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4.1. GROUNDWATER AND PALEOCLIMATE 127
4.1.7 Excess air formation at an artificial recharge site
Matthias Kopf (participating scientists: Laszlo Palscu, Werner Aeschbach-Hertig, Eric Zechner
(UBA))
Abstract Correlations between excess air and environmental conditions during groundwater recharge
are examined. A field experiment at an artificial recharge site as well as laboratory column experiments
are conducted in order to determine in detail how different parameters influence the formation
of excess air.
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Figure 4.8: Ne excess ∆Ne versus distance from the infiltration area in the test field Stellimatten
near Basel. A significant increase of ∆Ne takes place between surface water and the freshly infiltrated
water in the infiltration area followed by mixing with background groundwater.
Background Surface water infiltrating the
ground is equilibrated with the atmosphere, its
dissolved noble gas concentrations reflecting the
soil temperature because of the temperature dependence
of the Henry coefficients. However, as
the groundwater level increases as a result of the
infiltration, bubbles of air are entrapped in the
groundwater, adding an excess component to the
dissolved gases (the so-called excess air). In order
to determine paleo recharge temperatures from
the equilibrium component, the accumulation of
excess air has to be understood. Column tests
with sand of different granulation and field experiments
in areas with well-known recharge conditions
are performed to study the correlation of
excess air with parameters such as pressure variation
during groundwater level changes.
Methods and results A field experiment has
been conducted at an artificial recharge site (Stellimatten)
operated by the water works of the city
of Basel. The sequence of alluvial deposits at
this site ranges from silty clay to coarse-grained
gravel with hydraulic conductivity values between
9 · 10 −7 m/s and 2 · 10 −3 m/s. The study area lies
in a steady groundwater stream, to which periodically
every 30 days Rhine water is added by
flood irrigation. This infiltration lasts 10 days,
afterwards the flooded area is allowed to regenerate
for 20 days. During such a cycle, samples for
radon, dissolved noble gases, stable isotopes and
SF6 were taken. Groundwater level changes, temperature,
and conductivity were monitored.
Very well visible is the accumulation of radon in
the infiltrated Rhine water and the mixing with
the existing groundwater. At the surface, the
infiltrating Rhine water has an activity of < 6
Bq/l, increasing to < 20 Bq/l as it reaches the
aquifer. At the end of the test field the water
has an activity of < 70 Bq/l, comparable to the
steady groundwater stream (A ≈ 80 Bq/l), indicating
mixing with the groundwater. The stable
isotopes show a clear separation of the groundwater
stream ( δ 2 H≈-68�) from the Rhine water
( δ 2 H ≈-77�), mixing with increasing distance
from the infiltration point. In addition, an accumulation
of the relative Ne excess ∆Ne (a measure
of excess air) along the flow path is noticed, starting
at nearly zero ∆Ne in the infiltrating Rhine
water. Immediately after infiltration the ∆Ne increases,
indicating formation of excess air. In the
flow path, mixing between groundwater and infiltrated
Rhine water takes place, seen in the dispersion
of ∆Ne (see figure). The groundwater level
changes show a correlation with ∆Ne and therefore
excess air.
Outlook/Future work In the data obtained
from Stellimatten we look for further correlations
of excess air with parameters naturally influencing
its formation (e.g. geology, biology). The lab
experiments will allow us to explore such correlations
under even better controlled conditions.
If robust correlations are found, dissolved noble
gases in groundwater may not only be used to
derive paleotemperatures but also as a proxy for
other recharge conditions (e.g. water level fluctuations)