張 希,楊 靜,劉 敏*,陳 星,吳 建

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上海交通沿線農(nóng)田土壤中PAHs分布特征及源解析

張 希1,楊 靜1,劉 敏1*,陳 星1,吳 建2

(1.華東師范大學(xué)地理科學(xué)學(xué)院,教育部地理信息科學(xué)重點(diǎn)實(shí)驗(yàn)室,上海 200241；2.上海市環(huán)境科學(xué)研究院,上海 200233)

為探討交通干線對(duì)周圍農(nóng)田土壤環(huán)境中16種優(yōu)先多環(huán)芳烴(PAHs)累積的影響,采集了上海市交通干線旁與道路垂向分布的70個(gè)農(nóng)田土壤表層樣品及2個(gè)土壤柱樣品,系統(tǒng)分析了土壤中16種優(yōu)控PAHs的含量、組成及來源.結(jié)果表明上海交通干線旁農(nóng)田表層土壤PAHs含量范圍為23.16~21250.25ng/g,平均值為928.16ng/g,且隨著與防護(hù)林距離的增加呈現(xiàn)出先下降后增加而后又下降的趨勢(shì),農(nóng)田土壤柱中PAHs含量則表現(xiàn)出隨著距土壤深度的增加而上升的趨勢(shì).基于正定矩陣因子分析(PMF)模型及特征因子比值法的來源辨析結(jié)果,表明表層土壤和土壤柱中的PAHs均主要來源于煤、生物質(zhì)的燃燒及交通排放.

交通干線；農(nóng)田土壤；多環(huán)芳烴；分布特征；源解析

隨著全球城市化浪潮的快速推進(jìn),城市機(jī)動(dòng)車保有量快速增長,其引發(fā)的交通擁堵造成交通運(yùn)輸業(yè)能源(主要為油品)迅猛消耗[1].交通排放(尾氣排放、輪胎碎屑和瀝青路面等)產(chǎn)生的具有“三致”效應(yīng)的多環(huán)芳烴(PAHs)污染物會(huì)隨著大氣沉降、道路揚(yáng)塵和路面徑流等途徑進(jìn)入周邊農(nóng)田土壤,造成土壤PAHs污染,進(jìn)而通過作物吸收和食物鏈傳遞影響農(nóng)產(chǎn)品質(zhì)量及人體健康[2-3].近年來,PAHs在交通干線兩側(cè)耕地系統(tǒng)中的污染效應(yīng)逐漸成為研究熱點(diǎn),國內(nèi)外學(xué)者對(duì)高速公路、主次干道等公路沿線農(nóng)田土壤中PAHs的空間累積特征、來源解析及風(fēng)險(xiǎn)評(píng)價(jià)等方面進(jìn)行了一些有益的探索[4-6].朱利中等[7]采集并測(cè)定了不同燃料油品、車輛類型和行駛公里數(shù)的汽車尾氣中的14種多環(huán)芳烴,發(fā)現(xiàn)汽車在30min內(nèi)排放的14種PAHs濃度為1211~4153mg/m-3,揭示了汽車尾氣已經(jīng)成為公路旁土壤中多環(huán)芳烴污染的重要來源.道路沿線土壤既是PAHs的“匯”,也是PAHs的“源”,土壤中的PAHs通過降雨、降雪以及地表徑流的沖刷作用,成為水體及其他環(huán)境介質(zhì)中PAHs的重要污染源[8-9],在人口相對(duì)集中、道路分布過于密集的城市區(qū)域,PAHs等污染物因其自身的生物積累性、生物毒性和難降解等特性,長期存在于環(huán)境中將對(duì)環(huán)境和人體健康產(chǎn)生威脅[10].總體來說,道路兩旁農(nóng)田土壤PAHs污染范圍和污染程度的空間差異可能與道路車流量、通車年限、車輛類型、路邊障礙物、氣象、地形等因素有關(guān).一般來講,車流量越大、通車年限越長、重型車越多、障礙物越少,路邊農(nóng)田土壤PAHs含量就越高.

