污灌区地下水硝酸盐污染来源的氮同位素示踪

为了识别石家庄市南部污灌区地下水硝酸盐污染来源, 采集5种潜在污染源和19组地下水样用于化学和氮同位素分析.灌溉污水NH₄+的δ15N值较低(4.0‰), 施化肥土壤和粪堆下土壤NO₃-的δ15N值分别为1.4‰和12.4‰; 仅施厩肥的蔬菜种植区下伏近30 m厚包气带沉积物NO₃-的δ15N分布显示, 来自动物粪便的NO₃-已运移到11.5 m以下包气带, 均值10.9‰; 污水灌溉农田下伏厚层包气带沉积物样品分析结果指示, 土壤层下伏包气带沉积物δ15N值变幅较小, 均值5.7‰.污灌区内除一深井外, 其他水井地下水硝酸盐浓度变化在52.6~124.5 mg/L之间, 均值79.72 mg/L, δ15N值变化在5.3‰~8.3‰之间, 均值7.0‰.污灌区地下水的δ15N值较污灌区土壤层下伏包气带沉积物的δ15N值高, 表明地下水NO₃-除了来自灌溉的污水外, 还有δ15N值更高的其他来源, 这些来源主要是人和动物粪便.利用线性混合模型计算, 污灌区地下水NO₃-来自灌溉的污水, 约占76%, 而来自人和动物粪便的NO₃-约占24%.为控制污灌区地下水NO₃-浓度进一步增长, 不仅要加强污水灌溉管理, 还要加强人和动物粪便的管理. 

Nitrogen Isotope Tracing of Sources of Nitrate Contamination in Groundwater from Wastewater Irrigated Area

In order to identify sources of nitrate in groundwater from the wastewater irrigated area, the southern part of Shijiazhuang City, 5 soil/wastewater samples from potential contamination sources and 19 groundwater samples were collected for chemical and nitrogen isotopic analyses. Irrigation wastewater has relatively low δ15N value of 4.0‰, and soil applied with commercial fertilizer and beneath animal waste piles has δ15N values of 1.4‰ and 12.4‰, respectively. The distribution of δ15N values of sediment from about 30 m-thick vadose zone beneath the vegetable growth plot only applied with animal wastes shows that NO₃- derived from animal wastes has transported to the lower vadose zone with the mean δ15N value of 10.9‰. Sediment samples collected from the thick vadose zone beneath the farmland only irrigated with wastewater indicates that the deep vadose zone below the soil layer has narrow range of δ15N values with the mean δ15N value of 5.7‰. Groundwater from the wastewater irrigated area has NO₃- concentration ranging from 52.6 mg/L to 124.5 mg/L with a mean value of 79.72 mg/L, and δ15N values of NO₃- ranging from 5.3 to 8.3‰ with a mean value of 7.0‰ except a sample from a deep well. δ15N values of groundwater from the wastewater irrigated area are higher than those from the deep vadose zone beneath the wastewater irrigated area, which indicates that other sources with higher δ15N values contribute to groundwater NO₃-. The other sources are mainly human and animal wastes. NO₃- in groundwater from the wastewater irrigated area are derived from irrigation wastewater accounting for about 76% and from human and animal wastes accounting for about 24%, respectively. Wastewater irrigation and human/animal wastes management should be strengthened to prevent groundwater NO₃- contamination in the wastewater irrigated area.