王洪偉,王少明,張 敏,胡曉康,湯夢(mèng)瑤,楊凡艷,鐘繼承*

春季潘家口水庫(kù)沉積物-水界面氮磷賦存特征及遷移通量

王洪偉1,2,王少明3,張 敏1,2,胡曉康1,4,湯夢(mèng)瑤1,2,楊凡艷1,鐘繼承1*

(1.中國(guó)科學(xué)院南京地理與湖泊研究所湖泊與環(huán)境國(guó)家重點(diǎn)實(shí)驗(yàn)室,江蘇 南京 210008；2.中國(guó)科學(xué)院大學(xué)資源與環(huán)境學(xué)院,北京 100049；3.水利部海河水利委員會(huì)引灤工程管理局,河北 遷西 064309；4.安徽師范大學(xué)環(huán)境科學(xué)與工程學(xué)院,安徽 蕪湖 241000)

以春季潘家口水庫(kù)沉積物為研究對(duì)象,分析了水庫(kù)整體營(yíng)養(yǎng)鹽污染現(xiàn)狀及內(nèi)源釋放特征.通過(guò)高分辨率間隙水采樣器(HR-Peeper)獲得沉積物間隙水,以此分析營(yíng)養(yǎng)鹽垂向分布特征及空間差異性;以原柱樣沉積物靜態(tài)釋放試驗(yàn)獲取沉積物-水界面營(yíng)養(yǎng)鹽遷移通量,并分析了潘家口水庫(kù)內(nèi)源負(fù)荷特征.結(jié)果表明:庫(kù)區(qū)沉積物營(yíng)養(yǎng)鹽釋放風(fēng)險(xiǎn)較高,TN、TP含量分別為3701.59～8221.28和756.28～1696.15mg/kg.通過(guò)C/N比確定2017年以前的網(wǎng)箱養(yǎng)殖殘留的飼料和魚(yú)糞是水體富營(yíng)養(yǎng)化的主因.原柱樣靜態(tài)釋放結(jié)果表明,NH4+-N、NO3－-N、NO2－-N、SRP交換通量分別為23.71～156.80,-7.37～-161.78,1.64～33.4和0.56～2.86mg/(m2·d),潘家口水庫(kù)內(nèi)源負(fù)荷相對(duì)較高.該結(jié)果與水庫(kù)本身的高有機(jī)質(zhì)及氮磷賦存量、生物分解耗氧及春季逐步提高的水溫有關(guān),導(dǎo)致?tīng)I(yíng)養(yǎng)鹽加速釋放進(jìn)入上覆水柱.潘家口水庫(kù)的內(nèi)源負(fù)荷能夠加速水庫(kù)的富營(yíng)養(yǎng)化進(jìn)程,應(yīng)采取措施控制潘家口水庫(kù)內(nèi)源負(fù)荷.

潘家口水庫(kù)；沉積物-水界面；營(yíng)養(yǎng)鹽；內(nèi)源負(fù)荷；擴(kuò)散通量

在湖庫(kù)生態(tài)系統(tǒng)中,沉積物作為污染物的內(nèi)源,有著較為復(fù)雜的地化循環(huán)途徑,是營(yíng)養(yǎng)鹽遷移轉(zhuǎn)化的重要介質(zhì):水體中氮磷等營(yíng)養(yǎng)鹽通過(guò)河流及地表徑流等方式以溶解態(tài)及顆粒物態(tài)進(jìn)入水體,最終沉積于底泥中.底泥中累積的高濃度氮磷也會(huì)在一定的物理、化學(xué)及生物過(guò)程作用下進(jìn)入沉積物間隙水,并以間隙水為媒介,通過(guò)梯度擴(kuò)散、生物擾動(dòng)及再懸浮等過(guò)程重新進(jìn)入水體[1-3].因此,沉積物既是外源污染物的“匯”,亦是水環(huán)境污染潛在“源”.

現(xiàn)階段,由于國(guó)內(nèi)大量湖庫(kù)富營(yíng)養(yǎng)化問(wèn)題嚴(yán)重,沉積物內(nèi)賦存高濃度營(yíng)養(yǎng)鹽,使得內(nèi)源氮磷釋放成為湖泊富營(yíng)養(yǎng)化的主因[4-5].有研究表明,即使有效的控制了湖泊外源營(yíng)養(yǎng)鹽輸入,沉積物內(nèi)源依舊會(huì)導(dǎo)致嚴(yán)重的富營(yíng)養(yǎng)化問(wèn)題,帶來(lái)高危害的藻類爆發(fā)[6].其中,對(duì)于湖庫(kù)水體環(huán)境的高精度模擬,是評(píng)估界面間營(yíng)養(yǎng)鹽遷移通量的基本要求.在這一過(guò)程中,沉積物內(nèi)間隙水富集的高濃度營(yíng)養(yǎng)鹽通過(guò)沉積物-水界面擴(kuò)散進(jìn)入上覆水,是沉積物營(yíng)養(yǎng)鹽釋放的關(guān)鍵介質(zhì),其本身亦受沉積物結(jié)構(gòu)、pH、離子濃度、水動(dòng)力條件等影響[7-9].因此其內(nèi)部營(yíng)養(yǎng)鹽濃度垂向分布特征及界面通量是解釋湖泊污染過(guò)程的重要一環(huán).

