Mineralogical controls on PFAS and anthropogenic anions in subsurface soils and aquifers

Domain: earth_science
Published: 2025-04-01
Editorial outcome: minor_revision

Abstract
Per-andpolyfluoroalkylsubstances(PFAS)migrateintotheenvironmentthroughvariousmeans,e.g.,soil-amendmentimpuritiesandambientatmo-sphericdeposition,potentiallyresultinginvegetativeuptakeandmigrationtogroundwater.ExistingapproachesformodelingsorptionofPFAScommonlytreatsoilasanundifferentiatedhomogeneousmedium,withdistributionconstants(e.g.,K ,K )generatedempiricallyusingsurfacesoils.Consideringthelimitedmineralvarietyexpectedinweatheredgeologicmedia,PFASmobilitycanbebetterunderstoodbyaccountingforpredictablemineralassemblagesthatareubiquitouslydistributedinUSsoils.HereweexploretheroleofmineralsandelectrostaticsorptionincontrollingPFASmobilityinsubsurfacesettingsatcontaminatedagriculturalsitesbymeasuringgeochemicalparametersandPFAS,andcalculatingpH-dependentmineralsurfacechargesthroughfullsoilandaquifercolumns.Thesedatasuggestsubsurfacemobilityofshort-chainPFASlargelyiscontrolledbyaluminum-oxidemineral(oid)electrostaticsorption,whereaslong-chainPFASmobilityiscontrolledbyorganicmatterandair-waterinterfacialarea.

Introduction
Ourterrestrialenvironmentisubiquitouslyimpactedbyawidearrayofanthropogeniccompounds,themajorityofwhichhavebeenproducedfortheirbioactiveorotherfunctionalproperties.Remotefromtheirintended use, in the environment, these chemicals maintain the potentialtoexerteffectsontheirsurroundings,potentiallyincludinghumans,fauna,flora,fungi,andmicrobiomes.Tosustainourwayoflife, insecticides, acaricides, herbicides, and fungicides are applied liberallytoagriculturalcropsandsoils.Livestockarefedantibiotics,steroids and other agents, and their waste and other solid wastesarespreadonagriculturalfields.Inrecentyears,morethanhalfofthebiosolidsgeneratedinmunicipalwastewater-treatmentplantsintheUSwereappliedasamendmentstoagriculturalandothersoils1.Often thesebiosolidsbearawitches’brewofoursocietalwaste,e.g.,pharmaceuticals, PFAS, and plasticizers2. Still more anthropogenic compoundshavebeendocumentedtoundergolong-rangeatmospherictransportfromairemissionsatmanufacturingfacilities3,4,aswellastransportanddepositionofcontaminantcoatedaerosolsduringsandandduststorms5.Releasedtotheland’ssurface,thesepesticides,andmanure-, biosolids- and atmospheric-borne chemicals interact intimatelywithsoils,whichinitiallyactassinksfromtheaqueousphase,servingassizablereservoirs,andultimatelytime-delayedsourcesformoderatedconcentrationsofrecalcitrantcontaminants,perhapstobeaccumulatedinvegetationorsoilorganisms6ortoleachthroughthevadosezoneandmigratingwithgroundwater7,8.Critically,asgroundwatersuppliesroughlyhalfofpotabledrinkingwaterglobally9,thesefugitivesolutespresentaconsiderablepotentialrouteofexposuretoglobalpopulations.

Prominent among organic contaminants are per- and poly-fluoroalkylsubstances(PFAS),whichhavebeenfoundinbiosolids10and biosolids-amended agricultural fields11,12. PFAS, an extensivefamily of anthropogenic compounds found in both industrial andconsumerproducts13,usuallyarecomposedoffluorinatedaliphaticchainsterminatinginrelativelyhydrophilicfunctionalheadgroups14,imparting high resistance to thermal, chemical, and microbialdegradation13, and leading to environmental persistence as well assubstantial challenges in modeling transport or large-scale remediation efforts13. Among PFAS uses, the widespread industries oftextile15, paper16, and carpet17 milling operations historically haveusedpolymericPFAS,e.g.,sidechainfluorotelomerpolymers,totreattheirproductsforanti-stainingproperties18.Thesepolymersslowlydegradebyabiotichydrolysis,withhalf-livesontheorderof55to90years under environmental conditions, to form fluorotelomer alcohols (n:2FTOHs–F(CF ) (CH ) OH, n~4–18 even integers) asfirst-generationproducts19,andultimatelyformingrecalcitrantperfluorocarboxylates (PFCAs–F(CF ) CO(OH), n~3–18) and relatedcompounds in the environment by microbially mediatedtransformations19,20. Along with recalcitrant perfluorosulfonates(PFSAs–F(CF ) SO H, n~4–12), PFCAs are among the most widelydistributedPFASintheenvironment13.

Despite this considerable potential for PFAS and other contaminants to migrate from soil to agricultural and water resources,currentlycontaminantsubsurfacemobilityoftenismodeledbytreatingsoils as homogeneous and isotropic bodies with simple approaches suchassingledistributioncoefficientsderivedfromorganics-richsurfacesoils13,e.g.,K orK values,torepresenttheentiresoilcolumn.However,thesubsurfaceiscomposedalmostentirelyofminerals,anditiswellestablishedingeochemistrythatsoilbodiesvaryconsiderablyandpredictablywithdepthinmineralogicassemblage,chemicalandphysicalproperties21.Whiletheroleofmineralsinthesubsurfacefateofionized contaminants (including PFAS) is appreciated conceptually22,the relative importance of specific minerals and assessment of theirpropertiesaffectingcontaminantfateinreal-worldsoilbodies,arelesswellexplored.

