387 Columbia 65 44.74 55–65 13.11 0.04 8/30/2010 10:49 N/C N/CMW-17 Dc43-28 2,398 Columbia 70 49.6 60–70 10.56 0.05 8/31/2010 16:59 N/C N/CMW-18 Dc43-27 2,150 Columbia 50 29.57 40–50 10.92 0.05 8/31/2010 16:52 N/C N/CMW-21 1,028 Columbia 18 7.41 10–20 2.82 0.17 8/31/2010 09:31 N/C N/CMW-22 1,159 Columbia 17 6.69 6.5–16.5 2.89 0.24 8/31/2010 17:22 N/C N/CMW-23 1,415 Columbia 26 6.87 16–26 2.47 0.34 8/31/2010 17:26 N/C N/CMW-31 1,829 Columbia N/A 49.16 N/A 11.77 0.02 8/30/2010 07:34 N/C N/CMW-33 1,785 Columbia N/A 45.72 N/A 14.52 0.01 8/31/2010 12:56 N/C N/CMW-37 Dc43-31 2,046 Columbia 51 4.11 30–50 1.51 1.18 8/31/2010 12:30 N/C N/CMW-38 2,262 Columbia 50 5.32 30–50 1.84 1.76 8/31/2010 12:27 N/C N/C

OR-6C 74 Columbia 51 44.09 40–50 2.27 0.05 8/28/2010 18:16 N/C N/C

PMW-41 2,175 Columbia N/A 45.80 N/A 13.03 0.02 8/30/2010 07:46 N/C N/CPMW-42 1,987 Columbia N/A 45.48 N/A 12.09 0.02 8/30/2010 07:49 N/C N/CPMW-43 1,981 Columbia N/A 49.79 N/A 15.25 0.02 8/30/2010 07:50 N/C N/CPMW-44 1,612 Columbia N/A 45.43 N/A 10.88 0.02 8/31/2010 14:07 N/C N/CPMW-47 1,153 Columbia N/A 45.87 N/A 4.48 0.15 8/31/2010 09:53 N/C N/CPMW-50 2,599 Columbia N/A 48.53 N/A 11.87 0.02 8/30/2010 08:17 N/C N/CPMW-51 2,410 Columbia N/A 48.97 N/A 12.21 0.03 8/30/2010 07:37 N/C N/C

PW-04S Dc53-84 2,644 top sand 80 53.87 75–80 13.25 0.06 8/31/2010 11:26 N/C N/CPW-07S Dc52-79 2,709 top sand 85 51.4 77–82 14.19 0.48 8/31/2010 15:39 N/C N/C