作為中國最重要的工業(yè)中心和經(jīng)濟(jì)中心,上海在過去30a里經(jīng)歷了快速的城市擴(kuò)張和土地利用變化,大量的農(nóng)業(yè)用地被轉(zhuǎn)化成工業(yè)用地、居住用地和道路交通用地[11].據(jù)統(tǒng)計(jì),上海從1990~2016年間道路面積年均增長率為10.15%,同時(shí)期機(jī)動(dòng)車保有量也從1990年的21.21萬輛迅速增長至2016年的322.94萬輛,年均增速達(dá)9.89%[12].上海道路面積的增長速率跟不上車輛的飛速增長,由此引發(fā)了嚴(yán)重的城市交通擁堵和機(jī)動(dòng)車污染排放問題.目前以上海地區(qū)交通干線農(nóng)田表層及深層土壤中PAHs的污染程度及分布規(guī)律的研究報(bào)道還較少[6,13].因此,本研究以上海市交通干線旁農(nóng)田土壤為研究對(duì)象,探究其表層及深層土壤中PAHs的含量、分布特征及其主導(dǎo)來源,以期為未來上海交通沿線農(nóng)田土壤PAHs的污染防控提供依據(jù),以保障農(nóng)田土壤環(huán)境質(zhì)量和農(nóng)產(chǎn)品安全.

1 材料與方法

1.1 采樣點(diǎn)設(shè)置

圖1 采樣點(diǎn)分布

表1 采樣點(diǎn)概述及土壤理化性質(zhì)

在參考上海市國控大氣污染點(diǎn)源分布的基礎(chǔ)上,選擇了7塊靠近交通干線且1km緩沖區(qū)內(nèi)無國控大氣污染源的大型農(nóng)田(其中寶山區(qū)樣點(diǎn)1個(gè),青浦區(qū)樣點(diǎn)1個(gè),浦東新區(qū)樣點(diǎn)1個(gè),金山區(qū)樣點(diǎn)2個(gè),奉賢區(qū)樣點(diǎn)2個(gè)),各樣點(diǎn)信息見圖1和表1.于2017年4月共采集70份表層梯度樣品,即在每個(gè)樣點(diǎn)公路一側(cè)設(shè)定10個(gè)與公路平行的取樣斷面,與公路防護(hù)林外沿距離分別為0,5,10,20,40,60,80,120,200, 500m,同時(shí)設(shè)置5個(gè)垂直取樣帶,每個(gè)取樣帶間隔20m.用不銹鋼鏟在水平與垂直取樣帶交叉處以梅花形布點(diǎn)法采集表層土壤(0~10cm),充分混合后用四分法保留100g左右樣品并裝入貼好標(biāo)簽的聚乙烯密封袋內(nèi).在位于農(nóng)業(yè)密集區(qū)且遠(yuǎn)離上海市國控大氣污染點(diǎn)源的S1和S6樣點(diǎn)處使用不銹鋼鉆各采集一根土壤柱,其中S1豆田土壤柱長70cm,S6水稻田土壤柱長100cm,按10cm間隔進(jìn)行分割,裝入寫好標(biāo)簽的聚乙烯密封袋內(nèi).所有樣品帶回實(shí)驗(yàn)室,用真空冷凍干燥機(jī)干燥,過100目標(biāo)準(zhǔn)篩,保存在-20℃冰箱中待用.

1.2 土壤理化性質(zhì)與PAHs分析

1.2.1 粒度與有機(jī)碳 稱取大約0.2g冷干的土壤樣品,加入10mL雙氧水(體積比10%)和10mL鹽酸溶液(1mol/L)去除有機(jī)質(zhì)組分和CaCO3等碳酸鹽物質(zhì),最后加入5mL的分散劑(六偏磷酸鈉,質(zhì)量分?jǐn)?shù)5%)放入超聲震蕩儀中超聲10min,利用激光散射粒度儀(LA-960,日本HORIBA)的濕法測(cè)試系統(tǒng)進(jìn)行粒度測(cè)定.將樣品冷凍、干燥、研磨過200目篩后準(zhǔn)確稱取0.1g左右的樣品放入總有機(jī)碳分析儀中(SSM-5000A,日本島津)中利用燃燒氧化-非分散紅外吸收法進(jìn)行測(cè)定TOC的含量.

