Rit (Vísindafélag Íslendinga) - 01.06.1976, Page 29

Thus it can be concluded that the evaporating rate from rivers
is so small, that enrichment of deuterium due to fractionation needs
no special consideration.
Where rivers are fed by lakes with a great surface and small out-
flow, the change in deuterium content due to fractional evapora-
tion may however rise to such a level that it needs to he considered.
3. 3 Thermal water.
Collecting samples of hot water for isotopic analysis is more diffi-
cult than collecting samples of cold water. In some cases it is impos-
sible to take a sample that maintains the original isotope ratio.
In the following discussion we will assume that the water is
heated at depth to a certain temperature, called the base tempera-
ture, after which it begins its upwards movement.
When the base temperature is less than 100°C, a reliable sample
may be collected where the water flows out from the hot spring or
the borehole.
In certain cases a sample from borehole with base temperature
higher than 100°C can be collected under pressure higher than the
vapour pressure at the respective temperature, thus avoiding iso-
topic fractionation due to boiling. This is for instance the case for
the Reykjavík boreholes. The water in the boreholes has a tempera-
ture of approximately 130°C and flows from the boreholes through
pipelines under a pressure greater than the vapour pressure at
130°C. Here the arrangements allow cooling of the water below
100°C, before the pressure is lowered.
In natural hot springs or open boreholes with base temperature
higher than 100°C and therefore discharging a mixture of steam
and water at 100°C and 1 atm pressure, only the water phase can
be collected. This water phase has then already been enriched in the
heavier isotopes deuterium and oxygen-18 due to evaporation.
To calculate the original deuterium content of the groundwater
from that in 100°C water phase it is necessary to know the amount
of water which has evaporated and the fractionation factor for the
deuterium between steam and water.
Water which flows upwards in a borehole or a hot spring channel
25