冬季大黑汀水庫沉積物-水界面氮磷賦存特征及交換通量

文帥龍,吳 濤,楊 潔,李 鑫,龔琬晴,鐘繼承

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冬季大黑汀水庫沉積物-水界面氮磷賦存特征及交換通量

文帥龍1,2,吳 濤3,楊 潔3,李 鑫1,4,龔琬晴1,2,鐘繼承1*

(1.中國科學(xué)院南京地理與湖泊研究所湖泊與環(huán)境國家重點(diǎn)實(shí)驗(yàn)室,江蘇 南京 210008；2.中國科學(xué)院大學(xué)資源與環(huán)境學(xué)院,北京 100049；3.天津市水利科學(xué)研究院,天津 300061；4.蘇州科技大學(xué)環(huán)境科學(xué)與工程學(xué)院,江蘇 蘇州 215009)

本文研究了大黑汀水庫表層沉積物碳氮磷污染負(fù)荷及分布特征,利用Peeper (pore water equilibriums)技術(shù)獲取沉積物-水界面氮磷剖面特征,分析大黑汀水庫間隙水氮磷分布的空間差異;采集沉積物無擾動(dòng)柱樣用靜態(tài)培養(yǎng)法對其水土界面氮磷交換速率進(jìn)行估算.結(jié)果表明:沉積物中TN、TP和TOC的含量分別在729~5894mg/kg、1312~2439mg/kg和0.5%~5.6%之間,沉積物中氨氮(NH4+-N)、硝酸鹽氮(NO3－-N)、亞硝酸鹽氮(NO2－-N)和活性磷(PO43－-P)含量分別在0.6~202.9、34.4~168.3、0.1~0.3和16.1~75.2mg/kg之間,主要表現(xiàn)為下游含量高于上游,空間分布特征明顯;沉積物C/N表明該水庫有機(jī)質(zhì)主要來源于水體內(nèi)部,與人類網(wǎng)箱養(yǎng)殖活動(dòng)有關(guān).間隙水中NH4+-N和PO43－-P濃度遠(yuǎn)高于上覆水,表明大黑汀水庫間隙水具有向上覆水體擴(kuò)散營養(yǎng)鹽的潛力.在垂直方向上間隙水中NH4+-N濃度隨深度的增加而變大,PO43－-P濃度具有在0~4cm快速增加,之后表現(xiàn)出逐漸降低的趨勢.靜態(tài)釋放結(jié)果表明,沉積物-水界面NH4+-N和PO43－-P的交換通量分別為3.5~110.5mg/(m2·d)和0.1~1.6mg/(m2·d),NO3－-N和NO2－-N交換通量在-112.5~157.2mg/(m2·d)和0.04~0.94mg/(m2·d)之間.NH4+-N、NO3－-N和PO43－-P在下游表現(xiàn)出較高的釋放速率.較高的沉積物內(nèi)源負(fù)荷使得大黑汀水庫沉積物具有較大的向上覆水釋放營養(yǎng)鹽的潛力,改善水庫沉積物污染狀況是治理大黑汀水庫水體環(huán)境的必要之舉.

大黑汀水庫；營養(yǎng)鹽；沉積物-水界面；間隙水；釋放通量

氮磷是水體環(huán)境中最主要的營養(yǎng)鹽,往往被視為水生生態(tài)系統(tǒng)初級(jí)生產(chǎn)力的限制因子,其在沉積物-水界面(Sediment-Water Interface,SWI)的遷移轉(zhuǎn)化過程在水生生態(tài)系統(tǒng)中扮演著重要的作用[1].湖庫沉積物中較高的營養(yǎng)鹽負(fù)荷在某些條件下會(huì)以間隙水為介質(zhì),通過擴(kuò)散、對流以及再懸浮等過程向上覆水體釋放,從而使得沉積物成為上覆水體的內(nèi)在污染源[2-3].內(nèi)源氮磷釋放風(fēng)險(xiǎn)與氮磷形態(tài)、上覆水氮磷質(zhì)量濃度、間隙水氮磷剖面特征、孔隙度和有機(jī)質(zhì)含量間的關(guān)系最為密切[4-5].各種因素之間的相互作用極大程度影響了沉積物中氮磷的形態(tài)和分布特征[6].近年來我國水庫的環(huán)境問題凸顯,保障水源型湖庫飲用水供給和控制水生生態(tài)系統(tǒng)富營養(yǎng)化水平,降低藻類爆發(fā)帶來的危害并改善水質(zhì),減少人為因素引起的營養(yǎng)鹽輸入已成為湖庫污染控制的共識(shí)[7-8].明晰湖庫水體富營養(yǎng)化現(xiàn)狀,底泥污染水平及內(nèi)源釋放強(qiáng)度,便于為后續(xù)的湖庫富營養(yǎng)化及污染治理提供決策依據(jù).然而現(xiàn)階段,我國關(guān)于沉積物污染及內(nèi)源釋放速率的研究主要集中在影響較大的湖庫,而許多與當(dāng)?shù)仫嬘盟┙o直接相關(guān)的水源性湖庫在沉積物污染及內(nèi)源釋放方面的研究還存在著比較大的缺失.

