亚热带水库水质特征及沉积物内源污染研究

王 斌,黄廷林*,陈 凡,杨鹏程,叶焰中,翟振起,周碧雯

亚热带水库水质特征及沉积物内源污染研究

王 斌1,2,黄廷林1,2*,陈 凡1,2,杨鹏程1,2,叶焰中3,翟振起3,周碧雯4

(1.西安建筑科技大学环境与市政工程学院,西北水资源与环境生态教育部重点实验室,陕西 西安 710055；2.西安建筑科技大学环境与市政工程学院,陕西省环境工程重点实验室,陕西 西安 710055；3.深圳市北部水源工程管理处茜坑水库管理所,广东 深圳 518110；4.深圳市楠柏环境科技有限公司,广东 深圳 518110)

为探究沉积物内源污染对亚热带分层型水源水库(茜坑水库)夏季水质的影响,采用现场监测和室内模拟相结合的研究手段,于2020年5～9月对茜坑水库深水区水温、溶解氧、氮磷等进行了监测,并采用静态实验模拟法分析了茜坑水库沉积物的耗氧速率及沉积物中氮磷的释放通量.原位监测结果表明,5～9月,茜坑水库水温和溶解氧均处于分层状态,该时期水库底层水体溶解氧含量较低,为沉积物内源污染物的厌氧释放提供了条件;分层期底层水体氨氮和总磷浓度显著高于表层和中层(<0.01),相应的表层水体氨氮和总磷平均浓度分别为0.062mg/L和0.033mg/L,中层为0.058mg/L和0.037mg/L,底层为0.242mg/L和0.052mg/L.静态模拟实验结果表明,水体及沉积物耗氧均符合零级反应动力学模型(R分别为0.987,0.989),其中沉积物的耗氧速率处于较高水平,为1.03g/(m2·d),约为水体的1.45倍;沉积物耗氧诱发等温层溶解氧降低并伴随沉积物内源污染释放,其中氨氮的释放极值为0.261mg/L,平均释放通量为7.36mg/(m2·d),总磷的释放极值为0.108mg/L,平均释放通量为2.20mg/(m2·d).内源氨氮和总磷的释放对水体贡献率分别可达27.98%和38.92%,沉积物氮磷释放对水库水质影响显著.

水库；热分层；水质特征；沉积物；内源污染

水库是城市的重要水资源,但我国多数水库正遭遇不同程度污染问题[1-2].沉积物是水库污染物的主要积蓄场所,在整个水体系统的物质循环过程中既充当“汇”,也充当“源”的角色[3].已有研究表明,在外源污染得到有效控制的情况下,沉积物内源氮磷的释放依旧会导致严重的水质污染[4].尤其对于分层型水库,分层期底层水体溶解氧被水体和沉积物中的还原物质及底栖生物呼吸作用耗尽,底层水体呈厌氧状态,沉积物开始向上覆水体释放氨氮、正磷酸盐等内源污染物,造成水体富营养化与藻类高发[5-6].因此,沉积物内源污染特征及其对水库水质影响的相关研究,对于水库水生态保护和供水安全保障,具有重要意义.

近年来对水库沉积物内源污染的研究主要集中在沉积物上覆水氮磷质量浓度、沉积物-水界面氮磷交换通量等方面[7].沉积物-水界面污染物交换通量大小与方向作为判断沉积物“汇”或“源”作用的重要指标,对于评估污染物生物地球化学循环速率和水生态系统生态风险评价至关重要[8].目前对沉积物污染物交换通量的研究方法主要有原位箱式法[9]、质量平衡法[10]、间隙水浓度扩散模型估算法[11]和实验室培养法[12].实验室培养法操作简单、方便,测定结果比较准确,被广泛用于沉积物氮磷释放相关研究.如文献[13-14]通过室内沉积物释放模拟实验,探究沉积物内源污染影响因素,量化沉积物-水界面氮磷交换通量,描述其释放风险.

随着粤港澳大湾区建设,支持深圳先行示范区等重大国家战略深入推进,水安全问题已成为影响经济增长与可持续发展的关键性瓶颈制约[15].茜坑水库作为深圳市西北部片区最重要的供水水库,近年常有局部蓝藻水华爆发,引起水库管理单位高度重视.夏季沉积物氮磷营养盐的释放可能是藻类爆发的重要诱因,但目前对于茜坑水库中下层水体及沉积物监测较少,沉积物内源释放对水体的影响尚不明确.本研究针对上述研究不足且为揭示茜坑水库沉积物内源污染特征,采用现场监测和室内模拟相结合的研究手段,于2020年夏季,对茜坑水库主库区深水区水质进行连续监测,并采用静态实验模拟法对沉积物耗氧速率及沉积物氮磷释放通量进行分析,以期为水库内源污染治理和水质原位改善研究提供有效数据基础.

