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Water-level, Floating Hydrocarbon, and Total Well-depth Measurements Before purging and sampling occurs, the depth to water, floating hydrocarbon thickness, and the total well depth will be measured using an oiltwater interface measuring system The oiltwater interface measuring system consists of a sonic probe that emits a continuous audible tone when immersed in a nonconductive fluid, such as oil or gasoline, and an intermittent tone when immersed in a conductive fluid, such as water The floating hydrocarbon thickness and water level will be measured by lowing the sonic probe into the well Liquid levels will be recorded relative to the tone emitted at the groundwater surface The sonic probe ' will be decontaminated by being rinsed with deionized water after each use Alternately, an electric sounder and a bottom-filling, clear Teflon® bailer may be used to record floating hydrocarbon thickness and depth to water The electric sounder is a transistorized instrument that uses a reel-mounted, two- conductor, coaxial cable that connects the control panel to the sensor Cable ' markings are stamped at 1-foot intervals The water level will be measured by lowering the sensor into the monitoring well A low-current circuit is completed when the sensor contacts the water, which serves as an electrolyte The current is ' amplified and fed into an indicator light and audible buzzer, signaling when water has been contacted A sensitivity control compensates for highly saline or conductive water The electric sounder will be decontaminated by being rinsed with deionized water after each use The bailer will be lowered to a point dust below the liquid level, retrieved, and observed for floating hydrocarbon ' Liquid measurements will be recorded to the nearest 0 01 foot in the field logbook The groundwater elevation at each monitoring well will be calculated by subtracting the measured depth to water from the surveyed elevation of the top of the well casing (Every attempt will be made to measure depth to water for all wells on the same day ) Total well depth will then be measured by lowing the sensor to the bottom of the well Total well depth, used to calculate purge volumes and to determine whether the well screen is partially obstructed by silt, will be recorded to the nearest 0 5 foot in the field logbook In the event of shallow wells where water surface is visible, direct measurement using a clean tape measure may be employed Well purging will be carried out ' prior to sampling for contamination 1 C - 3