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Energy 35 (2010) 4587–4591

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Energy

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Effects of CO2 gas as leaks from geological storage sites on agro-ecosystems

Ravi H. Patil a,*,1, Jeremy J. Colls a, Michael D. Steven b

a Division of Agricultural and Environmental Sciences, School of Biosciences, University of Nottingham, NG7 2RD, Nottingham, UKb School of Geography, University of Nottingham, NG7 2RD, Nottingham, UK

a r t i c l e i n f o

Article history:Received 15 October 2009Accepted 19 January 2010Available online 6 February 2010

Keyword:Carbon captureCO2 gas leaksAgro-ecosystemEnvironmental risks

* Corresponding author. Tel./fax: þ45 8999 1200.E-mail address: ravi.patil@agrsci.dk (R.H. Patil).

1 Present address: Department of Agro-ecology & Ecultural Sciences, Aarhus University, 8830 Tjele, Denm

0360-5442/$ – see front matter � 2010 Elsevier Ltd.doi:10.1016/j.energy.2010.01.023

a b s t r a c t

Carbon capture and storage in geological formations has potential risks in the long-term safety becauseof the possibility of CO2 leakage. Effects of leaking gas, therefore, on vegetation, soil, and soil-inhabitingorganisms are critical to understand. An artificial soil gassing and response detection field facilitydeveloped at the University of Nottingham was used to inject CO2 gas at a controlled flow rate (1 l min�1)into soil to simulate build-up of soil CO2 concentrations and surface fluxes from two land use types:pasture grassland, and fallow followed by winter bean. Mean soil CO2 concentrations was significantlyhigher in gassed pasture plots than in gassed fallow plots. Germination of winter bean sown in gassedfallow plots was severely hindered and the final crop stand was reduced to half. Pasture grass showedstress symptoms and above-ground biomass was significantly reduced compared to control plot. Anegative correlation (r¼�0.95) between soil CO2 and O2 concentrations indicated that injected CO2

displaced O2 from soil. Gassing CO2 reduced soil pH both in grass and fallow plots (p¼ 0.012). Thenumber of earthworm castings was twice as much in gassed plots than in control plots. This studyshowed adverse effects of CO2 gas on agro-ecosystem in case of leakage from storage sites to surface.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon capture and storage (CCS) technology is expected to helpmitigate climate change with existing technology [4], but there isno experience to date on the long-term fate and safety of injectedCO2 gas [7]. Some of the naturally occurring CO2 springs andvolcanic sites located across the world emit CO2 gas, but the risk ofemissions will increase as CCS technology to inject and store hugeamounts of CO2 gas is implemented [14]. There are potential risks offailures in injecting wells and leaks in transportation pipelines or ofCO2 migration to surface via faults and fractures, which could gounnoticed [11] defying the very purpose of geological carbonsequestration.

Many studies have been carried out at volcanic and geothermalactivity sites releasing large amounts of gas varying from hot,mixed efflux to pure, cold CO2 releases [5]. Earlier leaf senescenceand decreased photosynthetic capacity in plants at Bossoleto, Italy[16] and at a natural CO2 spring in Iceland [8] and death of vege-tation at Horseshoe Lake, USA [19] due to toxic levels of soil CO2 arereported. High concentrations of soil CO2 can cause tree mortality

nvironment, Faculty of Agri-ark.

All rights reserved.

either by preventing the tree roots from absorbing O2 needed forrespiration or by interfering with nutrient uptake [22]. Higher soilCO2 concentration is also reported to lower the pH of the soilsolution [7,24] and affect soil mineralisation as some studies havefound increased [23], or decreased mineralisation [10] or nodifferences at all [9] indicating conflicting results.

Hence, it is important to understand the effects CO2 leaks couldhave on agro-ecosystem, which are neither systematically studiednor well understood. This study was carried out at a special gassingfield site by injecting targeted rates of CO2 into soil with a hypoth-esis that injected CO2 gas will displace soil O2 to create anoxicconditions in rhizosphere affecting above-ground vegetationgrowth and biomass.

2. Methodology

The study was carried out in an open field, formerly used forlivestock grazing, at the Artificial Soil Gassing and ResponseDetection (ASGARD) site (52�49060N, 1�14060E, 48 m a.s.l.) of theUniversity of Nottingham, UK. The soil is a sandy loam down to ca.60 cm where 20 cm gravel layer overlays clay. Sixteen plots of2.5 m� 2.5 m were used representing two land use types; eightplots with perennial pasture grass and another eight fallow (baresoil). Eight out of these 16 plots (four pasture and four fallow),chosen randomly, were equipped for controlled CO2 release fromcentre of the plot at 0.6 m below the surface.

