基于土壤氣的場(chǎng)地VOCs污染刻畫及風(fēng)險(xiǎn)評(píng)估

郝辰宇,鐘茂生,姜 林,李吉鴻,馬 琳,汪 洋

基于土壤氣的場(chǎng)地VOCs污染刻畫及風(fēng)險(xiǎn)評(píng)估

郝辰宇,鐘茂生,姜 林*,李吉鴻,馬 琳,汪 洋

(北京市生態(tài)環(huán)境保護(hù)科學(xué)研究院,國(guó)家城市環(huán)境污染控制工程技術(shù)研究中心,污染場(chǎng)地風(fēng)險(xiǎn)模擬與修復(fù)北京市重點(diǎn)實(shí)驗(yàn)室,北京 100037)

在某場(chǎng)地苯重點(diǎn)污染區(qū)布置10個(gè)鉆孔,按規(guī)范采集檢測(cè)了63對(duì)土壤-土壤氣樣品,基于土壤和土壤氣VOCs濃度分別刻畫了場(chǎng)地污染并評(píng)估了風(fēng)險(xiǎn).結(jié)果顯示,土壤采樣和土壤氣采樣兩種方法揭露的污染垂向分布特征基本一致,10個(gè)鉆孔中采集的土壤氣樣品均存在一定程度超標(biāo),超過(guò)土壤氣篩選值1.242mg/m3的樣品比例為35%,最大超標(biāo)1000倍,但僅3個(gè)鉆孔中采集的土壤樣品存在超標(biāo),超過(guò)土壤篩選值1mg/kg的樣品比例為5%,最大超標(biāo)30倍.可見(jiàn),僅檢測(cè)土壤樣品可能低估場(chǎng)地中VOCs的污染范圍和程度,在以砂土為主的場(chǎng)地尤為明顯.采用線性分配模型基于土壤中苯檢出濃度預(yù)測(cè)的健康風(fēng)險(xiǎn)較基于實(shí)測(cè)土壤氣中苯濃度預(yù)測(cè)的風(fēng)險(xiǎn)總體高約1個(gè)數(shù)量級(jí),因?yàn)椤袄匣?、“鎖定”等環(huán)境行為導(dǎo)致實(shí)際污染場(chǎng)地中VOCs在土壤固-液-氣間的分配并不完全遵循瞬間平衡分配原理,線性平衡分配模型預(yù)測(cè)的土壤氣濃度顯著高于實(shí)測(cè)值.此外,不考慮苯系物這類易生物降解的VOCs在傳輸過(guò)程中的生物降解過(guò)程可導(dǎo)致場(chǎng)地實(shí)際健康風(fēng)險(xiǎn)被高估至少2個(gè)數(shù)量級(jí),評(píng)估結(jié)論會(huì)發(fā)生本質(zhì)的變化.因此,我國(guó)應(yīng)盡快啟動(dòng)基于土壤氣的VOCs場(chǎng)地調(diào)查與風(fēng)險(xiǎn)評(píng)估技術(shù)方法的系統(tǒng)研究及相關(guān)標(biāo)準(zhǔn)規(guī)范的制定,為場(chǎng)地環(huán)境管理提供科學(xué)支撐.

土壤氣；揮發(fā)性有機(jī)物；污染刻畫；蒸氣入侵風(fēng)險(xiǎn)

據(jù)估計(jì)中國(guó)污染場(chǎng)地?cái)?shù)量可能超過(guò)100萬(wàn)塊[1],其中揮發(fā)性有機(jī)物(VOCs)在污染場(chǎng)地中普遍存在,檢出率超過(guò)50%[2-3].相比重金屬和半揮發(fā)性有機(jī)物(SVOCs),VOCs具有更強(qiáng)的揮發(fā)性及遷移性,可通過(guò)擴(kuò)散或?qū)α鹘?jīng)建筑物地板裂隙等通道進(jìn)入室內(nèi)對(duì)人體健康造成危害,即蒸氣入侵[4-7].因VOCs易揮發(fā)及場(chǎng)地非均質(zhì)性等特征,場(chǎng)地中VOCs污染刻畫和風(fēng)險(xiǎn)評(píng)估存在較大不確定性.如常州常隆化工廠場(chǎng)地對(duì)其周邊常州外國(guó)語(yǔ)學(xué)校學(xué)生健康的影響,西南某化工場(chǎng)地修復(fù)后開(kāi)發(fā)的新建住宅室內(nèi)空氣質(zhì)量超標(biāo)等事件,均可能與場(chǎng)地中VOCs污染調(diào)查和風(fēng)險(xiǎn)評(píng)估方法不當(dāng)有關(guān).因此,如何開(kāi)展場(chǎng)地VOCs污染調(diào)查及精準(zhǔn)預(yù)測(cè)場(chǎng)地再開(kāi)發(fā)利用后的健康風(fēng)險(xiǎn)是我國(guó)目前面臨的重大挑戰(zhàn)[4].

