delaware geological survey open file report 45 summary ...plain facies in the updip section where it...

Delaware Geological Survey Open File Report 45 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. SUMMARY Open File Report 45 is a three panel poster presented at the Society for Sedimentary Geology (SEPM) Research Conference on Ancient and Modern Coastal Plain Depositional Environments: Aquifer Heterogeneity and Environmental Implications, March 24th - 27th, 2002 in Charleston, SC. The abstract is included below for convenience and is also included on Sheet 1 of the oversized sheets. DISTRIBUTION Open File Report 45 is a compilation of this summary file with 3 oversized sheets (posters) compiled into a single document in Adobe Portable Document Format (pdf). The report is available for downloading from the Delaware Geological Survey’s website (www.udel.edu/dgs under “Publications”). Printed copies are available upon request. ABSTRACT Fluvial sands of the subsurface Cretaceous Potomac Formation form a major aquifer system used by a growing population in the northern Coastal Plain of Delaware. The aquifer is extremely heterogeneous on the megascopic scale and connectivity of permeable fluvial units is poorly constrained. The formation is characterized by alluvial plain facies in the updip section where it contains potable water. While over 50 aquifer tests indicate high permeability (5x10-5 to 7x10-4 m/s), the formation is primarily composed of fine-grained silt and clay in overbank and interfluvial facies. Individual fluvial sand bodies are laterally discontinuous and larger-scale sand packages appear to be variable in areal extent resulting in a labyrinth style of heterogeneity. The subsurface distribution of aquifers and aquitards has been interpreted within a new stratigraphic framework based on geophysical logs and on palynological criteria from four cored wells. The strata dip gently to the southeast, with generally sandy fluvial facies at the base of the formation lapping onto a south-dipping basement unconformity. The top of the formation is marked by an erosional unconformity that truncates successively older Potomac strata updip. Younger Cretaceous units overly the formation in its downdip area. In the updip area, the formation crops out or subcrops under Quaternary sands. The fine-grained facies include abundant paleosols that contain siderite nodules and striking mottling that commonly follows ped faces and root traces. These paleosols may serve as regional aquitards. This geologic complexity poses a challenge for determining the magnitudes and directions of ground-water flow within the aquifer that are needed for making informed decisions when managing this resource for water supply and contaminant remediation.

Fluvial sands of the subsurface Cretaceous Potomac Formation form a major aquifer system used by a growing population in the northern Coastal Plain of Delaware. The aquifer is extremely

heterogeneous on the megascopic scale and connectivity of permeable fluvial units is poorly constrained. The formation is characterized by alluvial plain facies in the updip section where it

contains potable water. While over 50 aquifer tests indicate high permeability (5x10-5 to 7x10-4 m/s), the formation is primarily composed of fine-grained silt and clay in overbank and interfluvial

facies. Individual fluvial sand bodies are laterally discontinuous and larger-scale sand packages appear to be variable in areal extent resulting in a labyrinth style of heterogeneity. The

subsurface distribution of aquifers and aquitards has been interpreted within a new stratigraphic framework based on geophysical logs and on palynological criteria from four cored wells. The

strata dip gently to the southeast, with generally sandy fluvial facies at the base of the formation lapping onto a south-dipping basement unconformity. The top of the formation is marked by an

erosional unconformity that truncates successively older Potomac strata updip. Younger Cretaceous units overly the formation in it's downdip area. In the updip area, the formation crops out or

subcrops Quaternary sands. The fine-grained facies include abundant paleosols that contain siderite nodules and striking mottling that commonly follows ped faces and root traces. These

paleosols may serve as regional aquitards. This geologic complexity poses a challenge for determining the magnitudes and directions of ground-water flow within the aquifer that are needed

for making informed decisions when managing this resource for water supply and contaminant remediation.