Table 1Mean soil CO2 and O2 concentrations (%) from 15 cm and 70 cm measurement tubesin pasture grass gassed (GG), pasture grass control (GC), fallow gassed (FG), fallowcontrol (FC) and remotely controlled (RC) plots.

Plots (n) CO2 (%) O2 (%)

Min. Max. Mean (�S.E.) Min. Max. Mean (�S.E.)

GG-15 cm 104 19.50 76.25 45.85 (�0.98) 4.93 16.20 11.12 (�0.18)GG-70 cm 104 6.13 29.20 11.56 (�0.46) 13.88 19.40 17.83 (�0.10)GC-15 cm 104 0.40 2.92 1.87 (�0.05) 14.48 20.80 19.62 (�0.07)FG-15 cm 92 5.85 62.08 22.48 (�1.32) 7.62 19.62 15.58 (�3.54)FG-70 cm 92 2.60 69.20 12.59 (�1.69) 6.10 20.13 17.56 (�0.36)FC-15 cm 95 0.00 1.82 0.64 (�0.04) 18.35 21.68 20.14 (�0.05)RC-15 cm 95 0.03 2.55 0.76 (�0.05) 17.40 21.55 19.94 (�0.08)

Here ‘n’ represents the total number of measurements done during the entire studyperiod (May 2007 through January 2008). The values in parenthesis indicate stan-dard error.

y = -4.5738x + 93.791r = 0.97 (FG -15cm)

y = -4.5154x + 91.875r = 0.98 (FG - 70cm)

y = -5.0131x + 101.58r = 0.96 (GG - 15cm)

y = -4.0889x + 84.482r = 0.94 (GG - 70cm)

0

10

20

30

40

50

60

70

80

4 6 8 10 12 14 16 18 20 22

Soil O2 concentration (%)

OC li

oS

2)

%(

noi

ta

rt

ne

cn

oc

Fig. 1. Correlation between soil CO2 and O2 concentrations in pasture grass gassed (GG),and fallow gassed (FG) plots from measurement tubes installed at 15 cm and 70 cm fromcentre of plot. GG-15 cm (D), GG-70 cm (B) FG-15 cm (C) and FG-70 cm (-).

R.H. Patil et al. / Energy 35 (2010) 4587–45914588

Study was initiated (24 April 2007) by starting pure CO2 injec-tion @ 1 l min�1 to four pasture grass (GG) and four fallow plots(FG). This flow rate was chosen to achieve sufficiently highconcentrations (w50% CO2) in the centres of the gassed plots. Theremaining four grass pasture (GC) and four fallow (FC) were left un-gassed as control along with another four grass pasture plotslocated 10 m away from the main plots (as remote control – RC) tocompare if gassing in pasture grass and fallow plots (GG & FG) hadany influence on adjacent control plots (GC & FC).

To measure soil gas concentrations, plastic tubes (100 cm long,19 mm internal diameter) were installed vertically into soil toa depth of 30 cm at 15 and 70 cm from plot centre in GG & FG plots,and at 15 cm only in GC, FC, and RC plots, on a diagonal transect.The bottom of each sampling tube (at 30 cm depth) was sealed andthe lower 15 cm of the tube was drilled with 14 equally spacedholes (4.5 mm diameter) enabling free diffusion of gas from outsidesoil into the tube so as to attain equilibrium with the soil gas. Thetop end of the tube was sealed with a plastic on/off valveconnectable to a portable GA2000 gas analyser to measure soil CO2

and O2 in percent of total 100% by volume. Measurements weremade between 10:00 and 12:30 h on weekdays from May 2007through January 2008. On two occasions in October 2007 at oneweek’s interval, soil surface CO2 concentrations in pasture grass andfallow plots were measured (ppm/h) using Draeger tubes (DraegerSafety AG & Co., Germany,) placed in a grid on the surface ata spacing of 0.5 m� 0.5 m to investigate if the presence (or not) ofvegetation affects surface CO2 efflux.

Growth of pasture grass was monitored by collecting grasssamples from 20 cm� 20 cm area, taken at distances of 30, 60 and90 cm from the plot centre in all four directions, by cutting the grassto 1–2 cm above the soil surface and weighing for fresh and ovendry biomass. The rest of the grass plots were mowed aftereach sampling. Winter bean (Vicia faba Cv. Clipper) was sown(1 November 2007) into all the eight fallow plots @ 45 seeds m�2

and the gassing was continued in FG plots. First germination count(number of emerged seedling per plot) was recorded on 3December 2007 and the same was repeated at regular interval untilthe germination process was complete in both FG and FC plots.

