Scholarly article on topic 'Comparing the Residence Time of Deep vs Shallow Groundwater in the Karoo Basin, South Africa using 3H, 14C, 36Cl and 4He Isotopes'

Comparing the Residence Time of Deep vs Shallow Groundwater in the Karoo Basin, South Africa using 3H, 14C, 36Cl and 4He Isotopes Academic research paper on "Earth and related environmental sciences"

CC BY-NC-ND
0
0
Share paper
Keywords
{"Deep and shallow groundwater" / "Karoo Basin" / "radiometric isotopes" / "residence time."}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — K.A. Swana, J.A. Miller, A.S. Talma, T.H. Darrah, M. Butler, et al.

Abstract The Karoo Basin in South Africa is a water-stressed region but little is known about the deep groundwater in the region. Sub-thermal groundwaters, defined for the Karoo as >25°C, emanate from springs or boreholes but there is no direct access to deep (>1500m) Karoo groundwater. Sub-thermal groundwaters are therefore taken to represent deeper groundwater since there is no heat source in the Karoo other than that generated by burial. Analysis of sub-thermal groundwaters, as a precursor to possible shale-gas development in the Karoo, has produced a complete set of 14C, 3H, 36Cl, 4He isotope data for a range of different groundwater types and allows comparison of their effectiveness for evaluating the residence time of Karoo groundwater. 14C activities range from > 94 pmC to as low as 20 pmC, implying a range of apparent ages from modern to >20,000 years. 3He/4He ratios indicate a similar range of ages and in both cases the shallow groundwater is younger whilst the sub-thermal spring waters are generally older. 36Cl/Cl ratios showed comparable differences between shallow and deep groundwater. Variations in 3H activities are less clear but shallow groundwaters generally have higher 3H activities. Deviations from this pattern suggest a range of mixing processes for the different groundwater types. In general, the sub-thermal groundwater showed much longer residence times than shallow groundwater, implying that sub-thermal groundwaters are a reasonable proxy for deep groundwater in the Karoo.

Academic research paper on topic "Comparing the Residence Time of Deep vs Shallow Groundwater in the Karoo Basin, South Africa using 3H, 14C, 36Cl and 4He Isotopes"

Available online at www.sciencedirect.com

ScienceDirect

Procedía Earth and Planetary Science 13 (2015) 215 - 218

11th Applied Isotope Geochemistry Conference, AIG-11 BRGM

Comparing the residence time of deep vs shallow groundwater in the Karoo Basin, South Africa using 3H, 14C, 36Cl and 4He isotopes

K.A. Swanaa, J.A. Millera*, A.S. Talmab, T.H. Darrahc, M. Butlerd, K. Fifielde

aStellenbosch University, Private Bag X1, Matieland, 7602, South Africa bIndependent Researcher, PO Box 72906, Lynnwood Ridge, 0040,Pretoria, South Africa cSchool of Earth Sciences, The Ohio State University, Columbus, Ohio 43210, United States diThemba LABS, Johannesburg, 2050, South Africa e Research School of Physics and Engineering, Australian National University, Canberra, Australia

Abstract

The Karoo Basin in South Africa is a water-stressed region but little is known about the deep groundwater in the region. Subthermal groundwaters, defined for the Karoo as >25°C, emanate from springs or boreholes but there is no direct access to deep (>1500m) Karoo groundwater. Sub-thermal groundwaters are therefore taken to represent deeper groundwater since there is no heat source in the Karoo other than that generated by burial. Analysis of sub-thermal groundwaters, as a precursor to possible shale-gas development in the Karoo, has produced a complete set of 14C, 3H, 36Cl, 4He isotope data for a range of different groundwater types and allows comparison of their effectiveness for evaluating the residence time of Karoo groundwater. 14C activities range from > 94 pmC to as low as 20 pmC, implying a range of apparent ages from modern to >20,000 years. 3He/4He ratios indicate a similar range of ages and in both cases the shallow groundwater is younger whilst the sub-thermal spring waters are generally older. 36Cl/Cl ratios showed comparable differences between shallow and deep groundwater. Variations in 3H activities are less clear but shallow groundwaters generally have higher 3H activities. Deviations from this pattern suggest a range of mixing processes for the different groundwater types. In general, the sub-thermal groundwater showed much longer residence times than shallow groundwater, implying that sub-thermal groundwaters are a reasonable proxy for deep groundwater in the Karoo.

