uranium investigation in an alluvial aquifer with dp methods wes mccall, pg tom christy, pe geoprobe...
TRANSCRIPT
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
-100
-80
-60
-40
-20
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
-120
-100
-80
-60
-40
-20
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
-120
-100
-80
-60
-40
-20
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
-120
-100
-80
-60
-40
-20
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 ?
-120
-100
-80
-60
-40
-20
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
-120
-100
-80
-60
-40
-20
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 ?
-120
-100
-80
-60
-40
-20
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
-100
-80
-60
-40
-20
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.
-120
-100
-80
-60
-40
-20
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
-120
-100
-80
-60
-40
-20
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