Quantifying and monitoring
potential ecosystem impacts
of geological carbon storage
Fact Sheet 13
Biogeochemical impacts of leaking CO2
What happens when CO2 reacts with
sea water?
When CO2 dissolves into sea water it reacts to form
a weak acid (carbonic acid - H2CO3) that readily
releases hydrogen ions (H+), contributing to
increased acidity (= pH decrease) and an increased
concentration of bicarbonate (HCO3-) ions (figure 1).
Calcium carbonate-rich (CaCO3) sediment composed
of shells and skeletal material from marine animals
(e.g. mussels, snails, brittle stars, sea urchins etc.) may
start to dissolve in order to ‘compensate’ for the drop
in pH caused by the injected CO2 gas (figure 1). This
buffering process limits acidification but potentially
at the expense of the shells and skeletal structures in
marine animals.
Figure 1. How injected CO2 gas dissolves and reacts with water
within sediments and above the sea floor.
Biogeochemical impacts in the sediment
During the QICS controlled CO2 release experiment in Ardmucknish
Bay (Scotland), 4.2 tonnes of CO2 gas was injected at a depth of 12
metres below the sea floor over 37 days. Data recorded by CO2 sensors
and analysis of sediment cores collected by divers revealed that the
injected CO2 gas release had an impact on the acidity (pH), dissolved
CO2 and buffering capacity of the sea bed sediment (figures 2 & 3).
Figure 2. Variation in geochemical parameters in the
sediment during the QICS experiment.
Biogeochemical impacts of the injected CO2 gas were highly localised
and only observed in shallow sea bed sediments over the centre of
the injection after five weeks of gas release. At this stage the dissolved
CO2 concentrations and the sediment buffering capacity (i.e. total
alkalinity) increased by more than 10 times compared to background
values. pH initially dropped with increasing CO2 but then increased to
near baseline levels as the buffering increased, (figure 2). An increase
in the concentration of dissolved metals, such as calcium, iron,
manganese and some trace metals, were also observed during the
release period. All of these changes persisted for at least a week after
the gas release was stopped, however three weeks after CO2 injection
ceased these parameters returned to their normal values (figure 2).
www.qics.co.uk
Quantifying and monitoring
potential ecosystem impacts
of geological carbon storage
Figure 3. Showing from left to right: sea floor CO2 sensor; diver collecting sea water samples; two sea bed CO2 sensors near
a stream of CO2 bubbles; diver collecting sediment core samples.
Biogeochemical impacts & fluxes in the
water column
Elevated CO2 concentrations and lowered pH were measured
by CO2 sensors in the sea water near the CO2 bubble streams
(Figure 4). The extent of the high CO2/low pH sea water plume
was confined to an area approximately 25 metres in diameter
above the gas injection point and closely corresponded with the
visual observations of CO2 bubbles. The plume of sea water with
high CO2/low pH reached the sea-surface (approximately 12
metres above the sea bed) during low tide. The gaseous CO2 flux
into the water column was measured at ≤15% of the injected
volume of CO2, with 85% of the gas presumed to remain trapped
within the sediments, either dissolved in the sediment water
or trapped as either gas or in mineral form (figure 1). However
modelling estimates suggest that some dissolved CO2 escaped the
sediments, with only ~50% of the CO2 retained in the sediments.
Figure 4. A plot of the CO2 content of sea water (partial pressure,
pCO2) recorded by a sensor towed 1 metre above the sea floor,
across the centre of the release and the area unaffected by the
release. Inset: bathymetry survey data shows peaks of gas ‘flares’
that correspond to the plot of elevated CO2 content.
Summary points
•
The biogeochemical impacts of the injected CO2 on water within sea bed sediment and sea water overlying
the sea floor were clearly distinguished.
•
Only the area above the release point was effected and the biogeochemistry of the sea bed sediment and
sea water returned to normal values within weeks of switching off the CO2 gas.
•
Shells of calcium carbonate (CaCO3) within the sea bed sediment dissolved to compensate for acidification
caused by the injected CO2.
•
Only 15% of injected CO2 escaped (as gas bubbles) from the sea bed into the overlying sea water with
some rising to the sea surface to enter the atmosphere.
•
A significant proportion of the injected CO2, was retained within the sea bed sediment above the release
points in dissolved, gaseous or mineral form.
•
The chemical observations indicate that leakage will be readily detectable by chemical sensors in the water
column, but for leaks of this magnitude, only within a few metres of the release point.
QICS Project Office
www.bgs.ac.uk/qics/ www.qics.co.uk
Plymouth Marine Laboratory, UK
Project Leader: Jerry Blackford
QICS [email protected]
led by Plymouth Marine Laboraory in the UK, contact Jerry Blackford | [email protected] further information
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