美國(guó)環(huán)保署(US EPA)1989年首次發(fā)布的場(chǎng)地風(fēng)險(xiǎn)評(píng)估技術(shù)導(dǎo)則,推薦了基于土壤VOCs濃度的Hwang and Falco模型評(píng)估蒸氣入侵風(fēng)險(xiǎn)[8-9].1996年USEPA[10]采用了簡(jiǎn)化的Jury模型替代了Hwang and Falco[11],但這些模型并未考慮VOCs侵入建筑物的過(guò)程.1991年Johnson-Ettinger提出了基于土壤氣VOCs濃度并考慮VOCs在包氣帶擴(kuò)散遷移和經(jīng)由建筑物裂縫的擴(kuò)散與對(duì)流侵入室內(nèi)的蒸氣入侵評(píng)估模型[12],但該模型并不適用于當(dāng)時(shí)基于土壤VOCs含量的場(chǎng)地污染調(diào)查技術(shù)體系.1999年P(guān)ark提出了土壤VOCs線性三相分配模型[13],并嵌入Johnson-Ettinger模型,實(shí)現(xiàn)了通過(guò)線性分配模型將土壤VOCs含量轉(zhuǎn)換成土壤氣VOCs濃度,然后應(yīng)用Johnson-Ettinger模型計(jì)算蒸氣入侵風(fēng)險(xiǎn)的評(píng)價(jià)方法.該方法隨后被各國(guó)廣泛應(yīng)用[7,14-16].我國(guó)發(fā)布的場(chǎng)地調(diào)查與評(píng)估導(dǎo)則[15]主要也采用了該方法.但研究表明,基于土壤VOCs含量的場(chǎng)地污染刻畫和風(fēng)險(xiǎn)評(píng)估方法存在較大的不確定性[4,14-20].2000年后, US EPA開(kāi)始推薦基于土壤氣的場(chǎng)地VOCs污染刻畫,逐步建立了以土壤氣及人居暴露環(huán)境監(jiān)測(cè)為核心的多證據(jù)調(diào)查與風(fēng)險(xiǎn)評(píng)估方法體系[15-22],但相關(guān)技術(shù)導(dǎo)則直到2015年才正式發(fā)布[5-6].美國(guó)蒸氣入侵主要調(diào)查評(píng)估場(chǎng)地土壤或地下水中VOCs對(duì)污染區(qū)及周邊已建成建筑中活動(dòng)人群的健康危害.然而,與歐美國(guó)家不同,中國(guó)大量污染場(chǎng)地僅完成了用地功能置換,處于待開(kāi)發(fā)狀態(tài),難以建立基于建筑物底板下實(shí)測(cè)土壤氣及人居環(huán)境空氣的多證據(jù)調(diào)查評(píng)估技術(shù)體系.因此,模型預(yù)測(cè)是中國(guó)VOCs場(chǎng)地風(fēng)險(xiǎn)評(píng)估的重要工具[4].針對(duì)我國(guó)導(dǎo)則推薦方法存在的問(wèn)題,本文對(duì)比了基于土壤和土壤氣中VOCs檢測(cè)結(jié)果刻畫的場(chǎng)地污染狀況的差異,分析了基于土壤和土壤氣中苯檢測(cè)結(jié)果計(jì)算場(chǎng)地風(fēng)險(xiǎn)對(duì)評(píng)估結(jié)果的影響,討論了中國(guó)VOCs污染場(chǎng)地調(diào)查與風(fēng)險(xiǎn)評(píng)估存在的主要問(wèn)題,提出了下一步的研究建議,以期為我國(guó)場(chǎng)地調(diào)查評(píng)估技術(shù)規(guī)范的修訂提供支撐.