1 材料与方法

1.1 研究区域概况

茜坑水库(113.994～114.022°E,22.690～22.711°N)位于深圳市龙华区福城街道,始建于1993年4月,于2002年5月完成扩建.茜坑水库地处北回归线以南,属于南亚热带海洋性季风气候.夏季气温22～35℃,冬季气温10～22℃,年平均气温22℃.水库正常库容1857万m3,总库容1917万m3,最大水深为20m.水库水域面积1.6km2,集雨面积4.79km2,无入库河流,水源主要来自市外引水.目前整个水库集雨区内均没有较为明显的点源和面源的人为污染源[16-17].沉积物的内源污染可能是目前水库污染物的主要来源.

图1 茜坑水库平面图及采样点分布示意

1.2 样品采集与监测

沉积物样品:在坝前深水区采用彼得森抓泥斗采集沉积物3次,现场充分混匀后装入聚乙烯自封袋(排出空气),5℃密封保存带回实验室.

水体样品:于2020年5～9月在坝前深水区进行取样监测,频率每月2～4次.采用2L有机玻璃采水器对取样点表层(水下0.5m),中层(温跃层中部),底层(沉积物上方0.5m)三个不同深度水样进行采集,分别置于聚乙烯取样瓶后立即运回实验室,24h内完成总氮(TN)、总磷(TP)、氨氮(NH4+-N)和硝酸盐氮(NO3--N)的测定.在采集水样同时,对点位水温(T)和溶解氧(DO)等参数选用HACH Hydrolab DS5型多参数水质测定仪(美国哈希公司)垂向间隔为1m进行原位监测.

1.3 静态实验设计

以体积为32L的PVC圆柱作为静态实验装置,装置高1000mm,内径为200mm(图2).将混合均匀的沉积物平铺于装置B底层,沉积物厚度约为50mm,静置完全后,抽走上覆水,用虹吸法再重新向装置B中装入深度约为950mm的上覆水水样;装置A中装入深度约为1000mm高上覆水.装置顶部密封有黑色柔性可变形塑料薄膜用以避光和平衡因取水而造成的装置内外压力差.装置中的DO浓度用荧光法测量(HQ30d便携式分析仪),每小时监测一次.每两天取100ml水样,用于装置B中TP和NH4+-N等指标的测定.

图2 静态实验装置

1.4 水质分析方法

所有指标的测定方法参照国家标准方法进行测定[18],TN采用碱性过硫酸钾氧化-紫外分光光度法,TP采用过硫酸钾消解-钼锑抗显色分光光度法,NH4+-N采用纳氏试剂分光光度法,NO3--N采用紫外分光光度法.

1.5 沉积物耗氧速率及释放量计算

沉积物耗氧速率(SOD)参照文献[19]计算.

SOD=HOD-WOD (1)

式中:SOD为沉积物耗氧速率,mg/(L·h);HOD为等温层需氧量,由装置B中测得DO随时间变化关系得出;WOD为水体需氧量,mg/(L·h),由装置A中测得DO随时间变化关系得出,水体需氧量(WOD)符合零级反应动力学方程关系[20].经换算可由式(2)求出单位时间、单位面积的沉积物的耗氧速率:

式中:SOD为沉积物耗氧速率,g/(m2·d);为耗氧系数,mg/(L·h);为与水接触的沉积物面积,m2;为装置中原水的体积,L.

静态实验平均释放通量采用式(3)计算[21]:

式中:为静态实验氮磷释放通量,mg/(m2·d);1为最大释放质量浓度,mg/L;为装置体积,L;为装置横截面积,m2;o为实验进行时间,d.

为说明沉积物氮磷释放对水库水质的影响,参照文献[22],在沉积物向上覆水体的释放仅考虑分子扩散的作用下,沉积物氮磷释放对水体贡献率采用式(4)计算:

式中:为溶质扩散对上覆水的贡献率,%;为氮磷释放通量,mg/(m2·d);w为茜坑水库水体滞留时间,d.根据水库管理所资料,茜坑水库平均水力停留时间为46d;为底层水体水深,m.本研究选取5m,与厌氧区高度保持一致;表示底层水体氮磷平均浓度, mg/L.