1.3 正定矩陣因子分析模型

本研究采用美國環(huán)境保護(hù)署推出的PMF模型5.0版(USEPA 2014)對(duì)垂向土壤中PAHs的來源進(jìn)行辨析.其基本方程如式1所示,

=+(1)

式中:(濃度矩陣)是一組×的二維矩陣(為污染物的種類,為樣品的數(shù)量);(貢獻(xiàn)矩陣)為×的矩陣(為不同的污染物)的來源;(源成分譜矩陣)為×的矩陣;(殘差矩陣)的元素可由式2:

式中:、和分別為每個(gè)樣品和每種污染物的貢獻(xiàn)矩陣、源成分譜矩陣和殘差矩陣.PMF模型所得到的目標(biāo)函數(shù)如式3所示:

U是樣品中污染物的不確定性,當(dāng)樣品中的某個(gè)污染物濃度小于或者等于方法檢測(cè)限(MDL)時(shí),U的計(jì)算公式為式(4):

當(dāng)樣品中的某個(gè)污染物濃度大于方法檢測(cè)限(MDL)時(shí),U的計(jì)算方法為式(5):

式中:error fraction指通過分析重復(fù)樣品和標(biāo)準(zhǔn)物質(zhì)獲得的測(cè)量不確定度(20%).

基于SPSS19.0逐步法建立線性回歸方程,以標(biāo)準(zhǔn)化后的總PAH總濃度為因變量,PMF分析確定的標(biāo)準(zhǔn)化后的各個(gè)因子為自變量,擬合線性回歸方程,通過各個(gè)因子的線性回歸系數(shù)定量每個(gè)因子對(duì)PAHs總量的貢獻(xiàn)率.根據(jù)線性回歸系數(shù)計(jì)算每個(gè)因子(每種PAH來源)的貢獻(xiàn)率的公式如式(6):

式中:為因子()的線性回歸系數(shù);Σ為所有因子的線性回歸系數(shù)之和.

2 結(jié)果與討論

2.1 交通干線旁農(nóng)田表層土壤中PAHs的空間分布特征

上海交通干線旁70個(gè)農(nóng)田表層土壤中16種優(yōu)控PAHs均被檢出,總PAHs含量(∑16PAHs)范圍為23.16~21250.25ng/g,平均值為928.16ng/g(圖2). ∑16PAHs含量高于崇明道路旁農(nóng)田土壤(36.77~ 990.25ng/g)[14],主要是由于崇明人口密度稀疏且污染源較少.而與東部經(jīng)濟(jì)較發(fā)達(dá)城市農(nóng)田土壤中∑16PAHs含量保持一致,如蘇州(45.4~3703ng/g)[15]和南京(312.2~27580.9ng/g)[16].本研究中∑16PAHs最高值出現(xiàn)在金山區(qū)S3采樣點(diǎn)120m處,該樣點(diǎn)的∑16PAHs均值也最高(2887.73ng/g),最低值出現(xiàn)在青浦區(qū)S1采樣點(diǎn)20m處.∑16PAHs最低均值點(diǎn)出現(xiàn)在青浦區(qū)的S7采樣點(diǎn)(172.78ng/g),均值最高值是最低值的16.7倍.位于國道、省級(jí)高速公路附近的S1~4采樣點(diǎn)歸于高速公路類,位于城市主、次干道旁的S5~7采樣點(diǎn)歸為城市道路.高速路旁農(nóng)田平均∑16PAHs為966.39ng/g,城市道路旁農(nóng)田平均∑16PAHs為220.68ng/g,高速路的∑16PAHs是城市道路的4.39倍.結(jié)合上海交通局的資料,本研究中4條高速公路,車流量較大,大型客貨車的流量均在5000PCU/12h左右(PCU即標(biāo)準(zhǔn)車當(dāng)量,不同車型轉(zhuǎn)換為標(biāo)準(zhǔn)小汽車車型),占道路總交通流量的10.92%,三條城市道路由于道路管制等原因大型客貨車的流量一般在580PCU/12h左右,僅占道路總交通流量的5.17%.密集的車流量使得高速路旁農(nóng)田PAHs含量顯著高于城市道路旁農(nóng)田.于卿嬋等[17]也指出PAHs濃度與車流量呈正相關(guān)性,同時(shí)怠速車輛增多也使其濃度增大.

