Characteristics and evaluation for nitrogen pollution in water and surface sediments of Xixi Wetland

Transcription

1 2013 Third International Conference on Intelligent System Design and Engineering Applications Characteristics and evaluation for nitrogen pollution in water and surface Shan Jian-li a, Wei-qin Zhu a, Min Pan a, Ze-ming Zhou a, Shu-ying Li a a College of Life and Environmental Science, Hangzhou Normal University, Hangzhou, , China Abstract In this paper, it was investigated and evaluated about characteristics of nitrogen pollution for 11 sampling water or surface sediments (0~10cm) in Hangzhou Xixi Wetland, and correlation analysis was conducted amongst nitrogen indexes for water and that for surface sediments. The results revealed that nitrogen nutrient loading was generally serious in water of Xixi Wetland, total nitrogen(tn) and NH 4 + -N concentration exceeded the standard in most sampling water. Organic nitrogen(on) was the mainly existing nitrogen forms in surface, according to evaluation criteria for organic nitrogen (ON, %) in sediments, the content of organic nitrogen (ON, %) in all the sampling sediments exceeded the pollution level IV, and TN contents showed serious biological toxicity in most sampling site based on Ontario environmental quality standards. Significantly positive correlation was found between concentration of TN or NH 4 + -N in water and NO 3 - -N content in surface sediments, moreover, content of NO 3 - -N in surface sediments may be attributed to the decomposition of organic matter in sediments since significantly negative correlation was found between total organic carbon (TOC) content and NO 3 - -N in sediments. Therefore, It is of great significance for controlling and reduction of organic matter and TN content existed in surface sediment of Xixi Wetland. Keywords: Xixi Wetland; Water; Surface sediment; Nitrogen pollution I. INTRODUCTION As a special ecosystem, wetlands can provide varieties of valuable service for humanity such as environment, resources and sustainable development[1,2]. Located in western Hangzhou City, Zhejiang Province,China. Xixi Wetland is one of the important provincial wetlands and also the most important city wetlands in Hangzhou, Currently, it gradually developed into a secondary wetland from native wetlands because of human intervention[3]. Three environmental factors of wetland is water quality, aquatic organisms and sediments. Since pollutants in water can easily transferred into the sediments and lead to sediment contamination[4], fortunately, contaminated sediments will increase accumulation of nitrogen nutrients and other pollutants in water[5], thus, the sediment can indirectly reflect the status of water pollution [6]. With large number of nitrogen nutrients and biological debris discharged into water, many wetlands was submitted to varying degrees of damage such as contamination and serious eutrophication of water[7-9]. Being main sources of nutrients, nitrogen in sediments has an important contribution to water eutrophication and may be the dominant factors for water eutrophication in wetland[10-13]. Reports also revealed that nitrogen pollution in surface sediments of Longevity Lake was serious[14]. Therefore, in this research, it was studied about characteristics and evaluation for nitrogen pollution in water and, with the aim of demonstrating the cycling, transferring and accumulation process of nitrogen nutrients in ecosystem of Xixi Wetland. II. MATERIALS AND METHODS 2.1 Sample collection Water and sediments were sampled in 11 sampling sites of Xixi wetland from April 21 until April 29, 2011, and sampling time was set at around 10:00 am every day. The 11 sampling sites were located in the first phase of Xixi Wetland, using the GPS global positioning system, sampling started from the south gate and then circled back to the northern end along the main flow of the wetland, the sampling sites evenly distributed and included WuYi Bridge (WYQ), Puji Bridge (PJQ,), BoAn (BA), Outside BoAn (BAW), Lianbin