潘家口水庫(kù)是北方少見(jiàn)的梯級(jí)水庫(kù),作為引灤入津水利工程的水源地,供水下游的大黑汀、于橋水庫(kù),保障了唐山、天津兩市約2000萬(wàn)人口的生產(chǎn)生活用水需求.近20年來(lái),由于當(dāng)?shù)亟?jīng)濟(jì)發(fā)展的現(xiàn)實(shí)需求,其養(yǎng)殖業(yè)發(fā)展規(guī)模大、范圍廣、技術(shù)落后,水庫(kù)主體發(fā)展了大量污染嚴(yán)重的網(wǎng)箱養(yǎng)殖,由此導(dǎo)致了水庫(kù)底層魚(yú)食魚(yú)糞的嚴(yán)重堆積,近年來(lái)水體處于劣V類水平,富營(yíng)養(yǎng)化問(wèn)題逐步加重,對(duì)水源地的供水安全形成不可忽視的威脅[10].隨著全國(guó)性的網(wǎng)箱養(yǎng)殖治理工作的開(kāi)展,潘家口水庫(kù)于2016年對(duì)水庫(kù)內(nèi)的網(wǎng)箱養(yǎng)殖集中整治,到2017年5月份基本拆除了整個(gè)庫(kù)區(qū)的網(wǎng)箱,降低了養(yǎng)殖業(yè)帶來(lái)的營(yíng)養(yǎng)鹽輸入風(fēng)險(xiǎn).但沉積物中營(yíng)養(yǎng)鹽釋放帶來(lái)的內(nèi)源污染至今依然是水庫(kù)污染治理的核心問(wèn)題之一.然而,潘家口水庫(kù)這類典型的深水水庫(kù), 由于其泥水界面相對(duì)低溫、風(fēng)浪擾動(dòng)小、DO季節(jié)性變化大,使得沉積物的理化性質(zhì)較淺水湖泊更為復(fù)雜,內(nèi)部污染物的代謝降解更慢,污染的時(shí)間尺度更長(zhǎng),營(yíng)養(yǎng)鹽釋放風(fēng)險(xiǎn)性也更大[11-13].現(xiàn)有的深水水庫(kù)沉積物污染及內(nèi)源負(fù)荷的研究,主要聚焦于與上覆水充分接觸的表層沉積物[14];研究?jī)?nèi)源負(fù)荷及遷移過(guò)程常使用模型模擬,難以真實(shí)反映現(xiàn)實(shí)情況下?tīng)I(yíng)養(yǎng)鹽釋放的復(fù)雜過(guò)程[15].因此,本文利用原柱樣靜態(tài)釋放培養(yǎng)及高分辨率間隙水采樣技術(shù),研究沉積物-水界面間營(yíng)養(yǎng)鹽交換過(guò)程,并從機(jī)理角度分析界面間的理化過(guò)程,以了解深水水庫(kù)氮磷在沉積物-水界面的遷移過(guò)程及機(jī)制,為富營(yíng)養(yǎng)化水庫(kù)內(nèi)源負(fù)荷治理提供參考.

1 材料與方法

1.1 研究區(qū)概況

潘家口水庫(kù)(40°22'～40°35'N,118°13'～118°25'E)位于唐山市遷西縣與承德市寬城縣的交界處(圖1),屬暖溫帶半濕潤(rùn)大陸型季風(fēng)性氣候,是典型的山區(qū)水庫(kù).水庫(kù)于1979年建成蓄水,承接上游灤河來(lái)水,壩上控制面積33700km2,總庫(kù)容29.3億立方米,占灤河流域75%面積,下游主庫(kù)區(qū)均深超過(guò)30米,壩口建有水電站,發(fā)電量15萬(wàn)kW,為多年調(diào)節(jié)型水庫(kù)[16-17].水庫(kù)上游灤河與瀑河形成的匯流區(qū)附近基本為山區(qū)林地,有少量居民點(diǎn),主要污染來(lái)源為養(yǎng)殖及生活污水排放.

圖1 潘家口水庫(kù)采樣點(diǎn)分布

1.2 樣品的采集與處理

考慮到春季是潘家口水庫(kù)夏季富營(yíng)養(yǎng)化的過(guò)渡時(shí)期,對(duì)分析營(yíng)養(yǎng)鹽釋放的規(guī)律、詮釋深水水庫(kù)底層營(yíng)養(yǎng)鹽釋放有著重要意義.因此本研究于2020年5月進(jìn)行沉積物及水樣采樣,自下游到上游布設(shè)8個(gè)采樣點(diǎn).利用重力柱狀采樣器(直徑9cm、長(zhǎng)50cm)采集沉積物原柱樣,每個(gè)點(diǎn)位采集3根沉積柱樣作平行,將采集的原柱樣用原位底層水樣緩注滿后以橡膠塞封口,防止運(yùn)輸過(guò)程中的擾動(dòng)對(duì)實(shí)驗(yàn)結(jié)果造成影響.同時(shí)用抓斗式采泥器采集表層沉積物樣品,每個(gè)采樣點(diǎn)各采集3次,厚度約為10cm,將3次采集的表層沉積物混勻后保存.上覆水的水質(zhì)理化指標(biāo)通過(guò)多參數(shù)水質(zhì)儀(YSI)測(cè)定,同時(shí)采集8L原位底層水樣用于靜態(tài)釋放培養(yǎng)及水質(zhì)分析.采樣完成后將樣品快速運(yùn)輸?shù)轿挥谂思铱谒畮?kù)大壩附近的水利部海河水利委員會(huì)引灤工程管理局實(shí)驗(yàn)室進(jìn)行室內(nèi)的培養(yǎng)及分析.

