2-Jul-08

Is Purging Necessary? A Comparison of Groundwater Data from Pre and Post-

Purge Samples Alexander Oiffer a, James Armstrong b, Brent Moore c, Trevor Butterfield a

a WorleyParsons Komex b WorleyParsons Komex

c Devon Canada Corporation

Objective

To evaluate the ability of No-Purge sampling to yield representative groundwater samples within a long-term groundwater quality monitoring program.

Potential Advantages of No-Purge Sampling include:– Cost/Time savings– Decreased groundwater disposal costs

Outline

1. Overview of purging (Downfalls, Alternatives)

2. Review of previous no-purge sampling studies

3. Evaluation of no-purge sampling based on conceptual understanding

4. Evaluation of no purge sampling based on field data

5. Conclusions and Future Plans

Traditional Purging

A means to remove “stagnant”, “non-representative”waterPurge volumes commonly (and somewhat arbitrarily) designated as 3 “well volumes”. Can generate excessive turbidity Can induce aeration of the water column and volatilization of organic compounds. Similar effect from turbulent recharge.Time consuming and disposal can be problematic.

Alternatives to Purging

Passive Sampling DevicesLow Flow Sampling DevicesNo-Purge Sampling

Some Previous Studies

Komex (1999) – VOCs, DOC, Major Ions comparable in Pre/Post PurgeRobin and Gillham (1987) – Flushing of water column less rapid above well screen.USGS (2006) – Water quality comparable when purging 1 or 3 wellbore volumes.San Mateo Health Services Agency (1997) – No purge sampling recommended for unconfined aquifers and well screened across the water table.

No-Purge sampling has not been broadly accepted (e.g. Barcelona, 2000).

No-Purge Sampling

Evaluate No-Purge sampling based on:– Conceptual understanding of well hydraulics and mass transport

processes– Comparison of field samples collected prior to and following

purging

Well Hydraulics – Conceptual Model (Where Does the Water Come From)

d2d

Plan view of flowline convergence to a monitoring well (with no filter pack) under natural gradients

After Wilson et al. 1997

Well Hydraulics – Conceptual Model (Where Does the Water Come From)

d

1.2d

Plan view of the radius of influence of a monitoring well (with no filter pack) after purging three casing volumes.

-Assumes water level in screen

Assume n=30%

Well Hydraulics – Conceptual Model

Zone of less Active Mixing

Zone of Active Mixing

After Robin et al., 1987

Potential Pitfalls of No-Purge Sampling

Volatile Loss from Water Column

Calculated % Benzene Loss from a 1 m Thick Water Column

Benzene loss calculation after Schwarzenbach et al., 2003

C6H6

Study - Setting

Shallow monitoring wells (approx 5 to 10 m deep)Dataset generated from up to 18 wells for each parameterWater table within or immediately below the screen.Till, sand or sedimentary bedrock.Inertial pumps and bailers

5-10 m

Evaluation of Field Data

What is a significant difference?In this case assessed based on:

– If sample concentrations differ by more than the field duplicatepair

– If sample concentrations differ by more than five times the method detection limit (Zeiner, 1994) or a RPD of 20% (which ever is greater)

ResultsConservative and Semi-Conservative

Parameters

0

10

20

30

40

50

0 10 20 30 40 50Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

0

10

20

30

40

50

0

25

50

75

100

125

150

0 25 50 75 100 125 150Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

0

25

50

75

100

125

150

0

50

100

150

200

250

0 50 100 150 200 250Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

0

50

100

150

200

250Calcium

SodiumChloride

Key:Pre and Post Purge Samples

Field Duplicates

Pre and Post Purge Samples (water level occurs above well screen)

1 to 1 line

10 Relative Percent Difference

20 Relative Percent Difference

ResultsRedox Indicators

0.0

2.0

4.0

6.0

8.0

10.0

0.0 2.0 4.0 6.0 8.0 10.0Pre-Purge (mg/L)

Pre-

Purg

e (m

g/L)

0.0

2.0

4.0

6.0

8.0

10.0

0.0

10.0

20.0

30.0

40.0

50.0

60.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

0.0

10.0

20.0

30.0

40.0

50.0

60.0Iron Manganese

0

8

16

24

32

40

0 8 16 24 32 40Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

0

8

16

24

32

40SulphateKey:

Pre and Post Purge Samples

Field Duplicates

Pre and Post Purge Samples (water level occurs above well screen)

1 to 1 line

10 Relative Percent Difference

20 Relative Percent Difference

ResultsVOCs

Key:Pre and Post Purge Samples

Field Duplicates

Pre and Post Purge Samples (water level occurs above well screen)

1 to 1 line

10 Relative Percent Difference

20 Relative Percent Difference

Benzene

0

0.04

0.08

0.12

0.16

0 0.04 0.08 0.12 0.16Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

0

0.04

0.08

0.12

0.16Benzene

y

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14Ethylbenzene

Xylenes

0

0.09

0.18

0.27

0.36

0.45

0 0.09 0.18 0.27 0.36 0.45Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

0

0.09

0.18

0.27

0.36

0.45Xylenes

ResultsPolycyclic Aromatic Hydrocarbons

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00

Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

Differences in concentration between Purged and No-Purge samples are minimal relative to spatial and temporal heterogeneity.

– SO4 concentrations at DP2 and DP3 differed by 2-3 orders of magnitude.

– SO4 concentrations at MW1, in samples collected during consecutive sampling events were observed to vary from 400 to 100 mg/L (and vice versa).

Effect of Purging Relative to Spatial and Temporal Heterogeneity

Depth DP1 DP2 DP3 P34 MW1 MW2

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Conclusions

1. Conceptual model suggests No-Purge sampling most applicable in monitoring wells screened across the water table, completed in reasonably permeable formations.

2. Initial data shows good reproducibility, and promising for the application of No-Purge Sampling for compliance monitoring.

3. Comparability of Pre/Post Purge data consistent with previous field studies (e.g., Komex 1999)

4. Effect of purging on analyte concentrations less than effects of screen length, and spatial/temporal variability.

Future Plans

Study is OngoingAs the dataset grows, it will be possible to more fully evaluate the effect of physical variables (e.g., well screen position and porous medium material) on Pre and Post-Purge reproducibility

Questions?

Extra Slide(s)

Toluene

0

0.0005

0.001

0.0015

0.002

0.0025

0 0.0005 0.001 0.0015 0.002 0.0025Pre-Purge (mg/L)

Post

-Pur

ge (m

g/L)

0

0.0005

0.001

0.0015

0.002

0.0025