Geochemical Transactions BioMed Central

Research article Open Access 

Geochemical detection of carbon dioxide in dilute aquifers 

Susan Carroll*, Yue Hao and Roger Aines 

Address: Chemistry, Materials, Earth and Life Sciences Division, Lawrence Livermore National Laboratory, Livermore CA 94550, USA 

Email: Susan Carroll* - carroll6@llnl.gov; Yue Hao - hao1@llnl.gov; Roger Aines - aines1@llnl.gov 

* Corresponding author 

Published: 26 March 2009	 Received: 1 October 2008 

Accepted: 26 March 2009 Geochemical Transactions 2009, 10:4 doi:10.1186/1467-4866-10-4
 

This article is available from: http://www.geochemicaltransactions.com/content/10/1/4
 

© 2009 Carroll et al; licensee BioMed Central Ltd. 


This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), 


which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
 

Abstract 

Background: Carbon storage in deep saline reservoirs has the potential to lower the amount of 

CO2 emitted to the atmosphere and to mitigate global warming. Leakage back to the atmosphere 

through abandoned wells and along faults would reduce the efficiency of carbon storage, possibly 

leading to health and ecological hazards at the ground surface, and possibly impacting water quality 

of near-surface dilute aquifers. We use static equilibrium and reactive transport simulations to test 

the hypothesis that perturbations in water chemistry associated with a CO2 gas leak into dilute 

groundwater are important measures for the potential release of CO2 to the atmosphere. 

Simulation parameters are constrained by groundwater chemistry, flow, and lithology from the 

High Plains aquifer. The High Plains aquifer is used to represent a typical sedimentary aquifer 

overlying a deep CO2 storage reservoir. Specifically, we address the relationships between CO2 

flux, groundwater flow, detection time and distance. The CO2 flux ranges from 103 to 2 × 106 t/yr 

(0.63 to 1250 t/m2/yr) to assess chemical perturbations resulting from relatively small leaks that 

may compromise long-term storage, water quality, and surface ecology, and larger leaks 

characteristic of short-term well failure. 

Results: For the scenarios we studied, our simulations show pH and carbonate chemistry are good 

indicators for leakage of stored CO2 into an overlying aquifer because elevated CO2 yields a more 

acid pH than the ambient groundwater. CO2 leakage into a dilute groundwater creates a slightly 

acid plume that can be detected at some distance from the leak source due to groundwater flow 

and CO2 buoyancy. pH breakthrough curves demonstrate that CO2 leaks can be easily detected for 

CO2 flux ≥ 104 t/yr within a 15-month time period at a monitoring well screened within a permeable 

layer 500 m downstream from the vertical gas trace. At lower flux rates, the CO2 dissolves in the 

aqueous phase in the lower most permeable unit and does not reach the monitoring well. Sustained 

pumping in a developed aquifer mixes the CO2-affected water with the ambient water and 

enhances pH signal for small leaks (103 t/yr) and reduces pH signal for larger leaks (≥ 104t/yr). 

Conclusion: The ability to detect CO2 leakage from a storage reservoir to overlying dilute 

groundwater is dependent on CO2 solubility, leak flux, CO2 buoyancy, and groundwater flow. Our 

simulations show that the most likely places to detect CO2 are at the base of the confining layer 

near the water table where CO2 gas accumulates and is transported laterally in all directions, and 

downstream of the vertical gas trace where groundwater flow is great enough to transport 

dissolved CO2 laterally. Our simulations show that CO2 may not rise high enough in the aquifer to 

be detected because aqueous solubility and lateral groundwater transport within the lower aquifer 

unit exceeds gas pressure build-up and buoyancy needed to drive the CO2 gas upwards. 

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Background 

Carbon storage as a liquid, gas, dissolved carbon, or as 

carbonate minerals has the potential to significantly offset 

global warming caused by anthropogenic combustion of 

fossil fuels [1,2]. It is generally accepted that the most suitable systems for geologic storage are depleted oil and deep 

saline reservoirs, because they are not viable for domestic, 

industrial, and agricultural uses and they are separated 

from the atmosphere by 1000s of meters of geologic 

strata. Despite the physical separation between a storage 

reservoir and a useable aquifer, there is still concern that 

storage reservoirs may leak through abandoned wells or 

along faults and be released back to the atmosphere. Leakage of supercritical CO2 at depth will change to its gas state 

at lower pressures associated with shallow aquifers, and 

may be accompanied with deeper formation water. Leakage would reduce the efficacy of carbon storage, possibly 

leading to health and ecological hazards at the ground 

surface, and possibly negatively impacting water quality 

of near-surface dilute aquifers [3-8]. In order to assess risk 

and long-term liability it is important to utilize sensitive 

monitoring techniques that both detect leaks and quantify 

the magnitude of the leak [8]. 

Carbon dioxide leakage rates that may compromise storage effectiveness and safety can range over several orders 

of magnitude. We have chosen to use units of metric tons 

per year (t/y) to directly compare leakage rates with the 

amount of CO2 that may be injected from a large coal 

plant. At the lower end of the scale, a detectable anthropogenic CO2 flux from the subsurface must be greater than 

about 102 t/yr to exceed CO2 produced by biological mineralization of organic matter [9-12]. Anthropogenic CO2 

fluxes of 103 to 105 t/yr are above background and represent small leaks corresponding to only 0.01% to 1% of the 

annual amount of CO2 injected to the subsurface for a 

gigawatt coal plant that generates and sequesters 107 t/yr 

of CO2. Although these leakage rates are small relative to 

the amount of CO2 that can be stored in deep reservoirs, 

they are comparable with CO2 fluxes from abandoned 

wells to natural CO2 reservoirs and volcanic activity 

[7,13,14]. Slow leakage of CO2 gas from small leaks can 

present a hazard if it builds up in enclosed areas like the 

basements of buildings or holes in the earth. This would 

include fault leaks and slow well leaks, and has been the 

cause of ecological hazards and of human fatalities in 

areas with volcanic emissions of carbon dioxide [15,16]. 