圖2 上海交通干線旁農(nóng)田表層土壤PAHs隨防護(hù)林邊距變化

LMW代表2~3環(huán)低環(huán)PAHs;HMW代表4~6環(huán)高環(huán)PAHs

上海交通干線旁農(nóng)田表層土壤PAHs含量隨防護(hù)林邊距變化如圖2所示,在0m距離上土壤中∑16PAHs含量整體較高,在0~40m的范圍內(nèi)基本呈現(xiàn)下降趨勢(shì),隨后又出現(xiàn)上升的變化趨勢(shì),基本在200m處又出現(xiàn)高值,而后繼續(xù)下降.整體來看,7個(gè)樣點(diǎn)農(nóng)田土壤∑16PAHs的含量隨著距離防護(hù)林邊緣的增加整體呈現(xiàn)出先下降后增加而后又下降的趨勢(shì),其中高環(huán)(4~6環(huán))PAHs占主要比率(75.28%),其變化趨勢(shì)與∑16PAHs大致相同.低環(huán)(2~3環(huán))PAHs的變化趨勢(shì)則表現(xiàn)出不確定性,主要是由于2環(huán)PAHs(Nap)具有的易揮發(fā)性[18]導(dǎo)致其在土壤中的存在并不穩(wěn)定,在高溫環(huán)境下Nap更容易釋放到周圍的環(huán)境中.林道輝等[19]通過研究交通道路旁茶園表層土壤發(fā)現(xiàn),汽車尾氣在一定程度上會(huì)造成路旁茶園的PAHs污染,尤其是在路旁50m以內(nèi), PAHs含量也是隨距道路距離的增加而減少.黃翠香等[20]在研究了距離道路0~150m的果園表層土壤后指出,隨著距道路距離的增加,PAHs會(huì)出現(xiàn)降低-升高-再降低的趨勢(shì),且低環(huán)化合物的沉降速率相對(duì)較快.而樣點(diǎn)S1的0m斷面∑16PAHs含量出現(xiàn)高值點(diǎn)可能是因?yàn)樵摂嗝婢嚯x交通干線較近,交通源(尾氣排放、輪胎碎屑和瀝青路面等)排放的PAHs聚集于路邊灰塵中[21],灰塵粒徑越小其PAHs含量越高[22],而較小粒徑的灰塵更容易受到氣流的影響飄散至附近防護(hù)林或農(nóng)田中,盡管防護(hù)林高于農(nóng)田,但雨水會(huì)將地表和植被灰塵沖刷至此.采樣點(diǎn)農(nóng)田附近均有居民房聚集,且距離路基越遠(yuǎn),居民房屋越密集,民用燃煤也是其主要來源.采樣點(diǎn)農(nóng)田大部分以種植水稻作物為主,盡管中國嚴(yán)禁秸稈就地焚燒,但仍有少量村民會(huì)對(duì)廢棄的秸稈進(jìn)行焚燒并把燃燒產(chǎn)物混進(jìn)土壤中,而秸稈焚燒也是低環(huán)多環(huán)芳烴的主要來源之一[23].采樣點(diǎn)大部分農(nóng)田在200m斷面處有一條單車道小路,摩托車與拖拉機(jī)等機(jī)動(dòng)車尾氣的排放加劇了農(nóng)田土壤PAHs污染,從而造成200m斷面的高污染特征.

