atmosphere Article Natural Seepage of Methane and Light Alkanes at Three Locations in Southern California Brynne M. Weed, Gisselle D. Brambila and Lambert A. Doezema * Department of Chemistry and Biochemistry, Loyola Marymount University, Los Angeles, CA 90045, USA; [email protected] (B.M.W.); [email protected] (G.D.B.) * Correspondence: [email protected] Received: 29 May 2020; Accepted: 9 September 2020; Published: 12 September 2020 Abstract: Methane and light (C2–C5) alkane fluxes were measured from three geologic seepage sites in Southern California during May and June of 2019. Samples were collected from visible macroseeps in Carpinteria, McKittrick, and Ojai using an aluminum flux chamber with attached stainless-steel canisters and were analyzed for C1 to C5 alkanes via gas chromatography. Carpinteria fluxes were characterized by a lower percentage of volatile organic compounds relative to methane but greatly enhanced (~20:1) ratios of i-butane to n-butane. McKittrick and Ojai exhibited less methane-rich emissions and i-butane to n-butane ratios of less than 2:1. The differences between gas ratios observed at the surface and those previously reported from underground gas deposits at Ojai suggest that gases undergo alterations to their molecular composition between deposit and surface. The ratios of emitted gases in this study show that not only does geologic seepage have a much different volatile organic compound profile than oil and natural gas extraction and pipeline natural gas, but also that individual geologic seepage locations exhibit large variability. Keywords: geologic seepage; macroseepage; methane; light alkanes; oil and natural gas 1. Introduction Geologic seepage from underground oil and natural gas deposits is a source of methane and volatile organic compounds (VOCs) to the atmosphere that is sometimes overlooked. This seepage can occur in marine and terrestrial environments. Terrestrial seepage can occur in three ways, via microseepage, macroseepage, or miniseepage. Microseepage occurs when seepage from underground oil and natural gas deposits exit the earth’s surface with no accompanying visible signs. Macroseepage (Figure1) is observed when there is a visible escape of gases from underground deposits. Macroseepage is caused by pressure and buoyancy bulk fluid flow in locations where significant cross-stratal migration occurs [1]. It is capable of moving large amounts of liquids and gases and is often characterized by co-emission of oil or asphalt from the seeps [1]. Miniseepage is invisible seepage that occurs in an area where macroseeps are present. Though microseepage has the ability to occur globally in all places where underground deposits exist [2,3], macroseepage occurs in a much more limited number of discrete locations [3]. A recent database reports over 2100 global macroseeps in 89 countries [4]. However, even though macroseepage is more limited geographically than microseepage, individual macroseeps typically emit several orders of magnitude more methane and VOCs than microseeps [4,5]. Along with methane, the VOCs that are most commonly associated with geologic seepage are light alkanes, namely ethane, propane, i-butane, n-butane, i-pentane, and n-pentane. Although these gases react relatively slowly with hydroxyl radicals compared to other VOCs, they can nevertheless affect the hydroxyl radical reactivity, ultimately contributing to ozone formation in urban areas [6,7]. Atmosphere 2020, 11, 979; doi:10.3390/atmos11090979 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 979 2 of 13 Atmosphere 2020, 11, x FOR PEER REVIEW 2 of 13 FigureFigure 1.1. Example of macroseepage, asas thethe asphaltasphalt bubblebubble formingforming showsshows gasgas escape.escape. PicturePicture takentaken atat thethe CarpinteriaCarpinteria samplesample site.site. AA number of of studies studies measuring measuring terrestrial terrestrial micros microseepageeepage have have been been carried carried out in out Europe, in Europe, North NorthAmerica, America, and Asia. and These Asia. studies These studieshave observed have observed daily positive daily positive microseepage microseepage fluxes ranging fluxes rangingfrom μg 2 2 fromm−2 toµ hundredsg m− to hundreds of mg m− of2 [8]. mg A m smaller− [8]. number A smaller of number studies ofhave studies measured have measuredemissions emissionsfrom terrestrial from 2 terrestrialmacroseepage; macroseepage; these studies these have studies reported have daily reported fluxes daily from fluxes individual from individual seeps in the seeps kg inm− the2 range kg m [9].