1 方法與材料

1.1 樣品采集與檢測(cè)

圖1 采樣點(diǎn)分布及典型的水文地質(zhì)剖面

案例場(chǎng)地占地約0.09km2,1979～2014年主要生產(chǎn)各種型號(hào)瀝青混合料,原輔材料包括瀝青、重油、柴油、導(dǎo)熱油、砂石料、礦粉、改性劑和乳化劑等.在重點(diǎn)污染設(shè)施儲(chǔ)罐區(qū)布設(shè)了10個(gè)鉆孔(圖1(a)),結(jié)合地層結(jié)構(gòu)及現(xiàn)場(chǎng)便攜式光離子檢測(cè)器(PID)的測(cè)試結(jié)果,通過(guò)重力沖擊鉆在每個(gè)鉆孔不同深度采集5～7個(gè)土壤樣品(圖1(b)),同時(shí)在各土壤采樣位置布設(shè)土壤氣采樣探頭采集對(duì)應(yīng)位置的土壤氣樣品,共采集63對(duì)土壤和土壤氣成對(duì)樣品.其中,土壤樣品按《建設(shè)用地土壤污染狀況調(diào)查與風(fēng)險(xiǎn)評(píng)估技術(shù)導(dǎo)則》(DB11/T 656-2019)[23]中的要求采集,苯含量采用US EPA 8260C進(jìn)行測(cè)定.土壤氣按《污染場(chǎng)地?fù)]發(fā)性有機(jī)物調(diào)查與風(fēng)險(xiǎn)評(píng)估技術(shù)導(dǎo)則》(DB1278- 2015)[24]中的要求采集,采樣體積約1L,苯濃度采用HJ644-2013[25]進(jìn)行測(cè)定.

案例場(chǎng)地地下水埋深約24m,包氣帶土壤以中粗砂夾卵石為主,但在12～16m范圍內(nèi)分布有一層粉土和細(xì)砂層(圖1(b)).

1.2 基于土壤濃度的風(fēng)險(xiǎn)評(píng)估

《建設(shè)用地土壤污染風(fēng)險(xiǎn)評(píng)估技術(shù)導(dǎo)則》(HJ25.3-2019)[15]基于土壤中VOCs含量預(yù)測(cè)建筑物室內(nèi)呼吸暴露風(fēng)險(xiǎn)的模型,假設(shè)污染區(qū)土壤固相、液相及氣相中VOCs相間分配處于線性平衡,從而推導(dǎo)對(duì)應(yīng)土壤采樣點(diǎn)周邊土壤氣中VOCs濃度[15].吸入室內(nèi)空氣來(lái)自下層土壤中氣態(tài)污染物的致癌風(fēng)險(xiǎn)RIsub-s采用HJ25.3-2019的模型計(jì)算,如式(1)所示[15]:

式中:揮發(fā)因子VFsub-soil及暴露因子ExpF分別采用式(2)和式(3)計(jì)算[15]:

(2)

1.3 基于土壤氣濃度的風(fēng)險(xiǎn)評(píng)估

采用Johnson等在1991年提出以實(shí)測(cè)土壤氣濃度預(yù)測(cè)VOCs侵入建筑物室內(nèi)造成的健康風(fēng)險(xiǎn)的評(píng)估模型[12].揮發(fā)因子VFsub-sg可用式(8)計(jì)算[12]:

因此,基于實(shí)測(cè)土壤氣濃度的致癌風(fēng)險(xiǎn)RIsub-sg可用式(9)計(jì)算:

以上模型中參數(shù)的定義及取值見(jiàn)表1.

