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For a variety of fluids (fresh water, brines, AMD-impacted and co-produced waters) And many geologic rock types and materials Coal, shale, permeable limestone and sandstone aquifers Deep and shallow units Cements/grouts/combustion byproducts (coal fly ash)
Must be able to identify contaminant source as well as provide ongoing monitoring
Introduced tracers Major and trace element geochemical signatures
Natural isotopic signatures © Copyright 2014 EchelonAGC
Four naturally-occurring stable isotopes, including 87Sr and 86Sr
87Sr is supplemented by the slow decay of 87Rb (‘radiogenic isotope’) Half-life: 48.8 Ga
87Sr/86Sr increases with time
© Copyright 2014 EchelonAGC Capo et al., 1998
Rocks with different compositions and geological histories develop distinct 87Sr/86Sr ratios Reflect sources of Sr available during
formation
Waters which interact with these units can inherit their 87Sr/86Sr
87Sr/86Sr in geologic materials is an indicator of both age and geochemical origin
© Copyright 2014 EchelonAGC
© Copyright 2014 EchelonAGC
0.712 –
0.703 –
0.709 –
0.706 –
Millions of Years 0 200 400 600
– 40
– -80
– 0
– -40
87Sr/86Sr
(RIVERS)
Ubiquitous and abundant One of the most abundant
trace elements in crustal rocks
Systematics understood Proxy for calcium Sr substitutes for Ca in
feldspars, carbonates, sulfate minerals
© Copyright 2014 EchelonAGC
Does not fractionate appreciably during physical, chemical, or biological processes (e.g. evaporation, mineral dissolution/precipitation, oxidation, sorption, plant/animal uptake)
Isotopic composition not affected by dilution or mixing
© Copyright 2014 EchelonAGC
~2 µg (10-6 g) Sr Minimum amount of
sample to process: Carbonate: < 10 mg Shale: 20-50 mg Groundwater: 0.5 – 1L Produced water: <500 mL AMD: <500 mL Brines: <500 mL
© Copyright 2014 EchelonAGC Capo et al., 1998
Can be diagnostic of specific environmental reservoirs, both natural and anthropogenic
Allows for sensitive quantification of source inputs, especially if source 87Sr/86Sr are very different
Quantifies amount of mixing
Can be used to understand processes along migration pathways
© Copyright 2014 EchelonAGC
To use Sr isotopes as a tracer, you must have: The isotopic composition of all
sources of Sr to the system The extent of isotopic variation
within individual sources
© Copyright 2014 EchelonAGC
Since some samples might have very low Sr concentrations, it is important to have very clean sample bottles for collection HDPE bottles (Nalgene) Acid-washed (ex. p. 22-27,
http://water.usgs.gov/owq/FieldManual/chapter3/final508Chap3book.pdf)
Samples for Sr isotope analysis should not be acidified in the field unless ultrapure acids are available
When possible, separate aliquots should be taken for major/trace element concentrations, and Sr isotopes
© Copyright 2014 EchelonAGC
Solid samples (cores, well cuttings, ash, soil, rocks) must be pulverized and leached to release Sr Simulate fluids that may
be interacting with the rock
© Copyright 2014 EchelonAGC
Sr concentrations must be accurately measured before processing for Sr isotopes Sr separation performed in Class 100 clean lab
© Copyright 2014 EchelonAGC
Rapid Sr separation (5x faster than traditional methods) Vacuum-assisted Disposable columns
Isotope measurement by multicollector ICP-MS (4-5 samples/hr) Typical measurement
uncertainty: 0.002% See Wall et al., 2013
© Copyright 2014 EchelonAGC
Marcellus produced waters • Four Pennsylvania counties Bradford Westmoreland Washington Greene
• Different sample types:
Individual well single samples
Impoundment samples
Produced water time series
© Copyright 2014 EchelonAGC Chapman et al., 2012; Kolesar et al., 2013; Capo et al., 2014
0
4
8
12
16
2
6
14
10
10 20 30 40 50
Sr SW ε
Bradford Co. Bradford Co.
Washington Co.
Greene Co.
