Michael Lawson

EPA Hydraulic Fracturing Study Technical Workshop on Chemical and Analytical Methods24 – 25th February 2011 Arlington, Virginia

Isotopic Tracers of Groundwater Contamination: Techniques, applicatiand considerations
Talk structure

• Background related to hydraulic fracturing and the relations

groundwater resources

• Why should isotopic tracers be employed for determining th

and controls on, groundwater composition?

• Use of O, H and Sr isotopes to trace natural water bodies• Introduction of 3H-3He and 4He techniques to monitor evolu

groundwater age

• 13C systematics to constrain the origin of methane in groun

• Limitations and recommendations
Background

• A requirement exists for the implementation of techniques that verify and i

groundwater is not contaminated as a result of hydraulic fracturing.

• Groundwater contamination can derive from numerous sources which may 

distinct “fingerprint” that allows for identification of the source of contaminaprocess which gave rise to the contamination. With respect to hydraulic frdirect and indirect pathways could be investigated anytime contamination 

• Direct contamination –unconventional gas or fracturing fluids migrating into groun

degrading water quality. While EPA's 2004 study and others have found no direcontamination from the hydraulic fracturing process itself, the breakdown of opersafeguards, e.g. lack of well bore integrity, surface spills, etc. may present pathw

• Indirect contamination – e.g. migrating fluids drive (bio)geochemical processes th

the release of contaminants to the groundwater or naturally occurring species pregroundwater at concentrations that exceed threshold values. 

• Isotopic techniques can identify the source of contamination and yield info

process such as redox or chemical evolution, mixing and migration.

• The chemical composition of relevant end members (groundwaters, produ

and fracturing fluids) are needed to make best diagnostic use of isotope or geochemical techniques. 
Why use isotopic tracers?

• Major-ion chemistry of natural and injected fluids may 

sufficiently enough for identification of the source(s) of present in groundwaters.

Isotopic tracers allow for:• Identification of naturally occurring surface-water contributio• Identification of mixing of different groundwater bodies.• Identification of contributions and identification of the sourc

natural gas to shallow groundwaters.

• Is this a result of the migration from shales as a direct result of 

fracturing or is this naturally present e.g. through the biologicaproduction of methane in shallow groundwater?
Natural water compositions Natural water compositions

-80-60-40-20

0

20

-12 -10 -8 -6 -4 -2 0 2 

δ18

O

δ

D

GMWL

Palaeowater

Evaporation surface wate

Seawater

groundwater

Exchange with H2

S

LMWL

-80-60-40-20

0

20

-12 -10 -8 -6 -4 -2 0 2 

δ18

O

δ

D

GMWL

Palaeowater

Evaporation surface wate

Seawater

groundwater

Exchange with H2

S

LMWL

After Geyh (2000)
Elucidating mixing processes Elucidating mixing processes

-80-60-40-20

0

20

-12 -10 -8 -6 -4 -2 0 2 

δ18

O

δ

D

GMWL Palaeowater

Evaporation surface wat

Seawater

Groundwater B

mixing

mixing

Groundwater A

-80-60-40-20

0

20

-12 -10 -8 -6 -4 -2 0 2 

δ18

O

δ

D

GMWL Palaeowater

Evaporation surface wat

Seawater

Groundwater B

mixing

mixing

Groundwater A

After Geyh (2000)
-26 -24 -22 -20 -18 -16 -14 -12 

δ18O ‰ VSMOW

Modern recharge

ShiBri

Glacial meltwater

After Douglas et al., 2000

Groundwater Mixing 
Resolving small contributions

• Formation waters typically have 

higher Sr concentrations compared to surface waters and shallow groundwater.

• When Sr isotopic compositions 

vary between different water bodies, Sr isotopes (87Sr/86Sr) 

provide a sensitive indicator of the presence of low concentrations of differently sourced fluids. 

End Mem

End Member 2

After Pete
Tritium-3He

• Directly incorporated into water Directly incorporated into water 

molecule. molecule.

• Decays to Decays to 3He, t1/2 = 12.43 yrs. = 12.43 yrs.

• GW < 60 years. GW < 60 years.

4He

• Radiogenic He produced by U and Radiogenic He produced by U and 

Th decay from crustal minerals decay from crustal minerals and atmosphere. and atmosphere.

• Used to identify older GW (100 Used to identify older GW (100’s –

100,000 100,000’s years). s years).

Groundwater age Groundwater age
Examples of Biogenic vs. ThermogenicGas Interpretation Schemes

1.E+011.E+021.E+031.E+041.E+05

-100 -90 -80 -70 -60 -50 -40

δ13C methane(‰)

C

1(C2 + C3

)

Mixed

-30

Bacterial

Thermogenic

-20

-400 -350 -300 -250 -200 

Therm

Transition

Bio

(CO2 Re

Biogenic

(Fermentation) 

Maturation

Methane δD (‰)

After Bernard, 1978 
Technical Summary

• There is a need to establish natural baseline conditions to 

deviations from the natural system.

• Major-ion chemistry alone may not yield definitive results a

source(s) of any identified deviations.

• The application of multiple isotope systems allows for a mo

and comprehensive assessment of potential sources of contamination and allows for quantitative estimates of the contribution of these sources to be made.

• Sampling of potential end member fluids (surface waters, 

groundwaters and introduced fluids used during the fractuprocess) may provide important compositional information prove useful when attempting to identify the existence andof any perceived contamination.