Several different methods have been used for determining pH in these pro-
grammes. Most have focused on pore water in the soil, using either dipwells or
piezometers in waterlogged environments. Alternately, suction samplers that
allow sampling of pore water from soil above the ground water level. In some
studies pH has been measured in soil samples, typically in soil sampled dur-
ing the installation of monitoring equipment. Direct measurement of pH in
the soil has been tried at several waterlogged sites using electrodes installed
at a selected depth for 7–10 days.
Some of the problems with pH measurements include the oxidation of
reduced soil and water samples by atmospheric oxygen, normally causing acid
production and thus a decreased pH in the sample. They also include loss of
gaseous species such as carbon dioxide, ammonia or hydrogen sulfide during
and after sampling. This is due to the different partial pressures below and above
ground – e.g.the partial pressure of carbon dioxide is often 10–100 times higher
in the soil than in the atmosphere, due to root respiration and decomposition of
organic matter. The magnitude of these problems will vary between different
methods and sites. For instance, degassing of carbon dioxide during the use of
suction samplers can easily lead to pH increases of^1 – 2 –1 pH units and degassing
during drying of soil samples may give deviations as high as 2 pH units.
Deviations are also possible when water is standing in a dipwell open to the
atmosphere. Theoretically, the ideal solution is to measure the pH in situ in the
soil rather than taking samples. This approach has recently been evaluated mak-
ing in situ measurements of pH from the soil surface using a specially developed
steel probe incorporating an Ion Sensitive Field Effect Transistor (ISFET) elec-
trode, which is a very sturdy, solid-state pH electrode (Figure 1). This method
gave reliable results and avoided problems with oxidation and degassing of soil
or water samples, and allowed the measurement of quite detailed pH profiles
from the soil surface. It can also be recommended to use the ISFET electrode
during archaeological excavations as it allows an immediate evaluation of the
pH values and burial conditions in open soil profiles with a minimal disturbance
of the soil. However, in situmeasurements with an ISFET electrode should be
considered a supplement to, rather than substitution for, the existing methods.
For instance, the ISFET electrode used is not suitable for permanent installation.
3 DETERIORATION OF MODERN MATERIALS ANALOGOUS
TO ARTEFACTS
Further to monitoring the environment itself, modern materials placed on a site
can serve as analogues to the types of archaeological material found. In this way
their deterioration can be regularly monitored in order to better understand
the processes. Furthermore, these materials serve as a “proxy” indicator of the
environment, and can potentially serve as a warning should the environment
In-situ Preservation of Waterlogged Archaeological Sites 315