Soil samples, one composite sample from each plot, (0–30 cmdepth) were collected (1 June, 15 September and 15 December2007) and analysed for pH. The holes were backfilled with local soiland marked to avoid use of the same spots during subsequentsampling. To look at how gassing affects soil-inhabiting earthworm,their activities were monitored by counting the number of theircastings in each plot on three occasions at monthly time interval(21 September, 23 October and 29 November) during this studyperiod. Land use types; pasture grass and fallow were treated asmain plots, which received the treatment; gassed or un-gassed,each replicated four times totaling 16 plots. Randomised BlockDesign with GenStat, version 10.2, Lawes Agricultural Trust (Roth-amsted Experimental Station), registered to University of Notting-ham and Generalised Linear Model with SAS package registered toAarhus University was used to analyse the data.

3. Results and discussion

3.1. Land use type and soil gas concentrations

Gassing increased soil CO2 levels in GG and FG plots compared toGC and FC plots (p< 0.001), respectively. Between the land usetypes, gassing resulted in significantly higher (p< 0.001) soil CO2

levels in GG plots (45.85%) than in FG plots (22.48%) only at 15 cmmeasurement tubes (Table 1). Whereas, soil O2 levels were signif-icantly low (p< 0.001) in gassed plots (GG & FG) both at 15 cm and70 cm measurement tubes. A strongly negative relationship was

found between the soil CO2 and O2 concentrations confirming thatsoil O2 was displaced by the injected CO2 gas (Fig. 1).

The presence of vegetation influences the gas build up andescape from soils. Thickly matted roots in perennial pasture plotshold the soil particles closer together and close the pores therebyreducing gas emission, while at the same time presence of largeroots, if present, can provide pathways for gas escape [6]. The landuse types (pasture and fallow) had significant influence on soil gaslevels and dispersion through the soil.

The pasture grass first sown almost 10 years prior to this studyhad an extensive closely-matted root system. While in fallow plots,the grass was uprooted and soil had been mechanically cultivatedto keep it vegetation free. Hence the soil structure in fallow plotswould have been broken up allowing easy gas movement. Hence,the horizontal spread of released CO2 gas from within soil wasgreater in GG plots compared to FG plots. The extended horizontalsub-surface spread of CO2 gas significantly reduced the above-ground grass biomass up to 90 cm away from centre of plot in GGplots compared with the grass biomass in GC plots on all thesampling dates (Fig. 3). The Draeger tube measurements gavea better visualisation of gas dispersion within each plot and showedthat the zone of influence of the gassing had a diameter of between1.0 and 1.5 m both in GG and FG plots and was more inclinedtowards gas entry pathway, particularly in FG plots (Fig. 2). It wasexpected that CO2 gas from the release points would migrate iso-tropically through the soil profile creating a spherically symmetricgas plume as suggested by Hoeks [13]. However, insertion of the gas

Fig. 2. Mean Draeger tube CO2 concentrations (ppm/h) in pasture grass gassed (GG, n¼ 8) on the left side and fallow gassed (FG, n¼ 8) plots on the right side. The contour lines atthe centre represent higher concentration, which decrease towards edges of the plot.

5

15

25

35

45

55

65

75

85

GG-30cm GG-60cm GG-90cm GC-30cm GC-60cm GC-90cm

)g

( s

sa

moi

b d

nu

or

g e

vo

bA

1-Jul 15-Aug 30-Oct

Fig. 3. Above-ground pasture grass dry biomass (g/0.04 m2) in pasture grass gassed(GG) and pasture grass control (GC) plots at 30 cm, 60 cm, and 90 cm from centre ofplot (standard error bars are with n¼ 4) at three different sampling time during CO2

gassing periods.

7.0

R.H. Patil et al. / Energy 35 (2010) 4587–4591 4589

delivery tube may have disturbed the soil structure creating addi-tional gas pathways in that area of the plot. Similar patterns werealso reported by Adams and Ellis [1] and Smith et al. [20] for naturalgas.

The control plots (GC & FC) recorded slightly higher levels of soilCO2 when compared to RC plots (Table 1). This could be attributedto horizontal sub-surface movement or drift of soil CO2 from the

75

100

125

150

175

200

225

250

275

300

3-Dec

tol

p r

ep

sg

nild

ee

s f

o .o

N

No Gassing

Gassing

11-Feb4-Feb18-Jan2-Jan12-Dec10-Dec7-Dec5-Dec4-Dec

Fig. 4. Winter bean germination and seedling count (No./plot) in fallow gassed (FG)and fallow control (FC) plots. Vertical lines show standard error (n¼ 4).

gassed plots (GG & FG) entering into neighboring un-gassed plots(GC & FC) not in any particular pattern or rate. However, RC plots, asexpected, recorded the highest soil O2 (19.94%) and the lowest CO2

(0.76%) concentrations (Table 1).

