Abstract: Seasonal changes in nitrate concentration and table of groundwater in Hetao Irrigation District were investigated.Results show that in March the concentrations of NO^-₃-N and TN were significantly higher than those in May,July and September and the groundwater table was also lower than those in May and July.Sharp difference was found in N concentration between wells different in type.The NO^-₃-N concentration in the wells in farmlands and courtyards was significantly higher than those near villages,whilst the concentrations of NH⁺₄-N and TN in the wells in courtyards were higher than those in farmlands and near villages.The major N component of the N pollution in the groundwater was（NO^-₃-N） and about 17.1% of the groundabuter samples investigated were above 10 mg·L^-1 in NO^-₃-N concentration throughout the year with the highest reaching 184.4 mg·L^-1.When similar in irrigation rate and farming practice,65.5% of the groundwater samples in the fertilized zone were over 10 mg·L^-1 in NO^-₃-N concentration,whereas only 27.6% of the groundwater samples in the non-fertilized zone.In the former,the concentration of（NO^-₃-N） averaged（17.55±15.02） mg·L^-1,which was much higher than that in the latter（[（7.67±4.48）） mg·L^-1].Higher NO^-₃-N concentration came from N fertilizer and animal manure applied in farming production and courtyard economy production.Therefore,the problem of NO^-₃-N contamination of groundwater that occurred with the current local N application rate can′t be ignored.

Key words: Hetao Irrigation District, nitrate-nitrogen, total nitrogen, leaching