2.2 交通干線旁土壤柱中PAHs的分布特征

分別采集S1和S6樣點(diǎn)距防護(hù)林邊緣200m處的土壤柱,2樣點(diǎn)土壤柱中∑16PAHs含量分布見圖3.在S1樣點(diǎn)的豆田土壤柱中,0~30cm層PAHs含量隨取樣深度的增加呈現(xiàn)略微下降趨勢(shì),這可能與土壤翻耕有關(guān),隨后土壤柱中PAHs含量隨深度增加出現(xiàn)急劇下降,并在50cm后達(dá)到穩(wěn)定狀態(tài),近地層土壤柱中PAHs含量的均值是穩(wěn)定狀態(tài)的10.59倍.同樣,S6樣點(diǎn)水稻田的0~40cm層PAHs含量隨著距表層深度的降低表現(xiàn)出顯著地波動(dòng)式增長,可能也受到水稻田頻繁翻耕的影響,但整體也呈現(xiàn)隨深度增加而下降的趨勢(shì),表層0~20cm層PAHs含量是穩(wěn)定態(tài)的4.76倍.許峰等[24]在研究綿陽市土壤柱后也發(fā)現(xiàn)在0~20cm層PAHs含量較高,在40cm后含量減少,總體呈現(xiàn)出隨深度增加PAHs含量減少的趨勢(shì).

圖3 PAHs隨深度變化分布

采樣點(diǎn)代表10cm土壤柱

從圖3可知,低環(huán)與高環(huán)PAHs隨深度變化上有所不同,在S1和S6兩土壤柱0~40cm段∑16PAHs變化均較為明顯,且高環(huán)PAHs作為主要成分,分別占∑16PAHs的82.98%、81.79%.而在50cm深度以下,∑16PAHs逐漸趨于穩(wěn)定,低環(huán)化合物比例逐漸升高,分別占據(jù)2個(gè)樣點(diǎn)的49.98%、29.37%,其中三環(huán)PAHs變化最為顯著,穩(wěn)定層土壤中三環(huán)PAHs比例是近表層的2~3倍.相關(guān)研究[25]指出在越深的土壤層其年代越久遠(yuǎn),其能源結(jié)構(gòu)以煤炭、木材為主,主要為三環(huán)化合物.隨著近現(xiàn)代上海城市化和工業(yè)化的快速發(fā)展,能源結(jié)構(gòu)也發(fā)生了較大轉(zhuǎn)變,人均汽車擁有量的迅猛增長也導(dǎo)致道路污染源排放量不斷增加,相較之前農(nóng)業(yè)發(fā)展時(shí)期,PAHs在近表層土壤中的累積量會(huì)顯著增加.同時(shí)低環(huán)PAHs更易揮發(fā),主要存在于氣相中,并在大氣中發(fā)生光化學(xué)降解[26],由于氣體/顆粒分配理論,高環(huán)PAHs多賦存在于大氣顆粒相當(dāng)中[27].而表層土壤中90%以上的PAHs來源于大氣中顆粒物質(zhì)的干和濕沉降[28],因此在近表層土壤中高環(huán)PAHs占比較大.同樣低環(huán)PAHs具有較大的溶解性和活性[29],更易在淋溶作用下向更深層土壤遷移;而Pyr與Bap等高環(huán)PAHs具有較強(qiáng)的疏水性和親脂性[30],更易吸附在表層土壤中的有機(jī)質(zhì)中.費(fèi)佳佳等[31]在利用土壤柱進(jìn)行淋溶模擬實(shí)驗(yàn)中也發(fā)現(xiàn)相對(duì)高分子量的PAHs的遷移量明顯低于低分子量的PAHs.

2.3 土壤理化性質(zhì)與PAHs含量的相關(guān)性

各個(gè)采樣點(diǎn)的TOC含量與粒徑組成情況詳見表1.從空間分布來看,70個(gè)農(nóng)田表層土壤的TOC含量為0.43%~4.79%,平均值為2.03±1.01%,土壤主要由粉砂組成(4~63mm),其占比在60.33%~ 95.42%之間,平均值為87.28±7.20%.所有土壤柱樣品中TOC含量在0.27%~4.26%,平均值為1.85± 1.24%,同樣以粉砂質(zhì)土壤構(gòu)成,總體而言,表層土壤的TOC隨著距防護(hù)林邊緣距離的增加呈現(xiàn)出上升的趨勢(shì),土壤柱中的TOC整體隨著深度的增加而減少.