1.6 数据处理方法

水质参数采用Excel 2019软件建立数据库,绘图采用AutoCAD 2020和OriginPro 2018软件.

2 结果与分析

2.1 茜坑水库水质特征

2.1.1 水温、DO变化特征 5～9月,茜坑水库水温和DO均处于分层状态(图3),水库表层水温变化范围为28.32～31.89℃,底层为20.19～28.33℃;表层DO变化范围为7.71～11.28mg/L,底层为0～0.38mg/L.水温分层阻碍了DO的垂向传递,加之底层微生物和沉积物耗氧作用不断消耗氧气,造成水库底层0～5m处于缺氧或者厌氧状态.较低的DO将会影响氮磷等物质的循环过程,沉积物极有可能释放出内源污染物,造成上覆水体污染.

2.1.2 氮磷营养盐变化特征 5～9月,水库表层TP浓度为(0.033±0.013)mg/L;中层浓度为(0.038±0.014) mg/L;底层浓度为(0.054±0.016)mg/L(图4a).表层和中层TP平均浓度均超过《地表水环境质量标准》中Ⅱ类水的限值要求,底层TP平均浓度超过《地表水环境质量标准》中Ⅲ类水的限值要求,TP污染严重.底层水体TP浓度显著高于表层和中层(<0.01). NH4+-N在垂向上也存在同样的差异(<0.01),底层NH4+-N浓度为(0.243±0.111)mg/L,显著高于表层(0.062±0.038)mg/L和中层(0.059±0.049)mg/L(图4b).但底层水体TN和NO3--N含量并非显著大于表层和中层(>0.05)(图4c,4d),这可能是底层水体微生物发生反硝化作用导致的.

图3 茜坑水库水温、DO变化特征

图4 茜坑水库水库氮、磷变化特征

2.2 静态实验沉积物耗氧速率分析

如图5所示,装置中DO均随着时间的增加而降低.有关SOD的计算,Bowman等[23]研究表明,上覆水DO浓度在一定范围内,SOD为一常数,也即沉积物耗氧速率在上述范围内不依赖于上覆水溶解氧浓度的变化,为零级反应.但也有研究认为,SOD与上覆水DO浓度之间呈一定的幂函数关系[24-26].本研究将实验数据分别按零级和一级反应处理,经检验,装置B中DO浓度随时间变化呈零级反应动力学方程关系(2=0.987).计算得HOD为1.74g/(m2·d).WOD符合零级反应动力学(2=0.989),计算得WOD为0.71g/(m2·d).由式(1),SOD为1.03g/(m2·d).静态实验中SOD约为HOD的1.45倍,说明水体中溶解氧的降低主要是由于沉积物对氧气的消耗.

图5 静态实验DO变化

2.3 静态实验氮磷释放特征分析

图6 氮循环示意

上述结果表明,茜坑水库底层水体NH4+-N和TP浓度显著高于表层和中层水体,成为茜坑水库主要污染指标.为探明沉积物氮磷释放特征,对NH4+-N和TP的平均释放通量进行了分析.沉积物中NH4+- N、NO3--N、亚硝酸盐氮(NO2--N)随着上覆水DO、氧化还原条件等的变化进行硝化、反硝化作用[27-30](图6).静态实验表明,30d左右,氮磷释放达到平衡,沉积物中氮的释放主要以NH4+-N形式为主. NH4+-N的释放极值为0.261mg/L,由式(3),其平均释放通量为7.36mg/(m2·d).NH4+-N因矿化作用浓度逐渐升高,沉积物是上覆水NH4+-N的主要来源.静态实验TP的释放极值为0.108mg/L,平均释放通量为2.20mg/(m2·d).静态实验氮磷释放通量均为正值,表明沉积物是氮磷污染物的“源”.