PZ-03 2,163 Columbia N/A 50.70 N/A 16.33 0.02 8/30/2010 07:44 N/C N/CPZ-05 1,292 Columbia N/A 48.02 N/A 9.57 0.03 8/31/2010 10:28 N/C N/CPZ-06 1,241 Columbia N/A 49.79 N/A 9.16 0.04 8/31/2010 10:32 N/C N/CPZ-07 1,172 Columbia N/A 40.98 N/A 8.55 0.04 8/31/2010 10:24 N/C N/C
58 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12 Appendix 3. Well characteristics and water-level drawdown and recovery data for wells monitored as part of U.S. Geological Survey 72-hour pumping at well OR-6A near the Standard Chlorine of Delaware, Inc. Superfund Site, August 28–31, 2010.—Continued [DGS, Delaware Geological Survey; NGVD29, vertical elevation referenced to National Geodetic Vertical Datum of 1929; bls, below land surface; N/A, data not available; N/C, data not calculated; EST, eastern standard time; %, percent; Note: Wells are identified by local site name and DGS identification number if available.] Local site name DGS identifica- tion number Distance from pumping well (feet) Hydrologic unit Well depth (feet bls) Land surface elevation (feet NGVD29) Screened interval (feet bls) Pre-pump- ing static water level (NGVD29) Maximum drawdown (feet) Date and time of maximum drawdown (EST) Date and time of 90% recovery (EST) Time of 90% recovery (days) Discrete measurements—continued PZ-08 1,130 Columbia N/A 50.08 N/A 8.62 -0.02 8/31/2010 10:36 N/C N/C PZ-09 895 Columbia N/A 48.17 N/A 4.12 0.03 8/30/2010 06:56 N/C N/C PZ-10 955 Columbia N/A 47.48 N/A 8.05 0.02 8/31/2010 10:17 N/C N/C PZ-12 984 Columbia N/A 46.53 N/A 3.50 0.10 8/31/2010 09:45 N/C N/C PZ-13 1,027 Columbia N/A 47.18 N/A 6.37 0.07 8/31/2010 10:02 N/C N/C PZ-14 1,147 Columbia N/A 48.11 N/A 6.18 0.05 8/30/2010 06:21 N/C N/C PZ-15 1,070 Columbia N/A 46.33 N/A 3.93 0.11 8/31/2010 09:50 N/C N/C PZ-16 1,207 Columbia N/A 48.51 N/A 5.19 0.10 8/31/2010 09:53 N/C N/C PZ-17 1,523 Columbia N/A 42.78 N/A 7.12 0.05 8/31/2010 10:44 N/C N/C PZ-18 1,635 Columbia N/A 45.08 N/A 8.20 0.03 8/30/2010 06:47 N/C N/C PZ-19 1,615 Columbia N/A 48.36 N/A 9.69 0.03 8/31/2010 10:49 N/C N/C PZ-20 2,271 Columbia N/A 48.69 N/A 11.48 0.03 8/31/2010 13:54 N/C N/C PZ-21 2,358 Columbia N/A 48.39 N/A 14.24 0.02 8/30/2010 08:11 N/C N/C PZ-22 2,617 Columbia N/A 48.80 N/A 16.59 0.02 8/30/2010 08:19 N/C N/C PZ-23 2,706 Columbia N/A 58.90 N/A 17.75 0.02 8/30/2010 08:21 N/C N/C PZ-24 2,545 Columbia N/A 53.98 N/A 18.69 0.03 8/30/2010 07:39 N/C N/C PZ-27 2,089 Columbia N/A 47.61 N/A 11.16 0.04 8/30/2010 08:01 N/C N/C PZ-28 1,108 Columbia N/A 53.72 N/A 8.45 0.01 8/31/2010 10:14 N/C N/C PZ-30 1,436 Columbia N/A 46.28 N/A 8.59 0.03 8/30/2010 06:38 N/C N/C TW-28 1,513 Columbia N/A 49.35 N/A 10.17 0.03 8/30/2010 07:22 N/C N/C OR-6A Dc42-41 Pumping well upper Potomac–A 176 42.42 102–176 2.10 43.98 8/31/2010 13:31 9/2/2010 06:06 1.69 OR-6B Dc42-42 95 upper Potomac–A 184 44.23 103–176 2.02 24.50 8/31/2010 13:15 N/C N/C PW-02 Dc53-50 3,047 upper Potomac–A 150 60.39 140–150 1.77 6.28 8/31/2010 15:09 N/C N/C PW-05D Dc43-30 2,044 upper Potomac–A 99 3.89 93–98 2.08 3.21 8/31/2010 12:33 N/C N/C PW-08 Dc53-86 3,155 upper Potomac–A 158 50.58 152–157 1.37 4.83 8/31/2010 11:30 N/C N/C PW-09 Dc53-133 2,740 upper Potomac–A 138 50.17 133–138 1.70 6.44 8/31/2010 17:05 N/C N/C PW-10 Dc43-36 2,090 upper Potomac–A 107 27.59 100–105 1.77 6.43 8/31/2010 16:47 N/C N/C
Appendixes 59

Appendix 3. Well characteristics and water-level drawdown and recovery data for wells monitored as part of U.S. Geological Survey 72-hour pumping at well OR-6A near the Standard Chlorine of Delaware, Inc. Superfund Site, August 28–31, 2010.—Continued[DGS, Delaware Geological Survey; NGVD29, vertical elevation referenced to National Geodetic Vertical Datum of 1929; bls, below land surface; N/A, data not available; N/C, data not calculated; EST, eastern standard time; %, percent; Note: Wells are identified by local site name and DGS identification number if available.]

Local 

site 

name

DGS 

identification 

number

Distance from 

pumping well 

(feet)

Hydrologic unit

Well 

depth 

(feet 

bls)

Land 

surface 

elevation 

(feet 

NGVD29)

Screened 

interval 

(feet bls)

Pre-pumping static 

water 

level 

(NGVD29)

Maximum 

drawdown 

(feet)

Date and time 

of maximum 

drawdown 

(EST)

Date and time 

of 90% recovery 

(EST)

Time of 90% 

recovery 

(days)

Discrete measurements—continued

PW-10D Dc43-37 2,075 upper Potomac–A 176 27.38 169–174 1.32 5.22 8/31/2010 16:50 N/C N/C

