鄱阳湖流域典型水源水库水华暴发成因及氮污染溯源

Driving factors of algal bloom and nitrogen sources tracing of a drinking water source reservoir in Lake Poyang Basin

富营养化导致的藻类水华是水源地水库面临的主要生态问题之一。探究水源水库中水华暴发成因并开展污染物溯源研究,是预防水华暴发的重要前提。本文选择鄱阳湖流域赣江水系吉安市某典型水源水库为研究对象,通过分析无水华期和水华暴发期水环境特征及其演变规律,探讨了水华暴发的成因及其促发条件,并针对识别出的主要污染因子,开展了同位素溯源研究。结果表明:(1)水库水华暴发的主要原因为水体总氮浓度显著升高(由0.77 mg/L升高至1.57 mg/L),超出了国际公认的水华暴发总氮阈值(0.5~1.2 mg/L),加之夏季高温(32℃)、水库库容量较低,促进了水华暴发;(2)无水华期,水库表层水总氮主要赋存形态为硝酸盐氮(占比>90%),水华暴发期,表层水总氮主要赋存形态由硝酸盐氮转变为有机氮(占比~86%),氮赋存形态转化的主要原因为藻类同化吸收作用;(3)硝酸盐氮、氧同位素示踪结果表明,水库硝酸盐氮主要来源于上游稻田流失的氮肥(负荷贡献率64.45%),其次为上游山区土壤氮流失(14.08%)、大气沉降(12.35%)和农村生活污水(9.12%),因此水库总氮优先控制污染源为上游稻田流失的氮肥。

Algal bloom,caused by eutrophication,is one of the major ecological disasters threatening the safety of drinking water reservoirs.Investigating the driving factors of algal bloom in drinking-water reservoirs is needed for preventing and controlling algal blooms.In this study,environmental factors in a typical drinking-water reservoir in Lake Poyang Basin were analyzed before and during algal bloom to reveal the driving factors of algal blooms.In addition,isotopic tracing technique was used to trace the main sources of pollutants.The results showed that:(1)Algal blooms in the drinking-water reservoir were mainly caused by the increasing concentration of total nitrogen(TN)from 0.77 to 1.57 mg/L,which exceeded the internationally recognized threshold of TN(0.5-1.2 mg/L).High temperature in summer(32℃)and low reservoir capacity further promoted the growth and bloom of algae.(2)Before algal bloom,the main occurrence of TN in the reservoir was nitrate(NO-3-N,accounting for>90%of TN).In contrast,the main occurrence of TN was organic nitrogen(accounting for about 86%of TN)during the algal bloom period due to algae assimilation.(3)Nitrogen sources were identified based on nitrogen and oxygen isotopes of nitrate.NO-3-N in the reservoir was mainly from nitrogen fertilizer in the upstream paddy field(64.45%),followed by water and soil erosion in the upstream mountain area(14.08%),atmospheric precipitation(12.35%),and rural domestic sewage(9.12%).Therefore,controlling nitrogen fertilizer in the upstream paddy field is a priority strategy for controlling TN in the reservoir.