Higher carbon dioxide release rates on the order of 20% 

of the annual amount of injected CO2 are equivalent to 

the largest reported amount of CO2 released from catastrophic well failure that was mitigated within seven days 

[17]. Water chemistry associated with CO2 gas leakage 

into domestic groundwater resources maybe an important 

way to monitor the potential for further leakage to the surface over a wide range of fluxes, because the acidity associated with dissolved CO2 will alter the ambient water 

chemistry. 

In this paper we use static equilibrium and reactive transport simulations to test the hypothesis that perturbations 

in water chemistry associated with a CO2 gas leak into 

dilute groundwater are important measures for the potential release of CO2 to the atmosphere. Simulation parameters are constrained by groundwater chemistry, flow, and 

lithology from the High Plains aquifer. The High Plains 

aquifer is used to represent a typical sedimentary aquifer 

that may overly a deep CO2 storage reservoir. Specifically, 

we address the relationships between CO2 flux (103 to 2 × 

106 t/yr), detection time and detection distance. 

Static Equilibrium Model 

In this section we establish that increases in pCO2 perturb 

groundwater chemistry of domestic aquifers by comparing the geochemical signatures of the High Plains aquifer 

and a model aquifer column saturated with CO2(g) as a 

function of depth. This geochemical model is incorporated into the reactive transport calculations that describe 

the ability for detection of stored CO2 that has leaked into 

a domestic aquifer. 

Methods 

Equilibrium calculations were conducted using the Geochemist's Workbench software [18], the thermodynamic 

data listed in Table 1, and the Debye-Huckel activity coefficients to correct for ionic strength. The model aquifer 

volume is a 1 m2 × 200 m column containing 30% porosity, 66.5 vol % quartz sand, and 3.5 vol % calcite. Calculations are made from 40 m to 240 m, with a pH 7.6, 0.01 

m NaCl background electrolyte at 17°C to be consistent 

with the depth interval for the saturated zone, temperature, and ionic strength reported for the High Plains aqui­ - fer [19-21]. Bicarbonate concentration, HCO3 is adjusted 

to maintain charge balance, and pore waters are in equilibrium with respect to calcite and quartz. Silicate dissolution and precipitation kinetics and cation-exchange to 

layered silicates are not considered because their contribution to groundwater chemistry is expected to be minimal 

Table 1: Thermodynamic equilibrium constants from the 

Geochemists Workbench, thermo.dat database used to account 

for aqueous speciation [18]. 

Mass balance reactions log K(17°C) 

CaCO3 (Calcite) + H+ = Ca2+ + HCO3 

- 1.83 

SiO2 (Quartz) = SiO2 (aq) -4.15 

CO2(g) + H2O = H+ +HCO3 

- -7.77 

CO2(aq) + H2O = H+ + HCO3 

- -6.42 

CO3 

2- + H+ = HCO3 - 10.42 

NaHCO3 + = Na++HCO3 

- -0.20 

H2O = H+ + OH­ -14.27 

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compared to carbonate chemistry over the short time periods associated with leak detection (days to months). Calcite dissolution kinetics were not included in the 

simulation because preliminary calculations show that 

even at equilibrium only a small amount of calcite dissolves, as is discussed below. In these calculations we 

assume that the pore space is fully saturated with water 

and that the aquifer volume can accommodate increases 

in pCO2 equal to the total hydrostatic pressure of the aquifer (hydropressure gradient = 0.09667 atm/m below the 

water table). 

Geochemical response of CO2 leak 

Figure 1 compares the simulated geochemical response of 

a model sandstone aquifer saturated with respect to CO2 

gas with the ambient groundwater chemistry measured 

from the High Plains aquifer from 40 to 240 m below the 

ground surface. Groundwater chemistry for sites within 

the southern and central High Plains aquifer represents a 

reasonable range of water chemistry for dilute aquifers 

that are likely to be some 800 to 2000 meters above deep 

carbon dioxide storage sites. The study areas in the southern and central High Plains aquifer are primarily within 

the Ogallala Formation consisting of sands, gravel, siltstone and clay, and calcium carbonate cement [20,21]. 

Although both study areas have fairly dilute groundwater, 

the dissolved solids and alkalinity are about ten times 

more concentrated in the central study area than in the 

southern study area. Despite this marked difference in 

absolute concentrations, trends in the High Plains aquifer 

are independent of depth at each site. Groundwater pH, - alkalinity as HCO3 , pCO2 and total dissolved carbon are 

constant over the sampling depth (40 to 240 m) with the 

exception of one data point in the southern study area. 

The carbonate signature of the High Plains aquifer is distinct from a model sandstone aquifer exposed to CO2 gas 

that might leak from a much deeper storage reservoir. Figure 1 plots the pH, alkalinity as HCO -

3 , pCO2, total dissolved carbon concentrations, carbonate speciation, and 

the amount of calcite dissolved as a function of depth for 

the aquifer system equilibrated with pCO2 equal to the 

hydrostatic pressure. Equilibration of CO2 gas that might 

leak from a carbon sequestration aquifer would alter the 

ambient chemistry indicated here by the High Plains aquifer. As the CO2 gas leaks into the aquifer, it dissolves into 

the solution and drives the pH more acid and promotes 

some calcite dissolution according to the following mass 

balance reactions: 

CO2(g) = CO2(aq) (1) 

CO2(aq) + H2O = HCO3

- + H+ (2) 

CaCO - 3(calcite) + H+ = Ca2+ + HCO3 (3) 

