亞熱帶水庫水質特征及沉積物內源污染研究

王 斌,黃廷林*,陳 凡,楊鵬程,葉焰中,翟振起,周碧雯

亞熱帶水庫水質特征及沉積物內源污染研究

王 斌1,2,黃廷林1,2*,陳 凡1,2,楊鵬程1,2,葉焰中3,翟振起3,周碧雯4

(1.西安建筑科技大學環境與市政工程學院,西北水資源與環境生態教育部重點實驗室,陜西 西安 710055；2.西安建筑科技大學環境與市政工程學院,陜西省環境工程重點實驗室,陜西 西安 710055；3.深圳市北部水源工程管理處茜坑水庫管理所,廣東 深圳 518110；4.深圳市楠柏環境科技有限公司,廣東 深圳 518110)

為探究沉積物內源污染對亞熱帶分層型水源水庫(茜坑水庫)夏季水質的影響,采用現場監測和室內模擬相結合的研究手段,于2020年5～9月對茜坑水庫深水區水溫、溶解氧、氮磷等進行了監測,并采用靜態實驗模擬法分析了茜坑水庫沉積物的耗氧速率及沉積物中氮磷的釋放通量.原位監測結果表明,5～9月,茜坑水庫水溫和溶解氧均處于分層狀態,該時期水庫底層水體溶解氧含量較低,為沉積物內源污染物的厭氧釋放提供了條件;分層期底層水體氨氮和總磷濃度顯著高于表層和中層(<0.01),相應的表層水體氨氮和總磷平均濃度分別為0.062mg/L和0.033mg/L,中層為0.058mg/L和0.037mg/L,底層為0.242mg/L和0.052mg/L.靜態模擬實驗結果表明,水體及沉積物耗氧均符合零級反應動力學模型(R分別為0.987,0.989),其中沉積物的耗氧速率處于較高水平,為1.03g/(m2·d),約為水體的1.45倍;沉積物耗氧誘發等溫層溶解氧降低并伴隨沉積物內源污染釋放,其中氨氮的釋放極值為0.261mg/L,平均釋放通量為7.36mg/(m2·d),總磷的釋放極值為0.108mg/L,平均釋放通量為2.20mg/(m2·d).內源氨氮和總磷的釋放對水體貢獻率分別可達27.98%和38.92%,沉積物氮磷釋放對水庫水質影響顯著.

水庫；熱分層；水質特征；沉積物；內源污染

水庫是城市的重要水資源,但我國多數水庫正遭遇不同程度污染問題[1-2].沉積物是水庫污染物的主要積蓄場所,在整個水體系統的物質循環過程中既充當“匯”,也充當“源”的角色[3].已有研究表明,在外源污染得到有效控制的情況下,沉積物內源氮磷的釋放依舊會導致嚴重的水質污染[4].尤其對于分層型水庫,分層期底層水體溶解氧被水體和沉積物中的還原物質及底棲生物呼吸作用耗盡,底層水體呈厭氧狀態,沉積物開始向上覆水體釋放氨氮、正磷酸鹽等內源污染物,造成水體富營養化與藻類高發[5-6].因此,沉積物內源污染特征及其對水庫水質影響的相關研究,對于水庫水生態保護和供水安全保障,具有重要意義.

近年來對水庫沉積物內源污染的研究主要集中在沉積物上覆水氮磷質量濃度、沉積物-水界面氮磷交換通量等方面[7].沉積物-水界面污染物交換通量大小與方向作為判斷沉積物“匯”或“源”作用的重要指標,對于評估污染物生物地球化學循環速率和水生態系統生態風險評價至關重要[8].目前對沉積物污染物交換通量的研究方法主要有原位箱式法[9]、質量平衡法[10]、間隙水濃度擴散模型估算法[11]和實驗室培養法[12].實驗室培養法操作簡單、方便,測定結果比較準確,被廣泛用于沉積物氮磷釋放相關研究.如文獻[13-14]通過室內沉積物釋放模擬實驗,探究沉積物內源污染影響因素,量化沉積物-水界面氮磷交換通量,描述其釋放風險.

