Efficacy of Greenhouse Gas Removal via Enhanced Rock Weathering: impact of groundwater chemistry and flow

Project context.

Meeting the UK’s obligations for net-zero carbon emissions by 2050 requires capturing and storing residual emissions of approximately 130 Megatonnes of CO2,e (CO2 equivalents) yr-1 with greenhouse gas removal (GGR) technologies within decades. This challenge assumes achieving stringent mid-century emissions reductions without sacrificing economic development.  Enhanced rock weathering (ERW) is a prime GGR technology based on amending soils with crushed calcium- and magnesium-rich silicate rocks that accelerates natural CO2 sequestration processes with important co-benefits for crop production and restoration of soil health. The technology has the potential to remove up to 10% of the UK’s GGR net-zero target when applied to the UK’s agricultural land (12 million hectares).

Figure 1. Diagrammatic representation of Greenhouse Gas Removal by Enhanced Rock Weathering via application of crushed silicate rocks to agricultural land.

Carbon dioxide captured by reaction with the applied crushed silicate rock dust in soils is transformed into aqueous bicarbonate (HCO3 ions, alkalinity), via Pathway 1:

Pathway 1. CaSiO3 + 2CO2 + H2O → Ca2+ + 2HCO3 + H4SiO4

Ideally the resulting bicarbonate is transported to the oceans where it has a residence time of 100,000 years so is effectively removed from the atmosphere on the human timescale.  However, the process is partially reversed where freshwater carbonate mineral precipitation occurs before the bicarbonate ions can reach the coastal margins with resulting degassing of up to 50% of the captured C02, via pathway 2:

Pathway 2. Ca2+ + 2HCO3 → CaCO3 + CO2 + H2O

Degassing of C02 and freshwater carbonate mineral precipitation can happen within the soil zone (forming pedogenic or soil-zone carbonates), within groundwater aquifers, and within rivers. The extent of degassing via pathway 2 will depend on the background bicarbonate concentrations, which influence the degree of (over- or under-) saturation of bicarbonate, and the presence of substances that inhibit carbonate mineral precipitation, such as phosphate, suspended sediment and DOM (dissolved organic matter).  In groundwater, the potential for precipitation will depend on the underlying geology and groundwater residence time (carbonate geology and long groundwater residence times are likely to be unfavourable).

Figure 2. Left/upper panel: groundwater flow processes which can lead to degassing of CO2 and precipitation of freshwater carbonate minerals within aquifers and on emergence into streams, which removes net C from the atmosphere according to pathway 2, as described in the text. Right/lower Panel: groundwater-fed chalk stream at the Harpenden field site.

The proposed PhD project will focus on the impact of the underlying geology and groundwater conditions on the efficacy of CO2 removal via application of silicate minerals to soils at three field sites where the ERW approach is being trialled, and extrapolate the findings to predict these impacts United Kingdom-wide.  The work will feed into developing a modelling framework for determining the realistic contribution of Enhanced Rock Weathering to the UK’s net-zero target.  The proposed work feeds into a larger project forming part of the Greenhouse Gas Accelerator scheme funded by Research Councils UK.

Methodology and Approach.

Three field sites will be investigated that define upper and lower bounds for the envelope of UK riverine and groundwater conditions for carbonate mineral saturation. Acidic organic-matter rich waters of Plynlimon (North Wales) and North Wyke (Devon) define the extremes of bicarbonate under-saturation, while the Chalk carbonate environs at Harpenden (Hertfordshire, England) define the conditions most favourable for freshwater carbonate precipitation.

We propose to conduct field characterisation of the groundwater conditions at the experimental sites, followed by a modelling study to establish the efficacy of the Enhanced Rock Weathering approach UK wide, using the available information held by the regulatory authorities in each of the devolved UK nations.   Thus, work undertaken by the PhD student will include:

  1. Review of current state-of-the-art regarding factors influencing the degree of carbonate mineral precipitation/dissolution in groundwaters and resurgences.
  2. Characterisation of groundwater conditions at the experimental field sites via pre-existing data and field measurements. Estimation of the groundwater residence times using hydrogeological characterisation approaches such as borehole dilution testing, and application of hydro-geophysical modelling approaches.
  3. Supporting development of a UK-wide modelling tool, via applying 1-D reactive transport modelling calibrated with the field site data. The model will use high resolution geospatial dataset of soils and land use, and future UK climate change scenarios to 2050, to predict alkalinity of waters draining from soil based on crop and soil type (using USGS hydrogeological code PHAST43). Note that the soil-zone model will be developed by a postdoctoral researcher; the PhD student on this project will adapt it to include the groundwater transport element e.g. model carbonate chemistry reactions during groundwater transport to receiving streams. Results will feed into to determination of the resulting enhanced rock weathering alkalinity flux to coastal margins at national scale, and therefore the efficacy of the approach should the ERW technology be adopted nationwide.

Objectives.

  1. Identify key geochemical factors controlling carbonate mineral precipitation in groundwaters and at stream resurgences.
  2. Characterise groundwater residence times and baseline hydro-chemistry at the three field experimental sites where the technology of Greenhouse Gas Removal via Enhanced Rock Weathering is being trialled, using archived data and aquifer and stream characterisation at the field sites.
  3. Support the development of a UK-wide modelling tool, based on 1D reactive transport model (using USGS PHAST-3D) calibrated with field site data and extend it for predicting the extent of carbonate mineral precipitation and hence degassing of the captured CO2 during transport in groundwater and at emergence into streams.

Training.

Training will be undertaken in i) field hydrogeophysical and hydrochemical characterisation of groundwater systems and associated lab analysis methods ii) 1-D reactive transport modelling using USGS PHAST-3D.  This project will be run as a CASE studentship with the Environment Agency, Hertfordshire; the student will undertake a short work placement with the Agency.

References

Agbotui PY, West LJ, Bottrell SH. 2020. Characterisation of fractured carbonate aquifers using ambient borehole dilution tests. Journal of Hydrology. 589

Beerling, D.J., Leake, J.R., Long, S.P. et al. (2018). Farming with crops and rocks to address global climate, food and soil security. Nature Plants 4, 138–147.

Beerling, D.J., Kantzas, E.P., Lomas, M.R. et al. (2020). Potential for large-scale CO2 removal via enhanced rock weathering with croplands. Nature 583, 242–248.

Hartmann J, West AJ, Renforth P, Kohler R, et al.  2013. Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification. Rev. Geophys. 51, 113–149

Kantola Ilsa B., Masters Michael D., Beerling David J., Long Stephen P. and DeLucia Evan H. 2017. Potential of global croplands and bioenergy crops for climate change mitigation through deployment for enhanced weathering. Biol. Lett.13: 20160714.

Medici G, West LJ, Chapman PJ, Banwart SA. 2019. Prediction of contaminant transport in fractured carbonate aquifer-types; case study of the Permian Magnesian Limestone Group (NE England, UK). Environmental Science and Pollution Research. 26(24), pp. 24863-24884

Medici G, West LJ, Banwart SA. 2019. Groundwater flow velocities in a fractured carbonate aquifer-type: Implications for contaminant transport. Journal of Contaminant Hydrology. 222, pp. 1-16

Parker AH, West LJ, Odling NE. 2019. Well flow and dilution measurements for characterization of vertical hydraulic conductivity structure of a carbonate aquifer. Quarterly Journal of Engineering Geology and Hydrogeology. 52, pp. 74-82.

West LJ, Odling NE. 2007. Characterisation Of A Multi-Layer Aquifer Using Open Well Dilution Tests. Ground Water. 45