101
give rise to problems (e.g. sands from the shelf or
purely foraminiferal sands on the mid-ocean ridge).
The reason for this is, first of all, that these sediments cannot be compacted by pressure exertion or
centrifugation. Consequently, practically no water
can be obtained from them at all. It is of even more
importance that these sediments possess a high
degree of permeability so that as soon as the corer is
raised out of the water, the pore water (and sometimes the core material as well) is spilled out from the
bottom. A simple method for the extraction of pore
water from coarse, sandy material by centrifugation
is described by Saager et al. (1990). However, this
technique does not solve the problem of how to
bring a specimen of sand and its pore water into a
centrifuge unperturbed. Only the water which is
present at the sampling site at a certain point in time
can be collected, even if the abovementioned
rhizons are applied. However, it must not be the
same water which had been in situ at this location.
In situ measurements of pH-values and oxygen
with the aid of landers are certainly still possible (cf.
Sect. 3.5). But since the thin electrodes are likely to
break off upon examining such material, these
sediments are not very much favoured. Fortunately,
this particular sediment type often contains only a
small amount of decomposable organic matter, so
that the biogeochemical processes of diagenesis do
not display reaction rates as high as other sediment
types. Upon sparing these sediments, the pore water
geochemist usually suffers no great loss.
3.3.3
Storage, Transport and Preservation
of Pore Water
In this section, the vessels in which the pressed
or centrifuged pore water is kept until the time for
analysis arrives will be briefly discussed. In
Section 3.4 it will also be considered, in connection with the analytical methods, how much time
one may allow to pass between pore water preparation and analysis. Hence the decision depends on whether immediate analysis is necessary, or whether the analyses can be made at a
later time, provided that an appropriate preservation of the material has been accomplished.
Vessels, Bottles
Bottles made of polyethylene have proven to be
quite useful for the storage of filtered pore water
samples. In most cases the disposable 20 ml PEbottles which are used in scintillation measurements are very much suited to the purpose. These
bottles need not be cleaned prior to use, but are
just briefly rinsed with the first milliliters that run
out of the press or the rhizon. A second use does
not really make sense considering the low price of
these bottles and the obligatory amount of work
effort and detergent to be invested in their
cleaning. If several bottles are to be used for the
storage of one and same sample, for instance
when the sample volume is rather large due to
water-rich sediments, precautions should be taken
to be sure that the entire sample amount is homogenized prior to the analytical measurements,
since differences in contrations between the first
and the last drop of pore water are possible.
Preservation of Pore water Samples
Only the alkali-metals, sulfate, and the halogenides do not require any preservation of the samples prior to their analytical measurement at a later
date. Otherwise, pore water samples that are not
to be analyzed immediately can be preserved with
regard to some dissolved substances, allowing a
delayed analysis.
The concentrations of most cations - as long
as different valences of iron, for instance, are not
to be determined - are measurable after a longer
period of time when the samples’ pH is lowered by
the addition of acid (mostly HNO 3 ) to a value of at
least pH 2 or pH 3. Care should be taken that a
sufficient amount of ‘suprapur’ quality acid is
added. The required amounts can vary considerably depending on the alkalinity of the sample.
Usually, concentrated acid will be used in order to
prevent unnecessary dilution of the sample. It is
also important that the sample has been previously passed through a filter of 0.1 - 0.2 µm poresize, since the acid will dissolve all particles (and
certainly the colloids that pass the filter as well).
As for the substances that are altered in their
concentration by microbial activity, the toxification of sample is recommended. Mercury (II)
salts are not very much favored since the handling of these extremely toxic substances - which
are toxic also to man - requires special precaution
measures. Chloroform is banned from most laboratories because it is considered as particularly
carcinogenic. Thus, only the solvent TTE
(trifluorotrichloro-ethane) remains which is frequently used in water analyses. The addition of
one droplet usually suffices since only a small but
sufficient proportion dissolves whereas the rest
3.3
Sampling of Pore Water for ex situ Measurements
give rise to problems (e.g. sands from the shelf or
purely foraminiferal sands on the mid-ocean ridge).