PW-12 Dc52-127 2,294 upper Potomac–A 130 45.68 125–130 1.76 6.73 8/31/2010 12:15 N/C N/CPW-14 Dc42-57 1,031 upper Potomac–A 69 7.48 64–69 1.77 9.24 8/31/2010 09:30 N/C N/C

TY-116B 2,982 upper Potomac–A N/A N/A 112–122 3.13 1.32 8/31/2010 16:19 N/C N/CTY-116C 2,960 upper Potomac–A N/A N/A 60–70 1.74 3.24 8/31/2010 16:23 N/C N/C

TY-121A 4,026 upper Potomac–A N/A N/A 77–87 2.97 0.16 8/31/2010 16:07 N/C N/CTY-121B 4,023 upper Potomac–A N/A N/A 43–53 2.24 0.27 8/31/2010 16:10 N/C N/C

PW-16 Dc52-173 3,239 upper Potomac–B 238 61.06 233–238 -6.34 0.65 8/31/2010 15:30 N/C N/C

TY-116A 2,958 upper Potomac–C N/A N/A 221–231 -2.87 0.15 8/31/2010 16:20 N/C N/C

Continuous measurements

CW-11 Dc42-50 1,864 Columbia 33 15.61 28–33 5.37 0.08 8/31/2010 17:00 N/C N/C

ION-MW-03 Dc53-93 3,540 Columbia 39 49.89 29–39 17.29 0.05 8/31/2010 05:15 N/C N/C

PMW-48 1,973 Columbia 59 47.58 N/A 11.11 0.03 8/30/2010 08:04 N/C N/C

PZ-29 1,097 Columbia 74 47.71 69–74 4.78 5.26 8/31/2010 09:59 N/C N/C

MW-11 Dc52-56 2,724 upper Potomac–A 136 51.78 126–136 2.06 7.36 8/31/2010 14:30 9/6/2010 01:30 5.46

MW-12 Dc43-20 2,513 upper Potomac–A 142 53.7 130–140 1.43 4.32 8/31/2010 16:30 9/7/2010 01:15 6.36

PW-03 Dc53-51 3,570 upper Potomac–A 138 49.96 125–135 4.10 3.65 8/31/2010 22:30 9/12/2010 06:15 11.32

PW-04D Dc53-85 2,654 upper Potomac–A 145 54.14 137–142 1.63 5.96 8/31/2010 15:00 9/6/2010 16:15 6.05

PW-05S Dc43-29 2,050 upper Potomac–A 68 3.82 63–68 2.07 3.13 8/31/2010 12:32 9/5/2010 12:15 4.99

PW-06D Dc42-44 35 upper Potomac–A 177 43.79 172–177 2.04 28.25 8/31/2010 12:53 N/C N/C

PW-06S Dc42-43 44 upper Potomac–A 108 44.11 102–107 2.03 28.18 8/31/2010 13:30 9/3/2010 08:30 2.79

PW-11 Dc42-51 1,865 upper Potomac–A 135 15.33 130–135 1.75 5.91 8/31/2010 13:30 9/4/2010 21:00 4.31

PW-13 Dc42-56 1,440 upper Potomac–A 166 46.35 161–166 2.15 10.29 8/31/2010 13:30 9/4/2010 16:30 4.13PW-17 Dc42-58 1,288 upper Potomac–A 98 10.43 93–98 1.99 7.76 8/31/2010 13:30 9/3/2010 21:30 3.33

PW-02D Dc53-187 3,039 upper Potomac–B 240 60.02 235–240 -4.55 2.44 9/1/2010 05:45 9/13/2010 13:00 12.30

PW-11D Dc42-52 1,873 upper Potomac–C 235 15.07 230–235 -12.64 0.18 8/31/2010 13:45 9/16/2010 09:00 15.80

Tidewater 12 Dc53-07 3,363 lower Potomac 701 55.36 534–539 -56.12 0.00 8/28/2010 13:45 N/C N/C
60 Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12

Appendix 4. Water-level data for non-U.S. Geological Survey network observation wells monitored as part of U.S. Geological Survey 

72-hour pumping at well OR-6A near the Standard Chlorine of Delaware, Inc. Superfund Site, August 26–September 1, 2010.