1.3 沉積物-水界面氮磷通量測(cè)定

采集的沉積物原柱樣運(yùn)回實(shí)驗(yàn)室后通過(guò)虹吸法排干上覆水,用醫(yī)用輸液管將過(guò)濾后的原位底層水樣沿壁緩慢無(wú)擾動(dòng)地注入沉積物柱中,當(dāng)上覆水高度達(dá)到20cm(1135mL)后停止.根據(jù)以往研究及實(shí)地采樣驗(yàn)證,5月份潘家口水庫(kù)沉積物上覆水處于低溫好氧狀態(tài),因此將沉積柱敞口轉(zhuǎn)移至恒溫低溫循環(huán)器(STIK,ILB-008-03)進(jìn)行約5℃的低溫培養(yǎng)[18].在設(shè)定好的取樣間隔(0,12,24,36,48,60,72h)用50mL注射器連接輸液軟管從沉積物-水界面以上5cm處采集上覆水50mL,采集的水樣收集于50mL聚乙烯瓶中,及時(shí)放入4℃冰箱保存,采樣結(jié)束后立即用過(guò)濾后的原位水樣補(bǔ)充到20cm高度以保持培養(yǎng)過(guò)程中水量一致.釋放通量計(jì)算公式如下[13]:

式中:為釋放速度,mg/(m2·d);為培養(yǎng)柱樣中上覆水體積,L;C、0、C-1為第次、初始和-1次采樣時(shí)氮磷組分含量,mg/L;C為添加的原位水樣中氮磷組分含量,mg/L;為沉積物-水界面表面積,m2;為靜態(tài)釋放時(shí)間,d.用該方法得到的氮磷交換通量為培養(yǎng)3d內(nèi)得到的平均交換通量.

1.4 高分辨率間隙水氮磷剖面測(cè)定

在上述的靜態(tài)釋放試驗(yàn)結(jié)束后,把高分辨率透析式間隙水采樣器(HR-Peeper)[19]插入沉積物中獲取間隙水,其垂向分辨率可達(dá)4mm,可更精準(zhǔn)的表征沉積物間隙水NH4+-N及SRP賦存特征.具體制作方法及原理詳見(jiàn)參考文獻(xiàn)[17].

1.5 樣品分析方法

將表層沉積物完全填滿20mL坩堝,經(jīng)105℃烘干后稱重,計(jì)算出沉積物的含水率、容重及孔隙率.另取沉積物濕樣20g左右冷凍干燥,研磨后過(guò)100目篩,過(guò)篩后的沉積物樣品用于測(cè)定燒失量(LOI)、總氮(TN)、總磷(TP)、NH4+-N、NO3－-N、NO2－-N及易交換態(tài)磷.沉積物燒失量(LOI)的測(cè)定是將研磨后的沉積樣品放入馬弗爐,在550℃條件下灼燒4h后,冷卻測(cè)定灼燒過(guò)程中的質(zhì)量損失.沉積物總有機(jī)碳(TOC)采用重鉻酸鉀-硫酸亞鐵滴定法測(cè)定.水樣及沉積物樣的TN、TP采用過(guò)硫酸鉀氧化法測(cè)定,沉積物樣品活性氮(NH4+-N、NO3－-N、NO2－-N)采用2M KCl提取,沉積物樣品易交換態(tài)P使用1M NH4Cl于180r/min震蕩2h提取,4000r/min離心10min后過(guò)濾待測(cè)定.水樣及提取液中的氮和磷分別使用納氏試劑比色法,紫外分光光度法、重氮偶合分光光度法、鉬藍(lán)比色法測(cè)定.

1.6 數(shù)據(jù)處理與分析

應(yīng)用Origin8.0和ArcGIS10.2軟件進(jìn)行相關(guān)圖形的繪制,并應(yīng)用Excel 365和SPSS 22.0軟件進(jìn)行數(shù)據(jù)處理和統(tǒng)計(jì)分析.

2 結(jié)果

2.1 水柱及沉積物基本理化性質(zhì)

如表1所示.潘家口水庫(kù)表層水體pH值范圍在8.60～8.80之間,均值為8.66,水體整體呈弱堿性.TN與h分別為4.09～4.31mg/L和12～77.2mV,濃度及氧化還原電位值在上游至下游有逐漸減小的趨勢(shì),其中NO3－-N作為最主要的無(wú)機(jī)氮形態(tài),占TN比重最高,占比為78.00%～78.67%, NH4+-N與NO2－-N范圍分別0.09～0.14 和0.01～0.04mg/L在溶解性無(wú)機(jī)氮(DIN)中占比及差異性較小,.而SRP自上游至下游有明顯的增大趨勢(shì),濃度為0.002～0.03mg/L,占TP的比例為5.4%～75%.TP、DO空間差異不顯著.