表1 參數(shù)定義及取值

2 結(jié)果與討論

2.1 場(chǎng)地污染刻畫

2.1.1 VOCs濃度及空間分布 檢測(cè)結(jié)果統(tǒng)計(jì)顯示(表2),研究區(qū)土樣中苯檢出率17.2%,但僅4.7%的樣品苯含量超過(guò)了《土壤環(huán)境質(zhì)量建設(shè)用地土壤污染風(fēng)險(xiǎn)管控標(biāo)準(zhǔn)(試行)》(GB36600-2018)[26]中居住用地篩選值1mg/kg.土壤氣樣品中苯檢出率54.7%,遠(yuǎn)高于土壤中苯檢出率,且34.4%的樣品中苯濃度超過(guò)了《污染場(chǎng)地?fù)]發(fā)性有機(jī)物調(diào)查與風(fēng)險(xiǎn)評(píng)估技術(shù)導(dǎo)則》(DB11/T 1278-2015)[24]中居住用地情景下土壤氣中苯篩選值1.242mg/m3.

表2 樣品苯檢測(cè)結(jié)果統(tǒng)計(jì)

濃度垂向分布特征顯示(圖2(a)),研究區(qū)淺層土壤受污染較輕,0～12m深度內(nèi)采集的30個(gè)土樣中苯含量均低于檢出限0.05mg/kg.深度12m以下土壤中苯檢出率33.3%,最大值30.70mg/kg,平均值3.67mg/kg,主要集中在12～16m的粉土層, 16m以下土樣中苯基本未檢出.此深度范圍內(nèi)3個(gè)土樣中苯含量超過(guò)篩選值,涉及S8～S10號(hào)鉆孔.含量最高的樣品出現(xiàn)在S10,超標(biāo)29.7倍,該點(diǎn)臨近場(chǎng)地歷史生產(chǎn)過(guò)程中重油儲(chǔ)罐區(qū)域中心位置,樣品深度12.5m,S8位于場(chǎng)地儲(chǔ)罐區(qū)域的西北側(cè),超標(biāo)樣品深度15.4m.因此,這兩個(gè)點(diǎn)位超標(biāo)可能是由于儲(chǔ)罐或這一區(qū)域的地下連接管線泄漏所致.S9位于儲(chǔ)罐區(qū)域北側(cè)邊界處,超標(biāo)樣品深度13.5m,推測(cè)其超標(biāo)原因可能是場(chǎng)地12～16m深度范圍的粉土層在場(chǎng)地內(nèi)不連續(xù),歷史上場(chǎng)地所在區(qū)域地下水水位高過(guò)12～16m位置的粉土層,儲(chǔ)罐區(qū)污染物滲漏進(jìn)入地下水后隨地下水遷移至S9所在區(qū)域,導(dǎo)致地下水水位下降后污染物依然被粉土層吸附而出現(xiàn)超標(biāo).

土壤氣中苯濃度垂向分布特征和土壤基本一致(圖2(b)),0～9m土壤氣中苯基本未檢出,9～16m土壤氣中苯濃度0.14～794.94mg/m3,平均濃度 112.2mg/m3,呈隨深度增加濃度總體升高的趨勢(shì),檢出濃度較高的土壤氣樣品主要集中在16m附近的粉土層.與土壤樣品檢測(cè)結(jié)果相比,9～12m土壤氣樣品苯檢出率較高,可能是12m之下土壤氣中苯垂直向上遷移所致.16m以下土壤氣中苯濃度0.09～ 1179.37mg/m3, 平均濃度 93.8mg/m3,總體隨深度增加濃度呈降低趨勢(shì),但這一深度范圍內(nèi)土壤苯基本未檢出,可能是滲漏初期污染主要集中在12～16m粉土層,土壤氣中苯在濃度梯度驅(qū)動(dòng)下向下傳輸導(dǎo)致污染物在16m以下的砂土層中逐步富集.滲漏污染切斷后,賦存在粉土層中的苯逐步被生物降解而濃度降低,但16m以下的砂土層因微生物活性較低,污染物一直維持在較高濃度.

對(duì)比顯示,研究區(qū)域34.4%的土壤氣樣品中苯濃度超過(guò)了居住用地篩選值,最大超標(biāo)約948.6倍,顯著高于土壤樣品的最大超標(biāo)倍數(shù).此外,10個(gè)鉆孔在16m附近均存在超標(biāo)的土壤氣樣品,揭露的污染范圍明顯大于利用土壤超標(biāo)點(diǎn)刻畫的污染范圍.因此, 相比于目前國(guó)內(nèi)推薦的基于土壤含量調(diào)查的方法,基于場(chǎng)地土壤氣濃度的調(diào)查方法可更加靈敏的反應(yīng)場(chǎng)地污染狀況,這一結(jié)論和Zhang等[27-28]的研究類似.