[NGVD29, altitude referenced to National Geodetic Vertical Datum of 1929; EST, eastern standard time]

Date and time 

(EST)

Local 

site

name

Altitude 

of water 

level 

(feet 

NGVD29)

Date and time 

(EST)

Local 

site

name

Altitude 

of water 

level 

(feet 

NGVD29)

Date and time 

(EST)

Local 

site

name

Altitude 

of water 

level 

(feet 

NGVD29)

8/26/2010 15:46 MW-10 10.63 8/28/2010 10:54 PMW-43 15.25 8/29/2010 12:43 PZ-06 9.17

8/27/2010 6:18 MW-10 10.61 8/29/2010 13:42 PMW-43 15.24 8/30/2010 7:17 PZ-06 9.14

8/28/2010 8:14 MW-10 10.60 8/30/2010 7:50 PMW-43 15.23 8/31/2010 10:32 PZ-06 9.12

8/28/2010 14:55 MW-10 10.59 9/1/2010 10:57 PMW-43 15.22 9/1/2010 8:03 PZ-06 9.12

8/29/2010 14:42 MW-10 10.60 8/27/2010 9:30 PMW-44 11.11 8/27/2010 7:45 PZ-07 8.50

8/30/2010 6:39 MW-10 10.60 8/28/2010 10:41 PMW-44 10.88 8/28/2010 9:34 PZ-07 8.55

8/31/2010 12:59 MW-10 10.56 8/29/2010 13:46 PMW-44 10.91 8/29/2010 12:49 PZ-07 8.54

9/1/2010 9:05 MW-10 10.58 8/30/2010 7:56 PMW-44 10.91 8/30/2010 7:00 PZ-07 8.52

8/27/2010 6:06 MW-13 15.39 8/31/2010 14:07 PMW-44 10.86 8/31/2010 10:24 PZ-07 8.51

8/28/2010 8:04 MW-13 15.40 9/1/2010 10:52 PMW-44 10.90 9/1/2010 8:09 PZ-07 8.51

8/29/2010 14:52 MW-13 15.41 8/27/2010 6:25 PMW-47 4.50 8/27/2010 7:40 PZ-08 8.71

8/30/2010 6:35 MW-13 15.40 8/28/2010 8:58 PMW-47 4.48 8/28/2010 9:36 PZ-08 8.62

8/31/2010 12:56 MW-13 15.38 8/29/2010 12:03 PMW-47 4.45 8/29/2010 12:52 PZ-08 8.67

9/1/2010 9:00 MW-13 15.38 8/30/2010 6:20 PMW-47 4.40 8/30/2010 7:11 PZ-08 8.65

8/27/2010 9:44 MW-31 11.80 8/31/2010 9:53 PMW-47 4.33 8/31/2010 10:36 PZ-08 8.64

8/28/2010 11:39 MW-31 11.77 9/1/2010 7:23 PMW-47 4.31 9/1/2010 8:22 PZ-08 8.62

8/29/2010 13:18 MW-31 11.78 8/27/2010 8:48 PMW-50 11.89 8/27/2010 7:17 PZ-09 4.14

8/30/2010 7:34 MW-31 11.75 8/28/2010 11:15 PMW-50 11.87 8/28/2010 9:59 PZ-09 4.12

8/31/2010 14:19 MW-31 11.75 8/29/2010 14:11 PMW-50 11.87 8/29/2010 13:07 PZ-09 4.10

9/1/2010 11:33 MW-31 11.73 8/30/2010 8:17 PMW-50 11.85 8/30/2010 6:56 PZ-09 4.09

8/26/2010 15:33 MW-33 14.56 8/31/2010 13:40 PMW-50 11.85 8/31/2010 10:19 PZ-09 4.10

8/27/2010 6:10 MW-33 14.53 9/1/2010 11:22 PMW-50 11.84 9/1/2010 8:14 PZ-09 4.08