Characterization of the Potomac Aquifer, an extremely

heterogeneous fluvial system in the Atlantic Coastal Plain of DelawareThomas E. McKenna, Peter P. McLaughlin, and Richard N. Benson

Introduction / Main Issues8 Aquifers in the Potomac Formation are the primary ground-water reservoirs

in northern Delaware

8 High population growth and increasing water demand make aquifer

management critical

8 Heterogeneous nature of aquifers complicate determination of magnitudes

and directions of ground-water flow

Objectives

Stratigraphic Framework

8 Potomac Formation overlies a basement composed of crystalline rock or saprolite with significant paleotopographic relief

8 Three pollen zones and several subzones constrain stratigraphic correlations

8 Top of the formation is marked by a significant (10 m.y.) erosional unconformity with some erosional relief

8 The Potomac Formation crops out and subcrops under Quaternary surficial sediments just south of the Fall Line

8 Recently updated stratigraphic framework indicates Potomac onlaps basement and is increasingly truncated updip

by younger formations - previous aquifer model for area (Martin, 1984) assumed basement-parallel stratigraphy

8 Sand correlation is complicated by laterally discontinuous nature of the fluvial facies

Abstract

8 The Potomac Formation is the lowermost Cretaceous stratigraphic unit in area

8 It overlies basement and is capped by a major unconformity

8 The aquifers are developed in non-marine facies that include fluvial

and overbank deposits with extensive paleosol development

Geological Background Database8 Detailed sedimentologic analysis on 3 continuously cored holes

8 Geophysical and lithologic logs for more than 50 wells

8 Palynology in 5 coreholes (3 continuously cored, 2 split-spoon)

8 250 aquifer test analyses (70 with full suite of data)

8 Establish accurate stratigraphic framework as basis for characterizing

depositional and aquifer architecture

8 Calibrate facies type to geophysical log character using core data

8 Estimate distribution of facies types within the updated stratigraphic framework

8 Assess aquifer characteristics (permeability, storage properties) and

interconnectivity of facies types based on available aquifer test results

75o 74o76o77oW

38o

39o

41oN

40o

Delmarva

Peninsula

New

Jersey

Atlantic Ocean

Cenozo

ic su

bcrop

Delaware

Delaw

are BayC

hes

apea

ke

Creta

ceous s

ubcrop

pre-C

reta

ceous

Bay

Maryland

Pennsylvania

study area

DE

LAW

AR

E

MA

RY

LAN

D

NEW J

ERSEY

PENNSYLVANIA

A

A'

B

B'

C

C'

D

D'

E

E'

C & D Canal

Fall Line

DELAWAR

E

RIV

ER

DELAWARE

0 5

Kilometers

Split Spoon Cores

Continuous Wireline Cores

Wells with geophysical logsCores with Pollen

Bethke, C. M., Lee, M. K., Quinodoz, H. A. M., and Kreiling, W. N., 1993, Basin modeling with Basin2, A guide to using Basin2: Urbana-Champaign, University of Illinois, 225 p.

Dickinson, G., 1953, Geological aspects of abnormal reservoir pressure in Gulf Coast Louisiana: AAPG Bulletin, v. 37, p. 410-432.

Fogg, G. E.,1989, Stochastic Analysis of Aquifer Interconnectedness: Wilcox Group, Trawick Area, East Texas: Univ. TX Bureau of Economic Geology, Report of Investigations 189, 68 p.

Galloway, W. E. and Hobday, D. K., 1996, Terrigenous clastic depositional systems: New York, Springer-Verlag, 2nd ed., 489 p.

Jordan, R. R., 1983, Stratigraphic nomenclature of nonmarine Cretaceous rocks of inner margin of Coastal Plain in Delaware and adjacent states: Delaware Geological Survey Report of Investigations No. 37, 43 p.

Martin, M. M., 1984, Simulated ground-water flow in the Potomac Aquifers, New Castle County, Delaware: USGS Water Resources Investigations Report 84-4007, 85 p.

Reading, H. G., 1986, Sedimentary Environments and Facies: Oxford, Blackwell Scientific Publications, 2nd ed., 615 p.

Retallack, G. J., 1990, Soils of the Past: Boston, Unwin Hyman, 520 p.

References

DELAWARE GEOLOGICAL SURVEY

UNIVERSITY OF DELAWARE, NEWARK

John H. Talley, State Geologist


OPEN FILE REPORT NO. 45

SHEET 1

Cd42-16

Cd52-14 Cd51-21 Dc15-06 Dc34-05 Dc43-09 Dc43-08

Dc53-07

800

700

600

500

400

300

200

100

0 -SEA LEVEL

800

700

600

500

400

300

200

100

0SEA LEVEL-

BASEMENT

FEET FEET

0

0

1 MILE

1 KILOMETER

QUATERNARY

Kmv

KetKmt

Kml

Kpt

QUATERNARY

UKLK

a

b

A

B

C

D

E

F

100 100

Ec14-01

Potomac Formation Sands

New Castle area Delaware City area

Zone III pollen (Cenomanian: 92-96 Ma)