The chemical response of the aquifer to the leak increases 

with depth because higher hydrostatic pressures allow for 

higher pCO2. Thus solution pH decreases from pH 7.6 to 

pH 5.2, alkalinity as HCO3

- increases from 10-2.6 to 10-1.3 

molal, total dissolved carbon increases from 10-2.6 to 1 

molal, and the pCO2 increases from 10-2.5 to 101.3 atm 

from the water table to 240 m below the surface in this 

calculation. It is clear from the comparison of the measured High Plains aquifer chemistry and the calculated 

aquifer water chemistry, that pH is a robust indicator of an 

influx of CO2 into a dilute aquifer from a much deeper 

carbon sequestration reservoir. There is a shift of about 2 

pH units between the measured values in the High Plains 

aquifer and those in the leak simulation at depths 60 m 

below the water table. Although calculated HCO3

- values 

fall within the range measured in the High Plains aquifers, 

alkalinity is equally important parameter to measure 

because when combined with pH it can be used to calculate the pCO2 (sum equations 1 and 2): 

CO2(g) + H2O = HCO3

- + H+ (4) 

to confirm that the acidity is due to an influx of CO2 and 

to estimate the magnitude of the leak when measured over 

time and area. Note that in dilute-aquifers we assume that 

the dominant component to alkalinity is HCO3

- concentration. This may not be the case in groundwater with significant concentrations of organic acids or sulfides. 

The reason the calculated HCO -

3 concentrations fall 

within the range of those measured in the High Plains 

aquifer is that only minor amounts of calcite dissolve in 

response to the acidity generated by the CO2 leak. About 

1% of the calcite dissolved in the deeper portion of the 

aquifer where pCO2 is higher, pH is more acid and 

CO2(aq) is the dominant carbonate species (Figure 1e, f). 

This suggests that calcite will effectively buffer acidity 

associated with a carbon dioxide leak in sandstone aquifers with minor amounts of calcite as well as in limestone 

and dolomite aquifers with major amounts of carbonate 

minerals. 

Reactive Transport Model 

In this section we explore the effects of CO2 flux, monitoring well location, pumping rate, and gravity-driven 

groundwater flow on leak detection using 3-D reactive 

transport simulations. We assume CO2 gas reaches the 

aquifer through an abandoned well or along a fault and is 

then transported in gas and fluid phases through the sedimentary layers. We use pH as the key geochemical indicator of carbon dioxide transport in the dilute aquifer. 

Simulation results are shown as pH breakthrough curves, 

contour plots at discrete time steps to capture transport 

details between the leak and monitoring well, and 

selected full-scale simulations shown as movies in additional files. 

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The geochemical response of carbon dioxide gas in a model sandst fer (symbols, [20,21]) and simulated groundwater chemistry (solid line) as depth ve one aquifer is shown by differen rsus pH (a), log [HCO ce between High Plains aqui- 3

-

] (b), log pCO2 log ΣC (c), aq (d), fraction calcite dissolved (e) and dissolved carbon speciation (f)

Geochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

Figure 1
 

The geochemical response of carbon dioxide gas in a model sandstone aquifer is shown by difference between 


High Plains aquifer (symbols, [20,21]) and simulated groundwater chemistry (solid line) as depth versus pH (a), 


log [HCO3

-

] (b), log pCO2 (c), log ΣC (d), fraction calcite dissolved (e) and dissolved carbon speciation (f).
 aq 

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The 3-D geologic model (10 × 10 km2 [20,21] and 240 m deep) is based on the central High Plains aquifer sand and clay lithology 

Geochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

Method 

The numerical simulations were performed using a parallel-version of the Nonisothermal Unsaturated-Saturated Flow 

and Transport code (NUFT) to handle large 3-D reactive 

flow and transport calculations [22]. The NUFT code is a 

highly flexible software package for modeling multiphase, 

multi-component heat and mass flow and reactive transport in unsaturated and saturated porous media. An integrated finite-difference spatial discretization scheme is 

used to solve mass and energy balance equations in both 

flow and reactive transport models. The resulting nonlinear equations are solved by the Newton-Raphson method. 

The NUFT code is capable of running on PCs, workstations, and major parallel processing platforms. Some of 

the application areas include: nuclear waste disposal, CO2 

sequestration, groundwater remediation, and subsurface 

hydrocarbon production [23-26]. 

We simulate the release of varying fluxes of CO2 gas into a 

saturated 3-D subsurface flow system that is 10 × 10 km2 

× 240 m deep represented by grid size varying from about 

50,000 to 150,000 cells with finer grid spacing near the 

leak source and the monitoring well. Note that refined 

meshes near the evolving interface between CO2 plume 

and ambient groundwater with dynamic gridding techniques (e.g. adaptive mesh refinement) were not used 

because it was beyond the scope and computational 

budget of this study. A simulation area of 10 × 10 km2 was 

chosen to remove boundary artifacts observed for smaller 

areas due to the extensive lateral transport of CO2 at high 

fluxes. A schematic of the subsurface model and the permeability structure based on the High Plains aquifer is 

shown in Figure 2. We constrain groundwater flow using 

lithology depth profiles and the horizontal hydraulic gradient, because the vertical and lateral heterogeneity of the 

High Plains aquifer permeability, porosity, and groundwater flow are not known at sufficient detail for the scale 

of the simulations. The High Plains aquifer generally 

flows from west to east over approximately 457,000 km2 

with an average groundwater flow of 0.3 m/d [27,28]. A 

0.3% horizontal hydraulic gradient is estimated from the 

depth of the water table documented for four lithology 

profiles over a distance of 150 km within the central High 

Plains aquifer [20,21]. The main aquifer is locally confined by impermeable clay layers near the water table and 

at the base of the permeable sandstone units as shown in 

Figure 2. The unsaturated zone lying above the confined 

aquifer is also included in the model for the sake of complete representation of a subsurface system; however, it is 

not the focus of the study since the low permeable layer 

prevents the CO2 vertical transport to the ground surface. 