污染物在沉積物-水界面的遷移轉(zhuǎn)化過程十分復(fù)雜,認(rèn)識(shí)沉積物-水界面結(jié)構(gòu)以及污染物在沉積物-水界面的擴(kuò)散過程,對探討污染物環(huán)境行為具有重要的理論和現(xiàn)實(shí)意義[9].間隙水作為水土界面交換的重要介質(zhì),在溫度、溶氧、pH值、鹽度、有機(jī)質(zhì)、生物擾動(dòng)等因素的作用下,其營養(yǎng)鹽濃度梯度會(huì)發(fā)生顯著的變化,進(jìn)而影響在沉積物-水界面的交換通量[9-10].目前國內(nèi)外研究沉積物-水界面擴(kuò)散通量的方法有孔隙水濃度梯度法[10-11]、原位箱測定法[12-13]、質(zhì)量平衡法[9]和實(shí)驗(yàn)室培養(yǎng)法[14-15].孔隙水濃度梯度法操作交簡單,但是容易忽略底棲擾動(dòng)、水動(dòng)力等因素的影響,而且在濃度梯度的選取模擬上存在一定的主觀性[9].原位箱測定法相對準(zhǔn)確,但實(shí)際操作比較復(fù)雜,對條件要求較高.質(zhì)量平衡法相對工作量大,誤差也大,適用于大范圍水域的估算[16].而實(shí)驗(yàn)室培養(yǎng)法相對容易操作,變量可控,也能較好的反映泥水界面營養(yǎng)鹽交換的源匯關(guān)系.

大黑汀水庫承載著唐山、天津兩市居民的生產(chǎn)生活用水的需求,其上接潘家口水庫來水,下通過灤河干流向唐山、天津兩市進(jìn)行供水.在過去幾十年中,大黑汀水庫由于長期的網(wǎng)箱養(yǎng)殖,水庫水體及底泥污染嚴(yán)重,水質(zhì)日益惡化,嚴(yán)重威脅了唐山、天津兩市生產(chǎn)生活用水需求.為了防止水體進(jìn)一步惡化,改善水體生態(tài)環(huán)境,該水庫管理處于2016年開展了整個(gè)庫區(qū)養(yǎng)魚網(wǎng)箱拆除工作,意在減少人為活動(dòng)對水體的影響.然而由于長期的污染物積累,大黑汀水庫內(nèi)源污染負(fù)荷嚴(yán)重.當(dāng)外源污染逐步控制后,如何有效控制來自沉積物的內(nèi)源氮磷負(fù)荷就成為了湖庫水體治理的關(guān)鍵[17].本文對大黑汀水庫沉積物營養(yǎng)鹽賦存特征、間隙水營養(yǎng)鹽剖面特征進(jìn)行了系統(tǒng)的研究,并基于實(shí)驗(yàn)室培養(yǎng)法研究了沉積物-水界面營養(yǎng)鹽擴(kuò)散通量,以期闡明大黑汀水庫沉積物污染負(fù)荷、內(nèi)源營養(yǎng)鹽釋放水平及潛在的釋放風(fēng)險(xiǎn).本文采用了高分辨率(mm級(jí))透析式間隙水被動(dòng)采樣裝置(Peeper)對沉積物-水界面間隙水微剖面特征進(jìn)行了采樣及表征,該技術(shù)能夠更好地刻畫沉積物-水界面微環(huán)境營養(yǎng)鹽的賦存特征及遷移潛力.本文的研究結(jié)果能夠辨識(shí)大黑汀水庫網(wǎng)箱拆除后沉積物污染現(xiàn)狀及內(nèi)源負(fù)荷,為后續(xù)的水體富營養(yǎng)化控制及污染底泥治理提供參考依據(jù).

