petroleum impacted potable water wells in northern...

Petroleum impacted potable water wells in Northern Virginia:

structural geology as a tool to predict petroleum contaminant movement in groundwater

Alex Wardle, Randy Chapman, Dell Cheatham, Joe Glassman, Jay Green, Kurt Kochan. Petroleum Program, Northern Regional Office, Department of Environmental Quality, Commonwealth of Virginia

National Tank Conference, Atlanta, Georgia, March 2008

Summary

• Why review potable wells affected by petroleum?

• How did we review those potable wells?• What did we find?• Case studies on how to predict

contaminant movement in bedrock • Conclusions

The problem• “Groundwater flow

conditions in the fractured bedrock media in the area of the site are generally undefinable.”

• “… the existing groundwater contamination onsite could, with little or no reduction in concentration, impact nearby water wells and environmental receptors”A. Consultant, 2000(-2007)

The problem

• Virginia is a risk based program with “zero tolerance” for petroleum polluted domestic wells.

• If groundwater flow in bedrock is “undefinable”, how do we assess risk?

• If groundwater flow is “undefinable”, then wells are probably impacted at random.

• If groundwater flow is not random, then there should be a pattern to wells that are impacted.

The survey

• We identified potable wells in fractured bedrock containing petroleum

• We reviewed relationships between the release and affected wells– Geology,– Distance, – Age, – Orientation.

Northern Virginia

Coastal PlainPiedmont

Blue Ridge

Triassic or “Mesozoic” Basin

Petroleum releases and polluted wells

Well review• Geologic province and structural dip

direction• Type of petroleum (non regulated

heating oil or regulated gasoline)• Age of well • Distance from release• Direction from release to well• Concentration of MTBE or benzene

in well• Wells included in the survey were

those assessed, treated or replaced under DEQ’s alternative water supply program

NBearing

Distance

Wells

• 155 wells polluted by 114 releases• from before 1900 to as late as 2005• 10 feet to 650 feet deep• 10 feet to 1500 feet distance from the

release• Mostly private wells so low flow

Wells and Geology

• 96 in Blue Ridge and Piedmont (65 releases)

• 41 in Triassic Basin (32 releases)• 18 in Coastal Plain and Fall Zone (17

releases)

Well Age

Age of impacted wells in the Triassic Basin

0

5

10

15

20

25

30

35

40

45

1900 to 1920 1921 to 1940 1941 to 1960 1961 to 1980 1981 to 2007

Age of impacted wells in Piedmont/Blue Ridge

0

5

10

15

20

25

30

35

40

1900 to 1920 1921 to 1940 1941 to 1960 1961 to 1980 1981 to 2007

Wells and DistanceTriassic Basin: Distance between release and

impacted well

0%

5%

10%

15%

20%

25%

30%

0 to 5

010

1 to 1

5020

1 to 2

5030

1 to 3

5040

1 to 4

5050

1 to 6

0070

1 to 8

0090

1 to 1

000

Distance, ft

Piedmont and Blue Ridge: distance between release and impacted well

0%

5%

10%

15%

20%

25%

30%

0 to 5

051

to 10

010

1 to 1

5015

1 to 2

0020

1 to 2

5025

1 to 3

0030

1 to 3

5035

1 to 4

0040

1 to 4

5045

1 to 5

0050

1 to 6

0060

1 to 7

0070

1 to 8

0080

1 to 9

0090

1 to 1

000

1000

+

Distance, ft

All heating oil cases: Distance between release and impacted well

0%

10%

20%

30%

40%

50%

60%

0 to 50 51 to 100 101 to 200 201 to 400

Distance, ft

Triassic Basin

0

200

400

600

800

1000

0 200 400 600 800 1000ft

ug/l

MTBEBenzene

Blue Ridge/Piedmont

0

1000

2000

3000

4000

5000

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

ftug

/l

Concentration and Distance of Release

Distance and DirectionDirection and Distance from release Blue Ridge-Piedmont

020406080

100120140160180200220240260280300320340360

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

Distance from release, ft

Bea

ring

from

rele

ase,

deg

rees

Direction and Distance from release, Triassic Basin

020406080

100120140160180200220240260280300320340360

0 100 200 300 400 500 600 700 800 900

Distance from release, ft

Bea

ring

from

rele

ase,

deg

rees

Direction and Concentration

Triassic Basin: MTBE and bearing from release

0

100

200

300

400

500

600

700

800

900

1000

0 90 180 270 360

Bearing from release

MTB

E (u

g/l)

Blue Ridge-Piedmont MTBE and bearing of release

0

500

1000

0 90 180 270 360

Bearing from release

MTB

E ug

/l

Wells and Direction

Triassic Basin: direction to impacted well

10

5

0

5

10

10 5 0 5 10

N

S

W E

Piedmont/Blue Ridge: direction to impacted well

(10)

(5)

0

5

10

(10) (5) 0 5 10

N

S

W E

10

5

0

5

10

10 5 0 5 10

N

S

W E

(15)

(10)

(5)

0

5

10

15

(15) (10) (5) 0 5 10 15

What’s going on? Geology, geometry, stress

• GEOLOGY: Flow controlled by regional NE/SW fractures (Piedmont/Blue Ridge, lesser degree Triassic)

• STRESS: In-situ geologic stresses determine which fracture sets are more consistently open

• GEOMETRY: Preferential flow offset parallel to strike

• Picture “fuzzy” as within 50 feet weathering profile and flow in unsaturated zone perhaps more important than structure?

Geologic Summary

From Diecchioand Gottfried


(10)

(5)

0

5

10

(10) (5) 0 5 10

N

S

W E

Simple FractureStrike

Dip

Release

“Exclusion” zone

Triassic Basin: direction to impacted well

10

5

0

5

10

10 5 0 5 10

N

S

W E

Complex systemStrike

Dip

Release


In-situ stress

• Fractures parallel to maximum shear stress are more likely to be open fractures

• Figures from Rogers 2003

Reinecker, J., Heidbach, O., Tingay, M., Sperner, B., & Müller, B. (2005): The 2005 release of the World Stress Map (available online at www.world-stress-map.org).

Northern Virginia principal stress

25’ to 50’


(10)

(5)

0

5

10

(10) (5) 0 5 10

N

S

W E

In-situ stress

• In Northern Virginia, in-situ stress regime would tend to open fractures off-set to the regional strike (NE-SW), and close fractures perpendicular to strike (SE-NW)

• Migration in unsaturated zone (or of DNAPLs) can follow dip direction in open fractures.

Way forward

• Combine structure, topography, groundwater vectors to predict probable contaminant flow

• Use surface and downhole geophysics to establish primary geologic structures of interest

• Predict “exclusion” zone where contamination likely

• Drill monitoring, remediation or replacement wells as appropriate

• Test model…

Rectortown


Deep weathering

Belvoir


Leesburg

Conductive feature


Groundwater flow

Dip direction

Bedrock dip

Conclusions• There is a pattern to impacted wells in northern

Virginia• That pattern is consistent with regional geologic

structure and in situ geologic stresses• Wells roughly along strike are more likely to be

polluted at higher concentrations than other wells closer to the release

• Geologic structure and history can be used to assess most probable routes for contaminant migration

petroleum impacted potable water wells in northern...

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