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.
•Low flow breakthrough of most PFAAs in sorbents with foc 0–0.017 exhibited equilibrium transport.•Rate-limited transport impacted breakthrough in a subset of PFAAs and sorbents.•Models incorporating mass transfer coefficients led to improved prediction of PFAA breakthrough.
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 K oc 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.
CO2 leakage from underground CO2 sequestration and storage poses potential risks to degradation of water quality in shallow aquifers. Increased CO2 concentrations can result in decreased pH and lead ...to subsequent metal release from mineral dissolution or desorption from mineral surfaces. Dissolution of carbonate minerals present in aquifer sediments or rocks will buffer pH and is generally thought to reduce the potential risk of metal release in the event of a CO2 leak. As a result, much of the research on geochemical impacts of CO2 leakage has focused on siliciclastic aquifers with little to no carbonate minerals present. However, carbonate minerals contain trace amounts of metals in their crystal structure that will be released into solution with dissolution and may pose a risk to drinking water quality. Here, we perform laboratory water–rock experiments to analyze the potential for metal release due to carbonate mineral dissolution in limestone aquifers. Rock samples from three limestone aquifers were dissolved in batch reactors with varying partial-pressures of CO2 (from 0.01 to 1bar) in the headspace. As CO2 dissolved into the fluid and decreased the pH, the carbonate minerals dissolved and released metals into solution. The concentrations of calcium, magnesium, strontium, barium, thallium, uranium, and cobalt increased but remained below any regulatory limits. The concentrations of arsenic and nickel increased and exceeded primary drinking water standards set by the USEPA and the State of California, respectively. Potential sources of metals in the rocks were determined through detailed sample characterization using sequential extractions, laser ablation inductively coupled mass spectrometry, and high resolution mineralogical mapping with QEMSCAN. We found that calcite dissolution released more metals to solution than pyrite dissolution or metal desorption from mineral surfaces in these experiments. Geochemical models based on the experimental data were used to evaluate the relative importance of calcite dissolution versus pyrite dissolution over a 30-year time frame. Under both oxic and sub-oxic conditions, calcite dissolution is the dominant source of metals to solution immediately after exposure to CO2. Pyrite dissolution becomes the dominant source at later times as the fluid reaches equilibrium with respect to calcite. For all model scenarios, the cumulative contribution of metals to solution was dominated by calcite dissolution. Results from this study suggest that the pH-buffering benefit of carbonate mineral dissolution in the event of a CO2 leak may be offset by the potentially negative effect of trace metal release from the crystal structure. This study highlights the need for detailed sample characterization at individual sites to identify sources of metals when assessing the potential risk of CO2 leakage into shallow aquifers.
•We analyzed metal release from natural limestone rocks at elevated pCO2.•Calcite dissolution and desorption controlled metal release.•We simulated kinetic calcite, pyrite dissolution under oxic and suboxic conditions.•Calcite will release more metals than pyrite in carbonate aquifers across 30years.
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.
•We analyzed metal release from natural dolomite rocks at elevated pCO2.•Carbonate mineral dissolution dictated aqueous Co, Mn, Ni, Sr, Tl and Zn levels.•Aqueous concentrations of most metals ...decreased after system decompression.•Reactive dolomite surface area was orders of magnitude less than BET area.
The potential for metal release associated with CO2 leakage from underground storage formations into shallow aquifers is an important consideration in assessment of risk associated with CO2 sequestration. Metal release can be driven by acidification of groundwaters caused by dissolution of CO2 and subsequent dissociation of carbonic acid. Thus, acidity is considered one of the main drivers for water quality degradation when evaluating potential impacts of CO2 leakage. Dissolution of carbonate minerals buffers the increased acidity. Thus, it is generally thought that carbonate aquifers will be less impacted by CO2 leakage than non-carbonate aquifers due to their high buffering potential. However, dissolution of carbonate minerals can also release trace metals, often present as impurities in the carbonate crystal structure, into solution. The impact of the release of trace metals through this mechanism on water quality remains relatively unknown. In a previous study we demonstrated that calcite dissolution contributed more metal release into solution than sulfide dissolution or desorption when limestone samples were dissolved in elevated CO2 conditions. The study presented in this paper expanded our work to dolomite formations and details a thorough investigation on the role of mineral composition and mechanisms on trace element release in the presence of CO2. Detailed characterization of samples from dolomite formations demonstrated stronger associations of metal releases with dissolution of carbonate mineral phases relative to sulfide minerals or surface sorption sites. Aqueous concentrations of Sr2+, CO2+, Mn2+, Ni2+, Tl+, and Zn2+ increased when these dolomite rocks were exposed to elevated concentrations of CO2. The aqueous concentrations of these metals correlate to aqueous concentrations of Ca2+ throughout the experiments. All of the experimental evidence points to carbonate minerals as the dominant source of metals from these dolomite rocks to solution under experimental CO2 leakage conditions. Aqueous concentrations of Ca2+ and Mg2+ predicted from numerical simulation of kinetic dolomite dissolution match those observed in the experiments when the surface area is three to five orders of magnitude lower than the surface area of the samples measured by gas adsorption.
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. PUBLICATION ABSTRACT
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.
CO2 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.
Nitrogen cycling in clay-textured soils with onsite wastewater treatment systems (OWTS) is studied and modeled much less often than sand- and loam-textured soils because there is little data on OWTS ...performance in these soils. Information on the nitrogen loads from these systems is needed for quantification of total maximum daily loads (TMDLs). The objective of this study was to calibrate a 2D HYDRUS model using experimental soil pressure head and vadose zone nitrogen (N) and chloride (Cl) data from a conventional OWTS that was installed in a clay soil in the Piedmont region of Georgia. An N chain model with water-content dependent first-order transformation rates for nitrification and denitrification was developed. The overall predicted soil pressure heads and solute concentrations were similar to data collected from the field experiment. The calibrated model made it possible to estimate water and solute fluxes in the drainfield and N losses from the OWTS. The estimated annual N loss from leaching at the lower boundary of the experimental drainfield was 3.8 kg yr-1. Scaled up to an OWTS size typical for GA and a zoning density of 5 homes ha-1, the N load to groundwater would be 57.4 kg ha-1 yr-1, which is comparable to agricultural production losses to groundwater. The model predicted 52% of the N removal in the system was from denitrification, whereas plant uptake and change in N storage accounted for ≤5% of the N loss. These estimates were specific to clay-textured soils and should be valuable to TMDL developers who need to predict load allocations for nonpoint sources in the Piedmont.
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