modeling of the potential for vertically downward saltwater migration from a dredge pond
DESCRIPTION
Modeling of the Potential for Vertically Downward Saltwater Migration from a Dredge Pond. Peter F. Andersen 1 , Lisa M. Grogin 1 , and Ronald L. Bartel 2 1 GeoTrans, Inc., Roswell, Georgia, United States 2 Northwest Florida Water Management District, Havana, Florida, United States. - PowerPoint PPT PresentationTRANSCRIPT
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Modeling of the Potential for Vertically Downward Saltwater Migration from a Dredge Pond
Peter F. Andersen1, Lisa M. Grogin1, and Ronald L. Bartel2
1GeoTrans, Inc., Roswell, Georgia, United States2Northwest Florida Water Management District, Havana,
Florida, United States
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Problem Statement
• Desire to deposit dredge materials from saltwater body into inland barrow pit
• Two concerns:– Saltwater (from dredge slurry) will migrate
vertically downward and contaminate a public supply well 1800 ft away (short-term + density dependent)
– Dredge materials will migrate from sediments into aquifer (long-term + non-density dependent
• Are current and future hydraulic gradients sufficient to contain the dredge materials and prevent contamination of the well and aquifer?
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Conceptual Model of Question to be Answered
Dredge Pond Pond
Water Supply Well Stream
To Well?
To Stream?
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Approach
• Characterization of flow system• Time series analysis of hydraulic
gradients• Numerical modeling to assess and
demonstrate physics of flow• Development of metrics for:
– Allowing dredging to begin– When dredge deposition would need to stop
• Development of groundwater and surface water monitoring plans
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Project Location
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Model Construction
(Plan View)
Dredge Pond
Stream
Ponds
Water Supply Well
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Model Construction: Vertical
Surfacia lZone(SZ)
M ainProducing
Zone(M PZ)
Low Perm eability Zone
Layer
1
2
3
4
5
6
7
8
9
10
11
South Pond
North Pond
Jackson Branch
1 20 500 1000
SCALE IN FEETVertical Exaggeration x10
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Model Details
• SEAWAT-2000• Calibration (steady state flow)
– 17 heads, 3 multilevel wells– ME = 0.19 ft; MAE = 0.35 ft (7.8% of variation)
• Predictive simulations– Lower head in pond from 3.7 ft to 2 ft– Concentration of saltwater in pond = 12,700 ug/L– Saltwater concentration active for 6 months
• 10 weeks dredging• 16 weeks residence time
– Various configurations of boundary heads to represent:
• Regional water level changes• Well pumping
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Scenarios Evaluated
• Minimum Operating Conditions– Base case boundary heads
– Conservative 0.5 ft upward head gradient at monitor well (1 ft required; 3-6 ft typical)
• Explicit Inclusion of Pumping Well– Implicitly incorporated in BCs
– Additionally included by superimposing Hantush-generated heads on BCs
• Worst Case– 2 ft downward gradient occurs immediately
– Lasts entire 6 months of dredging
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Minimum Operating Conditions
Concentration of Chloride Plume
0
50
100
150
200
250
300
350
0 200 400 600 800 1000 1200
Time (d)
Co
nce
ntr
atio
n (
mg
/L)
Far Side of Creek
Upgradient of Creek
1 20 500 1000
SC ALE IN FEET
Far S ide of C reek Location
Creek R iver C ells
LEGEND
Peoples #8
180 Days
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Explicit Inclusion of Pumping Well
Concentration of Chloride Plume
0
50
100
150
200
250
300
350
400
450
0 200 400 600 800 1000 1200
Time (d)
Co
nce
ntr
atio
n (
mg
/L)
Far Side of Creek
Upgradient of Creek
1 20 500 1000
SC ALE IN FEET
Far S ide of C reek Location
Creek R iver C ells
LEGEND
Peoples #8
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Worst Case (Head Gradient Reversal)
Concentration of Chloride Plume
0
500
1000
1500
2000
2500
0 200 400 600 800 1000 1200
Time (d)
Co
nce
ntr
atio
n (
mg
/L)
Far Side of Creek
Upgradient of Creek
1 20 500 1000
SC ALE IN FEET
Far S ide of C reek Location
Creek R iver C ells
LEGEND
Peoples #8
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Performance Metrics for Dredging
• Condition below which dredging could not commence:– 2.5 ft NGVD in DW3– 2 std dev below wet season
high; 2 std dev above drought condition low
• Condition that dredging would stop:– Upward gradient of < 1 ft at
PZ-19 / DW3 monitor well
-2.00
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
06
/26
/02
07
/26
/02
08
/25
/02
09
/24
/02
11
/11
/02
12
/11
/02
01
/10
/03
02
/09
/03
03
/11
/03
04
/10
/03
05
/10
/03
06
/09
/03
07
/08
/03
08
/08
/03
09
/07
/03
10
/07
/03
11
/06
/03
12
/06
/03
01
/05
/04
02
/04
/04
03
/05
/04
Sta
ge
(fee
t, N
AV
D 1
988)
Deep Well (DW3)
Shallow Well (PZ-19)
X
X
X - Deep Well - taped measurement X - Shallow Well - taped measurement
Top of DW3 well casing 6.67'DW3 well flowing, level above well casing (6.67')
Well casing extended, continuous recording resumed
X
X
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Postscript
• Dredging finally commenced March 28, 2008 and was completed 3 weeks later
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Postscript
• An upward gradient between the Deep and Shallow system was present during the entire operation
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
3/13/2008 3/23/2008 4/2/2008 4/12/2008 4/22/2008 5/2/2008 5/12/2008
Time
Sta
ge
(ft)
Deep Well
Shallow Well
Pond
Pump-dow n priorto dredging
Dredging Begins
Dredging Ends
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Postscript
• An estimated 200,000 cu yds of sediment was removed from Bayou Chico
• Monitoring will continue for 3 years:– Groundwater levels– Groundwater concentrations– Surface water concentrations
• Remediation could be triggered by exceeding of groundwater concentration limits
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Conclusions (1)
• An unusual saltwater intrusion problem!
• Numerical model was useful to assess and demonstrate effects of scenarios
• Model helped fill in gaps monitoring network
• Only worst case scenario—a reversal of gradient—created a potential problem
• Head and gradient metrics were established to prevent loss of containment from pond
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Conclusions (2)
• Example of use of modeling, time series analysis, and monitoring to ensure success
• Actual result was positive and (so far) in line with model predictions
• Length of dredging was overestimated: 10 weeks vs 3 weeks actual; another conservative modeling assumption
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Questions?
Pete AndersenGeoTrans, Inc.Roswell, Georgia, USA
E-mail: [email protected]
Phone: 770 642 1000