Metal release from sandstones under experimentally and numerically simulated CO2 leakage conditions.

Leakage of CO2 from a deep storage formation into an overlying potable aquifer may adversely impact water quality and human health. Understanding CO2-water-rock interactions is therefore an important step toward the safe implementation of geologic carbon sequestration. This study targeted the geochemical response of siliclastic rock, specifically three sandstones of the Mesaverde Group in northwestern Colorado. To test the hypothesis that carbonate minerals, even when present in very low levels, would be the primary source of metals released into a CO2-impacted aquifer, two batch experiments were conducted. Samples were reacted for 27 days with water and CO2 at partial pressures of 0.01 and 1 bar, representing natural background levels and levels expected in an aquifer impacted by a small leakage, respectively. Concentrations of major (e.g., Ca, Mg) and trace (e.g., As, Ba, Cd, Fe, Mn, Pb, Sr, U) elements increased rapidly after CO2 was introduced into the system, but did not exceed primary Maximum Contaminant Levels set by the U.S. Environmental Protection Agency. Results of sequential extraction suggest that carbonate minerals, although volumetrically insignificant in the sandstone samples, are the dominant source of mobile metals. This interpretation is supported by a simple geochemical model, which could simulate observed changes in fluid composition through CO2-induced calcite and dolomite dissolution.

Evidence of remediation-induced alteration of subsurface poly- and perfluoroalkyl substance distribution at a former firefighter training area.

Poly- and perfluoroalkyl substances (PFASs) are a class of fluorinated chemicals that are utilized in firefighting and have been reported in groundwater and soil at several firefighter training areas. In this study, soil and groundwater samples were collected from across a former firefighter training area to examine the extent to which remedial activities have altered the composition and spatial distribution of PFASs in the subsurface. Log Koc values for perfluoroalkyl acids (PFAAs), estimated from analysis of paired samples of groundwater and aquifer solids, indicated that solid/water partitioning was not entirely consistent with predictions based on laboratory studies. Differential PFAA transport was not strongly evident in the subsurface, likely due to remediation-induced conditions. When compared to the surface soil spatial distributions, the relative concentrations of perfluorooctanesulfonate (PFOS) and PFAA precursors in groundwater strongly suggest that remedial activities altered the subsurface PFAS distribution, presumably through significant pumping of groundwater and transformation of precursors to PFAAs. Additional evidence for transformation of PFAA precursors during remediation included elevated ratios of perfluorohexanesulfonate (PFHxS) to PFOS in groundwater near oxygen sparging wells.

Subsurface transport potential of perfluoroalkyl acids (PFAAs): Column experiments and modeling.

Transport of ten perfluoroalkyl acids (PFAAs) was studied with one-dimensional (1-D) saturated column experiments using four soil types with an organic carbon fraction (foc) range of ~0-0.045. Columns were operated under conditions relevant to aqueous film-forming foam (AFFF)-impacted fire protection training areas to determine the ability of equilibrium transport parameters to describe 1-D PFAA transport, if rate-limited sorption influences PFAA transport, and if kinetic parameters can be used to evaluate factors causing rate-limited sorption. Results of initial screening of PFAA breakthrough found that over half of the breakthrough curves deviated from equilibrium transport and merited further investigation. Subsequent analysis showed that, in many cases, these deviations could be accounted for by considering the range of applicable equilibrium Kd values (i.e. based on standard deviation) applicable to the solid phase. Thus, transport of the majority of PFAAs in 3 soils with foc of 0-0.017 was not impacted by rate-limited sorption. Further, low sorption led to transport that was essentially simultaneous for the majority of PFAAs in these porous media. Exceptions were observed for long-chain PFAAs, and also in a fourth soil with foc of 0.045, which indicated the potential for rate-limited sorption to impact transport in some scenarios. Subsequent flow interruption experiments isolating kinetic behavior confirmed rate-limited sorption caused nonequilibrium transport. Linear free energy relationships (LFERs) developed in previous work to predict the inverse relationship between mass transfer coefficients (k) and sorption parameters (i.e., Kd) were used to estimate values of k for PFAAs in this study. Resulting k values were 10-3 to 10-8 h-1, consistent with previously measured kinetic parameters for other polar and anionic compounds. Models incorporating estimated k values resulted in improved predictions of breakthrough observed in nonequilibrium scenarios (R2 0.83-0.98), but k values will require further validation prior to broader application. This work illustrates rate-limited sorption considerations are needed to describe 1-D column saturated transport for some PFAAs and solid phases. At field scales, subsurface heterogeneity and PFAA precursor transformation may be equally or even more important in determining saturated PFAA transport, but kinetic parameters in this study may help to determine relative contributions of rate-limited sorption to overall transport.

Geochemical implications of brine leakage into freshwater aquifers.

CO(2) injection into deep saline formations as a way to mitigate climate change raises concerns that leakage of saline waters from the injection formations will impact water quality of overlying aquifers, especially underground sources of drinking water (USDWs). This paper aims to characterize the geochemical composition of deep brines, with a focus on constituents that pose a human health risk and are regulated by the U.S. Environmental Protection Agency (USEPA). A statistical analysis of the NATCARB brine database, combined with simple mixing model calculations, show total dissolved solids and concentrations of chloride, boron, arsenic, sulfate, nitrate, iron and manganese may exceed plant tolerance or regulatory levels. Twelve agricultural crops evaluated for decreased productivity in the event of brine leakage would experience some yield reduction due to increased TDS at brine-USDW ratios of < 0.1, and a 50% yield reduction at < 0.2 brine-USDW ratio. A brine-USDW ratio as low as 0.004 may result in yield reduction in the most sensitive crops. The USEPA TDS secondary standard is exceeded at a brine fraction of approximately 0.002. To our knowledge, this is the first study to consider agricultural impacts of brine leakage, even though agricultural withdrawals of groundwater in the United States are almost three times higher than public and domestic withdrawals.