Jökull - 01.12.1988, Page 45

• Very strong concentration of the groundwater
flow towards preferred spring areas (yields
exceeding 1 m3/s).
• A high storage coefficient in the pyroclastic rocks
(0.1 - 0.5), but a low one in the lavaflows (0.01 -
0.1).
The central volcanoes in the young, recently
active formations play a similar role as the ones in
the older formations. They are irregular in structure,
the rocks have often been strongly altered and the
permeability has been drastically reduced over parts
of them, especially when compared to the high per-
meabilities found outside the central volcanoes.
High-temperature geothermal fields, connected with
these volcanoes, are the most important sources of
geothermal heat in Iceland. Fresh water is not abun-
dant in these areas, because of the geothermal
impact on its chemistry and the reduction in permea-
bility, even at the surface. Most of them are also
situated far away from the areas of major settle-
ments, where other sources of fresh water may be
more abundant and better accessible.
THE QUANTITY OF THE GROUNDWATER
As indicated above the discharge of springs and
wells is one or even two orders of magnitude less in
the Tertiary - Early Quatemary regions than in the
Late Quatemary - Recent ones. Discharge of springs
from rockslides, screes and (often temporary) from
river gravels is commonly in the range 1-10 1/s.
The total discharge from a single rockslide or any
other delimited area or system of aquifers seldom
exceeds 100 1/s. Yields of successful wells or
boreholes in river gravels are usually in the range 5 -
20 1/s. Drilled wells in unaltered and untightened
Tertiary basalts have given higher yields, but it is
not at present known how constant these yields will
remain in the course of time.
The springs from fissure zones and lavafields in
the younger formations have often a discharge in the
range 10 - 1,000 1/s, while the total discharge from
spring areas may be as high as 1 - 100 m3/s (see Fig.
4). Wells drilled in postglacial, fissured lavafields
are known to yield more than 100 1/s and wells in
interglacial lavafields often have yields in the range
10 - 100 1/s. The groundwater flow in these young
formations is very strong as a result of the high
infiltration ratio and the high precipitation (see Fig.
1). Due to the volcanic build-up in the active zones,
especially the heaping up of pyroclastics, these
zones are mountainous, giving rise to a strong oro-
graphic precipitation. Iceland lies in the path of
many of the cyclones of the Northem Atlantic -
Polar Front and receives a good deal of the precipi-
tation released from these. On the glaciers in the
southeastem part of the country the annual precipi-
tation is considered to exceed 4,000 mm and on the
mountainous Reykjanes Peninsula in the southwest
it probably also reaches 4,000 mm.
Many areas in Iceland are thus abundantly sup-
plied with groundwater. The groundwater outflow
from the northem part of the Reykjanes Peninsula in
southwest Iceland, in the vicinity of Reykjavík, is
probably 10-20 m3/s, a great part of which is
flowing out at the coast below sea level. An outflow
of groundwater of a similar volume has been esta-
blished for the westem part of the Reykjanes Penin-
sula, while even more outflow is expected on the
southeastem side of it (Sigurðsson, 1986). More
than 60 % of the total population of the country
lives on or close to the Reykjanes Peninsula, where
they are well provided for with fresh groundwater.
In the Öxarfjörður region in Northeastem Iceland
near to 50 m3/s may flow out in springs, but this
region is only inhabited by 200 - 300 people. On the
Northwestem Peninsula (Vestfirðir), and in some
other regions on the older formations, groundwater
may be deficient, at least locally and especially at
the more populous settlements.
QUALITY OF THE GROUNDWATER
Chemistry of the fresh groundwater — The strong
winds blowing inland from the Atlantic Ocean
around Iceland carry with them a lot of sea spray
and salt particles, which the precipitation brings
down to the land surface. The concentration of this
marine component is increased by the evapotran-
spiration of a part of the precipitation back into the
air. A comparison of the chemistry of precipitation
and local groundwater indicates that this increase
JÖKULL, No. 38, 1988 43