To better understand the complex processes involved in PFASfate in the terrestrial subsurface, we investigated two sites thatreceived sludge applications. The sludge originated from a now-defuncttextilemill,theformerGaley&LordfacilityinSouthCarolina,which was designated an EPA Superfund site in 2022 (EPA IDSCD058189622). At each study site, roughly 50 soil-core segmentswerecollectedfromthesurface,throughthevadosezoneandwater-tableaquifer,toanaquicludeat18m,andanalyzedforPFASandanexhaustive suite of geochemical and mineralogic properties (seeSupplementary Information section SI I Supplementary Methods).Here we report PFAS distribution patterns through the subsurface,documenttheubiquitousdistributionofauthigenicmineralsinsoilsacrosstheUnitedStates,andexplorehowthesegloballydistributedmineralsandothergeologicsoilparameterscontroldistributionandfateofPFASinthesubsurface(SIIIDataSummary).

Results
Geochemistryofmineralweathering
Soils, sediments, and weathered-aquifer zones form from igneous,metamorphic,andsedimentaryfaciesranginginnormativeelementalcompositions from granitic (alumino-silicic rich) to basaltic (ferro-magnesianrich)mineralassemblages23.Mostmineralsthatarethermodynamicallyorkineticallyfavoredduringrockformationarenotstable when subjected to weathering, undergoing dissolution orrecrystallizationatvaryingrates24.Whilesomeofthesemineralsareresistate, reacting slowly to weathering, e.g., commonly occurringquartz (SiO ), most weather at rates that are fast relative to soil-forming processes (SI III Extended Geochemical Summary). Asweathering of primary rock minerals proceeds, a series of a fewauthigenicmineralfamiliesformbyprecipitation,thenwithcontinuedweatheringmaydissolvetoformothers,sothatweatheredsettingscan be classified by mineral assemblages (Fig. 1). These weatheringregimeshavebeenclassifiedasincipient,intermediate,advanced,andextreme25 (Fig. 2A), and their distribution in the United States isexhaustively mapped by United States Department of Agriculture(USDA)SoilOrder,e.g.,inceptisols,alfisols,ultisolsandoxisols,amongothers26 (Fig. 2A). In stable soil profiles, these weathering regimescommonlyvarywithdepth,incipientatthebedrockinterface,gradingto intermediate in B horizons, and perhaps advanced in the upper-mostAandEhorizons.Overdecadesofeffort,theUSDAhasaccumulated an extensive soil database27 including the content of thesemineral phases in US soils, documenting nearly total ubiquity ofauthigenic mineralsacrossall Soil Orders(e.g., Table1 and Supplementary Tables 19–21) and trends in assemblage with increasingweathering(SupplementaryFig.6).

Basedupongeochemicalprinciples(SIIIIExtendedGeochemicalSummary)andasdocumentedintheUSDAdatabase27,incipientandintermediateweatheringregimescommonlyareenrichedinclayssuchasillite,vermiculite,smectites(Fig.1andTable1)whichbearaper-manentnegativesurfacechargeofferingthepotentialforelectrostaticsorption of cationic compounds and/or anionic compounds withcation bridging. Advanced and extreme weathering regimes commonlyareenrichedinkaoliniticclaysand(hydr)oxideminerals,e.g.,goethite, hematite, gibbsite24,25 (Fig. 1 and Table 1), bearing pH-dependent,variablesurfacechargesthatcommonlyarepositiveundersoil conditions (SI III Extended Geochemical Summary), therebyofferingpotentialsorptionsubstratesforanioniccompounds.

Critically with regard to anionic-compound mobility, crystallineauthigenicmineralsgenerallydonotnucleatedirectlyfromaqueoussoilsolutionspoisedatthecuspofmineralsaturation.Instead,becauseof high nucleation barriers for thermodynamically stable mineralphases, metastable amorphous phases (mineraloids) nucleate first,heterogeneouslyonexistingsurfaces28,inducingprecipitationofthecrystallinemineralphasesinaprocessknownasOstwaldripening28.Prominentamongtheseamorphousphases,hydrousaluminumoxide(HAO; roughly Al(OH)) and hydrous ferric oxide (HFO, roughlyFe(OH) ),haveparticularlyhighsurfaceareas(SupplementaryTable5)withpH-andionic-strength-dependentelectrostaticsurfacechargesthatofferrichsorptioncomplexesforchargedcompounds29,30.ThesemetastableamorphousmineraloidsarenearlyubiquitouslydistributedinUSsoils(Table1andSupplementaryTables19–21)and,byinference,elsewhereglobally.

SurfaceandsubsurfacePFASdistribution
Interrestrialsettings,PFASpartitionbetweentheaqueousphaseandsolidsurfaces,includingNOMthatisconcentratedinsurfacesoilsandorganic-richsediments31.Bearinghydrophobicchains,invadosesettings,PFASalsohavebeenshowntosorbtotheair-waterinterface32.SomefractionofPFASbindingtotheseinterfacesisreversiblysorbed,and PFAS uptake into vegetation, including agricultural foodproducts33,and/orpercolationtoaquifers7hasbeenwidelyreported34.However,insubsurfacesettings,whereNOMisgenerallymuch lessconcentrated than in surface soils, details regarding sorption andreleaseofPFAS,whichgoverntheseuptakeandpercolationprocesses,areill-defined.