8/28/2010 8:08 MW-33 14.52 8/27/2010 9:01 PMW-51 12.22 8/27/2010 7:33 PZ-10 8.07

8/29/2010 14:40 MW-33 14.55 8/28/2010 11:05 PMW-51 12.21 8/28/2010 9:57 PZ-10 8.05

8/30/2010 6:44 MW-33 14.54 8/29/2010 13:32 PMW-51 12.21 8/29/2010 13:00 PZ-10 8.07

8/31/2010 12:56 MW-33 14.51 8/30/2010 7:37 PMW-51 12.18 8/30/2010 7:05 PZ-10 8.04

9/1/2010 9:02 MW-33 14.51 8/31/2010 13:43 PMW-51 12.18 8/31/2010 10:17 PZ-10 8.03

8/27/2010 9:14 PMW-41 13.04 9/1/2010 11:14 PMW-51 12.17 9/1/2010 8:17 PZ-10 8.03

8/28/2010 10:56 PMW-41 13.03 8/27/2010 9:16 PZ-03 16.34 8/27/2010 6:16 PZ-12 3.50

8/29/2010 13:38 PMW-41 13.03 8/28/2010 10:58 PZ-03 16.33 8/28/2010 8:54 PZ-12 3.50

8/30/2010 7:46 PMW-41 13.01 8/29/2010 13:30 PZ-03 16.34 8/29/2010 11:49 PZ-12 3.49

8/31/2010 14:15 PMW-41 13.02 8/30/2010 7:44 PZ-03 16.31 8/30/2010 6:17 PZ-12 3.45

9/1/2010 10:59 PMW-41 12.99 8/27/2010 7:50 PZ-05 9.60 8/31/2010 9:45 PZ-12 3.40

8/27/2010 9:19 PMW-42 12.11 8/28/2010 9:31 PZ-05 9.57 9/1/2010 7:17 PZ-12 3.36

8/28/2010 10:51 PMW-42 12.09 8/29/2010 12:41 PZ-05 9.58 8/27/2010 6:15 PZ-13 6.36

8/29/2010 13:41 PMW-42 12.11 8/30/2010 7:15 PZ-05 9.55 8/28/2010 9:07 PZ-13 6.37

8/30/2010 7:49 PMW-42 12.07 8/31/2010 10:28 PZ-05 9.54 8/29/2010 11:52 PZ-13 6.39

8/31/2010 11:00 PMW-42 12.08 9/1/2010 8:06 PZ-05 9.53 8/30/2010 6:29 PZ-13 6.33

9/1/2010 10:55 PMW-42 12.07 8/27/2010 7:53 PZ-06 9.18 8/31/2010 10:02 PZ-13 6.30

8/27/2010 9:22 PMW-43 15.27 8/28/2010 9:40 PZ-06 9.16 9/1/2010 7:31 PZ-13 6.30
Appendixes 61

Appendix 4. Water-level data for non-U.S. Geological Survey network observation wells monitored as part of U.S. Geological Survey 

72-hour pumping at well OR-6A near the Standard Chlorine of Delaware, Inc. Superfund Site, August 26–September 1, 2010.

—Continued

[NGVD29, altitude referenced to National Geodetic Vertical Datum of 1929; EST, eastern standard time]

Date and time 

(EST)

Local 

site

name

Altitude 

of water 

level 

(feet 

NGVD29)

Date and time 

(EST)

Local 

site

name

Altitude 

of water 

level 

(feet 

NGVD29)

Date and time 

(EST)

Local 

site

name

Altitude 

of water 

level

(feet 

NGVD29)