Specific lithologies reported in a single borehole from Liberal, Kansas [20,21] were matched to midrange permeability and porosity values from Freeze and Cherry [29] to 

create the permeability and porosity depth profiles used 

in the simulations, because direct measurements are not 

available. The horizontal hydraulic gradient combined 

with the permeability and porosity profiles yields an average groundwater flow of 0.3 m/d in agreement with 

regional groundwater flow for the High Plains aquifer. 

Figure 2 

The 3-D geologic model (10 × 10 km2 and 240 m deep) is based on the central High Plains aquifer sand and clay 

lithology [20,21]. The main aquifer unit dips gently with a slope equal to 0.3% and is locally confined by low permeable clay 

layers at the top and base of the aquifer. 

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System permeability ranges from 10-17 to 2.5 × 10-10 m2 

and porosity ranges from 30 to 55%. Infiltration is not 

explicitly accounted for in the simulations, but it is 

reflected in the average groundwater flow of 0.3 m/d. 

Table 2 lists the hydrologic properties and parameters 

used in the reactive transport simulations. The leak source 

used in the simulations is small (40 × 40 m2) compared 

to the size of the storage reservoir and represents a focused 

leak. The gas leak is placed just above the lower clay layer 

at about 170 m below ground surface into saturated 

groundwater. The leak is then transported in both the gas 

and fluid phases. Gas phase CO2 leakage fluxes equal to 

103,104, 105, and 2 × 106 t/yr are used to assess chemical 

perturbations resulting from leakage rates that may compromise storage effectiveness and safety as outlined in the 

background section. When normalized to the source area, 

leakage rates range from 0.63 to 1250 t/m2/yr. The temperature is held constant at 17°C throughout the domain. 

The flow and transport models are coupled with the same 

geochemical equilibrium model as described in Table 1. 

In the simulation equilibrium conditions are assumed for 

partitioning components between the gas and fluid 

phases. The hydrostatic pressure and constant geochemical conditions are assigned along the lateral boundary. 

The model domain horizontal hydraulic gradient is 

imposed by changing the gravity vector direction. The 

relationships between capillary pressure, permeability, 

and saturation are described by the van Genuchten formulation with parameters m and α specified as 0.4 and 6.6 × 

10-4 Pa-1, respectively. The gas residual saturation is chosen to be 0.05 and the irreducible water saturation is 0.2. 

Initially a very small amount of less condensable gas is 

maintained in the aquifer domain (e.g. 0.01 gas phase saturation near the top of the aquifer and less than 0.002 

elsewhere). The presence of the less condensable gas has 

no effect on the simulation other than to improve numerical performance (enabling larger time steps in the simulation). 

The effect of groundwater pumping in developed aquifers 

on CO2 transport and detection is simulated by placing 

wells (equivalent diameter ~0.56 m) that are screened at 

about 110 m in the permeable zone above the CO2 leak 

source or at 100, 200, and 500 m downstream from the 

leak source. Pumping rates range from 0 to 1893 L/min to 

represent both undeveloped and developed aquifer 

regions. Typical irrigation rates for the High Plains aquifer 

vary from 379 to 1893 L/min (100 to 500 g/min) [27,28]. 

Overview of CO2 transport 

The spatial and temporal development of a CO2 plume is 

the result of several concurrent processes: CO2 solubility, 

leak flux, CO2 buoyancy, and groundwater flow, where 

groundwater flow depends on the hydraulic gradient, permeability, porosity, and gas saturation. Aqueous solubility transfers CO2 from the gas phase to the groundwater 

and is in equilibrium with the pCO2 and carbonate mineralogy. Gas pressure and its inherent buoyancy relative to 

water transport CO2 gas vertically once the gas pressure 

exceeds the hydrostatic pressure until the gas reaches 

impermeable lithologies such as the clay layers near the 

water table in the High Plains aquifer. Gas pressure creates 

a vertical trace from the leak source to the top of the aquifer, where CO2 gas saturation builds up and is transported 

laterally along the base of the impermeable lithology. 

Groundwater flow moves the CO2-rich groundwater 

down gradient from the gas trace. Groundwater pumping 

for domestic, agricultural, or industrial uses tends to mix 

the CO2-affected waters with the ambient groundwater 

(see Developed Aquifers). 

Figure 3 shows that after 6 months of CO2(g) leakage into 

the water saturated aquifer, gas pressure is slightly elevated over ambient values within the vertical trace and 

ranges from 0.3 to 0.6 at the base of the impermeable 

layer that defined the pre-leak water table for CO2 fluxes 

above 104 t/yr. The magnitude of the gas phase saturation 

increases with the CO2 leak flux rate. CO2(g) saturation is 

a measure of the relative amounts of gas and water within 

the aquifer because CO2 dominates the gas composition 

Table 2: Hydrologic properties and parameters used for reactive transport simulations 

Parameter Value 

Permeability (m2) 10-17 – 2.5 × 10-10 

Porosity (%) 30 – 55 

Gas residual saturation (Sgr) 0.05
 

Irreducible water saturation (Slr) 0.2

van Genuchten parameter m 0.4
 

van Genuchten parameter α (Pa-1) 6.6 × 10-4 

CO2 leakage flux (t/yr) 103, 104, 105, and 2 × 106 (0.63 to 1250 t/m2/yr) 

Source Leak (m2) 40 × 40 

Average groundwater flow (m/d) 0.1, 0.3 

Distance of the well from the leak (m) 0, 100, 200, 500 

Well pumping rate (L/min) 0, 379, 757, 1893 

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CO2 gas phase saturation profiles after 6 months leakage for CO2 flux = 103, 104, 105, and 2 × 106 t/yr (0.63 to 1250 t/m2/yr)

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Figure 3 

CO2 gas phase saturation profiles after 6 months leakage for CO2 flux = 103, 104, 105, and 2 × 106 t/yr (0.63 to 

1250 t/m2/yr). 

in our simulations. The higher gas saturation at the base 

of the impermeable layer lowers the water table and 

results in some downward groundwater flow. At low CO2 

flux = 103 t/yr there is minimal impact of the gas phase on 

groundwater flow. 