We studied PFAS distribution through full soil columns at
a PFAS impacted colluvial-soil site (Field 1) and residual-soil site(Field 2), characterizing subsurface vadose and aquifer samplesfor PFAS and a full range of geochemical/mineralogical properties(SIISupplementaryMethods).Atourstudysites,PFCAscomprisedthedominantclassofPFAS(SIIIDataSummary),86%and 93%oftotaltarget PFAS in surface soils in Fields 1 and 2 respectively (SupplementaryTables14and15),consistentwithexpectationsforsoilshaving received textile-based sludge applications. Wastewater biosolidsand sludges, including urban (e.g., New York City) and industriallyimpacted,havebeenwidelydocumentedtobehighinPFAS35.Amongsludge sources, sludges originating from textile operations can beparticularly high in PFAS15,36, largely as a consequence of textilescommonlyhavingbeentreatedforwater-repellencyusingsidechainfluorotelomerpolymers37,38,whichhavebeendocumentedtodegradetoPFASintheenvironment19,20asdescribedabove.

Followingatleast16yearssincethemostrecentapplicationofsludge(SII,SupplementaryFig.1),PFCAsarestillmostconcentratedin the surface soil compared to subsurface soil, with [PFCAs]~ 225µg/kg for colluvial Field 1 (dry-soil mass; Fig. 3A and Supplementary Fig. 14). Below the surface soil, subsurface maxima arepresent in both study sites at roughly 1–2m depth (Fig. 3A andSupplementaryFig.20),adepthintervaltypicalforBhorizonsoilswhereauthigenicsoilmineralsoftenaremostconcentrated(Table1andSupplementaryTables19–21),butthesesubsurfacePFASmaximaareroughlysixfoldlowerthanatthesurface(e.g.,forcolluvialField1[∑PFCAs] =34.1µg/kg).
1.67m
In terms of distribution among individual PFCAs, the surfacesoils are dominated by the longer chain compounds (Fig. 3B),withperfluorodecanoicacid[PFDA,C10(totalcarboncount,includingthecarboxylatecarbon)]throughperfluorotetradecanoicacid(PFTeA,C14) comprising >90% of PFCAs. In contrast, for the subsurface,shorterchainPFCAsdominate(Fig.3B)withperfluorobutanoicacid(PFBA, C4) through perfluorooctanoic acid (PFOA, C8) comprising>90% of PFCAs. At the nexus between the poorly mobile long-chain PFCAs (C10–C14) and relatively more mobile short-chainPFCAs (C4–C8), perfluorononanoic acid (PFNA, C9, bearing (CF ) )28exhibits a distribution intermediate between these compounds,with detectable concentrations throughout the soil column, albeitat lower fractions than its shorter homologues (Fig. 3B andSupplementaryFig.22).

WithsludgefromatextilemillasthesourceofthesePFAS,thehistoricallyusedsidechainfluorotelomerpolymersareinferredtobe
thedominantsourceofthePFCAs18,38.Then:2FTOHsboundtothesepolymersbearevennumbersnof(CF ) groups,typicallyontheorder2n3% n=6, 50% n=8, 30% n=10, 10% n=12 and 3% n=1420. Underenvironmentalconditions, these FTOHs aresubjectto β- and lesseramountsofα-oxidationtoformthe(CF ) =(n-1)and(n)PFCAs,for2nexample 8:2FTOH → PFOA (C8; n-1)+PFNA (C9; n)20,39. Accordingly,summingtheobservedPFCAsatourstudysitebythisformula,compared totypical sidechain fluorotelomerpolymers,surface soils arericherinlong-chainPFCAs(CF ) n=9–13(Fig.3C)andsubsurfacesoils2nare richer in short-chain PFCAs (CF ) n=3–7 (Fig. 3D), reflecting2npreferentiallyhighmobilityofshort-chainscomparedtolong-chains.

However, when PFCAs are summed over the entire soil column,weightedbysamplingincrement,thehomologueprofileforthepoorlymobilelong-chains(CF ) n=10–14comparesquitecloselywiththe2nsidechain fluorotelomer polymericsource (Fig. 3E). The homologuedistributionforthewholesoilcolumnisslightlydepletedinPFOA+PFNA compared to the sidechain fluorotelomer polymer (Fig. 3E),reflecting PFOA’s relatively highermobility(Fig. 3B). The whole soilcolumn also sums to higher short chains than the historic longersidechainfluorotelomerpolymers(Fig.3E),perhapsreflectingminortransformationreactionsthatcangenerateshorterchainPFCAs13,20oruse of newgeneration sidechain fluorotelomer polymers bearingshorterchains40.