分別對(duì)表層土壤和土壤柱中TOC及粒徑組成與PAHs含量進(jìn)行相關(guān)性分析,分析結(jié)果見表2.對(duì)于表層土壤,TOC與∑16PAHs、低環(huán)、高環(huán)PAHs均呈現(xiàn)顯著正相關(guān)(=0.328~0.477,<0.01),同樣地,土壤柱中TOC含量也與∑16PAHs、低環(huán)、高環(huán)PAHs有顯著的正相關(guān)(=0.350~0.519,<0.01),表明TOC是PAHs富集的影響因素,同樣也暗示了TOC與PAHs共同轉(zhuǎn)移運(yùn)輸?shù)目赡苄?有研究表明土壤中PAHs會(huì)隨著土壤粒徑的變化而發(fā)生改變[32],本研究中土壤柱PAHs含量與各個(gè)粒度組成并無顯著相關(guān)性,而表層土壤中各個(gè)粒度組成均與∑16PAHs和高環(huán)PAHs呈現(xiàn)顯著地相關(guān)性,其中粉砂與PAHs之間的相關(guān)性最好,有研究表明[33],粉砂中有機(jī)質(zhì)的芳香結(jié)構(gòu)對(duì)PAHs有較高的的親和力,是PAHs的優(yōu)先吸附劑.低環(huán)PAHs與黏土之間沒有相關(guān)性,由于低環(huán)PAHs易揮發(fā),在生成的過程中僅有少部分或者完全不能吸附在土壤顆粒上,與此同時(shí)又參與到土壤-空氣的分配過程中,使得相關(guān)性較差.

表2 農(nóng)田土壤樣品中TOC、粒徑及PAHs之間的Spearman相關(guān)分析

注:**在置信度(雙測(cè))為0.01時(shí),相關(guān)性是顯著的.* 在置信度(雙測(cè))為0.05時(shí),相關(guān)性是顯著的.

2.4 交通干線旁農(nóng)田土壤PAHs源解析

2.4.1 表層土壤PAHs源解析 利用約束PMF受體模型并設(shè)定5%的建模不確定性,獲得的表層農(nóng)田土壤中多環(huán)芳烴的源解析結(jié)果如圖4所示,因子1(1)中主要由Ant、Fl、Phe構(gòu)成,其分別占據(jù)74.74%、73.93%、62.35%,有研究表明Ant、Phe、Pyr、Flu是熱電廠、工業(yè)鍋爐、家庭燃煤取暖的主要示蹤標(biāo)記[34],同樣,Ant、Ace、Phe、Pyr也用來指示生物質(zhì)的燃燒[35],因此推斷因子1代表煤、生物質(zhì)的燃燒.因子2(2)以Nap為主(84.30%),煤焦油和石油蒸餾中的提取物含有大量的Nap,主要用于合成鄰苯二甲酸酐、染料的中間產(chǎn)品,橡膠品及殺蟲劑等多種產(chǎn)品[36].因此因子2定義為化學(xué)生產(chǎn)和金屬冶煉.在因子3(3)中,Acy、Fl和Ace占比較大(分別為36.76%、23.73%、21.35%).大量的文獻(xiàn)顯示,2~3環(huán)PAHs單體多來源于石油產(chǎn)品(包括原油、機(jī)用潤滑油及其他衍生產(chǎn)品)[37-38],推斷因子3與石油的揮發(fā)泄漏有關(guān).在因子4(4)中,Bghip、InP、DahA的貢獻(xiàn)率較高,有60%以上貢獻(xiàn)給因子4,同時(shí)BaP、BkF、BbF也分別達(dá)到55.18%、53.80%、51.66%的比重,由于InP及Bghip是汽車尾氣燃燒源的重要標(biāo)志物[39],BaA、Chr、BbF、BkF是汽油燃燒的主要指示物[40],BaA、Chr、BbF同樣也在柴油燃燒的廢氣中被廣泛發(fā)現(xiàn)[41-42],因此主要由高環(huán)PAHs構(gòu)成的因子4指示交通排放源.