Figure 4 and Movie 1 (Additional file 1) provide an example of the development of a pH plume in response to a 

CO2 leak equal to 105 t/yr at the base of an aquifer unit. 

We use pH as indicator of CO2 transport because pH is 

directly related to the pCO2 (Eqn. 4). A vertical plume 

trace from the leak source to the top of the aquifer evolves 

within 6 days as the pressure build-up, caused by the CO2 

flux along with buoyancy, rapidly exceeds the local hydrostatic pressure. pH decreases with depth because the pCO2 

is higher at depth in agreement with the static equilibrium 

model (Figure 1). In the first few days, the CO2 vertical 

transport is dominant over lateral plume spreading. The 

low permeable clay layer at the top of the aquifer acts as a 

barrier to release to the atmosphere (in the absence of the 

confining layer CO2 gas would diffuse through the vadose 

zone and to the atmosphere [30,31]). Within 40 days the 

effect of groundwater flow is apparent in the high permeability layers. This is most obvious as a finger-shaped 

plume in the most permeable layer (K = 2.5 × 10-10 m2) in 

the middle of the aquifer where the monitoring well is 

screened, but it also occurs within the slightly less permeable zones within the aquifer. Comparison of Figure 3 and 

4 show that even modest groundwater flow (0.3 m/d) 

effectively transports CO2 downstream along high permeable zones allowing it to be detected by a change in pH 

within a few months. The impact of elevated CO2(g) saturation at the top of the aquifer on downward groundwater 

flow is shown by the slight downward expansion of the 

pH plume from the impermeable layer into the upper 

most permeable later (k = 3.0 × 10-11 m-2). 

In our simulations, lateral dispersion at the top of the 

aquifer and within the highly permeable layers persists 

over time, because heterogeneity reflecting fingering and 

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Evolution of pH plume from a CO2 flux = 105 t/yr (62.5 t/m2/yr) and 0.3% hydraulic gradient

Geochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

Figure 4
 

Evolution of pH plume from a CO2 flux = 105 t/yr (62.5 t/m2/yr) and 0.3% hydraulic gradient. CO2 source is at 170 


m depth in an aquifer bounded by relatively impermeable clay layers. Plots show plume details between the CO2 leak source 


and sampling well at discrete time steps. The full lateral extent of the plume is shown in Movie 1 (see Additional file 1). The
 

plots are also identified on the pH breakthrough curves in Figure 5.
 

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pH breakthrough curves in undeveloped aquifers as a function of CO2 flux and distance of the monitoring well from the CO2 leak

 

Geochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

lateral discontinuities common in sedimentary formations was not incorporated into the geologic model. It is 

likely that a pH plume would be more complex when CO2 

is diverted by less permeable regions due to lateral heterogeneity present in actual aquifer lithology. We did not 

include lateral heterogeneity in the geologic model 

because it could not be represented with the available data 

on the scale of our simulations. 

Undeveloped aquifers 

In this section we explore the possibility of detecting CO2 

leakage from a change in pH in undeveloped aquifers as a 

function of the leakage rate and the location of the monitoring well. We define undeveloped aquifers as those in 

which there is no sustained pumping for domestic, agricultural, or industrial use. Figure 5 shows pH breakthrough curves for CO2 flux = 103, 104, 105, 2 × 106 t/yr at 

0, 100, 200, and 500 m away from the leak source. In all 

cases the screened zone of the monitoring well is within 

the highly permeable zone (note that positioning the well 

at different depths would yield different results). The simulation results show that the hydraulic gradient is sufficient to transport the CO2 plume so that it can be readily 

detected by measuring pH at some distance from the leak 

source within a few months for CO2 fluxes equal to and 

above 104 t/yr. As would be expected, breakthrough time 

decreases as the size of the leak increases and the distance 

of the well location decreases. Here, breakthrough is 

defined as the time needed for the plume to achieve a pH 

mid-point between the ambient and the CO2 affected 

Figure 5 

pH breakthrough curves in undeveloped aquifers as a function of CO2 flux and distance of the monitoring well 

from the CO2 leak. CO2 = 103, 104, 105, and 2 × 106 t/yr (0.63 to 1250 t/m2/yr). 

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groundwater at the monitoring well. Plume breakthrough 

corresponds to pH = 6.55 for a steady-state pH = 5.5. If the 

monitoring well happens to be directly above the leak, 

then breakthrough decreases from about 30 days at CO2 = 

104 t/yr to less than 7.5 hours at CO2 flux = 2 × 106 t/yr. If 

the sample well is 500 m from the source, then breakthrough decreases from 7.7 months at CO2 = 104 t/yr to 

5.7 months at CO2 flux = 2 × 106 t/yr. This is not the case 

for CO2 = 103 t/yr, where the monitoring well does not 

detect any change in pH over two-year period even when 

the sampling well is directly over the leak source. 

