Keith C. Hackley, Todd M. Coleman, Steven R. Pelphrey

Stray Gas Conference

2012

Collection and Analysis of Gas Samples from Groundwater

Interest in Dissolved Gas analysis

1. Addressing concerns of contamination (source identification)

2. Monitoring changes in concentration over time

3. Understanding the geochemical processes

4. Dating groundwater

5. Establishing background databases

Example from aquifer in central

IL.

Hackley et al., 2010, GSA Bulletin

Gas analysis & interpretation:

1. For good interpretations you need good data

2. For good data you need good samples

3. Good samples depend on your sample collection methods & containers

Domestic Well Modified from D.D. Coleman, 2011

Methane Solubility in mg/L (ppm) at 12 deg. C 0 300 400

Pres

sure

in a

tmos

pher

es (a

bsol

ute)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0

100

200

300

400

500

Water D

epth in Feet

100 200

Under Saturated

27 ppm (41 cc/L)

Completely dissolved, one phase

Gases may be under saturated or over saturated

Modified from D.D. Coleman, 2011

Use of bottle for sample collection

• For conditions where gas is below the saturation limit, bubbles will not form and gas will leave solution only by diffusion.

• Water can be collected in a non-permeable bottle (not LDPE) by minimizing contact with air.

• Basically filling the sample bottle immersed in a bucket filled w/ formation water

• Refrigeration or a preservative to prevent bacterial activity prior to laboratory analysis.

Sampling Procedures – under saturated

Examples of bottles used for dissolved gas collection.

Modified from D.D. Coleman, 2011

Addition of bactericide to prevent microbial degradation

Bactericide, with & without:

without

with

Headspace Equilibration In the laboratory, some of the water is withdrawn and replaced with a helium headspace

The headspace and the water are equilibrated at constant temperature

Analytical Procedures

Modified from D.D. Coleman, 2011

• The headspace is then analyzed

• The volume of water is measured by weighing

• The concentration of gas in the water is calculated using Henry’s law.

Headspace Equilibration Analytical Procedures

Modified from D.D. Coleman, 2011

This method is valid so long as:

1. Accurate measurements are made

2. The sample was adequately preserved

3. No gas was lost during collection of the sample

4. No air contamination was introduced when the sample was collected.

Under saturated conditions (Single Phase Sample)

Bottle – headspace equilibration

As the pressure on water is reduced by bringing it to the surface, a gas phase (bubbles) will form when the combined partial pressures of the dissolved gases exceeds the confining pressure.

Example of gas effervescing

from groundwater sampled from domestic well.

Water Displacement – gas phase This method has been in use for over 50 years

Sampling Procedures

• Allows collecting large gas samples for multiple analyses

• Focuses on the gas phase • Does not give an accurate concentration of

the amount of gas in the water

Water Displacement technique

• To adequately assess the gas concentration in water that is saturated at depth, it is necessary to either maintain the water at pressure to prevent a gas phase from forming, or collect both the water and the gas phase.

Sampling Procedures - over saturated conditions

• Evacuated gas bags can be used for collecting water for these conditions.

Sampling Procedures – over saturated conditions

Gas bags

Using Gas bags • IsoBagsTM are transparent, we

can see if there is a separate gas phase, and if none exists, helium can be injected as with the headspace equilibration

• The gas can be measured and analyzed and the gas concentration calculated with Henry’s Law.

• With gas bags, the pressure is easy to measure accurately

Analytical Procedures

Sampling Procedures Bottle vs. IsoBagTM

• Gas bags can be used for either over- or under- saturated conditions, BUT • They are delicate and easily broken • They are more expensive than bottles • They do not allow collecting as much water and thus

detection levels are not as low as with large bottles

• Currently - experimenting with a new container that will hopefully be • easier to use in the laboratory, • easier to use in the field, • will allow one container to be used for multiple

sampling methods • will provide reliable data on dissolved gas

concentrations while also providing sufficient sample for gas characterization

Alternative Container

The IsoFlask™

•Collapsible like a gas bag, can be evacuated

•Will stand up like a bottle

•More rugged than a gas bag or a glass bottle

•Impermeable to hydrocarbon gases

•Transparent

•Allows introduction of bactericide capsules

Sampling with the IsoFlask™

Methane Ethane Propane i-Butane n-ButaneCylinder -42.609 -28.74 -31.924 -30.92 -33.628IsoPak #1 -42.417 -28.498 -31.584 -30.709 -33.474IsoPak #2 -41.899 -28.464 -31.822 -30.908 -33.621IsoPak #3 -42.117 -28.322 -31.77 -30.836 -33.301

-45

-43

-41

-39

-37

-35

-33

-31

-29

-27

-25

Car

bon

Isot

ope

Valu

es (

‰)

Carbon isotope data after 90 days in IsoPaks™(error bars indicate the analytical level of precision; +/- 0.3 ‰)

Summary

1. Good sampling techniques are required for good data and accurate interpretations.

2. Standard headspace equilibration using the bottle method is effective when gas concentrations are under saturated at atmospheric pressure (<27ppm @ 12°C).

3. The water displacement technique is good for obtaining gas samples of sufficient size but does not result in accurate concentration measurements.

Summary

4. The current transparent bags allow collecting gas samples at under- and over- saturated conditions but are rather delicate and do not collect as much water as the bottles.

5. The initial data on the prototype of the IsoFlaskTM look promising and this container should make sampling simpler and analyses more accurate.

Thank You