如圖2所示,潘家口水庫(kù)表層沉積物TN含量為3701.59～8221.28mg/kg,均值4765.06mg/kg,從上游至下游含量逐漸增大.其中作為無(wú)機(jī)氮(DIN)的重要組分,NH4+-N含量在158.92～773.63mg/kg之間,均值318.05mg/kg,最高點(diǎn)出現(xiàn)在2號(hào)點(diǎn)位,整體上空間差異不大,NO3－-N含量在76.43～336.83mg/kg,均值218.21mg/kg,自上游至下游含量逐漸增大,NO2－-N濃度均小于1mg/L,在DIN中占比也小于1%,范圍為0.38～0.96mg/kg,均值0.58mg/kg,同樣是下游高于上游.三者組成的DIN含量為368.78～1093.54mg/kg,占TN比例為7.88%～19.26%.

水庫(kù)表層沉積物TP含量為756.28～1696.15mg/ kg,均值946.51mg/kg,位于下游壩前區(qū)域的1號(hào)點(diǎn)位,TP濃度顯著高于其他點(diǎn)位,其余點(diǎn)位濃度空間差異不大.易交換態(tài)磷(Labile-P)含量為0.92～ 9.18mg/kg,均值4.60mg/kg,占TP含量的0.11%～ 1.05%,表現(xiàn)出從上游灤河到下游壩前增加的趨勢(shì),潘家口水庫(kù)春季沉積物C/N為17.9～37.3之間,均值25.2,水庫(kù)的C/N較高.

表1 上覆水基本理化指標(biāo)

2.2 間隙水營(yíng)養(yǎng)鹽分布特征

如圖3所示,在空間分布上,各點(diǎn)位之間營(yíng)養(yǎng)鹽濃度存在顯著差異.在深度0～32mm,即上覆水中NH4+-N和SRP濃度差別不大,而在沉積物-水界面以下的沉積物中,除4號(hào)點(diǎn)的SRP外,兩種營(yíng)養(yǎng)鹽均呈現(xiàn)顯著的增大趨勢(shì),且濃度顯著高于上覆水.沉積物中的SRP在20mm左右達(dá)到峰值,隨后變化趨于穩(wěn)定.相較于SRP,NH4+-N在整個(gè)84mm的沉積物剖面中,濃度穩(wěn)步增大.在沉積物內(nèi),NH4+-N濃度范圍為0.38～2.2mg/L, SRP濃度范圍為0.10～0.95mg/L.在整體趨勢(shì)上,間隙水NH4+-N濃度呈現(xiàn)出點(diǎn)位4>2>7>5>6>8>3>1的特征,最高點(diǎn)出現(xiàn)在原圍網(wǎng)養(yǎng)殖區(qū)域的4號(hào)點(diǎn).考慮到潘家口水庫(kù)2017年以前普遍存在的圍網(wǎng)養(yǎng)殖,而低溫環(huán)境不利于有機(jī)物降解,因此各點(diǎn)位間雖空間差異大,但無(wú)量級(jí)上的差距.SRP濃度則呈現(xiàn)出點(diǎn)位3>1>2>8> 7>6>5>4的特征,下游顯著高于上游.

2.3 沉積物-水界面氮磷遷移通量

整體上,除NO3－-N外,潘家口水庫(kù)沉積物-水界面NH4+-N,NO2－-N,SRP均為正通量(圖4),即沉積物中的營(yíng)養(yǎng)鹽向上覆水體釋放.而NO3－-N在各個(gè)點(diǎn)位均表現(xiàn)為負(fù)通量,即上覆水向沉積物中遷移.沉積物-水界面NH4+-N釋放通量在23.71～156.80mg/ (m2·d)之間,下游顯著高于上游,這與表層沉積物NH4+-N含量趨勢(shì)相同,其中最高的通量出現(xiàn)在3號(hào)點(diǎn).而8號(hào)點(diǎn)相較于上游其他點(diǎn)位NH4+-N通量更高,NO3－-N遷移通量在-7.37～-161.78mg/(m2·d)之間,3號(hào)點(diǎn)出現(xiàn)了NO3－-N的最高負(fù)通量,該點(diǎn)也是最高的NH4+-N通量.整體上NO2－-N負(fù)通量越高的點(diǎn)位,其NO3－-N的遷移通量越低.NO2－-N遷移速率在1.64～33.45mg/(m2·d)之間,其中3號(hào)點(diǎn)位最低,空間差異性不明顯,對(duì)DIN釋放通量貢獻(xiàn)很小.對(duì)于SRP的釋放通量,其范圍為0.56～2.86mg/(m2·d)之間.除6號(hào)點(diǎn)以外,整體上依舊是下游通量高于上游.

圖4 靜態(tài)釋放試驗(yàn)中沉積物-水界面營(yíng)養(yǎng)鹽通量

3 討論

3.1 水柱及沉積物營(yíng)養(yǎng)鹽分布特征

水庫(kù)表層水柱中TN、TP、SRP、DIN濃度與間隙水間有著數(shù)量級(jí)上的差距,暗示著沉積物有著較高的營(yíng)養(yǎng)鹽擴(kuò)散潛力.同時(shí),上覆水中營(yíng)養(yǎng)鹽的濃度差異與沉積物中大體趨同,也體現(xiàn)出沉積物內(nèi)營(yíng)養(yǎng)鹽擴(kuò)散對(duì)庫(kù)區(qū)整體水質(zhì)的影響.水體中TN與NO3－-N濃度相較于其他水庫(kù)及天然湖泊偏高,是典型的富營(yíng)養(yǎng)化水體,而TP及SRP濃度相較N較小,顯示出春季潘家口水庫(kù)富營(yíng)養(yǎng)化問(wèn)題的主因在于高濃度的N污染.