Bridge (LBQ), Jianxia Bridge (JJQ,) the Xixi Botanical Gardens (ZWY), Buffering District (HCQ), Xixibinshui Bridge (BSZ), the Xixicaotang Bridge (CTQ) and the Jiangxianggong Bridge (JXG ). Water samples was collected at 0.5m depths of each sampling sites and preserved into 300mL polyethylene bottle, then the sampling water was carried back to the laboratory, physical and chemical characteristics was determined on the same day. Surface Sediment (about 0~10cm) samples was collected with the trumpet Peterson Borrow devices (20cm*30cm*60cm, weighed 5.5kg), and each sample was collected into a clean, sealed plastic bags (soaked with 20% nitric acid and then washed with ultrapure water prior to use) and stored at darkness, then the sediment samples was placed into freezer when returned to laboratory, after freezing for 24h, the samples were freeze-dried for one week and then crushed and passed through 100 mesh sieve with agate mortar. 2.2 Analysis methods Contents of NH 4 + -N and NO 3 - -N both in water and sediment samples were determined by Nessler's reagent colorimetric method and UV spectrophotometry respectively [15], contents of total nitrogen (TN) in the water was determined by method of potassium persulfate oxidation - UV spectrophotometry. Content of total organic carbon (TOC) in water was analyzed using a Corresponding author, TEL: , FAX: zzhwq2000@163.com /12 $ IEEE DOI /ISDEA

2 Shimadzu Total Organic Carbon Analyzer (TOC-L) analyzer, and contents of total nitrogen(tn) and total organic carbon(toc) in sediment was determined by elemental analyzer (Vario MACRO, German). III. Results and discussion 3.1 Evaluation for nitrogen pollution in water of Xixi Wetland Concentration of total nitrogen, NH 4 + -N and NO 3 - -N can reflect the degree of nitrogen pollution in water, since NO 2 - -N content is very low and can be easily transformed into NO 3 - -N in natural water[16], NO 2 - -N concentration in water of Xixi wetland was not determined. As shown in Table 1 and Table 2, among the 11 sampling sites, relatively low concentration of total nitrogen (TN) were found in water of LBQ, JJQ, BSZ and CTQ, which conformed to class II ~ III standards for environmental quality of surface water (lakes and reservoirs), however, concentration s of total nitrogen (TN) exceeded the class V standards in water of PJQ, BA, BAW, JXG and WYQ, and the highest concentration of total nitrogen(tn) was found in water of WYQ, being 4.29mg/L. Also can be seen in Table 2, among the 11 sampling sites, concentration of NH 4 + -N in water of WYQ, BAW and JXG was 2.35,2.02, 2.06 mg/l, respectively, which all exceeded the class V standard for NH 4 + -N concentration in water, much lower concentration of NH 4 + -N was found in water of LBQ, JJQ, HCQ and CTQ, reaching to class II to class III standards for environmental quality standards (lakes and reservoirs) of the surface water, and the concentration of NH 4 + -N in other four sampling water were closer to 1.04 ~1.40mg/L, reaching to standards of class III to class IV for surface water. Moreover, NO 3 - -N concentration in all the 11 sampling water were within the limits of NO 3 - -N in drinking surface water. In addition, NO 3 - -N concentration was about 75%~84% of the total nitrogen concentrations for the nine sampling sediments, which showed that NO 3 - -N was the main nitrogen form in water of Xixi Wetland. Thus, concentrations of total nitrogen (TN) and NH 4 + -N in water of Xixi Wetland obviously exceeded the standards of environmental quality for surface water, and it should be strengthen for monitoring and management of TN and NH 4 + -N pollution in water of Xixi Wetland. Table 1. Environmental Quality standards for Surface Water and additional limits for centralized drinking surface water. Class-I Class-II Class-Ш Class- Class- TN NH + 4 -N Limited NO - 3 -N concentration for centralized drinking surface water sources (mg/l) 10 Table 2. Nitrogen concentration and its pollution evaluation in water of Xixi Wetland. TN NH + 4 -N NO - 3 -N