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1 oxygen sensitive, the same precautions used for collection of DO samples should be followed (groundwater samples taken separately for ORP, measured ex situ). In dilute samples, it can take some time for the reading to be stabilize. minimize mixing to prevent oxygen introduction A flow-cell system will prevent mixing and minimize oxygen Interference. Redox probes can foul ' Nitrate Nitrate can be measured by colorunetnc methods, by use of a specific electrode, or by ion ' chromatography. Ion chromatography, a lab based method, is recommended. Although field colorimetric kits are available, the method Is sensitive to interference from dissolved constituents often found In groundwater. The presence of dissolved iron can give a false positive on the ' colorimetric method Nitrate concentrations can change due to bioactivity. Check with the analytical laboratory concerning the use of field preservatives. LSulfate Sulfate can be measured by colorimetric (field) methods or in the laboratory by ion chromatography. Due to potential interferences in the field methods for nitrate, it is probably best to measure sulfate and nitrate concurrently by the appropriate laboratory methods. Check ' with the analytical laboratory concerning the use of field preservatives. • Ferrous Iron ' In intrinsic evaluations, the form of Iron of interest is dissolved ferrous Iron (W). To separate the dissolved iron from precipitated iron, the sample should be filtered through a 0.45 micron ' filter immediately after collection. Time between collection and filtermg is Important si tre ferrous iron will react with oxygen and precipitate as ferric iron (Fe). If a pump is used for sample collection, an Inline filter should be considered. If total iron is required, a second non- filtered sample should also be collected. The sample should be stored In plastic bottles containing nitric acid as a preservative. All the samples containers and filters should be washed with a dilute nitric acid solution and rinsed with distilled water prior to taking into the field. Analysis should be done in the lab by TCP (Inductively Coupled Plasma). The concentration of other metals can be taken on the sample If required. HACH® colorimetric methods are available for ferrous and total iron. Since ferric iron does not react in the ferrous method, filtration Is not required unless the sample is turbid. If the sample is turbid it should be filtered through a 0.45 micron filter. In the total iron method all the iron is chemically reduced to ferrous iron and measured. Both these methods are pH sensitive. Follow the Instructions for controlling and measuring the pH. pH control is achieved by acidifying with hydrochloric acid. Correct the measured concentrations for the dilution caused by acid addition. Since ferrous Iron will oxidize and precipitate in the presence of oxygen, completely fill the . collection bottle to prevent headspace. Analyze the samples within six hours. �J 11