A DECADE OF ENVIRONMENTAL ISOTOPE RESEARCH IN A LOW PERMEABILITY
L. I. WASSENAAR Environment Canada
4. COMPARISON OF MULTI-TRACER AGES AND TIMING AND THEIR IMPLICATIONS
4. COMPARISON OF MULTI-TRACER AGES AND TIMING AND
these simulations yield our best estimates of the onset of: (1) till deposition (in the case of 4He and δD), (2) the Holocene (δD), and (3) post glacial soils development (14C-DIC and 14C-DOC). Further, 14C-DIC, 14C-DOC and 36Cl measurements provide estimates of the age of the glaciogenic water in the till (between 23 and 38 m depth) and 36Cl measurements were used to estimate of the age of the porewater near the top of the clay aquitard (91 m depth).
These estimates are shown in Table 1. The 14C-DOC age for the glaciogenic zone and the best-fi t simulations for the development of the 4He profi le suggest the Battleford till was deposited on the clay between 10 and 20 ka BP (more likely 15 to 20 ka). This range in values is consistent with current estimates of the timing of the deposition of this till by [41] who determined the 14C age on a sample of woody organic matter deposited below the Battleford till to be ~18ka BP. Although the range in 14C-DIC ages for the glaciogenic water from [12] is great (15 to 31ka BP), it does bracket the known timing of till deposition. The broad range in 14C-DIC ages can be attributed to a poorer understanding of its A0 value than for 14C-HMW, as noted by [12] who suggest the 14C-DIC age for
TABLE 1. SUMMARY OF INDEPENDENT ESTIMATES OF LONG- TERM GROUND WATER VELOCITIES (V), TIMING OF TILL DEPOSITION, AGES OF GLACIOGENIC WATER, AND THE ONSET OF THE POST-GLACIAL EVENTS1
Profi le V Till
deposition
Post-glacial timing
Comment on post glacial-timing hydrogeology 0.5–0.8
δD2 0.8–1.0 20–30 7–12 Onset of Holocene
δD3 0.5–0.8 15–20 5–8 Onset of Holocene
14C-DIC 15–31 (156) 7–10 Development of soil zone
14C-HMW 15–17 9–11 Development of soil zone
4He 10–20
Cl– 1–2 >54
36Cl <30
porewater
chem5 9 Development of soil zone
1 All velocities are reported in m per 10 ka and timings are reported in ka BP
2 using a laboratory determined De value for δD of 1.7 × 10–10 m2 s–1 [5]
3 using an in situ determined De value for δD of 3.5 × 10–10 m2 s–1 [34]
4 based on modeling of the Cl– profi le beneath the sand layer at 13 m depth.
5 from [6]
6 from [12]
the glaciogenic water is more likely about 15ka BP based on a reconsideration of Ao. This value is in keeping with the results of 14C-DOC and 4He.
Best-fi t simulations of the isotopic and chemical profi les that evolved from the top of the till aquitard and were associated with the onset of the Holocene and the development of a soil zone after deglaciation (δD, 14C-HMW and -DIC) yielded internally consistent results with respect to the timing of the onset of the Holocene, suggesting it occurred between 7ka and 12ka BP. Simulations of depth profi les of major ions through the upper 45 m of till supported this determination, yielding estimates of the onset of geochemical changes attributed to the Holocene of about 9 ka BP [6] (Table 1). These values are in keeping with the development of Prairie soil zones prior to the Hypsithermal (ca. 7.5–5.0 ka BP) [42], knowledge of glacial retreat about 12,000 a BP [43], and the transition from cold climatic conditions of late Pleistocene to warm climatic conditions of the Holocene about 12,000 to 10,000 a BP [44].
Unlike the distribution of δD, 14C-DIC and -DOC, the Cl- and (other halides) and δ37Cl exhibited atypical vertical profi les through the till, representative of the presence of the sand layer at about 13 m depth [9, 16]. This suggests the lack of deviation in one or more parameters from a well defi ned diffusive-depth trend cannot be used to rule out the presence of heterogeneity (e.g., a permeable sand layer), which could be an important consideration for solute transport.
The estimated timing of events using isotopic and hydraulic methods are in overall agreement. The small differences (about ±25%) in ages between isotopic methods can be attributed at least in part to the De values used. This is exemplifi ed by the differences in ages from the δD profi le presented in [30] and [34] (Table 1). The sole difference between these models was the difference in the measured (laboratory vs. in situ) De values used. The difference between the two De values was small, differing by a factor of two. If more accurate estimates of ages are required, additional comparisons of De values determined for various solutes under both in situ and laboratory conditions are warranted.
Comparisons between the extensive data sets collected at the King site and data from other surfi cial clay aquitards in Canada suggest the diffusive transport conditions described for the King site are likely common in thick aquitards. Our fi ndings clearly show that accurate prediction of the long-term migration of solutes through media is possible for time frames of up to 20 ka.
As a result, clay-till aquitard systems may be promising candidates for long- term containment of contaminants, including hazardous and nuclear wastes.
ACKNOWLEDGEMENTS
Long-term fi nancial support was provided by the Natural Sciences and Engineering Research Council of Canada and Cameco Co. Ltd. through the NSERC-Cameco Research Chair and by the Saskatchewan Potash Producers Association and Environment Canada. The assistance of numerous collaborators and students who contributed to the content of this paper is gratefully acknowledged.
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