基于氮氧同位素分馏和贝叶斯稳定同位素混合模型的流域尺度水体硝酸盐氮污染源解析研究

Water nitrate pollution has become a global environment issue of public concern. The nitrate source apportionment in polluted watershed is most important within the research areas of source pollution control. Due to the uncertainty arose from isotope fractionation and overlaying during the transformation processes of nitrate, previous studies have not yet effectively solved the key problem regarding accurately quantitative source apportionment. Taking West Tiaoxi catchment, one of the important upstream branches of the Taihu Lake, as a case study area, this project will focus on the nitrate source apportionment in typical polluted watershed. In contrast with the previous relevant work limited in qualitative level, this project emphasizes to identify the nitrate pollution sources quantitatively. To achieve this goal, the dual-isotope tracer technique will be used to evaluate the spatial and temporal variation of nitrate, to establish nitrate isotope end-member database for the effective determination of the environmental factors underpinning the accumulation of nitrate in the polluted watershed. Special laboratory control experiments will also be conducted to identify different isotope fractionation factors during nitration and denitrification processes that regulate the accumulation of nitrate. Based on the above research results, the bayesian modeling based methods will be used to minimize the uncertainty resulted from isotope fractionation and overlaying during the transformation processes of nitrate, and to evaluate quantitative contribution of each pollution source. All the research work will lead to the development of a precise quantitative method and improved methodology for nitrate source apportionment in the typical nitrate polluted watershed.

硝酸盐氮污染源解析是当前水体污染阻控领域的重要研究内容。已有研究较少考虑硝酸盐在转化过程中存在的同位素分馏和叠加效应，限制了源解析精准度的提高。本项目拟选取太湖上游重要支流西苕溪流域为代表，基于氮氧稳定同位素技术，解析流域内溶解性硝酸盐15N 和18O同位素的时空季节变化特征，构建氮氧同位素各端元组分数据库；结合对流域内土地利用方式和水化学变化特征的分析，识别流域不同河段硝酸盐的主要污染源排放清单；通过室内模拟试验，探明硝酸盐在硝化/反硝化过程中的同位素分馏效应；基于贝叶斯稳定同位素混合模型，最大程度减小由同位素分馏和叠加效应引起的源解析不确定性，建立适合于典型流域的硝酸盐氮污染源综合解析方法，最终实现各污染源贡献值和分担率的精准定量解析。预期成果有望进一步提升硝酸盐氮污染源定量解析的精准度，对流域水体硝酸盐污染的源头阻控具有重要意义，并可为太湖乃至长江中下游水体的富营化防治提供科学依据。

加强饮用水水源地环境保护，防范饮用水水源污染风险，保障饮用水安全，对保证居民身体健康和经济建设发展至关重要。而当前部分饮用水水源地因硝酸盐污染而产生水体富营养化，已严重威胁周边居民和水生生物的健康。污染末端治理不如源头控制，合溪水库流域作为饮用水水源保护区，在浙江省长兴县经济生态发展中扮演着重要的角色。因此，追溯合溪水库流域硝酸盐来源，并加以控制是防治水体富营养化和排除潜在污染风险的关键。本研究在2015年11月至2017年1月期间，于合溪水库流域采集了水体样品（包括河水、地下水和水库水共计259个）和潜在氮污染源样品（包括氮肥、粪肥、生活污水、工业污水、降水和土壤样品共计176个），利用细菌反硝化法测定液体样品的硝酸盐氮氧同位素，依据潜在氮源的氮氧同位素组成特征和水体硝酸盐氮氧同位素的时空变异特征，结合基于差异的样点源同位素组成的贝叶斯混合模型，解析水体硝酸盐潜在氮源的贡献及时空差异，根据解析结果提出合理的氮污染防控对策。