Westmoreland Co. n
0.710 0.711 0.712 87Sr/86Sr
© Copyright 2014 EchelonAGC Chapman et al., 2012
© Copyright 2014 EchelonAGC Chapman et al., 2012
15 20 25 30 35 40 45 50 55 60 0.0
0.1
0.2
0.3
0.4
Sr SW ε
Sr/C
a
Bradford Co. PW Westmoreland Co. PW Greene Co. PW Washington Co. PW Pittsburgh Coal AMD Stream water
to Venango brine (0.015, +111)
Combination of εSr and Sr/Ca likely to distinguish between sources in nearly all cases Even small amounts (<0.1%) of Marcellus produced waters
can significantly shift stream εSr
© Copyright 2014 EchelonAGC
87Sr/86Sr of Marcellus produced waters allow for extremely sensitive tracking
Potential applications: Verification of safe water disposal Determination of origin of dissolved
constituents in surface and ground waters affected by multiple sources Quantification of mixing
Produced water reflects: Original composition of injection water Mobilized constituents from shale Formation waters liberated by fracing
Sr measured in flowback up to 27 months post-frac
© Copyright 2014 EchelonAGC Capo et al., 2014
© Copyright 2014 EchelonAGC Capo et al., 2014
Strontium concentrations plateau within the first year of flowback
© Copyright 2014 EchelonAGC Capo et al., 2014
εSr continue to increase even after 2 years
Greene A
Greene B
Greene C
© Copyright 2014 EchelonAGC Capo et al., 2014
Linear correlations indicate the increase in εSr is due to mixing of two endmembers
Produced water
Leachates
25 © Copyright 2014 EchelonAGC
Stewart et al., in press © Copyright 2014 EchelonAGC
Higher- εSr endmember most likely pore/formation water rather than soluble salts
© Copyright 2014 EchelonAGC
Sr isotopes show a change over time from frac fluid + formation water primarily formation water Mixing models combined with leachate geochemistry
suggest that dissolved salts in produced water originate in formation water rather than as soluble salts within the shale
Greene County site with: Six Marcellus laterals One vertical Marcellus well Four Upper Devonian (UD) gas wells One shallow groundwater spring
Sr measured before and after hydraulic fracturing of laterals
© Copyright 2014 EchelonAGC Kolesar Kohl et al., 2014
© Copyright 2014 EchelonAGC
Most UD wells show no change after fracturing (p values >0.05)
For isotopic shifts to be considered significant enough to suggest Marcellus fluid incursion, εSr would need to decrease by 1-3 units
Kolesar Kohl et al., 2014
© Copyright 2014 EchelonAGC
Sr isotope values fall between Marcellus and Upper Devonian values
Values shift on a semiannual basis (±0.8 from the mean)
Spring water contains very little Sr
Very sensitive to any potential mixing with produced water
Kolesar Kohl et al., 2014
© Copyright 2014 EchelonAGC
The only well in the study that showed a significant change in Sr isotope values after horizontal wells were fractured (from +33.8 to +35.9)
Sr concentration also increased by ~200 mg/L
New pathways within the Marcellus were opened up by fracturing
Kolesar Kohl et al., 2014
© Copyright 2014 EchelonAGC
Calculated mixing models between produced waters and spring water
Most sensitive elements: Ba, Br, Cl, Sr
Elemental ratios (Sr/Ca, Br/Cl) less sensitive than absolute concentrations
Kolesar Kohl et al., 2014
© Copyright 2014 EchelonAGC
Greater sensitivity than elemental conc., especially in waters with natural seasonal variation
Unlike elemental concentration, Sr isotopes can distinguish between UD and Marcellus produced waters
Kolesar Kohl et al., 2014
© Copyright 2014 EchelonAGC
Subsequent to hydraulic fracturing, no significant migration of Marcellus-derived fluids was observed in Upper Devonian or shallow groundwater units Shift in Sr isotopes of vertical Marcellus well suggests
fracturing opened new flowpaths within the unit Sr isotopes show greater sensitivity to potential brine
migration than elemental concentrations or ratios
History of fossil fuel activities: Oil and gas extraction since mid-1800’s, Upper Devonian sands (Bradford,
Venango) Hilltop strip mines (Clarion, Brookville coals)
© Copyright 2014 EchelonAGC Chapman et al., 2013
Unplugged wells Artesian flow (>20 gpm) High total dissolved solids (TDS) Sulfate-dominated >100 mg/L iron
© Copyright 2014 EchelonAGC
Gas well discharges: High sulfate, iron Low sodium, chloride
AMD: High sulfate Low iron
Oil and gas brines: High sodium High chloride
© Copyright 2014 EchelonAGC
Siderite nodules (FeCO3) Coal overburden
Siderite cement Shallow sandstone drinking
water aquifers
© Copyright 2014 EchelonAGC
Chapman et al., 2013 © Copyright 2014 EchelonAGC
1-3: Gas well discharges
4-7: AMD
8: Unaffected aquifer water
9-10: Venango oil and gas brines
11-13: Siderite nodules
14-16: Siderite cement
87Sr/ 86Sr Sr SW ε
- 0.710
- 0.712
- 0.714
- 0.716
- 0.718 Venango brines
Siderite nodules
Siderite cement
Gas well discharges AMD
Unaffected aquifer
© Copyright 2014 EchelonAGC
87Sr/ 86Sr
Siderite cement
- 0.719
- 0.718
- 0.717
- 0.716
- 0.715
© Copyright 2014 EchelonAGC Chapman et al., 2013
Supported by saturation indices generated by PHREEQc modelling software
© Copyright 2014 EchelonAGC Chapman et al., 2013
Variety of sample types are often necessary to understand subsurface processes
Strontium isotopes able to: Differentiate between coal- and oil/gas-related
inputs Distinguish mineralogically identical but
genetically different sources
© Copyright 2014 EchelonAGC
Inactive coal mine partially filled by injection of grout to mitigate AMD 98% coal utilization byproducts (CUB) 2% Portland cement
Mine sealing unsuccessful – AMD still leaking 3 years later Major/trace element chemistry not sufficient to
distinguish between discharges which interacted with grout and those that did not
45 © Copyright 2014 EchelonAGC
Hamel et al., 2010
Sr isotopes show that discharges received 30-40% of their total Sr from grout material Grout is chemically eroding at a rate of approx.