2.1.2 土壤VOCs采樣檢測(cè)的不確定性 成對(duì)土壤-土壤氣樣品中苯檢測(cè)結(jié)果分析顯示(圖3),土壤中苯檢出且含量超過(guò)篩選值的樣品主要為粉土,但不同土質(zhì)周圍的土壤氣樣品中均檢出苯.其中,粗砂層土壤氣平均濃度3.7mg/m3,最高51.9mg/m3.細(xì)砂層土壤氣平均濃度86.3mg/m3,最高1179.4mg/m3.粉土土壤氣平均濃度83.0mg/m3,最高794.9mg/m3.可見(jiàn),土壤氣中苯雖然主要賦存在細(xì)砂和粉土層,但部分粗砂層土壤氣中苯也有檢出.以S2和S4號(hào)鉆孔為例,由圖2可知,這兩個(gè)鉆孔主要污染深度12～20m范圍內(nèi)土壤中苯含量均低于檢出限0.05mg/kg,但同一采樣位置土壤氣中苯濃度為1.36～27.23mg/m3,高于檢出限27～544倍.此外,S10號(hào)鉆孔在18.5m處的土壤中苯未檢出,但對(duì)應(yīng)位置土壤氣中苯濃度高達(dá)1179.4mg/m3,兩者揭露的污染水平差異顯著.結(jié)合土壤質(zhì)地分析發(fā)現(xiàn),土壤中苯未檢出但土壤氣中苯檢出的樣品均為細(xì)砂或粗砂,因此進(jìn)一步對(duì)所有土壤和土壤氣成對(duì)樣品檢測(cè)結(jié)果進(jìn)行分析顯示,土壤和土壤氣中苯均有檢出的樣品主要為粉土,而土壤氣中苯有檢出但土壤中苯未檢出的樣品主要為細(xì)砂和粗砂.造成上述現(xiàn)象的原因主要包括:(1)細(xì)砂和粗砂孔隙度相對(duì)較大,有機(jī)質(zhì)含量低,土壤氣相中污染含量相對(duì)較高,即便按規(guī)范采用了低擾動(dòng)鉆探采樣技術(shù),但在實(shí)際采樣過(guò)程中土壤孔隙氣體中的污染物更容易逃逸[27-30]; (2)土壤樣品檢測(cè)結(jié)果一般只代表采樣點(diǎn)的濃度,但研究發(fā)現(xiàn),場(chǎng)地土壤的異質(zhì)性導(dǎo)致污染物濃度往往在dm級(jí)范圍內(nèi)可相差數(shù)個(gè)數(shù)量級(jí),非連續(xù)的土壤采樣可能會(huì)錯(cuò)過(guò)高值點(diǎn)[29],土壤氣樣品檢測(cè)結(jié)果代表的是采樣點(diǎn)一定范圍內(nèi)土壤氣中污染物的平均濃度,因此,受異質(zhì)性影響相對(duì)較小;(3)分配模型計(jì)算顯示,當(dāng)土壤中苯含量達(dá)到檢出限0.05mg/kg時(shí)對(duì)應(yīng)土壤氣中苯濃度可達(dá)10mg/m3,高于土壤氣檢出限0.05mg/m3的200倍.綜上,由于土壤采樣過(guò)程中的逃逸、土壤污染分布異質(zhì)性以及檢出限的靈敏性,無(wú)論水平分布還是垂向分布,基于土壤氣苯濃度刻畫的檢出范圍、超標(biāo)程度和超標(biāo)范圍都大于基于土壤苯含量刻畫的結(jié)果.因此,目前國(guó)內(nèi)基于土壤中VOCs濃度的場(chǎng)地調(diào)查技術(shù)可能會(huì)低估場(chǎng)地VOCs的污染水平,特別是以砂土為主的污染場(chǎng)地,僅采集分析土壤樣品中的VOCs含量存在不能客觀的反應(yīng)場(chǎng)地VOCs實(shí)際污染狀況的風(fēng)險(xiǎn).

