Field-Scale Demonstration of PFAS Leachability Following In Situ Soil Stabilization.

A field-scale validation is summarized comparing the efficacy of commercially available stabilization amendments with the objective of mitigating per- and polyfluoroalkyl substance (PFAS) leaching from aqueous film-forming foam (AFFF)-impacted source zones. The scope of this work included bench-scale testing to evaluate multiple amendments and application concentrations to mitigate PFAS leachability and the execution of field-scale soil mixing in an AFFF-impacted fire-training area with nearly 2.5 years of post-soil mixing monitoring to validate reductions in PFAS leachability. At the bench scale, several amendments were evaluated and the selection of two amendments for field-scale evaluation was informed: FLUORO-SORB Adsorbent (FS) and RemBind (RB). Five ∼28 m3 test pits (approximately 3 m wide by 3 m long by 3 m deep) were mixed at a site using conventional construction equipment. One control test pit (Test Pit 1) included Portland cement (PC) only (5% dry weight basis). The other four test pits (Test Pits 2 through 5) compared 5 and 10% ratios (dry weight basis) of FS and RB (also with PC). Five separate monitoring events included two to three sample cores collected from each test pit for United States Environmental Protection Agency (USEPA) Method 1315 leaching assessment. After 1 year, a mass balance for each test pit was attempted comparing the total PFAS soil mass before, during, and after leach testing. Bench-scale and field-scale data were in good agreement and demonstrated >99% decrease in total PFAS leachability (mass basis; >98% mole basis) as confirmed by the total oxidizable precursor assay, strongly supporting the chemical stabilization of PFAS.

Introduction

Historical discharges of aqueous film-forming foam (AFFF), primarily at former fire training areas (FTAs), resulted in per- and polyfluoroalkyl (PFAS) source zones within vadose zone soils and shallow groundwater.[1−8] Studies have demonstrated that PFAS retained in FTA source zone soils can sustain groundwater plumes potentially for decades.[9−12] While authoritative methodologies to determine and quantify the source-strength that account for the relevant retention and transformation processes are still very much actively being researched by the academic community,[13] many of these AFFF-impacted source zones will likely require degrees of active management to comply with increasingly conservative groundwater criteria. Therefore, cost-effective management strategies that specifically target vadose zone sources and shallow groundwater are urgently needed.

Currently, multiple technologies are used to consolidate large volumes of marginally contaminated matrices into small, highly concentrated volumes for energy-intensive destruction.[14] Commercially available technologies to address AFFF-impacted source zones, however, are limited[15−20] to in situ stabilization[21] and excavation with offsite disposal/destruction or onsite management using soil washing[20] or thermal desorption.[22] Ex situ technologies require waste handling, disposal in a landfill, or separating the PFAS from the soil in favor of adsorption/consolidation technologies relevant to liquids. Once concentrated in a liquid, PFAS destruction challenges remain.[23,24] PFAS has shown recalcitrance to chemical mineralization,[25−27] and no complete biodegradation mechanism has been universally established.[28] Incomplete destruction and unknown intermediates associated with incineration of PFAS in liquid waste have been discussed.[23,24] Hydrothermal alkaline treatment and super critical water oxidation,[29,30] sonochemical degradation,[31] electrochemical oxidation,[32] and advanced reduction processes (e.g., nonthermal plasma,[33] electron beam,[34] and/or ultraviolet-activated catalysts and/or reductants[35,36]) show promise for PFAS destruction at the end of a treatment train.

For in situ applications, the injection of an adsorbent or complexing agent has been evaluated.[37−42] Injected adsorbents are relevant to mitigating PFAS migration within the saturated zone with much emphasis on subsurface distribution. For AFFF-impacted source zones (i.e., vadose zone soils), considerable PFAS mass is likely present within the near surface due to retention associated with organic matter, mineral phases, and the gas–liquid interface.[3−5,12,43−50] This renders PFAS within the vadose zone inaccessible to adsorbents injected within the saturated zone, thus uniquely challenging injected adsorbents to adequately mitigate PFAS leachability.

In situ stabilization and solidification (S/S) is a well-established source zone technology for numerous constituents within saturated and unsaturated impacted media.[51−56] Stabilization uses a chemical amendment to reduce and/or eliminate contaminant leaching. Solidification encapsulates soils and decreases the surface area exposed to infiltrating water. S/S is expected to be a cost-efficient and sustainable management strategy for AFFF-impacted source zones. As S/S homogenizes geological anisotropy during amendment addition via soil mixing, it facilitates access to PFAS associated with low permeable strata or adsorbed at phase interfaces.[57,58] Amendments are not yet available to destroy PFAS, and the current focus is the mitigation of PFAS leaching. S/S has been shown to decrease PFAS leaching from AFFF-impacted soils.[21,59−64] Relevant studies have included laboratory-based trials to evaluate the efficacy of stabilization to decrease PFAS leachability from FTA-specific or spiked soils[21,59] and to simulate weathering under controlled conditions.[60−64] These studies consistently demonstrated decreases in PFAS leachability of greater than 99% (mass basis) using stabilizing amendments such as biochar, activated carbon, montmorillonite, compost soil, pulverized zeolite, chitosan, layered double hydroxides (LDHs), aluminum hydroxide, bentonite, and calcium chloride.[21,59−64] Weathering under field conditions represents a gap in understanding the long-term performance of S/S for PFAS[60,61] and is the motivation of this study.

