Groundwater degassing

Hanor (1978) also discussed the mechanisms that may be responsible for degassing of groundwaters in the supratidal zone. His ranking of processes in order of probable importance was 1) tidally-induced oscillations of the groundwater table, 2) tidal pumping at the sediment-atmosphere interface, 3) a decrease in the average depth of the water table, and 4) a seaward increase in porosity. 

Figure 7.14. Calculated Pco2 an< calcium carbonate ion activity products for mixtures of groundwater and seawater. Arrows indicate degassing with time. (After Hanor, 1978.)...

The basic idea is to reconstruct geochemical evolution of the groundwater from its chemical composition. For example, knowing the chemical composition of a well on the one hand and an analysis of the rainwater on the other, it will be possible to reconstruct which geological formation the rainwater must have passed after its infiltration to change its chemical composition as the result of reactions with mineral and gas phases (dissolution, precipitation, degassing) in a way that accounts for the composition of the water from the well. 

Torgersen T. and Ivey G. N. (1985) Helium accumulation in groundwater II. A model for the accumulation of crustal He degassing flux. Geochim. Cosmochim. Acta 49, 2445-2452. 

Clarke and Kugler (1973) reported excesses of He in the one hole sampled near the Nordic mine at Elliott Lake, Ontario (1.56-8.69 pL/L He), and in several holes in a potential ore body in Labrador (0.05-28 pL/L He). Similar results were reported by Dyck et al. (1977) from Key Lake, Saskatchewan, where up to 20 pL/L He occurred in deep holes (60-100 m) intersecting mineralisation. Both studies showed that, in many holes. He contents increased with depth, presumably due to upward degassing and equilibrium with the atmosphere. Groundwaters away from mineralisation at Key Lake contained less than 0.043 pL/L He, suggesting that high values in the U deposits were due to the mineralisation itself, but the data are too few to be conclusive. 

Much of the Russian literature on He geochemistry has discussed the association of high He concentrations in groundwaters with deep faults. In general, samples have been taken from pumped bores at depths of 60-100 m. Where shallower waters have been sampled, results are less certain because of upward-degassing gradients and dilution near the surface (Eremeev et al., 1973 Bashorin, 1980). The conclusions of much of this work are that the He content of the groundwaters is less dependent on the radioactivity of the... 

In a study of lakes worldwide (4665 samples from 1835 lakes) Cole et al. (1994) found the pH ranged from about 4 to 10 and CO- pressures from about 10" to 10 ° bar. Sources of CO2 in lakes include its inflow in surface waters and groundwaters and respiration and decay related to lake sediments and biota. The overall mean CO2 pressure of 1.049 x 10" (10" ° ) bar in lakes was three times greater than the atmospheric value. This reflects a CO2 production and accumulation rate that exceeds its rates of consumption in the growth of aquatic plants and rate of degassing to the atmosphere. 

Since CAH are characterized by very much lower water solubiUties in contrast to MTBE CAH remain existent as an own phase in the aquifer over very long periods, in extreme cases decades to centuries. The high Henry constants of CAH principally promote a transfer from the groundwater into the seepage zone. Concentration reduction can take place due to degassing into the unsaturated zone, a process which is supported by the high vapor pressures of most CAH. 

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