圖3 不同土質(zhì)土壤和土壤氣中苯濃度

2.2 健康風(fēng)險(xiǎn)評(píng)估

2.2.1 基于土壤和土壤氣VOCs的風(fēng)險(xiǎn)比較 案例場(chǎng)地未來(lái)將作為居住用地開(kāi)發(fā).因土壤和土壤氣中苯主要在地面以下12～20m中的土層中檢出,因此選定距地表12～20m的污染土層作為蒸氣入侵的污染源,其頂部埋深設(shè)置為12m.考慮到未來(lái)建筑地下室層高2.2m,建筑物底板厚度0.35m,則污染源頂部距未來(lái)建筑底板的距離為9.35m.污染源強(qiáng)度采用各鉆孔12～20m深度范圍內(nèi)樣品的最高濃度.除鉆孔S2和S4因土壤中苯均未檢出未計(jì)算健康風(fēng)險(xiǎn)外,對(duì)其余8個(gè)鉆孔分別基于土壤和土壤氣中苯檢測(cè)結(jié)果計(jì)算風(fēng)險(xiǎn)并進(jìn)行比較.

結(jié)果顯示,按照導(dǎo)則HJ25.3-2019中基于土壤苯含量計(jì)算的致癌風(fēng)險(xiǎn)為2.6×10?7～4.7×10?5,平均風(fēng)險(xiǎn)8.0×10-6,除S1號(hào)鉆孔外,其余7個(gè)鉆孔的風(fēng)險(xiǎn)均超過(guò)了可接受水平10-6.基于實(shí)測(cè)土壤氣中苯濃度計(jì)算的健康風(fēng)險(xiǎn)為1.8×10?8～9.4×10?6,平均風(fēng)險(xiǎn)2.1×10-6,僅S6,S8,S9和S10這4個(gè)鉆孔的風(fēng)險(xiǎn)值超過(guò)了可接受水平10-6.對(duì)比顯示(圖4),同一點(diǎn)位基于土壤苯含量預(yù)測(cè)的健康風(fēng)險(xiǎn)基本高于基于實(shí)測(cè)土壤氣中苯濃度的預(yù)測(cè)結(jié)果,其差別總體在1個(gè)數(shù)量級(jí)以內(nèi),但最高可達(dá)64.7倍.

圖4 基于土壤和土壤氣濃度的風(fēng)險(xiǎn)比較

如圖4所示,盡管土壤鉆探采樣過(guò)程存在的污染物逃逸可能導(dǎo)致場(chǎng)地污染水平被低估,但采用導(dǎo)則基于土壤中苯含量的預(yù)測(cè)模型進(jìn)行健康風(fēng)險(xiǎn)評(píng)估依然可能高估風(fēng)險(xiǎn).對(duì)比各鉆孔土壤苯含量最高的樣品對(duì)應(yīng)位置實(shí)測(cè)土壤氣中苯濃度與采用導(dǎo)則模型基于土壤中苯含量預(yù)測(cè)的土壤氣苯濃度的結(jié)果顯示(圖5),基于土壤中苯含量預(yù)測(cè)的對(duì)應(yīng)位置土壤氣中苯濃度是實(shí)測(cè)值的0.9～64.7倍,平均12倍.原因是導(dǎo)則中的風(fēng)險(xiǎn)評(píng)估方法假設(shè)VOCs在土壤固-液-氣三相間始終保持線性平衡分配狀態(tài),因此推薦采用線性分配模型基于土壤VOCs含量預(yù)測(cè)對(duì)應(yīng)采樣位置土壤氣中VOCs濃度.但研究表明VOCs在土壤中的相間分配屬于非線性平衡[31-34],部分VOCs會(huì)被老化鎖定在土壤有機(jī)質(zhì)中,解吸速率降低,甚至難以解吸,因此導(dǎo)則中推薦的線性瞬間平衡分配模型的計(jì)算結(jié)果通常高于實(shí)測(cè)結(jié)果[35-36],最終造成基于土壤中VOCs含量預(yù)測(cè)的蒸氣入侵風(fēng)險(xiǎn)通常過(guò)于保守,進(jìn)一步增加了僅采集檢測(cè)土壤中VOCs含量的場(chǎng)地調(diào)查與風(fēng)險(xiǎn)評(píng)估方法的不確定性.針對(duì)目前采用線性平衡分配模型基于土壤中VOCs含量評(píng)估健康風(fēng)險(xiǎn)存在的問(wèn)題,Zhang等[27,37]開(kāi)發(fā)了耦合非線性和氣-固界面吸附過(guò)程的相間分配模型,Man等[38]開(kāi)發(fā)了融合更多土壤理化參數(shù)的機(jī)器學(xué)習(xí)算法,均可在一定程度上提高土壤氣中VOCs濃度的預(yù)測(cè)精度,但與實(shí)測(cè)土壤氣濃度仍存在一定差異.

