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I The use of a flow-cell probe system (Figure 1) attached to theum outlet can atmospheric contact A simple flow-cell system can be made by allowing the water tminimize slowly overflow a container with an immersed probe (Figure 2). The flow-cell reduces the potential to ' introduce atmospheric oxygen. A similar system can be used for monitoring redox potential. DO pmbe ex situ - Oxygen specific probes work by detecting the diffusion of oxygen across a ' membrane. There are a variety of oxygen probes on the market. The instructions supplied with the probe should be followed. To obtain accurate information, the probe should be calibrated regularly A probe that allows for two point calibration, air saturated and zero oxygen, will allow ' for more accurate readings at the low concentrations encountered in contaminated ground water The membrane of an oxygen probe is subject to fouling and should be replaced whenever fouling is suspected DO probes themselves use a small amount of oxygen, therefore the water should be gently mixed or the probe kept in motion to prevent mass transfer effects at the membrane surface. Be careful as overmixmg can introduce atmospheric oxygen. To 'nimiye contamination by air, there are probes that ft snugly into the mouth of a special sample bottle. DO probe in well - With enough cord, probes can be used directly in the well Carefully lower the probe to prevent air introduction, aend g ntly move�the ' probe to m m:;ze the effect of probe oxygen utilization. Nonaqueous phase liquid (free product) can foul the probes membrane. t. A drawback of using the probe is that it can take some time to stabilize at the correct reading. This is especially true at the low concentrations encountered in groundwater. During this time concentration changes can occur due to the introduction of atmospheric oxygen or depletion of oxygen due to the probe Use of a flow-cell system will minimize this effect. Field test kit - The chemical Winkler method for dissolved oxygen determination is a two step process. In the first step, chemicals are added to the solution to precipitate the available oxygen (fixation). The amount of trapped oxygen is then detected by a process called titration. In the second step, the addition of a titrant causes the solution to change color, according to the amount of titrant added The amount of titrant required is proportional to the amount of oxygen originally present A field kit is available which simplifies this method (HACH® Winkler Titration Kit) Field experience suggests it is accurate to low concentrations (0.2 mg/L). Another field test kit is the CheMetricsD Rhodazme-D Colorimetric Kit. Oxidation-Reduction Potential (ORP or Eh) A portable probe similar to a pH probe is used to measure ORP. It is important to read and ' understand the instructions that come with the probe. Although all redox probes give results in millivolts (mV), the relative reading depends on both the type of probe and sample temperature. A probe specific correction factor is required to convert the raw mV reading into Eh (calculated oxidation-reduction potential). When data is recorded it should be clear whether 't 's the raw readin or the corrected Eh value. The measurement should be taken in the field, as the ORP can ,. change due to oxidation of the groundwater sample. ORP s e u,�-io P ample measurements should be taken before, during, and after well purging using a direct-reading meter. Since the reading is 9