A standardized method to evaluate the leaching potential of PFAS from solid matrices is under development to account for the unique properties of PFAS. Most of the PFAS-relevant S/S studies equilibrated stabilizing amendments with AFFF-impacted soil at a prescribed soil-to-liquid ratio under agitation and then analyzed the liquid for PFAS after a solid–liquid separation. Kabiri and McLaughlin[62] evaluated USEPA Method 1314 (Leaching Environmental Assessment Framework [LEAF])[65] and the Australian Standard Leaching Procedure[66] to represent a practical leachability assessment. The LEAF methods involve subjecting a predetermined mass of a solid material at a specified liquid-to-surface-area ratio to successive leaches over time. LEAF presents a commercially available methodology to realistically evaluate the leachability of PFAS from solid matrices without imposing biases, such as particle size reduction[65] or acidic leaching conditions.[67]

The objective of this study was to evaluate S/S under field conditions at an AFFF-impacted source zone to understand the magnitude and longevity of decreased PFAS leachability. Specific technical objectives included a comparison of commercially available stabilization amendments and an evaluation of how the concentrations of PFAS in leachate from the stabilized test pits changed after more than 2 years using a sequential leaching procedure. To the best of our knowledge, this study provides the first field-scale evaluation of S/S as a management strategy for an AFFF-impacted source zone.

Results and Discussion

Baseline Characterization Results

Soil samples collected in the vicinity of the test pits had an average percent solids of approximately 82.5% (n = 30 and σ = 3.1%), with greater than 90% characterized as coarse sands per classical geological description, the Udden-Wentworth grain size classification system, and grain size distribution per American Society for Testing and Materials (ASTM) D422.

Of the polyfluorinated compounds analyzed, 8:2-fluorotelomer sulfonic acid (FTSA) and perfluorooctane sulfonamide (PFOSA) were the only compounds quantified above analytical detection limits (Table S1). Detections of 8:2-FTSA ranged from 3.3 J micrograms per kilogram (μg/kg) to 33.6 μg/kg (n = 12; μ = 10.5; and σ = 9.1). Detections of PFOSA ranged from 0.66 J to 41.6 μg/kg (n = 19; μ = 7.6; and σ = 10.3). For all but two results, the highest detections of 8:2-FTSA and PFOSA were found in the samples at 1.5 m below the ground surface (m bgs). These polyfluorinated compounds will not be discussed further as postsoil mixing performance monitoring data represent post total oxidiziable precursor (TOP) assay data, but the data are consistent with observations in the literature of polyfluorinated compounds being associated with shallow soils. 8:2-FTSA would be expected to transform into similar- or shorter-chain perfluorocarboxylic acids,[68−70] (PFCAs) and PFOSA is a known precursor to perfluorooctane sulfonate[71] (PFOS).

The frequency of perfluoroalkyl acid (PFAA) results above analytical detection limits in soils deemed relevant for this discussion was defined as more than 90% of the total samples (n = 30). Those PFAAs classified as infrequently detected include C9 and C10 perfluorosulfonic acids (PFSAs) and C11 through C14 PFCAs. Considering postsoil mixing performance monitoring data continued these observations of infrequently detected PFAAs, these PFAAs are not discussed further (Table S2). Those PFAAs classified as frequently detected include C4–C8 PFSAs and C4–C9 PFCAs. However, on average, perfluorohexane sulfonic acid (PFHxS), PFOS, perfluorohexanoic acid (PFHxA), and perfluorooctanoic acid (PFOA) comprised 98.2% of the total PFAS reported (PFHxS and PFOS comprised 95.7%; Table S2).

Groundwater samples were analyzed post TOP assay, and therefore, no polyfluorinated compounds were detected (Table S3). Similar to the analytical results of the soil, PFHxS, PFOS, PFHxA, and PFOA represented an average of 89.9% of the total PFAAs detected in groundwater. The observation of the most detected PFAAs represented by PFHxS, PFOS, PFHxA, and PFOA was continued through the postsoil mixing performance monitoring data, and therefore, they are the focus of the rest of the discussion.

The spatial variation of observed PFAS magnitudes is indicative of the difficulty to predict the nature of source zone FTAs due to the release mechanism (i.e., surface infiltration) and varied historical release. While not definitive, the larger percentage of PFHxS and PFOS, as well as the frequent detection of PFOSA, may indicate an electrochemical fluorination as the source of historical AFFF, which is anticipated to have less polyfluorinated compounds.[49,71]

No separate phase hydrocarbon was encountered, and the fraction of organic carbon (fOC) and target analyte list (TAL) metals were not confounding (i.e., not at such high concentrations that they would influence PFAS adsorption). There are reports in the literature of adsorptive retardation being correlated with natural organic matter [represented by total organic carbon (TOC)] and clays (i.e., metal oxide surfaces).[5,44,72] The fOC was quantified to be between approximately 0.002 and 0.006 (n = 10; μ = 0.003; and σ = 0.001), a moderate concentration comparatively.[73] TAL metal analysis suggests that aluminum, calcium, iron, magnesium, and sodium comprised 99.2–99.9% (n = 10; μ = 99.7%, and σ = 0.3%) of the detected metals and that calcium represented 63.8–96.4% (n = 10; μ = 85.4%, and σ = 12.9%). The divalent cationic speciation of calcium presents an intriguing possibility to serve as an electrostatic linkage between anionic functional groups common to PFAAs and is theorized to promote the formation of supramolecular structures.[74] Aluminum and iron could also represent charged surfaces (cationic or anionic), which could also promote electrostatic adsorption.

