Recently a couple of bore water samples came through the lab. By coincidence although they came from very different locations they had some distinct similarities. These were elevated dissociated carbon dioxide, elevated soluble iron and elevated manganese. But they both had another similarity – low ORP or redox value. The sample with the highest metal levels had an ORP of around 110 mV. In the other the ORP was around 160 mV. I usually expect ORP levels of between 200 – 400 mV in good quality water.
ORP is a measure of oxidizing capacity. A low ORP especially below 0 indicates reducing conditions. In the sample above with the lowest ORP I aerated the water with an aquarium pump for 5 hrs but could only get the ORP to around 300 mV. After that it slipped back to around 200 mV. What an ORP probe measures is the average effect of usually many different redox reactions pulling the mV value up and down. Oxygen supply is thought to mainly influence higher ORP values but lower values are created by reduction of oxygen supplying species (molecules / substances) such as sulphate and oxides of metals like iron and manganese.
For more detail on why measuring redox is important please see pH and ORP in water.