3.2. Weather variables and soil gas concentrations

Temporal changes in the soil gas levels (results not shown here)may have been due to changes in the moisture content of the soilopening and closing pathways, or by movement of soil organismssuch as earthworms. Hence, multiple regression analysis wascarried out between weather variables and soil gas CO2 levels andfound that only the rainfall events had a significant effect on soilCO2 levels (p¼ 0.046) (data not shown here). Smith et al. [20] foundcorrelations between gas flow or concentration and atmosphericvariables, mainly relating to injection of CH4 gas under pressure.

The most affected patch of grass matched to the region ofmaximum soil CO2 levels recorded by Draeger tube measurements(Fig. 2). Grass in this patch was much shorter, looked yellow and drythan the surrounding grass. However, this effect gradually reducedtowards edges of the plot. The interaction between treatment(gassing) and sampling period was also significant (P¼ 0.02)showing the compounding effect of higher soil CO2 levels andweather factors. Previous studies at sites of natural geological CO2

efflux have shown reduced plant growth, disrupted photosynthesis,inhibition of root respiration, and even plant death [8,15–17,25,26].

4.5

5.0

5.5

6.0

6.5

June September December

Hp li

oS

GG GC FG FC RC

Fig. 5. Soil pH as influenced by CO2 gassing into pasture grass (GG & GC) and fallow(FG & FC) plots (standard error bars are with n¼ 12) at three times during the studyperiod.

0

50

100

150

200

250

300

350

GG Plots GC Plots RC Plots

tol

p r

ep

sg

nit

sa

c m

ro

w f

o .o

N

21-Sep 23-Oct 29-Nov

Fig. 6. Mean number of earthworm castings per plot (n¼ 4) recorded in pasture grassplots (GG, GC and RC) at three times during CO2 gassing period. Standard error bars arewith n¼ 4.

R.H. Patil et al. / Energy 35 (2010) 4587–45914590

It has been reported that leaking natural gas causes stress tovegetation because the gas displaces O2 from the soil [3,12,20,21],thus inhibiting root respiration affecting uptake of water andnutrients from the soil. Adamse et al. [2] suggested that for theproper functioning of a healthy root system, a minimum soil O2

content of 12–14% is needed, which was not the case in this study atthe centre of GG plots (Table 1).

3.3. Response of winter bean to gassing

Gassing CO2 significantly (p¼ 0.015) reduced the number ofwinter bean seeds germinated in FG plots (131.50) compared to theFC plots (264.50) (Fig. 4). Plants are more sensitive to anaerobicconditions during early growth stages [12] and that might be thereason why CO2 gassing had lethal effect on winter bean seedgermination than on a well established pasture grass. Severalresearchers have reported stress effects on vegetation due toleaking natural gas with symptoms including reduced emergence,yellowing of the leaves or a shift in the developmental stage[1,3,18,20,21].

3.4. Gassing and the soil environment

Gassing lowered the soil pH (p¼ 0.017) both in GG and FG plotsin comparison to their respective control plots (GC & FC, respec-tively) (Fig. 5). Celia et al. [7] suggested that higher soil CO2 levelsacidify the soil solution. In fallow plots (FG and FC), no worm castswere recorded. Whereas in pasture grass plots, GG plots hadsignificantly greater number of casts than GC plots (Fig. 6). Smithet al. [20] also reported higher worm casts in grass plots injectedwith natural gas.

4. Conclusions

This study showed the potential ecological risks of CO2 leakagefrom geological storage sites agro-ecosystems. Even the low level ofgas leakage applied in this study greatly increased soil CO2

concentrations by displacing soil O2 and adversely affected thegrowth of pasture grass, severely hindered the germination andestablishment of winter bean crop, reduced soil pH and influencedthe activities of soil-inhabiting earthworms. However, we believe,while this study increases our understanding of the effects of belowground CO2 leaks, long-term studies to evaluate the potential long-term consequences on ecosystem are needed for decision makingand management.

Acknowledgements

This research was partly supported by the Royal Society, Londonthrough its International Fellowship program to the first author. Wewish to acknowledge Dr. Karon Smith, Ms. Manal Al-Traboulsi andJohn Alcock, University of Nottingham for their technical guidanceand help/assistance during the study period.

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