Laboratory Leaching Analysis

Batch Testing

The addition of RemBind (RB) and FLUORO-SORB (FS) resulted in approximately 99.3 and 99.5% decreases (mass basis) in the sum of detected aqueous PFAS in comparison to the soil-only control, respectively, regardless of the stabilizer ratio (Table S4), and is consistent with other studies.[21,59−64] In comparison to these larger and consistent decreases, observed decreases of 35.4–83.0% (mass basis) for LDH and the proprietary sealant were eliminated from consideration and are not discussed further. The batch testing identified RB and FS as the field-scale stabilizers.

Within the soil-only control, PFOS, PFHxS, and PFHxA comprised 91.8% of the sum of quantifiable PFAS, which is consistent with the characterization results. A noteworthy observation is the difference in the magnitude of detected PFAS in groundwater [μ = 1192.4 ng per liter (ng/L); n = 2; and σ = 1.26] versus that of the soil-only control (μ = 15,855.7 ng/L; n = 2; and σ = 361.5). The difference in magnitude is likely attributable to the liquid extractant accessing a greater surface area of the soil over the 48 h agitation for the soil-only control. PFHxA was observed to leach and increase the leachate concentration by greater than 400% in the soil-only control, which likely represents desorbed/dissolved polyfluorinated compounds subsequently transformed into PFHxA via the TOP assay.

Specifically for PFHxS, PFOS, PFHxA, and PFOA, leachability on a mass basis was observed to decrease by greater than 99% (Figure ), though detectable concentrations were observed in both RB and FS trials.

Figure 1

Percentage of PFHxS, PFOS, PFHxA, and PFOA leached for a soil-only control in comparison to treatment conditions with 5, 10, and 20% dry wt FS and RB, respectively, in batch testing.

PFHxS and PFOS were observed in multiple RB and FS trials at concentrations ranging from 6.3 to 61.5 ng/L, with no discernible trends with respect to the stabilizer ratio. PFHxA was detected in all but one RB and FS trials at concentrations ranging from 7.8 to 29.6 ng/L. There was no discernible trend associated with respect to the RB ratio, but PFHxA was observed to decrease slightly with higher FS ratios. These detections represent sizable decreases in comparison to the soil-only control (PFHxS: n = 2; μ = 2690 ng/L; and σ = 20; PFOS: n = 2; μ = 10,010 ng/L; and σ = 290; and PFHxA: n = 2; μ = 1860 ng/L; and σ = 60).

Both RB and FS demonstrated greater than 40% decreases in perfluorobutanoic acid (PFBA) leachability; however, PFBA proved to be the most difficult PFAA to stabilize. PFBA was detected in multiple RB and FS trials at concentrations ranging from 23.5 to 103 ng/L, with no discernible trend with respect to the stabilizer ratio. PFBA is characterized by only three fluorine-saturated carbons within its alkyl chain and often presents a challenge to adsorptive media (both hydrophobic and electrostatic adsorption) given its miscibility.[75,76] Batch-testing PFAS analysis was performed post TOP assay, and the TOP assay is expected to convert polyfluorinated compounds into PFCAs.[72] Consistent detections of PFHxA and PFBA for RB and FS trials could, however, suggest that polyfluorinated compounds were less effectively stabilized.

Given the apparent decreases in PFAS leachability associated with the addition of RB and FS seemingly insensitive to the stabilizer ratio, the 20% wt ratio was eliminated from consideration based on cost. The 5 and 10% wt stabilizer ratios for RB and FS were selected for the intact core testing and the field-scale soil mixing.

Intact Core Testing

5% wt Portland cement (PC) was determined to develop adequate unconfined compressive strength (UCS) as confirmed by 28-day peak stress testing (approximately 415 kPa) and a hydraulic conductivity of 6 × 10–5 cm per second (cm/s). While 5% wt addition to FS trials developed a similar UCS, RB trials did not, and therefore, higher percentages of PC were necessary in RB trials (Tables S5 and S6). A lower UCS in the presence of RB used for this work was observed previously.[60] The added PC corresponded to an order of magnitude lower hydraulic conductivity (10–6 cm/s vs 10–5 cm/s for the PC control and FS trials).

All PFASs (post TOP assay) within the T48h and T63d leachates (see Materials and Methods) were below analytical detection limits for RB and FS trials (Table S7). Perfluorononanoic acid (PFNA) was consistently detected in pre TOP assay only T63d leachate at concentrations ranging from 3.95 to 11 ng/L. These detections are likely associated with laboratory bias (PFNA detection in the laboratory blank sample of 9.7 ng/L) as PFNA was not detected in analysis post TOP assay.

