Uranium Investigation in an Alluvial Aquifer with DP Methods

Wes McCall, PG

Tom Christy, PE

Geoprobe Systems

Tom Christopherson, Prgrm. Mgr. Well Stds.

Howard Isaacs, Prgrm. Mgr. Mon. & Compliance

Nebraska, DHHS

HPT logging at Clarks

GWMR Vol. 29, No. 1, pp 42-44

Winter 2009

Salina Journal April 24, 2008 Page A5

Salina, KS

EPA Radionuclides Rule amendment establishes MCL for Uranium at 30 ug/l (mass)

Why ?

Where ?

Village of Clarks

Merrick County, NE

Alluvial deposits of the Platte River

Local bedrock is the Niobrara Chalk.

Nebraska Geologic Bedrock Map after Burchett & Pabian, 1991.

Background on Clarks PWS

Wells

• Old, shallow wells in town abandoned due to natural Uranium between 100-200ug/L

• Two new, deeper PWS wells installed outside of town after test wells indicate U nondetect

• New north PWS well exceeds U MCL at initial sampling event

• Short time later south PWS well exceeds U MCL

New Clarks PWS South

New Wells

-120

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-80

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0

0 50 100 150 200

South PWS well and 4” test well construction based on drillers logs

Gravel Pack

Concrete

Bentonite Seal

Dep

th (

feet

)

psi

PWS 4” Test

How ?• Hypothesis: Elevated oxygen in zone(s) of aquifer leading to dissolution of Uranium into the groundwater

• HPT logging for hydrogeology

• HPT logs guide well placement

• Install wells using DP methods

• Develop wells

• Low flow sampling - DHHS

• Nebraska DHHS Lab analysisGeoprobe Model 8040 advances 2.25” rods for DP well installation

Hydraulic Profiling Tool (HPT) Logs

Geoprobe 6625 Unit advanced HPT probe to obtain logs

HPT Operational Theory

• Hammer or push probe at constant rate (2cm/sec)

• Inject water at low flow rate (~300ml/min)

• Measure injection pressure with in-line transducer

Flow

Water

HPT Probe

Injection Screen

E-log Wenner Array

Basics of an HPT Log(South PWS Well)

Flow

• Electrical Conductance – EC (mS/m)

Higher EC >>> clay

Lower EC >>> sand/gravel

• Pressure (psi /100 max)

Higher P >>> lower perm

Lower P >>> higher perm

Hydrostatic P >>> rising baseline

• Flow (ml/min) --- ~ constant

Higher flow >>> higher perm

Lower flow >>> lower perm

De

pth

(ft

)

Pressure

EC

N

A4A3A2A1

B1B2B3B4B5

PWS Well South (Pumping)

4” Test Well South

PWS Well North (Inactive)

4” Test Well North

C1

C2

PWS wells are ~500ft apart

DP wells separated by ~5 ft

Site Sketch Map

(not to scale)

HPT log locations

DP Wells

Gravel Pad

HPT Cross Section C1-C2 Looking NWC1 C2

Presence and thickness of silt-clay layers varies vertically and laterally across the area. Most clay layers are not continuous.

Selected Screen Intervals

A-Group Wells

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0

10 30 50 70 90 110

Dep

th (

ft)

Pressure (psi)

A1 = 103-108

A4 = 33-38

A3 = 55-60

A2 = 75-80

Use HPT logs to guide screen interval selection.

Lower Pressure intervals = sand & gravel.

Target sand layers between larger clay layers/lenses

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0

0 20 40 60 80 100 120

Selected Screen Intervals

B-Group Wells

Dep

th (

ft)

Pressure (psi)

B1 = 105-110

B4 = 35-40

B3 = 65-70

B2 = 83-88

B5 = 15-20

Does ground water chemistry change with depth?

DO?

ORP?

Uranium ?

Cations & Anions?