The transport of CO2 from small leaks within an aquifer is 

distinct from higher fluxes, because the pressure build-up 

and buoyancy that drive CO2 upward are overcome by 

aqueous solubility and lateral groundwater flow. This is 

clearly seen in Figures 3 and 6 where the CO2(g) plume is 

limited to the source volume and the pH plume covers a 

much larger volume. Figure 6 and Movie 2 (see Additional 

file 2) shows plume geometry in a series of pH contour 

plots for CO2 = 103 t/yr. Vertical heterogeneity across the 

main aquifer yields non-uniform gravity-driven groundwater flow. Our aquifer model has a 40 m section of fairly 

permeable sandstone (K = 5.4 × 10-11m2) at the depth of 

the leak as seen in Figure 2. The sandstone layer is overlain 

by silt-rich sandstone with reduced permeability (K = 5 × 

10-12 m2). The pH plume is largely contained within 

hydrologic layer at the base of the main aquifer unit, 

because CO2 transport is controlled by groundwater flow 

and aqueous solubility. The lateral hydrologic transport 

suppresses vertical gas transport in this example. Had the 

monitoring well been screened at greater depth towards 

the base of the aquifer unit, then the pH perturbation 

would have been large enough to detect. 

Groundwater flow plays an important role in CO2 transport, because it dictates what fraction of CO2 will be transported down gradient as aqueous species within 

permeable zones and what fraction will be transported up 

towards the base of top impermeable lithologies for a 

fixed CO2 flux. This is particularly the case for small leaks 

and is illustrated by comparing simulation results for CO2 

flux = 103 t/yr for scenarios with dip slopes equal to 0.3% 

and 0.1%, corresponding to average groundwater flow 

velocities of about 0.3 m/d and 0.1 m/d, respectively (Figures 6 &7, Additional files 2 &3). Even a small slope 

change leads to a significant flow-pattern change and 

directly affects leak detection by change in pH. The pH 

contour plot shows that the slower aquifer flow does not 

"trap" the CO2 as aqueous species within the lower permeable unit, as is the case for faster flow. Instead, the pressure 

build-up along with buoyancy moves CO2 vertically until 

it reaches the most permeable layer, where CO2 plume 

moves downstream towards the monitoring well. In addition to transport within the highly permeable layer, gas 

pressure and buoyancy continue to drive the CO2 to the 

base of the clay layers at the top of the aquifer. The lower 

slope and the slower groundwater flow yield a pH breakthrough of 6.55 after 1.4 years and pH = 5.7 within 2 years 

of the start of the leak (Figure 8). 

Developed aquifers 

Sustained pumping on CO2 transport mixes ambient 

groundwater with CO2-rich groundwater, where the pH 

depends on the leakage rate and the location of the well. 

For small leaks where the lateral hydrologic transport 

exceeds gas buoyancy, sustained pumping enhances leak 

detection because CO2-rich waters that are being transported along the base of the aquifer are drawn up to the 

permeable units where the well is located. Figure 9 shows 

pH breakthrough curves for CO2 flux = 103 t/yr at 0, 100, 

200, and 500 m away from the leak source for pumping 

rates from 0 to 1893 L/min. In all cases the screened zone 

of the sampling well is within the highly permeable zone. 

In the absence of pumping, lateral transport of CO2-rich 

water within the sandstone unit at the base of the aquifer 

is dominant over vertical CO2 transport towards the top of 

the aquifer. Thus the CO2 gas never reaches the screen 

depth of the monitoring well even though the well is 

located in the most permeable unit within the aquifer. 

However, sustained pumping draws the CO2-rich waters 

up towards the well yielding earlier pH breakthrough with 

increased pumping rate than for undeveloped aquifers. 

Even though sustained pumping brings the plume to the 

well, the simulations suggest that detection of small leaks 

may be difficult because steady-state pH of the pumped 

waters are only about one pH unit lower than ambient 

groundwater pH. 

The effect of sustained pumping on plume geometry on 

small leaks is shown in Figure 10 and Movie 4 (see Additional file 4) for a well with a high pumping rate (1893 L/ 

min) that is 200 m from the leak source. Initially, CO2 is 

transported down gradient in the permeable units at the 

base of the aquifer similar to case for the undeveloped 

aquifer (Figure 6). After one year the plume is beneath the 

well. At this point, the plume is drawn toward the well, 

where it is then transported within the most permeable 

sandstone unit as well as being removed from the aquifer 

system by the well. Similar to the example for undeveloped aquifers, detection of a CO2 leak by measuring pH 

depends on the depth of the screened well. If the well were 

screened at a lower level, then the pH signature would be 

more acid and easier to detect than in the example shown 

here. 

For leakage rates greater than 104 t/yr, sustained pumping 

effectively mixes the ambient water with the CO2-rich 

water and lowers acidity at the well. Figure 11 shows pH 

breakthrough curves for CO2 flux = 105 t/yr at 0, 100, 200, 

and 500 m away from the leak source for pumping rates 

from 0 to 1893 L/min. Pumping rate has minimal effect 

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Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 0.3% hydraulic gradient

Geochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

Figure 6
 

Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 0.3% hydraulic gradient. CO2 source is at 170 


m depth in an aquifer bounded by relatively impermeable clay layers. Plots show plume details between the CO2 leak source 


and sampling well at discrete time steps. The full lateral extent of the plume is shown in Movie 2 (see Additional file 2). The
 

plots are also identified on the pH breakthrough curves in Figure 5.
 

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Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 0.1% hydraulic gradient

Geochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

Figure 7
 

Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 0.1% hydraulic gradient. CO2 source is at 170 


m depth in an aquifer bounded by relatively impermeable clay layers. Plots show plume details between the CO2 leak source 


and sampling well at discrete time steps. The full lateral extent of the plume is shown in Movie 3 (see Additional file 3). The
 

plots are also identified on the pH breakthrough curves in Figure 8.
 