T48h leaching concentrations for the 5% PC control were at least 1 order of magnitude lower than batch testing results for the soil-only control, suggesting that the PC may have mitigated some PFAS leachability, if only by decreasing the hydraulic conductivity. Of the six distinct PFAAs detected in the pre TOP assay, T48h leachate concentration for the 5% PC control, PFHxS, PFOS, PFHxA, and PFOA represented 90% of the sum of PFAAs. Increases in PFBA and PFHxA in PFAS analysis post TOP assay again indicate some degrees of leachable polyfluorinated compounds. Other observations of leachable polyfluorinated compounds include 6:2-FTSA (13.1 ng/L) and PFOSA (14.9 ng/L) detected in PFAS analysis pre TOP assay T48h leachate data from the 5% RB trial and the 10% FS trial, respectively. For all RB and FS trials, pre and post TOP assay T48h and T63d leaching concentrations for PFHxS, PFOS, PFHxA, and PFOA were below analytical detection limits.

Field-Scale Leaching Analysis

The T48h leaching results from all sampling events (SEs) are presented for PFHxS, PFOS, PFHxA, and PFOA (post TOP assay) per mass of soil leached, where the concentration leached from soil has been corrected to remove the added amendment mass (Figure ). Test Pit 1 is the 5% PC control, Test Pit 2 is 5% PC/10% FS, Test Pit 3 is 5% PC/5% FS, Test Pit 4 is 15% PC/10% RB, and Test Pit 5 is 10% PC/5% RB and SEs were conducted 5, 12, 16, 22, and 28 months postsoil mixing.

Figure 2

Scatter plots of all T48h leachate results for PFHxS, PFOS, PFHxA, and PFOA from Test Pits 2 through 5 vs Test Pit 1. Data are presented as nanomole PFAA per kg soil. The 1:1 line is the Test Pit 1 plotted against Test Pit 1. One-half of the analytical detection limit was used when results were below the analytical detection limits.

The data demonstrate comparative decreases in PFHxS, PFOS, PFHxA, and PFOA leachability from Test Pits 2 through 5 compared to Test Pit 1 under relevant field-scale weathering conditions for greater than 2 years. The sustained decreases in PFAS leachability via the T48h leachate (post TOP assay) suggest that S/S could be a pragmatic approach to manage AFFF-impacted source zones. An apparent exception to this observation is PFHxA from Test Pits 2 and 3 for two data points from SE 12mo; however, these data are all below analytical detection limits (Table S8), and these data points are suspected to be unrepresentative of Test Pit 1.

Over the 28-month monitoring period, the summation of PFAS in the T48h leachate from Test Pit 1 ranged from 195.7 to 15,645.7 ng/L (n = 13; μ = 5436.4 ng/L; and σ = 3842). The average SE 12mo T48h leachate from Test Pit 1 is 95.7% different than the average T48h leachate from Test Pit 1 for all other SEs. Geochemical indicator parameters [pH and specific conductivity (SC)] collected from sample cores TP-1-4 and TP-1-5 (Figure S1 and Table S9) suggest lower average pH and SC when compared to the other 10 sampling grids (data not shown). The data from SE 12mo are not discarded as outliers and rather serve as a demonstration of the importance of homogeneous mixing for S/S to be a successful long-term PFAS source zone management strategy. The comparatively low PFAS result of SE 12mo is contrasted with the comparatively high PFAS result of SE 28mo, where the T48h leachate concentration from TP-1-10 (Figure S1 and Table S9) was 15,645.7 ng/L, which was higher than any other total PFAS result from SEs and is comparable to the soil-only control (n = 2; μ = 15,855.7; and σ = 361.5) from the batch testing.

The PFASs detected in the T48h leachate from Test Pit 1 suggest that PC alone was marginally effective in comparison to the soil-only control from batch testing at mitigating PFAS leachability (Figure ).

Figure 3

Percentage of PFHxS, PFOS, PFHxA, and PFOA leachability observed after SE 5mo, 12mo, 16mo, 22mo, and 28mo after soil mixing/stabilizer amendment addition. The soil-only control from batch testing data is repeated for comparison purposes. The percentage leached is calculated using one-half of the analytical detection limit when results were below the analytical detection limits.

PFHxS, PFOS, PFHxA, and PFOA represented 78.3% (n = 13 and σ = 10.9%) of the detected PFAS in T48h leachate from Test Pit 1, which is comparatively lower than the characterization and laboratory testing. Leaching data associated with Test Pit 1 show consistent detections of PFCAs [specifically, PFBA, perfluoropentanoic acid (PFPeA), and perfluoroheptanoic acid (PFHpA)] throughout the SEs, which represented 57.5% (n = 13 and σ = 13.8%) of the detected PFAS. The higher percentage of PFCAs detected in post TOP assay T48h leachate from Test Pit 1 is likely an indication of polyfluorinated compound leachability.

