Ground and surface waters in urban areas are susceptible to nitrate contamination from septic systems, leaking sewer lines, and fertilizer applications. Source identi®cation is a primary step toward a successful remediation plan in aected areas. In this respect, nitrogen and oxygen isotope ratios of nitrate, in conjunction with hydrologic data and water chemistry, have proven valuable in urban studies from Austin, Texas, and Tacoma, Washington.In Austin, stream water was sampled during strem¯ow and base¯ow conditions to assess surface and subsurface sources of nitrate, respectively. In Tacoma, well waters were sampled in adjacent sewered and un-sewered areas to determine if locally high nitrate concentrations were caused by septic systems in the un-sewered areas. In both studies, sewage was identi®ed as a nitrate source and mixing between sewage and other sources of nitrate was apparent. In addition to source identi®cation, combined nitrogen and oxygen isotopes were important in determining the signi®cance of denitri®cation, which can complicate source assessment by reducing nitrate concentrations and increasing d15 N values. The two studies illustrate the value of nitrogen and oxygen isotopes of nitrate for forensic applications in urban areas.# Published by Elsevier Science Ltd. on behalf of AEHS.