Zone II pollen (Albian: 96-108 Ma)

Estimated Zone II-III boundary line

(approximate Upper/Lower Cretaceous

[UK/LK] boundary)

Cd42-16

Cd52-14 Cd51-21 Dc15-06 Dc34-05 Dc43-09 Dc43-08

Dc53-07

800

700

600

500

400

300

200

100

0 -SEA LEVEL

800

700

600

500

400

300

200

100

0SEA LEVEL-

BASEMENT

FEET FEET

0

0

1 MILE

1 KILOMETER

QUATERNARY

Kmv

KetKmt

Kml

Kpt

QUATERNARY

UKLK

a

b

A

B

C

D

E

F

100 100

Ec14-01

E'E

G gamma log

R resistivity log

S spontaneous potential log

Kml Mount Laurel Fm.

Kmt Marshalltown Fm.

Ket Englishtown Fm.

Kmv Merchantville Fm.

Km Magothy Fm.

Kpt Potomac Fm.

G S R

G G S R G S R G S R G S R

G S R

G

a, b, A, B, ... stratigraphic correlation lines

Potomac Sedimentology

Facies

(Environmental

Interpretation)

Sedimentological

Characteristics

Core Photo Geophysical Log Character

Thick Sands

(Isolated Channels)

8 individual sands 5-20 ft thick

8 occur within 10-30-ft-thick fining-upward packages

8 sands fine upward from fine to medium

(less commonly coarse to very coarse) sand

to silty very fine sand, fines above are preserved

8 sedimentary structures uncommon, most

commonly cross-bedding

8 mud chips and scattered charcoal may occur

8 bell-shaped log patterns

Thin Sands

(Crevasse Splay/

Proximal Levee)

8 sand intervals <10 ft thick, usually 1-3 ft

8 grain sizes range from coarse to

very fine, fining-upward

8 current ripples evident in places

8 mud clasts and charcoal are common

8 irregular log pattern

Interlaminated

Sand and Silt

(Distal Levee/

Flood Plain)

8 centimeter-scale beds and thinner laminae

8 includes sandy silt, silty sand, and

silty clay, fines predominate

8 current ripples evident in some

thin sand beds

8 often associated with abundant charcoal,

mud clasts in places

8 irregular log pattern

Mottled Silts & Clays

(Weathered Flood

Plain with Paleosols)

8 predominantly variegated silts and clays,

light gray, olive-tan, red, orange, purple,

or whitish gray

8 sandy lithologies commonly contain a

significant component of silt or clay

8 exhibits extensive mottling or irregular banding

8 contains sphaerosiderite in places

8 irregular log pattern, reduced gamma in paleosols

Amalgamated Sands

(Amalgamated Channels)

8 sand intervals 30-70 ft thick

8 sharp base, top abrupt, fines at top usually

thin or not preserved

8 mostly fine to medium sand, with less common

coarse and very coarse sand

8 fining-upward packages within these sands

commonly 3-10 ft thick

8 sedimentary structures uncommon, include

planar parallel and cross-bedding

8 blocky log patterns

GAM(NAT) SP RES(16N)FEET

API-GR0 180 MV-120 -30 OHM-M5 500

200

210

220

230API-GR0 180


API-GR0 180 MV-10 60 OHM-M3 300

150

160

170


API-GR0 180 MV-10 60 OHM-M3 300

320

330

340


API-GR0 180 MV-120 -30 OHM-M5 500

70

80

90

100

110

120


API-GR0 180 MV-120 -30 OHM-M5 500

280

290

300

API-GR0 180 MV-120 -30 OHM-M5 500

90

80

100

105

110

95

85

290

280

300

295

285

215

205

225

220

210

160

150

170

165

155

Core Log

depths in feet

330

320

340

335

325

Core Log Legendclay silt

very fine to

fine sand

fine to

medium sand

coarse to

very coarse sandhematite cement mottling mud clast charcoal

Amalgamated Sands

(Amalgamated Channels)

8 Deposition across broad (~ 5 km /

3 mi wide) fluvial valleys

yields high lateral continuity

8 Clean fine and medium sands have

good porosity and permeability

8 Few aquitards within sand

intervals due to amalgamation of

channel sands

Thick Sands

(Isolated Channels)