The reason for this is, first of all, that these sediments cannot be compacted by pressure exertion or
centrifugation. Consequently, practically no water
can be obtained from them at all. It is of even more
importance that these sediments possess a high
degree of permeability so that as soon as the corer is
raised out of the water, the pore water (and sometimes the core material as well) is spilled out from the
bottom. A simple method for the extraction of pore
water from coarse, sandy material by centrifugation
is described by Saager et al. (1990). However, this
technique does not solve the problem of how to
bring a specimen of sand and its pore water into a
centrifuge unperturbed. Only the water which is
present at the sampling site at a certain point in time
can be collected, even if the abovementioned
rhizons are applied. However, it must not be the
same water which had been in situ at this location.
In situ measurements of pH-values and oxygen
with the aid of landers are certainly still possible (cf.
Sect. 3.5). But since the thin electrodes are likely to
break off upon examining such material, these
sediments are not very much favoured. Fortunately,
this particular sediment type often contains only a
small amount of decomposable organic matter, so
that the biogeochemical processes of diagenesis do
not display reaction rates as high as other sediment
types. Upon sparing these sediments, the pore water
geochemist usually suffers no great loss.
3.3.3
Storage, Transport and Preservation
of Pore Water
In this section, the vessels in which the pressed
or centrifuged pore water is kept until the time for
analysis arrives will be briefly discussed. In
Section 3.4 it will also be considered, in connection with the analytical methods, how much time
one may allow to pass between pore water preparation and analysis. Hence the decision depends on whether immediate analysis is necessary, or whether the analyses can be made at a
later time, provided that an appropriate preservation of the material has been accomplished.
Vessels, Bottles
Bottles made of polyethylene have proven to be
quite useful for the storage of filtered pore water
samples. In most cases the disposable 20 ml PEbottles which are used in scintillation measurements are very much suited to the purpose. These
bottles need not be cleaned prior to use, but are
just briefly rinsed with the first milliliters that run
out of the press or the rhizon. A second use does
not really make sense considering the low price of
these bottles and the obligatory amount of work
effort and detergent to be invested in their
cleaning. If several bottles are to be used for the
storage of one and same sample, for instance
when the sample volume is rather large due to
water-rich sediments, precautions should be taken
to be sure that the entire sample amount is homogenized prior to the analytical measurements,
since differences in contrations between the first
and the last drop of pore water are possible.
Preservation of Pore water Samples
Only the alkali-metals, sulfate, and the halogenides do not require any preservation of the samples prior to their analytical measurement at a later
date. Otherwise, pore water samples that are not
to be analyzed immediately can be preserved with
regard to some dissolved substances, allowing a
delayed analysis.
The concentrations of most cations - as long
as different valences of iron, for instance, are not
to be determined - are measurable after a longer
period of time when the samples’ pH is lowered by
the addition of acid (mostly HNO 3 ) to a value of at
least pH 2 or pH 3. Care should be taken that a
sufficient amount of ‘suprapur’ quality acid is
added. The required amounts can vary considerably depending on the alkalinity of the sample.
Usually, concentrated acid will be used in order to
prevent unnecessary dilution of the sample. It is
also important that the sample has been previously passed through a filter of 0.1 - 0.2 µm poresize, since the acid will dissolve all particles (and
certainly the colloids that pass the filter as well).
As for the substances that are altered in their
concentration by microbial activity, the toxification of sample is recommended. Mercury (II)
salts are not very much favored since the handling of these extremely toxic substances - which
are toxic also to man - requires special precaution
measures. Chloroform is banned from most laboratories because it is considered as particularly
carcinogenic. Thus, only the solvent TTE
(trifluorotrichloro-ethane) remains which is frequently used in water analyses. The addition of
one droplet usually suffices since only a small but
sufficient proportion dissolves whereas the rest
3.3
Sampling of Pore Water for ex situ Measurements