FollowingPFCAs,PFSAswerethesecond-mostdominantfamilyofPFASinField1(SupplementaryTables14and15)butnotField2.Field1received textile sludge more recently than Field 2 (SupplementaryFig. 1), likelywhen alternative textile treatments were inuse. PFSAsarenotunexpectedintextilewasteassomesidechainpolymersusedin textile industry use sulfonamido-ethanol sidechains which candegrade to PFSAs18,38. Bolstering this premise, intermediate compounds including N-ethyl perfluorooctane sulfonamidoacetic acid13weredetectedinthesurfacesoils(SupplementaryTable14).IncontrasttoPFCAs,thelargestpartofPFSAshadchainlengthsof(CF )≤8.2In the absence of longer chain lengths, the longest PFSA detectedinFields1and2are(CF ) n=10andn=8,the∑PFSAswerehighest2nintheshallowsubsurface(<3m),generallyatsimilardepthincrements to the PFCA subsurface maxima at the B-horizon depths (Supplementary Fig. 21). Additionally, the PFSAs showed similar relativemobilitypatternstothePFCAsamonghomologuelengths;forexample,perfluorobutanesulfonate(PFBS,C4)maximumfellat1.5to3m,whilst perfluorooctane sulfonate (PFOS, C8) maximum was at <1m(SupplementaryFig.21).

ThepatternsobservedinrelativemobilitiesamongsthomologuesofPFCAsandPFSAsalsoarepresentinthegroundwater,withshort-chainsdetected(C4-C10forPFCAs,C5toC9forPFSAs),butlongerhomologues remaining at non-detectable aqueous concentrations(Fig. 3F). In terms of potential exposure from ingestion of groundwater, the observed concentrations of the short-chain PFCAs andPFSAs are considerably higher than EPA’s 2016 and supersedinginterim2022healthadvisories41,42(Fig.3G).Thecombinedfactorsof(i)thedetectionofshort-livedprecursorsobserved16yearssincethelastsludge application, (ii) the persisting close comparison of the soil-column homologue distribution with sidechain fluorotelomer polymersusedtotreattextiles,and(iii)thegroundwatershortchainPFASconcentrationsexceeddrinking-waterhealthadvisoriesbyordersofmagnitude, in concert suggest that slow degradation of sidechainfluorotelomerpolymersmaycontributePFAStosoilandgroundwaterfor decades or longer into the future, consistent with our previouslaboratoryexperimentsandmodeling19,20.

We employed the generalized two-layer model of Dzombakand colleagues29,43 to calculate (Supplementary Eqs. 1–16) thepH-dependent electrostatic charge (Fig. 4A, B) on the aluminummineral(oid)shydrousaluminumoxide(HAO),gibbsiteandkaolinite,andtheferricmineral(oid)shydrousferricoxide(HFO)andgoethite(SI I Supplementary Methods). As expected, these phases presentconsiderablepositivesurfacechargesthroughthesoilcolumn,offeringarichpotentialsorptionmatrixfortheanionicPFASpresentinourstudyandwhicharethePFASmostcommonlyencounteredincontaminatedenvironments13.Consistentwithgeochemicalprinciples(SIIIIExtendedGeochemicalSummary)andtheextensiveUSDAdatabaseofUSsoils(Table1andSupplementaryTables19–21),theamorphousmineraloids’ HAO and HFO charge contributions are most concentrated at the soil-column surface, and the crystalline mineralsgoethiteandgibbsitedominateinsubsurfacesoil(Figs.2Band4C).Whereastheferricsolids,whicharesubjecttoreductivedissolution(SIIII Extended Geochemical Summary), are most concentrated in theoxygen-richvadosezone(Fig.4D),thealuminumsolids(notsubjecttoreductive dissolution) present considerable electrostatic chargethroughthevadosezoneandintotheaquifer(Fig.4E).

Figure5presentsaheatmapsurveyingrelationshipsindistribution of PFAS with composite surface and vadose subsurface geochemical properties. In overarching perspective, Fig. 5 depicts twoprominentclusters,(i)thelong-chainPFASareclusteredontheair-waterinterface,totalorganiccarbonandHFOconcentration,whilst(ii)short-chain PFAS are strongly clustered on the aluminum-oxidemineral(oid) and clay concentrations, and electrostatic charges. Weestimatedair-waterinterfacialareausingthemethodsofBrusseau44(SupplementaryEqs.17and18)andtheverticalpositionofsoilsamplesabove the free-standing water table (SI I Supplementary Methods).ConsistentwiththefindingsofBrusseau32,ourlong-chainPFCAsandPFSAs(CF ≥8forourdata)clusteronair-waterinterfacemorepro-2minentlythanforshorterchains(Fig.5).Inaddition,ourlong-chainPFASclusterontotalorganiccarbonandtotalnitrogen(Fig.5),whichlargelyarecomposedofNOMthatusuallyconcentratesinsurfacesoilsasitdoesinourstudysites(SupplementaryFigs.20–22).Long-chainsalso cluster with HFO concentration but not HFO charge, likelyreflecting HFOs concentrating in surface soil (Fig. 4C) and the welldocumentedintimaterelationsofincipientferricoxideswithNOM45,46.Inverselong-chaincorrelationsontheclayfractionsinFig.5reflectthedominantpresenceoflong-chainsatthesandysurface,butauthigeniccrystallineoxidesandclaysaccumulatinginthesubsurface.Whereas
the long-chain PFAS clustering on air-water interface and organicmatter comportswithwidelyheldconceptualmodels,theshort-chainPFAS(CF ≤7)clusteringheavilyonconcentrationofoxideminerals2andclays,andtheelectrostaticchargesofHAO,gibbsite,andkaolinite,hasnotbeenwidelyreportedasadominantsinkinsubsoils(Fig.5).HAO,gibbsiteandkaolinitesharepositiveAlsurfacesatpH≤7(Fig.4BandSupplementaryTables10and11),therebydirectlyattractingthenegative charge of most recalcitrant PFAS13, and these authigenicmineral(oid)shavebeendocumentedtobenearlyubiquitousinUSsoilprofiles(Table1andSupplementaryTables19and20).Regarding
short-chain clustering on illite, which bears permanent negativelychargedsurfaces,anionicPFASsorptiononnegativelychargedillitefacetsmightbefosteredbybridgingofdivalentsolutioncations47,48under the low-NOM conditions characteristic of subsurface soils,sorbingatthepH-dependentbroken-bondsheetedgesofillite49,and/or diffusion into the clay interlayer. In any case, such a stronglyexpressed cluster between short-chain PFAS and these globally distributed authigenic mineral(oid) concentrations/charges is highlysuggestiveoftheirprominentroleintheterrestrialfateofPFAS,aswellas other anionic anthropogenic contaminants, in the subsurfaceenvironments ofthe United States and elsewhere globally.

