POTENTIAL EFFECTS OF GEOSEQUESTERED CO2 LEAKAGE INTO OVERLYING HYPOXIC GROUNDWATERS

To simulate the geochemical effects of CO₂ leakage from geosequestration sites into drinking water aquifers, year-long incubations of aquifer formation sediments submerged in water were continuously bubbled with CO₂ and monitored for pH, alkalinity, dissolved ions and trace metals. Results revealed harmful increases of dissolved metals, highlighting a potential threat to aquifers (Little and Jackson, 2010). Here we report initial results from a set of longer-running incubations redesigned to promote reducing conditions. Three localities representing the Ogallala aquifer, revisited from the 2010 study, were incubated alongside four localities of the Central Valley aquifer system of California. A control group was maintained under the same conditions without CO₂ bubbling.

After 420 days of incubation, CO₂-bubbled (+CO₂) samples show a mean pH of 5.5 relative to the control group pH of 7.5. The +CO₂ waters are hypoxic and are more reducing than control waters. The mean Eh of initial +CO₂ and control waters was 220 mV and 117 mV. As of July 2012, +CO₂ and control waters exhibited mean Eh values of 164 mV and 180 mV respectively. Ogallala +CO₂ waters yielded the lowest Eh values, occasionally becoming negative. The +CO₂ waters generally exhibit DO concentrations less than 1 mg L^-1.

In comparison with control group, the +CO₂ waters showed rapid twofold increases in alkalinity, which then remained relatively stable throughout the experiment. Alkali earths and metals (Li, Ca, Mg, Rb and Sr) gradually increased in +CO₂ waters over the first 100 days and remained stable or slightly decreased. Trace elements generally increased under +CO₂ conditions, with concentrations dependent on locality. Several element concentrations (As, Sb, Th and V) rapidly decreased during the experiment, suggesting adsorption onto the sediments. Ogallala samples revealed alarming increases in Ni and Co, with values of Co as high as 2 ppm, well above EPA recommendations. These results illuminate the impact of aquifer redox conditions on a potential CO₂ leakage scenario. These data are currently being incorporated into PHREEQC and MODFLOW to examine the geochemical and spatiotemporal nature of a potential CO₂ leak. Initial results will be presented.