Pilot Testing of Groundwater Circulation Well Technology for a Caustic Groundwater Plume in a Coastal Aquifer

Pilot Testing of Groundwater Circulation Well Technology for a Caustic Groundwater Plume in a Coastal Aquifer

presented by

Lynda Smithard, P.Eng.URS Corporation, Vancouver, BC

Background

a portion of the groundwater beneath chlor-alkali plant has pH values in excess of 7.0 (caustic)

Client spent 5 years investigating and monitoring the groundwater within this area of the site

From June 04 to April 05 a pilot test was conducted to assess a GCW designed to neutralize the high pH groundwater

pH Plume Distribution

pH Plume Distribution

Summary of Receiving Environment Study Results

pH Plume

Discharge area located by direct push sampling from a barge

Area of discharge approximately 75 m2

Groundwater discharging from freshwater and saline zones of coastal aquifer

Shallow Discharge Zone

Deep Discharge Zone

Off-Site Receiving Environment Groundwater Discharge Area

Initial System Design

Initial System Design

Remedial System Troubleshooting

Several start-up issues due to scaling of equipment and safety aspects associated with HCL injection

System had to be re-configured in June and a new pump installed

Remedial System Monitoring

Performed monthly monitoring of pH in system wells

pH observed to vary seasonally and over the daily tidal cycle

Comparison of annual pH values shows a decrease in some wells

pH over Daily Tidal Cycle

Fluctuation in Field pH with Time and Water Level at PZ-2

0.00

0.20

0.40

0.60

0.80

1.00

1.20

8:24 9:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00

Time (hh:mm)

Gro

undwat

er E

leva

tion (m

asl)

8.

8.5

9.

9.5

10.

10.5

11.

11.5

12.

pH

Water Elevation

pH

Seasonal pH measurements

MW22S pH vs. time

12.14

11.6511.55

11.3 11.22

12.0

11.65

11.38 11.3211.24 11.20

6

7

8

9

10

11

12

13

14

14-Jan-04 23-Apr-04 1-Aug-04 9-Nov-04 17-Feb-05 28-May-05 5-Sep-05

Time

May 2004 pH = 12.14May 2005 pH = 11.20

Seasonal pH measurements

PZ1 pH vs. time

12.13

11.63 11.5611.42

10.6

9.69

11.27

9.93

10.93

9.73

10.63

11.18

6

7

8

9

10

11

12

13

14

14-Jan-04 23-Apr-04 1-Aug-04 9-Nov-04 17-Feb-05 28-May-05 5-Sep-05

Time

May 2004 pH = 12.13May 2005 pH = 11.18

Seasonal pH measurements

PZ2 pH vs. time

12.29

11.6711.55

10.63 10.55

12.56

11.79

11.09

10.22

8.94 9.02

6

7

8

9

10

11

12

13

14

14-Jan-04 23-Apr-04 1-Aug-04 9-Nov-04 17-Feb-05 28-May-05 5-Sep-05

Time

May 2004 pH = 12.29May 2005 pH = 9.02

Remedial System Troubleshooting

Pumping rate observed to decrease over duration of pilot trial

Performed groundwater modeling to predict groundwater treatment zone at the end of the pilot trial

Figure 4. GCW Flow vs. Time

0

10

20

30

40

50

60

70

80

90

100

6/ 17/ 2004 7/ 17/ 2004 8/ 17/ 2004 9/ 17/ 2004 10/ 17/ 200 11/ 17/ 2004 12/ 17/ 200 1/ 17/ 2005 2/ 17/ 2005 3/ 17/ 2005

Time (months)

Remedial System Troubleshooting

Decrease in pumping rate attributed to scale formation

Scale issues associated withextraction of groundwater high in pH and salinity

Bench tests using soil and groundwater from transition and saline zones produced a gel (salt) and white precipitate

Remedial System Troubleshooting

Bench tests using soil and groundwater from freshwater zone produced no precipitates

Bench tests indicated scale issues related to pH reduction in saline water

Supplemental Pilot Trial

Re-configured the system in 06 to assess feasibility of intermediate injection point (well hydraulics / fouling / scaling)

Installation of an intermediate injection well above the depth of saline groundwater

Monitored pump rates, water levels and pH in surrounding monitoring wells

Supplemental Pilot Trial – Average Daily Pump Rates

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

9/25

/200

69/

27/2

006

9/29/

2006

10/1/

2006

10/3/

2006

10/5

/2006

10/7

/200

610

/9/2

006

10/1

1/200

610

/13/

2006

10/1

5/200

610

/17/

2006

10/1

9/200

610

/21/

2006

10/2

3/20

0610

/25/

2006

10/27

/2006

10/2

9/200

610

/31/

2006

11/2

/200

611

/4/2

006

11/6

/200

611/

8/20

0611/

10/2

00611/

12/2

00611

/14/

2006

11/1

6/200

611

/18/

2006

11/20

/2006

11/2

2/200

611

/24/

2006

11/2

6/200

611

/28/

2006

11/3

0/20

0612/

2/20

0612

/4/2

00612

/6/2

006

Date

Pum

p R

ate

(gpm

)

Supplemental Pilot Trial Results

Initial pumping rate between 30 and 40 gpm

Decreased capacity of injection well required old intermediate extraction well to be used for overflow

Final pumping rate = 8 gpm

Transducer data suggested fouling in new injection well however no scale noted in video inspection completed at end of test

Pilot Trial Conclusions

GCW technology is not viable at the site

Groundwater pumping is a viable remediation method for the shallow pH plume

GCW pilot test fouled the deep soil formation creating a zone of low conductivity

CO2 injection is an effective method of pH reduction