1 材料與方法

1.1 研究區(qū)概況

大黑汀水庫(40°11'~40°21'N,118°15'~118°19'E)位于唐山市遷西縣城北5km,潘家口水庫下游30km處,1986年建成投產(chǎn),總庫容為3.37億m3,主要承接潘家口水庫的調(diào)節(jié)水量及灤河支流灑河來水,有效庫容2.24億m3,正常蓄水位133m,全長約22km,平均水深13.84m.大黑汀水庫控制流域面積35100km2,占灤河流域面積的78%,多年平均徑流量28.28億m3.大黑汀水庫的作用是承接潘家口水庫的調(diào)節(jié)水量,調(diào)節(jié)灑河來水,抬高水位,下接還鄉(xiāng)河和陡河,通過引灤入津輸水工程向天津、唐山市供水.大黑汀水庫及上游潘家口水庫保障了津、唐兩市供水區(qū)630萬人的生活飲用水和生產(chǎn)水源供給,對當(dāng)?shù)氐纳a(chǎn)發(fā)展和經(jīng)濟(jì)建設(shè)起著非常重要的作用.

1.2 樣品的采集與處理

野外的采樣工作于2017年12月中旬開展,采樣點(diǎn)分布如圖1所示,自水庫下游壩前至上游設(shè)置8個(gè)采樣點(diǎn),用重力采樣器(直徑為90mm)采集柱狀沉積物,每點(diǎn)采集4根柱子作為平行樣,并用抓斗式采泥器采集表層沉積物樣品;在每個(gè)采樣點(diǎn)用多參數(shù)水質(zhì)儀(YSI)探測表層水柱及沉積物-水界面處的理化參數(shù).同時(shí)采集底層原位上覆水10L備用.采集的柱狀沉積物上部覆蓋有約20cm深的原位底層水,然后用橡膠塞封口避免在轉(zhuǎn)移的過程中發(fā)生擾動(dòng)及氧化.樣品采集后迅速運(yùn)輸?shù)轿挥诖蠛谕∷畮靿吻暗囊秊垂こ坦芾砭执蠛谕∷畮旃芾硖帉?shí)驗(yàn)室進(jìn)行前處理及靜態(tài)釋放試驗(yàn).

圖1 大黑汀水庫采樣點(diǎn)分布 Fig.1 Distribution of sampling sites in Daheiting Reservoir

采集的水樣運(yùn)到實(shí)驗(yàn)室后立即放入冰箱中4℃低溫保存.沉積物-水界面靜態(tài)釋放實(shí)驗(yàn)在室內(nèi)將沉積物柱狀樣中上覆水用虹吸法抽去,再用虹吸法沿壁小心滴注已過濾的原采樣點(diǎn)水樣,至液面高度距沉積物表面20cm處停止(此時(shí)水柱體積為1135mL),標(biāo)注刻度,將所有柱狀樣蔽光室內(nèi)按照水庫原位溫度進(jìn)行恒溫培養(yǎng).此后在指定時(shí)間(0、12、24、36、48、60、72h)用移液管于水柱中靠近沉積物表層5cm處取樣,每次取樣體積為50mL,同時(shí)用各原位初始過濾水樣補(bǔ)充至刻度以保持水量平衡,實(shí)驗(yàn)于72h(3d)后終止,培養(yǎng)過程中采集的水樣用干凈的100mL塑料瓶盛裝并及時(shí)放入冰箱中進(jìn)行冷凍保存.待實(shí)驗(yàn)結(jié)束后樣品迅速帶回位于南京的湖泊與環(huán)境國家重點(diǎn)實(shí)驗(yàn)室分析.

沉積物-水界面營養(yǎng)鹽交換速率()按下式計(jì)算[14]:

式中:為平均交換通量,mg/(m2·d);為柱中上覆水體積,L;c、0、c-1為第次、0次(即初始)和-1次采樣時(shí)某物質(zhì)質(zhì)量濃度,mg/L;a為添加水樣中的物質(zhì)質(zhì)量濃度,mg/L;V-1為第-1次采樣體積,L;為柱樣中水-沉積物接觸面積,m2;為釋放時(shí)間,d.所計(jì)算的營養(yǎng)鹽釋放速率均為3d平均表觀交換通量.