隨著粵港澳大灣區建設,支持深圳先行示范區等重大國家戰略深入推進,水安全問題已成為影響經濟增長與可持續發展的關鍵性瓶頸制約[15].茜坑水庫作為深圳市西北部片區最重要的供水水庫,近年常有局部藍藻水華爆發,引起水庫管理單位高度重視.夏季沉積物氮磷營養鹽的釋放可能是藻類爆發的重要誘因,但目前對于茜坑水庫中下層水體及沉積物監測較少,沉積物內源釋放對水體的影響尚不明確.本研究針對上述研究不足且為揭示茜坑水庫沉積物內源污染特征,采用現場監測和室內模擬相結合的研究手段,于2020年夏季,對茜坑水庫主庫區深水區水質進行連續監測,并采用靜態實驗模擬法對沉積物耗氧速率及沉積物氮磷釋放通量進行分析,以期為水庫內源污染治理和水質原位改善研究提供有效數據基礎.

1 材料與方法

1.1 研究區域概況

茜坑水庫(113.994～114.022°E,22.690～22.711°N)位于深圳市龍華區福城街道,始建于1993年4月,于2002年5月完成擴建.茜坑水庫地處北回歸線以南,屬于南亞熱帶海洋性季風氣候.夏季氣溫22～35℃,冬季氣溫10～22℃,年平均氣溫22℃.水庫正常庫容1857萬m3,總庫容1917萬m3,最大水深為20m.水庫水域面積1.6km2,集雨面積4.79km2,無入庫河流,水源主要來自市外引水.目前整個水庫集雨區內均沒有較為明顯的點源和面源的人為污染源[16-17].沉積物的內源污染可能是目前水庫污染物的主要來源.

圖1 茜坑水庫平面圖及采樣點分布示意

1.2 樣品采集與監測

沉積物樣品:在壩前深水區采用彼得森抓泥斗采集沉積物3次,現場充分混勻后裝入聚乙烯自封袋(排出空氣),5℃密封保存帶回實驗室.

水體樣品:于2020年5～9月在壩前深水區進行取樣監測,頻率每月2～4次.采用2L有機玻璃采水器對取樣點表層(水下0.5m),中層(溫躍層中部),底層(沉積物上方0.5m)三個不同深度水樣進行采集,分別置于聚乙烯取樣瓶后立即運回實驗室,24h內完成總氮(TN)、總磷(TP)、氨氮(NH4+-N)和硝酸鹽氮(NO3--N)的測定.在采集水樣同時,對點位水溫(T)和溶解氧(DO)等參數選用HACH Hydrolab DS5型多參數水質測定儀(美國哈希公司)垂向間隔為1m進行原位監測.

1.3 靜態實驗設計

以體積為32L的PVC圓柱作為靜態實驗裝置,裝置高1000mm,內徑為200mm(圖2).將混合均勻的沉積物平鋪于裝置B底層,沉積物厚度約為50mm,靜置完全后,抽走上覆水,用虹吸法再重新向裝置B中裝入深度約為950mm的上覆水水樣;裝置A中裝入深度約為1000mm高上覆水.裝置頂部密封有黑色柔性可變形塑料薄膜用以避光和平衡因取水而造成的裝置內外壓力差.裝置中的DO濃度用熒光法測量(HQ30d便攜式分析儀),每小時監測一次.每兩天取100ml水樣,用于裝置B中TP和NH4+-N等指標的測定.

圖2 靜態實驗裝置

1.4 水質分析方法

所有指標的測定方法參照國家標準方法進行測定[18],TN采用堿性過硫酸鉀氧化-紫外分光光度法,TP采用過硫酸鉀消解-鉬銻抗顯色分光光度法,NH4+-N采用納氏試劑分光光度法,NO3--N采用紫外分光光度法.

1.5 沉積物耗氧速率及釋放量計算

沉積物耗氧速率(SOD)參照文獻[19]計算.

SOD=HOD-WOD (1)

式中:SOD為沉積物耗氧速率,mg/(L·h);HOD為等溫層需氧量,由裝置B中測得DO隨時間變化關系得出;WOD為水體需氧量,mg/(L·h),由裝置A中測得DO隨時間變化關系得出,水體需氧量(WOD)符合零級反應動力學方程關系[20].經換算可由式(2)求出單位時間、單位面積的沉積物的耗氧速率:

式中:SOD為沉積物耗氧速率,g/(m2·d);為耗氧系數,mg/(L·h);為與水接觸的沉積物面積,m2;為裝置中原水的體積,L.