Sampling site Contents Contents Pollution class Contents Pollution Pollution class WYQ 4.29 V 2.35 V 2.71 PJQ 3.37 V 1.39 III-IV 2.52 BA 2.64 V 1.08 III-IV 2.31 BAW 2.12 V 2.02 V 1.26 LBQ 0.86 II-III 0.86 II-III 0.97 JJQ 0.90 II-III 0.85 II-III 0.97 None ZWY 1.25 III-IV 1.35 III-IV 1.40 HCQ 1.09 III-IV 0.86 II-III 1.27 BSZ 0.86 II-III 1.04 III-IV 0.84 CTQ 0.98 II-III 0.96 II-III 1.03 JXG 3.76 V 2.06 V Contents of different nitrogen forms in surface As shown in Figure 1, obvious difference was found among total nitrogen (TN) contents in the 11 sampling surface sediments, the highest content of total nitrogen (TN) was found in the sediment of JJQ, which was 6.685mg/g; while the lowest content of total nitrogen (TN) was 1.715mg/g in the sediment of PJQ. Content of NH N in the sediment of BAW reached to the highest point of 0.472mg/g, and insignificant difference was found for the NH 4 + -N content in the sediments for the other ten sampling sites, which ranged from 0.131mg/g to 0.300mg/g. The highest NO 3 - -N content was 16.65mg/kg and found in surface sediments of PJQ while the minimum of NO - 3 -N content was 4.19mg/kg for sediments of CTQ. In addition, As shown in Figure 1, both the ratio of NH + 4 -N to TN and that of NO - 3 -N to TN were very low in the surface sediments for the 11 sampling sites, which ranged from 2.3% to 20.3% and from 0.1% to 1% respectively, and indicating that organic nitrogen (ON) could be the main nitrogen forms existing the surface. 3.3 Evaluation of nitrogen pollution and its ecological toxicity in surface sediment of Xixi Wetland Based on the similar data and literature about lake and other wetland [17-19], organic index and organic 420

3 nitrogen content was often used as indicators for evaluating environmental state and level of nitrogen pollution in sediments. Criteria for organic index and organic nitrogen and their evaluation results in sediments was shown in Table 3 and Table 4, respectively. environmental quality standards from department of Environment and Energy affiliated to Ontario, Canada, toxiceffects will be produced to benthic organisms when content of TN in sediments exceeded some certain level, the standard was divided into three class: (1) security level, which showed no toxic effects on aquatic organisms; (2) the lowest level, the sediment has been contaminated, but the majority of benthic organisms can withstand; (3) serious level, the benthic community has suffered significant damage [20]. As shown in Table 5, Content of total nitrogen (TN) in the surface sediments were greater than 550mg/kg for all the 11 sampling sites, and the ecotoxicity reached the lowest level, which demonstrated that surface sediment of Xixi Wetland was contaminated by total nitrogen (TN), but the vast majority of benthic organisms could withstand. However, content of total nitrogen (TN) in sediment of JJQ reached up to 6685 mg/kg, which were much greater than 4800 mg/kg and reached the degree of pollution, already posing severe toxicity on the benthos in sediments, thus, much more attention should be paid on this site. Table 3. Evaluation standards of organic index and organic nitrogen in sediments Pollution level Organic index Content of organic nitrogen(on) /% clean/ <0.05 <0.033 cleaner// fairly clean/ш polluted / >0.5 >0.133 Note: the organic index = organic carbon (%) of organic nitrogen (%), organic nitrogen= total nitrogen* Correlation between the indexes of nitrogen in water and that in surface sediment of Xixi Wetland Figure 1. Content of TN, NH 4 + -N and NO 3 - -N in surface As shown in Table 3 and Table 4, the highest organic index was found in sediments of JJQ, which exceeded six times than standards of nitrogen pollution for sediments, the lowest organic index was found in sediments of BAW and PJQ, which reached to the level of cleaner/ii. Moreover, similar results was found for organic nitrogen content in sediments of the 11 sampling sites, however, the content of organic nitrogen (ON,%) all exceeded the level of pollution/iv for the 11 sampling