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

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?

Conceptual Model of Question to be Answered

Dredge Pond Pond

Water Supply Well Stream

To Well?

To Stream?

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

Project Location

Model Construction

(Plan View)

Dredge Pond

Stream

Ponds

Water Supply Well

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

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

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

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

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

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

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

Postscript

• Dredging finally commenced March 28, 2008 and was completed 3 weeks later

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

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

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

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

Questions?

Pete AndersenGeoTrans, Inc.Roswell, Georgia, USA

E-mail: pandersen@geotransinc.com

Phone: 770 642 1000