潘家口水庫(kù)屬于典型的深水水庫(kù),相較于淺水水庫(kù),風(fēng)浪擾動(dòng)作用小.同時(shí),水庫(kù)為多年調(diào)節(jié)型,因此底泥再懸浮造成的污染風(fēng)險(xiǎn)相對(duì)較低.水庫(kù)的上游水源來(lái)自于灤河,水質(zhì)相對(duì)較好[20].因此整個(gè)庫(kù)區(qū)最大的污染風(fēng)險(xiǎn)還是來(lái)自于沉積物內(nèi)源.

對(duì)于水庫(kù)沉積物,沉積物C、N、P循環(huán)是營(yíng)養(yǎng)鹽遷移轉(zhuǎn)化機(jī)制的核心.潘家口水庫(kù)內(nèi),相較其他富營(yíng)養(yǎng)化水庫(kù),TN含量相對(duì)較高(表2).同時(shí)沉積物無(wú)機(jī)氮含量占TN的比例為7.88%～19.26%,這表明有機(jī)氮占氮素的主導(dǎo)地位.這些有機(jī)氮的重要來(lái)源是水庫(kù)圍網(wǎng)養(yǎng)殖時(shí)期沉積于水庫(kù)底層的大量魚(yú)食魚(yú)糞帶來(lái)的高含量有機(jī)質(zhì).有機(jī)質(zhì)在一定條件下能誘導(dǎo)微生物產(chǎn)生胞外酶,催化分解后產(chǎn)生DIN,參與微生物的化能合成作用,加劇水體富營(yíng)養(yǎng)化[21].潘家口水庫(kù)TN與DIN含量顯著高于下游的大黑汀水庫(kù)[27],表明在營(yíng)養(yǎng)鹽污染尺度上,水文及地形條件更為復(fù)雜的潘家口水庫(kù)污染程度較高.

水庫(kù)除1號(hào)點(diǎn)TP濃度顯著較高外,其余點(diǎn)位空間差異不大.其原因可能在于水庫(kù)電站運(yùn)行調(diào)控等因素定期放水,另外再加上下游之間水力作用,導(dǎo)致底泥沖擊擾動(dòng)后沉積于壩前區(qū)域,使得整體營(yíng)養(yǎng)鹽含量偏高.沉積物中活性磷作為即時(shí)有效磷,通過(guò)吸附作用與沉積物顆粒結(jié)合[28],本身不穩(wěn)定,易釋放進(jìn)入上覆水,參與生物的代謝分解.采樣時(shí)發(fā)現(xiàn),帶有大量魚(yú)糞和浮泥等有機(jī)質(zhì)的沉積物含水率高,孔隙度大,表層理化性質(zhì)與底層沉積物差別很大.研究表明,有機(jī)質(zhì)含量對(duì)沉積物營(yíng)養(yǎng)鹽通量影響很大,高濃度的有機(jī)質(zhì)會(huì)改變沉積物-水界面的pH值、h與DO,影響磷的遷移轉(zhuǎn)化[29].

表2 國(guó)內(nèi)湖庫(kù)沉積物營(yíng)養(yǎng)鹽賦存量

有機(jī)碳與氮素的比值(C/N),常用來(lái)表征水體內(nèi)有機(jī)質(zhì)的來(lái)源[30].C/N較大,表示陸源有機(jī)物占優(yōu)勢(shì),C/N較小表示有機(jī)物的主要來(lái)源是水體內(nèi)部[31].潘家口水庫(kù)春季C/N為17.9～37.3之間,均值25.2,水庫(kù)的C/N非常高,表示沉積物中有機(jī)質(zhì)的主要組分來(lái)源于外源輸入,即養(yǎng)殖區(qū)域內(nèi)飼料及魚(yú)糞的堆積降解.TOC含量及N污染程度自下游至上游整體呈遞減趨勢(shì).其主要原因在于:①水庫(kù)本身自上游到下游,存在顯著的水力搬運(yùn)過(guò)程,這在梯級(jí)水庫(kù)中尤為明顯[32].②潘家口水庫(kù)下游的主庫(kù)區(qū),由于其水面較為寬闊,圍網(wǎng)養(yǎng)殖的密度更高,使得養(yǎng)殖過(guò)程中的飼料糞便堆積層更厚.

3.2 沉積物-水界面N遷移特征及機(jī)制

N、P作為營(yíng)養(yǎng)鹽的核心組分,其地化循環(huán)過(guò)程差異性較大.對(duì)N而言,無(wú)機(jī)氮組分中的NH4+-N, NO3－-N與NO2－-N,以微生物為介導(dǎo),在特定的物化條件下產(chǎn)生硝化、反硝化、氨化作用,彼此間相互轉(zhuǎn)化,整個(gè)過(guò)程復(fù)雜多變[33-34].相較于P,微生物作用對(duì)其遷移的影響更明顯.

