level in a dipwell can be made using a “dipmeter”, consisting of a probe on a
graduated cable or tape measure, which gives an audible signal when the probe
comes into contact with water. Another method is simply to use a piece of plas-
tic tubing taped to the side of an extending ruler and lower this into the dipwell.
By blowing through the tubing, bubbles can be heard when the end of the tube
is in contact with the water in the dipwell. Other methods, which have success-
fully been used are datalogging devices, which give continual measurement of
the water level. Dataloggers are expensive but give a better understanding of
the dynamics of the system, for instance during particularly wet or dry periods.
2.2 Pore Water Composition
Literature on the biodeterioration of organic materials and the corrosion of
metals show that parameters such as pH, alkalinity, conductivity, oxygen, sul-
fate, sulfide, ammonium, nitrate, nitrite, phosphate, potassium, chloride, iron
and manganese may be relevant to monitor. Some of the parameters are sus-
pected to affect the deterioration of archaeological materials directly, while
others reflect the ongoing chemical and biological processes in the soil. Fol-
lowing the creation of a waterlogged environment, oxidised species within the
soil rapidly decrease due to their utilisation by chemical reactions and
microorganisms. The lack of dissolved oxygen suppresses the activity of aer-
obic microorganisms. Thus, there is an increase in the activity of anaerobic
microorganisms, which in turn leads to the production of chemical species in
their reduced form. For example dissolved oxygen will tend to be depleted
within the first few centimetres of soil below the water level. Nitrate, sulfate and
ferric iron levels decrease, and an increase is seen in their reduced forms such as
ammonium, sulfide and ferrous iron.
Ideally oxygen, along with other parameters, should be measured directly
in situin the soil. The electrodes used for oxygen measurements directly in the
soil have to have extremely low or no oxygen consumption, otherwise they
may use up oxygen faster than it is supplied in the soil, thus always giving a zero
oxygen reading after a short while. Clark-type microelectrodes, which were
developed for measurements in sediment, have been used with success, due
to their fine size (sub-millimetre) and design they do not require stirring to
obtain reliable readings (Figure 1). With this concept, it is possible to get an
immediate idea of the oxygen concentrations in soil. It can give a very
detailed picture of the vertical variation at a site with a minimum of physical
disturbance. The electrodes used are not meant for permanent installation,
but the measured concentration profiles can give an indication of where, and
at what depths, e.g.water suction samplers should be installed. Continuous
logging of dissolved oxygen is possible using optical type oxygen sensors, yet
such devices are still relatively new on the market.
In-situ Preservation of Waterlogged Archaeological Sites 311