8 Deposition in isolated channels

yields poor lateral continuity

8 Clean fine and medium sands have

good porosity and permeability

8 Commonly fine upward into silts and

clays which may form aquitards

Thin Sands

(Crevasse Splay / Proximal Levee)

8 Near-channel flood-plain deposition by

overbank events yields local sand

bodies with poor lateral continuity

8 Variable sand permeability, with thin,

permeable sand making local aquifers

8 Silts and clays within and enclosing this

facies are aquitards across parts of the

study area

Interlaminated Sand and Silt

(Distal Levee / Flood Plain)

8 Sands may have broad lateral extent

on flood plain, but thin-bedded character

makes continuity difficult to establish

8 Most sands are silty and less permeable,

not aquifer quality

8 Silts and clays within and enclosing this

facies are aquitards across parts of the

study area

Mottled Silts & Clays

(Weathered Flood Plain with Paleosols)

8 Important aquitards across study area

(and perhaps regionally)

8 Dense clays and hard paleosols are not

fractured and have very low permeability

8 Interbedded thin sands and silts may

allow some leakage across aquitards

Aquifer / Aquitard Characteristics

of Potomac Facies



SHEET 2




DGSID: Cd51-21

85

90

100

105

114.3

111

110

depths in feet

120

DGSID: Cd51-21

290

280

298

300

DGSID: Cd51-23

170

155

160151.25

170

DGSID: Cd51-21

205

210

220

225

DGSID: Cd51-23321.24

330

325

338

344.3

321.2

Example

Example

Example

Example

Example

Hydraulic Conductivity

0

5

10

15

20

25

30

35

1-5 x10-5

5-10 x10-5

1-5 x10-4

5-10 x10-4

1-5 x10-3

Horizontal Hydraulic Conductivity (m/s)

Fre

qu

en

cy

Hydraulic Characteristics

Horizontal Hydraulic Conductivity = 10-5 - 10-3 m/s (3 - 300 ft/day)

Transmissivity = 4 - 1900 m2/day (40 - 20,000 ft2/day)

Storativity = 10-4 - 10-5

Thickness = 3 - 30 m (10 - 100 ft )

Stacked Amalgamated and Isolated ChannelsAquifers

Thickness

0

5

10

15

20

25

10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89 90-99 100-

109

Thickness (ft)

Fre

qu

en

cy

Vertical Hydraulic Conductivity from tests

= 10-8 - 10-5 m/s (10-3 - 3 ft/day)

Highly variable (over 3 orders of

magnitude range from 1 test at 3

different wells)

Distal Levee / Flood Plain,Weathered Flood Plain with Paleosols

Aquitards

Mudrocks containing paleosols are

overcompacted and are estimated to

have very low local permeability

(10-15 - 10-12 m/s; 10-10 - 10-7 ft/day)

Hydraulic Conductivity

0

1

2

3

4

5

6

7

8

1x10-8

1x10-7

1x10-6

1x10-5

Verical Hydraulic Conductivity (m/s)

Fre

qu

en

cy

1 2 3

Bulk Density (g/cc)

3000

2000

1000

0

De

pth

(ft

)

Dickinson (1953) model

Bethke (1993) model

Potomac Fm. measurements

DE

LAW

AR

E

MA

RY

LAN

D

NEW J

ERSEY

PENNSYLVANIA

C & D Canal

Fall Line

DELAWAR

E

RIV

ER

DELAWARE

0 5

Kilometers

Aquifer Test

Locations of Aquifer Tests

Aquifer Framework

200

180

160

140

120

100

80

60

Dep

th (

ft)

K = 8x10-5 m/s

T = 80 m2/day

Dc31-10

SP Res

1amalgamated sands

320

300

280

260

240

220

K = 1X10-4 m/sT= 220 m2/day

Dc51-04

SP Res

2stacked thick and

amalgamated sands

540

520

500

480

460

440

420

400

380

360

340

320

300

Dep

th (

ft)

SP Res

3Dc41-04K = 5 x 10-5 m/s

T = 100 m2/day

stacked thick and

amalgamated sands

700

680

660

640

620

600

580

560

540

520

500

480

Dep

th (

ft)

K= 5x10-5 m/sT = 70-120 m2/day

SP Res

8Ec13-06

amalgamated sands

with fining at top

700

650

600

550

500

Dep

th (

ft)

K = 9 x10-5 m/sT = 95 m2/day

Ec12-03

SP Res

9amalgamated sands

with fining at top

Gam

720

700

680

660

640

620

600

580

De

pth

(ft)

