Scholarly article on topic 'O, H, CDIC, Sr, B and 14C Isotope Fingerprinting of Deep Groundwaters in the Karoo Basin, South Africa as a Precursor to Shale Gas Exploration'

O, H, CDIC, Sr, B and 14C Isotope Fingerprinting of Deep Groundwaters in the Karoo Basin, South Africa as a Precursor to Shale Gas Exploration Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — J. Miller, K. Swana, S. Talma, A. Vengosh, G. Tredoux, et al.

Abstract The possibility of shale-gas development in the environmentally sensitive Karoo Basin, South Africa has created the need to develop a hydrochemical baseline for deep Karoo groundwater. Little is known about the composition of deep (>1500m) groundwater in the Karoo because there are no functional boreholes that tap these depths. This study examined whether sub-thermal spring waters, defined as groundwater with a temperature >25°C, are suitable proxies for deep Karoo groundwater. On the basis of temperature, major cations and anions and 14C, three groups of groundwaters were defined: (1) shallow (cold, young); (2) deep (sub-thermal, old); and (3) mixed (sub-thermal or cold, intermediate age). δ18O, δ2H, δ13CDIC, δ11B and 87Sr/86Sr ratios for the three groups indicate that the sub-thermal groundwaters may be suitable proxies for deep groundwater but also that mixing already occurs between the deep and the shallow groundwater systems. This does not impact on the overall groundwater quality but could leave the shallow groundwater vulnerable to future contamination should shale gas development proceed.

Academic research paper on topic "O, H, CDIC, Sr, B and 14C Isotope Fingerprinting of Deep Groundwaters in the Karoo Basin, South Africa as a Precursor to Shale Gas Exploration"

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Procedía Earth and Planetary Science 13 (2015) 211 - 214

11th Applied Isotope Geochemistry Conference, AIG-11 BRGM

O, H, CDiC, Sr, B and 14C isotope fingerprinting of deep groundwaters in the Karoo Basin, South Africa as a precursor to

shale gas exploration

J. Millera*, K. Swanaa, S. Talmab, A. Vengoshc, G. Tredouxd, R. Murraye, M. Butlerf

aStellenbosch University, Private Bag X1, Matieland, 7602, South Africa b Independent Researcher, PO Box 72906, Lynnwood Ridge, 0040,Pretoria, South Africa c Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, North Carolina 27708, USA

d Independent Researcher, Cape Town, South Africa eGroundwater Africa, 54 Irene Road, Somerset West, 7130, South Africa f iThemba LABS, South Africa

Abstract

The possibility of shale-gas development in the environmentally sensitive Karoo Basin, South Africa has created the need to develop a hydrochemical baseline for deep Karoo groundwater. Little is known about the composition of deep (>1500m) groundwater in the Karoo because there are no functional boreholes that tap these depths. This study examined whether sub-thermal spring waters, defined as groundwater with a temperature >25°C, are suitable proxies for deep Karoo groundwater. On the basis of temperature, major cations and anions and 14C, three groups of groundwaters were defined: (1) shallow (cold, young); (2) deep (sub-thermal, old); and (3) mixed (sub-thermal or

1 Q O 1 O 11 07 Q/T

cold, intermediate age). 81 °O, 5 H, CDIC, 81 B and 0/SrrDSr ratios for the three groups indicate that the subthermal groundwaters may be suitable proxies for deep groundwater but also that mixing already occurs between the deep and the shallow groundwater systems. This does not impact on the overall groundwater quality but could leave the shallow groundwater vulnerable to future contamination should shale gas development proceed.

© 2015The Authors.PublishedbyElsevier B.V. Thisis an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of AIG-11

Keywords: Karoo Basin, isotope fingerprinting, deep groundwater, shale gas exploration,

* Jodie Miller, Tel.: +27-21-808-3121. E-mail address: jmiller@sun.ac.za

1878-5220 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of AIG-11

doi:10.1016/j.proeps.2015.07.050

1. Introduction

Over the last ten years considerable effort has been put into shale-gas exploration in order to provide an alternative source of energy. The process of extracting the shale-gas is however not without risks and has been shown in some instances to cause serious contamination issues through spills and disposal of wastewater, shale gas well integrity associated with stray gas leaks and contamination (Vengosh et al., 2014). The Karoo Basin in South Africa is one of the largest sedimentary basins in the world and is currently the target of a number of shale-gas exploration projects. However, there is considerable concern amongst different sectors of the population as to how shale-gas extraction might impact on groundwater quality. Such concerns are heightened by the fact that South Africa is a naturally dry country and the Karoo in particular is a water stressed region. Shale-gas in the Karoo Basin is thought to exist in the Ecca Group sediments, specifically the Whitehill Formation shales, which occur at depths in excess of 2500 metres. Although there is a wide network of boreholes shallower than 300 m throughout the Karoo that provide domestic and agricultural water, there is currently only one borehole that is known to go to depths greater than 2500 m and hence the nature of deep groundwater in the Karoo is largely unknown.

