text: * [http://www.science.uottawa.ca/~eih/ch1/ch1.htm Environmental Isotopes]   (University of Ottawa)
  * [http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html Fundamentals of Isotope Geochemistry]  (C. Kendall & E.A. Caldwell, chap.2 in Isotope Tracers in Catchment Hydrology [edited by C. Kendall & J.J. McDonnell], 1998)
  * [http://pubs.usgs.gov/info/seal2/ Stable Isotopes and Mineral Resource Investigations in the United States]  (USGS)