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- 2 - <br /> Determination of Sheen <br /> After depth to water and free-product thickness measurements are performed, a <br /> test for the presence of sheen is conducted. A transparent bailer is lowered into <br /> the well in a manner such that only part of the bailer is submerged The bailer is <br /> withdrawn from the well and the surface of the water in the bailer is examined for <br /> the presence of sheen as determined by the presence of iridescence or <br /> emulsification Presence of sheen is not investigated if it is determined that free <br /> product is present in the well. Use of a cleaned dark colored bucket is helpful for <br /> sheen-determination purposes. <br /> Total Well Degth Measurement <br /> Once the test for sheen presence has been conducted, the total depth of the well <br /> is measured Total well depth is determined by measuring from the reference <br /> elevation described in the section for Water Level Measurement, above, to the <br /> depth at which tension in the tape measure (to which the electric probe is <br /> attached or the steel tape is attached) becomes slack. The tape measurement is <br /> corrected for probe length Total well depth measurements are made to the <br /> nearest 0 05 feet <br /> Well Purging <br /> In order to obtain a representative sample of the water in the aquifer being <br /> sampled, stagnant water in the well casing must be removed to permit well <br /> recharge with non-stagnant aquifer water The removal of stagnant water is <br /> accomplished by the removal of the water to the surface where it will be either <br /> disposed of or stored for future disposal <br /> The purging rate used at a particular monitoring well will depend on the expected <br /> or known hydraulic yield of the well <br /> In moderate to high yield formation wells the purging device will be placed near <br /> the top of the screened interval of the well to ensure that non-stagnant formation <br /> water will move upward in the screened interval When purging low-yield <br /> formation wells, water will be removed from the bottom of the screened interval. <br /> When purging low-yield wells (wells which yield less than 3 casing volumes), the <br /> wells will be purged to dryness once As soon as the well has recovered to a <br /> volume sufficient for sampling, samples will be collected At no time will a well be <br /> purged to dryness if the rate of recharge is such that formation water will cascade <br /> down the sides of the casing. <br /> During purging operations, the field parameters of pH, temperature, and electrical <br /> conductivity (EC) in the purged water will be monitored and recorded at a <br /> minimum rate of two (2) readings per casing volume purged. Stabilization of the <br /> parameters of pH, temperature and EC will be used to indicate that the well has <br /> been sufficiently purged for sampling. Parameter stabilization will be indicated <br /> by at least three near-constant pH, temperature, and EC values for a minimum of <br /> i one (1) casing volume The acceptable range of values for stabilization of the <br />