圖5 模型預(yù)測(cè)與實(shí)測(cè)土壤氣苯濃度對(duì)比

2.2.2 土壤氣傳輸過(guò)程生物降解對(duì)風(fēng)險(xiǎn)的影響 好氧環(huán)境下苯系物易被微生物降解[39],但現(xiàn)有國(guó)家導(dǎo)則推薦的基于土壤VOCs含量的風(fēng)險(xiǎn)預(yù)測(cè)方法未考慮VOCs在土層傳輸過(guò)程中的生物降解.研究區(qū)土壤氣中苯濃度垂向分布特征顯示(圖2(b)),土壤氣中苯自16m遷移至12m的過(guò)程中濃度顯著下降,存在生物降解的可能.采用Pasteris等[40]的方法對(duì)各鉆孔土壤氣中苯濃度分布擬合求取苯垂向遷移過(guò)程中的表觀一級(jí)生物降解系數(shù),結(jié)果顯示(圖6),其幾何均值為0.0013h-1(0.0001～0.029h-1),低于DeVaull等[41]報(bào)道的0.79h-1(0.06～11h-1)約2個(gè)數(shù)量級(jí),可能是因?yàn)檠芯繀^(qū)以砂質(zhì)粉土為主,微生物數(shù)量較低.

進(jìn)一步采用Yao等[42]開(kāi)發(fā)的耦合生物降解過(guò)程的PVI-2D模型計(jì)算每個(gè)鉆孔考慮苯垂向遷移時(shí)微生物等降解過(guò)程的蒸氣入侵風(fēng)險(xiǎn),結(jié)果顯示(圖7)考慮生物降解后苯的健康風(fēng)險(xiǎn)進(jìn)一步降低至5.5×10-32～5.6×10-9(平均值6.2×10-10),均遠(yuǎn)低于可接受致癌風(fēng)險(xiǎn)水平1×10-6,也低于未考慮生物降解時(shí)的健康風(fēng)險(xiǎn)2～26個(gè)數(shù)量級(jí).因此,如考慮生物降解,案例場(chǎng)地?zé)o需采取相應(yīng)的風(fēng)險(xiǎn)管控或治理修復(fù),與不考慮生物降解情形下的評(píng)估結(jié)論有本質(zhì)區(qū)別.可見(jiàn),目前我國(guó)關(guān)于蒸氣入侵風(fēng)險(xiǎn)預(yù)測(cè)模型中針對(duì)苯系物等易生物降解的VOCs不考慮其微生物降解將嚴(yán)重高估場(chǎng)地的實(shí)際風(fēng)險(xiǎn),造成管理決策保守.

圖6 生物降解系數(shù)擬合

圖7 生物降解對(duì)風(fēng)險(xiǎn)計(jì)算結(jié)果的影響

3 結(jié)論

3.1 僅采集檢測(cè)土壤不能精準(zhǔn)刻畫場(chǎng)地中VOCs污染程度及空間分布,可能低估風(fēng)險(xiǎn),以砂土為主的場(chǎng)地這一問(wèn)題尤為突出,監(jiān)測(cè)土壤氣能很好的解決這一問(wèn)題.

3.2 采用線性平衡分配模型基于土壤VOCs含量預(yù)測(cè)土壤氣VOCs濃度的風(fēng)險(xiǎn)評(píng)估方法可能高估室內(nèi)蒸氣入侵風(fēng)險(xiǎn),基于實(shí)測(cè)土壤氣中VOCs濃度的風(fēng)險(xiǎn)評(píng)估方法能很好的解決這一問(wèn)題.

