Unfortunately, electrodes forin situmeasurements are not available for all
parameters yet, and the price of the electrodes may be prohibitive. Instead,
analysis of water samples from dipwells or piezometers is a reasonable alter-
native if precautions are taken during sampling. It is essential to purge them
prior to sampling, in order to minimise problems with oxygen uptake and oxi-
dation of reduced species in any stagnant water. However, purging alone does
not solve all problems related to dipwells. It may actually lead to instantly
increased oxygen concentrations in the water. This is probably due to the large
surface area of the water trickling into the emptied dipwell, increasing the
atmospheric contact and oxygen uptake. Within a few hours or days the oxygen
312 Chapter 12
Figure 1Equipment used for measuring dissolved oxygen and pH profiles from the soil
surface. On the left is shown, the specially-designed steel probe (with white
markings for each 5 cm), with the inner rod which has the measuring sensor
attached. The middle image shows the dissolved oxygen micro-sensor with
protective hypodermic needle. The image to the right shows the pH electrode