© 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

Keywords: Deep and shallow groundwater; Karoo Basin; radiometric isotopes; residence time.

* Jodie Miller, Tel.: +27808 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.2015.07.051

1. Introduction

Two of the key questions in water resource management are how old is the resource and how long will it last. Often when these questions are answered it is using one radiometric dating scheme and it is often taken to imply the length of time that the groundwater has been in the aquifer. Groundwater systems are complex and diverse in character and the "age" of the groundwater could mean different things. For 14C, the "age" of the water is taken to be the point at which it becomes isolated from the atmosphere. However, this does not necessarily equate to the residence time because it does not take into account transport time through the unsaturated zone. Moreover, different radiometric isotopes have different half-lives and different points of isolation from the atmosphere. Therefore, the "age" calculated by each radiometric isotope may be different. The purpose of this project is to compare the residence time as well as the aging processes of 3H, 14C, 36Cl and 4He, and to locate where the points of isolation from the atmosphere of each isotope occurs in the Karoo Basin. This information is used to determine the residence time of deep Karoo groundwaters and what this implies about a recharge processes to the deeper aquifer systems throughout the Karoo Basin.

2. Methods

Eight locations throughout the Karoo Basin were sampled. At each location a sub-thermal spring or borehole was sampled along with a corresponding cold borehole to represent the shallow groundwater. Temperature, pH, electrical conductivity and alkalinity of all samples were measured in the field and samples were collected for major cations and anions, and 3H, 14C, 36Cl and 4He isotopes. Cations (ICP-MS and ICP-AES) and anions (IC) were measured at Stellenbosch University, South Africa. 14C (for samples where the alkalinity is >100 mg/L HCO3-) and 3H were measured at iThemba LABS in Johannesburg, South Africa. 14C was analysed via liquid scintillation counting and 3H was analysed using a Packard Tri-Carb 2770TR/SL, a low level liquid scintillation analyser. 14C samples with alkalinity <100 mg/L HCO3-, were analysed by Beta Analytic in Miami, USA, via accelerator mass spectrometry (AMS). 36Cl was measured via AMS at the Australian National University, Canberra, Australia. 4He was measured via a Helix SFT noble gas mass spectrometer at Ohio State University, USA.

3. Results and Discussion

All the sites selected were classified as either deep or shallow, based on the temperature of the water emanating from the surface at the collection point, the Stiff diagram shape and the 14C activities. Some samples did not fit within either the deep or shallow groupings and a mixed group was defined. The data presented below is discussed in terms of these three groups and whether the mixed group does in fact represent a mixture of shallow and deep groundwater.

3.1 Carbon-14 and Tritium

As expected, the 14C activities of the deep waters are lower than the shallow waters, and that the 14C activities of the samples from the mixed group fall between these two (Fig 1a). In general there is a good relationship between temperature and the 14C activity of the waters (Fig 1a). The shallow group has the coldest temperatures (16-22°C) and highest 14C activities (74-94 pmC), indicative of shallow and relatively young groundwater. The temperature of the deep group ranges between 20-30°C and they have the lowest 14C activities (20-53 pmC). The mixed group plots in between the deep and shallow group with moderate to high 14C activities that slightly overlap with the other two groups (50-74 pmC) as well as moderate to warm temperatures. Analysis of the relationship between 3H and 14C activities (Fig 1b) shows a good correlation, with shallow sites having both higher 3H and 14C activities in comparison to deep sites with low 3H and 14C activities. The mixed group have moderate to high 3H and 14C activities, supporting the idea that these sites deliver a mixture of both deep and shallow groundwater. There are however a couple of outliers to this general trend and the relationship between 3H and 14C activities is not a linear one. Figure 1b shows a mixing line derived from an exponential mixing model that is widely used for the evaluation of time related tracers (Zuber and Maloszewski 2001). The good fit of the project data to this exponential mixing line indicates that the samples are likely to be well-mixed groundwaters of various residence times (Talma and

Weaver 2003). The wide range of 14C activities suggest a wide range of apparent ages between recent (<2,000 years) and much older groundwaters (>20,000 years). As mentioned above, the mixing trend of 3H and14C shown in Figure 1b shows two outliers (samples labelled as CRS1 and FLS1). These samples have moderate to low 14C activities which suggest an older apparent age, however, the samples also have moderate to high 3H activities. This could be the result of either mixing or contamination and further work and processing will assess this.

