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Water-level, Floating Hydrocarbon, and Total Well-depth !Measurements Before purging and sampling, the depth to water, floating hydrocarbon thickness, and the total well depth were measured using an interface measuring system. The oil/water interface measuring system consists of a 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 were measured by lowering the probe into the L well. Liquid levels were recorded relative to the tone emitted at the ground-water surface. The probe was decontaminated by being rinsed with delonized water or steam cleaned after each use. Alternatively, an electric ' sounder and a bottom-filling Teflon bailer may have been 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 was ' measured by lowering the sensor into the monitoring well. A low-current circuit was completed when the sensor contacted the water, which served as an electrolyte. The current was amplified and fed into an indicator light and audible buzzer, signaling when water had been contacted. A sensitivity control compensated for highly saline or conductive water. The electric sounder was decontaminated by being rinsed with deionized water after ' each use. The bailer was lowered to a point just below the liquid level, retrieved, and observed for floating hydrocarbon. Liquid measurements were recorded to the nearest 0.01 foot in the field logbook. The ground-water elevation at each monitoring well was calculated by subtracting the measured depth to water from the surveyed elevation of the top of the well casing. Total well depth was then measured by lowering the sensor to the bottom of the well. Total well depth, used to calculate purge volumes and to determine whether the well screen was partially ' obstructed by silt, was recorded to the nearest 0.1 foot in the field logbook. Well Purging ' Before sampling occurred, a centrifugal pump, submersible pump, pneumatic displacement pump, or bailer was used to purge standing water ' in the casing and gravel pack from the monitoring well. Monitoring wells were purged according to the protocol presented in Figure D-1. In most monitoring wells, the amount of water purged before sampling was greater than or equal to four casing volumes. Some monitoring wells were expected to be evacuated to dryness after removing fewer than four casing volumes. pi010f9010f900020.1 ag D - 3 Rev. 0 December 15, 1992