表層農(nóng)田土壤PAHs的線性回歸方程為

基于PMF模型的結(jié)果,不同距離處因子貢獻(xiàn)率的空間變化表現(xiàn)出不同的分布特征.根據(jù)因子貢獻(xiàn)率的平均比率,在靠近道路的土壤與遠(yuǎn)離道路的土壤各因子占比不同(圖5).根據(jù)上海2017統(tǒng)計(jì)年鑒[11]顯示,2016年共消耗4625.62萬t煤炭及3632.98萬t石油,煤炭及石油的燃燒排放都是上海市PAHs污染的重要來源.生活燃煤及生物質(zhì)燃料是農(nóng)田周邊居民做飯、取暖的重要燃料,有研究指出,大量低效率的生物質(zhì)燃燒會(huì)產(chǎn)生并釋放出PAHs[43],同時(shí),農(nóng)作物及水稻秸稈的焚燒也會(huì)產(chǎn)生一定的PAHs.Wang等[44]在探討上海城市和郊區(qū)干濕沉降顆粒物中PAHs來源后指出,交通源和燃煤源是干濕沉降顆粒物中PAHs的主要來源,而郊區(qū)地區(qū)有更多的燃煤源貢獻(xiàn).在0~40m靠近交通道路的土壤受到交通排放的影響較大(38%),而在遠(yuǎn)離交通道路的土壤由于附近居戶生活燃煤的影響,則以煤和生物質(zhì)燃燒源為主(37%).

圖4 PMF模型辨析農(nóng)田表層土壤中PAHs來源

Fig.4 Source identification of PAHs in farm surface soils based on PMF model

圖5 PMF模型辨析表層土壤中PAHs來源貢獻(xiàn)率

2.4.2 土壤柱PAHs源解析 由于土壤柱樣品數(shù)量不足50個(gè),PMF模型對(duì)于不足50個(gè)樣本的運(yùn)行有較大的不穩(wěn)定性,因此采取特征因子比值法對(duì)縱向土壤進(jìn)行源解析.研究表明,Flu、Pyr、InP、BghiP在環(huán)境中的降解速率相對(duì)較低,且特征比值的變化不大,能夠較為完整的保留初始信息,是進(jìn)行判源較理想的特征化合物[45].本文選用Flu/(Flu+Pyr)及InP/ (InP+BghiP)來解析PAHs的來源.當(dāng)Flu/(Flu+Pyr)的比值<0.4時(shí),指示石油泄漏或揮發(fā)源,比值位于0.4~0.5之間時(shí),指示石油燃燒源,當(dāng)比值>0.5時(shí)指示煤、生物質(zhì)燃燒源;當(dāng)InP/(InP+BghiP)的比值<0.2時(shí),指示石油泄漏或揮發(fā)源,比值在0.2~0.5之間時(shí),指示石油燃燒源,當(dāng)比值>0.5時(shí)同樣指示煤、生物質(zhì)燃燒源[46].這2種特征比值在S1與S6樣點(diǎn)土壤柱中隨深度的變化見圖6.

從圖6中可以看出絕大部分樣點(diǎn)的Flu/(Flu+ Pyr)的比值在0.5以上,表明其來源是煤、生物質(zhì)燃燒.在S1樣點(diǎn)土壤中,絕大多數(shù)土壤的InP/(InP+ BghiP)比值>0.5,表明煤、生物質(zhì)燃燒是其主要來源, S6樣點(diǎn)的上層土壤(0~40cm)中的InP/(InP+BghiP)比值大部分<0.5,指示石油燃燒源,深層土壤的InP/ (InP+BghiP)比值均>0.5,指示煤、生物質(zhì)燃燒源.通過各土壤柱中特征比值對(duì)比結(jié)果發(fā)現(xiàn)一般在上層土壤中PAHs污染主要源自于石油燃燒源與煤燃燒源的混合源,而下層土壤均指示煤、生物質(zhì)的燃燒.上層土壤屬于耕作層,每年會(huì)有不同程度的翻作,同時(shí)交通源排放的PAHs也會(huì)沉降在表層土壤中,并隨著雨水淋溶及土壤層翻作進(jìn)入更深的土壤層中;而50cm以下的土壤層一般不會(huì)進(jìn)行土壤翻作,且年代較為久遠(yuǎn)[24],過去以煤炭為主要能源,農(nóng)村地區(qū)是以木材等生物質(zhì)作為日常燃料,使得深層的土壤中的PAHs主要源自于煤、生物質(zhì)的燃燒.