Figure 1. (a) 14C versus temperature for the three depth groups (b) 3H as a function of 14C for the three groups of samples. The heavy dashed line represents typical groundwater mixing using an exponential mixing model (Talma and Weaver, 2003). The 3H content of present-day rainfall in the study area is 2-3 TU. The light dashed line shows one possible mixing line to illustrate how the 'anomalous' high 3H can be modelled as two-component mixing of young and old water.

3.2 Chlorine-36

Comparison of the 36Cl/Cl ratio with 3H and 14C activities (Fig 2) indicates that the deep groundwater group is distinct from both the mixed and the shallow groups. The deep group, with low 14C activities, also have low 3H activities and 36Cl/Cl ratios and are considered to be relatively old. The shallow group have high 14C activities, 3H activities and 36Cl/Cl ratios and are therefore interpreted to be relatively young. The trend between 36Cl/Cl and 14C (Fig. 2b) is almost identical to the trend seen between 3H and 14C (Fig. 1b). However, the mixing relationship indicated by samples labelled FLS1 and CRS1 in Figure 1b is not seen in Figure 2b. This may represent the more complex flow systems in the springs, the greater difficulty of disturbing the 36Cl value than the 3H value, as well as the significant difference in their half-lives (~301 000 years for 36Cl and ~12.5 years for 3H). The deep groundwater group tends to have higher chloride concentrations however, and therefore in these groundwaters the contribution of leached chloride from host rocks will need to be assessed before 36Cl values can be used to estimate residence times.

Figure 2. 36Cl/Cl- versus (a) 3H and (b) 14C

3.3 Helium-4

The distribution of 4He for the different sampling sites shows that for waters defined as shallow, the 4He content is low (Fig 3b). Deep waters have high 4He contents caused by either gradual accumulation of 4He as a result of long residence times or mixing with deeper, older fluids (Fig 3b). The distribution of 3He/4He ratios follows the same pattern in that the mixed and deep waters have similar values which are different from the shallow waters, but that there are a couple of exceptions (Fig 3a,b). Assuming a standard crustal release model, the 4He concentrations suggest the deep groundwaters have ages in excess of 15 000 years whilst the shallow groundwaters are relatively modern. Further work is being undertaken to better understand the accumulation times for 4He and thus the estimated residency based on 3He/4He ratios and what they might mean for the Karoo Basin (e.g. Darrah et al., 2014).

Figure 3. 3He/4He versus (a) 14C and (b) 4He

Conclusions

A clear distinction between the sub-thermal and cold groundwaters is recorded by 14C and 36Cl activities, with lower values recorded in the sub-thermal waters. With a few exceptions, the 3H and 4He activities show similar differences in values. The wide range in activities relates to a wide range in residence times ranging from moderately recent to greater than 20 000 years with the sub-thermal waters being older, implying that they could be a viable proxy for the deep groundwater in the Karoo. The use of 14C, 3H, 36Cl and 4He isotopes together has highlighted where "anomalous" values of some of these parameters are present and probably indicate that the spring and groundwater flow patterns are more complex than the standard models allow for. A site specific conceptual model for the residence time of deep and shallow groundwater for the Karoo Basin is the ultimate goal of this study in part to help understand the interconnectivity of groundwater systems in the Karoo. This information would be of paramount importance should shale-gas development in the Karoo commence.

References

Darrah T.H., Vengosh, A., Jackson, R.B., Warner, N.R., Poreda, R.J. (2014). Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and Barnett Shales. Proceedings of the National Academy of Sciences 111(39) 14076-14081.

Talma, A., & Weaver, J. (2003). Evaluation of groundwater flow patterns in fractured rock aquifers using CFCs and isotopes. WRC Report No 1009/1/03.

Zuber, A. & Maloszewski, P., 2001. Lumped-parameter models. Technical Documents in Hydrology, 6(1), pp. 5-35.