表1 茜坑水库沉积物静态实验计算结果

3 讨论

3.1 SOD对茜坑水库水体DO的影响

沉积物耗氧是影响水体溶解氧的重要因素,对于研究水体氧收支平衡具有重要意义[31].研究表明[32],沉积物耗氧能占到整个水体耗氧的90%以上,对上覆水溶解氧有很大的影响.水体中的溶解氧一方面受表层水体与大气进行气体交换复氧和浮游植物光合作用增氧影响,另一方面受微生物呼吸作用耗氧和沉积物耗氧作用的影响.热分层期底层水体厌氧环境的形成是水体耗氧和复氧失衡导致的[33-34].本研究证实了以上结论,茜坑水库热分层期SOD约为WOD的1.45倍(图5),SOD是导致茜坑水库底层呈厌氧状态的主要因素,暗示热分层的存在阻碍了上下层水体溶解氧的传递.余晓等[35]对潘家口水库的研究表明,水库底层耗氧物质是造成热分层期间底层溶解氧浓度降低的重要原因.Müller等[36]也指出,水库底层厌氧区的耗氧主要是沉积物-水界面耗氧.苏露等[19]对金盆水库的研究也表明,金盆水库沉积物对氧气的消耗量约为水体的2～6倍,等温层中溶解氧的降低主要是由于沉积物对氧气的消耗.

相比于国内其他地区水库沉积物的耗氧速率(表2),茜坑水库沉积物的耗氧速率高于山西汾河水库、山东周村水库,但与气候相近的厦门西港水库沉积物的耗氧速率相近,这可能是夏季茜坑水库地处南亚热带,日平均气温高使得水库底层水温较高,沉积物-水界面的生物、化学反应活性增强使得其耗氧速率相对较大.

表2 不同水库沉积物耗氧速率

3.2 沉积物氮磷释放对茜坑水库水质的影响

沉积物是湖泊、水库等水体中营养盐的汇与源,沉积物中累积的氮、磷等污染物质可以在厌氧条件及再悬浮等作用下释放重新进入水体,对水质造成影响[40-41].茜坑水库底层水体厌氧区的出现为沉积物污染物的释放提供了有利条件.现场监测数据表明,夏季茜坑水库热分层期,底层水体厌氧导致沉积物中氨氮和总磷大量释放,底层水体氨氮平均浓度为0.242mg/L,总磷平均浓度为0.052mg/L,斜温层的稳定存在阻断了表层和底层水体的对流交换,底层水体氮磷浓度显著高于表层和中层水体.徐进等[21]对李家河水库的研究表明,热分层期,水库底部会出现季节性缺氧现象,沉积物附近氧化还原电位降低,不同形态的磷不断转化释放进入上覆水体中,使得底部水体总磷浓度迅速增大.夏品华等[42]对红枫湖水库季节性分层的水环境质量响应研究表明,分层期总磷具有上低下高的分布特征.

表3 茜坑水库沉积物营养盐扩散对水体的贡献率

注:为底层水体水涤;表示底层水体N、P平均浓度;为溶质扩散对上覆水的贡献率.

静态实验氨氮和磷的平均释放通量表明,沉积物向上覆水体释放氮磷污染物,沉积物是氮磷污染物的“源”.内源氨氮和磷的释放对水体的贡献率分别可达27.98%和38.92%(表3),沉积物氮磷对茜坑水库水质有较大的影响.

相比于国内其他类型湖库(表4),茜坑水库氨氮的磷的平均释放通量处于较高水平,茜坑水库沉积物氮磷的释放对水体造成的影响不容忽视.

表4 不同湖库沉积物氮磷释放通量

4 结论

4.1 夏季茜坑水库水体处于热分层状态,热分层的存在阻碍了DO的传递,底层水体DO含量较低,底层0～5m处于厌氧或缺氧状态,有利于沉积物污染物的释放.

4.2 静态实验沉积物的耗氧速率为1.03g/(m2·d),水体中溶解氧的降低主要是沉积物对氧气的消耗.

4.3 静态实验显示,沉积物中氨氮和总磷的平均释放通量为7.36mg/(m2·d)和2.20mg/(m2·d),表明沉积物是水库内源氮磷污染的“源”.沉积物氮磷的释放是底层水体氨氮和总磷的浓度显著高于表层和中层水体重要原因.消除水库底层厌氧区,控制沉积物内源污染,是改善茜坑水库水质的有力措施.

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本实验的现场采样工作由深圳市北部水源工程管理处茜坑水库管理所的工作人员协助完成,在此表示感谢.

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,内蒙古自治区鄂尔多斯市人,西安建筑科技大学硕士研究生,主要研究方向为水源水库污染物演替及水质改善.

2021-03-15

国家重点研发计划(2019YFD1100101);国家自然科学基金资助项目(51979217)

* 责任作者, 教授, huangtinglin@xauat.edu.cn