NITROGEN ISOTOPE INDICATORS OF SEASONAL SOURCE VARIABILITY TO GROUNDWATER

A nitrogen isotope study of soil water and groundwater in southern Ind iana, USA, in 1991-1992 demonstrated considerable variations in nitrat e degradation processes compared to an earlier investigation in 1986-1 987. Although N-fertilizers were applied in May 1991, the delta(15)N v alues in soil water decreased in February 1992, indicating its delayed release into the system after substantial rainfall. The delta(15)N va lues of groundwater decreased from + 12.3 parts per thousand in Novemb er 1991 to + 11.3 parts per thousand in February 1992, and to + 7.5 pa rts per thousand in March 1992. The increased residence time of nitrat e in the soil resulted in increased denitrification, ammonia volatiliz ation and plant uptake, and reduced threat to the groundwater quality. The 1986-1987 study in the area reported that excessive rainfall duri ng the summer rapidly transported the nitrate to deeper horizons and d rastically reduced volatilization and microbial reduction of nitrate, thus increasing the immediate threat to the groundwater quality in the area. The present study demonstrated that nitrogen isotopic signature s can be used to determine the effects of local soil type, rainfall, a nd land-use practices on the fate of nitrate in the subsurface.