Setting the DP WellsDrive Cap

O-Ring Seals

Probe Rod

(2.25” OD x 1.5” ID)

Expendable/Anchor Point

(ASTM D6725)

ProtectorDP Well Construction

Tremie Grouting

25% solids bentonite grout ¾” PVC

Grout barrier: fm natural collapse

Nominal ¾” Prepacked Screen x 5ft

(not to scale)

DP Well Initial

Development

Development with Check Valve Set within Screen

Check valve

Early purge water

Low Flow Sampling with Mechanical Bladder Pump

Final Development : with Mechanical Bladder Pump

Monitor Water Quality Parameters

N

A4A3A2A1

B1B2B3B4B5

PWS Well South (Pumping)

4” Test Well South

PWS Well North (Inactive)

4” Test Well North

C1

C2

DP wells separated by ~5 ft

Site Sketch Map

(not to scale)

HPT log locations

DP Wells

Gravel Pad

-120

-100

-80

-60

-40

-20

0

0 50 100 150 200

A-Group & South 4” Well

A4

S4”

A1

A2

A3

Sp Cnd = 1043 uS/cm

Sp Cnd = 698

Sp Cnd = 530

Sp Cnd = 405

Sp Cnd = 1115

DO < 0.1 mg/l all wells

ORP ~ -200 to -300 mV all DP wells

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-80

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0

0 50 100 150 200

B5

N4”

B4

B3

B2

B1

Sp Cnd = 770

Sp Cnd = 805

Sp Cnd = 795

Sp Cnd = 605

Sp Cnd = 372

Sp Cnd = 885 uS/cm

B-Group & N. 4” Well

DO < 0.1 mg/l all wells

ORP ~ -200 to -350 mV all wells

Where’s the DO ?

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0

0 50 100 150 200

A4

S4”

A1

A2

A3

A-Group & S. 4” WellsUranium Data

(ug/L)

U = 32.2

U = 15.1

U = < 1

U = < 1

U = 168

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-80

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0

0 50 100 150 200

B5

N4”

B4

B3

B2

B1

U = 15.1

U = 1.8

U = 1.3

U = 376

U = 98.7

U = 124

B-Group & N. 4” WellsUranium Data

(ug/L)

What does this mean ?

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0

0 50 100 150 200

Low permeability layer(s)

South PWS & 4” Test Well Construction With

HPT Log

Both filter packs penetrate low permeability layers allowing for “short circuiting”

Filter packs behave as a preferential flow path.

U = 376 ug/l

U = 98.7 ug/l

U = N

D

-120

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0

0 50 100 150 200

Summary - Conclusions• HPT logs provide detailed

information on hydrogeology

• DP wells yield discrete interval samples for water quality assessment

• Uranium can be mobile in low DO and low ORP environments … dependent on water chemistry

• Uranium distribution at this site is heterogeneous in vertical and horizontal dimensions

• Investigate before Investing

Uranium ?

Low DO and Low ORP …

DO < 0.1 mg/l in all wells

ORP ranges from ~ -200 to -300mV

Uranium geochemistry indicates it should be reduced (U+4) and form insoluble precipitates (Pitchblende/Urananite)

But …

Why is Uranium Mobile here?

NE DHHS Team samples DP wells with Mechanical Bladder Pump

Increase in pCO2 lowers ORP (Eh) at which U+4 oxidizes to U+6

Sufficient CO2 will lower this to < -300mV

Iron and Manganese can behave as electron acceptors for U+4 oxidation to U+6

Available Calcium can result in formation of soluble Ca-U-CO3 complexes

Geochemistry at the Clarks Well Field …

Geochemistry in the local aquifer is consistent with having uranium in solution even with low DO and Low Eh conditions observed.

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0

0 50 100 150 200

A-Group & S. 4” WellsSodium & Sulfate

(mg/L)

A4

S4”

A1

A2

A3

Na = 98.1 SO4 = 239

Na = 43.7 SO4 = 121

Na = 39.8 SO4 = 113

Na = 22.1 SO4 = 46.4

Na = 107 SO4 = 257

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0

0 50 100 150 200

B-Group & N. 4” Well

Sodium & Sulfate (mg/L)

B5

N4”

B4

B3

B2

B1

Na = 56.8 SO4 = 136

Na = 66.8 SO4 = 180

Na = 72.9 SO4 = 160

Na = 45.7 SO4 = 149

Na = 29.4 SO4 = 98.7

Na = 67.9 SO4 = 170