利用透析式間隙水采樣技術(shù)(Pore Water Equilibriums, Peeper)[18]獲取沉積物-水界面氮磷剖面特征,分析大黑汀水庫間隙水氮磷垂向分布特征及庫區(qū)空間差異.將高分辨率間隙水采樣器(Peeper)垂直插入柱狀沉積物中,待平衡3d后取出Peeper用酶標(biāo)儀(BiOTek,USA)檢測間隙水中的NH4+-N和PO43－-P濃度.該P(yáng)eeper由30個(gè)隔室組成,每個(gè)隔室之間間隔2mm,分辨率可達(dá)4mm,具體制作方法及工作原理詳見參考文獻(xiàn)[19].

1.3 樣品測定方法

表層沉積物樣品含水率通過把濕泥于105℃下烘干至恒重測得,濕沉積物經(jīng)冷凍干燥后研磨過100目篩后,總氮(TN)總磷(TP)采用過硫酸鉀氧化法測定,沉積物中總有機(jī)碳(TOC)采用重鉻酸鉀氧化-硫酸亞鐵滴定法測定, 無機(jī)氮(DIN)采用Bremner氯化鉀[(KCl)=2mol/L]提取,磷酸鹽用1mol/L的NH4Cl提取,提取液經(jīng)離心后取上清液過濾[20].水樣用0.45μm濾膜(Whatman GF/F)過濾后,其中磷酸鹽(PO43－-P)、氨氮(NH4+-N)、硝酸鹽氮(NO3－-N)和亞硝酸鹽氮(NO2－-N)的分析采用流動(dòng)注射分析儀進(jìn)行分析(SkalarSanplus, 荷蘭),間隙水中NH4+-N和PO43－-P分別用納氏試劑比色法和鉬藍(lán)比色法測定.

1.4 數(shù)據(jù)處理與統(tǒng)計(jì)分析

所有實(shí)驗(yàn)數(shù)據(jù)使用Excel 2016進(jìn)行預(yù)處理,用SPSS 22.0進(jìn)行單因素方差分析和相關(guān)分析,使用Origin 2017和ArcGIS 10.2軟件進(jìn)行繪圖.

2 結(jié)果與討論

2.1 上覆水及表層沉積物基本理化性質(zhì)

大黑汀水庫上覆水基本理化性質(zhì)見表1,其水體TN濃度較高,在9.43~11.14mg/L之間,DIN主要以NO3－-N為主,其中NH4+-N濃度在中游明顯高于上下游.TP濃度在0.03~0.08mg/L之間,溶解性磷酸鹽濃度在中游最高,這些可能與中游較高的溶解氧有關(guān).

大黑汀水庫表層沉積物氮磷和有機(jī)質(zhì)含量及分布特征見圖2,沉積物中TN含量為729~ 5894mg/kg,平均值為2881.6mg/kg;TP為1312~ 2439mg/kg,平均值為1980.5mg/kg;TOC含量在0.5%~5.6%之間,平均值為2.1%.表層沉積物TN含量自下游到上游逐漸減小,空間差異性顯著(=0.008), TP含量上下游差異不大,空間差異不顯著(=0.112> 0.05),TOC含量與TN含量表現(xiàn)出相似的空間分布特征,自下游到上游逐漸降低,空間差異性顯著(=0.031).通過表2可以看出,大黑汀水庫相較于其他湖庫水體,其表層沉積物中TN、TP含量相對較高.

表1 上覆水基本理化指標(biāo) Table 1 Basic physical and chemical properties of overlying water

