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TitleRegional-scale mapping of buried, surface-connected, karstic groundwater systems using dissolved gases and hydrochemical tracers
DownloadFree download (whole publication) (pdf 1047 KB)
AuthorHamilton, S M; Brunton, F R; Priebe, E H
SourceRegional-scale groundwater geoscience in southern Ontario: an Ontario Geological Survey and Geological Survey of Canada groundwater geoscience open house; by Russell, H A J; Ford, D; Priebe, E H; Geological Survey of Canada, Open File 8212, 2017 p. 19, https://doi.org/10.4095/299775
Year2017
PublisherNatural Resources Canada
MeetingOntario Geological Survey and Geological Survey of Canada groundwater geoscience open house; Guelph; CA; March 1-2, 2017
Documentopen file
Lang.English
RelatedThis publication is contained in Russell, H A J; Ford, D; Priebe, E H; (2017). Regional-scale groundwater geoscience in southern Ontario: an Ontario Geological Survey and Geological Survey of Canada groundwater geoscience open house, Geological Survey of Canada, Open File 8212
ProvinceOntario
NTS30; 31C; 31D; 40; 41A; 41G; 41H/03; 41H/04; 41H/05; 41H/06; 41H/12; 41H/13
AreaSouthern Ontario
Lat/Long WENS -84.0000 -76.0000 46.0000 41.5000
Subjectshydrogeology; geochemistry; environmental geology; groundwater; groundwater resources; aquifers; groundwater regimes; karst topography; mapping techniques; gas analyses; carbon dioxide; oxygen; groundwater geochemistry; tritium; nitrate; bacteria; isotopes; bedrock geology; lithology; sedimentary rocks; carbonates; glacial deposits; glaciofluvial deposits; hydraulic conductivity; soil gas geochemistry; water wells; water quality; pollutants; Lucas Formation; Dundee Formation; groundwater recharge; Phanerozoic; Cenozoic; Quaternary; Paleozoic
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Location
 
Natural Resources Canada Library - Ottawa (Earth Sciences)
 
Released2017 02 22
AbstractIn 2008, the Ontario Geological Survey (OGS) released a map of known and potential karst in southern Ontario. Known karst occurs mostly in areas where glacial overburden is thin (<1m) or absent and in many places the top of bedrock shows evidence of active, ongoing solution-enhancement. Data from deep bedrock boreholes indicate that karst in Ontario's carbonate terrains is much more widespread than can be observed on surface and that it extends into areas covered by thick glacial sediments. Direct evidence for this pre-glacial karst includes: (1) televiewer logs and drill-core showing solution-enhanced bedding planes and sequence boundaries; (2) drill records reporting large cavities in bedrock that are open or filled with glacio-fluvial sediments; (3) epikarst and bedrock conduits in quarries; (4) drift thickness mapping with extensive buried valleys and canyons, some that appear to form natural bridges; and (5) thick glacial sediment-covered areas with extensive interpreted karst rubble at the bedrock surface. Traditional methods for local mapping of subsurface karst and its effects on groundwater cannot be easily adapted for regional-scale studies. Physical techniques include tracer tests and piezometric logging of monitoring wells to detect conduits (by the rapid rise and fall of water levels); chemical techniques include monitoring, at spring vents, of pH, Ca2+, HCO3 - and saturation indices of carbonate minerals to determine the degree of corrosiveness of groundwater. However, these are all proximal techniques that characterize individual, known systems. To date, there are no well-developed regional techniques that can map areas where groundwater is influenced by buried karst over a wide area.
Here we describe a methodology that uses dissolved CO2 and O2 in groundwater to map areas in buried karstic carbonates that have a rapid hydraulic connection to surface. O2 originates in the atmosphere and has no geological sources. CO2 in groundwater originates largely in the soil zone and has few other geogenic sources in non-tectonic settings. Because both parameters are attenuated with increasing distance from their sources, a CO2/O2 factor allows for an objective description of how well connected these buried karstic groundwater systems are to meteoric and soil zone recharge sources. An empirically derived lower threshold for the CO2/O2 factor delineates a number of large regions in southern Ontario where groundwater is elevated in either or both gases; all of these are centred on areas of known karst. Groundwater analysis using tritium, nitrate and bacteria (for samples collected from secure-cap water supply wells) show that these areas have younger, more recently recharged groundwater with a relatively rapid connection to the surface environment. Regional CO2/O2 and other chemical, isotopic and bacteriological data were purpose-filtered from the large, publically accessible OGS Ambient Groundwater Geochemistry database. This is an exceptionally well characterized groundwater geochemical dataset for samples collected on a uniform grid from domestic, farm and monitoring wells across southern Ontario. Mapping areas of groundwater vulnerability to surface contamination due to karstic flow systems is another of the many possible uses for this database.
GEOSCAN ID299775