To address this, a study was initiated to fingerprint the deep groundwater in the Karoo and to differentiate deep groundwater from shallow groundwater in order to provide a baseline for future contamination monitoring. Since no direct access to the deep groundwater exists, the study investigated the sub-thermal springs within the Karoo to assess whether they can be used as proxy for deep groundwater and how their chemistry compares to the shallow groundwater system. Since there are no obvious heat sources in the Karoo Basin, such as high heat-producing igneous rock suites, groundwater with an elevated temperature is interpreted to have originated from greater depth. A wide spectrum of geochemical tracers were used to evaluate source depth and this contribution reports principally on the application of the stable isotopes of O, H, C, Sr and B along with radiometric C. In combination with temperature, major cations and anions the results show there is generally a clear distinction between sub-thermal and shallow groundwaters in the Karoo and that most sub-thermal groundwaters have relatively long residence times implying they are derived from deeper source regions. This difference in geochemical character facilitates modelling of mixing relationships between deep and shallow groundwater and could provide a geochemical framework for future assessment of the impact of hydraulic fracturing on the quality of groundwater in the Karoo.

2. Methods

Eight locations, widely dispersed throughout the Karoo Basin, were sampled. At each location a sub-thermal spring or borehole site was sampled along with a corresponding cold water (< 25°) borehole site. All the sites were measured for temperature, pH, electrical conductivity and alkalinity in the field and samples were collected for major cations and anions, stable isotopes of O, H, C, Sr and B as well as 14C. Cations (ICP-MS and ICP-AES) and anions (IC) were measured at Stellenbosch University, South Africa. 818O and 82H ratios were measured at iThemba LABS in Johannesburg using a Finnigan GasBench II IRMS. 811B ratios were determined on a Thermo Fisher Triton at the TIMS Laboratory at Duke University, USA. 87Sr/86Sr ratios were measured on a NuPlasma HR MC-ICP-MS at the University of Cape Town, South Africa. For samples with alkalinity >100 mg/L HCO3- , 14C and 813Cdic were measured at iThemba LABS Johannesburg. 14C was analysed via liquid scintillation counting and 8 CDIC by IRMS. For samples where alkalinity was <100 mg/L HCO3-, 14C and 813Cdic were analysed by Beta Analytic in Miami, USA, via AMS.

3. Results and Discussion

Groundwater from each of the sampling sites was initially classified as either warm or cold based solely on the temperature of the water at the collection point. A temperature of > 25 °C was used to define deep groundwater. To test the validity of this initial temperature-based deep versus shallow classification, the data was put through a second assessment using standard Stiff diagrams. Three Stiff diagram shapes were found. A "Y" shaped pattern in general associated with the warm groundwater, a hexagonal pattern in the cold groundwater and a variable pattern that was neither a "Y" shape nor a hexagonal shape. In addition to this, 14C values for these three groups are markedly different. The warm groundwater generally had very low 14C values of between 20-53 pmC, the cold groundwater between 74-94 pmC, and group with variable Stiff diagram shapes had 14C values between 50-74 pmC.

From these three assessments, the groundwaters were classified into three groups: deep (sub-thermal, old), shallow (cold, young) and mixed (above and below 25 °C, intermediate residence time) groundwater. These three groups were then used as a framework to assess suitability of O, H, C, Sr and B isotopes as deep-flow indicators.

Comparison of S2H and S18O values shows the deep group is depleted in the heavy isotopes with respect to the shallow group with a relatively narrow range of S2H and S18O values at approximately -39%o and -7.7%o respectively for the deep groundwater. The shallow group plots higher up the global meteoric water line (GMWL), with a considerably wider range in S2H and S18O values extending up to 0%o for both isotopes (Fig. 1a). The mixed group plots in between the deep and shallow groups also with a relatively narrow range of values at around -28% and -5.2%. Treating the one high 62H and 618O point as an anomaly, the shallow and mixed groups plot along the GMWL. The deep group in addition being more depleted also appears to plot along a shallower trend and together these features are consistent with recharge in the past (>6000 yrs) when climatic conditions (temperature, rainfall, evaporation) were different from the present (Talma and van Wyk, 2013).