從圖2可以看出沉積物TN和TOC之間呈現(xiàn)相同的波動(dòng)特征,其相關(guān)性分析表明TN和TOC之間高度相關(guān),相關(guān)系數(shù)達(dá)0.974(<0.01),表明有機(jī)質(zhì)是氮素的主要來源,TP與TOC的相關(guān)性不高,說明了沉積物中的TP與TN及有機(jī)質(zhì)在來源上的差異,沉積物中TP來源可能受其他因素的影響[21].沉積物中NH4+-N、NO3－-N和NO2－-N的含量分別在0.6~ 202.9、34.4~168.3和0.1~0.3mg/kg之間,NH4+-N和NO3－-N自下游到上游逐漸減小,NO2－-N空間差異性不顯著,各點(diǎn)位DIN之和占TN的比例在2.6~8.1%之間.沉積物中用1mol/L的NH4Cl提取態(tài)磷常被稱為可交換磷、松散結(jié)態(tài)磷、不穩(wěn)性磷、弱吸附性磷及溶解性磷等,可以統(tǒng)稱為活性磷,它是一種即時(shí)有效態(tài)磷,而且其含量常隨著季節(jié)波動(dòng).在本研究中,大黑汀水庫活性磷的含量在16.1~75.2mg/kg之間,占TP的比例在0.6%~4.5%,表現(xiàn)出中下游含量較高,上游含量相對較小的特征.沉積物中營養(yǎng)鹽(N和P)通常以無機(jī)和有機(jī)的形式存在,在沉積物中營養(yǎng)鹽的積累過程中,有機(jī)態(tài)的營養(yǎng)鹽占了很大比例[22].該水庫沉積物中的氮主要以有機(jī)氮的形式存在,沉積物營養(yǎng)鹽的吸附特性與沉積物有機(jī)質(zhì)含量密切相關(guān)[23-24].氮磷在沉積物-水界面的吸附特性被認(rèn)為是一個(gè)重要的過程,其直接影響水生生態(tài)系統(tǒng)中沉積物營養(yǎng)鹽含量及其遷移和生物可利用性[8].大黑汀水庫由于長時(shí)間的網(wǎng)箱養(yǎng)殖,大量的餌料和魚類糞便沉入水底,是沉積物中有機(jī)質(zhì)的主要來源.有機(jī)質(zhì)的分解會(huì)直接向水體釋放營養(yǎng)鹽,有機(jī)質(zhì)的礦化也會(huì)間接改變沉積物-水界面的氧化還原條件和pH值[25],進(jìn)而影響沉積物-水界面的氮磷循環(huán).

表2 不同湖庫表層沉積物氮磷含量 Table 2 Contents of nitrogen and phosphorus in surface sediments of different lakes and reservoirs

由于不同有機(jī)質(zhì)類型中氮的釋放與轉(zhuǎn)化穩(wěn)定性不同,C/N也常用來揭示有機(jī)質(zhì)的來源及類型, C/N也是沉積物氮負(fù)荷影響因素之一[30].大黑汀水庫表層沉積物C/N在4.8~9.5之間,平均值為6.7.研究表明較大的C/N一般表明為陸源有機(jī)質(zhì),較小的C/N代表有機(jī)質(zhì)主要來源于水體內(nèi)部[28,31],該水庫的C/N相對較小,表明該水庫表層沉積物中的有機(jī)質(zhì)主要來源于水體內(nèi)部,即來自水體中的浮游動(dòng)植物以及餌料、魚糞等的氧化分解,大黑汀水庫網(wǎng)箱養(yǎng)魚多集中于中下游,上游沉積物中較低的TOC含量進(jìn)一步說明人為活動(dòng)尤其是網(wǎng)箱養(yǎng)魚對水庫水體造成的污染.

2.2 間隙水中氮磷分布特征

利用高分辨率透析式間隙水采樣器(Peeper)獲得的大黑汀水庫間隙水中NH4+-N和PO43－-P分布特征見圖3.在空間分布上,8個(gè)采樣點(diǎn)的沉積物間隙水中營養(yǎng)鹽的分布存在較大差異,這與8個(gè)點(diǎn)所處的位置條件、氧化還原環(huán)境及微生物活動(dòng)有關(guān).間隙水中NH4+-N的含量在1.0~22.1mg/L之間,其質(zhì)量濃度在空間上呈現(xiàn)出點(diǎn)位3>2>1>8>7>6>4(5)的分布特征,總體下游NH4+-N濃度大于上游.PO43－-P的濃度在0~7.5mg/L之間,在空間上呈現(xiàn)出點(diǎn)位2>3>1>6>7>4>5(8)的分布特征,這與NH4+-N的分布特征略有差異,但總體也是下游含量最高,而水庫上游8號(hào)點(diǎn)位PO43－-P含量最低.