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Comparison of pH breakthrough curves in undeveloped aquifers for CO2 = 103 t/yr (0.63 t/m2 away from the leak source /yr) with monitoring well 200 m 

Geochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

Figure 8 

Comparison of pH breakthrough curves in undeveloped aquifers for CO2 = 103 t/yr (0.63 t/m2/yr) with monitoring well 200 m away from the leak source. 

on pH breakthrough times when sampled at a fixed distance from the leak source. The larger impact is that 

steady-state pH after breakthrough is more neutral with 

increased pumping due to mixing of a larger fraction of 

the ambient groundwater at greater distances from the 

leak source, as well as removing CO2 affected water at 

higher pumping rates. In practical terms, dilution resulting from groundwater pumping tends to decrease the likelihood of detecting leaks as the distance and the pumping 

rates increase. The steady-state pH of the CO2 affected 

waters in actively pumped aquifers ranges pH 5.5 for wells 

above the leak regardless of the pumping rate to about pH 

6.6 units for wells 500 m from the leak source with high 

pumping rates (1893 L/min). 

Implications for Measurement, Monitoring, and 

Verification (MMV) Plans 

Carbon dioxide leaking from a deep storage reservoir is 

likely to intercept groundwater resources before breeching 

the surface and reaching the atmosphere. The ubiquity of 

water wells may provide a simple means to test for such 

leakage before it reaches the surface. Occasional chemical 

testing for pH and alkalinity in water wells would indicate 

if carbon dioxide were entering the groundwater and if it 

is in danger of reaching the surface nearby. Much of the 

appeal of using pH and carbonate chemistry to detect CO2 

leakage is that the chemical and hydrological processes 

governing detection are well understood and that it uses 

readily available technology. Groundwater pH and carbonate chemistry are good indicators for leakage of stored 

carbon into an overlying aquifer because elevated CO2 

yields a more acid pH than the ambient groundwater. pH 

and alkalinity are good first level monitoring tools 

because these parameters capture the carbonate geochemistry associated with excess carbon dioxide in a dilute 

aquifer. It is important that pH and alkalinity be measured in the field, because their values will change as CO2­

rich waters degas and carbonate minerals precipitate 

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pH breakthrough curves in response to CO2 = 103 t/yr (0.63 t/m2 tance of the monitoring well from the CO /yr) in developed aquifers as a function pumping rate and dis- 2 leak

Geochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

Figure 9 

pH breakthrough curves in response to CO2 = 103 t/yr (0.63 t/m2/yr) in developed aquifers as a function pumping rate and distance of the monitoring well from the CO2 leak. 

[32,33]. A successful monitoring program includes both 

pre- and post-injection sampling. It is important to assess 

the baseline water chemistry and mineralogy in the dilute 

aquifer of concern, because identification of a CO2 leak 

and any associated hazard to the aquifer will be made 

against the baseline measurements. Knowledge of the 

groundwater flow, permeability, porosity and lithology 

depth profiles can be used to assess if pre-existing wells 

can be used for monitoring or if new wells will be needed. 

Reactive transport modeling of site geochemistry and 

hydrology is a highly useful for the design of effective 

MMV plans for carbon storage. Our simulations of the 

chemical perturbations associated with a CO2 gas leak 

into dilute groundwater suggest that more than one monitoring well is needed to detect leaks, because differences 

between leak flux, CO2 buoyancy, groundwater flow, and 

aquifer permeability yield asymmetric plumes over time. 

As a result, leaks have a higher likelihood of being 

detected if the monitoring well is down gradient from the 

vertical plume trace and samples the most permeable 

units. This is seen in Figures 4, 6, 7, 10 and Movies 1–4 

(see Additional files 1, 2, 3, 4), where wells within the 

most permeable units (K = 2.5 × 10-10 m2) and at the base 

of the aquifer unit for small leaks (CO2 = 103 t/yr, K = 5.4 

× 10-11 m2) would miss the leak altogether if the well is up 

gradient from the leak trace. One exception would be 

wells that sample groundwater near the top of the aquifer 

just below the confining layer near the water table (Additional file 1). CO2 spreads laterally along the top of the 

aquifer, because the confining layer prevents the CO2 

from diffusing through the vadose zone to the atmosphere. The extensive transport of the CO2 at the top of the 

aquifer allows CO2 to be detected far from the leak source 

irrespective of the direction of groundwater flow. After 

injection, it is important to sample over time and over a 

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Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2 tained pumping (1893 L/min) /yr) and 0.3% hydraulic gradient in a developed aquifer with susGeochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

Figure 10 

Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 0.3% hydraulic gradient in a developed 

aquifer with sustained pumping (1893 L/min). CO2 source is at 170 m depth in an aquifer bounded by relatively impermeable clay layers. Plots show plume details between the CO2 leak source and sampling well at discrete time steps. The full lateral extent of the plume is shown in Movie 4 (see Additional file 4). The plots are also identified on the pH breakthrough 

curves in Figure 9c. 

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pH breakthrough curves in response to CO2 = 105 t/yr (62.5 t/m2 tance of the monitoring well from the CO /yr) in developed aquifers as a function pumping rate and dis- 2 leak

Geochemical Transactions 2009, 10:4 http://www.geochemicaltransactions.com/content/10/1/4 

Figure 11 

pH breakthrough curves in response to CO2 = 105 t/yr (62.5 t/m2/yr) in developed aquifers as a function pumping rate and distance of the monitoring well from the CO2 leak. 

large enough sampling grid to capture pH breakthrough. 