« -20

□ Deep A Mixed ♦ Shallow

O D. -5

to -20

-6 -4 -2 ölsO (%oSMOW)

: □ B

: ♦ D « ; i? . ♦

- □□

-6 -4 -2 S180 (%o SMOW)

Figure 1. (a) 518O versus 52H. The global meteoric water line is plotted on the graph as reference; (b) 613CDIC ratios versus 518O. The wide distribution of 613CDIC values is likely the result of very low C contents reflected in very low alkalinity for the deep groundwaters.

3.1 SO. S2H and S13C

S13CDIC values (Fig. 1b) do not show the same distinction between the shallow and the mixed groups. However,

S13CDIC values for the deep boreholes are much more variable and range from < -25 % to over 0%. The range in

values for the deep boreholes more likely reflects the difficulty in determining S13CDIC values for groundwaters with very low alkalinities, in this case less than 100 mg/L HCO3- and these samples had poor reproducibility between

repeats. For the shallow and mixed groundwaters, S13CDIC values are dominantly indicative of a recharge regime

associated with C4 plants consistent with the geology and vegetation of the Karoo although the spread to values less

than -10%o suggests other factors are also contributing to the S13CDIC values.

3.3 Boron and SUB

Boron concentrations and 811B ratios are loosely correlated with increasing chloride (Fig. 2a) with the deep

groundwater generally having higher chloride and boron than the shallow groundwater. Boron isotope ratios show a

wide variation in values with 811B in the deep groundwater ranging from +5 to +30 % and in the shallow

groundwater from +27 to +35 % (Fig. 2a). Seawater remnants are expected to have 811B of >39%, and hence the

lower 811B values in the groundwater in this study more likely reflects mobilization of exchangeable boron from

marine clay minerals (Vengosh et al., 1994). Two sub-thermal (deep) sites that are suspected to come from basement

rocks underlying the Karoo Basin (see Sr isotopes below) have the lowest 811B values for the deep group and these

Figure 2. (a) SnB versus Cl concentrations for the three groundwater types. Note that one deep groundwater sample with a chloride concentration of 3879 mg/L is not shown; (b) S"B versus 87Sr/86Sr.

may reflect a contribution from crustal rocks. Overall, comparison of B concentrations, 811B values and chloride concentrations allows differentiation of deep from shallow groundwaters, but not mixed from deep groundwaters.

3.3 Strontium and ' 87Sr/86Sr

Sr concentrations in the deep and shallow groups are correlated with Cl concentrations but the Sr/Cl molar ratio is markedly different between the groups (0.001 for the deep groundwater compared to 0.009 for the shallow groundwater). 87Sr/86Sr ratios for the deep and shallow groundwaters overlap considerably and vary between 0.7089 and 0.7186. Two of the deep groundwater samples have highly radiogenic 87Sr/86Sr ratios of 0.7772 and 0.7539 (Fig. 2b) and these are associated with higher chloride, strontium and boron. These samples are the two northern-most sites in the study area, where the base of the Karoo Basin shallows and at least one of these boreholes is known to intersect basement consisting of mafic igneous rocks. The lack of clear definition in 87Sr/86Sr ratios between the deep and shallow groundwaters suggests this determinand may not be a good groundwater depth indicator for much of the Karoo Basin but can assess where boreholes are intersecting groundwater derived from underlying basement.

4. Conclusions

Using temperature, major cations and anions and 14C, along with stable isotopes of O, H, C, Sr and B it is possible to differentiate deep (sub-thermal) groundwaters from shallow (cold) groundwaters although a mixed groundwater group was also defined. O and H isotopes were the most effective at differentiating the three groups. B isotopes differentiated the shallow from the deep group but the deep group could not be differentiated from the mixed group. Sr and C isotopes did not differentiate any group. The mixed group suggests there is already interaction between the deep and shallow groundwaters but the nature of this interaction is unclear. Mixing does not appear to significantly impact on the existing groundwater quality in the Karoo but highlights that this interconnection will facilitate contamination by fracking fluids if these are not properly contained. Overall, the complexity of the chemical and isotopic variations require a careful examination and application of multiple isotopic tracers for delineating possible water quality modification upon shale gas operations in the Karoo Basin

References

Talma, A.S. and Van Wyk, E. (2013). Rainfall and Groundwater Isotope Atlas. In: T Abiye (editor). The use of isotope hydrology to characterize and assess water resources in southern Africa, 83-110. Report TT570/13, Water Research Commission, Pretoria

Vengosh, A., Jackson, R., Warner, N., Darrah, T., & Kondash, A. (2014). A critical review of the risks to water resources from nnconventional shale gas development and hydraulic fracturing in the United States. Environmental Science and Technology , A-O.