The importance of considering rock-water interaction when studying potential 

impacts of geological CO2 sequestration on drinking water resources 

Changbing Yang, Jean-Philippe Nicot, Rebecca C. Smyth, Katherine D. Romanak, and 

Susan D. Hovorka 

Summary 

It is of fundamental importance that water-rock reactions be taken into account in the 

interpretation of ground water chemistry. Not considering the rock contributions to the 

rock-water-CO2 reactive system can lead to incorrect extrapolations and overemphasize 

the risk to water. 

Conclusions of modeling exercises must also be seriously weighted by the soundness of 

the model assumptions. For example, we think that the role of sulfides in terms of risk to 

water as a consequence of CO2 leakage in the report of Birkholzer et al. (2008) has been 

overemphasized, as these species are not the minerals commonly observed in fresh-water 

aquifers. 

Introduction 

Researchers from the Bureau of Economic Geology (BEG) at the University of Texas at 

Austin are conducting experimental and modeling studies relevant to geologic 

sequestration of CO2 (CO2GS) with specific regard to potential impacts to shallow 

groundwater resources. In addition, BEG researchers have recently completed a fieldbased groundwater sampling study at the SACROC oilfield in west Texas and are 

continuing research on CO2GS at the Cranfield oilfield in Mississippi. Other researchers 

in the field of geologic carbon sequestration have recently published work on potential 

impacts to water resources as a result of CO2 injection directly into deep brine 

formations, or by accidental release of injectate CO2 into drinking water resources either 

through laboratory experiments or groundwater modeling studies. 

Themes we want to emphasize as being critical to studies of potential impacts to drinking 

water resources from CO2 injection are: 

•	 CO2 occurs naturally in shallow aquifers. CO2 concentrations in a single aquifer 

can vary temporally and spatially as a result of agricultural and industrial 

practices. For example, Scanlon et al. (2005, 2007) have documented enhanced 

recharge from land use changes. Enhanced recharge of groundwater through the 

vadose zone can potentially change CO2 concentrations in the saturated zone. 

•	 The capacity for carbonate minerals (carbonate rocks and carbonate cement in 

clastic rocks) to buffering changes in pH with influx of CO2 is under represented 

in the literature (i.e. Carroll et al., 2009). An exception to this statement is 

Keating et al. (2009) who documented nicely the buffering capacity of carbonate 

cement in a high-CO2 concentration natural analog system in New Mexico. 

•	 Realistic rock-water reactions are critical to accurate prediction of impacts to 

groundwater as a result of CO2 injection (numerous articles referenced herein). 

For example, Romanak et al. (in preparation) have also documented the 

importance of carbonates but in a more complex fashion through the incongruent 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

dissolution of dolomite as a process dominating the hydrochemistry of the 

Dockum aquifer that overlies the SACROC oilfield in west Texas. 

The following text is a review of recently published studies on potential impacts to 

groundwater from GS with some examples from BEG’s on-going and soon to be 

published work. We start by citing published work on the importance of rock-water 

interactions in both deep and shallow aquifers for interpreting results of groundwater 

modeling. We then focus on one specific study (Birkholzer et al., 2008) that has been 

completed by researchers at the Lawrence Berkeley National Laboratories (LBNL). 

CO2-water-rock interactions discussed in literature 

Rock-water reactions are ubiquitous in groundwater systems and cannot be excluded 

from models used to predict impacts to groundwater from introduction of CO2. Work of 

Kaszuba et al. (2003) concludes that because of the importance of rock-water reactions in 

potential geologic sequestration systems, much more than simple aqueous dissolution of 

CO2 in water must be factored into any realistic model of potential impacts to deep 

formation fluids or freshwater aquifers from introduction of anthropogenic CO2. 

Earlier work by Zhu and Burden (2001) states that the complexity of aquifer fluid 

chemistry is defined by the degree of interaction with aquifer mineralogy, which includes 

solid matrix, grain-coatings and colloids. They are critical of groundwater modeling 

efforts that only consider pore-fluid compositions and ignore or greatly simplify aquifer 

mineralogy. Conclusions from Zhu and Burden (2001) are that contaminant fate and 

transport model predictions depend upon mineral composition of the aquifer. In fact, they 

state that the amount of calcite in the aquifer matrix determines the distance of 

contaminant plume migration. 

Wigand et al. (2008) go on to state that not only does introduction of supercritical CO2 

into geologic systems affect brine composition through aqueous dissolution, it also 

affects the intensity of rock-water interactions. The phenomenon of increased rock-water 

interaction with increase in dissolved CO2 in brine is becoming common knowledge 

through DOE/NETL- and EPA-sponsored research on geologic sequestration (i.e., 

Keating et al. 2009). 