Whiletheroleofauthigenicaluminumoxidemineral(oid)sinthe
terrestrialfateofPFASandotherorganiccontaminantshasnotbeen
deeplyexplored,theroleofthesesolidsinthedistributionofinorganicanionsandtracemetalshasbeenstudiedformorethanacentury29,43.Modeling of inorganic ion sorption on metal oxides includes bothelectrostaticandintrinsiccomponents29,43.InFig.5,short-chainPFASloadingonaluminumsolidscommonlyfallsinthesamegeneralsignificancelevelforbothconcentrationandelectrostaticcharge,suggesting a potentialintrinsic contribution of PFAS sorptionon thesesolids.InearlyresearchonPFASinteractionswithminerals,Hellsinget al. studied sorption of four PFAS to alumina (positive surfacecharge) and silica (negative surface charge) crystals and observedsorption to Al O but not SiO, reflecting the role of electrostatic2 3 2interactionswereporthere50.StudyingsorptionofPFOAandPFOSonamorphous(highsurfacearea)SiO ,Changetal.51reportedconsider-2ablenon-linearsecond-ordersusceptibility,indicatingintimatesurfacecontact and suggesting a measurable intrinsic component for PFASsorptiononmineralsurfaces.Bolsteringthispremise,Wahmanetal.52studied the sorption selectivity, relative to chloride, of nine monovalentPFASonthreeion-exchangeresinsanduniformlyfounda1-to5-order-of-magnitudepreferenceforPFASsorptionoverCl-;ifsorption were purely electrostatic, the large, diffusively charged PFASwouldnotbesostronglypreferredoverCl−.Unfortunately,unlikethechemicallyindifferentforcesoftheelectrostaticinteractionsexploredhere,theintrinsiccomponentofsorptionisionandsurfacespecific,andthereforeremainsachallengeforfutureinvestigations.

Despite that ferric oxides have been shown to electrostaticallysorb PFAS53 and exhaustively documented to sorb inorganicanions30,43,inourdatasetcharacterizingfullsoilprofiles,PFASdonotclusteruponferric-oxidecharge(Fig.5),likelylargelyasaconsequenceof the restricted vertical distribution of ferric oxides dominantly atshallowBhorizondepths(Figs.2Band4D,andSupplementaryFig.5).In contrast, aluminum-oxide charge achieves its maximum at typicalB-horizon depths but sustains considerable charge contributionsthrough most of the aquifer column (Figs. 2B and 4E, and Supplementary Fig. 5) thereby offering governance of PFAS distributionthroughoutthesamplinginterval.Thisrestrictedverticaldistributionofferric oxides at our study sites, relative to aluminum oxides, mightberelatedtothepotentialforreductivedissolutionofferricoxidesunderlow-oxygenconditions(SupplementaryFig.6),forwhichaluminumoxidesarenotsusceptible(SIIIIExtendedGeochemicalSummary).However,ferricoxidescanbeverticallydistributedmuchmoreextensivelyinsomesettings54.Regardless,theseresultshighlighttheneedtodocumentthedistributionofpotentialsorbentsaswellastheircapacityinassessingfateoforganiccontaminantsinthesubsurface.

Methods
Allstudymethodsaresummarizedhereanddescribedindetailintheon-lineSupplementaryInformation(SI).

Sampling
Two agricultural fields in the South Carolina coastal plain consisting of marine sediments, that had received sludge from thenow defunct Galey and Lord textile mill, were selected to studythefateofPFAScontaminantspresentinthesludge.Ateachfield,a Geoprobe drill rig was used to drive cores through the soilprofile and underlying water-table aquifer to a clay aquiclude. Inaddition to collecting solid core samples, at each site, two temporary water wells were installed with screens located near theshallow water table (at 3.7 to 4.6m) and the deep aquiclude(~18m),followed bywatersampling for analysis.