潘家口水庫(kù)整體水深超過(guò)30m,沉積物-水界面處于低溫狀態(tài).自4月中旬開(kāi)始,水庫(kù)底部的滯溫層水溫緩慢升高,微生物活性得到增強(qiáng),水體DO開(kāi)始逐步下降,這也被看作夏季藻類爆發(fā)的初始階段[18].從間隙水NH4+-N剖面可知(圖3),上覆水與間隙水之間的NH4+-N含量存在著直觀的濃度梯度,且在整個(gè)84mm的沉積物剖面中,NH4+-N隨著沉積物的深度增加不斷增大.這一方面反映了營(yíng)養(yǎng)鹽在沉積物-水界面遷移過(guò)程中,間隙水由于濃度梯度存在著遷移擴(kuò)散的能力,是水柱營(yíng)養(yǎng)鹽的來(lái)源之一;另一方面,沉積物內(nèi)部NH4+-N濃度隨深度增加的特性,則是由于深層沉積物厭氧狀況不斷加深,導(dǎo)致好氧菌被抑制,厭氧菌占主導(dǎo),深層沉積物中硝化作用減弱,反硝化及氨化作用增強(qiáng),且厭氧環(huán)境有利于NH4+-N的積累,從而使得NH4+-N逐步升高[35-36].在空間分布上,間隙水NH4+-N濃度也呈現(xiàn)出獨(dú)特的分布規(guī)律,4、2、7、5四個(gè)點(diǎn)位的NH4+-N濃度較高,而1、3兩個(gè)典型的下游點(diǎn)位,其NH4+-N濃度反而最低.這與沉積物中TN和DIN的分布規(guī)律不一致.很大原因在于,4、2、7、5這四個(gè)點(diǎn)位是取締前的圍網(wǎng)養(yǎng)殖區(qū),在采集的沉積柱中,發(fā)現(xiàn)大量魚(yú)食和魚(yú)糞構(gòu)成的結(jié)構(gòu)松散發(fā)臭的浮泥,表層疏松多孔且高污染的特性促進(jìn)了有機(jī)質(zhì)向NH4+-N的轉(zhuǎn)化.

下游同水源的于橋水庫(kù),其表層沉積物內(nèi)間隙水中NH4+-N濃度遠(yuǎn)高于本研究,而上覆水體的NH4+-N濃度均更低[37].兩項(xiàng)實(shí)驗(yàn)沉積柱采集與間隙水的獲取方法類似,而沉積物-水界面的釋放差異性較大.這意味著營(yíng)養(yǎng)鹽釋放通量,不僅僅受沉積物-水界面的濃度梯度控制.通過(guò)與其他富營(yíng)養(yǎng)化湖庫(kù)對(duì)比發(fā)現(xiàn)(表4),潘家口水庫(kù)沉積物NH4+-N釋放速率遠(yuǎn)高于大黑汀水庫(kù)、衡山水庫(kù)、駱馬湖及南四湖等富營(yíng)養(yǎng)化湖庫(kù),而與太湖及滇池重度富營(yíng)養(yǎng)化湖灣相似,這個(gè)結(jié)果表明潘家口水庫(kù)內(nèi)源氮負(fù)荷較為嚴(yán)重.

靜態(tài)釋放實(shí)驗(yàn)揭示了更深層次的DIN轉(zhuǎn)化機(jī)理,對(duì)于DIN而言,其生物循環(huán)過(guò)程受到上覆水、沉積物及沉積物-水界面之間的理化性質(zhì),如溫度、h、pH值、DO、沉積物物理結(jié)構(gòu)等共同影響[38-39].因此,分析沉積物-水界面DIN釋放通量,需要結(jié)合潘家口水庫(kù)特殊的底層理化性質(zhì)分析.沉積物-水界面NH4+-N遷移通量下游顯著高于上游,這與表層沉積物NH4+-N含量趨勢(shì)相同.潘家口水庫(kù)下游水深普遍高于上游,水溫較低,而水體溶氧差距不大,理論上更不利于NH4+-N釋放入上覆水,其根本原因還是在于下游相對(duì)于上游的高污染狀況.而8號(hào)點(diǎn)相較于上游其他點(diǎn)位NH4+-N濃度更高,則是因?yàn)?號(hào)點(diǎn)處于最上游,其受上游灤河的影響較大.3號(hào)點(diǎn)出現(xiàn)了NO3－-N的最高負(fù)通量,該點(diǎn)也是最高的NH4+-N通量.這很大程度上是由于深層水庫(kù)的水溫較低,自春季開(kāi)始有機(jī)質(zhì)分解速率加強(qiáng),氨化作用得到增強(qiáng),促進(jìn)了NH4+-N的轉(zhuǎn)化釋放.同時(shí),厭氧狀態(tài)下硝化反應(yīng)受限,反硝化過(guò)程占主導(dǎo),使得沉積物-水界面NO3－-N被轉(zhuǎn)化為NO2－-N,并進(jìn)一步還原為N2與N2O[40],本研究中沉積物-水界面NO3－-N通量各個(gè)點(diǎn)位均為負(fù)的通量也佐證了這一點(diǎn).這也解釋了釋放水樣里,整體上NO2－-N負(fù)釋放通量越高的點(diǎn)位,其NO3－-N的釋放通量越低. NO2－-N釋放速率在3號(hào)點(diǎn)最低,空間差異性不明顯,這也是由于NO2－-N作為中間產(chǎn)物,濃度主要取決于沉積物與上覆水之間的硝化與反硝化強(qiáng)度,對(duì)DIN釋放通量影響很小.

