Figure 6. Nitrogen and oxygen isotopic composition of public supply well nitrates. Error bars show the 2-sigma uncertainty in the analyses. Nitrate in the public supply wells is primarily derived from nitrification of ammonium in the soil. The nitrification field delineates the range of isotopic compositions for nitrate produced by nitrification of ammonium in the soil based on the isotopic composition of atmospheric oxygen and that of Long Island groundwater. Sample C was collected downgradient from a former sewage treatment plant. Based on discussionwith Heide Flatt at EIL, we believe that the elevated oxygen isotopic composition of this sample may reflect interference in the analysis from dissolved organic carbon.