This paper summarizes various lines of evidence, including new geophysical and geochemical surveys indicating the discharge of naturally occurring saline formation water from Cretaceous and Devonian formations to the Athabasca River downstream of Fort McMurray — an active oil sands extraction area. The following features are indicative of saline water discharge: (i) the hydrogeological setting of the reach which is situated near the western, up-dip, and subcropping, edge of the Western Canada Sedimentary Basin; (ii) springs and seepage along area rivers and tributaries that have been observed and reported in previous studies; and (iii) a significant increase in dissolved solids in the river, particularly chloride, occurring in a downstream direction from Fort McMurray. Further evidence of the saline groundwater discharge was obtained from electromagnetic surveys conducted along a 125 km reach from the Clearwater River to the Firebag River. This technique was used to map the distribution of saline water in the riverbed hyporheic zone, and revealed broad zones of generally high terrain electrical conductivity values in deeply incised Cretaceous- and Devonian-aged subcrop areas, but with numerous point-source and lineal anomalies attributed to occurrence of saline water discharge in less incised areas. Porewater sampling using drive-point piezometers was then used to confirm the presence of saline water in selected zones. Depth-wise gradients in chemical parameters observed in the riverbed porewaters in these zones are interpreted as evidence of upward movement of saline formation water mixing with the Athabasca River. Geochemical properties of the porewater are consistent with natural sources of groundwater flow from the Cretaceous- and Devonian-aged formations discharging along various reaches of the river.

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