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Oak Ridges Moraine Groundwater Program

Isotope summary, a regional look

    Isotopes are atoms with the same number of protons, but not neutrons. They share similar chemical properties but differ in mass and by extension physical properties. An isotopic signature is expressed as the ratio between the amount of various isotopes of an element. Isotopes are used as tracers for many types of investigations including, but not limited to, groundwater provenance, relative ages, recharge processes, subsurface processes, and geochemical reactions including reaction rates (Clark and Fritz, 1997). This snapshot provides a regional overview of some of the isotope data held in the ORMGP database - specifically the stable isotopes $^{18}O$ and $^2H$ (deuterium) and the radioactive isotope $^3H$ (tritium). Within the study area tritium is used to provide an understanding regarding if groundwaters have a component recharged in modern or recent times, approximately within the last 50 years. $^{18}O$ and $^2H$ provide information regarding recharge sources and conditions such as temperature, as well as potential differences in water types and relative ages. Further information regarding the use of isotopes in water investigations are available in Clark and Fritz (1997) and Kendall and McDonnell (1998). A brief summary of historical data for the study area is included in Gerber et al., 2009.

Isotope Measurements

In the maps below, isotope measurements currently in the ORMGP database for $^{18}O$, $^2H$ (Deuterium) and $^3H$ (tritium) are plotted. The isotopic data that have currently been incorporated into the ORMGP database include:

  1. Data from research investigations (Aravena and Wassenaar, 1993; Aravena et al., 1993; 1995; 2004; Gerber, 1999; Gerber and Howard, 1996);
  2. Data from landfill siting investigations (M.M. Dillon Limited, 1990; IWA, 1994a-d);
  3. Data from municipal pumping and monitoring wells (Regions of Halton, York, Peel and Durham);
  4. Data from the Provincial Groundwater Monitoring Network (PGMN) operated by the Province of Ontario and Conservation Authorities;
  5. Data from the Province of Ontario Drinking Water Surveillance Program (DWSP);
  6. Data from regional groundwater quality surveys (2007-2019) conducted by the Ontario Geological Survey (Hamilton, 2021); and
  7. Data from ongoing University of Waterloo/ORMGP geochemical and isotopic analyses initiated in 2008.

click on any circle to reveal station summaries. Full-screen available in the top-left corner

Figure 1: Measured tritium ($^3H$) in groundwater. Shallow wells have top of screen <=20 mbgs. Deep wells have top of screen >20mbgs. See also Priebe and Hamilton (2022) for a discussion on tritium concentrations in groundwater in southern Ontario.

Figure 2: Measured $\delta$$^{18}O$ in groundwater. Shallow wells have top of screen <=20 mbgs. Deep wells have top of screen >20mbgs.

Figure 3: Measured deuterium ($\delta$$^2H$) in groundwater. Shallow wells have top of screen <=20 mbgs. Deep wells have top of screen >20mbgs.

Figure 4: $\delta$ $^2H$ versus $\delta$ $^{18}O$. GMWL = Global Meteoric Water Line from Craig, 1961. LMWL = Local Meteoric Water Line (Simcoe, Ontario) from Fritz et al., 1987. Study area results occur above both the GMWL and LMWL due to deuterium enrichment from processes associated with Great Lakes humidity and vapour exchange (Gat et al., 1994).


Aravena, R. and Wassenaar, L.I. 1993. Dissolved organic carbon and methane in a regional confined aquifer, southern Ontario, Canada: Carbon isotope evidence for associated subsurface sources. Applied Geochemistry, 8, 483-493.

Aravena, R., L.I. Wassenaar, and J.F. Barker. 1993. Distribution and isotopic characterization of methane in a confined aquifer in southern Ontario, Canada. Journal of Hydrology, 173, 51-70.

Aravena, R., Wassenaar, L.I. and Plummer, L.N. 1995. Estimating 14C groundwater ages in a methanogenic aquifer. Water Resources Research, 31 (9), 2307-2317.

Aravena, R., L.I. Wassenaar, and E.C. Spiker. 2004. Chemical and carbon isotopic composition of dissolved organic carbon in a regional confined methanogenic aquifer. Isotopes Environ. Health Studies, 40(2), 1-12.

Clark, I.D, and Fritz, P. 1997. Environmental Isotopes in Hydrogeology. Lewis Publishers. 328p.

Craig, H. 1961. Isotopic variations in meteoric water. Science, 133, 1702-1703.

Fritz, P., Drimmie, R.J., Frape, S.K., and O’Shea. 1987. The isotopic composition of precipitation and groundwater in Canada. In Isotope Techniques in Water Resources Development. IAEA Symposium 299, March 1987, Vienna. 539-550.

Gat, J.R., Bowser, C.J., and Kendall, C. 1994. The contribution of evaporation from the Great Lakes to the continental atmosphere: estimate based on stable isotope data. Geophysical Research Letter, 21(7), 557-560.

Gerber, R.E. 1999. Hydrogeologic behavior of the Northern till aquitard near Toronto, Ontario. Ph.D. thesis, University of Toronto, 172p.

Gerber, R.E., and K.W.F. Howard. 1996. Evidence for recent groundwater flow through Late Wisconsinan till near Toronto, Ontario. Canadian Geotechnical Journal, v. 33, p. 538-555.

Gerber, R.E., Holysh, S., Doughty, M., Frape, S., Henkemans, E., and Kemp, W. 2009. Flow system dynamics: isotope tracer studies, south-central Ontario. In proceedings GeoHalifax 2009: 62nd Canadian Geotechnical Conference & 10th Joint CGS/IAH-CNC Groundwater Conference, Halifax, 1203-1213.

Hamilton, S.M. 2021. Ambient Groundwater Geochemical and Isotopic Data for Southern Ontario, 2007-2019. Ontario Geological Survey, Miscellaneous Release – Data 283 – Revision 2.

Interim Waste Authority. 1994a. EA document IV, geology/hydrogeology, technical appendix 1: Site T1 for Durham Region landfill site search. Prepared by M.M. Dillon Limited, February, 1994, 90 p.

Interim Waste Authority (IWA). 1994b. EA document IV, geology/hydrogeology, technical appendix 2: Site EE4 for Durham Region landfill site search. Prepared by M.M. Dillon Limited, February, 1994, 70 p.

Interim Waste Authority (IWA). 1994c. EA document IV, geology/hydrogeology, technical appendix 3: Site EE10 for Durham Region landfill site search. Prepared by M.M. Dillon Limited, February, 1994.

Interim Waste Authority (IWA). 1994d. IWA landfill site search, Durham Region. Detailed Assessment of the Proposed Site EE11, Appendix C Geology/Hydrogeology. Prepared by M.M. Dillon Limited, October, 1994, 197p.

Kendall, C., and McDonnell, J.J. (Editors). 1998. Isotope Tracers in Catchment Hydrology. Elsevier, 839p.

M.M. Dillon Limited. 1990. Regional Municipality of Durham Contingency Landfill Site Assessment Technical Support Volume B, Technical Report, Hydrogeology. September 1990. Published by Ontario Ministry of the Attorney General. 588p.

Priebe, E.H., and Hamilton, S.M. 2022. Tritium in shallow groundwater of southern Ontario. Ontario Geological Survey, Groundwater Resources Study 20.