Experimental study of sediment-CO2 reactions with application to changes in groundwater quality due to leakage of sequestered CO2

Geologic sequestration of CO2 requires storage of buoyant, immiscible supercritical CO2 in the subsurface for long periods of time. In the viability assessment of sequestration, one of the most significant risk concerns is damage of shallow drinking water aquifers from potential CO2 leakage. The groundwater quality concerns are primarily due to acidification, increase in total dissolved solids (TDS), and the potential mobilization of hazardous trace metals. Previous studies of this issue include natural analogs of CO2-groundwater interactions where there can be uncertainty as to whether observations reflect variations in groundwater quality unrelated to CO2 and numerical studies of CO2-brine interactions with shallow aquifers where there is uncertainty as to the geochemical mechanisms of trace element mobilization. In this study, we conduct experiments of CO2-water-sediment interactions from the Chimayo region in north-central New Mexico. This region includes a natural CO2 seep which we have used in natural analog studies (Keating et al. 2009) and the presence of elevated arsenic and uranium concentrations. The experiments were designed to address the question of whether CO2 reactions with aquifer sediments could account for the observed water quality problems in Chimayo and to consider the more general question of the nature of trace element mobilization by CO2-induced reactions. The experiments consisted of periodically sampled batch reactions of water and sediment reacted with a continuous source of 1-atm CO2. The sediments are quartz-rich alluvial fan deposits that include feldspars and clays and were characterized by XRD, XRF, and by sequential extraction to determine trace metal content. The water was created to represent the background major ion chemistry of groundwater in the region. The sediments were initially equilibrated with the synthetic water prior to introduction of CO2. Reaction with CO2 was monitored over 10 days with periodic fluid sampling. The results showed a rapid decrease in pH from 7.8 to 5.8, which slowly rose over the course of the experiment to 6.4. TDS increased from 900 to 2000 ppm and was driven primarily by an increase in dissolved bicarbonate. The impact of CO2 on dissolved metals was strongly dependent on the sediment source. The sandy lithology showed little reactivity. In the following, the impact on the clay-rich lithology is discussed in more detail. Of the major ions, the largest change was observed for Ca which increased from 240 to 450 ppm. Of the trace elements, U, Ba, Mn, Ni and Zn all showed an increase in concentration with CO2, while As, Cd, Co, Cr, Cs, Cu, Pb, Se and V showed no change or remained below detection levels. None of the elements exceeded EPA’s maximum contaminant levels (MCL). However, of particular note, uranium increased from 4 to 14 ppb and Ba increased from 80 to 200 ppb. The results indicate that at Chimayo the most reactive sediments did not exceed EPA drinking water standards and the less reactive sediments showed little change except in pH and TDS. Acknowledgments: We thank the Fossil Energy Program of DOE for funding.