USE OF NITROGEN ISOTOPES TO STUDY GROUNDWATER NITRATECONTAMINATION IN SHIJIAZHUANG CITY

摘要

Groundwater is commonly contaminated by nitrate. Identification of sources of nitrate contaminations is the first step for therngroundwater management. Nitrogen isotope techniques provide a direct means of the source identification. This paper analyzed conditions ofrnsoil organic nitrogen as a source of nitrate contamination to groundwater. 50 groups of samples of groundwater and various potential rechargernsources were collected to identify sources of nitrate contaminations of groundwater beneath Shijiazhuang City by nitrogen isotope techniquesrncombined with hydrochemistry. Results revealed that soil organic nitrogen was not a major source of nitrate contamination of groundwaterrnbeneath Shijiazhuang City through analyses of lithology, organic matter and nitrate contents of soils in the unsaturated zone. 85.7% of totalrngroundwater samples collected in 1991 had δ15N values ranging from +6.1~+8.4‰ under low nitrate concentrations, indicating major sourcesrnof nitrate contamination derived from mixsures of fertilizer and animal wastes, while all samples collected in recent years had mean values ofrnδ15N (+9.9±4.4‰) more than +8‰ and more than half numbers of samples (65%) had δ15N values greater than +8‰ under current highrnnitrate concentrations, being indicative of major sources of nitrate contamination derived from animal wastes or sewages. Samples collected inrnthe northwestern part had d15N values ranging from +6 to +7‰, indicating NO3rn- in groundwater derived from fertilizers applied in farmland,rnand samples of multi-point contamination occurring in the northern alluvial flat of Hutuo river and point contamination occurring in thernwestern part with high NO3rn- concentration had relative low d15N values (+4~+6‰), indicating that NO3rn- in groundwater derived fromrnindustrial effluents of weak ammonia volatilization, rapid filtration, discharged from fertilizer plants combined with analysis of Cl- and landrnuses.