For Test Pit 2 (5% PC/10% FS), all T48h leachate concentrations of PFHxS, PFOS, and PFOA were below analytical detection limits (n = 12). Detected PFAAs included PFBA, PFPeA, and PFHxA, suggesting a potential for polyfluorinated compounds to leach from Test Pit 2; however, the concentrations were an order of magnitude less than those from Test Pit 1 (specifically for PFPeA and PFHxA), demonstrating some degrees of mitigation of such leaching. The highest magnitude PFAA detected was PFHxA (n = 10; μ = 216.3 ng/L; and σ = 96.9) and was significantly less than Test Pit 1 [one-tailed Student t-test (p = 0.0002)]. The sum of all detected PFAS concentrations in the T48h leachate represents 95.7 and 98.5% decreases in detectable PFAS leachability (mass basis) as compared to Test Pit 1 and the soil-only control, respectively.

For Test Pit 3 (5% PC/5% FS), there were detections of PFOS (31 ng/L) and PFOA (15 J and 127 ng/L) during SE 5mo and SE 22mo. There were no detections of PFHxS above analytical detection limits. Like Test Pit 2, PFCAs (PFBA, PFPeA, and PFHxA) were detected, with PFHxA being of the largest magnitude (n = 13; μ = 127 ng/L; and σ = 169). PFHxA detected in T48h leachate from Test Pit 3 was significantly less than that from Test Pit 1 [one-tailed Student t-test (p = 0.00002)]. T48h leachate results from TP-3-1 (Figure S1 and Table S9) collected during SE 5mo had the greatest number of PFAA detections (6), represented the only PFOS detection, and represented the highest PFBA, PFHxA, and PFOA detections. Geochemical indicator parameters (pH and SC) from sample core TP-3-1 were comparatively lower than those from the other 15 samples (TP-3-2–TP-3-12, plus duplicates; data not shown). The summation of the detected PFAAs from TP-3-1 was 1341 ng/L compared to an average of 122 ng/L for all other samples (n = 12 and σ = 72). These observations may suggest that the location represented by sample TP-3-1 was poorly mixed. The results from TP-3-1 and the J-flagged estimate of PFOA during SE 22mo represent the only observed differences between the 10 and 5% wt FS stabilizer ratios. If the TP-3-1 sample represents poor soil mixing and FS distribution, this would suggest that there is no added benefit between 5 and 10% wt stabilizer ratios, and further optimization of lower-stabilizer ratios may be possible. The sum of all detected PFAS concentrations in the T48h leachate represents 96.7 and 98.9% decreases in detectable PFAS leachability (mass basis) as compared to Test Pit 1 and the soil-only control, respectively.

For Test Pits 4 and 5 (15% PC/10% RB and 10% PC/5% RB), all PFAAs in T48h leachate were below analytical detection limits except for SE 28mo results from Test Pit 5, which demonstrated J-flagged detections of PFPeA and PFHxA. There is insufficient data to reliably infer if the SE 28mo detections of PFCAs represent a similar potential for polyfluorinated compound leachability; however, the similarity of the detected PFAAs to Test Pits 1 through 3 seems to suggest this. This would represent the only minor difference between 5 and 10% wt RB stabilizer ratios, suggesting that there was no added benefit to 10% versus 5% wt for this specific demonstration study. With respect to Test Pit 4, 100.0% decrease in detectable PFAS leachability (mass basis) was observed as compared to Test Pit 1 and the soil-only control. With respect to Test Pit 5, the sum of all detected PFAS concentrations represents 99.9 and 100.0% decreases in detectable PFAS leachability as compared to Test Pit 1 and the soil-only control, respectively.

Attempted Mass Balance

The attempted mass balance is presented in Table S10, and masses are calculated from PFAS analysis performed post TOP assay. The attempted mass balance was performed on sample cores collected during SE 12mo and is a comparison of the soil analysis for PFAS prior to leaching (M1) to the summation of the soil analysis for PFAS after leaching (M3) and a composite analysis of the nine sequential leaches (M2).

The differences in M1 and M3 do not appear to be entirely associated with PFAS leaching from the soil. The differences (M3 – M1) range from −50,301.7 to 27,542.5 ng and are far greater in magnitude than any M2 reported for Test Pits 2 through 5. This may suggest that analytical limitations could better explain the observed differences. The PFASs were extracted from soil using a triple methanol rinse, and sonication or other extraction methods may have been more appropriate.[77] While this challenges the precise interpretation of the mass balance, a comparison on an order of magnitude basis showing 105 ng total PFAS associated with the pre- and postleached soil versus 102–103 ng total PFAS observed in the composite leachates strongly suggests that chemical stabilization decreases PFAS leachability into groundwater as well as extractability with methanol rinse. The calculated percentage of PFHxS, PFOS, PFHxA, and PFOA leachability demonstrates that in comparison to the soil-only control from the batch testing, stabilizers reduced the leachability of these PFAAs (Figure ).

