Why does ORP seemingly dive at times just when the other parameters are stable, is there some thing the fuel tank is doing to cause the shift?

The answer the second half of the question is no, the fuel tank is not doing anything wrong. ORP is a difficult parameter to measure accurately in the field because in fact, ORP is a net measure of all the oxidizing and reducing processes that are occurring within the groundwater. As we continue to purge the well, we may see stabilization of these parameters or may see them change. Unfortunately, the other parameters such as conductivity and dissolved oxygen may be stable and, all of a sudden, the ORP changes. This is problematic if you are required to monitor it but it is not an issue in terms of getting a representative sample. Since it’s a net measurement you will see it go up or down during purging, it doesn’t follow a specific pattern. As long as your other parameters, such as conductivity and dissolved oxygen, remain stable take the samples and do not be concerned about the fluctuating ORP value.