在垂向分布上,上覆水中NH4+-N濃度普遍低于間隙水,這暗示著沉積物中間隙水是上覆水中NH4+-N主要的“源”.各點(diǎn)位上覆水體中NH4+-N濃度在沉積物-水界面上部變化不大,處于動(dòng)態(tài)平衡波動(dòng)中,在沉積物-水界面下,點(diǎn)位1、2、3中間隙水NH4+-N濃度隨深度的增加而逐步升高,其最大值在13.9~22.1mg/L之間,而4~8號(hào)點(diǎn)位沉積物間隙水中NH4+-N濃度與上覆水體中NH4+-N濃度差異不大,在1.2~4.0mg/L之間波動(dòng).與表層沉積物相比,深層沉積物往往處于厭氧狀態(tài),硝化作用較弱[32],更有利于厭氧微生物活動(dòng),反硝化和氨化過程被促進(jìn),加上幾乎無擾動(dòng),較深處沉積物更有利于NH4+-N的積累[33],沉積物中有機(jī)質(zhì)礦化促進(jìn)NH4+-N的產(chǎn)生, NH4+-N在厭氧環(huán)境下進(jìn)入間隙水并成為間隙水中DIN的主要組分,這是間隙水中NH4+-N濃度隨深度增加而變大的原因.

PO43－-P的垂直分布特征在各點(diǎn)呈現(xiàn)高度的一致性.PO43－-P在上覆水中的質(zhì)量濃度相對較低,而在間隙水中濃度較大,隨著深度的增加總體表現(xiàn)為先增大后減小的趨勢.PO43－-P在沉積物-水界面下0~4cm處濃度增大,在4cm以下逐漸減小,這點(diǎn)表現(xiàn)出了與NH4+-N不同的垂向特征.間隙水與上覆水中營養(yǎng)鹽濃度的差異反映了沉積物-水界面營養(yǎng)鹽主要以間隙水向上覆水遷移及擴(kuò)散為主,沉積物起到了“源”的作用.沉積物接收來自上覆水和懸浮顆粒物的溶解性或非溶的有機(jī)/無機(jī)磷,并以PO43－-P的形式釋放到間隙水中[34].間隙水中較高的PO43－-P濃度與微生物作用下的有機(jī)質(zhì)降解有關(guān)[35],與鐵氧化物的吸附解析也有很大的關(guān)系,在地質(zhì)成巖作用下,鐵的氫氧化物在相對還原的條件下脫氧,使得與氫氧化鐵結(jié)合的磷溶解并釋放[36],這也解釋了水庫下游較深的水深條件下伴隨較低的溶氧,間隙水中PO43－-P濃度較大.此外,在水庫中上游部分區(qū)域,水體底部生長有大量的水草,上游區(qū)域間隙水中相對較低的PO43－-P濃度一方面與沉積物中活性磷含量有著直接的關(guān)系(圖2),另外也與水體中沉水植物的吸收消耗有關(guān).

圖3 沉積物-水界面營養(yǎng)鹽垂直分布特征 Fig.3 Vertical characteristics of nutrients distribution in sediment-water interface

2.3 沉積物-水界面營養(yǎng)鹽交換通量

靜態(tài)培養(yǎng)過程中,NH4+-N釋放速率為正值表示在沉積物-水界面以沉積物向上覆水體釋放,總體釋放速率在3.5~110.5mg/(m2×d)之間,其中1~3號(hào)點(diǎn)釋放速率最高,在55.8~110.5mg/(m2×d)之間,平均值為90.8mg/(m2×d),4~8號(hào)點(diǎn)釋放速率較低,平均值為7.8mg/(m2×d),下游釋放通量是中上游的11.6倍.1~3號(hào)點(diǎn)位于水庫下游,表層沉積物及間隙水中NH4+-N含量很高,此外該處水深較深,表層沉積物處于厭氧環(huán)境,在還原條件下其NH4+-N釋放速率較高.NO3－-N在沉積物-水界面表現(xiàn)出不同的遷移特征,在1~5號(hào)點(diǎn)位,NO3－-N主要從沉積物釋放到上覆水體,釋放速率在10.1~157.2mg/(m2×d)之間,其中1~3號(hào)點(diǎn)位釋放速率較大,4號(hào)和5號(hào)點(diǎn)位釋放速率較小.6~8號(hào)點(diǎn)位則表現(xiàn)出相反的遷移特征,即NO3－-N主要從上覆水?dāng)U散至沉積物間隙水中,此時(shí)NO3－-N在沉積物-水界面表現(xiàn)出吸附的特性,且其速率較大,在-112.5~-60.5mg/(m2×d)之間.相對而言,NO2－-N的釋放速率較小,表現(xiàn)為從沉積物向上覆水釋放,總體釋放速率在0.04~0.94mg/(m2×d)之間,同樣表現(xiàn)為1~3號(hào)點(diǎn)釋放速率較大,4~8號(hào)點(diǎn)釋放速率較小.NH4+-N和NO3－-N是DIN在沉積物-水界面的主要交換形式,由NO2－-N引起的DIN交換差異很小,但NO2－-N作為硝化和反硝化作用的中間產(chǎn)物,其作用依然十分重要[37].NH4+-N來源于有機(jī)氮的礦化或者由微生物厭氧還原有機(jī)氮形成,一些被沉積物吸附的NH4+-N釋放到上覆水并通過硝化作用氧化成NO3--N和NO2－-N[38],與此同時(shí),沉積物-水界面的厭氧環(huán)境有利于反硝化作用,NO3－-N或NO2－-N被還原為N2和N2O.有機(jī)質(zhì)含量對沉積物-水界面DIN的遷移轉(zhuǎn)化過程起了較為關(guān)鍵的作用,有機(jī)質(zhì)的礦化分解改變沉積物-水界面的氧化還原環(huán)境,并通過硝化-反硝化、厭氧氨氧化等過程來改變界面處NH4+-N、NO3－-N和NO2－-N之間的遷移轉(zhuǎn)化.

