In Situ ${\mathsf{CO}}_{\mathsf{2}}$ Sparging. II: Groundwater Mounding and Impacts on Aquifer Properties

In situ sparging of gaseous ${\mathrm{CO}}_2$ was demonstrated through laboratory and pilot-scale testing to be an effective means to neutralize a caustic brine plume (CBP) and to reduce levels of mercury and other heavy metals in groundwater. The CBP exhibits high pH levels ranging from 10.5 to 12, densities as high as $1.06\mathrm{g}/\mathrm{mL}$, high dissolved silica concentrations, and mercury ranging from 50 to $\mathrm{1,000}\mathrm{\mu g}/\mathrm{L}$. The CBP lies at the base of a moderately permeable aquifer at depths ranging from 30 to 50 ft below ground surface. The pilot test involved a single sparge well and 13 monitoring wells screened at varying depths and radial distances up to 100 ft. The pilot test demonstrated that pH within an aquifer volume of approximately $184{\mathrm{m}}^3$ ($6,500{\mathrm{ft}}^3$) could be reduced to near neutral pH. Sparging caused intermittent mounding of groundwater levels, particularly in the piezometric surface of the basal portion of the aquifer, where the sparge well was screened. The piezometric surface in the basal portion of the aquifer rose to several feet above the ground surface during active sparging and then rapidly declined upon cessation of sparging. However, the rise in the overlying groundwater table was considerably muted by the lower vertical hydraulic conductivity of the aquifer. At its peak during the sparging, the groundwater table rose to within approximately a foot of the surface. Pre- and post-sparging aquifer testing indicated that the transmissivity of the aquifer was reduced by approximately 75% and the storativity was substantially increased. These changes in aquifer properties are believed to be primarily associated with the residual saturation of ${\mathrm{CO}}_2$ gas from the sparging and exsolution of ${\mathrm{CO}}_2$ gas from the groundwater in areas immediately adjacent to ${\mathrm{CO}}_2$ channels where groundwater is expected to have near saturation levels of ${\mathrm{CO}}_2$.