靜態實驗平均釋放通量采用式(3)計算[21]:

式中:為靜態實驗氮磷釋放通量,mg/(m2·d);1為最大釋放質量濃度,mg/L;為裝置體積,L;為裝置橫截面積,m2;o為實驗進行時間,d.

為說明沉積物氮磷釋放對水庫水質的影響,參照文獻[22],在沉積物向上覆水體的釋放僅考慮分子擴散的作用下,沉積物氮磷釋放對水體貢獻率采用式(4)計算:

式中:為溶質擴散對上覆水的貢獻率,%;為氮磷釋放通量,mg/(m2·d);w為茜坑水庫水體滯留時間,d.根據水庫管理所資料,茜坑水庫平均水力停留時間為46d;為底層水體水深,m.本研究選取5m,與厭氧區高度保持一致;表示底層水體氮磷平均濃度, mg/L.

1.6 數據處理方法

水質參數采用Excel 2019軟件建立數據庫,繪圖采用AutoCAD 2020和OriginPro 2018軟件.

2 結果與分析

2.1 茜坑水庫水質特征

2.1.1 水溫、DO變化特征 5～9月,茜坑水庫水溫和DO均處于分層狀態(圖3),水庫表層水溫變化范圍為28.32～31.89℃,底層為20.19～28.33℃;表層DO變化范圍為7.71～11.28mg/L,底層為0～0.38mg/L.水溫分層阻礙了DO的垂向傳遞,加之底層微生物和沉積物耗氧作用不斷消耗氧氣,造成水庫底層0～5m處于缺氧或者厭氧狀態.較低的DO將會影響氮磷等物質的循環過程,沉積物極有可能釋放出內源污染物,造成上覆水體污染.

2.1.2 氮磷營養鹽變化特征 5～9月,水庫表層TP濃度為(0.033±0.013)mg/L;中層濃度為(0.038±0.014) mg/L;底層濃度為(0.054±0.016)mg/L(圖4a).表層和中層TP平均濃度均超過《地表水環境質量標準》中Ⅱ類水的限值要求,底層TP平均濃度超過《地表水環境質量標準》中Ⅲ類水的限值要求,TP污染嚴重.底層水體TP濃度顯著高于表層和中層(<0.01). NH4+-N在垂向上也存在同樣的差異(<0.01),底層NH4+-N濃度為(0.243±0.111)mg/L,顯著高于表層(0.062±0.038)mg/L和中層(0.059±0.049)mg/L(圖4b).但底層水體TN和NO3--N含量并非顯著大于表層和中層(>0.05)(圖4c,4d),這可能是底層水體微生物發生反硝化作用導致的.

圖3 茜坑水庫水溫、DO變化特征

圖4 茜坑水庫水庫氮、磷變化特征

2.2 靜態實驗沉積物耗氧速率分析

如圖5所示,裝置中DO均隨著時間的增加而降低.有關SOD的計算,Bowman等[23]研究表明,上覆水DO濃度在一定范圍內,SOD為一常數,也即沉積物耗氧速率在上述范圍內不依賴于上覆水溶解氧濃度的變化,為零級反應.但也有研究認為,SOD與上覆水DO濃度之間呈一定的冪函數關系[24-26].本研究將實驗數據分別按零級和一級反應處理,經檢驗,裝置B中DO濃度隨時間變化呈零級反應動力學方程關系(2=0.987).計算得HOD為1.74g/(m2·d).WOD符合零級反應動力學(2=0.989),計算得WOD為0.71g/(m2·d).由式(1),SOD為1.03g/(m2·d).靜態實驗中SOD約為HOD的1.45倍,說明水體中溶解氧的降低主要是由于沉積物對氧氣的消耗.

圖5 靜態實驗DO變化

2.3 靜態實驗氮磷釋放特征分析

圖6 氮循環示意

上述結果表明,茜坑水庫底層水體NH4+-N和TP濃度顯著高于表層和中層水體,成為茜坑水庫主要污染指標.為探明沉積物氮磷釋放特征,對NH4+-N和TP的平均釋放通量進行了分析.沉積物中NH4+- N、NO3--N、亞硝酸鹽氮(NO2--N)隨著上覆水DO、氧化還原條件等的變化進行硝化、反硝化作用[27-30](圖6).靜態實驗表明,30d左右,氮磷釋放達到平衡,沉積物中氮的釋放主要以NH4+-N形式為主. NH4+-N的釋放極值為0.261mg/L,由式(3),其平均釋放通量為7.36mg/(m2·d).NH4+-N因礦化作用濃度逐漸升高,沉積物是上覆水NH4+-N的主要來源.靜態實驗TP的釋放極值為0.108mg/L,平均釋放通量為2.20mg/(m2·d).靜態實驗氮磷釋放通量均為正值,表明沉積物是氮磷污染物的“源”.