8/27/2010 6:27 PZ-14 6.18 8/31/2010 13:54 PZ-20 11.45 8/30/2010 6:38 PZ-30 8.56

8/28/2010 9:00 PZ-14 6.18 9/1/2010 10:44 PZ-20 11.44 8/31/2010 10:42 PZ-30 8.56

8/29/2010 12:05 PZ-14 6.18 8/27/2010 8:42 PZ-21 14.27 9/1/2010 8:30 PZ-30 8.56

8/30/2010 6:21 PZ-14 6.13 8/28/2010 11:19 PZ-21 14.24 8/27/2010 7:57 TW-28 10.18

9/1/2010 7:28 PZ-14 6.09 8/29/2010 13:59 PZ-21 14.24 8/28/2010 9:25 TW-28 10.17

8/27/2010 6:23 PZ-15 3.95 8/30/2010 8:11 PZ-21 14.22 8/29/2010 12:40 TW-28 10.18

8/28/2010 8:56 PZ-15 3.93 8/31/2010 13:56 PZ-21 14.22 8/30/2010 7:22 TW-28 10.14

8/29/2010 12:12 PZ-15 3.94 9/1/2010 10:45 PZ-21 14.21 8/31/2010 10:30 TW-28 10.14

8/30/2010 6:18 PZ-15 3.87 8/27/2010 8:47 PZ-22 16.61 9/1/2010 8:00 TW-28 10.12

8/31/2010 9:50 PZ-15 3.82 8/28/2010 11:16 PZ-22 16.59 8/28/2010 10:19 TY-116A -2.87

9/1/2010 7:25 PZ-15 3.78 8/29/2010 14:12 PZ-22 16.59 8/29/2010 10:30 TY-116A -2.88

8/27/2010 6:34 PZ-16 5.20 8/30/2010 8:19 PZ-22 16.57 8/31/2010 13:10 TY-116A -3.00

8/28/2010 9:03 PZ-16 5.19 8/31/2010 13:39 PZ-22 16.57 8/31/2010 16:20 TY-116A -3.02

8/29/2010 12:00 PZ-16 5.17 9/1/2010 11:24 PZ-22 16.56 9/1/2010 8:30 TY-116A -3.07

8/30/2010 6:22 PZ-16 5.13 8/27/2010 8:53 PZ-23 17.77 8/28/2010 10:18 TY-116B 3.13

8/31/2010 9:53 PZ-16 5.09 8/28/2010 11:11 PZ-23 17.75 8/29/2010 10:32 TY-116B 2.98

9/1/2010 7:30 PZ-16 5.07 8/29/2010 14:07 PZ-23 17.76 8/31/2010 13:11 TY-116B 1.89

8/27/2010 6:54 PZ-17 7.13 8/30/2010 8:21 PZ-23 17.73 8/31/2010 16:19 TY-116B 1.81

8/28/2010 10:15 PZ-17 7.12 8/31/2010 13:35 PZ-23 17.76 9/1/2010 8:31 TY-116B 1.60

8/29/2010 12:22 PZ-17 7.11 9/1/2010 11:20 PZ-23 17.73 8/28/2010 10:20 TY-116C 1.74

8/30/2010 6:40 PZ-17 7.09 8/27/2010 8:57 PZ-24 18.72 8/29/2010 10:28 TY-116C 0.56

8/31/2010 10:44 PZ-17 7.07 8/28/2010 11:02 PZ-24 18.69 8/31/2010 13:09 TY-116C -1.42

9/1/2010 8:32 PZ-17 7.06 8/29/2010 13:28 PZ-24 18.69 8/31/2010 16:23 TY-116C -1.50

8/27/2010 7:07 PZ-18 8.20 8/30/2010 7:39 PZ-24 18.66 9/1/2010 8:32 TY-116C -0.74

8/28/2010 10:25 PZ-18 8.20 8/27/2010 9:51 PZ-27 11.31 8/28/2010 10:05 TY-121A 2.97

8/29/2010 12:28 PZ-18 8.20 8/28/2010 11:36 PZ-27 11.16 8/29/2010 10:12 TY-121A 2.93

8/30/2010 6:47 PZ-18 8.17 8/29/2010 13:52 PZ-27 11.15 8/31/2010 13:24 TY-121A 2.83

8/31/2010 10:46 PZ-18 8.20 8/30/2010 8:01 PZ-27 11.12 8/31/2010 16:07 TY-121A 2.81

9/1/2010 8:37 PZ-18 8.15 8/31/2010 14:00 PZ-27 11.13 9/1/2010 8:20 TY-121A 2.78

8/27/2010 6:59 PZ-19 9.71 9/1/2010 11:28 PZ-27 11.12 8/28/2010 10:04 TY-121B 2.24

8/28/2010 10:27 PZ-19 9.69 8/27/2010 7:38 PZ-28 8.54 8/29/2010 10:11 TY-121B 2.19

8/29/2010 12:31 PZ-19 9.70 8/28/2010 9:49 PZ-28 8.45 8/31/2010 13:22 TY-121B 2.00

8/30/2010 6:51 PZ-19 9.68 8/29/2010 12:55 PZ-28 8.46 8/31/2010 16:10 TY-121B 1.97

8/31/2010 10:49 PZ-19 9.66 8/30/2010 7:09 PZ-28 8.49 9/1/2010 8:21 TY-121B 1.98

9/1/2010 8:39 PZ-19 9.65 8/31/2010 10:14 PZ-28 8.44

8/27/2010 8:35 PZ-20 11.50 9/1/2010 8:20 PZ-28 8.43

8/28/2010 11:21 PZ-20 11.48 8/27/2010 6:51 PZ-30 8.60

8/29/2010 14:02 PZ-20 11.47 8/28/2010 10:12 PZ-30 8.59

8/30/2010 8:10 PZ-20 11.46 8/29/2010 12:18 PZ-30 8.74

Prepared by USGS West Trenton Publishing Service Center.

Edited by Valerie M. Gaine.

Graphics and layout by Timothy W. Auer.

For additional information, contact:

Director, MD-DE-DC Water Science Center

U.S. Geological Survey

5522 Research Park Drive

Baltimore, MD 21228

or visit our Web site at:

http://md.water.usgs.gov
Brayton, M.J., and others—Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow, SCD Superfund Site, Del., 2005–12—Scientific Investigations Report 2014–5224

Printed on recycled paper

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20145224