PO43－-P在沉積物-水界面也表現(xiàn)出向上覆水體釋放的特征,總體釋放通量在0.1~1.6mg/(m2d)之間,其中2、3號(hào)點(diǎn)位釋放速率最大,1、7號(hào)次之,4~6、8號(hào)點(diǎn)位釋放速率較小,總體表現(xiàn)位水庫下游釋放通量較高.由圖3可知,PO43－-P在間隙水和上覆水之間存在較大的濃度差(釋放潛力),研究表明有機(jī)質(zhì)組成、粒度、氧化還原條件、底棲生物活動(dòng)等也會(huì)影響沉積物-水界面PO43－-P的釋放[39],在好氧條件下沉積物表層毫米級(jí)別的好氧層及沉積物-水界面處的底層擴(kuò)散邊界層會(huì)阻止間隙水磷向上覆水體遷移,當(dāng)上覆水水體中溶解氧降低,好氧層或者擴(kuò)散邊界層變薄或消失,間隙水中的溶解性磷會(huì)隨著濃度梯度更容易向上覆水體擴(kuò)散[40].

通過表3可以看出,類似性質(zhì)的富營養(yǎng)化湖庫氮磷釋放速率都很高.有研究表明溫度升高會(huì)促進(jìn)沉積物-水界面氮磷向上覆水釋放,夏季的釋放速率往往高于冬季[37,41],本研究采樣時(shí)間為冬季,水溫較低,大黑汀水庫所處的區(qū)域四季分明,一年之中溫差較大,本研究所得到的沉積物-水界面氮磷釋放速率處于一年之中最低水平,預(yù)計(jì)該水庫在夏季會(huì)有更大的氮磷釋放速率.

表3 國內(nèi)外湖庫氮磷擴(kuò)散通量比較[mg/(m2·d)] Table 3 Comparison among fluxes of ammonia and orthophosphate of domestic and foreign lakes and reservoirs [mg/(m2·d)]

綜上所述,冬季大黑汀水庫沉積物主要表現(xiàn)為“源”向上覆水體釋放NH4+-N和PO43－-P,NO3－-N在水庫上下游表現(xiàn)相反的遷移特征,NO2－-N也表現(xiàn)為向上覆水體釋放為主,可見內(nèi)源釋放在大黑汀水庫水體富營養(yǎng)化進(jìn)程中扮演了重要角色.大黑汀水庫總體表現(xiàn)出下游釋放通量高,上游釋放通量小的特征,這與沉積物中TN及TP、DIN和PO43－-P含量以及間隙水中NH4+-N和PO43－-P含量的分布特征相一致.通過對比國內(nèi)外湖泊水庫營養(yǎng)鹽釋放通量可知該水庫NH4+-N和PO43－-P釋放通量較高.因此在大黑汀水庫治理進(jìn)程中應(yīng)重點(diǎn)關(guān)注水庫下游的污染治理,但即使是上游相對較低的氮磷含量,也高過許多其他富營養(yǎng)化湖庫水體,所以進(jìn)一步阻止外源污染輸入、改善庫區(qū)沿岸帶環(huán)境、采用底泥疏浚及生態(tài)修復(fù)等多種手段對水庫進(jìn)行綜合治理對于改善水庫水體環(huán)境至關(guān)重要.