− rangeRecent [ 9studies]. Recent have studies estimated have estimatedthat global that macros globaleepage macroseepage contributes contributes 3–4 Tg of 3–4methane Tg of annually, methane annually,compared compared to an annual to an global annual contribution global contribution of 25 Tg of 25methane Tg of methanefrom microseepage from microseepage [10]. Though [10]. Thoughprevious previous studies studieshave mainly have mainlyfocused focused on methane on methane emissions, emissions, it has been it has estimated been estimated that annual that annual global globalethane ethane and propane and propane emissions emissions from all from types all typesof geol ofogic geologic seepage seepage are in arethe inrange the range of 2–4 of Tg 2–4 and Tg 1–2.4 and 1–2.4Tg respectively Tg respectively [11]. [11]. PreviousPrevious geologicgeologic seepage seepage studies studies have have been been carried carried out out by by our our research research group group at the at the La BreaLa Brea Tar PitsTar inPits Los in Angeles, Los Angeles, California California [12,13]. The[12,13]. second Th ofe thesesecond studies of these measured studies not measured only macroseepage not only butmacroseepage also associated but also miniseepage associated at miniseepage La Brea. This at La study Brea. estimated This study that estimated approximately that approximately 1000 kg of methane,1000 kg of more methane, than 10more kg ofthan ethane, 10 kg and of ethane, more than and 4 more kg of than propane 4 kg wereof propane emitted were daily emitted from the daily La Breafrom area. the La Another Brea area. finding Another from thesefinding La from Brea studiesthese La was Brea that studies ratios was of emitted that ratios gases of not emitted only varied gases atnot the only surface, varied but at also the di surface,ffered significantly but also differed from alkane significantly ratios that from are typicallyalkane ratios associated that are with typically oil and naturalassociated gas with production oil and andnatural pipeline gas production natural gas and [14– pipeline16]. Furthermore, natural gas the [14–16 gas wetness,]. Furthermore, or the relative the gas amountwetness, of or C the2–C 5relativealkanes amount to methane, of C2–C diff5 eredalkanes throughout to methane, the samplingdiffered throughout locations. Mostthe sampling notably, thelocations. largest Most seep notably, measured the was larg significantlyest seep measured less methane-rich was significantly and gave less a methane-rich much lower iand-butane gave to a nmuch-butane lower ratio i-butane than other to n-butane seeps that ratio were than studied. other seeps The gas that ratios were atstudied. this largest The gas seep ratios were at very this similarlargest toseep the were ratios very of gases similar in theto the underground ratios of gases reservoir, in the underground which had been reservoir, measured which in a previoushad been studymeasured [17]. Thisin a previous suggested study that the[17]. gas This from suggested this largest that seep the underwentgas from this less largest alteration seep betweenunderwent the less reservoir alteration and thebetween surface, the as reservoir the gases and had the a moresurface, direct, as the and gases thus had faster, a more pathway direct, to and the thus surface. faster, This pathway alteration to the or ‘processing’surface. This of alteration the gas as or it ‘proce makesssing’ its way of the up gas through as it makes the soil its wasway suggestedup through as the a contributing soil was suggested factor foras a the contributing anomalous factor ratios for of gasesthe anomalous emitted at ra thetios surface of gases seeps. emitted at the surface seeps. TheseThese findingsfindings atat LaLa BreaBrea werewere notnot necessarilynecessarily novelnovel asas similarsimilar uncommonuncommon ratiosratios hadhad beenbeen previouslypreviously observedobserved in other seepage studies. Unusually Unusually high high i/ in/n-butane-butane ratios ratios of of approximately approximately 5 5were were found found in ina marine a marine seep seep off o theff the coast coast of ofAlaska Alaska [18]. [18 These]. These findings findings fit with fit with other other studies studies that thatsuggest suggest that thatn-alkanesn-alkanes are arepreferentially preferentially degraded degraded by by microbes microbes compared compared to to ii-alkanes-alkanes [19,20]. [19,20]. Additionally,Additionally, studies studies of leakageof leakage gas fromgas undergroundfrom underground storage tanksstorage have tanks noted have chemical noted fractionation chemical duringfractionation migration during that migration can preferentially that can remove preferentially heavier remove (C2+) alkanes heavier compared (C2+) alkanes to methane compared [21]. to methaneThe current[21]. study seeks to further expand the database of macroseepage emissions of methane and CThe2–C current5 alkanes. study Three seeks macroseepage to further expand sites