3.3 沉積物-水界面P遷移特征及機(jī)制

相較于N,沉積物-水界面P的遷移轉(zhuǎn)化受到沉積物-水界面理化性質(zhì)的影響更大.SRP作為藻類能直接利用的磷素,是其生長(zhǎng)繁殖過(guò)程中必須的營(yíng)養(yǎng)鹽來(lái)源[41].潘家口水庫(kù)表層水體中SRP占TP的均值為37.58%(表3),一方面,這表明潘家口水庫(kù)磷素具有較高的生物有效性,而另一方面,相較于水庫(kù)整體較為均衡的TP濃度,SRP的空間差異性明顯,因此,沉積物-水界面的SRP通量是分析這一問(wèn)題的關(guān)鍵點(diǎn)之一.

表3 國(guó)內(nèi)湖庫(kù)水柱SRP 濃度及占比

沉積物表面存在著P的擴(kuò)散邊界層,其結(jié)構(gòu)與表層DO的含量密切相關(guān),好氧情況下,擴(kuò)散邊界層能有效阻止沉積物內(nèi)部高濃度的SRP向水體釋放,而厭氧條件下,邊界層結(jié)構(gòu)被破壞,對(duì)SRP的阻隔消失,大量SRP釋放入水柱造成水柱SRP濃度升高,從而補(bǔ)充水柱中SRP供給初級(jí)生產(chǎn)對(duì)于水柱磷的需求[45-47].潘家口水庫(kù)沉積物-水界面基本位于較深的滯溫層,其水體擾動(dòng)較小,沉積物表層的DO主要受水體擴(kuò)散的還原物質(zhì)氧化所決定[48].

由于常年網(wǎng)箱養(yǎng)殖,潘家口水庫(kù)所積累的有機(jī)質(zhì)提供了充足的氧化物.隨著春季末期溫度的升高,沉積物有機(jī)質(zhì)礦化反應(yīng)加強(qiáng),從而使得更多的SRP生成及解析到間隙水中,這也解釋了間隙水剖面中SRP僅在2cm左右就達(dá)到峰值,隨后變化趨于穩(wěn)定(圖3).同時(shí),厭氧狀態(tài)的沉積物也會(huì)促進(jìn)其內(nèi)部鐵磷的轉(zhuǎn)化,使得Fe3+被還原為Fe2+,釋放部分SRP進(jìn)入上覆水[49].水庫(kù)表層沉積物TP含量差異性不大,而間隙水剖面卻體現(xiàn)出明顯的空間差異性.考慮到水庫(kù)底層DO、溫度等影響P遷移轉(zhuǎn)化的關(guān)鍵理化條件大致相當(dāng)[18],推測(cè)圍網(wǎng)養(yǎng)殖造成的有機(jī)質(zhì)空間分布差異,是導(dǎo)致SRP濃度差異的決定性因素之一.4號(hào)點(diǎn)的樣品采集中發(fā)現(xiàn),沉積柱內(nèi)的表層沉積物孔隙度非常高,在柱樣釋放試驗(yàn)中甚至出現(xiàn)表層4～5cm沉積物完全漂浮于上覆水中.這是典型的劣質(zhì)化底泥,分析其成因在于投放飼料后圍網(wǎng)養(yǎng)魚(yú)被取締,使得疏松多孔的魚(yú)飼料覆蓋在沉積物表面,導(dǎo)致了間隙水與上覆水體內(nèi)SRP的低差異性.

從間隙水?dāng)?shù)據(jù)(圖3)中可以看出,潘家口水庫(kù)內(nèi),沉積物間隙水積累了高濃度的SRP,同時(shí)沉積物自身的磷素濃度也維持在較高的水平(表2),客觀上有著較高的磷釋放潛力.然而,沉積物-水界面的離子交換過(guò)程較為復(fù)雜,以分子擴(kuò)散模型模擬的通量誤差較大.同時(shí)潘家口水庫(kù)屬于多年調(diào)節(jié)型水庫(kù),換水周期長(zhǎng),水相流動(dòng)性小,沉積物內(nèi)TP濃度與水相磷之間大多不存在顯著的相關(guān)關(guān)系[43].因此,靜態(tài)釋放的通量研究能更為客觀的反映釋放過(guò)程中,沉積物-水界面的營(yíng)養(yǎng)鹽交換過(guò)程.

潘家口水庫(kù)水溫自4月開(kāi)始逐步上升,相對(duì)較高的水溫促進(jìn)有機(jī)質(zhì)分解.作為湖庫(kù)中磷最重要的再生庫(kù),沉積物中的有機(jī)態(tài)磷分解并從沉積物解吸進(jìn)入間隙水中,同時(shí)分解反應(yīng)的耗氧也使得擴(kuò)散邊界層的厭氧狀況加深[49].沉積物-水界面SRP的釋放通量已不容忽視.

從釋放通量上來(lái)看,SRP通量下游高于上游,整體與有機(jī)質(zhì)的分布規(guī)律趨同.相較于下游水庫(kù),以及類似的大型深水水庫(kù),潘家口水庫(kù)SRP的內(nèi)源負(fù)荷相對(duì)較高,實(shí)際的釋放通量對(duì)水柱的磷素影響也更為明顯(表4).