K = 4x10-5 m/sT=110 m2/day

Ec14-01,-07

SP Res

10stacked amalgamated sands

with some upward fining

amalgamated sands

300

280

260

240

220

200

De

pth

(ft

)

K = 6x10-5 m/sT = 40 m2/day

Dc51-03

SP Res

5

280

260

240

220

200

180

De

pth

(ft

)

K = 2 x10-5 m/s

T = 17 m2 /day

Dc51-08

SP Res

4amalgamated sands

340

320

300

280

260

De

pth

(ft

)

Dc52-06K = 6x10-5 m/sT = 95 m2/day

SP Res

6amalgamated sands

with fining at top

600

580

560

540

520

De

pth

(ft

)

K = 8e-5 m/sT = 130 m2/day

Ec12-02

SP Res

7amalgamated sands

Stratigraphic correlations indicate the Potomac Fm. onlaps basement.

A previous aquifer model (Martin, 1984) assumed basement-parallel

stratigraphy. This has signficiant implications for modeling recharge.

stratigraphic unit / aquifer

Basement

strat. unit / aquifer

Surficial Aquifer

Basement



Surficial Aquifer

Direct recharge to upper aquifer from

surficial aquifer, limited recharge

to lower aquifer from surficial aquifer

Our ModelDirect recharge to aquifers from

surficial aquifer

Previous Model

Conclusions 8 Framework developed for stratigraphic correlation in the Potomac Formation using palynology and geophysical

log correlations tied to three continuous cores

8 Potomac Formation onlaps basement and is increasingly truncated updip by younger formations

8 Potomac Formation characterized by five facies and environments: 1) amalgamated sands (amalgamated

channels), 2) thick sands (isolated channels), 3) thin sands (crevasse splay / proximal levee), 4) interlaminated

sand and silt (distal levee / flood plain), 5) mottled silts and clays (weathered flood plain / paleosols)

8 Major aquifers composed of stacked amalgamated and isolated channel sands

8 Major aquitards composed of flood-plain silts and clays with abundant paleosols

Implications8 These interpretations of depositional environments within well-defined stratigraphic boundaries,

when combined with aquifer tests and potentiometric-surface response to pumping,

will increase our ability to determine the magnitudes and directions of ground-water flow.

8 Stratigraphy: refine correlations based on new understanding of sedimentology; more

palynology; explore paleosols, heavy minerals, and clays

8 Sedimentology: increase areal coverage based on log characteristics; map depositional

environments within sequences

8 Hydrogeology: aquifer-test compilation and comparison with depositional-system maps,

detailed study on smaller areas with aquifer tests, pumping schedules / water levels; map

recharge areas (sand subcrop)

Future Directions

amalgamated sands

Recharge

8 Best aquifers are characterized as stacked amalgamated and thick channel sands

8 Estimated 2 - 3 mile (3 - 5 kilometer) horizontal sand dimension within a stratigraphic interval

8 50 - 100 feet (15 - 30 meter) vertical sand dimension

8 Highest transmissivity zones along section are in lower part of Potomac Formation, below UK / LK horizon

8 Highest transmissivity is in stacked sands (e.g. near the base of the Potomac Formation in the southeast part of the cross-section)

B’BCb54-49

Ec14-07,01

Ec13-06

Db25-07

Dc31-10,12

Dc41-04

Dc52-08Dc52-01

SEALEVEL

FEET

Sands of potential aquifer quality

BASEMENT

UK

LK

B

Upper Cretaceous

Lower Cretaceous

Correlation line

UKLK

B

UKLK

B

UK

LK

B

QUATERNARY

KM0 5

1

98

765

4

3

210

SEALEVEL

fee

t

100

100

800

700

600

500

400

300

200

SEALEVEL

fee

t

100

100

800

700

600

500

400

300

200

NW SE

Potomac

High Transmissivity

Low Transmissivity

screened interval (projected onto cross-section)

Db25-07 DGS well identifier

= aquifer

Presented at: Society for Sedimentary Geology (SEPM) Research Conference on Ancient and Modern Coastal Plain Depositional Environments:

Aquifer Heterogeneity and Environmental Implications, March 24 - 27, 2002, Charleston, SC.

Dep

th (

ft)



SHEET 3




delaware geological survey open file report 45 summary ...plain facies in the updip section where it...

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