Analysis
Allsampledsoilandaquifersolids(~50ateachfieldsite)wereanalyzedfor conventional soil-characterization parameters (e.g., pH, cationexchangecapacity,basesaturation,limerequirement,Mehlichmetals,water-extractable anions, total organic carbon, total nitrogen, andparticle size) at the University of Georgia cooperative extensionlaboratories.Soilandaquifersolidsweresubjectedtobulkx-raydiffractionanalyseswithsemi-quantitationofmajormineralphasesattheUniversity of Georgia Department of Geology. Solids samples weresubjected to selective extractions to quantitate the amorphoushydrous-oxide mineraloid fractions of ferric iron, aluminum, andmanganese.SolidssampleswereanalyzedforPFASaccordingtotheASTM D7968 method and water samples were analyzed using theASTM 7979 method, all using liquid-chromatography, tandem massspectrometry.Nontargetedanalysiswasperformedonselectextractsby liquid-chromatography high resolution quadrupole time-of-flightmassspectrometry.

Summarizingcontentofmineralsubiquitouslypresentin
USsoils
MineralogydataofUSsoilswereobtainedviaon-linequeryoftheUSDANatural Resources Conservation Service (NRCS)National CooperativeSoilSurvey(NCSS)SoilCharacterizationDatabase.Dataonmajorsoilauthigenicminerals,quartz,amorphoussolidsofferricironandaluminum,andtotalorganiccarbonwereextractedandgroupedbyUSDASoilOrderandmajorpedogenichorizonsofA,B,andC.TheresultingsummaryofUSsoilsconsistedof>60000datapoints.

Calculationofvariableelectrostaticcharge
Forthecommonsoilmineral(oid)sthatpossessavariableelectrostaticcharge,includinggoethite,gibbsite,kaolinite,HAOandHFO,wecalculated charge for all solids samples as a function of pH and ionicstrengthusingageneralizedtwo-layermodel29,43.NecessaryinputsforthesecalculationswereculledfromliteratureandaresummarizedasSupplementary Table 5. This approach yielded ~500 values of electrostaticcharge(~100samples×5mineral(oid)s)(SI).