淺水水庫(kù)由于水深較淺,水體內(nèi)部的水溫分層現(xiàn)象基本上不存在,其受溫度驅(qū)動(dòng)更為明顯:氣溫越高,營(yíng)養(yǎng)鹽的釋放量通常越大[50].然而,對(duì)比處于夏季的淺水水庫(kù)(表4),春季潘家口水庫(kù)的SRP釋放通量整體依舊較高.其中少部分富營(yíng)養(yǎng)化嚴(yán)重的湖庫(kù),雖然SRP的釋放通量高于潘家口水庫(kù),但也基本處于同一數(shù)量級(jí),這意味著潘家口水庫(kù)的磷素內(nèi)源負(fù)荷相對(duì)較高.隨著夏季到來(lái),更高的底層水溫與更低的底層DO也意味著更大的SRP釋放風(fēng)險(xiǎn).鑒于潘家口水庫(kù)內(nèi)源負(fù)荷現(xiàn)狀及水庫(kù)自身的環(huán)境條件,建議嘗試通過(guò)原位覆蓋及底泥疏浚等技術(shù)控制水庫(kù)內(nèi)源負(fù)荷.

表4 國(guó)內(nèi)湖庫(kù)NH4+-N及SRP釋放通量

4 結(jié)論

4.1 潘家口水庫(kù)整體富營(yíng)養(yǎng)化程度較重,水體及沉積物營(yíng)養(yǎng)鹽含量高.沉積物中TN均值4765.06mg/kg, TP均值946.51mg/kg.整體趨勢(shì)上,自下游到上游,營(yíng)養(yǎng)鹽污染逐步降低,沉積物DIN與易交換態(tài)P含量相對(duì)較高.庫(kù)區(qū)氮磷及有機(jī)質(zhì)主要來(lái)源于庫(kù)區(qū)曾經(jīng)的網(wǎng)箱養(yǎng)殖.

4.2 潘家口水庫(kù)沉積物間隙水中的NH4+-N和SRP,濃度遠(yuǎn)高于上覆水,表明存在較大的釋放潛力.總體來(lái)說(shuō)下游高于上游.在垂向剖面上,NH4+-N隨深度逐步增長(zhǎng),SRP在表層2cm處達(dá)到峰值,隨后趨于穩(wěn)定.

4.3 水庫(kù)沉積物-水界面的氮磷通量較高. NH4+-N, NO3－-N、NO2－-N及SRP交換通量分別為23.71～ 156.80,-7.37～-161.78,1.64～33.45和0.56～2.86mg/ (m2·d).總體來(lái)說(shuō),下游釋放通量高于上游釋放通量,這與沉積物中氮磷含量相一致,是河流-水庫(kù)系統(tǒng)水力條件引起的大壩對(duì)于營(yíng)養(yǎng)鹽的截留作用及庫(kù)區(qū)曾經(jīng)的網(wǎng)箱養(yǎng)殖共同作用的結(jié)果.根據(jù)現(xiàn)有污染狀況分析,應(yīng)當(dāng)采用措施控制潘家口水庫(kù)內(nèi)源負(fù)荷.

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本文的研究工作受到了水利部海河水利委員會(huì)引灤工程管理局的諸多幫助,在此深表謝意.

Occurrence characteristics and transport fluxes of nitrogen and phosphorus at sediment-water interface of Panjiakou Reservoir in spring.

WANG Hong-wei1,2, WANG Shao-ming3, ZHANG Min1,2, HU Xiao-kang1,4, TANG Meng-yao1,2, YANG Fan-yan1, 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.Bureau of Luanhe Diversion Project, Haihe Water Conservancy Commission, Ministry of Water Resources, Qianxi 064309, China；4.School of Environmental Science and Engineering, Anhui Normal University, Wuhu 241000, China)., 2021,41(9)：4284～4293

The sediments from Panjiakou Reservoir were selected as the research object. The overall status of nutrient pollution and the release characteristics of internal loading of the reservoir were analyzed. The vertical distribution characteristics and spatial differences of nutrient were analysed by obtaining interstitial water in sediments via high-resolution interstitial water samplers (HR-Peeper). The transport fluxes of nutrient at the sediment-water interface were obtained by static release test of intact sediment cores, and the internal loading characteristics of Panjiakou Reservoir were analysed. The release risk of nutrient in the sediments was relatively high, and the TN and TP content was 3701.59～8221.28 mg/kg and 756.28～1696.15 mg/kg, respectively. According to the C/N ratio, the main cause of eutrophication in Panjiakou Reservoir was the residual feed and fish manure from cage culture before 2017. The static release results of the intact sediment cores showed that the exchange fluxes of NH4+-N、NO3—-N、NO2—-N and SRP were 23.71～156.80, -7.37 ～ -161.78, 1.64 ～ 33.4, and 0.56 ～ 2.86mg/(m2·d), respectively, and the internal loading of Panjiakou Reservoir was relatively high. The results are related to the high contents of organic matter, nitrogen and phosphorus, oxygen consumption by biological decomposition and gradually increasing water temperature in spring, which leads to the accelerated release of nutrient into the overlying water column. The internal loading of Panjiakou Reservoir can accelerate the process of eutrophication, and measures should be taken to control the internal loading of Panjiakou Reservoir.

Panjiakou Reservoir；sediment-water interface；nutrient；internal loading；diffusive flux

X524

A

1000-6923(2021)09-4284-10

王洪偉(1995-),男,安徽安慶人,中國(guó)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?kù)內(nèi)源負(fù)荷及污染控制.發(fā)表論文6篇.

2021-02-03

國(guó)家自然科學(xué)基金資助項(xiàng)目(41771516);水利部海河水利委員會(huì)引灤工程管理局潘家口水庫(kù)內(nèi)源污染專項(xiàng)

*責(zé)任作者, 副研究員, jczhong@niglas.ac.cn