圖6 土壤柱中PAHs的特征比值

3 結(jié)論

3.1 上海市交通干線旁農(nóng)田表層土壤PAHs濃度隨著距防護(hù)林距離的增加,整體呈現(xiàn)出先下降后增加而后又下降的趨勢(shì),農(nóng)田土壤柱中PAHs濃度則呈現(xiàn)出隨著土壤深度的增加而下降的趨勢(shì).同時(shí)表層土壤和土壤柱中PAHs濃度均與TOC含量具有顯著相關(guān)性,暗示TOC是土壤PAHs富集的重要控制因子.

3.2 PMF模型揭示出上海道路沿線農(nóng)田土壤中PAHs主要來自于為煤、生物質(zhì)燃燒和交通排放,其中在0~40m靠近道路的土壤受到交通排放的影響較大,而對(duì)于遠(yuǎn)離道路的土壤(40~500m)由于附近居戶生活的影響,煤、生物質(zhì)燃燒源占比較大.特征因子比值法揭示出0~30cm耕作層土壤中PAHs主要來源于石油與煤燃燒的混合源,而更深層的土壤均指示煤和生物質(zhì)燃燒源.由于從源到匯的過程中不同組分PAHs的降解速率不同,PMF模型及特征因子比值法均有一定的不確定性,未來已期利用同位素標(biāo)記法更為準(zhǔn)確的運(yùn)行PAHs的溯源分析.

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Distribution characteristics and source analysis of PAHs in farmland soils along Shanghai traffic artery.

ZHANG Xi1, YANG Jing1, LIU Min1*, CHEN Xing1, WU Jian2

(1.Key Laboratory of Geographic Information Science of the Ministry of Education, School of Geographic Sciences, East China Normal University, Shanghai 200241, China；2.Shanghai Acadamy of Environmental Sciences, Shanghai 200233, China)., 2019,39(2)：741~749

In order to investigate the influence of traffic emission on the accumulation of 16 polycyclic aromatic hydrocarbons (PAHs) known as priority pollutants in the surrounding farmland soils, seventy soil surface samples and two soil cores samples were collected within 500m of the Shanghai traffic artery. The concentrations, compositions and sources of 16PAHs were systematically analyzed. The results showed that the concentrations of Σ16PAHs in the surface soils of the farmland along the Shanghai traffic artery was 23.16~21250.25ng/g, with an average of 928.16ng/g. Along the increase of distance from roadside shelterbelts, the concentrations of Σ16PAHs showed a trend of decrease first remove then increase, and finally decrease. The concentrations of Σ16PAHs in farmland soil cores showed an increasing trend with the decreasing distance from the soil surface. Positive definite matrix factor analysis (PMF) models and isomer ratios were used to distinguish PAHs sources in soil surfaces and core samples. It was found that they were mainly derived from the combustion of coal and biomass and traffic emissions.

main lines of communication；farmland soil；polycyclic aromatic hydrocarbon；distribution characteristics；source analysis

X142

A

1000-6923(2019)02-0741-09

張 希(1994-),女,河南平頂山人,華東師范大學(xué)碩士研究生,主要從事城市環(huán)境地球化學(xué)過程研究.

2018-07-21

國家自然科學(xué)基金資助項(xiàng)目(41730646,41761144062, 41601526);上海市環(huán)境保護(hù)局重大項(xiàng)目(滬環(huán)科[2016]第5號(hào))

* 責(zé)任作者, 教授, mliu@geo.ecnu.edu.cn