Quantifying saline groundwater seepage to surface waters in the ...

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6 S. Jasechko et al. / Applied Geochemistry xxx (2012) xxx–xxx Saline GW as proportion of river discharge (%) Saline GW discharge (L s -1 ) 10 1 10 10 0 1 10 -1 0.1 10 -2 10 -3 10 -4 10000 1000 100 10 1 0.1 (a) 1999 1987 2010 1987 2010 1987 2010 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 03 02 04 05 06 07 08 09 87 10 88 89 90 91 92 93 94 95 96 97 98 99 00 01 03 02 04 05 06 07 08 09 87 10 88 89 90 91 92 93 94 95 96 97 98 99 00 01 03 02 04 05 06 07 08 09 10 (b) 1999 CONTROL REACH CONTROL REACH CONTROL REACH CONTROL REACH 1987 2010 1987 2010 1987 2010 Hinton to Town of Athabasca Athabasca to Ft. McMurray Ft. McMurray to Old Fort 87 88 89 90 91 92 93 94 95 96 97 98 00 99 01 02 03 04 05 06 08 07 09 10 87 88 89 90 91 92 93 94 95 96 98 97 99 00 01 02 03 04 06 05 07 08 09 10 87 88 89 90 91 92 93 94 95 96 98 97 99 00 01 02 03 04 06 05 07 08 09 10 Fig. 5. Calculated saline water discharges along three reaches of the Athabasca River from 1987 to 2010. (a) Upper plots show this flux as a percentage of Athabasca River discharge. (b) Lower plots show saline groundwater discharge in m 3 /s. All plots are semi-logarithmic, highlighting the order of magnitude difference between upstream reaches (‘‘control reaches’’) and the lowermost reach between Ft. McMurray and the Peace-Athabasca Delta. A five-point running mean using the average Devonian- and Cretaceous-aged formation water Cl concentration (CG) is marked by a thick black line and two white squares. δ 2 H (‰ SMOW) -25 -50 -75 -100 -125 -150 -175 SYNCRUDE SUNCOR CNRL SHELL Springs (near Ft. McMurray) Lakes (near Ft. McMurray) Upper Athabasca River Lower Athabasca River Subsurface brines Oil sands tailings δ18 -200 -30 -25 -20 -15 -10 -5 0 5 10 O (‰ SMOW) Fig. 6. Plot of stable isotope ratios of H and O (d =[R sample/R standard 1] 1000; R: 2 H/ 1 Hor 18 O/ 16 O) in natural and process-affected waters within the Athabasca River basin. Data for the Athabasca River is divided into upper (dark gray squares, Hinton and Athabasca, n = 42) and lower Athabasca River stations (white squares, Fort McMurray and Old Fort, n = 31). Riverbank groundwater seepage data from the Fort McMurray area from Devonian formations are plotted as white stars (data from Grasby and Chen, 2005). Devonian brine samples from the Edmonton region from Connolly et al. (1990) and Simpson (1999) are represented by crosses. Data for 345 lake samples collected within 200 km of Fort McMurray between 2002 and 2008 are represented by small white circles (data from Gibson et al., 2010). Results for process-affected water samples are marked by white diamonds (data from Gibson et al., 2010 and Abolfazlzadehdoshanbehbazari, 2011). oil sands region (Ozoray, 1974; Gibson et al., 2011). Despite this seemingly small contribution of groundwater, the reach exhibits a fourfold increase in Cl concentration over 200 km, suggesting a profound impact of groundwater discharges on the chemistry of the Athabasca River. Fig. 7 highlights the seasonal variability of saline groundwater discharges as a proportion of Athabasca River discharge. The influence of saline groundwater discharges on Athabasca River chemistry is greatest in winter months when Athabasca River discharge is low (0.1–3%), whereas this impact is diluted in June and July during peak river discharge (
Saline groundwater discharge (% of Athabasca River discharge) isotopic compositions of river seeps and groundwater samples relative to modern precipitation. Advection of a denser, more saline groundwater to the surface requires a hydrologic gradient directed toward the surface. The mechanism for an ‘‘upward’’ hydrogeologic gradient could include relict subglacial pressures, or the regional topographically-driven flow system that recharges at higher altitudes in the Rocky Mountains. Moreover, the Athabasca, Clearwater and Christina river incisions expose aquifers bearing saline waters to the surface, thereby aiding the discharge of saline formation waters. Actual surface 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Month: Fort McMurray to Old Fort Athabasca discharge ± 1σ Saline GW discharge (%) 1 2 3 4 5 6 7 8 9 10 11 12 Fig. 7. Monthly saline groundwater discharges as a proportion of Athabasca River discharge (%) for the river reach between Ft. McMurray and Old Fort are shown as black squares and associated error bars (all data for 1987–2010 are plotted). The long-term mean discharge for the Athabasca River is represented by a white line (10 day bins). Shaded gray areas represent one standard deviation from the long-term mean discharge for each 10 day bin. Elevation (m.a.s.l.) Avg. chloride (mg L-1 ) Saline groundwater input (L s-1 ) 2000 1000 0 -1000 20 10 0 10000 1000 100 10 1 0 Hinton Foreland orographic belt: Tertiary, Cretaceous 20th Mean 80th S. Jasechko et al. / Applied Geochemistry xxx (2012) xxx–xxx 7 L Athabasca Devonian-Silurian o w e r C r e t a c e o u s 500 Prairie Evaporites CONTROL REACH CONTROL REACH Distance Downstream (km) Fort McMurray 2500 2000 1500 1000 500 0 seeps are likely to be a mixture of modern recharge, river water, and relict Pleistocene waters, supported by the presence of 3 H. Subsurface–surface hydrologic connections are a primary water contamination concern for ecosystems in the lower Athabasca River basin as bitumen production continues. Natural sources of contaminants – notably naphthenic acids – include weathering of bitumen outcrops along river banks, and the seepage of water that has been in prolonged contact with bitumen. A standalone technology to quantitatively decouple natural contributions from those associated with bitumen development has remained elusive, Athabasca River discharge (m 3 /s) Precambrian 1000 (a) Old Fort Fig. 8. Synoptic profile of the Athabasca River showing (a) bedrock geology, (b) Athabasca River Cl concentrations, and (c) saline groundwater discharges to the Athabasca River. Lower Cretaceous- and Devonian-aged formations bearing saline waters subcrop between Ft. McMurray and Old Fort. Average Cl concentrations increase progressively downstream, with the largest concentration increase occurring between Ft. McMurray and Old Fort (average increase of 18 mg L 1 ). Average saline groundwater discharges calculated for the period 1987–2010 are shown here. Discharges are shown for three groundwater salinity scenarios: 80th percentile, average, and 20th percentile of Devonian and Cretaceous formation water (Cl ). The largest groundwater discharges occur between Ft. McMurray and Old Fort (10 3 Ls 1 ±half an order of magnitude). Upper reaches of the Athabasca are labelled as controls, as saline groundwater discharges for these reaches are expected to be near-zero. The lower groundwater discharges calculated for these upper reaches ( 10 L s 1 ) supports the calculation presented. Please cite this article in press as: Jasechko, S., et al. Quantifying saline groundwater seepage to surface waters in the Athabasca oil sands region. Appl. Geochem. (2012), http://dx.doi.org/10.1016/j.apgeochem.2012.06.007 (b) (c)
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