The magnitude of the PFAS mass associated with Test Pit 1 is most likely an underrepresentation, based on the previously discussed differences in T48h leaching results in comparison to other SEs. The mass of PFAS leaching from Test Pit 1 based on the T48h leaching results from SEs 5mo, 16mo, 22mo, and 28mo ranges from 1474.9 to 6430.4 ng (n = 11; μ = 2623 ng; and σ = 1401.8). This range and average of T48h leachate mass are similar to Test Pit 1 M1 and M3; however, based on data from the other test pits, they should not be similar. The mass of PFAS within the T48h leachate for Test Pits 2 through 5 from all SEs ranges from 43.7 to 583.5 ng (n = 52; μ = 114.5 ng; and σ = 84.6) and is 3–4 orders of magnitude less than the corresponding M1 and M3. Despite the underrepresented magnitude of PFAS mass from Test Pit 1, the largest percent difference (Table S10) between M1 and the summation of M2 and M3 is expected and consistent with the higher T48h leachate concentrations observed during other SEs. The greatest percent leaching being associated with Test Pit 1 is also expected as only PC was added to this test pit, and a comparison of the percent of PFAS leaching between Test Pit 1 and Test Pits 2 through 5 supports that stabilization amendments influenced PFAS leachability.

Non-T48h PFAS Leaching Evaluation

To evaluate any bias imposed by using the T48h leachate, composite samples of the T2h–T24h and T5d–T63d leaches were analyzed from Test Pits 1, 3, and 5 during SE 28mo (Table S11). These test pits were selected because Test Pit 1 was the control and Test Pits 3 and 5 both represented the lowest stabilizer ratio (5% wt FS and RB).

The most notable PFAS leaching was observed in T2h–T24h and T5d–T63d composites from Test Pit 1 and was predominantly composed of PFHxS, PFOS, PFHxA, and PFOA. In comparison to the composite leachate from the mass balance (detected PFAS ranging from 15.2 to 183 ng/L; Table S12), the concentrations of detected PFAS within the T2h–T24h and T5d–T63d composite leachates were more aligned with the expected results (10.2 J to 8890 ng/L). The associated leached PFAS masses for T2h–T24h, T48h, and T5d–T63d from Test Pit 1 were approximately 7290 ng, μ = 3192 ng (n = 3 and σ = 5574), and 48,815 ng, respectively. A PFAS mass leaching rate can also be approximated based on the leaching durations of each composite (7290 ng/d for T2h–T24h, 3192 ng/d for T48h, and 800 ng/d for T5d–T63d). While these are expected results given the skepticism of PC to facilitate chemical stabilization, it does suggest that more PFASs leached from cores collected from Test Pit 1 that were not adequately represented by the T48h leachate.

Composite leachates T2h–T24h and T5d–T63d from Test Pit 3 show detections of PFBA, PFHxA, and PFOS that are comparable to the composite leachate from the mass balance. Similar PFAS leachability over approximately 16 months provides some perspective that an equilibrium was established at Test Pit 3, which is supportive of the long-term leachability reduction associated with stabilization. The mass of PFAS leached for Test Pit 3 specific to samples collected during SE 5 was approximately 199 ng in T2h–T24h, 159 ng in T48h, and 858 ng in T5d–T63d, with associated PFAS mass leaching rates of 199, 159, and 14 ng/d, respectively.

Composite leachates T2h–T24h and T5d–T63d from Test Pit 5 show estimated detections of PFBA and PFOS, which is comparable to the composite leachate from the mass balance and implies a similar observation of established equilibrium as Test Pit 3. The mass of PFAS leached from Test Pit 5 specific to samples collected during SE 5 was approximately 125 ng in T2h–T24h, 123 ng in T48h, and 382 ng in T5d–T63d, with associated PFAS mass leaching rates of 125, 123, and 6.3 ng/d, respectively.

In comparison to the composite leachate analysis from Test Pit 1, composite leachability results from Test Pits 3 and 5 are notably less. This is an expected result due to the chemical stabilization of the FS and RB and is consistent with the comparison of T48h leachate data throughout the other SEs. This evaluation confirms that some leachable PFASs were missed by selecting T48h leachate; however, the magnitude of PFAS mass leaching rates between T2h–T24h, T48h, and T5d–T63d suggests that T48h may suffice as a qualitative comparison of PFAS leaching for the purposes of assessing the long-term leachability decreases associated with stabilization.

Implications and Future Work

The data presented here strongly suggest that PFAS leaching can be mitigated via chemical stabilization and should be considered as an effective alternative for AFFF source zone management. While the stabilizers used here (FS and RB) resulted in significant decreases in PFAS leachability compared to a PC control, discrete sample data throughout the test pits reinforce the importance of uniformity in stabilizer distribution via proper soil mixing. Unpublished work to optimize the stabilizer ratio suggests that lower ratios (e.g., 2–3% wt) may achieve targeted leachability goals assuming homogeneous distribution. The stabilizer ratio can be a considerable cost for larger AFFF source zones, and predesign optimization testing is recommended. Where immediate load bearing capacity is not necessary, mechanical compaction of stabilized soil may eliminate the need for PC to further optimize costs. Homogeneous distribution of stabilizers is essential to reliably optimize S/S to balance the risk of PFAS leaching as the amendment ratios are refined.

Collectively, these data demonstrate a net reduction of PFAS leachability from stabilized test pits. However, the mechanism of retention is unknown and could involve surface adsorption, complexation, fluorophilic interactions, and/or chemisorption. Future work should utilize more advanced microscopic and/or spectroscopic methods specifically to probe these molecular interactions. Additional SEs to evaluate PFAS leachability at the subject site may be considered and would strengthen the defensibility of S/S as a long-term remedial solution.