3 結(jié)論

3.1 大黑汀水庫沉積物內(nèi)源污染負(fù)荷較重,TN、TP和有機(jī)質(zhì)含量較高,其中碳氮表現(xiàn)出水庫下游>中游>上游的空間分布特征,TP空間差異不顯著.沉積物有機(jī)質(zhì)主要來源于水體內(nèi)部.

3.2 大黑汀水庫間隙水中NH4+-N和PO43－-P表現(xiàn)出相同的空間分布特征,即水庫下游(1~3)大于水庫中上游(4~8),垂直方向上NH4+-N隨著深度的增加濃度逐漸變大,PO43－-P則在沉積物-水界面下0~4cm處隨深度的增加而變大,在4cm以下隨著深度的增加而減小.

3.3 沉積物-水界面NH4+-N和PO43－-P的釋放通量分別為3.5~110.5mg/(m2·d)和0.1~1.6mg/(m2·d), NO3－-N和NO2－-N釋放通量在-112.5~157.2mg/ (m2·d)和0.04~0.94mg/(m2·d)之間.其中NH4+-N和NO3－-N是沉積物-水界面處主要的DIN遷移形態(tài).NH4+-N和PO43－-P在空間上主要表現(xiàn)為下游釋放通量高,上游釋放通量低的空間特征,且各點(diǎn)沉積物均表現(xiàn)為“源”,表明沉積物是水庫上覆水中營養(yǎng)鹽的重要來源.

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致謝：本文的研究工作受到了水利部海委引灤工程管理局大黑汀水庫管理處的大力協(xié)作,在此深表謝意.

Distribution characteristics and exchange flux of nitrogen and phosphorus at thesediment-water interface of Daheiting Reservoir in winter.

WEN Shuai-long1,2, WU Tao3, YANG Jie3, LI Xin1,4, GONG Wan-qing1,2, ZHONG Ji-cheng1*

(1.State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Science, Nanjing 210008, China；2.College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China；3.Tianjin Hydraulic Research Institute, Tianjin 300061, China；4.School of Environmental Science and Engineering, Suzhou University of Science and Technology, Suzhou 215009, China)., 2019,39(3)：1217~1225

In this paper, the pollution load and distribution characteristics of carbon, nitrogen and phosphorus in surface sediments of Daheiting Reservoir were studied. The characteristics of nitrogen and phosphorus profiles at the sediment-water interface were obtained by pore water equilibriums technology, and the spatial differences of NH4+-N and PO43－-P in the interstitial water of Daheiting Reservoir were analyzed. The results showed that the contents of TN, TP and TOC in sediments were 729~5894mg/kg, 1312~2439mg/kg and 0.5%~5.6%, respectively. The contents of NH4+-N, NO3－-N, NO2－-N and PO43－-P in sediments were 0.6~202.9, 34.4~168.3, 0.1~0.3 and 16.1~75.2mg/kg, respectively, and the spatial distribution was significantly different. Sediment C/N ratio indicated that the substances inwater were the source of organic matter because of human cage culture activities. The concentration of NH4+-N and PO43－-P in the interstitial water was much higher than that in the overlying water, indicated that nutrients have the potential to diffuse from interstitial water to overlying water. In the vertical direction, the concentration of NH4+-N in interstitial water increases with depth, and PO43－-P increases rapidly at 0~4cm, then decreases gradually. The exchange fluxes of NH4+-N and PO43－-P across the sediment-water interface were 3.5~110.5mg/(m2·d) and 0.1~1.6mg/(m2·d), respectively. The exchange fluxes of NO3－-N and NO2－-N were -112.5~157.2mg/(m2·d) and 0.04~0.94mg/(m2·d), respectively. NH4+-N, NO3－-N and PO43－-P showed higher exchange fluxes downstream. Higher sediment endogenous load made the sediments of Daheiting Reservoir have greater potential to release nutrients to overlying water,so controlling the pollution state of sediment is a necessary measure to manage the water environment of Daheiting Reservoir.

Daheiting Reservoir；nutrients；sediment-water interface；interstitial water；diffusive flux

X142

A

1000-6923(2019)03-1217-09

文帥龍(1993-),男,河南禹州人,中國科學(xué)院南京地理與湖泊研究所碩士研究生,研究方向?yàn)楹此h(huán)境生態(tài)修復(fù).

2018-07-23

國家自然科學(xué)基金資助項(xiàng)目(41771516);天津市水利科學(xué)院研究院項(xiàng)目(2017SZC-C-89)

*責(zé)任作者, 副研究員, jczhong@niglas.ac.cn