Assessing the source of nitrate pollution in water using stable N and O isotopes

We used the isotopic composition of nitrogen ( delta N-15) and oxygen ( delta O-18) in water nitrate ( NO3-) to assess water pollution. delta N-15 and delta O-18 values in drainage water nitrate of a conventionally managed field and the adjacent surface waters were measured for 6 weeks during the main discharge period of the hydrological year 2003/2004. We hypothesized that this approach could provide more information about the impact of drainage water containing high nitrate loads on the following surface water bodies than common measurements of concentrations. The nitrate concentrations ranged between 686 and 2040 mu M in the tile drain outlets and were positively correlated to the tile drain discharge. The low delta O-18-NO3- values, from 1.8 to 4.3 parts per thousand, indicated that most of the nitrate derived from the nitrification process in the agricultural soils. The high delta N-15-NO3- values, from 8.5 to 15.0%, reflected the long-term fertilizing practice which was carried out for several years with inorganic as well as organic fertilizers. In the adjacent ditch and the brook nitrate concentrations were lower but showed a similar development to the tile drain outlet. The delta N-15-NO3- values ( 7.2-12.1%) and delta O-18-NO3- values ( 2.4-9.1%) in the ditch and the brook indicated that the nitrate from tile drain discharge is the major N source for the adjacent surface water bodies.