Chemical evolution of the Mt. Hekla, Iceland, groundwaters: A natural analogue for CO2 sequestration in basaltic rocks

doi:10.1016/j.apgeochem.2008.12.031

Applied Geochemistry

Volume 24, Issue 3, March 2009, Pages 463-474

https://doi.org/10.1016/j.apgeochem.2008.12.031 Get rights and content

Abstract

A detailed study of the chemical composition of the groundwater surrounding the Mt. Hekla volcano in south Iceland was performed to assess fluid evolution and toxic metal mobility during CO₂-rich fluid basalt interaction. These fluids provide a natural analogue for evaluating the consequences of CO₂ sequestration in basalt. The concentration of dissolved inorganic C in these groundwaters decreases from 3.88 to 0.746 mmol/kg with increasing basalt dissolution while the pH increases from 6.9 to 9.2. This observation provides direct evidence of the potential for basalt dissolution to sequester CO₂. Reaction path calculations suggest that dolomite and calcite precipitation is largely responsible for this drop in groundwater dissolved C concentration. The concentrations of toxic metal(loid)s in the waters are low, for example the maximum measured concentrations of Cd, As and Pb were 0.09, 22.8 and 0.06 nmol/kg, respectively. Reaction path modelling indicates that although many toxic metals may be initially liberated by the dissolution of basalt by acidic CO₂-rich solutions, these metals are reincorporated into solid phases as the groundwaters are neutralized by continued basalt dissolution. The identity of the secondary toxic metal bearing phases depends on the metal. For example, calculations suggest that Sr and Ba are incorporated into carbonates, while Pb, Zn and Cd are incorporated into Fe (oxy)hydroxide phases.

Introduction

A large current effort is being made to identify and optimize CO₂ sequestration technologies to address the potential dangers associated with increased atmospheric CO₂ content (IPCC, 2005, Oelkers and Schott, 2005, Oelkers and Cole, 2008). One such technology involves the injection of CO₂ into basaltic rocks (McGrail et al., 2006, Gislason et al., 2007, Matter et al., 2007, Oelkers et al., 2008). This method offers several potential advantages including the availability of divalent metal cations such as Ca²⁺ and Mg²⁺ which could provoke the precipitation of stable carbonate minerals (Walker and Hays, 1981, Gaillardet et al., 1999, Brady and Gislason, 1997, Wolff-Boenisch et al., 2006). One method to assess both the potential and the risks associated with CO₂ sequestration in basaltic rocks is through the study of natural analogues. One such analogue is the Mt. Hekla groundwater system. The groundwaters surrounding Mt. Hekla experience large inputs of magmatic gases dominated by CO₂ (Kjartansson, 1957, Gislason et al., 1992, Flaathen and Gislason, 2007). A study of the chemical composition of these groundwaters should, therefore, provide insight into the fate and consequences of injecting CO₂ into basaltic rocks. Taking advantage of this natural analogue, waters have been regularly sampled from 26 springs surrounding Mt. Hekla. Analyses of these waters, together with reaction path modelling, suggest that (1) CO₂ is readily sequestered, via fluid–basalt interaction through carbonate mineral precipitation and (2) although they may be liberated due to basalt dissolution, toxic metals are readily reincorporated into solid phases as the basalt neutralizes the initially CO₂-rich fluid. The purpose of this paper is to present the results of this combined field and modelling study providing insight into the consequences of injecting CO₂ into basaltic rocks.

Section snippets

Geological background: Hekla volcano and its groundwater system

The Mt. Hekla volcano (63.98°N, 19.70°W) is a ridge built up by repeated fissure eruptions. The volcano strikes N 65°E and is located where the eastern volcanic zone, meets the South Iceland seismic zone (Gudmundsson et al., 1992). It is one of Europe’s most active volcanoes with 18 eruptions during the last 900 a (Gronvold et al., 1983). The most recent eruptions occurred during 1970, 1980, 1991 and 2000. The bulk of the erupted material during the last 900 a is of basaltic andesite composition (

Water samples from springs

A total of 111 samples from 26 springs surrounding Mt. Hekla were collected during 1988, 1991, 1992 and 2006. The locations of the sampling sites are shown in Fig. 1. Each spring was collected for 1–6 times except spring 18, which was sampled 19 times. The samples were taken during all seasons. The water samples were filtered immediately after sampling through 0.2 μm Millipore cellulose acetate membranes into high density polyethylene bottles. Samples taken for pH and dissolved inorganic C (DIC)

Main hydrogeochemical features

The aqueous concentrations of major elements of all samples can be seen in the Appendix. The pH and alkalinity/dissolved inorganic C (DIC) of these waters range from 7.3 to 9.2 and 0.75 to 3.88 meq/kg, respectively. The spatial distribution of these pH and DIC values are shown in Fig. 1. DIC decreases while pH increases with increased distance from the volcano. Total dissolved solids (TDS) range significantly with the highest concentrations close to the volcano.

δ¹⁸O and δ²H were measured in the

Controls on major element mobility

Insight into metal mobility can be obtained from the results of reaction path modelling. The concentrations of major elements in the springs are compared with those from the reaction path modelling in Fig. 8. As can be seen in Fig. 8, the concentrations of major elements tend to be close to those estimated from the model calculation. In some cases however, there are important difference between model calculations and spring water concentrations. For example, spring water Si concentrations are

Conclusions

The results presented above illuminate the fate of both CO₂ and dissolved metals during the interaction of CO₂-rich rainwater and basaltic rocks. The major conclusions of this study include:

1.
Results indicate that the neutralization of CO₂-rich waters by their interactions with basalt in the subsurface may provide an effective means to fix CO₂ as carbonate minerals. This process proceeds by the combination of Ca and Mg liberated to solution through basalt dissolution driven by dissolved CO₂ to

Acknowledgements

We thank Luigi Marini and Stefano Caliro for constructive reviews that led to significant improvements of the manuscript. We are grateful to many friends and colleagues for their help. Specifically we would like to thank Guðmundur B. Ingvarsson for helpful discussions and Rósa Ólafsdóttir for assisting with GIS. We would also like to thank the Carb-Fix consortium; Wallace S. Broecker, Juerg M. Matter, Hólmfriður Sigurðardóttir, Andri Stefánsson, Domenik Wolff-Boenisch, Einar Gunnlaugsson,

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Chemical evolution of the Mt. Hekla, Iceland, groundwaters: A natural analogue for CO2 sequestration in basaltic rocks

Abstract

Introduction

Section snippets

Geological background: Hekla volcano and its groundwater system

Water samples from springs

Main hydrogeochemical features

Controls on major element mobility

Conclusions

Acknowledgements

Geochim. Cosmochim. Acta

J. Volc. Geothermal Res.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

J. Volc. Geothermal Res.

Chem. Geol

Chem. Geol.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Waste Manage.

J. Hydrol.

Geochim. Cosmochim. Acta

Chem. Geol.

Lithos

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Eruptive and diffusive emissions of CO2 from Mount Etna

Nature

Andisols from four different regions of Iceland

Soil. Sci. Soc. Am. J.

Groundwater systems in Iceland traced by deuterium

Isotopic variations in meteoric waters

Science

Effect of coagulant treatment on the metal composition of raw water

Water SA

Local effects of volcanoes on the hydrosphere: example from Hekla, southern Iceland

Chemical evolution of the Mt. Hekla, Iceland, groundwaters: A natural analogue for CO₂ sequestration in basaltic rocks

Eruptive and diffusive emissions of CO₂ from Mount Etna