Abstract
Groundwater contains dissolved He, and its concentration increases with the residence time of the groundwater. Thus, if the 4He accumulation rate is constant, the dissolved 4He concentration in ground-water is equivalent to the residence time. Since accumulation mechanisms are not easily separated in the field, we estimate the total He accumulation rate during the half-life of 36Cl (3.01 × 105 years). We estimated the 4He accumulation rate, calibrated using both cosmogenic and subsurface-produced 36Cl, in the Great Artesian Basin (GAB), Australia, and the subsurface-produced 36Cl increase at the Äspö Hard Rock Laboratory, Sweden. 4He accumulation rates range from (1.9±0.3) × 10−11 to (15±6) × 10−11 ccSTP·cm−3·y−1 in GAB and (1.8 ±0.7) × 10−8 ccSTP·cm−3·y−1 at Äspö. We confirmed a ground-water flow with a residence time of 0.7-1.06 Ma in GAB and stagnant groundwater with the long residence time of 4.5 Ma at Äspö. Therefore, the groundwater residence time can be deduced from the dissolved 4He concentration and the 4He accumulation rate calibrated by 36Cl, provided that 4He accumulation, groundwater flow, and other geo-environmental conditions have remained unchanged for the required amount of geological time.
Highlights
Long-term isolation of high-level radioactive wastes in deep, stable geological formations has been proposed as an effective way to protect human life and the human genetic effects from radiation, and sequestration of large amounts of CO2 in such formations has been proposed for counteracting anthropogenic carbon dioxide emissions to reduce global warming
We propose the use of dissolved chlorine-36 (36Cl) and helium-4 (4He) as environmental tracers for estimating groundwater residence times longer than 2 million years, a period long enough for all cosmogenic 36Cl in groundwater to decay or to reach secular equilibrium with subsurface production of 36Cl
ER2010 the accumulation rate from crustal He components depends on the diffusion rates of ancient He left in crustal rock and of the mantle component as well as advection by groundwater flow, which cannot be estimated in the field individually
Summary
Long-term isolation of high-level radioactive wastes in deep, stable geological formations has been proposed as an effective way to protect human life and the human genetic effects from radiation, and sequestration of large amounts of CO2 in such formations has been proposed for counteracting anthropogenic carbon dioxide emissions to reduce global warming. ER2010 the accumulation rate from crustal He components depends on the diffusion rates of ancient He left in crustal rock and of the mantle component as well as advection by groundwater flow, which cannot be estimated in the field individually. The total He accumulation rate during the half-life of 36Cl (3.01 × 105 years) can be estimated, provided that the 4He accumulation mechanisms, groundwater flow conditions, and other geo-environmental conditions have not changed over the required amount of geological time. We summarize a new, practical 4He–36Cl dating method developed as part of our research activities in the Great Artesian Basin (GAB), Australia [4, 5] and at the Aspo Hard Rock Laboratory (HRL), Sweden [6, 7]
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