金佛山世界遗产地岩溶地下河系统硝酸盐来源与转化

Sources and transformations of nitrate of the subterranean river system in Jinfoshan Karst World Heritage

地下水硝酸盐污染已成为一个普遍的环境问题.为研究重庆金佛山水房泉岩溶地下河系统的硝酸盐来源与转化,于2017年4-10月每24d左右对地下河系统内的某酒店自来水、化粪池、1^#落水洞、水房泉4个采样点开展监测,进行水化学和δ^15Nnitrate、δ^18Onitrate同位素分析.某酒店污水经化粪池处理后,由1^#落水洞排入地下河,最后在水房泉排泄.结果表明:①水房泉NO-3浓度范围为4.65~10.20mg/L,相对于我国生活饮用水标准处于较低水平;化粪池、1^#落水洞、水房泉3个采样点电导率和NO3^-、Cl^-浓度的高值期与游客人数增多对应关系较好.②某酒店自来水δ^15Nnitrate值为3.7‰~5.8‰、δ^18Onitrate值为1.6‰~2.7‰,说明硝酸盐主要来源为土壤有机氮,处于自然背景值;1^#落水洞δ^15Nnitrate值为14.4‰~21.1‰、δ^18Onitrate值为3.5‰~11.2‰,显示硝酸盐主要来源为粪肥污水;化粪池和水房泉的δ^15Nnitrate值为3.7‰~17.0‰、δ^18Onitrate值为-9.0‰~7.3‰,表明硝酸盐主要来源为土壤有机氮与粪肥污水,显示其硝酸盐主要污染源是酒店生活废污水.③某酒店自来水、水房泉地下水的硝酸盐转化过程以同化作用为主;化粪池污水以硝化作用为主,是岩溶地下河系统硝酸盐的重要来源之一;1^#落水洞污水表现为反硝化作用.④基于SIAR模型对水房泉的硝酸盐来源进行定量解析,发现大气降水、土壤有机氮和粪肥污水的贡献率分别为28%、36%和36%左右.

Nitrate pollution in groundwater has become a common environmental problem.From April to October in 2017,four sampling sites (a hotel's tap water,septic system,Sinkhole 1^# and Shuifang spring) in Jinfoshan Karst,Chongqing were monitored every 24 days to investigate the sources and transformations of nitrate.The hydrochemistry and δ^15 N nitrate,δ^18 O nitrate isotopic analyses were carried out.The sewage from hotel was degraded in the septic system,then discharged into the subterranean river by the Sinkhole 1^#,and finally drained at Shuifang spring.Results showed that the concentrations of nitrate ranged from 4.65 to 10.20 mg/L at Shuifang spring,which was at a low level comparing with the standard of drinking water in China.The three high-value periods of the electrical conductivity and concentrations of nitrate and chloride at septic system,Sinkhole 1^# and Shuifang spring corresponded well to the increase in the number of tourists.The isotopic nitrate compositions of hotel's tap water were found to range from 3.7 ‰ to 5.8 ‰ for δ^15 N nitrate and from 1.6 ‰ to 2.7 ‰ for δ^18 O nitrate,indicating that the nitrate mainly originated from soil organic nitrogen maintaining at a natural background value.The δ 15 N nitrate and δ^18 O nitrate at Sinkhole 1^# varied from 14.4 ‰ to 21.1 ‰,and from 3.5 ‰ to 11.2 ‰,respectively,revealing that manure & sewage was the main source of nitrate at Sinkhole 1 #.The δ^15 N nitrate and δ^18 O nitrate at septic system and Shuifang spring varied between 3.7 ‰ and 17 ‰ for δ^15 N nitrate value, and -9.0 ‰ and 7.3 ‰ for δ^18 O nitrate value,suggesting that soil organic nitrogen and manure & sewage were the major sources of nitrate which was dominantly derived from the domestic sewage of the hotel.In hotel's tap water and groundwater from Shuifang spring,assimilation is the main process of nitrate transformation.While effluent in septic system,one of the important sources of nitrate in the subterranean river system was affected by nitrification.The wastewater at Sinkhole 1^# was characterized by denitrification.A SIAR model was used to estimate the contribution of each nitrate source to total nitrate at Shuifang spring,which indicated that the contribution rate of atmospheric precipitation,soil organic nitrogen and manure & sewage were roughly 28 %,36 % and 36 %,respectively.