3.3 對(duì)于苯系物等易生物降解的VOCs,不考慮生物降解將顯著高估實(shí)際風(fēng)險(xiǎn).

4 建議

我國(guó)應(yīng)盡快啟動(dòng)基于土壤氣的場(chǎng)地VOCs污染調(diào)查與風(fēng)險(xiǎn)評(píng)估技術(shù)方法的系統(tǒng)研究及標(biāo)準(zhǔn)制定,以科學(xué)支撐場(chǎng)地環(huán)境管理, 包括但不限于:1)適用于不同水文地質(zhì)條件的土壤氣調(diào)查技術(shù)方法及配套的設(shè)備材料,如適用于低滲透高含水率地層的定量被動(dòng)土壤氣監(jiān)測(cè)方法;2)耦合實(shí)際場(chǎng)地中VOCs多相多界面分配與傳輸降解反應(yīng)過(guò)程的精準(zhǔn)風(fēng)險(xiǎn)評(píng)估模型.

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Characterization and vapor intrusion risk assessment of VOCs in contaminated sites based on soil gas.

HAO Chen-yu, ZHONG Mao-sheng, JIANG Lin*, LI Ji-hong, MA Lin, WANG Yang

(Beijing Key Laboratory for Risk Modeling and Remediation of Contaminated Sites, National Engineering Research Centre of Urban Environmental Pollution Control, Beijing Municipal Research Institute of Eco-Environmental Protection, Beijing 100037, China)., 2023,43(11)：5700～5708

Contamination characterization and vapor intrusion risk evaluation based on volatile organic compounds (VOCs) in bulk soil were recommended in the technical guidelines of China. However, these guidelines are found to be full of uncertainties in practical use. To evaluate its effectiveness, ten boreholes were drilled within an area contaminated by benzene in a decommissioned site, and 63 paired soil and soil gas samples were collected and analyzed following the technical guidelines. The results revealed that benzene profiles in the soil were similar, as revealed by both the soil and soil gas analyses. The benzene content in 35% of soil gas samples was above the screening level of 1.242mg/m3, and it was distributed in all the ten boreholes. The maximum concentration was nearly 1000 times the screening value. However, in only 5% of soil samples, benzene content was above the soil screening value of 1mg/kg, and the maximum concentration was about 30 times the screening value. Therefore, it can be concluded that analyzing the bulk soil may underestimate the VOC contamination in sites, especially for sites composed of sandy formations. The risk quantified based on soil contents using the linear partition equation was about one order of magnitude higher than that estimated based on measured soil gas concentration. The reason is that the partition of VOCs among soil solid, water, and vapor phases does not follow the linear equilibrium partition exactly due to the aging and sequestration of contaminants by organic matter in the soil. The vapor concentration is highly overestimated using the linear equilibrium equation. Additionally, risk can be overestimated by over two orders of magnitude without considering biodegradation throughout the intrusion process for VOCs (e.g., BTEX) that can be degraded. The results may be entirely different. Therefore, systematic research on VOC contamination and risk characterization, as well as relevant technical guidelines based on soil gas, is suggested to be initiated.

soil gas；volatile organic compounds (VOCs)；contamination characterization；vapor intrusion risk

X511

A

1000-6923(2023)11-5700-09

郝辰宇(1996-),女,陜西榆林人,研究實(shí)習(xí)員,碩士,主要從事場(chǎng)地污染物歸趨模擬與風(fēng)險(xiǎn)評(píng)估研究.發(fā)表論文2篇.haochenyu@cee.cn.

郝辰宇,鐘茂生,姜 林,等.基于土壤氣的場(chǎng)地VOCs污染刻畫及風(fēng)險(xiǎn)評(píng)估 [J]. 2023,43(11):5700-5708.

Hao C Y, Zhong M S, Jiang L, et al. Characterization and vapor intrusion risk assessment of VOCs in contaminated sites based on soil gas [J]. China Environmental Science, 2023,43(11):5700-5708.

2023-03-16

國(guó)家自然科學(xué)基金資助項(xiàng)目(42177404)

* 責(zé)任作者, 研究員, jianglin@cee.cn