Calculationofvadosemoisturecontentandair-waterinterface
We calculated air-water interfacial area of the soil-column samplesusingthemedian-particleapproachofBrusseau44.Topopulateinputsforthisequation,weusedourmeasuredUSDAsoiltextures.WeusedliteraturevaluesofmedianparticlediameterreportedforeachUSDAsoil texture58. We assumed equilibrium water tension in the vadosesone relative to elevation above the water table, and employed theBoltzmannDistributionequation,parameterizedforeachUSDAsoiltexture59, allowing us to estimate air-water interfacial area for allvadosesamples.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript titled \"Controls on the Fate of PFAS and Anthropogenic Anions in the Terrestrial Subsurface\" presents a detailed examination of the mechanisms governing the mobility and fate of PFAS in soil and aquifer environments. It highlights the significant role of mineralogy, particularly authigenic minerals, in adsorbing PFAS, thereby influencing their environmental behavior and distribution.\nStrengths:\nThe study's use of a wide array of geochemical and mineralogical analyses across different soil layers enhances the depth and reliability of the findings. This robust methodological framework provides a solid basis for the conclusions drawn.\nThe application of advanced geochemical modeling to predict PFAS interactions with soil minerals adds a valuable dimension to the research, offering insights that are both predictive and explanatory.\nThe detailed discussion on the impact of specific minerals like kaolinite and gibbsite on PFAS sorption is a strong point, providing clarity on the molecular interactions that govern PFAS mobility in subsurface environments.\nThe manuscript effectively uses data to challenge and refine existing theories on PFAS behavior, particularly concerning their interactions with different soil minerals.\nWeaknesses:\nWhile the data is generally well presented, some figures and tables are densely packed with information. Simplifying these or providing supplementary online material could help maintain the reader's focus on key results.\nThe manuscript could enhance its scope by incorporating more discussion on biological factors affecting PFAS fate, such as microbial degradation of precursors, which are equally crucial in subsurface environments.\nWhile the study presents detailed data, the generalization of results across varied environmental conditions could be better addressed to clarify the scope of application of the findings.\nThe heavy use of technical jargon and complex descriptions may make the paper less accessible to a broader audience for Nature Communications. Please include a nomenclature section to aid understanding.\nThe manuscript's references are up-to-date; however, incorporating discussions on 'older' but relevant studies, such as Xiao et al., 2015, on PFAS in surface and subsurface soils could enhance its thoroughness.\nAlthough the study provides a comprehensive current assessment, it could offer more explicit suggestions for future research directions, particularly in addressing the gaps identified in PFAS behavior under varying environmental conditions.", "reviewer_2": "General comments:\n1. This dataset is significant and of high quality. I also think that it’s inclusion of impacts from the textiles industry (not a major focus of the paper) is quite unique, and I wonder if there are aspects of the release type (textiles, side chains, etc.) that have yet to be explored in the manuscript. I think there’s a lot that could be done with these data, and I’m excited to see where the authors take the story based on reviewer feedback.\n2. The significance of this study relative to prior work really needs to be clarified. As currently written, it is difficult to ascertain what this study contributes over prior work. Is it a conceptual model of PFAS sorption in the vadose zone and saturated zones with depth? Is it confirmation of a conceptual model that already existed with the extensive dataset the authors are presenting? There are a number of prior studies focused on PFAS sorption to minerals. The authors cite some of it (mostly in the supplementary material), but in my view the outcomes of this prior work were not thoroughly considered and were not used to identify a clear knowledge gap that is addressed here. It would be helpful if the authors outlined a clear conceptual model of the role of minerals in PFAS sorption based on outcomes of prior work, identified clearly knowledge gaps, and then provided clear hypotheses or objectives for the current study.\nIn case they are helpful, here are a few additional papers related to PFAS interactions with minerals that I’m not sure were considered:\n• https://doi.org/10.1016/j.chemosphere.2016.06.016\n• https://doi.org/10.1021/acs.est.0c01646\n• https://doi.org/10.1021/acs.est.2c01054\n3. Another aspect of significance that needs to be clarified is the importance of PFAS interactions with minerals in governing the overall distribution of PFAS. Showing correlations of PFAS with various mineral components is useful and interesting, but it does not inform the magnitude of the interactions of PFAS with those mineral phases (e.g. the magnitude of equilibrium sorption coefficients [Kd] values). Studies have shown that equilibrium sorption coefficients of PFAS to mineral phases can be quite low, and overall, the interactions of short-chain PFAS with any phase are quite low. So will these mineral phases really lead to a meaningful change in PFAS transport via sorption? Could we achieve similar predictions of transport, if we just assumed that there was no sorption to these phases? What about the role of other processes like matrix diffusion into/out of clays? I suspect the latter would be much more important in influencing PFAS transport over sorption to the minerals.\n4. This study is very geology-oriented, uses a lot of the associated terminology, and it seems written for a geology audience over the type of broad readership often targeted by a journal such as Nature Communications. It will be difficult for a broader scientific audience to understand, and currently the relevance of this topic to a broad scientific audience isn’t clear. PFAS are contaminants that are of global concern, but as written this manuscript seems better suited to a journal more focused on hydrogeology or environmental engineering. Clarifying the significance (as noted above) might go a long way to address this, but I also think that the final paragraphs of the paper need to do more to address the significance to a multidisciplinary readership.\nBelow are just a few, additional line by line comments.\nLines 17-31 (the abstract) are all background and a brief summary of the study objective. I suggest including a description of the significant outcomes of the study. Due to word limitations this may necessitate reducing the amount of background.\nLine 42: Minor comment that “and the like” is a vague way to end the sentence. Plus this information is unsupported by references.\nLine 50: Why only advection? Why not dispersion, back-diffusion, and other mechanisms?\nLine 64 mentions fluorotelomer polymers, and I think it would be beneficial to denote that these are also PFAS (i.e., polymeric PFAS) that can degrade to non-polymeric PFAS. As written it implies that these are polymers (not PFAS) that can degrade into PFAS.\nLines 78-80: Please consider revising as this is not a complete sentence and it is unclear what the authors are trying to say.\nLines 117-135: Some of this information was already mentioned in the introduction, and this is just more general background on PFAS. If this space is to be used to focus on PFAS background, then I suggest using to define why this study is needed. So far the authors have simply said that current work focuses on surface soils, which are higher in Natural Organic Matter (NOM) than deeper soils. However, they haven’t said anything about why understanding interactions with minerals in more weathered depths is important or what kinds of interactions are not already understood via prior work.\nLine 240 discusses the strong correlation between negatively charged illite and concentrations of short-chain (and anionic) PFAS. Multiple hypotheses are given regarding the cause, but none address non-sorptive mechanisms such as the diffusion of PFAS into clay, which would presumably occur faster for short-chain PFAS due to low molecular weight.\nFigure 1: Panel C could be better utilized. For example, A and B show details of composition that support the caption, but there is nothing in panel C that really represents what is described in the caption for that panel. I.e., the panel doesn’t really reflect anything about, “Shallow-aquifer flow zones tend to weather less extensively than the vadose zone; in reducing waters, Fe (hydr)oxides may undergo reductive dissolution thereby not contributing to the potential exchange complex, but Al (hydr)oxides and kaolinite can sorb short-chain PFAS regardless of oxidation state.”