sites, which were mostly serious than that of the organic index. Based on the As shown in Table 6, significantly positive correlation was found between concentration of TN and that of NO 3 - -N or NH 4 + -N in water of Xixi Wetland, with the correlation coefficient (r) being (p <0.01) and (p <0.01) respectively, which indicated that NH 4 + -N and NO 3 - -N was the main existing forms of nitrogen in water of Xixi Wetland. Further analysis showed significantly positive correlation between content of NO N in surface sediments and concentration of TN or NH 4 + -N in water, with the correlation coefficient (r) being (p<0.05) and (p<0.05) respectively, which revealed that NO 3 - -N may be the main forms of nitrogen released into overlying water from surface sediments. Moreover, highly significant positive correlation (r = 0.845, p <0.01) was shown between content of TN and that of TOC in surface sediment, while significantly negative correlation was found between content of TOC and that of NH 4 + -N (r = , p <0.05) or NO 3 - -N (r= , p <0.05) in surface sediments, indicating that NH 4 + -N and NO 3 - -N in surface sediment of Xixi Wetland mainly stem from the decomposition of the organic nitrogen components. Therefore, it should be emphasized about controlling pollution of organic matter and nitrogen nutrition in surface sediments for reduction of nitrogen nutrients in water bodies of Xixi Wetland. 421

4 Table 4. Content of organic nitrogen (ON), organic index in surface sediments and their pollution level Sampling site TOC(%) TN (%) Organic index and its pollution level Organic nitrogen(on) content and its pollution level Organic index pollution level Content of organic nitrogen(%) pollution level WYQ fairly clean/ш polluted/ PJQ cleaner/ polluted/ BA polluted/ polluted/ BAW cleaner// polluted/ LBQ polluted/ polluted/ JJQ polluted/ polluted/ ZWY polluted/ polluted/ HCQ polluted/ polluted/ BSZ polluted/ polluted/ CTQ fairly clean/ш polluted/ JXG polluted/ polluted/ Note: data listed in the table are the average for each sampling site. Table 5. The toxic levels of TN(%) for surface sediment and their ecological toxicity standards. Sampling site TN(mg/kg) Toxic level WYQ 2100 the lowest PJQ 1715 the lowest Evaluation standards BA 3000 the lowest TN mg/kg Ecological toxicity standards BAW 1815 the lowest / security LBQ 3695 the lowest JJQ 6685 serious 550 the lowest ZWY 3015 the lowest HCQ 2900 the lowest 4800 serious BSZ 3405 the lowest CTQ 2215 the lowest JXG 2490 the lowest Table 6. Correlation matrix for nitrogen indexes from water and surface sediment of Xixi Wetland materials indicators TN Water Sediment Water NH + 4 -N NO 3 - -N TN mg/kg Note: * and ** indicates a significance level of 0.05 (r=0.602) and 0.01(r=0.735) for each correlation coefficient, NH 4 + -N (mg/kg) Sediment NO 3 - -N \(mg/kg) TN(mg/L) 1 NH 3-N (mg/l) 0.817** 1 NO 3-N(mg/L) 0.956** 0.647* 1 TN(mg/kg) NH 3-N(mg/kg) NO 3-N(mg/kg) 0.648* 0.653* TOC(mg/g) ** TOC (mg/g) IV. CONCLUSION Nitrogen nutrient loading was much more serious in water of Xixi Wetland, mainly existing form of nitrogen nutrients in the surface sediments was organic nitrogen(on), and the content of organic nitrogen (ON) in surface sediments exceeded the level of pollution/iv and was close to serious biological toxicity criteria. However, concentration of TN and NH 4 + -N in water of Xixi Wetland was positively correlated with NO 3 - -N content in surface sediments, while NO 3 - -N in sediment originated from the decomposition of organic matter in sediments. Therefore, it is of great significance to control and reduce the content of organic matter and total nitrogen (TN) in the surface sediment of Xixi Wetland. ACKNOWLEDGEMENTS This research was supported by the Project of Zhejiang Key Scientific and Technological Innovation Team (2010R50039). REFERENCES [1] Lang H Q, Lin P, Lu J J. China wetland research and protection. Shan Hai: East China normal university press,1998. (in Chinese) 422

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