[Temporal and spatial variations of the nitrate-nitrogen sources in an underground river using 15N isotope technique]

Groundwater in Qingmuguan underground river was monitored using hydro-chemical and 15N isotope techniques to investigate temporal and spatial variations of nitrate-nitrogen and its possible sources from October, 2007 to October, 2008. The results show that nitrate concentrations are 3.20 mg/L of the inlet (D1) and 20.35 mg/L of the outlet (S2) of the underground river. Affected by the fertilizers in agricultural field and the rainfall flush and dilution,nitrate concentrations of groundwater are higher but less stable from April to July, 2008 than that from October, 2007 to March, 2008. They are elevated during August and September, 2008 due to the residual fertilizers and decreased rainfall events. According to the NO3- -delta 15N values of -0.857% per hundred +/- 2.01% per hundred (n=9), the sources of nitrate-nitrogen of D1 are dominated by residual fertilizers from the paddy fields from October, 2007 to March, 2008 and from July to October, 2008, while the NO3- -delta 15N values of 2.50% per hundred +/- 0.29% per hundred (n=3) demonstrate that the mixture of soil organic nitrogen and fertilizers are the sources from April to middle and late June, 2008. And the NO3- -delta 15N values of -3.74% per hundred in late May and 0.52% per hundred in early June indicate that the nitrate-nitrogen comes from fertilizers applied in the paddy fields in 2008. The nitrate-nitrogen of S2 is partly from fertilizers in the upper basin, and also from fertilizers and soil organic nitrogen carried by lateral fissure and soil permeation water of forest and farm land in the middle and lower basin. The NO3- -delta 15N values of 4.77% per hundred (n=9) show nitrate-nitrogen of S2 mainly originates from fertilizers from October, 2007 to March, 2008 and from July to October, 2008, while NO3- -delta 15N values of 3.16% per hundred +/- 0.39% per hundred (n=5) explain that the nitrate-nitrogen derives from the mixture of soil organic nitrogen and fertilizers from April to June, 2008.