\nFigure 3. There is an indication of a kaolinite line in Figure 3, panel C, but the kaolinite line is not visible.", "reviewer_3": "The authors present an interesting dataset combining field data from a large number of soil cores (~50/site) with extensive geological and geochemical information complemented with detailed PFAS analysis. These data and explaining the interesting trends in CF2 chain length dependent distribution, especially in deeper, carbon depleted soil and aquifer layers, form the core of the manuscript, which is very interesting and valuable on its one.\nHowever, the authors could have substantially increased the value of their work, had they further validated their interpretations with laboratory scale experiments (e.g. PFCAs formation rates from precursors as well as sorption and desorption batch or column experiments at relevant conditions as partially suggested by the authors in line 296ff). This would have helped complementing their field data with some sort of kinetic modeling of PFCAs fate (formation, transport sorption in dependence of water saturation, soil/aquifer composition, AEC, CEC, pH, ionic strength ect), to better contextualize, translate and enable extrapolation of the interesting findings shown in Figure 4. Such complementary activities would have given the study more weight and could have facilitated the translation of these findings to other sites and scenarios.\nAs the manuscript stands now, it is hard to say if the trends and findings on chain length dependent behavior and the importance of electrostatic interactions in carbon depleted matrices are a “just” really interesting observation from two field sites, or if -and how- these findings can be generalized and used for future modeling, risk assessment, and potentially prioritization of mitigation measures across the large number of potentially relevant sites mentioned by the authors in line.\nMinor remarks:\nLine 52-56: a bit of an overstatement on the lack of studies on minerals and NOM, as these aspects have been the focus of a substantial number of studies over the last decade(s), including for charged and ionizable compounds.\nLine 62: a sentence contextualizing the focus on sidechain fluorotelomer within the larger family of PFAS may be useful for some readers.\nLine 73: rephrase “in soils in our soils and soils”\nLine 219: this paragraph could be extended somewhat.\nLine 261: some systematic experiments would have been possible and highly appreciated to get a better understating on the sorption processes. Please also see my general comments."}, "author_response": "The authors have provided a point-by-point response to the reviewers in the subsequent round."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript has undergone significant improvements, with additional information included and a thorough response to the prior comments, which have enhanced the clarity and depth of the study.\nOverall, the authors have effectively addressed concerns about the dense packing of figures and tables. They have simplified the presentation and have relocated extensive data to supplementary material, which helps maintain focus on the key results.\nBy incorporating more discussion on the biological factors affecting PFAS fate, such as microbial degradation, the manuscript offers a more comprehensive view of the mechanisms governing PFAS behavior in subsurface environments.\nThe authors have made considerable efforts to simplify the language and structure of the manuscript, making it more accessible to a broader audience without sacrificing technical depth.\nGiven the comprehensive nature of the revisions and the manuscript's alignment with the journal's scope, particularly in terms of its interdisciplinary approach and potential for broad interest, I recommend this manuscript for publication.", "reviewer_2": "I would like to thank the authors for simplifying the use of technical jargon, improvements to figures, and improved framing of the contribution of their work. As I understand it, important contributions of this work include use of calculated, pH-dependent charge when correlating subsurface PFAS concentrations with geochemical parameters, an improved conceptual model of PFAS interactions with subsurface soils that includes the relative importance of various minerals, and an evaluation of the global ubiquity of these minerals based on the United States Department of Agriculture (USDA) soils database. I agree that these results offer important considerations for those evaluating PFAS fate and transport either through laboratory experiments or modeling. However, I think it is the application of these results that will be of most interest to a broad, multidisciplinary audience consistent with Nature Communications. For example, it would be great to see a quantitative analysis (e.g., fate and transport model- could be a simplified one) that demonstrates how consideration of these mineral-specific interactions influence the distribution of short-chain PFAS vs. a model that considers homogenous surface soil composition vs. (perhaps) a model that considers little to no sorption to minerals. Without that application, it is difficult to frame the importance of the authors’ advancements to the PFAS sorption conceptual model. I have provided some point by point feedback within my original comments and the authors’ responses in the document below. There are some additional, minor comments unique to this version beneath this point by point response. All line numbers reference the track changes version of the manuscript.\n[...Author responses omitted for brevity in this field...]\nAdditional line-by-line comments for revision\nLine 52. I suggest changing “biosolids-applied agricultural fields” to “biosolids-amended agricultural fields.”\nLine 69 should be edited to state, “…and it is well established…”\nLine 239. It isn’t clear how concentrations in excess of drinking water advisories informs slow degradation of side-chain polymers. Perhaps the authors intended to make a point about persistent concentrations that pose a health risk, but if so, this point is never made.\nLine 284. Should this be “number of?”\nLine 286. Should this be “implications for?”\nLine 328. “So, the subsequent fate of PFAS in the subsurface, e.g., uptake by deeply rooted plants and/or leaching to groundwater resources, remains poorly predicted by these surface-soil data.” I feel that the authors could be overstating things here. There are many reasons why leaching of PFAS to groundwater resources could be poorly predicted. Is it really the lack of info on interactions with the mineral phase? If so, is there data to support this? Since the current study is not applied in a predictive capacity (i.e., to model transport), it isn’t clear if this knowledge gap is addressed by the current study.\nFigure 3 panel f. It seems like it would be more intuitive to plot Cs/Cw so that the plot can also give a sense for Kd without having to reverse the depicted trends during figure interpretation. Also, I suggest marking the eluviated horizons, depth typical of authigenic minerals, etc. on Figure 3 (at least on Panel A).\nFigure 3 panel g. Please considering using the MCL of 4 ng/L as opposed to the interim advisory of 0.004 ng/L.\nFigure 5 states “Heat map of nominal correlation coefficients for geochemical properties (columns), grouped according to close geochemical relationships, to PFCAs…” I assume you mean PFCA concentrations? It would be better to specify. Same comment for PFSAs.\nSupplementary Figures 2 and 3 would benefit from a legend that defines what I is represented by each color.\nSupplementary Figure 11. It is impossible to correlate the colors in the graphs to the individual PFAS for PFCAs, PFSAs, or other PFAS. Please consider use of patterning or colors that can be distinguished from one another enough to be matched to the legend.", "reviewer_3": "As mentioned in the previous round of my review, the detailed PFAS analysis coupled to extensive geological and geochemical information are of high interest and value. The authors well addressed all of my minor comments and I look forward to seeing future work towards a more generalizable translation of their findings."}, "author_response": "We thank all of the Reviewers for their thorough reviews, thoughtful edits and comments, and careful consideration of our manuscript. For clarity, new Reviewer comments intended for response are highlighted in green and new Author responses are in bold. [Full text of rebuttal included in the provided source]"}]

Decision letter