CO2 Geological Storage & Groundwater Resources (EPA-NETL)
Task A. Understanding Groundwater Quality Changes in
Case of CO2 Intrusion.
Proper siting and management of geological storage projects
will ensure that the risks of carbon sequestration to human
health and the environment are low. However, there are
realistic scenarios under which CO2 could migrate from
the deep storage formation(s) to shallower aquifers, and
very little research has been done regarding the potential
impact of such leakage. In this project, the consequences
of CO2 leakage
on groundwater quality are further evaluated to provide
sound scientific information to regulators and the public.
Injection of high-pressure CO2 could impact
shallow aquifers through several processes (see below figure):
(1) CO2 gas
migrating from depth could reach a USDW (underground source
of drinking water) and dissolve in the water, with an increased
acidity that could enhance the solubility of inorganic
hazardous constituents. (2) In deep storage formations,
the enhanced solvent properties of CO2 likely
lead to the leaching of organic compounds from organic
material in (e.g., benzene and toluene). Subsequent transport
of the contaminated CO2 from
depth and intrusion into a USDW could result in the contamination
by these hazardous organic compounds. (3) Contaminants
such as H2S, a byproduct of coal gasification, could be
co-injected with CO2. H2S would preferentially partition
into the formation brine, but H2S-bearing CO2 could also
leak into USDWs and adversely affect water quality. H2S could furthermore interact with organic matter in the CO2 reservoir.
Schematic of potential impact of CO2 storage
on groundwater quality
The following three subtasks are conducted during this three-year project
to evaluate the potential hydrochemical impact of CO2 storage
projects on USDWs.
Sub-Task A1: CO2-Related Dissolution of Heavy Metals
and Other Constituents in USDWs
In this ongoing sub-task, we investigate the water quality changes
upon intrusion of CO2 into potable groundwater. The intruding
CO2 would lower groundwater pH and thereby enhance the solubility
of hazardous inorganic constituents (including heavy metals). How and to what
extent groundwater quality would be affected depends largely on the initial
abundance and distribution of these constituents in the aquifers, as well
as on the aquifer mineralogy and the oxidation state. Using the
USGS NWIS (National Water Quality Information System) data base, we have
conducted a systematic evaluation of more than 38,000 groundwater quality
analyses from aquifers throughout the United States that report non-zero
concentrations of selected hazardous constituents. The results of
the evaluation are employed to set up an equilibrium geochemical model
of the aquifer chemistry in order to estimate the distribution of each
heavy metal between the aqueous phase and adsorption and ion exchange
sites, and in solid solution in primary and secondary minerals. Important
qualitative conclusions can be drawn immediately from this evaluation
regarding the geochemical vulnerability of the groundwaters.
For quantitative evaluation, we use the equilibrium geochemical model
as a starting point for reactive geochemical transport simulations that
predict the impact of CO2 intrusion into a fresh-water aquifer
and the related changes to the host rock mineralogy and water chemistry. To
validate and support the numerical results, preliminary laboratory experiments
are currently conducted, where CO2 is injected into vessels
initially filled with rock fragments and water and periodic aqueous samples
are taken to measure changes in the concentrations of the heavy metals. Our
findings will help to understand (1) which aquifer systems and regions
of the country might be vulnerable in case of CO2 intrusion,
and (2) which inorganic constituents might adversely affect water quality
and to what extent.
Sub-Task A2: Leaching of Organic Compounds
In this future task, we will review and evaluate the distribution
coefficients of selected organic compounds (e.g., benzene, toluene)
and other potentially hazardous compounds to be identified following
further review. The conditions assumed are representative of a deep
saline aquifer. We will establish whether the solvent properties of
CO2 to hazardous organic compounds constitute
a potential risk to USDW contamination. For example, we will
investigate if solved organic compounds may be transported
with leaking CO2 or may be left behind, e.g., at phase change
from supercritical to gaseous CO2.
Sub-Task A3: Impact of Co-Injection of H2S
In this future task, we will model the injection and transport
of CO2 containing co-injected contaminants into a deep saline
aquifer. We will investigate the progressive partitioning of
H2S into the brine during transport and the extent to which
H2S is removed from the CO2 phase with time and
distance. The role of H2S in inducing precipitation
of hazardous metals and sulfides, or their leaching due to sulfide complexation,
will be evaluated. This study may be expanded to include other co-injected
hazardous trace constituents. The reactive transport code TOUGHREACT
will be used for this purpose. Depending on H2S partitioning
results, we will study the impact of H2S-bearing CO2 migrating into USDWs. |