Rit (Vísindafélag Íslendinga) - 01.06.1976, Page 104

uppermost young rock formations, the oldest groundwater in the
deep thermal systems of the Tertiary formations.
Tómasson (1971) studied the permeability of postglacial lava
flows and found the hydraulic conductivity in such rock formation
to lie in the range k = 1 cm/sec to k = 10 cm/sec. This suggests
the use of Darcy’s law to estimate the flow velocity of water in
young rock formations.
Darcy’s law can be written as
where q
k
- dh/dl
The specific discharge is defined by
where Q: the discharge of water
A: the total cross-section area of the porous media
The mean velocity, vmean, of the water can then be found by divid-
ing the percolation velocity by the effective porosity netf,
Vmean q/neff
where neff.- the volume of mobile water in the rocks divided by
the total volume.
Now assuming that cold groundwater flows through a postglacial
lava field for a distance of 100 km with an altitude difference of
600 m. This is equivalent to groundwater flowing from the north-
ern side of the Vatnajökull glacier through the postglacial lava field
Ódáðahraun and into the lake Mývatn. There exist no measure-
ments of how much water is contained in such lava fields, but here
we assume the effective porosity to be approximately 0.2. Using
the lower value of k = 1 cm/sec, the mean flow velocity of the
water would be vmean = 9 km/year. Thus under these conditions it
will take approximately 10 years for the water to flow a distance of
100 km. Using the higher value of k = 10 cm/sec, the water would
travel the 100 km distance in approximately one year.
specific discharge
hydraulic conductivity
hydraulic gradient
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