Materials and Methods

Investigation Site

The FTA selected for demonstration is in the southeastern United States at an Air Force installation that was previously the subject of a high-resolution site characterization. Site selection was based on a well-established conceptual site model, a shallow groundwater aquifer, low total dissolved solids/salinity and TOC, coarse-grained lithology with a relatively flat hydraulic gradient, and concentrations of PFAS in soil and groundwater in the μg/kg and micrograms per liter (μg/L) range, respectively. Historical firefighting training included burning fuels and other flammable/combustible liquids within an unlined, earth-bermed circular pit, followed by extinguishment with dilute AFFF.

Baseline Data Collection

In February 2018, baseline soil and groundwater samples were collected throughout the FTA from 10 locations (DPT-01 through DPT-10). The groundwater table was determined to be approximately 1.5 m bgs. The soil borings were advanced to a depth of approximately 6 m bgs, and three soil samples (approximately 1.5, 3, and 4.6 m bgs) were collected from each boring (30 soil samples total). One groundwater sample was collected from each boring. Soil and groundwater analyses were conducted by SGS AXYS in Canada and SGS Accutest Orlando in Florida (Tables S13 and S14). All PFAS analyses associated with the work summarized herein were done in accordance with quality protocols specified in the Department of Defense/Department of Energy Consolidated Quality Systems Manual (QSM).[78]

Soil was composited in two 18.9 L plastic Department of Transportation-rated buckets. Approximately 9.5 L of site groundwater was collected into a plastic container. The composited soil and the container of groundwater were shipped to the Arcadis Treatability Laboratory (ATL) in Durham, North Carolina for bench-scale testing.

PFAS Analytical Quality Assurance and Quality Control

For liquid and solid PFAS analysis performed at SGS Accutest Orlando, all analyses followed modified USEPA Method 537 with quality protocols specified in QSM 5.3. For pre TOP analysis performed at SGS AXYS in Canada, analysis essentially followed modified USEPA Method 537 (SGS Method MLA-110), which meets or exceeds quality protocols specified in QSM 5.3. For post TOP analysis performed at SGS AXYS in Canada, analysis was performed according to SGS Method MLA-111, which similarly meets or exceeds quality protocols from QSM 5.3 but is not Department of Defense accredited as no accreditation is available for post TOP analysis. All methods use isotope-dilution high-performance liquid chromatography with tandem mass spectrometry after solid-phase extraction. Procedural blanks and ongoing precision and recovery quality control samples were included. PFASs detected above the analytical detection limit but below the limit of quantification were assigned “J” flags, indicating that the value is detected but with a degree of increased uncertainty.

Bench-Scale Testing

Amendment Selection

Five potential stabilizers were considered to varying extents during the bench-scale testing: RB, FS, pyrolyzed cellulose, an LDH, and an unnamed proprietary product. RB is a powdered reagent composed of a proprietary blend of clays, aluminum hydroxide (as pseudoboehmite) and carbons, and it was obtained from Ziltek.[79] FS is a modified bentonite clay, and it was obtained from CETCO.[80] The LDH was hydrotalcite (CAS number 11097-59-9), and it was obtained from Sigma-Aldrich and was included based on promising performance in previous trials in Australia.[81] Pyrolyzed cellulose was not included in the bench-scale testing because, at the time (2018), it was not cost feasible if selected. A proprietary sealant was included at a commercial request. PC was selected as the solidifying amendment.

Sample Preparation

Soil was stored under ambient laboratory temperature (approximately 23 °C), while groundwater was refrigerated (<4 °C) until further use. Duplicate subsamples of air-dried soil were analyzed for PFAS, TOC, TAL metals, and grain size. Groundwater was analyzed for PFAS, total TAL metals plus major cations, TOC, nitrate, nitrite, and alkalinity. A subsample of groundwater was filtered and submitted for dissolved TAL metals plus major cations. Analysis for PFAS incorporated the TOP assay.[82]

100 g of wet soil (gravimetric moisture content 11.6% and 88.4 g dry soil), targeted dry soil weight amendment ratios, and 200 mL of groundwater were placed in 250 mL polypropylene centrifuge tubes with polyethylene caps and thoroughly shaken by hand (Table S15). The centrifuge tubes were then placed on a roller table for 48 h to provide continuous agitation. After 48 h, the tubes were centrifuged (Sorvall Instruments Model RC3C; 30 min; and ∼3700 relative centrifugal force) to separate the stabilized soil from groundwater and then sacrificially sampled. Two 60 mL high-density polyethylene bottles were filled from each centrifuge tube and submitted for aqueous-phase PFAS analysis, including the TOP assay.[82] The pH of each tube was measured using an Accumet pH electrode.

Intact Core Testing

While solidification (i.e., intentional permeability reduction via PC addition) was not the focus of this work, due to a sensitive ecological receptor at the selected site, some degrees of rapid load-bearing capacity were required. Therefore, strength development testing evaluated various PC/soil ratios to develop an acceptable UCS. Each test mix was packed into a series of approximately 5 cm diameter by 10 cm long plastic geotechnical molds. After the molds were filled, they were placed into plastic Ziploc bags to retain humid/moist conditions and left to cure at ambient laboratory temperature. At 28 days of curing, UCS testing was conducted on one mold per test mix (ASTM D1633).

