基于δ15N和δ18O的农业区地下河硝酸盐污染来源

为研究岩溶区农业活动为主导的地下河流域硝酸盐污染来源,于2017年5~10月每24 d左右对重庆青木关流域6个采样点进行监测,利用~(15)N和~(18)O同位素技术对示踪硝酸盐来源进行解译,应用IsoSource模型计算出不同端元硝酸盐的贡献率.结果表明:(1)青木关农业区地下河系统存在较大的硝酸盐污染风险,大部分采样点出现不同程度NO_3~--N浓度超标现象.(2)空间上,青木关地下河中NO_3~--N浓度整体呈现由上游向下游升高的趋势.时间上,上游鱼塘和岩口落水洞以及下游姜家泉样点NO_3~--N浓度在5~6月因受农业施肥的影响,均呈上升趋势,6~9月受降水影响而出现不同程度升高或降低,9月之后随着农业活动减少而逐渐降低;中游土壤点NO_3~--N浓度保持较高值;中下游大鹿池NO_3~--N浓度较低且变幅不大.(3)通过硝酸盐~(15)N和~(18)O同位素分析,表明上游鱼塘和岩口落水洞的硝酸盐源于土壤有机氮、动物粪便及污废水混合;中游土壤点硝酸盐源于土壤有机氮、降水和肥料中NH_4~+;中下游大鹿池中硝酸盐来源于动物粪便及污废水、土壤有机氮、降水和肥料中NH_4~+的混合作用.地下河出口处姜家泉硝酸盐污染严重,其源于土壤有机氮、降水和肥料中NH_4~+、动物粪便及污废水、大气沉降的综合作用.(4)基于IsoSource模型对地下河出口处硝酸盐来源进行定量分析,发现动物粪便及污废水贡献率占46.4%,土壤有机氮占32.6%,降水与肥料中NH_4~+占18.6%,大气沉降仅占2.4%.

Nitrate-Nitrogen Pollution Sources of an Underground River in Karst Agricultural Area Using 15N and 18O Isotope Technique

The objectives of this study were to reveal the sources of nitrate and the ratio of karst in an agricultural basin based on a ¹⁵N and ¹⁸O isotope technique and quantitative calculation of the IsoSource model. From May to October 2017, six sampling points in the Qingmuguan river basin, Chongqing, were monitored every 24 d. Results showed that there was a great risk of nitrate pollution in the underground river system, because most NO₃^--N concentrations of the sampling points exceeded the threshold. Spatially, NO₃^--N concentrations in the underground river increased from upstream to downstream. Temporally, NO₃^--N concentrations of Fishpond and Yankou Ponor upstream and Jiangjia Spring downstream were impacted by agricultural fertilizer from May to June and fluctuated from June to September due to precipitation. With decreased agricultural activities, NO₃^--N concentrations gradually decreased after September. NO₃^--N concentrations were high in midstream soil water. Daluchi, in the middle and lower reaches, maintained relatively low NO₃^--N concentrations with stable fluctuations. Dual ¹⁵N and ¹⁸O isotopic compositions suggested that the upstream nitrates were derived from soil organic nitrogen and a mixture of manure and sewage. The midstream nitrates originated from soil organic nitrogen and NH₄⁺ from fertilizer and rain. Nitrates in the middle and lower reaches were derived from the mixing of manure and sewage, soil organic nitrogen, and NH₄⁺ from fertilizer and rain. Jiangjia Spring, the outlet of the underground river, was seriously polluted by nitrates. It is believed that soil organic nitrogen, NH₄⁺ in fertilizer and rain, the mixing of manure and sewage, and NO₃^- in precipitation were the main nitrate sources in the outlet. Nitrate source contribution of the outlet was calculated with the IsoSource model. The calculation results showed that manure and sewage, soil organic nitrogen, NH₄⁺ in fertilizer and rain, and NO₃^- in precipitation contributed 46.4%, 32.6%, 18.6%, and 2.4%, respectively.