On elevated fluoride and boron concentrations in groundwaters associated with the Lake Saint-Martin impact structure,Manitoba

Hydrogeological investigations conducted by the Geological Survey of Canada in the Lake Saint-Martin region of Manitoba have confirmed earlier reports of naturally elevated F⁻ and B concentrations in local groundwaters. Fluoride and B concentrations are highly correlated (r² = 0.905) and reach 15.1 mg/L and 8.5 mg/L, respectively. Virtually all groundwaters with F⁻ concentrations greater than the drinking water limit of 1.5 mg/L are from wells within the Lake Saint-Martin impact structure, a 208 Ma complex crater 23 km in diameter underlying a large part of the study area. The high-F⁻ groundwaters can be classified into two groups according to their anionic and isotopic compositions. Group I samples consist of Na-mixed anion groundwaters, with Cl greater than 100 mg/L and highly depleted ¹⁸O compositions indicative of recharge under much cooler climatic conditions than at present. Samples belonging to this group exhibit a striking relationship to crater morphology, and are found in an arcuate belt within the southern rim of the impact structure. Group II high-F⁻ samples consist of Na–HCO₃–SO₄ groundwaters, with little Cl, and less depleted ¹⁸O compositions. Samples belonging to this group are associated with groundwaters recharged locally, on a low ridge within the impact structure. This paper traces the probable source of high-F⁻ groundwaters to phosphatic pellets in shales of the Winnipeg Formation, a regional basal clastic unit which sub-crops at shallow depth beneath the crater rim as a result of more than 200 m of structural uplift associated with the impact event. This extensive aquifer is known elsewhere in southern Manitoba for its naturally-softened groundwaters and locally elevated F⁻ concentrations. Group I groundwaters are interpreted as discharge from the Winnipeg Formation where it abuts against crater-fill deposits. Group II high-F⁻ groundwaters are interpreted as modern recharge from within the impact structure, displacing Group I groundwaters. Thus, elevated F⁻ and B concentrations observed in groundwaters of the Lake Saint-Martin area represent the geochemical signature of upwelling from a deep regional aquifer. The previously unsuspected discharge zone occurs within an isolated sub-crop of the aquifer formed as a result of structural uplift caused by the impact event.