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

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)