Stabilizer and PC ratios (based on dry weight) identified as the best performers were then combined into intact core leach testing (Table S5). Soil was blended with appropriate quantities of stabilizers, PC, and water in a stainless-steel bowl using a Hobart model HL-200 planetary mixer. After thoroughly blending, test mixes were packed into a series of 5 cm diameter by 10 cm long plastic geotechnical molds and cured similarly to the UCS development testing. At 7 days of curing, one mold from each test mix was submitted for UCS testing. At 28 days of curing, molds from each test mix were submitted for UCS testing and tested for hydraulic conductivity (ASTM D5084). At 34 days of curing, one mold from each mix was sized (5 cm diameter by 8.9 cm long) and shipped for monolithic leachability evaluation by LEAF USEPA Method 1315.[83] During the leaching procedure, samples were placed into 2 L plastic wide-mouth jars and exposed to scheduled 1.8 L leachate exchanges (∼9.9 mL/cm2). Leachate samples from the third (T48h; 2 days) and ninth (T63d; 63 days) leaching intervals were collected and submitted for the analysis of PFAS concentrations (pre and post TOP assay), TAL metals, and TOC. Leachates from T63d were also analyzed for SC and pH. T48h was selected to qualitatively match the 2-day batch testing, and T63d was selected to conservatively, yet cost-effectively, assess leaching, given the highly asymmetric elution tailing observed for select PFAS (e.g., PFOS) during transport studies.[84]

Field-Scale Demonstration

Soil Mixing

Field-scale implementation used two distinct stabilizer ratios per selected commercial product (RB and FS) and PC ratios according to UCS testing results (Table S6). Five approximately 3 m long by 3 m wide by 3 m deep test pits (approximately 28 m3) to an approximate depth of 4.5 m bgs were mixed in July 2018 (Figure S1). Approximately 1.5 m of overburden soil was temporarily removed from each test pit to create room for anticipated bulking. A process pass using an excavator loosened the soil within the test pits. Stabilizers (FS and RB) and PC were measured and mixed into the soil in situ with a bucket mixer and/or rotary mixer. The site was graded using the excess overburden soil. Slump tests were completed on test pits to verify the consistency after mixing. No sample cores were collected immediately following the completion of soil mixing (i.e., no zero-time soil samples).

Performance Monitoring

SEs were conducted 5-, 12-, 16-, 22-, and 28-month post mixing from July 2018 through October 2020 to assess PFAS leachability (Table S9). A 12-area grid was created for each test pit (Figure S1). A total of 65 sample cores were collected during the approximate 28-month duration, including five duplicate samples (Table S9). Sample cores were collected from the middle portion of the test pit grids, biased to the most uniformly mixed intervals. Intervals that were noticeably friable (i.e., crumbly) or appeared to be poorly mixed via visual inspection were avoided. Due to intentionally targeting a low UCS, sample cores were poorly formed and were inappropriate for intact core leaching. Therefore, granular sequential leaching was used for analysis.

Each test pit sample core was sent to the ATL for processing into granular samples prior to packaging for shipment to Eurofins Test America. Eurofins Test America packed the granular samples into 7.6 cm molds provided by the ATL before performing LEAF Method 1315 analysis. Nine sequential leachate samples (T2h through T63d) of approximately 411 mL (equivalent to 9 ± 1 mL/cm2 leaching surface area as specified in USEPA Method 1315) each were produced per granular sample, and the leached sample mass ranged from 260 to 527 g of soil. Wet chemistry data (pH, oxidation reduction potential, and SC) were collected for each leachate sample as well as the blank laboratory water used for the analysis. Leachate from the T48h interval and the method blank was analyzed for PFAS (post TOP assay) and for metals and TOC. For SE 12mo and SE 28mo, leachates were composited (SE 12mo: all nine leachates composited together and SE 28mo: T2h–T24h and T5d–T63d; Table S9).

Mass Balance

Sample cores collected during SE 12mo (Table S9) were analyzed for PFAS soil concentrations (post TOP assay) pre- and post-leaching. Additionally, the T2h-T63d leaches were composited together and analyzed for PFAS (post TOP assay).

Theory/Calculation

The percentage of PFAS leached during the batch testing was calculated after Kabiri and McLaughlin[62] as followswhere Ctotal is the PFAS concentration in soil (μg/g) and Cw (μg/g) is the concentration of the PFAS leached, calculated as followswhere C (μg/L) is the concentration of PFAS in the leachate, V (L) is the volume of the leachate, and m (g) is the mass of the soil leached. The corresponding mass of the amendments was subtracted from the mass of the soil leached to eliminate dilution concerns. Where data were reported to be less than the analytical detection limit, one-half of the reported detection limit was used for calculation purposes.

The attempted mass balance is a comparison of the soil analysis for PFAS prior to leaching (M1) to the summation of the soil analysis for PFAS after leaching (M3) and a composite analysis of the nine sequential leaches (M2) calculated as follows

The percent difference was calculated as follows

Statistical one-tailed Student t-tests were performed via Microsoft Excel.