Jökull - 01.12.1988, Page 42

weathering cover. The principal aquifers are the
rockslides and the river gravels. Some rockslides
have extensions of some km2 and thicknesses of
more than 10 m. Springs issuing from them have a
discharge up to some tens of 1/s, which is the same
order of magnitude as that of other "great" springs in
the Tertiary.
In the older formation the bedrock is more or less
impermeable, spring-fed rivers are rare and glacial
rivers only cross these formations on their way from
the geologically younger, glacier capped highlands.
The surface runoff is high, the rivers are liable to
flooding and the gravel plains at the valley bottoms
correspondingly voluminous, although often divided
into basins. In the lower courses of the rivers, and
on all the glacial rivers, the sediments tend to
become finer, reducing the permeability and making
the construction of wells difficult. Seasonal variation
in the rivers is very strong and so are the variations
in the groundwater flow in the river gravels,
although tempered by the storage effect of the
aquifer. Seasonal changes in temperature of the
water are also great, which may cause some
difficulties during the winter time.
In the Late Quatemary - Recent regions spring-
fed rivers and glacial rivers are predominant, the
latter ones often being reduced to their spring com-
ponent during the winter season. During the
snowmelt in spring - early summer and after rain-
storms most rivers are in flood, but those without a
steady supply often almost dry up in between.
Potential sedimentary aquifers may also dry up,
because of the high permeability of the bedrock. In
these regions the groundwater extraction is almost
exclusively from springs or highly permeable rocks.
LATE QUATERNARY AND RECENT BEDROCK
The late Quatemary and Recent formations are
also of volcanic origin and mostly basaltic in com-
position. Quite a number of central volcanoes have
also been active in these younger geological periods
and some are still active, e.g. Hekla, Katla, Askja
and other intemationally well known volcanoes
(Fig. 3).
Pyroclastics, lavafields andfissure swarms — A pec-
uliarity of the volcanism during the glacials is the
subglacial heaping up of fragmentary rocks, tuffs,
breccias and pillow lavas, due to extrusion under an
ice cover and rapid chilling of the lava in the
subglacial/glacial meltwater. These rocks often
appear as steep-sided mountains, but are also found
as extensive layers, perhaps of a secondary origin.
The porosity of these rocks is high, up to 50 %, but
the permeability is strongly varying. The fine-
grained tuffs have in most cases narrow pores and
low permeability, which may be further reduced by
alteration, either contemporary to the eruption, post-
volcanic or due to later geothermal activity. The
same applies to the glassy matrix of breccias. The
highest original permeability is to be found in the
pillow lavas, but it may also have been subjected to
reduction through alteration, especially when the
pillow lavas contain a high proportion of glassy
matrix.
The postglacial lavaflows have very high permea-
bility (conductivity 0.001 -1.0 m/s). Like their Terti-
ary counterparts they have a relatively low storage
coefficient (0.01 - 0.1), in contrast to the fragmen-
tary, "pyroclastic" rocks, described above. The per-
meability of the interglacial lavaflows has often
been reduced by glacial tightening, razing off of the
scoriaceous top layers and perhaps some deforma-
tion or rearrangement of joints under the glacial ice
cover. In any case their permeability is as a mle less
than in the postglacial lavas, the difference being
one or even more orders of magnitude.
Both the postglacial and the interglacial lavafields
reach from the volcanically active highlands into the
older lowlands, some porphyritic basaltic lavaflows
being known to have reached more than 100 km
away from the eruption sites. In many cases great
springs issue from these lavastreams, with a
discharge of tens or hundreds 1/s, i.e. an order of
magnitude higher than the greatest springs in the
older formations (Fig. 4).
Great springs and spring areas — The total
discharge of a spring area of only some few km2
extent can be some tens of m3/s, as is the case at the
northem end of the lake Þingvallavatn. Quite a
40 JÖKULL, No. 38, 1988