Review of LBNL shallow groundwater modeling 

LBNL (Birkholzer et al., 2008) evaluated mobilization of a set of trace elements (called 

hazardous constituents in the report1

), such as arsenic, barium, cadmium, mercury, lead, 

uranium and zinc from a set of mineral species that may potentially be mobilized in the 

event of CO2 leak from the deep storage reservoirs using a comprehensive computational 

model. The mineral assemblages in the aquifer rocks were selected indirectly upon 

evaluation of 38,000 groundwater samples from aquifers throughout the United States, 

retrieved from the Unites States National Water Information System (NWIS). The report 

concluded that some trace elements may be mobilized and exceed the maximum 

1 We disagree with the term “hazardous constituents” used in the Birkholzer et al., (2008) report because 

the trace elements occur in groundwater naturally even if their concentrations may increase owing to CO2 

intrusion. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

contamination levels (MCL) resulting from enhanced dissolution of the mineral 

assemblages caused by CO2 intrusion into shallow groundwater aquifers. However, the 

model results rely on a fundamental assumption with large uncertainties in model 

parameters. We disagree with the assumption for constructing the EQ3/6 models and the 

methodology they used for investigating mobilization of trace elements from the aquifer 

rocks. We believe that this report illustrates the trappings of solely using groundwater 

compositions to estimate impact of CO2 on the system. 

The fundamental assumption in the Birkholzer et al. (2008) report for constructing the 

EQ3/6 models is that mobilization of trace elements is mainly dominated by enhanced 

dissolution of a set of specific mineral assemblages (for instances, sulfides in Table 3.6, 

p37 in the report). This set of specific mineral assemblages was selected from 

mineralogical analysis of coal seam aquifers, which formed in very strong reducing 

environments. Based on such an assumption, the authors calculated saturation indices of 

the selected mineral assemblages over chemical analyses of 38000 groundwater samples 

and determined whether a specific mineral phase would be present in a rock in 

equilibrium with the water sample. In a second step, assuming this phase is present, the 

authors computed the amount of trace elements that could be released under addition of 

CO2. This assumption is unreasonable and not well supported at all in the literature. 

There are many other factors that could affect mobilization of the tracer elements, such as 

aqueous complexation (by ligands), ad/desorption from metal oxide and clay surfaces, 

and solid solutions in common minerals (carbonates, feldspars, and clay minerals). For 

instance, behavior and contribution of arsenic in natural water are related to MeOx (either 

simple desorption in oxidizing conditions or reducing conditions) (for example, Smedley 

and Kinniburgh, 2002, Scanlon et al. 2009). 

BEG researchers did a set of batch experiments by reacting rock samples taken from 

major aquifers in Texas with water and CO2 gas at a pressure of 1 atm (Lu et al., 2009, 

Smith etal. 2009). They found that concentrations of some trace elements may be 

elevated after CO2 intrusion, such as arsenic (Figure 1). However, no evidences of XRD 

and SEM analysis indicated that arsenopyrite existed in the rock samples. Lead, which 

according to the Birkholzer (2008) could be the most problematic element after CO2 

intrusion, however, is non-detectable in the 10 batch experiments. Of the 114 analyses of 

potable groundwater in the vicinity of the SACROC oilfield in west Texas, 100 samples 

(88% of the total samples) had Pb concentrations below a detection limit of 0.0002 

milligrams per liter (mg/L) (Smyth et al., in preparation). Our experiences indicate that 

Pb in drinking water resources commonly comes from corrosion of submersible water 

pumps and accompanying conduit pipes and not from the aquifer rocks. 

In conclusion, the methodology employed in the Birkholzer (2008) report on potential 

impacts of CO2 addition to shallow groundwater resources by using only numerical 

modeling techniques is questionable and may be misleading if the controlling minerals/or 

controlling phases assumed in the numerical models are not the right ones. In addition, 

large uncertainties in the model parameters, such as Eh values and solubility constants 

may lead to wrong conclusions. Regulations of CCS based on such conclusions drawn 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

from the model results presented in the Birkholzer (2008) report could be biased and may 

be impractical when applied to actual sites. 

1.E-01 

1.E+00 

1.E+01 

1.E+02 

-12 -8 -4 0 4 8 12 16 

Time (days) 

As ppb 

Batch 1 

Batch 2 

Batch 3 

Batch 4 

Figure 1. Evolution of arsenic concentration in four batch experiments (time 0 

represents introduction of CO2 into the batch reactor) 

References 

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Technical Report LBNL-1251E, 352p. 

Carroll, S., Hao, Y., and Aines, R., 2009, Geochemical detection of carbon dioxide in 

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analog site and implications for carbon sequestration, under review. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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