表1 茜坑水庫沉積物靜態實驗計算結果

3 討論

3.1 SOD對茜坑水庫水體DO的影響

沉積物耗氧是影響水體溶解氧的重要因素,對于研究水體氧收支平衡具有重要意義[31].研究表明[32],沉積物耗氧能占到整個水體耗氧的90%以上,對上覆水溶解氧有很大的影響.水體中的溶解氧一方面受表層水體與大氣進行氣體交換復氧和浮游植物光合作用增氧影響,另一方面受微生物呼吸作用耗氧和沉積物耗氧作用的影響.熱分層期底層水體厭氧環境的形成是水體耗氧和復氧失衡導致的[33-34].本研究證實了以上結論,茜坑水庫熱分層期SOD約為WOD的1.45倍(圖5),SOD是導致茜坑水庫底層呈厭氧狀態的主要因素,暗示熱分層的存在阻礙了上下層水體溶解氧的傳遞.余曉等[35]對潘家口水庫的研究表明,水庫底層耗氧物質是造成熱分層期間底層溶解氧濃度降低的重要原因.Müller等[36]也指出,水庫底層厭氧區的耗氧主要是沉積物-水界面耗氧.蘇露等[19]對金盆水庫的研究也表明,金盆水庫沉積物對氧氣的消耗量約為水體的2～6倍,等溫層中溶解氧的降低主要是由于沉積物對氧氣的消耗.

相比于國內其他地區水庫沉積物的耗氧速率(表2),茜坑水庫沉積物的耗氧速率高于山西汾河水庫、山東周村水庫,但與氣候相近的廈門西港水庫沉積物的耗氧速率相近,這可能是夏季茜坑水庫地處南亞熱帶,日平均氣溫高使得水庫底層水溫較高,沉積物-水界面的生物、化學反應活性增強使得其耗氧速率相對較大.

表2 不同水庫沉積物耗氧速率

3.2 沉積物氮磷釋放對茜坑水庫水質的影響

沉積物是湖泊、水庫等水體中營養鹽的匯與源,沉積物中累積的氮、磷等污染物質可以在厭氧條件及再懸浮等作用下釋放重新進入水體,對水質造成影響[40-41].茜坑水庫底層水體厭氧區的出現為沉積物污染物的釋放提供了有利條件.現場監測數據表明,夏季茜坑水庫熱分層期,底層水體厭氧導致沉積物中氨氮和總磷大量釋放,底層水體氨氮平均濃度為0.242mg/L,總磷平均濃度為0.052mg/L,斜溫層的穩定存在阻斷了表層和底層水體的對流交換,底層水體氮磷濃度顯著高于表層和中層水體.徐進等[21]對李家河水庫的研究表明,熱分層期,水庫底部會出現季節性缺氧現象,沉積物附近氧化還原電位降低,不同形態的磷不斷轉化釋放進入上覆水體中,使得底部水體總磷濃度迅速增大.夏品華等[42]對紅楓湖水庫季節性分層的水環境質量響應研究表明,分層期總磷具有上低下高的分布特征.

表3 茜坑水庫沉積物營養鹽擴散對水體的貢獻率

注:為底層水體水滌;表示底層水體N、P平均濃度;為溶質擴散對上覆水的貢獻率.

靜態實驗氨氮和磷的平均釋放通量表明,沉積物向上覆水體釋放氮磷污染物,沉積物是氮磷污染物的“源”.內源氨氮和磷的釋放對水體的貢獻率分別可達27.98%和38.92%(表3),沉積物氮磷對茜坑水庫水質有較大的影響.

相比于國內其他類型湖庫(表4),茜坑水庫氨氮的磷的平均釋放通量處于較高水平,茜坑水庫沉積物氮磷的釋放對水體造成的影響不容忽視.