In our example, the aquifer contains a very permeable unit 

that allows the CO2 leak to be detected for CO2 flux = 104 

t/yr within a 15-month time period. CO2 detection in less 

permeable aquifers might be achieved over longer sampling interval. These data could then be used in reverse or 

stochastic simulations to quantify the location, extent, 

and magnitude of the leak. A large enough sampling grid 

over time ensures that the MMV plan can capture the CO2 

leak and its growth, thus avoiding false positive/negative 

results from single point measurements. Equally as 

important, time-series chemical contour maps indicate 

the aquifer capacity for the CO2 leak and its potential flux 

to the surface. As an example, the IEA GHG Weyburn carbon dioxide monitoring and storage project injected 

about 5000 t/d of carbon dioxide into dolomite and limestone oil reservoirs in the southeast corner of Saskatchewan in Western Canada to study carbon dioxide – 

enhanced oil recovery at this site and to study monitoring, 

site selection, risk and other issues for CO2 storage [34]. 

The Weyburn project sampled groundwater chemistry 

prior to CO2 injection and 11 times over a four-year 

period after injection from about 35 to 60 wells per 25 

square kilometers to create the chemical contour maps 

showing spatial and temporal trends in carbonate chemistry due to the injection of supercritical CO2 in the reservoir [35,36]. Data of this kind for an overlying aquifer can 

be used to inform storage efficiency and future monitoring, remediation, and mitigation programs. 

Currently, there are no standards for the containment of 

stored CO2 in the subsurface. The United States Environmental Protection Agency (US EPA) is currently proposing 

that owners and operator's demonstrate that geologic 

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Geochemical Transactions 2009, 10:4	 http://www.geochemicaltransactions.com/content/10/1/4 

storage of CO2 does not endanger US drinking water for a 

50 year-time frame prior to closure of the site [37]. The US 

EPA recommends groundwater geochemistry as a monitoring tool, however assessment of leak magnitude leading to endangerment of drinking waters is still an area of 

active research [3-8]. One area of concern is the release of 

toxic metals in acidified water. Our simulations show that 

CO2 generates a slightly acid solution near pH 5.5. Sorption experiments suggest that a wide range of metals could 

be desorbed from the iron hydroxides common in sedimentary rocks as the pH is reduced from ambient conditions to pH 5.5 in response to leaking CO2 [38]. It is 

expected that metals would re-sorb to iron hydroxide 

phases as the solution is neutralized by reaction or mixing 

with the ambient groundwater, thus limiting the longterm hazard. The extent of re-sorption will depend on the 

concentration of other ions in solution. For example, CO2 

leaks may be accompanied by higher salinity water found 

in the storage reservoir. Any metals released at depth or 

within the drinking water aquifer may remain in the aqueous phase as chloride or organic complexes. In light of the 

need to protect drinking water, it would be prudent to collect filtered, acidified water samples for additional analysis if required to assess contamination from metals 

dissolved from hydroxides or other phases present in the 

sedimentary rocks, as was seen in the CO2 sequestration 

field demonstration in the Frio Sandstone [39]. 

The US EPA regulations on CO2 sequestration do not 

address minimum standards for leakage on global warming or on ecological or human hazards should anthropogenic CO2 be released to the atmosphere. The aim is to 

store carbon dioxide in the subsurface for 100s of years. 

Although a range of geophysical techniques are used to 

track supercritical CO2 plumes at depth during the injection phase, these techniques are not sensitive enough to 

confirm effective storage by detecting small changes in the 

amount stored. It is generally believed that "above zone" 

monitoring is the best approach to account for the containment of CO2 in the subsurface, because very small 

releases of stored CO2 will yield large signals. This is 

clearly the case for CO2 leakage into dilute aquifers. Our 

simulations show that leakage of ≥ 0.1% (104 t/yr) of the 

annual amount of stored CO2 from a gigawatt coal fired 

power plant can be readily detected by changes in pH and 

carbonate chemistry (Figures 5, 11). Detection of lower 

fluxes is possible if the monitoring well grid can capture 

the pH plume at greater depths or if active wells draw the 

plume toward the well. Although it is possible to detect 

CO2 using gas samplers at the surface at similar rates, the 

sampling chambers must be directly over the surface 

expression of the leak [7]. The ability to detect CO2 leakage at a distance from the leak source is a key advantage 

over gas sampling at ground surface. 

Competing interests 

The authors declare that they have no competing interests. 

Authors' contributions 

SC provided the geochemical analysis and was the primary author, YH conducted the reactive transport simulations, and RA provided the larger context for the research. 

Additional material 

Additional file 1 

Evolution of pH plume from a CO2 flux = 105 t/yr (62.5 t/m2/yr) 

and 0.3% hydraulic gradient. 

Evolution of pH plume from a CO2 flux = 105 t/yr (62.5 t/m2/yr) and 

0.3% hydraulic gradient. 

Click here for file 

[http://www.biomedcentral.com/content/supplementary/1467­

4866-10-4-S1.wmv] 

Additional file 2 

Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) 

and 0.3% hydraulic gradient. 

Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 

0.3% hydraulic gradient. 

Click here for file 

[http://www.biomedcentral.com/content/supplementary/1467­

4866-10-4-S2.wmv] 

Additional file 3 

Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) 

and 0.1% hydraulic gradient. 

Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 

0.1% hydraulic gradient. 

Click here for file 

[http://www.biomedcentral.com/content/supplementary/1467­

4866-10-4-S3.wmv] 

Additional file 4 

Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) 

and 0.3% hydraulic gradient in a developed aquifer with sustained pumping (1893 L/min). 

Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 

0.3% hydraulic gradient in a developed aquifer with sustained pumping (1893 L/min). 

Click here for file 

[http://www.biomedcentral.com/content/supplementary/1467­

4866-10-4-S4.wmv] 

Acknowledgements 

This work was prepared by LLNL under Contract DE-AC52-07NA27344. 

We thank Andy Tompson and three reviewers for their comments, which 

significantly improved the quality of this manuscript. 

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