Regional-scale mapping of buried, surface-connected, karstic groundwater systems using dissolved gases and hydrochemical tracers

Abstract

In 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.

Septic System Impacts on Source Water: Two Novel Field Tracer Experiments in Fractured Sedimentary Bedrock

“…While no karst features have been reported on the reserve [50], the rapid arrival of tracers in the CMT may indicate that groundwater flow is influenced by nearby subsurface karst. The results of this research and the study conducted by Hamilton et al [89] suggest that more stringent regulations are needed for the design and use of septic systems in karst or non-karst fractured rock environments. In the source water protection plan for the Saugeen, Grey Sauble, and Northern Bruce Peninsula areas, it is recommended that the Ontario Building Code "require that advanced septic systems be required for new installations in vulnerable areas where an on-site sewage system is or would be a significant drinking water threat" [88] (pp.…”

“…A recent study by Hamilton et al [89] outlined extensive areas in Southern Ontario where subsurface karst is connected to the surface, resulting in rapid recharge. Hamilton et al greatly extended the areas known to be highly vulnerable due to surface-connected karst features, including the area near the boundary of Neyaashiinigmiing reserve.…”

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. It is recommended here that the definition of "drinking water threat" be extended beyond the areas that impact municipal drinking water sources to include areas of rapid recharge that may impact the drinking water sources of rural and First Nation residents, such as those areas outlined by Hamilton et al [89].…”