利用氮氧同位素解析赤水河流域水体硝酸盐来源及其时空变化特征

赤水河流域作为长江上游重要的水源涵养区,其生态环境状况及水环境质量备受关注。为了了解流域河水氮素来源,本次研究利用硝酸盐稳定同位素(~(15)N、~(18)O)示踪技术并结合流域土地利用类型空间分布分析了赤水河流域丰水期与枯水期干流及主要支流河水硝酸盐来源与转化过程。结果表明,流域水体NO_3~-浓度具有明显的时空变化,其中丰水期NO_3~-浓度要高于枯水期,喀斯特区域的NO_3~-浓度要高于非喀斯特区域。流域干、支流水体δ~(15)N-NO_3~-、δ~(18)O-NO_3~-季节性差异明显,丰水期支流δ~(15)N-NO_3~-差异较大,干流差异较小,而枯水期支流δ~(15)N-NO_3~-差异较小,干流差异较大。结合氮氧同位素和土地利用信息发现,丰水期支流NO_3~-受其土地利用方式的影响,其来源具有多样性;干流NO_3~-浓度则主要受支流混合作用影响。枯水期干流NO_3~-受流域人为活动影响较为显著,点源输入造成水体氮同位素分布范围较宽,主要来源表现为生活污水和土壤有机氮;而支流NO_3~-多表现为土壤有机氮来源,部分支流受流域内城镇影响,生活污水对河流NO_3~-贡献较大。流域水体氮污染控制应以农业面源氮流失为主,同时严格控制点源污染的输入。

Temporal and Spatial Variation of Nitrate Sources in the Chishui River Watershed based on Isotope Approach

The Chishui river watershed is an important water conservation area of the Yangtze River Basin with very high economic values, its ecological service function and water environment quality such have attracted more and more public attentions. To understand water quality of this watershed, the nitrate pollution resulted from human activities was studied from the perspectives of sources and transformation processes of nitrate using approaches of stable isotopes (¹⁵N, ¹⁸O) in nitrate and land uses during wet season and dry season. Nitrate concentrations showed significant spatial and temporal heterogeneity, nitrate concentrations were higher in wet season than in dry season and higher in karst areas than in non-karst areas. The seasonal variations of dual isotopes of nitrate in the mainstream and tributaries were different, for example, the variation of N isotope compositions was small in tributary samples and big in mainstream samples during wet seasons, but the scenario was reversed during dry seasons. The data of land use and dual nitrate isotopes suggested that nitrate sources of tributaries varied and depended largely on the land use, while nitrate sources of the mainstream were mixtures of tributaries contributions. During dry seasons, nitrate in tributaries derived mainly from soil organic nitrogen and showed obvious signals of anthropology activities since the tributaries located mostly in developed areas. To manage the nitrogen pollution in the Chishui river watershed, more attentions on the upper and middle stream where the karst landscape widely spreads should be paid. Meanwhile, the controlling of point sources in the downstream of the Chishui river watershed is also important.