0.04% (3 x 104 kg) per year
48 © Copyright 2014 EchelonAGC
Hamel et al., 2010
Sr isotopes able to distinguish waters interacting with grout material vs waters with no interaction Major/trace element chemistry unable to differentiate
Sr isotopes used to calculate rate of grout dissolution
49 © Copyright 2014 EchelonAGC
Hamel et al., 2010
Capo, R.C., Stewart, B.W., Rowan, E., Kolesar, C., Wall, A.J., Chapman, E.C., Hammack, R.W., and Schroeder, K.T., 2014. The strontium isotopic evolution of Marcellus Formation produced waters, southwestern Pennsylvania. International Journal of Coal Geology, available online 28 Dec 2013. Capo, R.C., Stewart, B.W., and Chadwick, O.A., 1998. Strontium isotopes as tracers of
ecosystem processes: theory and methods. Geoderma, v. 82, p. 197-225. Chapman, E.C., Capo, R.C., Stewart, B.W., Hedin, R.S., Weaver, T.J., and Edenborn,
H.M., 2013. Strontium isotope quantification of siderite, brine and acid mine drainage contributions to abandoned gas well discharges in the Appalachian Pleateau. Applied Geochemistry, v. 31, p. 109-118. Chapman, E.C., Capo, R.C., Stewart, B.W., Kirby, C.S., Hammack, R.W., Schroeder, K.T.,
and Edenborn, H.M., 2012. Geochemical and strontium isotope characterization of produced waters from Marcellus Shale natural gas extraction. Environmental Science & Technology, v. 46, p. 3545-3553. Chapman E.C., Capo, R.C., Stewart, B.W., Johnson, J.D., Graney, J.R., Hammack, R.W.,
2011. Geochemical and strontium isotope study of sequentially extracted metals from Marcellus Shale drill core. Geol. Soc. Am. Abstr. Prog., #236-3.
50
© Copyright 2014 EchelonAGC
Hamel, B.L., Stewart, B.W., Kim, A.G., 2010. Tracing the interaction of acid mine drainage with coal utilization byproducts in a grouted mine: Strontium isotope study of the inactive Omega Coal Mine, West Virginia (USA). Applied Geochemistry, v. 25, p. 212-223. Kolesar Kohl, C.A., Capo, R.C., Stewart, B.W., Wall, A.J., Schroeder, K.T.,
Hammack, R.W., and Guthrie, G.D., 2014. Strontium isotopes test long-term zonal isolation of injected and Marcellus Formation water after hydraulic fracturing. Environmental Science & Technology, v. 48, p. 9867-9873. Kolesar, C.A., Capo R.C., Wall, A.J., Stewart, B.W., Schroeder, K.T., Hammack, R.W.,
2013. Using strontium isotopes to test stratigraphic isolation of injected and formation waters during hydraulic fracturing, AAPG Search and Discovery Article #90163. Stewart, B.W., Chapman, E.C., Capo, R.C., Johnson, J., Graney, J.R., Kirby, C.S., and
Schroeder, K.T., in press. Origin of brines, salts and carbonate from shales of the Marcellus Formation: Geochemical and Sr isotope study of sequentially extracted fluids, Applied Geochemistry.
51
© Copyright 2014 EchelonAGC