表4 不同湖庫沉積物氮磷釋放通量

4 結論

4.1 夏季茜坑水庫水體處于熱分層狀態,熱分層的存在阻礙了DO的傳遞,底層水體DO含量較低,底層0～5m處于厭氧或缺氧狀態,有利于沉積物污染物的釋放.

4.2 靜態實驗沉積物的耗氧速率為1.03g/(m2·d),水體中溶解氧的降低主要是沉積物對氧氣的消耗.

4.3 靜態實驗顯示,沉積物中氨氮和總磷的平均釋放通量為7.36mg/(m2·d)和2.20mg/(m2·d),表明沉積物是水庫內源氮磷污染的“源”.沉積物氮磷的釋放是底層水體氨氮和總磷的濃度顯著高于表層和中層水體重要原因.消除水庫底層厭氧區,控制沉積物內源污染,是改善茜坑水庫水質的有力措施.

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本實驗的現場采樣工作由深圳市北部水源工程管理處茜坑水庫管理所的工作人員協助完成,在此表示感謝.

Water quality characteristics and sediments endogenous pollution of subtropical stratified reservoir.

WANG Bin1,2, HUANG Ting-lin1,2*, CHEN Fan1,2, YANG Peng-cheng1,2, YEYan-zhong3, ZHAIZhen-qi3, ZHOUBi-wen4

(1.Key Laboratory of Northwest Water Resource, Environment and Ecology, Ministry of Education, Xi’an University of Architecture and Technology, Xi’an 710055, China；2.Shaanxi Key Laboratory of Environmental Engineering, School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China；3.Xikeng Reservoir Management Institute, North Water Resources Engineering Management Office, Guangdong, Shenzhen 518110, China；4.Shenzhen Nanbo Environmental Technology Co., Ltd., Guangdong, Shenzhen 518110, China)., 2021,41(10)：4829～4836

To explore the impact of sediment endogenous pollution on subtropical stratified water source reservoirs (Xikeng Reservoir), a combination of in-situ monitoring and indoor simulation was used to analyze the water temperature and dissolved oxygen, nitrogen and phosphorus in the deep water area of the Xikeng Reservoir from May to September 2020. The oxygen consumption rate of sediments in Xikeng Reservoir and the release flux of nitrogen and phosphorus in the sediments were analyzed by the static experimental simulation method. The results of in-situ monitoring showed that the water temperature and dissolved oxygen in Xikeng Reservoir were in stratified state from May to September, and the dissolved oxygen in the bottom of the reservoir was low during this period, which provides prerequisites for the anaerobic release of endogenous pollutants from sediments. In the stratification stage, the concentrations of ammonia nitrogen and total phosphorus in bottom water were significantly higher than those in surface and middle water (<0.01). The corresponding average concentrations of ammonia nitrogen and total phosphorus in surface water were 0.062mg/L and 0.033mg/L, respectively, while those in middle water were 0.058mg/L and 0.037mg/L, and those in bottom water were 0.242mg/L and 0.052mg/L. Static simulation experiments showed that the oxygen consumption of both water and sediments was in line with the zero-order reaction kinetics model (2was 0.987 and 0.989, respectively). The oxygen consumption rate of sediments was 1.03g/(m2·d), which was about 1.45times of that of water. The oxygen consumption of sediment induced the reduction of dissolved oxygenand the release of sediment endogenous pollution. The maximum release value of ammonia nitrogen was 0.261mg/L, and the average release flux was 7.36mg/(m2·d). The maximum release value of total phosphorus was 0.108mg/L, and the average release flux was 2.20mg/(m2·d). The release of endogenous ammonia nitrogen and total phosphorus contributed 27.98% and 38.92% to the water, and the release of nitrogen and phosphorus from sediments had a significant effect on the water quality of the reservoir.

stratified reservoir；thermal stratification；water quality characteristics；sediments；endogenous pollution

X524

A

1000-6923(2021)10-4829-08

王 斌(1995-),男,內蒙古自治區鄂爾多斯市人,西安建筑科技大學碩士研究生,主要研究方向為水源水庫污染物演替及水質改善.

2021-03-15

國家重點研發計劃(2019YFD1100101);國家自然科學基金資助項目(51979217)

* 責任作者, 教授, huangtinglin@xauat.edu.cn