SPATIAL VARIATION IN NITROGEN ISOTOPE VALUES BENEATH NITRATE CONTAMINATION SOURCES

Nitrogen isotope ratios of soil water and ground water nitrate have be en used to identify or implicate sources; however, lack of data on del ta(15)N in thick (>12 m) vadose zones beneath the source and potential effects of denitrification on delta(15)N signatures have raised quest ions about the meaning of delta(15)N values measured in ground water, In this study, nitrogen isotope ratios (delta(15)N) were measured on n itrate extracted from 218 core samples removed from the surface to the water table below natural (soil organic matter) fertilizer, onsite se wage disposal systems (septic tank effluent), and animal sources locat ed in Salinas and Sacramento Valleys, California. Additionally, spatia l variability of delta(15)N in the horizontal plane was measured benea th an agricultural field, and delta(15)N values of native geologic mat erials (organic-rich shales) were determined. In general, delta(15)N v alues throughout the vadose zone were consistent with the literature a nd remained fairly constant with depth, indicating little denitrificat ion during transport. The delta(15)N values from soil organic matter s ources varied from about 0 to +4 parts per thousand, and the mean was not significantly different from that of fertilizer sources. The delta (15)N values of animal sources varied from about +8 to +20 parts per t housand and were dependent on site and animal source. The delta(15)N o f onsite sewage disposal sources varied from about +2 to +12 parts per thousand, and the mean was significantly different from that of anima l sources at a 90% confidence level. delta(15)N Of organic-rich shales of the Panoche and Moreno Formations (Cretaceous; San Joaquin Valley) and soils derived from these rocks were generally between +4 and +8 p arts per thousand, Thus, it may be difficult to distinguish a geologic -N source from septic tank or commercial fertilizer sources using delt a(15)N. Little spatial variation was found in delta(15)N beneath the a gricultural field in the horizontal plane.