Rit (Vísindafélag Íslendinga) - 01.06.1976, Qupperneq 105

Theodórsson (1969) measured the flow velocity in a postglacial
lava field near Reykjavik, using radioactive I131 as a tracer. The
tracer was injected into the ground water flow through a drill hole.
Samples were then collected from several drill holes approximately
75 m from the point of injection. These experiments gave a flow
velocity of 20 m/day or approximately vmean — 7.5 km/year, which
is similar to the value calculated above using k = 1 cm/sec.
In all cases where tritium has been measured in cold ground
water systems the water shows an appreciable amount of tritium
already in the years 1960—1962 (Theodórsson 1967). The tritium
is produced by the hydrogen-bomb tests which began in 1952.
Consequently tritium measurements, tracer studies and studies
on the permeability coefficient all agree with the assumption that
the cold groundwater is relatively young.
There exist no direct measurements of the flow velocity of deep
circulating thermal water. Any attempt to estimate the age of ther-
mal water is therefore necessarily more or less speculative.
Einarsson (1942) suggested that the flow velocity of thermal
groundwater in Iceland might be as low as 3.6 m/year. Thus it
would take approximately 30 000 years for the water to travel
underground a distance of 100 km. Einarsson, however, pointed
out that this is likely to be the lowest possible velocity; the real
velocity could well be an order of magnitude higher, i.e. 36 m/year.
The best information about the flow velocity of deep circulating
thermal water is undoubtedly that obtained from experiments car-
ried out by Thorsteinsson et al. (1970) and Thorsteinsson (1975).
These authors used the relation between thermal groundwater level
and pumping rate to estimate the coefficient of transmissivity in
three thermal areas in Southwest Iceland. The three thermal areas
were: Laugames, Elliðaár and the westem part of the Mosfellssveit
area (see section 15. 10).
The coefficient of transmissivity is defined by
T = k • D
where
T: coefficient of transmissivity
k: hydraulic conductivity
D: total thickness of aquifers
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