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Evaluation of Naturaldttenuation: 7500 West Eleventh Street, Tracy, CA. Page 13 4.0 GROUNDWATER SAMPLING AND ANALYSIS Because the groundwater sampling program conducted at the Navarra Site over the period February 24 - March 16, 2005 was intended to support an evaluation of natural G' p rY pp attenuation of contaminants in the groundwater beneath the site as well as the standard quarterly round of groundwater-quality monitoring, it was necessary to develop sampling protocols that would avoid generating conditions by which certain key analytical results would be rendered inaccurate by the techniques used to -recover samples for analysis of other parameters. In particular, it was essential that measurements of parameters such as ? <� dissolved oxygen, chemical oxygen demand and ferrous oxide were not affected by entrainment of air into the water in the wells. To comply with the requirements described above, the order of sampling for the various parameters was strictly controlled. To achieve this, the samples for the various tests were recovered in the order that they appear in the following sections of this report, which also describes the purposes of the various analyses and the protocols by which they were performed_ The scope of the sampling was designed to permit analyses for all of the secondary indicators for natural attenuation processes that are included in ASTM '= standard E 1943-98 (American Society for Testing and Materials 2004). 4.1 Field Tests for Natural Attenuation Parameters Analysis of groundwater for ferrous oxide, dissolved oxygen, oxygen content, oxidation- G;; reduction potential (redox potential) provides data of primary importance to the development of secondary evidence that natural attenuation processes are active at a site. These parameters are measured in the field. It is standard practice to also measure the total dissolved solids, conductivity, pH and temperature of the groundwater at the same time. Because accurate measurement of the parameters involving oxygen and oxides requires that oxygen is not inadvertently entrained into the groundwater by the sampling process, SJC performed the field tests for the parameters cited above prior to the purging the monitoring wells, which is required prior to recovery of samples for analysis for components of fuel hydrocarbons. ' 4.1.1 Ferrous Oxide The Hach Method (8146), which measures the concentration of ferrous oxide, was used for field measurement of that compound in all 27 wells extant on the Navarra Site. The Method involves dispensing an ampule of 1,10 phenanthroline into a glass vessel containing the water sample. The 1,10 phenanthroline reacts with ferrous iron in the sample to form an orange color. A calibrated colorimeter is then used to determine the concentration of ferrous oxide in the sample. Samples of the groundwater in each well were pumped into the glass container used for the ferrous oxide test by means of a peristaltic pump fitted with a 0.45µ filter in its discharge line. To ensure that the pump lines were fully purged of air, the pump riser line was slowly lowered into the well and suspended several feet below the water table (but sic